This paper describes four decades of research carried out at the University of Michigan that has investigated the clinical alteration of the transverse dimension of the face, with specific attention given to orthodontic and orthopedic treatment of patients in the early mixed dentition. The results of a series of clinical studies beginning in the late 1970s are described that consider the etiology of dental crowding as well as treatment effects produced by expansion in the late mixed/early permanent dentition and the early mixed dentition. A treatment protocol for early treatment is described that includes the lower Schwarz appliance, a bonded acrylic splint expander and the placement of anterior brackets. A series of retrospective and prospective studies are presented, including a study investigating the spontaneous improvement of Class II malocclusion following increases in the transverse dimension. The effect of maintaining or increasing anterior arch length on subsequent mandibular second molar eruption is evaluated. The ideal timing for rapid maxillary expansion is before puberty. An individual assessment of the midpalatal suture using cone-beam computed tomography (CBCT) images can be useful when making a clinical decision between conventional and surgically assisted RME, especially for adolescent and young adult patients.
The focus of this paper is the alteration of the transverse dimension in patients treated at various stages of dental development, with emphasis on intervention in the early mixed dentition. Expansion treatment can be undertaken for a variety of reasons, including correcting posterior and/or anterior crossbite and increasing available arch space to avoid extraction. Other major goals for using this approach include correcting Class III malocclusions with a facial mask combined with a bonded expander, facilitating the eruption of impacted canines or incisors, encouraging spontaneous sagittal improvement of Class II malocclusion, “broadening the smile,” and enlarging the nasal airway.
A major reason why expansion protocols initially were of interest clinically was in the correction of unilateral or bilateral posterior crossbite by dividing the two parts of the maxilla orthopedically. A less obvious but far more frequently observed orthodontic problem related in part to the transverse dimension is the discrepancy between tooth size and the sizes of their bony bases. The most frequently observed type of malocclusion in routine orthodontic practice is dental crowding, an underlying imbalance between aggregate tooth size and available bony arch perimeter. This relationship occasionally may be expressed not as crowding but as protrusion and flaring of the teeth relative to their associated basal bone, particularly in the maxillary incisor region.
Preparing such a paper for publication in a themed issue of Seminars in Orthodontics has allowed us to write an overview of our research dealing with expansion that has been conducted over the last four decades. Our research projects have been undertaken in association with many investigators, including colleagues at the University of Florence in Italy and Guarulhos University in Guarulhos, Brazil, as well as with many former orthodontic residents at the University of Michigan who typically are listed as the lead author on the projects that they conducted.
The ideas emerging from our initial clinical experiences in a private practice setting have led us to ask series of questions that range from the etiology of dental crowding to the treatment effects produced following expansion in the mixed dentition by way of a bonded acrylic splint expander, with or without prior orthodontic expansion with a removable lower Schwarz appliance. Further investigations led to the examination of spontaneous improvement of Class II malocclusion during the post-expansion period when the patient was wearing a removable palatal plate to stabilize the treatment results achieved. We also investigated whether the eruption of the second molar is affected using a lower lingual arch, a lower Schwarz appliance, or both during the transition to the permanent dentition.
Overall, we estimate that at least 2500 patients have been treated in our Ann Arbor practice using our mixed dentition protocol that involves a lower Schwarz expander (40% of patients), a bonded acrylic splint expander (100% of patients) and “temporary braces” on the maxillary incisors (65% of patients); these are real numbers based on a census of our patient population. We have collected records prospectively on all early-expansion patients treated in the Ann Arbor practice beginning in 1982. Thus, our comments in this paper are based both on our clinical studies and our hands-on experiences.
Passive expansion with vestibular shields
Our first exposure to expansion during orthodontic treatment other than that produced by fixed appliances was the passive expansion of the dental arches that occurs with Function Regulator (FR) therapy developed by Rolf Fränkel. The unique vestibular shields of the FR appliance are constructed so that the shields are fabricated 3 mm away from the maxillary mucosa, allowing passive expansion of the dental arches as well as uprighting of the lower posterior teeth. The forces produced by the adjacent soft tissues are kept away from the developing dentition, allowing for the passive widening and uprighting of these teeth, especially in the maxilla.
The clinical results of FR treatment were observed first hand by the lead author in 1975 and again in 1979 when he visited Fränkel’s clinic in what was then the German Democratic Republic (East Germany). Observing an estimated 75 patients over the two visits and measuring available serial dental casts indicated that stable expansion of the dental arches was possible over the long-term. Subsequently, after conducting two clinical studies of our own FR patients, and with the simultaneous introduction of rapid maxillary expansion as a routine clinical procedure, we began to explore the possibility of orthodontic and orthopedic expansion in growing individuals, focusing on the short-term and long-term stability of expanded dental arches.
