Abstract
This study evaluated the differences in surgical changes and post-surgical changes between bi-cortical and mono-cortical osteosynthesis (MCO) in the correction of skeletal Class III malocclusion with bilateral sagittal split osteotomies (BSSOs). Twenty-five patients had bi-cortical osteosynthesis (BCO), 32 patients had mono-cortical fixation. Lateral and postero-anterior cephalometric radiographs, taken at the time of surgery, before surgery, 1 month after surgery, and on completion of orthodontic treatment (mean 9.9 months after surgery), were obtained for evaluation. Cephalometric analysis and superimposition were used to investigate the surgical and post-surgical changes. Independent t -test was performed to compare the difference between the two groups. Pearson’s correlations were tested to evaluate the factors related to the relapse of the mandible. The sagittal relapse rate was 20% in the bi-cortical and 25% in the mono-cortical group. The forward–upward rotation of the mandible in the post-surgical period contributed most of the sagittal relapse. There were no statistically significant differences in sagittal and vertical changes between the two groups during surgery and in the post-surgical period. No factors were found to correlate with post-surgical relapse, but the intergonial width increased more in the bi-cortical group. The study suggested that both methods of skeletal fixation had similar postoperative stability.
Bilateral sagittal split osteotomy (BSSO) of the mandible was first introduced by T rauner & O bwegeser . Many studies have reported post-surgical stability after BSSO setback in terms of the sagittal and vertical dimensions. In the sagittal aspect, the variation of the amount of relapse ranged from 0.2 to 4.2 mm with regard to pogonion (relapse rate from 2% to 91%) . In the vertical aspect, the amount of relapse showed a greater range of variation from −2.47 to 1.5 mm with regard to gnathion and B point respectively (relapse rate from −1040% to 1300%) .
Several factors affect the amount of skeletal relapse after mandibular setback, including paramandibular muscular and periosteal tissue action , the magnitude of setback during surgery , the method of bone fixation , the type of inter-maxillary fixation (IMF) , as well as pre-surgical orthodontic alignment and decompensation .
Wire fixation and IMF were used as osteosynthesis for BSSO at an early stage. Since the mid-1980s, rigid internal fixation (RIF) has commonly been used in orthognathic surgery instead of wire fixation. Two major RIF methods are now used: bi-cortical screw fixation (bi-cortical osteosynthesis, BCO) and plate fixation with mono-cortical screws (mono-cortical osteosynthesis, MCO).
Although plate fixation with mono-cortical screws offers some advantages such as reduction of inferior alveolar nerve injury, avoids scars on the face or neck through trans-cutaneous drilling, and prevents rotation of the mandibular condyles which bi-cortical screw fixation does not , surgeons still question whether MCO can provide equal strength and stability as BCO. Some in vitro studies have been carried out to evaluate the resistance characteristics and rigidity of the plate–bone–screw system. A nucul et al. compared the stability of bi-cortical 2.0 mm miniscrews with 2 mm 4-hole miniplates retained by mono-cortical screws. Biomechanical experiments on bovine bone showed that the force generated for osteosynthesis failure was three times greater than that of miniplate fixation. P istner et al. examined the biomechanical characteristics of various osteosynthesis procedures following BSSO on the lower jaw of pigs. The results indicated that the force required for failure of osteosynthesis with screw fixation was almost twice that for miniplate fixation.
T haranon compared the rigidity of bi-cortical screws and a miniplate for fixation of a mandibular setback after a simulated sagittal split osteotomy. Thirty hemi-mandibles from 15 human cadavers were used to measure the maximum resistance load (MRL). The results indicated the mean MRL of three bi-cortical screws at the superior border was greater than the mean MRL of a single 4-hole miniplate on the external oblique ridge.
M aurer et al. used the finite element (FE) method to compare the mechanical qualities of 2.0 mm titanium screws in a triangular configuration with those of a titanium miniplate with 2.0 mm mono-cortical screws following BSSO. The results demonstrated that the 2.0 mm titanium screws were able to withstand a maximum masticatory force of 167.5 N, but the miniplate with mono-cortical screws fixation tolerated a maximum of 124.6 N. The authors explained that there is no conspicuous clinical difference because the muscle force 6 weeks after surgery is only 65 ± 43 N. C huong et al. used a three-dimensional FE computer modelling technique to simulate BSSO stabilization with two fixation techniques (screw and plate fixation). The results indicated that using three bi-cortical titanium screws leads to smaller deflections at the central incisor, suggesting higher mechanical stability. It also generated lower mechanical stresses in the bone and in the implanted screws in bi-cortical screw fixation than in plate fixation.
