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Problem statement: There are two types of implant fixation in hip joint replacement-cemented and cementless. The cemented types are, in general, more popular due to concerns of possible inability of cementless implants achieving maximum primary stability for bone integration. The concern is more significant in cases where there are major losses of cancellous bone stock and thinning of the trabeculae due to osteoporosis. Approach: Three Computed Tomography (CT) images of human hip joints were obtained from a hospital. The first patient showed osteoporotic condition based on DEXA scan of the bone. The second was registered for total hip replacement due to significant deterioration of the cartilage covering the bone ends. The third dataset was from a patient with no reported skeletal diseases and was used as control. Three dimensional models of the femora were reconstructed from the CT images and hip arthroplasty using cementless stem was simulated. Finite element method was used to analyze the stability of the implant through a specialized algorithm to measure micromotion at the bone-implant interface. Bone properties were assigned on an element-by element basis and loads simulating stair climbing were used. Results: Hip stems fixed in the control and osteoarthritic femoral model showed minimum interface micromotion. For the osteoporotic bone there is a progressive reduction in surface area feasible for bone in growth. Conclusion/Recommendations: Bone quality affects the stability of femoral components used in hip replacement and therefore the bone-implant integration potential. Cementless hip stem should not be used in patients with osteoporotic condition as the deterioration of bone tissues lead to an increase in interface micromotion.

Problem statement: The design of hip prostheses has evolved over time due to various complications found after hip replacement surgery. The currently commercially available cementless femoral stems can be categorized into one of three major types, straight cylindrical, tapered rectangular and anatomical. Each type proposes a unique concept to achieve primary stability-a major requirement for bone healing process. Virtual analyses have been made on individual implants, but comparison between the three major types is required to determine the strength and weaknesses of the design concepts. Approach: Three types of implants were modeled in three dimensions-the straight cylindrical, rectangular taper and anatomical. The size of the three implants was carefully designed to fit and fill the canal of a femur reconstructed from a computed tomography image dataset. Hip arthroplasty was simulated virtually by inserting the hip stem into the femoral canal. Finite element method was used in conjunction with a specialized sub-routine to measure micromotion at the bone-implant interface under loads simulating physiological walking and stair-climbing. Another sub-routine was used to assign bone properties based on the grayscale values of the CT image. Results: All the three types of cementless hip stems were found to be stable under both walking and stair climbing activities. Large micromotion values concentrated around the proximal and distal part of the stems. Conclusion/Recommendations: The three major types of hip stems were compared in this study and all of them were found to be stable after simulated physiological activities.