Our objectives were to evaluate midfacial skeletal changes in the coronal plane and the implications of circummaxillary sutures and to localize the center of rotation for the zygomaticomaxillary complex after therapy with a bone-anchored maxillary expander, using high-resolution cone-beam computed tomography.
Fifteen subjects with a mean age of 17.2 ± 4.2 years were treated with a bone-anchored maxillary expander. Pretreatment and posttreatment cone-beam computed tomography images were superimposed and examined for comparison.
Upper interzygomatic distance increased by 0.5 mm, lower interzygomatic distance increased by 4.6 mm, frontozygomatic angles increased by 2.5° and 2.9° (right and left sides), maxillary inclinations increased by 2.0° and 2.5° (right and left sides), and intermolar distance increased by 8.3 mm ( P <0.05). Changes in frontoethmoidal, zygomaticomaxillary, and molar basal bone angles were negligible ( P >0.05).
A significant lateral displacement of the zygomaticomaxillary complex occurred in late adolescent patients treated with a bone-anchored maxillary expander. The zygomatic bone tended to rotate outward along with the maxilla with a common center of rotation located near the superior aspect of the frontozygomatic suture. Dental tipping of the molars was negligible during treatment.
We evaluated midface skeletal effects induced by a microimplant-supported skeletal expander in late adolescent patients in the coronal plane.
Several circummaxillary bones and sutures were modified by the bone-borne maxillary expander.
The zygomatic bone undergoes a considerably greater lateral displacement than previously reported for tooth-borne expanders.
Center of rotation for the zygomaticomaxillary complex was slightly above the frontozygomatic suture.
Microimplant-supported skeletal expander generated midface expansion in late adolescent patients.
It is believed that during rapid palatal expansion (RPE), the main resistance to the opening of the midpalatal suture is probably not in the suture itself but, rather, in the surrounding structures with which the maxilla articulates, particularly the sphenoid and zygomatic bones. Therefore, the expansion force might affect all circummaxillary sutures: internasal, nasomaxillary, frontomaxillary, frontonasal, frontozygomatic, zygomaticomaxillary, zygomaticotemporal, and pterygopalatine. This involvement has been hypothesized based on investigations that used histologic methods, radiologic imaging, photoelastic models, bone scintigraphy, and finite element methods.
Cranial sutures respond differently to external orthopedic forces depending on their anatomic location and degree of interdigitation, and different studies have indicated diverse regions of the midfacial skeleton as the most affected by RPE. Some authors cited the frontozygomatic, zygomaticomaxillary, and zygomaticotemporal sutures as the primary anatomic sites of resistance to RPE. Other clinical investigations have described greater changes in the sutures directly articulating with the maxilla than those indirectly articulatig. Finite element method analyses found high stress levels in the zygomatic process of the maxilla, external walls of the orbit, frontozygomatic suture, and frontal process of the maxilla.
For the rotational fulcrum of the maxillary bone during RPE, it is still being debated where it is located. Studies have established this center of rotation in different areas, frequently at the frontomaxillary suture. Other authors have identified the center of rotation close to the superior orbital fissure. In relation to the zygomatic bone, although high stress levels have been reported at the zygomatic sutures, no study has described its motion path during RPE and the location of its rotational fulcrum.
Analysis of the circummaxillary suture modifications during rapid maxillary expansion have been previously conducted using study models, 2-dimensional imaging, and, more recently, 3-dimensional (3D) imaging based on computed tomographic data. The introduction of cone-beam computed tomography (CBCT) and the development of new computer software allow obtaining multiplanar, 3D reconstructions, extending the possibilities for analysis of the craniofacial complex in living subjects.
Miniscrews have been added to RPE devices, as proposed by Wilmes et al in the hybrid hyrax appliance, to prevent buccal tipping of the lateral teeth and the negative consequences on their periodontal support. Furthermore, various miniscrew-assisted RPE appliances with different designs have been developed in recent years, with the goal to enhance the orthopedic effects of maxillary expansion. A maxillary skeletal expander (MSE) is a specific type of bone-borne expander that uses 4 miniscrews in the posterior part of the palate with bicortical engagement. The advantages of miniscrew-assisted RPE appliances over conventional expanders in achieving orthopedic changes are controversial in the literature. Comparisons between tooth-borne and bone-borne expanders have been published using CBCT technology, and different conclusions were drawn regarding the possibility for generating a greater orthopedic response with miniscrew-supported devices. The aim of this investigation was to further evaluate the skeletal changes in the midface and the implications of circummaxillary sutures during rapid maxillary expansion with MSE, by describing the magnitude and pattern of lateral movement of the zygomaticomaxillary complex in the coronal plane, using high-resolution CBCT.