Etiology of dental crowding
Before initiating a discussion of the treatment effects produced by expansion treatment, we asked a fundamental question as to the etiology of the underlying tooth-size/arch-size relationship. Is crowding caused by teeth that are too big, arches that are too small, or some combination of the two? One would assume that the last answer is correct, but surprisingly in most instances it is not.
In one of the first clinical investigations by our research group, Howe and co-workers compared pretreatment dental casts of 50 patients with erupted permanent second molars and severe crowding to the dental casts of 54 untreated individuals from the University of Michigan Growth Study who were classified as having ideal or near-ideal occlusions. The study evaluated both the sizes of the teeth and the dimensions of the bony bases.
In the first part of our study, individual tooth sizes were considered; the results were surprising. In no instance was there a statistically significant difference between the size of any given tooth when the crowded and uncrowded dental arches were compared. The mesiodistal sums of the teeth in both arches then were determined, as measured around the dental arches from the distal of the first molar to its antimere. Even though there was a slight tendency (∼1 mm) toward larger teeth in the crowded cases, there was no statistical difference in aggregate tooth size in either arch between the two samples. Thus, increased tooth size typically was not the cause of the crowding, although there were some individuals with larger than normal tooth size in the crowded group.
A relatively simple clinical guideline that we developed subsequently is based on the measurement of one permanent maxillary central incisor. If the upper central is wider than 10 .0mm, then the teeth of the patient may be considered larger than normal. The average mesiodistal diameter of the maxillary central incisors in Caucasian males and females is 8.9 ± 0.6 mm and 8.7 ± 0.6 mm, respectively. A central incisor width greater than 10 mm is two standard deviations larger than the norm; the possibility of tooth extraction should be considered in these patients.
Size of the bony bases
Significant differences were observed between the arch dimension of the crowded and uncrowded subjects. The crowded group had smaller dental arch dimension, especially in transpalatal widths and arch perimeter. For example, transpalatal width measured between the maxillary first molars was 37.4 mm in males and 36.2 mm in females in the uncrowded sample. In contrast, the same measurement in the crowded group was 31.3 mm in males and 30.8 mm in females. Thus, the differences between the two groups was on average 5–6 mm in the transverse dimension, indicating that decreased transpalatal width was a characteristic of dental crowding.
Significant differences were found in arch perimeter measurements as well. In the noncrowded group, the average mandibular arch perimeter was 88.1 mm in males and 84.6 mm in females. In contrast, in the crowded group, the same measurement in males was 83.7 mm in males and 79.6 mm in females, indication not only the presence of sexual dimorphism but also that there was at least 4.5 mm less arch perimeter available in the crowded individuals in comparison to the normal occlusion group.
The conclusion that can be drawn from the findings of this 1983 study is that a decreased size of the bony bases, rather than large aggregate tooth size is the primary factor in dental crowding. Therefore, expansion of the dental arches, particularly in the early mixed dentition, may be indicated in individuals with tooth-size/arch size discrepancies.
Expansion in the late mixed/early permanent dentition
Before beginning a discussion of treatment effects produced by expansion in the early mixed dentition, it is appropriate to present data from a study published by our group in 2003 that involved patients treated with a Haas-type expander followed by comprehensive edgewise orthodontics in patients in the late mixed or early permanent dentition. This unique treated group consisted of 112 subjects, all of whom were from the private practice of Drs. Robert and Thomas Herberger of Elyria, Ohio, and who were treated according to the RME protocol developed by Dr. Andrew Haas. These records were compared to those of 41 untreated subjects from the University of Michigan Growth Study and the University of Groningen Growth Study .
Serial dental casts were available at three different intervals: pre-treatment (T 1 ), after expansion and fixed appliance therapy (T 2 ), and at long-term observation (T 3 ). The mean duration of the T 1 –T 2 and T 2 –T 3 periods for the treatment group was 3years 2 months, and 6years 1 month, respectively, with the last observation interval at 20years of age.
Treatment by means of a Haas-type expander followed by fixed appliances produced significantly favorable long-term changes in almost all the maxillary and mandibular arch measurements. In comparison to controls, a net gain of 6.0 mm was achieved in the maxillary arch perimeter, whereas a net gain of 4.5 mm was found for the mandibular arch perimeter of treated subjects in the long term (about 20years of age). The net gain was determined by taking the increase in each arch dimension of the RME group and subtracting the gain or loss of the untreated group. Thus, at least in the samples studied, clinically meaningful increases in arch width and perimeter were observed in the treated group in comparison to what occurred in the untreated sample.
