Posterior crossbite correction is the most common reason for early transverse treatment. This chapter provides a summary of the etiology, diagnosis, and treatment of transverse problems ranging from one‐tooth dental crossbites in the primary dentition to bilateral skeletal crossbites in the permanent (adolescent) dentition.
- Q: Can you describe the ideal permanent first molar transverse relationship? Can you describe common first molar transverse malocclusions (assume an absence of centric relation [CR]‐centric occlusion [CO] shifts during closure)?
A: In the ideal permanent first molar transverse relationship (Figure 6.1a), maxillary first molar mesiolingual cusps are seated in the mandibular first molar central fossae, mandibular first molar distobuccal cusps are seated in the central fossae of the maxillary first molars, maxillary and mandibular first molar buccal and lingual cusps (Figure 6.1b) are oriented along a straight horizontal line, and posterior teeth are in a normal buccolingual inclination. Normal first molar transverse inclinations vary with age in individuals with normal occlusion, ranging from ~10 to ~4° of buccal inclination for maxillary molars (buccal crown torque decreases with age during growth) and from ~10 to ~3° of lingual inclination for mandibular molars (lingual crown torque decreases with age during growth) . Large samples have confirmed individuals with normal transverse occlusion show mild maxillary molar buccal crown torque and mild mandibular molar lingual crown torque with the ideal cusp‐fossae relationships noted above .
Variation in molar transverse occlusion can produce transverse malocclusion including:
- Excess maxillary buccal crown torque or excess mandibular lingual crown torque (Figure 6.1c, molars on left of illustration)
- Excess maxillary lingual crown torque or excess mandibular buccal crown torque (Figure 6.1d, molars on left of illustration)
- Inadequate posterior overjet (Figure 6.1e, bilateral)
- Lingual crossbite (Figure 6.1f, molars on right of illustration)
- Complete lingual crossbite (Figure 6.1g, molars on right of illustration)
- Excess posterior overjet (Figure 6.1h, bilateral)
- Complete buccal crossbite or scissors bite (Figure 6.1i, molars on the right of illustration)
Notes: Posterior crossbites can present as unilateral or bilateral malocclusions of the primary, mixed, or permanent dentitions. Canine involvement is often seen and considered part of a posterior crossbite even though canines are not posterior teeth.
Nomenclature is based on the position of the maxillary teeth. Posterior lingual crossbite, the most common posterior crossbite, exists when the buccal cusps of the maxillary teeth are lingual to the buccal cusps of the mandibular teeth. Posterior buccal crossbite occurs when the lingual cusps of the maxillary teeth are buccal to the opposing buccal cusps of the mandibular teeth.
- Q: The terms buccal crown torque and lingual crown torque were used in the previous question. This is confusing. Isn’t torque a physics concept, like force is a physics concept? In other words, force is an interaction that causes linear motion, and torque is an interaction that causes rotation. What does torque have to do with tooth position?
A: Good question. In physics, torque is indeed a tendency to cause rotation of a body. In fact, the term “torque” is often used interchangeably with other terms (e.g. moment or couple) to describe a tendency to cause rotation of a body. Likewise, in orthodontics, torque is a tendency to cause rotation of a tooth or jaw.
Now, by convention, many orthodontists will also use the term torque to describe the position of a tooth, buccolingually. For instance, they will state that a maxillary molar has excess buccal crown torque or a mandibular molar has excess lingual crown torque (Figure 6.1c).
- Q: In Figures 6.1a–6.1i, we assumed the absence of a lateral CR‐CO shift. But, what if that was not the case? What if a lateral CR‐CO shift was present? How could Figures 6.1e–6.1f be related if a lateral CR‐CO shift was present? How could Figures 6.1h–6.1i be related if a lateral CR‐CO shift was present?
A: Let’s start with Figure 6.1e, which illustrates inadequate posterior overjet. The bite is relatively unstable here because a cusp‐to‐fossa occlusion is absent. Instead, a cusp‐to‐cusp occlusion is present. In such cases, it is not unusual for the patient to close from CR to CO with a lateral shift into a more stable cusp‐to‐fossa relationship (Figure 6.1f).
Figure 6.1h illustrates excessive posterior overjet. Once again, the bite is relatively unstable because a cusp‐to‐fossa occlusion is absent. In such cases, it is not unusual for the patient to close from CR to CO with a lateral shift into a stable cusp‐to‐fossa relationship (Figure 6.1i).
- Q: What is the incidence of posterior crossbites in the primary or early mixed dentition stage of development?
- Q: How do transverse dental relationships develop?
A: Transverse (buccolingual) dental relationships develop in the same way, and for the same reasons, that incisor labiolingual dental relationships develop. Teeth tend to erupt along their long axes throughout life until sufficient occlusal, or soft tissue, forces prevent a further eruption. Their buccolingual inclination is governed by the soft tissue envelope (tongue‐cheeks‐lips), which directs them to erupt into occlusion with teeth from the opposing jaw. In the presence of an anteroposteriorly deficient maxilla, the tongue will tend to tip the maxillary incisors forward, and the mandibular lip will tend to tip the mandibular incisors lingually (anteroposterior dental compensations for an underlying skeletal discrepancy between the jaws). In this way, the incisors can erupt into normal occlusion.
In a similar fashion, in the presence of a transversely deficient maxilla, the tongue will tend to tip the maxillary molars buccally, and the cheeks will tend to tip the mandibular molars lingually. These are transverse dental compensations for an underlying transverse skeletal discrepancy between the jaws. In this way, the molars erupt into normal occlusion.
- Q: A seven‐year‐old girl presents to you with a posterior crossbite. Her parents ask, “Will our daughter’s crossbite correct spontaneously?” How do you respond?
A: Her posterior crossbite will probably not correct spontaneously. However, a range of spontaneous crossbite corrections (17–45%) [3, 4, 7, 8] has been reported. So, spontaneous correction is possible, just not probable. One option is to monitor her crossbite for three to six months to see whether it corrects spontaneously before treating.
- Q: Is it easier to treat a patient who presents with a lateral CR‐CO shift into a unilateral posterior crossbite (functional unilateral crossbite), or is it easier to treat a patient with a unilateral posterior crossbite without a CR‐CO shift (a true unilateral crossbite)?
A: It is easier to treat a patient with a lateral CR‐CO shift into a unilateral posterior crossbite. Why? When the lateral shift is eliminated, the transverse discrepancy becomes bilateral but smaller on the crossbite side. Also, bilateral arch forces to correct the bilateral discrepancy (expansion, constriction) are easier to generate than unilateral forces.
- Q: What fact of diagnosis does this concept underscore?
A: It underscores the importance of checking for CR‐CO shifts at every appointment. If a patient presents with a unilateral crossbite in CO, but shifts into a bilateral (smaller) transverse discrepancy in CR, then your transverse correction will generally be easier.
- Q: What may result if a unilateral crossbite resulting from a lateral CR‐CO shift (functional crossbite) is not corrected during growth?
A: A mandibular growth asymmetry may result. This fact stresses the importance of early correction of lateral shifts into crossbites.
The fact that most unilateral crossbites do not spontaneously correct and that functional shifts are rarely detected in adults with unilateral crossbites suggests that adaptive remodeling of the temporomandibular joint occurs and that children with unilateral crossbites and functional shifts develop asymmetries of the mandible [9–11].
- Q: A child presents to you with a developing mandibular asymmetry due to a lateral shift into crossbite. What does the literature tell us about the asymmetry if the crossbite is corrected early enough with maxillary expansion?
- Q: Anneke is a seven‐year‐old girl who presents to you in the early mixed dentition (Figure 6.2) with the parents’ chief complaint, “Anneke has a crossbite.” Past medical history (PMH) includes asthma. Past dental history (PDH) is within the range of normal (WRN), CR = CO, and upper dental midline (UDML) WRN. Temporomandibular joints (TMJs), periodontal tissues, and mucogingival tissues are WRN. Can you list her primary problems in each dimension (plus others)?