Many in vitro studies demonstrate that BCO can provide more strength than MCO, but this was not common in clinical studies. J ohan et al. studied 20 patients with BSSO advancement; the relapse rate (5%) of miniplate osteosynthesis was compatible with that occurring with BCO. B lomqvist & I saksson compared the postoperative stability of bi-cortical screw fixation and mono-cortical plate fixation in 38 cases with BSSO advancement (16 had screw fixation and 22 plate fixation). The results indicated that there was no significant difference of in post-surgical stability between the two groups.
F ujioka et al. reported two unusual cases with complete breakage of the miniplates after sagittal split osteotomies. They noted that the postoperative mandible shape tended to be more changeable with MCO. In 2000, they compared the postoperative stability following BSSO setback with both MCO and BCO . The mono-cortical fixation group had more postoperative change in gonial angle and proportion of upper face to total face height. They concluded that BCO was more rigid against this shear stress than MCO.
According to previous reviews, some authors reported that mono-cortical plate fixation provided less rigidity and was more susceptible to deformation than bi-cortical screw fixation. Some clinicians found no difference in post-surgical stability provided by either method. The conclusion is controversial and the performance of these two fixation methods remains inconclusive in terms of stability.
Transverse condylar displacement after BSSO has been studied. Several studies demonstrated that the use of RIF after BSSO resulted in greater transverse condylar displacement and more intergonial width change than the use of wire fixation. This suggested that the role of the fixation technique could be of importance and worth further investigation. Regarding plate fixation with mono-cortical screws, a different fixation technique for osteosynthesis, its clinical performance might be different with bi-cortical screw fixation especially in changes of transverse dimension.
The purpose of this study was to evaluate the difference in the surgical changes and post-surgical changes between BCO and MCO in the correction of skeletal Class III malocclusion with BSSO setback in three dimensions.
Materials and methods
Patients who received surgical-orthodontic treatment at Chang Gung Craniofacial Center from 2000 to 2004 were evaluated. Subjects were selected according to the following criteria: adult patients (aged >18 years); no history of trauma or associated craniofacial anomaly; underwent mandibular setback through BSSO surgical intervention with or without either maxillary surgery or genioplasty; fixation methods were BCO or MCO; cephalometric radiographs available before surgery, within 1 month after surgery, and at least 6 month after surgery.
Fifty-seven patients fulfilled the selection criteria. There were 32 patients (16 males and 16 females, 12 one-jaw and 20 two-jaw) in the MCO group, aged 18–34 years (mean 21.3 ± 3.4 years), and 25 patients (10 males and 15 females, 14 one-jaw and 11 two-jaw) in the BCO group, aged 18–33 years (mean 23.8 ± 4.3 years).
All patients had pre-surgical orthodontic treatment to level and align both arches. The orthodontic treatments were performed by the same orthodontist. For the orthognathic surgery, the mandible was osteotomized using the modification of the Obwegeser–DalPont sagittal split of the ramus. The masseter and medial pterygoid muscles from the proximal segment were detached in all subjects to avoid any stretching after the repositioning of the main fragments.
The decision regarding the fixation method depended on the surgeon’s choice. There were three senior surgeons in the orthognathic team and all had more than 15 years of experience in performing orthognathic surgery. Two surgeons (K.-T.C. and L.-J.L.) treated every patient with BCO and one surgeon (Y.-R.C.) treated every patient with MCO. In the BCO group, fixation was achieved with three bi-cortical screws (Leibinger, 2.0 mm in diameter) (Leibinger, Frieburg, Germany) placed in a triangular pattern on each side of mandible through a transfacial approach. For the MCO group, three 2-hole miniplates (Leibinger 3-D standard plate) and six mono-cortical screws (Leibinger, 2.0 mm in diameter) were used on each side of mandible. IMF was maintained for 1–3 weeks as needed after surgery. Post-surgical orthodontic treatment was completed for detailing the alignment, and functional interdigitation.
Cephalometric analyses
Lateral and postero-anterior (PA) cephalometric radiographs were taken under standardized conditions with the head in a natural position. Image magnification was 10.74%. The linear measurements were adjusted to true size. The radiographs were used to evaluate the surgical changes (T2–T1) and post-surgical relapse (T3–T2). The cephalometric landmarks and reference lines in the lateral and PA cephalograms are shown in Figs 1 and 2 . All the cephalometric tracings and superimpositions as well as measurements were investigated by the same observer.
Error study
The measurement errors were tested through repetitive measuring on the lateral and PA cephalograms. Ten randomly selected films were traced and measured twice 1 month apart on both lateral and PA cephalometric radiographs. The errors of the two sets of tracings and measurements were estimated by paired sample t test and the Dahlberg formula: <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='(∑D2/2N)’>(∑D2/2N)−−−−−−−−−−√(∑D2/2N)
( ∑ D 2 / 2 N )
. The errors ranged from 0.28 to 0.40 mm in linear variables and from 0.24° to 0.54° in angular variables.
