INTRODUCTION

Mandibular symphyseal distraction osteogenesis (MSDO) is an effective treatment option for the correction of mandibular transverse discrepancies.¹ This technique can be incorporated into a comprehensive orthodontic treatment plan for the correction of maxillo-mandibular transverse discrepancies, a narrow mandibular arch, and mandibular dental crowding.² The success of distraction osteogenesis is explained by the Law of Tension-Stress in which the controlled and gradual separation of two bone segments stimulates the formation of new bone. The first protocol for distraction osteogenesis was described by Ilizarov for use in limb lengthening.^3,4 His technique involved the surgical division of long bones, a latency period, a distraction phase that initiated the traction forces with distraction at 1 mm per day, and a consolidation period for the remodeling of the bony regenerate.⁵ Distraction osteogenesis provides the ability to gradually develop both bone and soft tissue, which are associated with increased stability.⁶ In comparison to acute orthopedic movements with fixation, this gradual development reduces the risks of relapse.⁷

Distraction osteogenesis has been incorporated into the treatment plan of craniofacial abnormalities for its ability to produce stable and dramatic changes in bone and soft tissue. A well-known application of distraction osteogenesis in dentistry is surgically assisted palatal expansion.⁸ The use of MSDO was first described by Guerrero.^9,10 Several devices, bone-borne, tooth-borne, and hybrid, are available to achieve MSDO and differ in their attachment sites. Following an osteotomy, the bone-borne device is placed on either side of the mandibular symphysis, the tooth-borne device is attached to selected mandibular teeth, and the hybrid device is attached to both bone and teeth. The tooth-borne and hybrid appliances used in this study are illustrated in Figures 1 and 2, respectively.

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Numerous studies describing the successful use of both tooth-borne and hybrid appliances have been published.^11–13 Successful dental and skeletal expansions have been demonstrated with both tooth-borne and hybrid distractors.¹⁴ Very few clinical studies evaluate the long-term stability following MSDO. Most studies with follow-up records show good long-term stability with negligible relapse; however, the limitations of these studies include short follow-up time frames, a lack of immediate posttreatment records as a reference, or small sample sizes.^12,13,15 A clinical trial comparing posttreatment to follow-up records evaluating a hybrid appliance concluded that no statistically significant transverse changes occurred during the mean follow-up time of 7.5 years after distraction.¹¹ No long-term direct comparison is available to evaluate the dental and skeletal stability of MSDO achieved with different appliances.

The purpose of this study was to evaluate and compare the long-term skeletodental stability of mandibular symphyseal distraction osteogenesis achieved with the use of tooth-borne vs. hybrid distraction appliances.

MATERIALS AND METHODS

The University of Tennessee Health Science Center Institutional Review Board granted approval 14-02923-XP to obtain and review the posttreatment and follow-up records of patients who underwent MSDO in conjunction with orthodontic treatment. Informed consent for MSDO as part of a comprehensive orthodontic treatment plan was obtained from 47 patients under the care of one privately practicing orthodontist. From this sample, patients were included in the present study if follow-up records were obtained a minimum of 2 years following the removal of all orthodontic appliances. Patients were excluded from the study if they were older than the age of 18 at the start of treatment or if they were provided fixed retention upon removal of appliances. A total of 33 patients (14 tooth borne, 19 hybrid) met the criteria and were included in the current study. The 14 patients who underwent MSDO with the use of a tooth-borne appliance were on average 12.92 years of age at the start of treatment and had a mean follow-up time of 5.08 years from the completion of treatment. The 19 patients who underwent MSDO with the use of a hybrid appliance were on average 13.54 years of age at the start of treatment and had a mean follow-up of 6.07 years. All patients were treated by the same orthodontist and oral surgeon. Each treatment plan involved predistraction orthodontics, a symphyseal osteotomy with the placement of the specified distraction appliance, a latency period, a distraction period, a consolidation period, and postdistraction orthodontics. Following the completion of orthodontic treatment, patients were provided removable retainers with instructions to wear them full-time for 6 months except when eating, brushing, and playing contact sports followed by 6 months at-home wear. Following the first year, patients were instructed to wear the retainers nightly.

Posttreatment and follow-up orthodontic records included study models, a lateral cephalometric radiograph, a postero-anterior (PA) cephalometric radiograph, and intraoral photographs. From these records, a total of 16 measurements were recorded. The study models provided eight measurements that included intersecond molar width, interfirst molar width, intersecond premolar width, interfirst premolar width, intercanine width, intercentral incisor width, irregularity index, and arch length. The PA cephalometric radiograph provided seven measurements that included bigonial width, biantegonial width, bicondylar width, interbone marker width, interincisor apices width, intercanine width, and intersecond molar width. The lower incisor to mandibular plane angle was measured from the lateral cephalometric radiograph. Descriptions of each measurement can be found in Table 1.