Table 6.1 Primary problems list for Anneke.
AP – Vertical OB 0% Transverse Left lingual crossbite
Maxillary first permanent molar buccal crown torque (Figure 6.2r)
Mandibular right first permanent molar lingual crown torque (Figure 6.2s)
Maxillary first permanent molar interlingual cusp distance = 34.9 mm
Mandibular first permanent molar intercentral fossa distance = 40.9 mm
Other Mildly proclined mandibular incisors (FMIA = 63°)
Edge‐to‐edge relationship of maxillary left primary central incisor and mandibular left permanent central incisor
2–3 mm maxillary anterior spacing
Mild (3–4 mm) mandibular incisor crowding
- Q: Can you more fully describe Anneke’s left crossbite?
A: It is a unilateral left lingual crossbite of her permanent first molars, primary canines, and primary molars (without CR‐CO shift).
- Q: We will return to Anneke later. For now, can you list at least four etiologic factors that cause transverse discrepancies?
- Ectopic tooth eruption. Figure 6.3a illustrates a developing (left to right) buccal crossbite resulting from an ectopic first permanent molar eruption.
- Soft tissue imbalance (e.g. resulting from prolonged digit sucking). Figure 6.3b shows the posterior crossbite (and anterior open bite) resulting from three decades of heavy thumb sucking. However, one study reported no difference in sucking habits of patients with or without spontaneous correction of a posterior crossbite .
- A large tongue carried low in the mouth can cause excessive mandibular transverse dental widening resulting in a lingual posterior crossbite .
- Prolonged retention of primary teeth.
- Asymmetric mandibular growth (Figures 6.3c–6.3e) – note the right chin deviation and right posterior unilateral crossbite that has resulted from asymmetric mandibular growth.
- A lateral CR‐CO shift without an underlying skeletal problem.
- Excess or deficient anteroposterior growth of the maxilla or mandible. Figures 6.3f–6.3h illustrate a 19‐year‐old male with lingual crossbite of the entire dentition due to maxillary anteroposterior deficiency/mandibular anteroposterior excess.
- Some TMD issues. Temporomandibular joint dysfunction has been associated with skeletal asymmetries and crossbite occlusion.
- Excess or deficient maxillary or mandibular transverse growth. Figures 6.3i–6.3k illustrate a left posterior crossbite developed in a cleft lip and palate patient due to deficient maxillary transverse growth.
- Q: A unilateral posterior crossbite often results from one of the two etiologies. What are they?
A: Either a lateral CR‐CO shift into crossbite or asymmetric mandibular growth.
- Q: Which of these two etiologies is the more difficult to treat?
A: A unilateral posterior crossbite resulting from asymmetric mandibular growth (Figure 6.3c–e) is more difficult to treat. Why? Correcting the crossbite early does not normalize growth. As asymmetric mandibular growth continues into adolescence, the unilateral crossbite is likely to return. This fact underscores the importance of determining whether a unilateral posterior crossbite is due to asymmetric mandibular growth or a CR‐CO lateral shift.
- Q: Can you discuss six factors that should be considered when formulating a diagnosis and treatment plan for patients with transverse discrepancies?
A: The following factors must be considered:
- Magnitude of transverse discrepancy: This is the single most important factor in your transverse treatment planning decision and will influence your decision to treat the child with orthopedics, masking/camouflage (adult dentition), or surgery (adult dentition).
As illustrated in Figures 6.4a–6.4c, the magnitude of a transverse discrepancy is not simply the linear difference between maxillary first molar intermolar width (lingual cusp to lingual cusp) and mandibular first molar intermolar width (central fossa to central fossa). This linear difference would reflect the magnitude of the discrepancy only if the molars displayed normal buccolingual inclinations (i.e. were initially upright).
Instead, transverse dental compensations (usually maxillary posterior buccal crown torque and mandibular posterior lingual crown torque) must be considered when determining the magnitude of the transverse discrepancy. Buccally tipped maxillary molar crowns will tend to move lingually as they are uprighted (unless a transpalatal arch is used to apply torque to upright their roots while maintaining the intermolar distance). Likewise, lingually tipped mandibular molar crowns will tend to move buccally as they are uprighted (unless a lower lingual holding arch is used to apply lingual root torque while maintaining the intermolar distance). Removal of these compensations by uprighting the molars can dramatically increase the transverse occlusal discrepancy magnitude and worsen a skeletal crossbite relationship.
Also, changes in the sagittal relationships between the jaws affect transverse relationships. If you move the maxilla or mandible relative to each other via orthopedics or surgery, you must anticipate how these movements will affect the resulting posterior transverse relationships (e.g. see absolute vs. relative transverse discrepancy below).
- Facial symmetry or asymmetry: The first thing to do when compiling a problem list in the transverse dimension is to examine the patient’s face in the frontal view. Unlike problems in either the anteroposterior or vertical dimensions, problems in the transverse dimension are frequently camouflaged by the overlying soft tissue. However, asymmetries can usually be spotted during a clinical examination. The use of a cone beam computed tomography (CBCT) scan can also be of value in assessing skeletal asymmetries and transverse skeletal discrepancies between the maxilla and mandible.
We do not recommend using a 2‐D posteroanterior cephalometric radiographs to determine transverse skeletal discrepancies between the maxilla and mandible because the alveolar process bone of the jaws, the bone that houses the roots of the teeth and is the bone of interest in diagnosing transverse skeletal discrepancies, is obscured in posteroanterior cephalometric films.
- Presence of a lateral CR‐CO shift: You should always check for a CR‐CO shift in every patient at every appointment. If you note an asymmetry, especially a deviation of the chin in the presence of a unilateral crossbite, try to establish whether the asymmetry is a result of the lateral shift or a developing mandibular growth asymmetry. In addition, check for lateral deviation upon opening.
Have the patient tip his chin upward and touch his tongue to the back of their palate while he opens and closes slowly until his teeth just touch, to help seat the condyles in the glenoid fossa. Then, ask the patient to close into maximal intercuspation to check for a CR‐CO shift. If there is any doubt about the presence or absence of a functional shift, you can place the patient on a flat‐plane bite plate for a week or two to disarticulate the occlusion and deprogram. However, compliance with a removable appliance is never certain. We prefer to deprogram using either a fixed expansion appliance (in the presence of a constricted maxilla while attempting orthopedics), or by leveling/aligning with fixed orthodontic appliances.
Principle of CR‐CO shifts and treatment planning: if you detect a sizable CR‐CO shift, inform the patient that you cannot establish your final treatment plan until the shift has been eliminated. It is only when the shift is eliminated that you can observe the true relationship between the jaws and formulate a rational treatment plan.
- Whether the transverse discrepancy is relative or absolute: Haas  introduced the terms relative and absolute transverse discrepancy. A relative transverse discrepancy exists when the posterior teeth do not coordinate in centric relation but do coordinate when the canines of the models are placed in Class I occlusion. For instance, Figure 6.5a shows the models of an adult patient with a severe Class II malocclusion in CR. Note the significant transverse discrepancy. The patient was treatment planned for mandibular advancement surgery. Figure 6.5b shows the same models advanced to a Class I canine relationship. Note that the transverse discrepancy has disappeared. This patient had a relative transverse discrepancy.
On the other hand, an absolute discrepancy exists when the posterior teeth do not coordinate even when the canines are placed into a Class I relationship. For instance, Figure 6.5c shows the models of a patient with a severe Class III malocclusion in a centric relationship. Note the significant transverse discrepancy. The patient was treatment planned for a mandibular setback osteotomy. Figure 6.5d shows the same models set back to Class I canines. Note that the transverse discrepancy is still present. In fact, the transverse discrepancy will be even worse when the mandibular molars are uprighted (Figure 6.5e, note mandibular molar lingual crown torque). This patient has an absolute transverse discrepancy.