A companion lateral cephalometric study of the long-term cephalometric effects of a subgroup of patients treated with the same protocol revealed that RME therapy used in the treatment of Class I and Class II patients does not have a significant long-term effect on either the vertical or the anteroposterior skeletal dimensions of the face when compared to a matched group of patients treated with fixed appliances alone or to untreated controls. For example, there was no opening of the mandibular plane angle and no forward or backward movement of Point A over the long term. Further, a related posteroanterior cephalometric study on the same patient sample demonstrated the long-term stability of the skeletal correction in the transverse dimension, when the expanded patients were ∼20years of age.
A much larger cephalometric study on a larger Herberger sample by Lineberger et al. was performed to evaluate the skeletal and dental changes in the short and long-term in hyperdivergent patients treated with rapid maxillary expansion (RME) and fixed appliances. The sample consisted of 143 patients who underwent RME with a Haas-type expander followed by edgewise therapy. Two groups were established: a normal vertical dimension group (mandibular plane angle [MPA] greater than 20° and smaller than 27°; N = 52) and a hyperdivergent group (MPA equal to or greater than 27°, N = 91). Lateral cephalograms were taken before treatment (T 1 , average age 11.5years in both groups) and after fixed appliance therapy (T 2 , average age 14.3years for the two groups).
No significant differences in treatment effects (T 2 –T 1 ) were found in any of the sagittal or vertical dentoskeletal variables examined. The long-term evaluation of the patients at five or more years post-treatment (T 3 –T 2 ) showed no significant skeletal changes. The results of this retrospective study indicate that rapid maxillary expansion can be carried out successfully in patients with increased vertical dimension without detrimental effects on the vertical skeletal relationships. Thus, an increased mandibular plane angle is not a contraindication to RME therapy.
Expansion in the early mixed dentition
Early treatment protocol
Since 1982, we have used the following protocol in the management of early mixed dentition patients with crossbites and tooth-size/arch-size discrepancies. The essential appliance is the bonded acrylic splint expander ( Fig.1 ) that is made from 3 mm Splint Biocryl™, softened and pressed on a stainless-steel wire framework using a thermal-forming machine (Biostar™; Great Lakes Orthodontic Products, Tonawanda NY). Substituting cold-cure acrylic for Splint Biocryl™ is not recommended because the former type of acrylic is too rigid, making bonded expander removal difficult.
Surprisingly, our protocol has not changed much over the years, except for the addition of the lower Schwarz appliance in 1986. We still use Excel™ adhesive (Reliance Orthodontic Products, Itasca, IL), a material specifically designed to bond large acrylic appliances with adequate working time. The bonded expander is activated once-per-day for 28 days; then the number of turns remaining is determined by the orthodontist. After the completion of expansion, the appliance is left in place for 5 additional months to allow for reorganization of the maxillary sutures to occur.
If an increase in lower anterior arch perimeter or an uprighting of lingually inclined lower posterior teeth is desired, a removable lower Schwarz appliance ( Fig.2 ) can be used for mandibular “dental decompensation”. The appliance is worn nearly full-time by the patient, including during eating and sleeping as well as during most other activities. The appliance is activated once quarter turn per week for 5months, and then the lower appliance serves as a retainer during the subsequent RME phase. The maxillary expander is removed after 6–7 months; usually the Schwarz appliance is discontinued at that time as well. The expanded maxilla is stabilized by having the patient wear a removable maintenance plate ( Fig.3 ) on a full-time basis for at least a year, after which it is worn on a full-time or part-time basis or discontinued.
In about two-thirds of the patients, preadjusted brackets (referred to as “temporary braces” in contrast to the full fixed appliances used in Phase II) also are placed on the upper anterior teeth for about 6 months to achieve incisor alignment ( Fig.1 ). The total treatment time for RME plus anterior brackets is 9 months; if a Schwarz appliance is added to the protocol, the total active treatment time is extended to 14months.
Before the loss of the maxillary deciduous molars, a transpalatal arch ( Fig.4 ) is used to rotate and deliver torque to the maxillary first permanent molars as needed. Because of the differences in the sizes of the second deciduous molars and the second premolars, simply placing a transpalatal arch to prevent the forward movement of the permanent maxillary first molar can save 2.0 mm of “leeway space” bilaterally. Similarly, a lower lingual arch ( Fig.5 ) can be used to prevent forward permanent molar movement in the mandible, resulting in a net gain in arch perimeter of 2.5 mm on each side. Phase II consists of full preadjusted edgewise appliances to optimize the occlusion, with an average treatment time of about 18 months.