Table 1. Descriptions of Measurements^a

^a Table modified from The Angle Orthodontist.¹⁴

^b PA indicates posterior-anterior.

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The 33 patients included in the study had similar orthodontic records acquired: (1) following a 7-day postosteotomy latency period (predistraction, T1), (2) at the beginning of the consolidation period (postdistraction, T2), (3) at the end of active orthodontic treatment (T3), and (4) after follow-up (T4). Comparison of the T1 and T2 records provided measurements of the amount of expansion achieved. In this retrospective study, no measurement of the distraction device was available.

RESULTS

Average measures at T1, T2, T3, and T4 for the tooth-borne and hybrid distractor groups are shown in Tables 2 and 3 and in Figure 3 (study model measurements only). Differences in the changes observed over time between the groups are presented in Table 4 and Figure 4 (study model measurements only).

Table 2. Tooth-Borne Distractor: Dental and Radiographic Data^a

^a Values are expressed as mean (standard deviation).

^b PA indicates posterior-anterior.

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

^a Values are expressed as mean (standard deviation).

^b PA indicates posterior-anterior.

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Table 4. Comparison of Tooth-Borne Distraction and Hybrid Distraction During Follow-Up^a

^a Values are expressed as mean (standard deviation [SD]). Positive values indicate greater change in the hybrid group. Negative vales indicate greater change in the tooth-borne group. Difference = hybrid changes − tooth-borne changes.

^b PA indicates posterior-anterior.

* P < .05. Sig indicates significance; NS, not significant.

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From postorthodontic records to follow-up records, both distraction groups shared several similarities. Study model analyses revealed that both groups experience changes in the irregularity index, arch length, and canine width. The irregularity index increased 2.0 mm in the tooth-borne group and 1.67 mm in the hybrid group. The arch length decreased 1.54 mm in the tooth-borne group and 1.39 mm in the hybrid group. The intercanine width also decreased 1.28 mm in the tooth-borne group and 0.64 mm in the hybrid group. From the PA cephalometric radiograph, the interincisal apices width increased, and from the lateral cephalometric radiograph, the mandibular plane to lower incisor (MP-LI) decreased. The interincisal apices width increased 1.11 mm in the tooth-borne group and 0.52 mm in the hybrid group. The MP-L1 decreased 1.25° in the tooth-borne group and 2.88° in the hybrid group. The only significant difference during the follow-up period was the central incisor contact point as measured from the study models. For this measurement, patients in the tooth-borne group showed a significant increase of 0.52 mm.

In the hybrid expander group, no significant correlation was found between the amount of expansion (increase in distance between bone markers T1–T2) and relapse of study cast measurements (P > .12, r-values: 0.0-0.4, Pearson correlation) and PA cephalogram parameters (P > .14, r- values: −0.4-0.3). Similarly, in the tooth-borne expander no significant correlation was found between the amount of expansion (increase in distance between bone markers T1-T2) and relapse of study cast measurements (P > .12, r-values: 0.1-0.4, Pearson correlation) and PA cephalogram parameters (P > .1, r values –.5 to .0).

The power analysis showed that everything that was concluded to be significant had a high enough power (>0.8) to confirm that the results were valid.

DISCUSSION

Both skeletal and dental expansion is achieved with MSDO using tooth-borne and hybrid appliances as part of a comprehensive treatment plan. As a result of the differing attachment sites of the distractors, the hybrid distractor's lower center of resistance produces a more parallel regenerate. The more alveolar placed tooth-borne distractor rotates the segments, resulting in a disproportionate regenerate with greater separation of the alveolus.¹⁷ Numerous clinical studies have demonstrated these differences in the bony regenerate produced by the different appliances. Hybrid distractors provide a more parallel expansion of basal and alveolar bone, whereas the tooth-borne distractors create a disproportionate bony regenerate with greater expansion of the alveolar bone.^12,13,18 A direct comparison between the two appliances also supports the fact that during distraction, a hybrid distractor creates a more parallel expansion of basal and alveolar bone than does a tooth-borne distractor. Furthermore, a hybrid device provides greater skeletal effects, whereas distraction with a tooth-borne device creates greater dental effects.¹⁴ The skeletal and dental differences between the effects of the two appliances leads to consideration of potential differences in long-term stability.

Mean follow-up records of 6.07 years from the completion of treatment for hybrid patients and 5.08 years for tooth-borne patients provide the longest follow-up study available for the direct comparison of posttreatment changes between tooth-borne and hybrid groups. A previous publication studying orthodontic relapse concluded that approximately half of the total relapse takes place in the first 2 years after retention.¹⁹ For this reason, no patients were included in the current study if the follow-up records were less than two years after the completion of orthodontic treatment. The same study further concluded that nearly all occlusal traits relapsed gradually over time, but remained stable from five years postretention. This follow-up range supports that the average follow-up records for the patients in our study should reflect most long-term changes that will occur.