- How future anteroposterior growth/treatment will affect the transverse discrepancy: Forward growth of the mandible relative to the maxilla brings a wider part of the mandibular arch forward relative to the maxillary arch. This relative mandibular arch forward movement can improve a posterior buccal crossbite but worsen a posterior lingual crossbite.
For example, if the patient shown in Figure 6.5a was still growing, then orthopedically restricting the maxilla with headgear (while allowing the mandible to grow forward) could result in improvement/elimination of the transverse discrepancy (Figure 6.5b).
- Magnitude of buccal corridors: When a patient smiles, the buccal corridors are spaces existing between the lateral surfaces of the posterior teeth and the inner commissures of the lips or cheeks. Usually, a patient with a constricted maxilla and narrow maxillary arch will have large buccal corridors. Conversely, a patient with a wide maxilla, and a broad maxillary arch, will have small buccal corridors. Moore and colleagues studied the effect of buccal corridor size on smile esthetics  and found that large buccal corridors are considered unaesthetic (compare Figures 6.6a and 6.6b). When examining a patient, we recommend recording the presence of large buccal corridors during a posed smile and sensitively discussing them with the patient.
- Whether the posterior crossbite (transverse discrepancy) is dental or skeletal: This factor will be considered in detail in the following series of questions.
- Q: Assume that a patient presents to you with a unilateral right lingual posterior crossbite of only one to two teeth (Figure 6.7). Further, assume that she is Class I without a functional shift. Is her crossbite a dental, or skeletal, crossbite? In other words, can dental crossbites be differentiated from skeletal crossbites by simply counting the number of teeth in crossbite? 
A: No – we cannot conclude that it is a dental crossbite because only one to two posterior teeth are in crossbite. Nor can we conclude that it is a skeletal crossbite if more than a few teeth are in crossbite.
- Q: Why are we not able to differentiate skeletal from dental crossbites simply by counting the number of teeth in crossbite?
A: Differentiation between dental and skeletal crossbites is not that simple. As recently reported , there is a large variation in transverse skeletal widths (and transverse dental compensations) in the absence of crossbites ranging from:
- Small maxillary skeletal widths compared to large mandibular skeletal widths (Figure 6.8a), to
- Comparable maxillary and mandibular skeletal widths (Figure 6.8b), to
- Large maxillary skeletal widths compared to small mandibular skeletal widths (Figure 6.8c).
In other words, a large transverse skeletal discrepancy can exist between the jaws in the absence of a crossbite, in the presence of only a few teeth in crossbite, or in the presence of a large number of teeth in crossbite. Counting teeth to determine the presence or absence of a transverse skeletal discrepancy is ill‐advised. You must take into account the presence and magnitude of dental compensations in order to determine whether a transverse skeletal discrepancy exists.
- Q: So, if we cannot simply count the number of posterior teeth in crossbite to determine whether it is dental or skeletal, then how do we differentiate between a dental and skeletal crossbite?
A: By visualizing what happens when we upright the molars (i.e. estimate the true intermolar width that results from eliminating transverse compensations in each arch). For instance, since maxillary and mandibular skeletal bases relate well in a dental crossbite, uprighting molars should improve (eliminate) a dental crossbite (Figure 6.9a). On the other hand, since maxillary and mandibular skeletal bases do not relate well in a skeletal crossbite, uprighting molars should worsen a skeletal crossbite (Figure 6.9b).
- Q: Let’s apply the above concept to Anneke (Figure 6.2). Is Anneke’s left lingual crossbite a dental crossbite or a skeletal crossbite?
A: It is a skeletal crossbite. If we upright Anneke’s buccally inclined maxillary first molars (Figure 6.2r), then their crowns will tend to move lingually, worsen her left crossbite, and minimize her right posterior overjet. If we upright her lingually inclined mandibular right first permanent molar (Figure 6.2s), then its crown will tend to move buccally – reducing her right posterior overjet. Since her transverse relationships worsen as the molars are uprighted, we conclude that her crossbite is skeletal (her skeletal bases do not relate well in the transverse).
- Q: List goals of transverse treatment.
A: Goals of transverse treatment include:
- Correcting posterior crossbites and lateral CR‐CO shifts
- Uprighting posterior teeth
- Creating a stable and coordinated posterior occlusion
- Reducing large buccal corridors
- Maintaining a healthy periodontium
- Increasing chewing efficiency (for example, in patients with scissor bites)
- Q: Isn’t cheek biting elimination a goal of transverse treatment?
A: Possibly, if the cause of cheek biting is the transverse discrepancy. In many patients, suggesting that they simply slow down when they chew can reduce/eliminate cheek biting.
- Q: When should posterior crossbites be corrected? Should you correct posterior crossbites in the primary dentition or wait until the mixed dentition?
A: If limited occlusal interferences are causing a lateral shift into posterior crossbite, then perform occlusal adjustment as early as possible, even in the primary dentition (Figures 6.10a–6.10d). Why? Limited occlusal adjustment can be performed with minimal time, compliance, and cost to the patient. However, high failure rates have been reported with occlusal adjustments [3, 7, 30, 31].
If extensive occlusal interferences are causing a lateral shift into crossbite, or if an underlying transverse skeletal discrepancy is causing crossbite, then you should not use occlusal adjustment. Instead, we recommend waiting until the permanent first molars erupt (early mixed dentition) to begin treatment. Why? Although maxillary expansion plates, composite buildups, and Porter W appliances can be used to treat posterior crossbites in the primary dentition [3, 32, 33], there is generally no urgency to treat posterior crossbites that early. Further, the permanent first molars provide excellent anchorage for maxillary expansion, cooperation should be better if the patient is a little older, and treatment relapse in the primary dentition may require later re‐treatment. We recommend waiting until the early mixed dentition to treat posterior crossbites with maxillary expansion .
Figures 6.10e–6.10j illustrate a seven‐year‐old girl in the early mixed dentition with a Class I right posterior lingual crossbite of all primary teeth and right first permanent molars. CR = CO. When would you recommend treatment of her crossbite?
Because a lateral shift is absent and she is facially symmetric, we see two options. You can wait to perform rapid maxillary expansion (RME) when she is a little older, say eight years of age, without harm. Or, you can proceed with RME now. We recommend speaking to the parents, assessing her maturity level, and using that information to decide which option to pursue.
- Q: A child presents to you with a posterior crossbite. Can you list options for dealing with it?
A: There is no single treatment approach for every patient. Options include:
- No treatment – leave the crossbite
- Dental crossbite – upright the involved teeth, thus improving/correcting the crossbite
- Skeletal crossbite – the same three general options are available for dealing with transverse skeletal discrepancies as for dealing with anteroposterior or vertical skeletal discrepancies:
- Orthopedics (attempting maxillary skeletal expansion)
- Masking (camouflage, increasing transverse dental compensations without addressing the underlying skeletal discrepancy)
- Orthognathic surgery
The challenge is deciding which option is the best option. You must consider all of the factors discussed earlier in coming to your decision: presence of a developing skeletal asymmetry, presence of a CR‐CO shift, magnitude of the transverse discrepancy, whether the crossbite is of dental or skeletal origin, whether the transverse discrepancy is relative or absolute, how orthopedic or surgical correction in the anteroposterior direction will affect the transverse, the presence and magnitude of dental compensations (torques), which jaw is at fault, the presence of large buccal corridors, the patient’s age, the condition of the periodontal tissues, and the patient’s desires.
- Q: Which jaw should be treated to correct a skeletal transverse discrepancy in a child?
A: If a child presents with a constricted maxilla, the obvious choice is to orthopedically expand the maxilla. If a child presents with an excessively wide mandible, a reasonable choice may still be to expand the maxilla. Surgery would be an option for treating an excessively broad maxilla (removing a wedge of bone to constrict the maxilla), or an excessively narrow mandible (midline osteotomy, with symphyseal distraction or graft).
