| Recently, the fixation treatment of femoral shaft fracture has changed greatly, which mainly reflects in the transition from mechanical fixation to the biological fixation and from absolute fixation to the relative fixation. Mechanical fixation pursues of strong internal fixation, ignoring the good protection of blood supply in the fracture and its surrounding soft tissue. Blood supply once destroyed, it is extremely easy to cause osteoporosis, delayed union or nonunion. It may cause infection. Clinical studies have shown that mechanical fixation don’t generate callus, osteon reconstructed over the fracture end in this fixation to realize the healing, which is called direct healing or primary healing. Direct healing is not solid, which is likely to cause secondary fracture after removal of the compressed plate. Biological fixation, considering good blood supply of the surrounding soft tissue, adopts the elastic fixed, which means there is micro motion between fracture blocks. This kind of micro motion can stimulate callus formation rapidly to promote fracture healing in clinical studies, the healing known as indirect healing or secondary healing. However, some literatures show that existing biological fixation treatment of femoral shaft fractures has not improve the fracture healing, whose nonunion rate was 16%-20%. Analyzing its reasons, the mainly one is that the micro motion caused by biological general locking screws fixation is asymmetric between fracture blocks, leading to the callus size away from the locking plate greater than it near the plate, and delayed union or nonunion happens near the plate.In order to solve this problem, a bone materials company plans to launch a new screw for fractures-dynamic locking screw. Based on the dynamic locking screw, this paper has carried on the preliminary theory study and experimental study, and adopted the experimental method and finite element method to implement the dual authentication, which includes dynamic pressure test and fatigue test on the synthetic femoral models, and the corresponding finite element analysis. According to comparing all the results of the ordinary locking screws and dynamic locking screws, the feasibility of the DLS design is evaluated.1. With two fracture models of dynamic locking screws and ordinary lock screws, speed load is adopted in the compression test. Based on tests, stiffness curves of two models are made and analyzed. The movement of fractures seam is observed in tests to confirm the parallel micro motion in dynamic locking screws model.2. With reference to setting parameters in the experimental test, finite element analysis are carried out respectively for two types of models, and its stiffness curves are made and analyzed. Choosing 5 moments, where femoral shafts don’t contact with each other, fracture seam distances of the two fracture models are compared under the same moment. The seam displacement changing over the model displacement and compression load near and away the locking plates in two kinds of fracture models are compared respectively. Under the load condition of 480 N, the stress distribution and deformation of locking plates in two kinds of fracture models are analyzed. The results of stiffness, displacement and compression load in the experiment and finite element analysis are compared, and the reasons for its differences are analyzed. In the basis of finite element analysis results, mechanical properties of dynamic locking screws are validated.3. The fatigue life of the two types of fracture models are calculated in tests and finite element analysis under cyclic loads 100/1000N and 140/1400N with pure titanium locking plates and 140/1400N,180/1800N and 210/2100N with titanium alloy plates, which will provide reference for the rational use of screws with locking plates to prevent breakage phenomenon and avoid secondary fractures.Through the test and finite element analysis, the following conclusion can be drawn that:compared with ordinary lock screws, dynamic locking screws have good biomechanical properties, mainly reflecting in the parallel micro motion> two-phase character of stiffness and the uniform stress distribution in DLS fracture model. Under the condition of initial lower load, initial stiffness makes the fracture seam move parallel, and promotes the callus formation and fracture healing well. Under the condition of larger load in the later treatment, the secondary stiffness can guarantee the stability femoral shaft fractures structure, which means that fractures are hard to shift in the healing. In theory, this paper confirms that, compared with biological general locking screws, the biological dynamic locking screw has excellent prominences in the fracture treatment of femoral shafts, looking forward to get better effect of fracture healing in clinic. |
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