INTRODUCTION

Distraction osteogenesis (DO) is the formation of new bone stimulated by the gradual separation of two bone segments. Ilizarov1, a Russian orthopedic surgeon, established the distraction protocol using the Law of Tension-Stress for the lengthening of limb bones. The concept of guided and controlled bone growth by DO was incorporated into the field of dentistry in the 1970s for surgically assisted, rapid palatal expansion.2 Following the introduction of DO to dentistry, mandibular symphyseal distraction osteogenesis (MSDO) was described to achieve mandibular widening.3,4 Since then, this procedure has been used as an effective treatment option for the correction of a narrow mandibular arch, mandibular crowding, maxillomandibular transverse discrepancies, and unilateral and bilateral posterior scissors bite.5

Traditional methods for the correction of mandibular dental space deficiency and crowding include extraction, interproximal reduction, dental expansion, and proclination of mandibular incisors. These approaches may not produce optimal results. Interproximal reduction is limited by the thickness of enamel.6 Excessive dental expansion and proclination can lead to periodontal problems.7 MSDO has been analyzed as an effective treatment alternative for the treatment of mandibular crowding.7 More recent research has supported MSDO as a treatment option when risks to periodontal health and esthetic outcomes contraindicate the use of more traditional approaches.8

At present, multiple distraction devices are available. Bone-borne, tooth-borne, and hybrid distraction devices are options for MSDO. As the names indicate, a bone-borne distractor is secured only by bone screws on both halves of the mandible. The tooth-borne distractor is attached to selected mandibular teeth (Figure 1A). The hybrid distractor is secured by two mandibular bone screws and bonded circumdental wires on the mandibular canines or premolars (Figure 1B). These appliances vary in patient acceptance, cost, invasiveness, and comfort. Because they are designed and attached differently, the distractors apply force at different locations; therefore, they are predicted to produce varying amounts of skeletal and dental expansion.9–11 The bone-borne and hybrid distractors are predicted to provide more parallel expansion of basal and alveolar bone, while the tooth-borne distractor should create a disproportionately bony regenerate with greater expansion of alveolar bone.12

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Past research has identified the differences in the bony regenerate produced by different appliances.13 Uckan et al.12 described the concept that if the force of distraction is concentrated above the center of resistance, greater separation will occur within the alveolar bone than in the basal bone. A comparison of tooth-borne and hybrid distraction should support that the lower point of force application obtained by the hybrid distractor produces a more parallel regenerate, while the more alveolar-placed tooth-borne distractor rotates the segments for a disproportionate regenerate, with wider separation at the alveolus.12 A triangular regenerate has been demonstrated when using a tooth-borne distractor,14 while parallel regenerate has been shown to result from hybrid distraction.15

In the current study, two large groups of patients who underwent tooth-borne and hybrid distraction were compared. The study differs from existing literature by directly comparing distraction of two appliances using a consistent treatment protocol with thorough record keeping. Differences in the outcomes between the two methods studied could potentially affect the long-term stability of the changes obtained by MSDO.16

MATERIALS AND METHODS

Informed consent for the use of MSDO was obtained from 47 patients under the care of one private-practice orthodontist in the Richmond, Va, area. A custom-fabricated hybrid distractor was first used to achieve distraction in 27 patients with a mean age of 13.31 ± 1.47 years. A tooth-borne distractor was then used to achieve distraction in 20 patients with a mean age of 12.88 ± 1.60 years at the time of banding. All patients underwent six phases of orthodontic treatment, including distraction osteogenesis. The sequence of treatment was predistraction orthodontics, the symphyseal osteotomy with placement of the distractor, a latency period, a distraction period, a consolidation period, and postdistraction orthodontics.17

Standard orthodontic records were taken at four time points: pretreatment (T1), predistraction (T2), postdistraction (T3), and posttreatment (T4). T1 records were obtained prior to comprehensive orthodontic treatment. T2 records were obtained immediately after the approximately seven-day latency phase following the osteotomy. T3 records were recorded at the end of the distraction phase and prior to the approximately 12-week consolidation phase. T4 records were obtained at the debonding appointment. Records included intraoral photographs, study models, postero-anterior (PA) cephalometric radiographs, and lateral cephalometric radiographs at each time point. Submental vertex (SMV) radiographs were taken at T2, T3, and T4 only.