- Q: Let’s spend some time discussing options for treating posterior crossbites in more detail. When is the option of leaving the child in crossbite (no transverse treatment) reasonable?
A: In contrast to adults, we generally do not consider this option in children. In children, we will almost always correct crossbites. Of course, this depends upon the transverse discrepancy magnitude, whether the crossbite is absolute or relative, current anteroposterior relationships, future growth potential, plans for future surgery, and periodontal biotype. Occasionally, we will finish a child’s treatment with a permanent second molar left in crossbite, if the remaining occlusion has excellent interdigitation (no CR‐CO shift), if correcting the crossbite would require placing too much second molar compensations, if the patient is not cheek biting with the second molars, and if the parents are in agreement with our decision.
In rare medical instances, we will elect to leave a child in crossbite. For example, we recently left a child in crossbite who suffered from chronic recurrent multifocal osteomyelitis. We were concerned with micro‐hemorrhaging (surgical insult), which would occur as the midpalatal suture separated. Our pediatric rheumatologist concurred with our decision. Ideally, in situations where we leave a child in posterior crossbite, we would at least finish treatment with canines in proper occlusion and with canine rise disclusion.
- Q: Figure 6.11 illustrates a female patient with a right posterior buccal crossbite of her first and second molars. She is Class I and CR = CO. Does she have a skeletal or dental posterior crossbite?
A: Her unilateral posterior crossbite is a dental crossbite. Why? Clearly, her mandibular right molars have erupted with significant lingual crown torque (Figures 6.10e and 6.10g), while her maxillary right second molar exhibits mild buccal crown torque (Figure 6.10f). Since uprighting these teeth will improve or correct her crossbite, we conclude that she has a dental crossbite.
- Q: In general terms, how would you correct her dental crossbite?
A: Since uprighting posterior teeth improves a dental crossbite (skeletal bases relate well in dental crossbites), you would move her right maxillary second molar crowns lingually and her mandibular right molar crowns buccally (Figure 6.12).
- Q: What problems do you anticipate when uprighting her right molars?
A: The major problem is the magnitude of right molar overbite. In other words, her right molars have erupted so far past each other (Figures 6.11h–6.11i) that her vertical dimension may be opened significantly as the right molars upright and are brought into occlusion. These molars may become the only teeth in contact, and she may not be able to tolerate such an increase in vertical dimension. She may require significant occlusal adjustment (enameloplasty) to reduce this increased vertical dimension. Depending upon the amount the right molars must be cut down, she may even require molar endodontics and crowns. She must be informed of these possibilities.
- Q: How would you proceed?
A: We placed fixed orthodontic appliances, and asked her to wear crossbite elastics from right mandibular molar lingual buttons to right maxillary molar buccal brackets. Surprisingly, no other treatment was necessary, not even an anterior biteplate to allow her right molars to pass across each other. She required no enameloplasty. Her deband photographs are shown in Figure 6.13.
- Q: Let’s discuss the treatment of skeletal crossbites, beginning with orthopedic treatment. What does transverse orthopedic treatment consist of?
A: Correcting dental arch width discrepancies by disarticulating the midpalatal suture with force and spreading the hemi‐maxillae apart.
- Q: Upon application of a transverse maxillary force (Figure 6.14a), what movements occur in the maxillary anchor teeth and in the maxilla?
A: The answer depends upon the magnitude of the force and the maturity of the facial skeleton [35–38]. If the force is too light to overcome the facial skeletal resistance, only buccal tipping/buccal translation of the maxillary posterior teeth will result. If the force is heavy enough to overcome the skeletal resistance, buccal tipping/buccal translation of the maxillary teeth will still occur, together with midpalatal suture disarticulation/separation (Figure 6.14b). This sutural separation may be asymmetric, depending upon the rigidity of the bony structures.
If the facial skeleton is fully mature, the skeletal resistance may be so large it will prevent midpalatal suture separation regardless of how great the force. In this instance, the alveolar process may fracture if a heavy enough force is applied.
- Q: Assume that you wish to orthopedically expand a maxillary arch in an eight‐year‐old child. You are considering three appliances (Figure 6.15) to achieve expansion: a split plate, a quad‐helix (Porter arch/W‐appliance), or a Hyrax expansion screw appliance. Which should you use?
A: Let’s first consider the effects of each appliance:
- Split plate – results in an increased rate of growth (widening) of the midpalatal suture when the screw is opened 0.5 mm/week . The split plate acts as a bite plate to deprogram any shift.
- Quad‐helix – can achieve true midpalatal sutural separation if the facial skeletal resistance is low enough [40, 41].
- Hyrax expansion screw appliance – virtually guarantees true midpalatal sutural separation with skeletal expansion at this age.
- Tipping/buccal translation of maxillary anchor teeth will occur with each of these appliances.
- The quad‐helix has significantly fewer failures, is more cost‐effective, and has a shorter treatment time than an expansion plate [42, 43]. Early treatment with the quad‐helix appliance is effective in increasing intermolar, palatal, and alveolar widths with the teeth moving through the alveolus, leading to substantial decreases in buccal bone thickness and increases in lingual bone thickness .
- No scientific evidence exists to show which treatment modality – expansion plate, quad‐helix, Hyrax expansion screw appliance, or enameloplasty – is the most effective in crossbite correction .
In summary, which appliance should we use? Well, each of the above appliances may be effective in widening, or at least increasing growth of, the midpalatal suture in an eight‐year‐old child. However, of the three appliances, we recommend using the Hyrax expansion screw appliance. Why? With a Hyrax expansion screw appliance, patient compliance is minimal and skeletal expansion is virtually guaranteed before puberty.
- Q: If the midpalatal suture disarticulates during RME, how do the hemi‐maxillae separate? In other words, what pattern do you see with this separation? How do the maxillary teeth separate?
A: Skeletal expansion during RME is greatest in the anterior (Figure 6.16a), creating a “V” expansion of the suture [45, 46]. On the other hand, transverse movement of the maxillary first molars is approximately twice that of the maxillary canines, leading to a “reverse V” expansion of the dental arch.
Maxillary anchor teeth tip buccally [45–47]. Viewed in the frontal plane (Figures 6.16b–6.16d), a pyramidal opening occurs during RME with the fulcrum at the frontomaxillary suture [26, 48–50]. The RME creates a significant increase in nasal width, and the intermaxillary and maxillary frontal nasal sutures are the sutures primarily affected .
- Q: When we apply a force during RME, the facial bones resist this force. What is the site of this facial skeletal resistance? Is the midpalatal suture the major site of facial skeletal resistance to RME force?
A: No, the midpalatal suture is not the major site of skeletal resistance to RME. If the midpalatal suture (Figure 6.17a) was the major site of skeletal resistance to RME, then once the suture separated, the skeletal resistance would decrease dramatically. However, Isaacson and Ingram  reported no significant change in the RME force during the time the midpalatal suture opened.
Instead, it has been suggested that the zygomatic buttress (Figure 6.17b) is the major site of skeletal resistance to RME . Other resistance sites include the palatal soft tissue, pterygomaxillary articulation, pterygoid plates (sphenoid bone), pyramidal processes (palatine bone), and pyriform aperture.
- Q: What problem do we face in attempting RME in postpubertal adolescents?
A: The problem we face is the increasing skeletal resistance to maxillary expansion that occurs with age. The most marked RME skeletal effect occurs before/during the pubertal growth spurt, and expansion after the pubertal growth spurt is mainly dentoalveolar (not orthopedic/skeletal) [18, 35, 50, 54–57]. In older adolescents, facial maturation may be too advanced and offer too much skeletal resistance to permit separation of the midpalatal suture.
In older adolescents, you may consider attempting RME at the rate of 0.25 mm per day, or 0.25 mm every other day, and monitor whether the midpalatal suture separates. On occasion, we have been able to achieve a midpalatal suture separation in women in their late teens or early twenties. The youngest person we were unable to achieve a midpalatal sutural separation in was a twelve‐year‐old girl.