Much research has focused attention on relapse following orthodontic treatment. Studies have found typical posttreatment changes to include increases in incisor irregularity and the decreases in intercanine width and arch length.²⁰ Findings of the present study¹⁴ are consistent with these changes in incisor irregularity, arch length, and intercanine width. A previous study with a slightly larger patient sample treated under the care of the same orthodontist as in the current study provides an example of the effects of distraction with the use of hybrid and tooth-borne appliances during treatment. Using the same treatment and record protocol, patients who underwent distraction with the use of a hybrid distractor had an irregularity index decrease of 6.6 mm. Those who underwent distraction with the use of a tooth-borne distractor had a decrease of 8.1 mm.¹⁴ The current study, with a subset of the same patient population, showed an increase in the irregularity index during the follow-up time period in both groups. The irregularity index increased an average of 1.68 mm in the hybrid group and 2 mm in the tooth-borne group. Both groups also shared a decrease in the MP-L1 measurement over time. A difference during retention was noted that patients in the tooth-borne group had a greater discrepancy of the central incisor contact point over time than patients in the hybrid group.

During review of our data, we questioned the existence of a correlation between the amount of expansion achieved with each device and the relapse occurring during the posttreatment interval of interest in this study. As would be expected with the hybrid expander, there was more expansion measured at the bony markers than occurred with the tooth-borne appliance. However, in the posttreatment period, there was no significant correlation between the amount of expansion and postorthodontic relapse of any of the parameters measured regardless of the type of expander.

Changes noted in our study are comparable to studies of the long-term stability of mandibular incisors.²⁰ The arch length decrease was comparable to relapse noted in previous studies with an average decrease of 1.39 mm in the hybrid and 1.54 mm in the tooth-borne groups. A meta-analysis, regarded as a highest level of evidence in research, reviewed postretention intercanine widths and is in agreement with the reduction in intercanine width found in our study. The meta-analysis concluded that mandibular intercanine width tends to decrease postretention by 1.2 to 1.9 mm, regardless of pretreatment classification or whether treatment was extraction or nonextraction.²¹ Our findings showed an average decrease of 0.64 mm in the hybrid group and 1.28 mm in the tooth-borne group. When comparing the preexpansion (T1) and the follow-up (T4) intercanine widths (ICW), we found that systemic differences between the two appliances do exist. In the tooth-borne appliance, most of the increase in intercanine width (measured on casts) achieved during expansion was lost in the postexpansion and follow-up period. However, a statistically significant increase of 1.2 mm (P = .0001, paired t-test) was still present during follow-up. However, this finding was not supported by the result from the PA cephalogram, which showed a complete relapse in the ICW in the follow-up time point. On the other hand, in the hybrid group, cast measurements showed a relapse of ICW to pretreatment measurement (P = .4, paired t-test). On the contrary, the ICW cephalogram measurements showed a net significant increase of about 3 mm between predistraction and follow-up appointments (P = .0001, paired t-test). This discrepancy can be attributed to the way the ICW was measured on the cast and the cephalogram and differences between the two expanders. The cusp tips were used in the cast measurements, whereas the lateral cementoenamel junctions (CEJs) were used in the PA measurements. This suggests that the increase in the cast-ICW in the tooth-borne appliance was mainly a result of the tipping of the canine crown with minor changes in the bony base dimension. On the other hand, in the hybrid appliance, our results suggest more bony expansion as evidenced by the increase in the ICW measured on the cephalogram. This change was not reflected in the hybrid cast measurement possibly because of the canines moving anteriorly and mesially during closure of the postdistraction midline space. In addition, canine tipping and error in the cephalogram measurements cannot be ruled out. This explanation corresponds with the results from the bone marker measurements that showed a net increase of 5.0 ± 1.5 mm in the hybrid group compared with 1.8 ± 1.4 in the dental expansion group.

Our study demonstrates changes following treatments that are common to other treatments. Change has also been shown to occur over time in untreated groups. A long-term comparison of treated and untreated groups concluded that from late adolescence through early to middle adulthood, lower incisor irregularity increased similarly in both groups.²²

A posttreatment finding unique to the current study was the increase in the distance of interinicisal apices. It is probable that this posttreatment change can be attributed to planned tooth movements during predistraction orthodontics. During this phase, divergence of lower incisors is created to provide space and reduce possible root damage during the osteotomy. Although complications are rare, damage to lower incisors has been encountered.¹ It is our recommendation that the prevention of damage during the osteotomy outweighs the relapse of interincisal apices posttreatment.

CONCLUSION

• Changes found in this study are consistent with those found in untreated and orthodontically treated individuals over time.

• The long-term changes in the current patient sample can be determined to be expected and acceptable.

• MSDO is a viable treatment option with the use of either a hybrid or tooth-borne appliance.