Material and methods
Institutional Review Board approval from the University of California at Los Angeles was obtained for this retrospective study, which included 15 patients (6 male, 9 female), consecutively treated with MSE (Biomaterials Korea, Seoul, Korea), with a mean age of 17.2 ± 4.2 years (range, 13.9-26.2 years) of predominantly Hispanic ethnicity. Of the 15 patients, 9 had bilateral posterior crossbite, 5 had unilateral crossbite, and 1 was diagnosed with maxillary transverse deficiency without a dental crossbite. Treatment for all patients was done at the orthodontic clinic of the School of Dentistry at our university. Expansion with MSE was started and completed before bonding any brackets or other appliances.
The inclusion criteria were the following: (1) diagnosis of a transverse maxillary deficiency based on a modified version of Andrews’ analysis of 6 elements, as elaborated below; (2) treatment with MSE as part of the overall treatment plan, (3) CBCT scans taken at 2 times: before treatment and within 3 weeks after active expansion; (4) no craniofacial abnormalities, and (5) no previous orthodontic treatment.
The method adopted to analyze the relationship between the maxillary and mandibular widths is described in Figure 1 . Maxillary width is represented by the distance between the right and left most concave points lying on the maxillary vestibule at the level of the mesiobuccal cusp of the first molars. Mandibular width is defined as the distance between the right and left WALA ridges located at the level of the mesiobuccal groove of the first molars. To assess transverse deficiency, we calculated the difference between the mandibular and maxillary widths, which ideally should have been equal. It also gave an estimate of the amount of maxillary skeletal expansion required ( Fig 1 ).
MSE was chosen instead of a traditional tooth-borne expander, based on the following criteria: patient maturity (appearance of secondary sexual characteristics including facial hair, voice changes, onset of menstruation, and cervical vertebral maturation stage higher than CS4), dolichofacial vertical pattern (based on high SN-GoGn and FMA angles), and positive history of nasal airway problems. At our Section of Orthodontics, dolichofacial patients are treated with MSE rather than tooth-borne expanders, because bone-borne appliances tend to yield less posterior mandibular rotation.
An MSE appliance ( Fig 2 , A ) consists of a jackscrew unit supported by 4 palatal microimplants and attached to the molars with connecting arms and molar bands. The rate of expansion was 2 turns per day (0.25 mm per turn) until a diastema appeared; then the rate changed to 1 turn per day. Expansion was stopped when the maxillary skeletal width, defined in Figure 1 , was equal to or greater than the mandibular width. After completion, the MSE was kept in place without further activation for at least 3 months to retain the expansion.
The amount of activation of the MSE jackscrew applied to the patients was calculated as follows: the distance between the 2 halves of the expansion screw was measured on the CBCT image taken after expansion ( Fig 2 , B ); the preexpansion distance was determined by taking a CBCT scan on an MSE appliance and measuring the distance 10 times. The preexpansion distance was subtracted from the postexpansion one, and the values were then averaged to obtain the mean and standard deviation.
The CBCT scans were taken at 2 times: before expansion and within 3 weeks after active expansion. The time between the scans was 5 ± 2 months, and this included the time for administrative procedures between the patient and the clinic’s office, as well as for appliance fabrication and delivery. Postexpansion scans were taken before the patient received any bonded brackets or other appliances, to analyze skeletal changes induced solely by MSE.
A scanner (5G; NewTom, Verona, Italy) was used for all patients, with an 18 × 16 cm field of view, 14-bit gray scale, and a standard voxel size of 0.3 mm. Configuration of the CBCT included scan time of 18 seconds (3.6 seconds emission time), with 110 kV. We used an automated exposure control system to detect the patient’s anatomic density and adjust the milliamperes accordingly.
OnDemand3D (Cybermed, Daejeon, Korea) is a software capable of superimposing the preexpansion and postexpansion CBCT images of the patient using the anatomic structures of the entire anterior cranial base in adults and the anterior cranial fossae in growing children, by automated processing in matching the voxel gray-scale patterns. Accuracy of the superimposition method has been recently validated. After superimposition of CBCT data sets, the following novel methodology was used to evaluate the skeletal changes in the midface. The maxillary sagittal plane was identified, passing through the anterior nasal spine, posterior nasal spine, and nasion on the preexpansion CBCT image ( Fig 3 ). Then the coronal zygomatic section ( Fig 4 ) was selected to evaluate the changes in the maxillary, zygomatic, frontal, and ethmoid bones. The section passes through the lowest point of the zygomaticomaxillary sutures and the uppermost point of the frontozygomatic sutures. In this coronal section, both linear and angular skeletal measurements were made ( Figs 5 and 6 ).