Are there nonsurgical techniques to help overcome skeletal resistance to RME in postpubertal adolescents and young adults? Yes, in addition to including both maxillary first premolars and first permanent molars as anchors, temporary anchorage devices (TADs) (mini‐screws) can be inserted into the palatal bone and attached to the Hyrax appliance (Figures 6.17c–6.17d). TAD‐supported RME may help overcome skeletal resistance to RME, protect anchor teeth, and reduce buccal tipping of the posterior dentoalveolar segment [58–60].
- Q: Can you offer age guidelines of when to use TAD‐supported RME?
A: At the University of Iowa, we use TAD‐supported RME in children older than fifteen years (especially in large‐framed individuals).
- Q: If you attempt RME in a child, how do you know whether the midpalatal suture has separated?
A: The most obvious sign of midpalatal suture separation is the development of a maxillary midline diastema (Figures 6.18a–6.18b) or radiographic evidence of midpalatal sutural separation (Figure 6.18d).
- Q: If you attempt RME, how do you know when the midpalatal suture is not separating and you are getting only dental tipping/translation?
A: Signs include:
- Absence of a midline diastema or absence of unequivocal radiographic midpalatal suture separation.
- Increasing impingement of the RME appliance’s metal arms on the palatal alveolar process soft tissue (indicating lateral dental movement and absence of maxillary bony expansion).
- Patient sensation of expansion pressure from the screw for more than five or ten minutes after expansion screw activation.
- Patient complains of pain.
- Q: When attempting RME, you must have a “fallback” plan if the midpalatal suture fails to separate (dental expansion only). Can you discuss such contingency plans?
A: Here is what we recommend:
- If the dental expansion already achieved is large compared to what you need to correct the crossbite, and if the maxillary posterior buccal periodontium is robust (thick biotype, thick buccal bone, thick keratinized attached tissue), and if you believe you can correct the crossbite by increasing posterior compensations further (masking), then your fallback plan may be continuing dental expansion slowly at the rate of 0.25 mm or 0.5 mm per week.
- If the dental expansion already achieved is small compared to what you need to correct the crossbite, or if the posterior buccal periodontium is of a thin biotype, then your fallback plan may be switching to a TAD‐supported RME, surgical intervention (SARME), or leaving the crossbite.
- Q: Is there a difference in outcome between rapid maxillary expansion (0.25–0.5 mm expansion per day) and slow maxillary expansion (0.25–0.5 mm or slower expansion per week)? Can you give a recommendation about which to use?
A: Since the 1800s, a debate has raged between those advocating rapid maxillary expansion and those claiming a slower process is less traumatic and more stable [61, 62]. The literature is mixed regarding this question:
- Hicks  compared his slow expansion patients to a subset of Krebs’  rapid expansion patients. Rapid expansion produced a greater maxillary skeletal width increase (45%) compared to slow expansion (28%). In a more recent study, RME skeletal width increase ranged from 38 (first molar) to 55% (first premolar) .
- More molar bodily displacement and bone loss have been reported with slow maxillary expansion along with more molar inclination with rapid maxillary expansion .
- Approximately, one millimeter expansion per week is the maximum rate tissues of the midpalatal suture can adapt to so that tearing and hemorrhaging are minimized .
- In a systematic review , studies on slow maxillary expansion failed to achieve a higher level of scientific evidence, and the authors could not make strong conclusions regarding dental or skeletal changes with slow maxillary expansion.
Recommendation – we use RME almost exclusively. However, because the skeletal and dental expansion components of RME long‐term are approximately 50–50 (about the same as when done more slowly – 1 mm per week), some authors consider slow expansion to be equally effective and less traumatic than RME .
- Q: In anticipation of transverse relapse and to permit post‐expansion maxillary molar uprighting, overexpansion during RME is necessary [35, 45]. How much overexpansion do you recommend?
A: Haas recommended 50% overexpansion . Assuming that any mandibular compensations have been removed (upright mandibular molars), we recommend overexpanding until the maxillary first permanent molar lingual cusps contact the mandibular molar buccal cusps (Figure 6.19).
- Q: As the maxilla is widened during RME, how can lingually torqued mandibular posterior teeth crowns be uprighted so as to maintain arch coordination?
A: Lingually inclined mandibular posterior teeth can be uprighted by wearing crossbite elastics from the maxillary expansion appliance to mandibular molar lingual buttons (Figure 6.20a), by inserting an expanded lower lingual holding arch (LLHA), or by using a Schwarz expansion appliance (Figure 6.20b).
- Q: What is the effect of RME on maxillary and mandibular anteroposterior or vertical positions?
A: The following effects have been reported during RME:
- Downward displacement of the maxilla [50, 66, 67]
- Forward displacement of the maxilla in some studies, but not all [50, 66–69]
- Downward and backward mandibular displacement (Figures 6.21a–6.21e) [50, 66–68].
- However, long‐term (> three to six years), RME has little effect on either vertical or anteroposterior facial dimensions, even in hyperdivergent patients [70–72].
- Q: Is long‐term retention recommended following RME? Why?
A: Yes, long‐term retention is recommended following RME. Let’s look at some facts:
- There is only second‐level evidence that maxillary expansion obtained with fixed appliances is stable long‐term, and there is only weak indirect evidence of long‐term stability using removable maxillary expansion appliances .
- The transversely displaced hemi‐maxillae require a lengthy period of rigid stabilization in order to allow sutural readjustment and dissipation of accumulated residual forces at the contiguous articulations of the maxilla .
- Intermolar and intercanine relapse following RME has been shown to continue for up to five years post‐RME .
- A meta‐analysis reports that following a mean expansion of 6, 4.9 mm was maintained while wearing retainers, and only 2.4 mm was maintained long‐term, post‐retention (which the authors stated was no greater than normal growth) .
- Based upon the above, we recommend lifelong retention for all RME patients.
- Q: Can you suggest a retention protocol post‐RME?
A: Post‐activation retention periods of three to six months, while the expansion appliance is still cemented in place, are normally recommended to allow reorganization and stabilization of rapidly expanded maxillary sutures . However, the longer we are in practice, the longer we keep the expansion appliance cemented in place. If the patient will tolerate the appliance, then we will leave it cemented in place for nine to twelve months post‐expansion. Removable retention following expansion seems less effective [35, 40, 54, 77].
In order to permit bracket bonding, leveling, and aligning of dental arches during the fixed retention period, we will convert the expansion appliance into a trans palatal arch (TPA) by cutting off the metal arms to the first premolars (Figures 6.22a–6.22b). The fact that the appliance jackscrew sits some millimeters away from the palate during this nine‐ to twelve‐month retention period may reduce eruption of the maxillary first molars via tongue pressure. This may be desirable in patients with excess vertical development but undesirable in patients with inadequate vertical development.
If the RME appliance must be removed, then another option to maintain the expansion is to insert a slightly expanded 0.032‐inch stainless steel overlay wire (Figure 6.22c) into the 0.045‐inch headgear tubes. Or, if the patient is wearing a headgear, simply expand the inner bow of the headgear.
Assuming the above protocol is followed post‐RME, the transverse correction should be relatively stable once you have leveled and aligned the maxillary arch to stainless steel archwires of 0.018 × 0.025 inches or greater. However, you must continue to monitor the transverse dimension (in CR) at each visit during active treatment. Following deband, the patient should be placed in a well‐fitting maxillary Hawley retainer to be worn at night for life.
- Q: Which expander provides greater skeletal expansion, a two‐banded maxillary RME appliance (maxillary first permanent molars banded for anchorage) or a four‐banded RME appliance (maxillary first molars and premolars banded for anchorage)?
A: Sutural expansion is 2.5 times greater with four bands [78, 79]. When maxillary premolars have erupted, we recommend RME using a 4‐banded appliance. When maxillary premolars have not yet erupted, we recommend banding maxillary permanent first molars and bonding maxillary primary molars to metal arms extending forward from the maxillary permanent first molars.
