| Objective:Intercalary endoprosthesis is one of the new approaches for the reconstruction of the bone defects of limbs in recent years. It has presented advantages such as simple operation, relatively small trauma and early ambulation of limbs after operation. Yet, due to biomechanical factors, the incidence rate of loosening endoprosthesis in an early stage is high. In this paper, on the basis of the stimulated stress environment for normal walking with the finite element approach, changes in the length of the shaft of the intercalary endoprosthesis as well as the regularity of stress distribution of the backbone and bone cement are explored and analyzed, intending to provide mechanical assistance for the guidance of the clinical operation and the improvement of the design of intercalary endoprosthesis.Objects and Methods:the total femoral information is obtained with the approach of CT. which is then saved in the format of DiCOM so as to conduct three-dimension reconstruction of the total femur with an overall length in MIMICS. Bone fractures in different areas of the femoral distal and proximal shaft are restructured with intercalary endoprosthesis.14femur-bone cement-intercalary endoprosthesis models with the shaft length of40mm,50mm,60mm,70mm,80mm,90mm and100mm are established. With Abaqus6.91finite element analysis, stress in the normal walking borne by the femur is loaded to the models, generating the peak values of stress and stress distribution nephogram of the femur, bone cement and intercalary endoprosthesis. The peak values of stress and stress distribution nephogram of the femur, bone cement and intercalary endoprosthesis in A zone (endoprosthesis of proximal femur) and C zone (endoprosthesis of distal femur)are compared and analyzed, obtaining the proper scope of application and objective stress data of A, C and B endoprosthesis (the middle of femoral diaphysis) in femoral shaft.Results:Intercalary endoprosthesis model, peak values of stress of the femur and bone cement are inversely proportional to the shaft length of endoprosthesis. The maximum peak value of stress of the femur and bone cement is70.27Mpa and11.6Mpa respectively, which appear in the model with a shaft length of40mm at the near-end of the femur. All peak values of stress of the femur are located at the interface of corticomedial part of femoral shaft and the restructured part of endoprosthesis, all within the vertical stress extremum of cortical bone. Peak values of stress of bone cement in all models are located near the restructured part of endoprosthesis of the intrathecal bone cement, not exceeding the fatigue strength of Palacos bone cement. Peak values of stress of intercalary endoprosthesis are inversely proportional to the shaft length of endoprosthesis. The maximum peak value of stress,245.3Mpa, appears in the model with a shaft length of40mm at the near-end of the femur. Peak values of stress of all models are located at the migration part of the inner side of the shaft and the restructured part. Two stress concentration areas are formed around the screw holes in the restructured part. All endoprosthesis models are within the maximum tensile strength of titanium. Compared with the comeout. peak values of stress of of the femur and bone cement of the endoprosthesis in A and C zone are lower, with a reduction of32.87±6.81Mpa. Peak values of stress are located at the interface of the outer cortical bone and the restructured part. An average reduction of5.882±1.473Mpa is found in peak values of stress of bone cement compared with endoprosthesis models without and all peak values of stress are located at the top (bottom) of bone cement theca. The stress concentration area is also formed here. Peak values of stress of intercalary endoprosthesis decrease by24.51±9.54Mpa. New stress concentration areas are also formed around bending areas of titanium plate and screw holes.Conclusions:1. In this experiment, with the approach of the finite element analysis, it is found that the changes in the shaft length and the changes of peak values of stress of the backbone, bone cement and endoprosthesis are closely related, that is. compared with endoprosthesis with longer shaft, endoprosthesis with shorter shaft can easily produce the fragmentation of the bone cement and shaft fractures. In the allowed extent of the remnants of the medullary cavity length, endoprosthesis with longer shaft should be utilized to improve the stability.2. The scope of application of A zone endoprosthesis is within70mm at the near end of the remnants of the medullary cavity and that of C zone endoprosthesis is within50mm at the far end of the medullary cavity of femur. The rest is the scope of application for B zone endoprosthesis.3. The application of A and C zone endoprosthesis significantly reduces peak stress of bone cement and the backbone, and rationalizes the stress distribution of backbone and bone cement so as to to reduce or avoid the risk of bone cement fragmentation and shaft fractures under long-term stress. Therefore, A and C zone endoprosthesis expand the scope of application in the femoral shaft with the intramedullary and extramedullary fixation.4. In the clinical use, the flare index of the femur may be attributed to weak bone cement theca stress. In other words, the higher the flare index, the higher risk the fragmentation and detachment of bone cement theca at an early stage.5. The restructured part of the intercalary endoprosthesis effectively reduces the stress concentration in the migration part of endoprosthesis shaft and the restructured part and around the screw holes by approaches such as increasing the sprons and the oval designing of the screw holes, hence decreasing the incidence of endoprosthesis fracture. For titanium plates and screws in A and C zone intercalary endoprosthesis. the lock mode can be adopted. Meanwhile, the thickness of the titanium plate and the screw diameter can be enhanced so as to reduce risks of screw detachment and endoprosthesis fracture. |
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