Skeletal linear measurements included the upper interzygomatic distance that extends from the most external point of the right frontozygomatic suture to the most external point of the left frontozygomatic suture, and the lower interzygomatic distance that extends from the most external point of the right zygomaticomaxillary suture to the most external point of the left zygomaticomaxillary suture ( Fig 5 ).
Skeletal angular measurements included the frontoethmoidal angle, frontozygomatic angle, zygomaticomaxillary angle, and maxillary inclination, as shown in Figure 6 .
The frontoethmoidal angle is formed by the lowest point of crista galli of the ethmoid bone and the most external points of the frontozygomatic sutures bilaterally. The frontozygomatic angle is formed by the lowest point of crista galli, the most external point of the frontozygomatic suture, and the most external point of the zygomaticomaxillary suture. The zygomaticomaxillary angle is formed by the same landmarks as above, located at the frontozygomatic and zygomaticomaxillary sutures, and by the point where the cortical bones of the maxillary sinus floor and the nasal floor merge. Maxillary inclination is the angle between 2 lines: one that connects the most lateral point of the maxillary bone and the point where the cortical bones that form the floor of the nasal cavity and maxillary sinus merge, and the other line represented by the maxillary sagittal plane.
For the dental analysis, a coronal section through the furcation of the roots and the central fossae of the maxillary first molars was used, called the coronal molar section ( Fig 7 ).
Dental measurements included the intermolar distance and the molar basal bone angle. The intermolar distance is measured at the level of the most occlusal point of the mesiopalatal cusp of the maxillary first molars, and the molar basal bone angle is the angle formed by the same horizontally oriented line used in maxillary inclination and the line connecting the central pit of the molar crown to the furcation of the roots.
All evaluated parameters are listed in Table I .
For each variable, the preexpansion value was subtracted from the postexpansion value. The mean change was compared with zero, and the P value was computed using the Wilcoxon signed rank test for paired data.
For the assessment of method reliability, measurements were obtained for all 12 variables on 8 randomly selected patients by 2 raters. Measurements were then repeated after 2 weeks by the same operators, after reorienting the skull according to the reference planes to compute reliability parameters that are the combination of error in identification of reference planes (coronal zygomatic section, maxillary sagittal plane, coronal molar section) and error in landmark localization. The calculated parameters were rater standard deviation, rater coefficient of variation, error standard deviation, error coefficient of variation, and intraclass correlation coefficient.
The average amount of activation of the MSE expansion jackscrew was 6.8 ± 1.9 mm (range, 4.1-10.5 mm). The duration of maxillary expansion ranged from 12 to 36 days.
For the skeletal linear measurements, both upper interzygomatic distance and lower interzygomatic distance significantly increased ( Table II ).
|Unit||Before expansion||After expansion||Treatment change||P value|
|Skeletal linear measurements|
|1 Upper interzygomatic distance||mm||98.18||2.93||98.70||3.09||0.52||0.37||<0.0001 ∗|
|2 Lower interzygomatic distance||mm||86.14||4.88||90.76||5.66||4.62||1.33||<0.0001 ∗|
|Skeletal angular measurements|
|3 Frontoethmoidal angle||°||169.71||6.51||169.53||6.52||-0.18||0.43||0.441|
|4 Right frontozygomatic angle||°||79.36||4.09||81.81||3.81||2.45||1.26||<0.0001 ∗|
|5 Left frontozygomatic angle||°||77.94||2.64||80.85||2.79||2.91||1.39||<0.0001 ∗|
|6 Right zygomaticomaxillary angle||°||103.80||5.52||103.50||5.68||-0.23||0.88||0.324|
|7 Left zygomaticomaxillary angle||°||105.80||5.51||105.50||5.29||-0.35||0.96||0.175|
|8 Right maxillary inclination||°||96.61||4.85||98.63||5.31||2.01||1.03||<0.0001 ∗|
|9 Left maxillary inclination||°||97.25||4.42||99.74||4.64||2.49||1.81||0.000 ∗|
|10 Intermolar distance||mm||38.58||3.53||46.91||3.46||8.33||2.29||<0.0001 ∗|
|11 Right molar basal bone angle||°||89.79||8.36||91.83||10.24||2.04||3.31||0.076|
|12 Left molar basal bone angle||°||90.33||8.45||92.15||11.50||1.83||4.26||0.144|