- Q: Can the maxilla be orthopedically expanded using only primary teeth for anchorage?
A: Yes, in young children, the facial skeletal resistance to expansion may be so small that the midpalatal suture will open using (nonmobile) maxillary primary second molars as anchor units (Figure 6.23). Using primary teeth exclusively for anchorage has been suggested as a way to avoid undesirable side effects of RME on maxillary permanent molars (e.g. root resorption) .
- Q: What do you do if the patient/parent overexpands the maxillary arch with the RME appliance (turns the screw too many times)?
A: First, attempt to turn the screw backward. We have had mixed results attempting this. Or, remove the expansion appliance and allow the expansion to relapse. If only the anchor teeth are overexpanded, you could remove the expansion appliance and place a Hawley retainer with the plastic selectively trimmed away lingual to the anchor teeth to permit relapse.
- Q: You receive a maxillary expansion appliance back from the lab. The appliance fits very nicely on the model from which it was fabricated, so you know the laboratory did a good job. You try to seat the appliance in the patient’s mouth but discover that it will not seat on the patient’s left side due to the path of draw (Figure 6.24). What do you do?
A: Because maxillary molars erupt with buccal crown torque, problems with seating banded maxillary appliances are not unusual (due to their divergent path of draw). In this particular case, you could try:
- Opening the expansion screw slightly to see if that helps seat the appliance.
- Section one, or both, premolar bands from the appliance but leave the metal arms lingual to the premolars intact. Then, roughen the surface of the metal arms and bond them to the premolars (and maxillary left primary second molar if it is nonmobile) to provide anchorage in addition to the permanent first molars.
- Using larger bands for all anchor teeth (providing additional “play” during appliance seating) and having the appliance remade.
- Use a bonded expansion appliance instead of a banded appliance.
- Q: What is the effect of RME on the periodontium?
A: Study results appear mixed. Of twenty eight children undergoing 4.6 mm of maxillary molar expansion , few children exhibited marked periodontal breakdown and minimal difference was found compared to a control group. However, in a group of eight girls, RME induced bony dehiscences on the buccal aspect of anchor teeth, especially in children with thinner buccal plates, when measured by CT three months post‐expansion . Finally, Timms and Moss found histological evidence of buccal bone deposition two years post‐expansion , and the possible long‐term recovery of buccal bone remains to be investigated.
- Q: Based on these mixed study results, do you have any recommendations regarding the condition of maxillary buccal periodontium before RME?
A: Before performing maxillary expansion, confirm that the child has a thick buccal periodontal biotype (thick buccal bone, thick keratinized attached tissue) and not a thin buccal biotype (thin buccal bone, thin keratinized attached tissue).
Figures 6.25a–6.25f illustrate a patient who presented with a thick maxillary buccal periodontal biotype, a bilateral posterior lingual crossbite, and underwent successful slow maxillary expansion. The maxillary buccal tissues were healthy before, and after, buccal expansion. On the other hand, Figures 6.25g–6.25h illustrate a patient with thin maxillary buccal bone plus gingival recession over the buccal of the maxillary left molar. Nonsurgical maxillary expansion in this patient would be ill‐advised.
Even if you begin RME with a thick buccal periodontal biotype, you should visually monitor, and palpate, the buccal periodontium during expansion. If you begin to feel the first molar roots, then you should consider discontinuing expansion.
- Q: Does RME cause root resorption?
A: Yes. Anchor premolars have been found to resorb with RME while non‐anchor premolars have been shown not to resorb . Further, the longer a tooth is held overcorrected, the greater the resorption. Two years after RME evidence of root resorption and repair are still present .
In our clinical experience the potential for significant root resorption from RME is small. However, the risk of root resorption from any orthodontic treatment must be discussed with the patient before treatment begins.
- Q: The Haas maxillary expander (Figure 6.26) is similar to the Hyrax expander but includes acrylic pads seated against palatal alveolar process shelves. Has the Haas expander been shown to provide more orthopedic correction than a Hyrax expander?
- Q: A child presents with a true unilateral crossbite (CR = CO, no lateral shift). Can you list the skeletal and dental differences between unilateral crossbites, bilateral crossbites, and control subjects?
A: Miner et al.  reported that the:
- Skeletal discrepancy in the bilateral crossbite group was due to a narrow maxilla and wider mandible .
- Molar inclination was not different between the control and bilateral crossbite groups.
- Skeletal discrepancy in the unilateral crossbite group was due to a wider mandible.
- Molar inclination on the crossbite side of the unilateral crossbite group was not significantly different from that of the control group.
- Dental compensations on the non‐crossbite side of the unilateral crossbite group resulted in a normal transverse dental relationship on that side.
- Q: A child presents with a true unilateral lingual posterior (dental) crossbite which has not resulted from asymmetric mandibular growth. CR = CO. How do you recommend treating it?
A: Figures 6.27a–6.27e illustrate models of a patient with a true unilateral right lingual posterior crossbite. Note the maxillary arch asymmetry (Figure 6.27d, maxillary right hour‐glass shape) which is often found in these patients. On the right crossbite side, the mandibular teeth have normal buccolingual inclination (no transverse dental compensations), while on the non‐crossbite side, the mandibular molars are torqued lingually (dental compensations).
First, confirm the absence of a lateral CR‐CO shift. Although various acrylic appliances have been proposed for unilateral crossbite correction (Figures 6.27f–6.27h), we recommend RME using a conventional Hyrax expander (Figures 6.27i–6.27k). When using a Hyrax expander, the non‐crossbite side of the maxillary arch will quickly become overexpanded. If the patient has lingual crown torque (transverse dental compensation) on the non‐crossbite side of the arch, place the patient on a crossbite elastic to upright the lingually torqued, non‐crossbite side mandibular first molar – thus, moving it buccally, uprighting it, reducing the non‐crossbite side overjet (Figures 6.27l–6.27m), and allowing continued bilateral maxillary expansion (overexpansion). Figure 6.27n shows the resulting transverse relationship at the end of maxillary expansion with even left and right side posterior overexpansion.
- Q: What are the advantages of bonded maxillary expanders compared to banded maxillary expanders?
- Act as bite plates to eliminate lateral CR‐CO shifts
- Offer the potential of intruding (or at least reducing the eruption of) posterior teeth due to stretching of the masseteric sling by this bite plate effect
- Eliminate the need to place separators and fit bands
- Open the bite so that anterior teeth can be tipped forward out of crossbite.
- Q: Does the biteplate effect of bonded RME appliances result in less vertical growth?
A: Yes, slightly. Studies report an average 0.8 mm maxillary first molar intrusion  and 0.5–1.5 mm reduction in downward movement of PNS [89, 90] with bonded RME appliances compared to banded RME appliances. Surprisingly, no significant difference in buccal tipping of posterior teeth has been demonstrated between bonded, and banded, RME .
- Q: How does adolescent maxillary expansion compare when using tooth‐borne versus bone‐borne appliances?
A: Results of studies appear mixed. One study showed similar outcomes with both appliances , including: midpalatal suture separation; greater dental crown expansion than apical/skeletal expansion; no difference in molar apical expansion; and no difference in molar tipping. However, another study  reported that tooth‐borne maxillary Hyrax expansion in adolescents produced more dental expansion, buccal rolling, and a greater increase in nasal width than did bone‐borne expansion. Both expanders produced basal bone expansion at the level of the hard palate.
- Q: What is the limit for maxillary arch expansion?
A: There are really two answers. First, a linear millimetric limit of maxillary arch expansion has never been established. At a Midwest Angle meeting, Dr. Andrew Haas (inventor of the Haas expander) once told me that 20 mm was the greatest maxillary expansion he had ever performed. We cannot recall ever expanding a maxilla more than this.
Second, another way of answering this question is to state that the limit for maxillary arch expansion is the mandibular arch width. The mandibular arch is the template for treatment. Ideal coordination and interdigitation with the mandibular arch are both the goals, and limits, for maxillary arch expansion at the time of deband.
