| Objective:Asymmetric mandibular prognathism,a type of dento-maxillofacial deformity which combines mandibular prognathism deformity and face asymmetry deformity,is a very common skeletal maxillofacial deformity.Bilateral Sagittal Splint Ramus Osteotomy(BSSRO)is thought to be the chief surgical method to treat the asymmetric mandibular prognathism deformity.At the same time some studies also claim that the unilateral Sagittal Splint Ramus Osteotomy(USSRO)can be valid for some patients with asymmetric mandibular prognathism deformity and gain good outcomes.Additionally,the USSRO can reduce the SSRO-associated complications by 50% compared to BSSRO.But USSRO is not widely accepted due to the possibility of condylar excessive torsion,which raises the local stress,leading to a higher risk of temporomandibular disorder.There have been plenty of studies concerning about the clinical applications and finite element analysis(FEA)of BSSRO but few about the condylar positional changes after USSRO and relevant FEA studies.The present study is conducted to evaluate three-dimensional positional changes of the condyle in patients with asymmetric mandibular prognathism after the BSSRO and USSRO respectively,seeking the proper indications for USSRO.Furthermore,three-dimensional finite element models of temporomandibular joints of asymmetric mandibular prognathism patients treated with BSSRO/USSRO are established.The biomechanical analysis is made to calculate the stress distribution and the changing trend of the temporomandibular joint structure under physiological movements in order to provide references for the clinical use of USSRO.Methods:1.A total number of 19 patients with asymmetric mandibular prognathism deformity were treated with SSRO in the hospital from January 2015 to December 2016.There were 12 cases of BSSRO and 7 cases of USSRO.The samples are consisted of 4 cases of male and 15 cases of female while the youngest 18.9 years old and the oldest 33.4 years old,with an average of 23.9 ± 3.8 years old.Three-dimensional spiral CT data were collected before and after operation.Mimics software was used to reconstruct the temporomandibular joint structure.The ANB angle,mandibular deviation,mandibular setback distance,bilateral condylar three-dimensional translocation,three-dimensional rotation angle were measured and comparatively analyzed.The same group of patients before and after surgery and the comparison of left and right mandibular of the same patients used paired t test.BSSRO group compared with the USSRO group used independent sample t test,while P <0.05 were considered statistically significant.2.According to three-dimensional spiral CT data of two asymmetric mandibular prognathism patients who underwent USSRO/BSSRO,three-dimensional models of maxillary,mandible,teeth,articular disc,temporomandibular ligament were reconstructed in Mimics et al.After reverse engineering process and assigned material,models were imported in the finite element analysis software Abaqus to define the boundary conditions and load conditions.3.Under the situations of clenching(IID = 0)and small opening(IID = 2mm,4mm,6mm,8mm),calculated and analyzed the Von Mises stress distribution and max stress of bilateral articular disc,condylar and temporomandibular ligament.Assumed that the IID is set to 0 when the mandible closing muscles constrict,and IID is 2mm,4mm,6mm,8mm when the mandible closing muscles constrict.Results:1.All 19 patients were successfully received surgery,with no complications after surgery.(P <0.05).There were no significant differences in ANB angle,ANB angle and ANB angle between the BSSRO group and the USSRO group before and after operation.The ANB angle of all the patients was significantly improved(P <0.05).(5.16 ± 2.98 mm vs 1.07 ± 0.55 mm,P = 0.00).The preoperative mandibular deviation of USSRO group was significantly higher than that of BSSRO group(4.06 ± 1.62 mm vs 7.04 ± 3.89 mm,P = 0.03).The change of mandibular deviation was significantly larger than that of BSSRO(2.97 ± 1.75 mm vs 5.99 ± 4.30 mm,P = 0.04).The mandibular setback distance of deviation side was significantly greater than that of the non-deviation side(4.95 ± 1.94 mm vs 8.31 ± 2.64 mm,P = 0.00).The mandibular setback distance of deviation side of BSSRO group was significantly higher than the USSRO group(5.90 ± 1.69 mm VS 3.33 ± 1.07 mm,P = 0.00).The setback ratio in USSRO group was significantly lower than that of BSSRO group(0.46 ± 0.16 vs 0.70 ± 0.14,P = 0.00).There were no significant differences in the translocation of the BSSRO group and the USSRO group on the XYZ axis(P> 0.05).There were no significant differences in the condylar rotation in the sagittal plane between the two groups(P> 0.05).The condyles of non-deviation side of BSSRO group rotated forward,and the condyles of non-deviation side of USSRO had a tendency to rotate forword(P> 0.05).On the coronal plane,the condyles of non-deviation side of the BSSRO group rotated inwardly(P <0.05),and the deviation side was not changed.The condyles of non-deviation side of the USSRO group rotated inwardly(P <0.05)and the condyles of deviation side rotated outwardly(P <0.05).There were no significant differences between the two groups.There were no significant differences between the non-deviation sides of two groups but there were significant difference between the deviation sides.On the cross plane,on both sides of the BSSRO and USSRO groups,bilateral condyles occurred significantly inward rotation(P <0.05).2.By combined use of Mimics,Abaqus and other series of softwares,the 3D finite element models of the temporomandibular joint structure of BSSRO/USSRO correction of asymmetric mandibular prognathism before and after surgery were built.The models showed good geometric accuracy and mechanical performance simulation.3.In the closed state,the maximum stress of the articular disc was compressive stress,and the maximum stress on the non-deviation side was greater than the deviation side.The maximum stress of the articular discs were increased with the degree of opening.The maximum stress on both sides of the articular disc became close postoperatively.Preoperative non-deviation side and deviation side condylar stress mainly occurred in the neck of condylar head.The strain occurred on the front side of condyle and the stress on the back side.From clenching state to IID = 8mm,the stress of non-deviation side kept greater than the stress of the deviation side.There was a higher stress on the temporomandibular ligament of non-deviation side than that of the deviation side.Postoperatively stress of bilateral temporomandibular ligament was balanced.Conclusion:1.According to the results of this study,the mandibular setback distance of the deviation side less than 3mm,setback ratio of the deviation side and the non-deviation side less than 0.40,and without obvious condyle dislocation during simulation model operations should be the indication of USSRO.USSRO has a practical clinical effect on asymmetric mandibular prognathism.2.This study built the 3D finite element models of the temporomandibular joint structure of BSSRO/USSRO correction of asymmetric mandibular prognathism.The models showed geometric accuracy and mechanical performance simulation.The simulation results of TMJ under physiological motion conditions can visually display the stress distribution and change the trend in the above structures.This technique is of great significance for the further development of BSSRO/USSRO biomechanical studies of the TMJ region. |
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