Institutional Review Board Approval was granted by Virginia Commonwealth University to evaluate and compare changes in these MSDO patients using the records previously obtained. In total, 18 measurements obtained from these records were used in this study. Measurements are described in Table 1. Eight measurements were made from study models with the use of a digital caliper (intersecond molar width, interfirst molar width, intersecond premolar width, interfirst premolar width, intercanine width, intercentral incisor contact point, irregularity index, and arch length). Eight measurements were made from the PA cephalometric radiographs (bigonial width, biantigonial width, bicondylar width, intersecond molar width, intercanine width, interincisor apices width, the width of bone markers placed at the time of surgery, and distractor screw width). The lateral cephalometric radiograph was used to measure the lower incisor to mandibular plane angle. The bicondylar width was also measured with the SMV cephalometric radiographs.

Table 1.  Descriptions of Measurements^a

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RESULTS

Average measures at each time point are shown for the tooth-borne and hybrid distractor groups in Tables 2 and 3, respectively. Differences in the changes observed over time between the groups are presented in Table 4.

Table 2.  Tooth-Borne Distraction: Dental and Radiographic Data

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Table 3.  Hybrid Distraction: Dental and Radiographic Data^a

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Table 4.  Comparison of Tooth-Borne Distraction and Hybrid Distraction During Each Phase of Treatment^a

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The results obtained using the hybrid distractor and the tooth-borne distractor shared several similarities. From preorthodontic records obtained at T1 to postorthodontic records obtained at T4, both distraction groups showed an increase in all widths measured from study models. Both groups also demonstrated a decrease in irregularity index during orthodontic treatment. Analysis of the PA cephalometric radiograph showed that the overall measurements from T1 to T4 were not different between distraction groups, with a decrease in bigonion and bicondylar widths for both. There was an increase in interincisor apices and intercanine width. From the lateral cephalometric radiograph, both groups of distraction patients showed an increase in lower incisor angulation (MP-L1).

Although both distraction groups shared similarities from the beginning to the end of orthodontic treatment, analysis of each treatment phase revealed many differences. During predistraction orthodontics from T1 to T2, there was a statistically significant difference in changes in intercanine width measured from the study models (P < .05) and in the interincisor apices width measured from the PA cephalometric radiograph (P < .01). For both of these measurements, there was a greater increase in width for patients that would receive tooth-borne distractors. Dental and radiographic measurements obtained from the use of the tooth-borne distractor can be viewed in Table 2.

During the distraction phase from T2 to T3, the hybrid distractor produced statistically greater increases in measurements. Dental and radiographic measurements obtained from the use of the hybrid distractor can be viewed in Table 3. The most significant changes were noted in the biantigonial and interincisor apices widths as measured from the PA cephalogram (P < .001). The intersecond molar width from the PA cephalogram and intersecond molar width from the study models both indicated a statistically greater increase in width for the hybrid distractor (P < .01). The hybrid distractor also showed significantly greater increases in interfirst molar, intersecond premolar, and interfirst premolar widths as measured from study models and the bigonion and intercanine widths of the PA cephalogram (P < .05).

From T2 to T3, tooth-borne segments tipped significantly more than in the hybrid group (P < .0001), and the hybrid was nearly parallel. When comparing the changes in bone marker widths with intercanine widths as measured from the PA cephalogram for hybrid distraction (Figure 2), there was a 92% skeletal-to-dental expansion of the bony regenerate. When comparing the changes in bone marker widths with intercanine widths during tooth-borne distraction, there was a 49% skeletal-to-dental expansion of the bony regenerate. These comparisons demonstrate a more parallel separation of bone during distraction with a hybrid distractor. Distraction with the tooth-borne distractor had a more disproportionate expansion, with a greater separation of the canines in alveolar bone than of the bone markers in basal bone.

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Postdistraction orthodontics from T3 to T4 showed significantly greater increases in measured widths in patients that received distraction with the tooth-borne device. The most significant differences were noted in the intercanine and interfirst premolar widths measured from study models (P < .001). The interfirst molar and intersecond premolar widths of the tooth-borne distractor increased more than those of the hybrid (P < .01). The intersecond molar width of the tooth-borne distractor also increased more than that of the hybrid (P < .05).