- Q: As a follow‐up to the previous question’s answer, what is the limit for mandibular arch expansion?
A: The limit for mandibular arch expansion is the point where the mandibular posterior teeth are upright. If the mandibular posterior teeth initially present with lingual crown torque, then it is reasonable to expand the mandibular posterior arch (assuming a thick buccal periodontal biotype) by moving the molar crowns buccally to the point where they are upright. The only way to achieve more mandibular expansion than this is to perform a symphyseal osteotomy and widen the mandible with a symphyseal bone graft or bony distraction.
- Q: Can you estimate the amount of maxillary arch perimeter increase for every 1 mm of maxillary arch width increase at the first premolars during RME?
A: It has been estimated that for every 1 mm of maxillary arch width increase at the first premolars during RME, the maxillary arch perimeter increases by 0.7 mm .
- Q: Should arch expansion be used to increase arch length (arch perimeter) in the absence of a crossbite?
A: Yes and no. Maxillary and mandibular expansion increases arch perimeter, but the mandibular arch limits the amount of maxillary expansion that can be achieved. Expansion of both arches (at deband) beyond the point where the mandibular molar crowns are upright and excellent occlusal interdigitation achieved, is inherently unstable and not recommended.
As previously stated, we reported that maxillary molars erupt with buccal crown torque and upright with age while mandibular molars erupt with lingual crown torque and upright with age . Because this is an inherent part of normal human facial growth, uprighting lingually inclined mandibular molars to a more upright position is a reasonable orthodontic treatment (assuming the other goals of transverse treatment presented earlier can be achieved).
Expansion resulting from mandibular molar uprighting will create an additional arch perimeter. In cases with lingually inclined mandibular molars, the molar roots are lateral to the crowns and moving the crowns to a position directly over the roots can be appropriate. However, if the mandibular posterior teeth are already upright, then arch expansion that results in labial crown torque is ill‐advised.
We again wish to emphasize that it is the intermolar width of upright mandibular molars that ultimately determines the limit of potential maxillary expansion. Unless the mandible is widened (surgically, e.g. symphyseal distraction), the mandibular basal bone width is fixed.
Finally, arch expansion (arch development) increases cheek pressure against the teeth, so instability can be a significant problem. As Little  has noted, arch development (widening arches) in the mixed dentition without lifetime retention yields unstable results.
- Q: What is a non‐extraction alternative to mandibular expansion to gain arch space?
A: Place an LLHA in the mixed dentition to preserve leeway space. Gianelly noted that 68% of patients with crowding will have adequate space for alignment if a lower lingual holding arch is used to preserve leeway space, and another 19% will have adequate space with only marginal crowding (up to 1 mm per side) . Further, for mixed dentition cases with favorable leeway space, treatment results using a lower lingual holding arch appear stable . Of course, interproximal enamel reduction (IPR, slenderizing) is another way to gain arch space.
- Q: Do mandibular arch intercanine and intermolar widths spontaneously increase with RME (Figure 6.29)?
A: Results of studies investigating the effect of RME on mandibular intercanine and intermolar widths are mixed. The concept behind this question is intuitively appealing because as the maxilla is expanded, the cheeks are pushed buccally by the maxillary teeth. So, the tongue should push the mandibular canines and posterior teeth buccally, increasing intercanine and intermolar widths. However, mandibular intermolar width increases with RME have been found to range from 0 to over 3 mm and mandibular intercanine width increases have been found to range from 0 to less than 1 mm [26, 47, 50, 69, 95–101]. We speculate that spontaneous mandibular effects relate to the magnitude of the maxillary expansion.
- Q: Is mandibular arch expansion stable when used in conjunction with RME?
- Q: Assume that you are seating an RME appliance and the maxillary left first premolar band splits (Figure 6.30a). What can you do?
A: Instead of remaking the expansion appliance, simply remove the split premolar band, prepare the lingual surfaces of the premolars for bonding, roughen the appliance steel arms next to the maxillary left premolars to better hold the bonding material, and bond the lingual of the premolars to the appliance arms (Figure 6.30b).
- Q: In addition to expanding the maxillary bones, why else did RME gain popularity?
A: A belief that it could improve nasal breathing .
- Q: What is the effect of RME on the nasal cross‐sectional area? On upper airway volume?
A: RME increases nasal width, nasal cross‐sectional area, [54, 55, 65, 106–111], and seems to be associated with an increase in the nasal cavity volume both in the short, and long, term . However, changes in nasal volume are small .
- Q: Should RME be used to treat obstructive sleep apnea in children?
A: We are fortunate that the American Association of Orthodontists provided the specialty with a White Paper discussing this topic . We recommend following these guidelines:
- The definitive diagnosis of obstructive sleep apnea is appropriately made by a physician. If the patient is found to have OSA, the physician should decide on an appropriate course of action for the treatment of OSA.
- The orthodontist may choose to work in a collaborative way with the physician, providing orthodontic treatment when necessary and when it does not interfere with ongoing medical treatment.
- The primary objective of the RME appliance should be to improve the occlusion and address the underlying skeletal discrepancy.
- Secondary effects of this treatment may result in the reduction of nasal airway resistance and an increase in the volume of the nasopharynx and nasal cavity.
- An OSA patient might be referred for expansion who does not have a transverse discrepancy. The treatment alternatives should be considered on a case‐by‐case basis by the medical and dental practitioners involved. In such situations, it is appropriate to prioritize the treatments to serve the best interests of the patient.
- Q: Does prophylactic RME prevent the future development of obstructive sleep apnea?
A: There is no indication in the literature that prophylactic maxillary expansion prevents the future development of OSA .
- Q: Has RME been suggested as a possible treatment for other medical problems?
A: Yes, RME has been suggested as a treatment for nocturnal enuresis (NE, bed wetting) [115–118]. NE is thought to result from an insufficient arousal response, insufficient vasopressin (ADH) production, increased intra‐abdominal pressure (pressure from respiratory efforts against an obstructed airway), and decreased oxygen saturation (resulting in myoclonus). Conventional NE treatments include ADH substitution and addressing any airway obstruction [119, 120].
- Q: Can RME change the soft‐tissue profile?
A: Yes, during RME use, but not post‐retention .
- Q: Some have suggested that in the absence of a posterior crossbite, RME be used to correct a Class II relationship. Is this notion true?
A: Since Class II functional appliances, which actively posture the mandible forward, do not enhance mandibular forward growth in the long term, it is doubtful that RME enhances mandibular growth. Any Class II improvement with RME in adolescence is probably due to unlocking of the occlusion. Since there is greater normal forward growth of the mandible during adolescence compared to the maxilla, unlocking the occlusion may allow the mandibular posterior teeth to move forward with mandible growth (improving the Class II relationship), instead of being held back by the maxillary teeth. Let’s discuss this in more detail.
Short‐term functional appliance studies by Stöckli and Willert , McNamara , and Woodside and coworkers  clearly demonstrate that if an appliance postures the mandible forward, then remodeling changes in the temporomandibular joint occur that tend to bring the mandible forward. In a magnetic resonance imaging study, Ruf and Pancherz demonstrated the same effect . Thus, in the short term, Class II functional appliances have been shown to remodel the joint (accelerate growth).
However, acceleration in the short‐term does not translate into enhanced growth, long term. DeVincenzo reported significant, short‐term increases in mandibular length when children wore the twin‐block appliance . However, compared with controls, this significant increase gradually diminished with time. By the fourth year after treatment with the twin‐block appliance, DeVincenzo reported no significant difference in mandibular length compared with controls. Wieslander, in a study of the headgear‐Herbst treatment followed by activator appliance treatment, reported that the mandibular protrusive effect of these appliances decreased to insignificance years after treatment . Wieslander stated that the long‐term skeletal effect did not come from enhanced mandibular growth but rather from maxillary restriction with the Herbst appliance and activator. Pancherz  reported no long‐term influence of Herbst treatment on mandibular growth. In other words, in the short term, Class II functional appliances enhance (accelerate) mandibular growth; but in the long term, controls catch up.
