Analyzing the anatomical features of the hip joint in 85 hips of 55 patients with osteoarthritic hip and 90 hips of 60 subjects with normal hip using three-dimensional CT images,we found that 9 different stem designs varying in width and proximal radius were required to accommodate the hips with secondary osteoarthritis.In normal hips,the proportionality of the anatomical features of the proximal femoral canal to the femoral length was observed.Thus,we manufactured prototype stems for femoral neck fracture and aseptic necrosis of the femoral head which proximal radius,isthmus position,off-set value and width would increase with the stem size,but the ratios of the increase were different.
By micromotion measurements of the prototype stems for femoral neck fracture and aseptic necrosis of the femoral head,Omniflex stem,Omniflex J-stem (Osteonics,Allendale,New Jersey) and AML stem (DePuy,Warsaw,Indiana), under the axial cyclic loading of 2000 N,maximum average of micromotion of the stems w
ere each 43.1mum,54.7mum,62.0mum and 56.2mum,that were all proximal micromotions in superior-inferior direction.Under the rotational loading of 20 Nm,maximum average of micromotions were each 38.0mum,48.0mum,55.2mum and 67.5 mum,that were all proximal micromotions in anterior-posterior direction.The proximal medial compression strains of the femur were each 18.6%,23.6%,25.9% and 44.6% of those of the intact femur under the 2000N axial loading.
By three-dimensional templating of femoral prostheses using computer simulation,canal filling ratios(ratio of the cross-section of the stem within the canal area) in proximal of the prototype stem designed for secondary osteoarthritis of the hip,Omniflex stem,Omniflex J-stem and AML stem were each 61.2%,59.8%,56.5% and 53.8%,and rasp ratios(ratio of the rasped cortex within the cortex area) in proximal of those were 1.10%,1.69%,1.30% and 5.17 %. Canal filling ratios in isthmus level of those stems were each 75.8%,68.1 %,68.6% and 74.1%,and rasp ratios in isthmus level were each 0.69%,0.68%,0.70% and 2.25%.
Therefore, our prototype stems designed using three-dimensional CT images had smallest micromotion,highest canal filling ratio and lowest rasp ratio,so obtained satisfied primaly fixation and compatibility to the femoral canal. Less