During orthodontic treatment from T1 to T4, there was a statistically greater increase in arch length and lower incisor angulation for patients that received distraction with the use of a tooth-borne device. All other changes in measurements were not determined to be statistically significant.

DISCUSSION

This retrospective study offers a comprehensive comparison during each stage of orthodontic treatment with MSDO between tooth-borne and hybrid distractors. Numerous publications have investigated differences between distraction appliances through the use of finite element models, case documentations, and smaller studies.9,11,12 Existing literature also documents the effects of individual distraction appliances.14,15 Little research, however, has compared directly the effects of different appliances using a consistent treatment protocol with thorough record keeping. Both tooth-borne and hybrid distraction devices are recognized for their effective treatment of mandibular crowding.8,14,15,18

Analysis of study models and radiographs supported the use of both tooth-borne and hybrid distractors to effectively resolve mandibular crowding. The results were consistent with previous literature stating that the greatest expansion takes place nearest to the site of distraction. Current results support that the most significant widening is localized to the mandibular symphysis (Table 4). Both the tooth-borne and hybrid distraction devices demonstrate a progressive increase in widening from the posterior to the anterior (Table 2 and Table 3).9,11

Alignment of teeth by dental expansion has been shown to be unstable.19–22 The MSDO results demonstrated that greater expansion was achieved dentally than skeletally when using a tooth-borne distractor (Table 4). With greater dental effects than skeletal effects, MSDO with a tooth-borne distractor may be less stable over time. Previous studies evaluating tooth-borne distraction have demonstrated a triangular regenerate with more widening superiorly. When comparing the changes in bone marker widths with intercanine widths as measured from the PA cephalogram in this study, there was a 49% skeletal-to-dental expansion of the bony regenerate. Other studies of tooth-borne distraction have also demonstrated dental changes through increased proclination of mandibular anterior teeth.14,18 DelSanto et al.14 showed a greater increase in lower incisor angulation for the tooth-borne distractor than for the hybrid distractor. The more disproportionate regenerate and the greater proclination of mandibular incisors suggest that expansion with a tooth-borne distractor is gained through greater dental effects than MSDO with a hybrid distractor. Despite these findings, MSDO with a tooth-borne distractor does have advantages. Tooth-borne distraction devices only require a single surgical procedure for the osteotomy, which can potentially decrease cost and increase comfort for patients. Hybrid distractors require a second surgical procedure to remove the device. For these reasons, tooth-borne devices continue to be favored by some practitioners.

Other experts have been attracted to the hybrid distractor for its potentially increased long-term stability. In a previous study,15 a tooth- and bone-borne hybrid distractor produced parallel widening of the mandibular frontal plane. This widening resolved crowding of mandibular anterior teeth by proportionately increasing skeletal and dental arch widths. The current results also support the findings of Malkoc et al.15 When comparing the changes in bone marker widths with those of intercanine widths as measured from the PA cephalogram, there was a 92% skeletal-to-dental expansion of the bony regenerate. The more parallel regenerate achieved with MSDO using a hybrid distractor and the disproportionate regenerate of the tooth-borne distractors can be viewed in Figures 3 and 4, respectively.

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While the cumulative effects of orthodontics and MSDO produced similar increases in interdental widths, there were differences regarding when the expansion was achieved. The hybrid device gained space during the distraction phase of treatment (Table 3). The tooth-borne group compensated for less gain during distraction with a greater gain during postdistraction orthodontics (Table 2). This could contribute to the final greater increase in lower incisor angulation and greater arch length gains observed with tooth-borne distraction. These measurements reflect greater dental effects through protrusive movements, with relatively less contribution from skeletal width changes. While the overall results support both appliances as effective strategies for creating space to relieve dental crowding, the skeletal and dental changes produced by the appliances were not equal. The timing of when space was gained reflects the differing amounts of skeletal and dental changes effected by each appliance. Future studies should evaluate whether these morphological differences also result in differences in periodontal outcome. Additionally, the stability of skeletal versus dental movements may influence the potential for relapse.

CONCLUSIONS

• Tooth-borne and hybrid distraction both resulted in significant skeletal and dental expansion.

• The skeletal component of the total expansion achieved was greater in the hybrid distractor group.

• The dental component of the total expansion achieved was greater in the tooth-borne distractor group.

• During distraction, the hybrid distractor effected more parallel expansion of basal and alveolar bone than did the tooth-borne distractor.