If in the long term, hyperpropulsive functional appliances do not enhance mandibular growth compared with controls, then how can one possibly anticipate improvement in Class II relationships through RME? There may be a simple explanation. Research has shown that during adolescence, the mandible usually grows forward more than the maxilla [129–131]. For this reason, orthodontists should typically see a spontaneous improvement in Class II patients during growth without any treatment at all. However, in fact, we do not see it. Most orthodontists would agree that Class II malocclusions are not self‐correcting, and studies have demonstrated that minimal change occurs in a Class II relationship in growing patients [132, 133]. Reasons for this phenomenon need further evaluation.
You et al.  compared mandibular growth in a sample of untreated Class II malocclusion children to a sample of norms. Their findings confirmed earlier studies that forward growth of the mandible during adolescence exceeded that of the maxilla (by over 4 mm). However, they also reported that the effect of forward growth of the mandible, which could potentially bring the lower dentition forward, vanished because of intercuspal locking. In other words, without treatment, as the mandible outgrew the maxilla, intercuspal locking caused the mandibular teeth to drag the maxillary teeth mesially, the maxillary teeth to drag the mandibular teeth posteriorly, and the Class II relationship to be left intact. Lager  recommended the elimination of intercuspal locking in a growing Class II patient with a biteplate to allow forward movement of the mandibular dentition (improvement in the anteroposterior relationship) with mandibular growth.
In summary, Class II improvement with RME in adolescence is probably due to unlocking of the occlusion and the greater normal forward growth of the mandible compared with the maxilla.
- Q: We have discussed the orthopedic treatment of skeletal crossbites. Let’s next discuss masking (camouflage) as a means to treat skeletal crossbites. What is masking in the transverse dimension?
A: Masking is the correction of skeletal crossbites with dental movement – without treating the underlying skeletal discrepancy. Masking usually involves placing (or increasing) posterior transverse compensations, often in combination with transverse bodily tooth movement. We introduce masking here, but masking is generally reserved for treatment in adult dentition during comprehensive orthodontics.
For example, masking is illustrated in Figure 6.31 as a way to correct a bilateral lingual crossbite caused by a skeletally narrow maxilla. The maxillary first molars are tipped buccally (Figure 6.31a, buccal crown torque) while the mandibular molars are tipped lingually (Figure 6.31b, lingual crown torque). These torquing movements correct the lingual crossbite (Figure 6.31c) without addressing the underlying narrow maxilla. Masking could also consist of translating maxillary first molars buccally and/or translating mandibular first molars lingually. The point is that masking consists of dental, not skeletal, movements to improve the occlusion.
- Q: Can you list five options for masking skeletal crossbites?
A: Options include the following:
- TPAs or LLHAs to bilaterally expand, or constrict, arches (Figure 6.32) by translating teeth through bone. We italicized the word bilaterally for a reason. In correcting unilateral crossbites, orthodontic residents will frequently suggest using a TPA or LLHA but forget that the expansive/constrictive forces are exerted bilaterally.
Note: If the skeletal resistance is low enough in a child, the bilateral expansive force of a TPA may be large enough to create midpalatal sutural separation (skeletal expansion).
- TPAs or LLHAs to increase molar compensations (buccal or lingual crown torques, Figures 6.31a– 6.31c) bilaterally, or unilaterally.
- Bilateral maxillary dental expansion using a Hyrax appliance (RME appliance) if skeletal resistance prevents the midpalatal suture from opening.
- Crossbite elastics (Figure 6.33) to either place compensations or remove compensations. Crossbite elastics can be used unilaterally, or bilaterally. If the crossbite involves permanent first molars and primary molars, then elastics should also include the primary molars. Why? Failure to correct the primary tooth crossbite will result in a high probability of the permanent premolars erupting into crossbite .
- Bilateral expansion (or constriction) using expanded (or constricted) archwires or overlay wires (Figures 6.34a–6.34c).
- Bilateral expansion using a maxillary split plate (Figure 6.15a) or mandibular Schwarz expansion appliance (Figures 6.35a–6.35b). Expansion with the Schwarz appliance is primarily dentoalveolar .
- Bilateral expansion (or constriction) using expanded (or constricted) archwires or overlay wires (Figures 6.34a–6.34c).
Removable expansion plates may be designed for correction of true unilateral crossbites involving one or two teeth. However, because removable plates exert equal but opposite bilateral forces when activated, an increasingly bilateral effect will result as more teeth are involved.
- Q: With each of the previous masking options, can you state whether the teeth will be translated laterally, torqued (tipped) laterally, or undergo a combination of translation and torquing?
A: The exact movement will depend upon the force and moment applied at the center of resistance (CR) of the tooth.
For example, in Figure 6.36a, a lingual force is applied to the maxillary first molar crown, and a buccal force is applied to the mandibular first molar crown. For the maxillary molar, the lingual force equates to (Figure 6.36b) a lingual force plus a CW moment at CR (yellow dot), which results in maxillary molar lingual translation and lingual crown tip. For the mandibular molar, the buccal force equates to a buccal force plus CW moment at CR which results in mandibular molar buccal translation and buccal crown tip.
If a maxillary appliance, such as a TPA, can apply this force to the molar crown and concurrently apply a CCW moment, then this CCW moment can cancel the CW moment at CR, leaving only a pure lingual force acting at CR and resulting in pure lingual maxillary molar translation. A TPA is capable of adding a CCW moment, but a removable split plate or crossbite elastic is not.
Likewise, if a mandibular appliance, such as an LLHA, can apply this force to the molar crown and concurrently apply a CCW moment, then the CCW moment could cancel the CW moment at CR, leaving only a pure buccal force acting at CR and resulting in pure buccal translation. An LLHA is capable of adding this CCW moment, but a Schwarz appliance or crossbite elastic is not.
- Q: Do you have a recommendation regarding masking and the periodontium?
A: We suggest the same recommendation that we made earlier (Figure 6.25) – before performing maxillary or mandibular buccal expansion, confirm that the patient has a thick buccal periodontal biotype (thick buccal bone, thick keratinized attached tissue) and not a thin buccal biotype (thin buccal bone, thin keratinized attached tissue). If you fail to follow this principle, you may end up with bony dehiscences, fenestrations, or gingival recession.
- Q: Do you have any recommendations regarding masking limits?
A: Yes, a 3–5 mm correction is a reasonable transverse correction to attempt with masking. We recommend avoiding masking with a thin buccal periodontal biotype.
The literature does not offer limits on the amount of buccolingual tip (torque) which can be placed in molars, since a large variation in transverse dental compensations exists in the absence of crossbites . There does not appear to be a scientific reason to prevent us from increasing posterior teeth compensations, even in children. However, molars normally upright with age ; upright molars are a standard required by the American Board of Orthodontics, and masking transverse discrepancies is a compromise treatment.
We may offer masking as a compromise crossbite treatment option for adults, but in children, we recommend crossbite correction with orthopedics whenever possible in order to increase posterior overjet, permit elimination of transverse dental compensations, and finish with upright molars.
- Q: Would you like to think outside the box? An older adolescent presents to you with a left scissor bite (left complete buccal crossbite), Class I relationship, and CR = CO (Figure 6.37a–c). Can you suggest masking options for correcting this scissor bite?
A: Options include:
- Aligning the mandibular arch with a broad arch form. Because the mandibular arch has an hour‐glass shape on the patient’s left side, and because the mandibular left first molar appears to be torqued/tipped lingually, the mandibular left premolars and molars should move buccally with a broad arch form. This buccal movement will improve/correct the left crossbite and upright the lingually torqued mandibular molar.
- Left crossbite elastics
- Q: Can you more fully describe Anneke’s left crossbite?