Statistical analysis
Standard descriptive statistics (means and standard deviations) were calculated for all the cephalometric measurements. The cephalometric measurements of the MCO and BCO groups were analysed and compared with independent t -tests. Pearson’s correlations were tested between variables to evaluate the factors affecting the post-surgical relapse. Statistical analysis was performed using the SPSS statistical program v12.0 (SPSS Inc., Chicago, IL, USA). Significance was determined if two-side p values were less than 0.05.
The study was approved by the Institutional Review Board and Medical Ethics Committee at Chang Gung Memorial Hospital. The authors have followed the guidelines of the Helsinki Declaration in this investigation.
Post-surgical complication assessment
Post-surgical complications, including inferior alveolar nerve disturbance/injury and recovery, infection, and secondary procedures for removing the hardware, were evaluated by reviewing the charts and confirming the findings with phone calls to the patients. Sensation recovery of the inferior alveolar nerve was categorized into 5 groups (no numbness, recovery in 1, 3, 6, and over 6 months; Fig. 3 ).
Results
Measurements of initial cephalometric films
There were no statistically significant differences between the BCO and MCO groups in all variables. The dento-facial characteristics in both groups presented no significant difference compared with their initial cephalometric measurements ( Table 1 ).
| Variables | BCO group | MCO group | p value | ||
|---|---|---|---|---|---|
| Mean | SD | Mean | SD | ||
| Horizontal | |||||
| ANSx | 70.3 | 5.3 | 70.4 | 4.6 | 0.942 |
| PNSx | 19.3 | 3.3 | 19.0 | 4.0 | 0.756 |
| Ax | 67.6 | 5.5 | 67.5 | 4.7 | 0.944 |
| U1x | 74.5 | 8.4 | 73.6 | 6.0 | 0.636 |
| L1x | 79.9 | 10.6 | 78.6 | 7.8 | 0.574 |
| Bx | 73.7 | 10.6 | 73.0 | 9.9 | 0.789 |
| Pogx | 73.2 | 12.6 | 73.2 | 11.3 | 0.986 |
| Cox | −11.0 | 4.7 | −12.5 | 3.5 | 0.164 |
| Vertical | |||||
| ANSy | 49.9 | 3.8 | 51.3 | 4.4 | 0.197 |
| PNSy | 47.4 | 3.9 | 48.5 | 3.4 | 0.248 |
| Ay | 57.2 | 4.2 | 58.5 | 4.7 | 0.275 |
| U1y | 81.3 | 6.4 | 82.9 | 6.3 | 0.357 |
| L1y | 82.4 | 9.0 | 84.4 | 7.1 | 0.357 |
| By | 101.7 | 9.8 | 104.5 | 7.3 | 0.213 |
| Pogy | 119.5 | 12.3 | 120.5 | 8.8 | 0.707 |
| Coy | 19.5 | 3.6 | 18.8 | 3.3 | 0.499 |
| Transverse | |||||
| Intergonial width | 104.9 | 8.1 | 104.2 | 7.2 | 0.714 |
| Right ramus angle | 80.6 | 4.3 | 81.4 | 5.1 | 0.508 |
| Left ramus angle | 81.8 | 5.2 | 83.0 | 4.1 | 0.331 |
| Linear measurement | |||||
| Mandibular length (Co–Gn) | 133.2 | 12.6 | 133.3 | 8.7 | 0.953 |
| Posterior facial height (S–Go) | 89.9 | 9.8 | 88.5 | 7.9 | 0.557 |
| Anterior facial height (N–Me) | 135.1 | 12.8 | 137.3 | 9.1 | 0.461 |
| Anterior lower facial height (ANS–Me) | 77.9 | 9.9 | 78.5 | 6.1 | 0.776 |
| Angular measurement | |||||
| SNA | 82.3 | 3.4 | 83.6 | 3.9 | 0.194 |
| SNB | 86.0 | 4.5 | 86.5 | 4.8 | 0.652 |
| Upper incisor angle (SN/U1) | 118.2 | 9.4 | 118.0 | 9.5 | 0.943 |
| Lower incisor angle (MP/L1) | 85.3 | 11.6 | 81.6 | 8.9 | 0.175 |
| Gonial angle (Co–Go–Me) | 131.2 | 8.3 | 133.0 | 6.4 | 0.373 |
| Sagittal ramus angle (SN/Co–Go) | 76.3 | 6.3 | 77.6 | 6.7 | 0.461 |
| Mandibular plane angle (SN/Go–Me) | 29.8 | 5.9 | 31.2 | 5.5 | 0.332 |
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