Biomechanical Analysis Of Various Fixation Pattern In Bilateral Sagittal Split Ramus Osteotomy

Aim: The stability between proximal and distal segment after bilateral sagittal splite ramus osteotomy(BSSRO)has a vital significance to the prognosis. The documents available debate mainly about how to obtain desirable stability and few about stress distribution pattern. So in this study we aim at stress distribution pattern of different fixation method after BSSRO. In this study, finite element and mechanical analysis method is used in trying to get a method through which we can get suitable fragement stability and stress distribution pattern at the same time, in order to hasten bone healing and minimize re-fracture and tempromandibular joint disease that arised from stress avoiding effect and the heavy burden loaded on condylar. By examing bone surface healing of experiment animal, we try to investigate the relationship between "fixation pattern—stress distribution—histological healing conditions". Method: In this study, we established finite element model of a normal mandible and fibres attached to condylar and mastication muscle are simulated. Then we analysed stress distribution of various slopes of the condylar and osteotomy area of ramus under the condition of physiologically occlusal force. With the same method, stress distributions of various fixation patterns after BSSRO are recorded. In this way we find a fixation pattern, which can produce a stress distribution similar to the physiological sitution. Physiological station of the mandible under occlusal force is simulated, we give a force up to 50N to the mandible after BSSRO and various fixation patterns, and positional changes in two directions of boney mark situated at distal area of the mandible are recorded. In the animal experiment, we operate on all dogs according to modified BSSRO, and different fixation pattern is given to various groups. One dog of each group is killed after six and twelve weeks of the operation, mandible is taken out, soft tissue and fixation appliance is get rid of. After routinely decalcification process, pathology slide of osteotomy area is made, which is observed under microscope later. Results: The results of FEM analysis demenstrated: the stress distribution of different fixation patterns are different markedly. Ramus of the normal mandible endure mainly lag and compression stress under physiological occlusal force, front slope of condylar endure mainly compression force and back slope lag force. Results of stability experiment indicated: with the same load, position change of boney mark is great markedly in wire fixation group than in other groups; position changes of boney mark between mini-plate,bi-cortical screw,wire fixation plus mini-plate groups have no statistical difference. Histological examination results of experiment animals demonstrated: indirect healing is obtained by wire fixation; direct healing is the main healing pattern in mini-plate,bi-cortical screw and wire fixation plus mini-plate groups, and local callus also can be found, that is to say, indirect healing exist in these group either some times. In the same group, histological appearance is similar at six and twelve weeks after operation, it is only a bit more mature at twelve weeks. From these results, we can draw following conclusions: 1.Different fixation pattern after BSSRO can produce a different stress distribution pattern which have a vital significance to the bone healing and mechanical property recovery of mandible. 2.Wire fixation method can't provide sufficient stability between fragments, and the stress distribution state is also unreasonable, which have no advantage to the bone healing, so, it should not be used alone. 3.Two rigid fixation methods can produce similar stability, bi-cortical screw seems better in this field, but no statistical difference was found between two groups. Either method can produce sufficient stability to resist occlusal force. Both methods have a same drawback when compare to wire fixation plus mini-plate method, the un-sufficient consideration to the stress distribution. 4.Wire fixation plus mini-plate method can produce the stability similar to two rigid fixation methods. The most advantage of wire fixation plus mini-plate method exists in the adjustment to the stress distribution, which is more accord with the mechanical theory of bone healing. The aim of fragment fixation during BSSRO lies mainly in the hope to minimize complications after operation and to ensure a fast bone healing. From inter-maxillary fixation to wire fixation and then to various rigid fixation pattern, during all these stages, stability between fragments is at the first consideration of scholars. However, from the view of mechanical theory during bone healing,stable contact and functional stress stimulus between fragments are two indispensable situations. Now, we have not given enough consideration to the relationship between fixation pattern and stress distribution in clinical procedure, and related article is rare. From clinical procedure and experimental study, we have found: rigid fixation have a better result than non-rigid fixation shortly after operation, but latterly, the stability between rigid and non-rigid fixation group have no difference, which remind us: stress avoiding effect of rigid fixation affect bone remodeling during the later stage of bone healing after operation, and then weaken the stability between fragments. In this study we examine the stability and stress distribution of various fixation pattern by biomechanical and FEM analysis methods. The results indicates: stability between fragments after wire fixation is not sufficient, and the stress distribution state is also unreasonable, so, wire fixation is not a desirable method and should not be used alone; Two rigid fixation method can provide enough stability, bi-cortical screw seems better in this field, but both methods have the same drawback, that is the avoiding of normal compression force conduction, so they are not idea fixation method either. In accordance to the result of this study, we put forward a fixation method, which provide not only enough stability between fragments but also adjust stress distribution. Using this method in experiment animal, we found: histological characters of bone healing are similar to RIF groups shortly after BSSRO, twelve weeks after operation, bone healing appears better than in RIF groups histologically. Now, we have notice the significance between bone healing and stress distribution, but articles related to the stress distribution after BSSRO and the effect on bone healing are rare. According to Wollf"s law, all animal have a...