A New Locking Compression Plate In Periprosthetic Proximal Femur Fractures-design And Biomechanical Testing

Objective To design a new locking compression plate(LCP)for fixation of periprosthetic proximal femur fractures(PPFF)and to evaluate its biomechanical stability and strength.Methods The distance between the femur prosthesis and anterior or posterior femoral cortex were measured on the postoperative lateral X-rays films of 90 cases that had primary total hip arthroplasty(THA).Five cadaver femoral specimens with different lengths(36.12~46.83cm)and perimeters(7.90~10.11cm)were implanted with femoral prosthesis stem according to the standard technique of artificial hip arthroplasty,then underwent gemstone CT spectral scan and three-dimensional reconstruction.The gemstone CT image data were imported MIMICS 15.0,and the distance between the femoral prosthesis stem and anterior or posterior femoral cortex were measured on the transective image.The position and angle of locking holes on the new LCP were designed in order that the locking screws can avoid the femoral prosthesis stem and achieve bilateral cortical fixation.In order to observe the number of the locking screws corresponding to anterior or posterior the femoral prosthesis stem,the length of femur for the distance greater than 6mm between the femoral prosthesis stem and femoral cortex were measured on the sagittal CT image reconstruction.And the sagittal curve of femur was assessed on three-dimensional reconstructive CT images,in order that the new LCP was consistent with the femoral lateral curvature.Besides,the shape of the proximal LCP was designed according to the anatomic morphology of proximal femur.Next the LCP were digital customized.After that,they were used to fix cadaveric femur specimens with different lengths and perimeters and femoral stem prosthesis implanted,and,the relation of the the locking screws on the LCP and the femoral prosthesis stem were observed by general view and image.The angle of locking screw which maybe contacted the femoral prosthesis were adjusted again.Then the improved LCP were used for fixing 10 cadaveric femur specimens of different lengths and diameters,the relation between the the locking screws on the LCP and the femoral prosthesis stem were further evaluated until the locking screws of LCP correspongding to the femur prothesis achieved double cortical fixation,and didn't contact with the prosthesis as well.At this point,the design of the new LCP is determined.Eight matched pairs of Vancouver B1 type cadaveric PPFF specimens were fixed with the new LCP and inverted distal femoral less invasive stabilization system(LISS)respectively,Four bicortical locking screws(LCP group)and four unicortical locking screws were used at the level of prosthesis stem,and 4 double cortical locking screws were used to fix the distal end of the fracture in two groups,the distance from the locking screws to the fracture were also equal.Four point bending test and torsion test were performed until the internal fixation failed.Statistical analysis of biomechanics of PPFF specimens fixed by new LCP was carried out using the paired t –test.Results The average distance between the proximal half of the prosthesis and the posterior cortex of the femur was 12.36±3.24 mm,and the average distance between the distal half of the prosthesis stem and the anterior femoral cortex was 8.14±1.21 mm.The length of femur was 69.20 ± 4.53 mm corresponding with the distance over 6mm between proximal half prosthesis and femoral cortex,and that was 57.31± 3.82 mm corresponding to the distance over 6mm between distal half prosthesis and femur cortex.The length of the new LCP corresponding to proximal half femur prosthesis were designed with 3 locking holes which can be angulated in a specific angle in posterior of the prosthesis stem,and the length corresponding to distal half femur prosthesis were designed with 3 locking holes that can be angulated in a specific angle in anterior of the prosthesis stem,so that the bicortical locking screws can be fixed in the rear or front of the femoral prosthesis stem.The curve of new LCP was consistent with the curve of the femur,and its proximal shape matched the proximal femoral anatomical structure.Besides,symmetric design of the new LCP was applied to the left and right side,to meet both sides.The maximum bending load,maximum bending displacement and bending stiffness of PPFF specimens fixed with new LCP group were all higher than those used with LISS,but the difference was not statistically significant(P >0.05).The maximum torque,maximum torsional angle and torsional stiffness of new LCP group were significantly larger than the LISS group;there was a significant difference between two groups(P < 0.05).Conclusion The bicortical locking screws of the new LCP can be angulated in the posterior and anterior of the femoral prosthesis.The stiffness of anti-torsion with new LCP were significantly better than that of LISS fixed with unicortical locking screws.Consequently,the new LCP have better biomechanical stability for PPFF.