The Effects Of Osteonecrosis Extent Of Femoral Head On The Stress Distribution In Proximal Femur After Hip Resurfacing

Hip replacement is the only effective methods to Osteonecrosis Of Femoral Head(ONFH). However, to young patients with ONFH, total hip arthroplasty(tha) has some shortcomings, such as shorter duration, revising hard and more complications. Hip resurfacing arthroplasty (HRA) is reported to preserve bone mass, revise easily, increase hip stability and range of hip motion, and preserve the normal joint mechanics. HRA is being increasingly considered as an effective alternative to total hip arthroplasty in young, active patients. Femoral neck fractures and aseptic loosening are the two main reasons after operation which affect survival rate. Although there already have some studies with respect to the failure mechanisms, investigation into the effect of bone defect in the weight-bearing region of the head, which exist in patients with ANFH and hip OA, on the stress distribution in proximal femur after HRA is not been reported. A three-dimensional (3D) finite element model (FEM) of the proximal femur was used to examine the effects of bone defect extent in the weight-bearing region of femoral head on the stress distribution in proximal femur after hip resurfacingObjective: Observe the effects of bone defect extent in the weight-bearing region of the head on the stress distribution in the femoral neck, bone-cement interface, head-neck junction and bone underlying the implant in relation to the prosthesis aseptic loosening or neck fracture after hip resurfacing and provide guidance to surgery.Methods: 1. Sophisticated measuring instrument and 3D designing software were used to construct 3D models, including proximal femur, femoral head resurfacing prosthesis, bone cement mantle, different extent bone defects region, resurfaced intact femoral head and resurfaced femoral head with defects in the weight-bearing region. 2. Finite element software was used to mesh and analyze each model, simulating single-leg stance phase, observing and studying the effects of bone defect extent (necrotic angle 60°.80°.100°.120°.140°respectively)on the stress distribution in the femoral neck, bone-cement interface, head-neck junction and bone underlying the implant.Results:1. FE models of proximal femur after resurfacing were constructed , with femoral head integrity and different defections in the weight-bearing region.2. The gross stress distribution in the proximal femur after resurfacingVon Mises equivalent stress peak values are experienced in the superior and inferior neck regions after resurfacing. Whether bone defects in weight-bearing region exist or not, only 2.7% differences are observed among the peak values in all models, bone defects in the weight-bearing region affect little on the stress distribution in the femoral neck. 2.88-16.4KPa observed in the proximal bone underlying the prosthesis in all models account for only 1/10-1/100 of the stress values in the distal femoral head. The stress values were reduced considerably, which show that resurfacing caused stress shielding to a great extent in the proximal femoral head underlying the prosthesis. The load transferred from prosthesis to bone cement, then through the distal femoral head to neck.3. The stress distribution in the resurfaced femoral headVon Mises equivalent stress peak values in the bone-cement interface are concentrated on the superior rim, top regions and medial-inferior regions of the cement mantle. Von Mises equivalent stress peak values in the femoral head underlying the prosthesis are experienced in the distal femoral head and the surrounding center channel, especially superior region of head-neck junction. Bone defects in the weight-bearing region affect the stress distribution in the two main regions above notably, which related to neck fractures and aseptic loosening.When the defect regions were necrotic bone, with the necrotic angle increasing, Von Mises equivalent stress peak values in interesting regions increase considerably. Among the total, peak values in the bone-cement interface superior rim increase 22.3% and 27.3% respectively when necrotic angle is 120°and 140°,while peak values in the head-neck junction superior rim increase 13.2% , 28.3% and 53.3% respectively when necrotic angle is 100°, 120°and 140°, but increase only 4.6% and 7.2% respectively when necrotic angle is 60°and 80°. However, when the defect regions were filled with bone cement, Von Mises equivalent stress peak values in main stress-concentrating regions increase slightly or reduce gradually.Conclusion:1. Physiological load transfer mechanisms and stress distribution were changed notably after hip resurfacing, inducing significant stress shielding in the proximal femoral head underlying the prosthesis.2. Different extent bone defects in the weight-bearing region of the femoral head affect the stress distribution in the bone-cement interface and head-neck junction in relation to prosthesis aseptic loosening or neck fracture after hip resurfacing. When the necrotic angle is less than or equal to 80°,Von Mises equivalent stress peak values in the bone-cement interface and head-neck junction change slightly, however, When the necrotic angle is more than or equal to 100°, Von Mises equivalent stress peak values increase notably.3. Stresses in the bone underlying the prosthesis were reduced and significant stress shielding was induced, which may be in favor of necrotic bone repairing, stress shielding will enable a small defect in the weight-bearing regions of the femoral head(the necrotic angle less than or equal to 80°), which has little effect on the mechanics in proximal femur.4. When the necrotic angle is less than or equal to 80°, whether the necrotic bone remain or is cleared and filled with bone cement, effects on the mechanics in proximal femur change little. While when the necrotic angle is more than or equal to 100°, stresses increase significantly in the in the bone-cement interface and head-neck junction, which may be one of the cause inducing femoral neck fracture or prosthesis loosening. More normal stress distribution in proximal femur can achieve when clearing necrotic bone and filling with bone cement, which may be an effective method. But further efforts should be made to investigate whether other disadvantages were induced.