Date on Master's Thesis/Doctoral Dissertation

12-2006

Document Type

Master's Thesis

Degree Name

M. Eng.

Department

Mechanical Engineering

Committee Chair

Hnat, William Patrick

Subject

Femur--Effect of implants on

Abstract

Modular hip implants are currently being used in total hip arthroplasty revision procedures because it is common to have little or no bone left in the proximal region of the femur and it is difficult to accurately size implants based off of radiographs or other medical imaging techniques. The effect of modular systems using tapered fluted stems on proximal stress shielding in the human femur is the focus of this study. Seven modular implants were press-fit into seven femurs, where there was little or no contact between the modular body and the test femur. Finally, hydroxyapatite bone cement was added to create an interface between the modular body and femur. Stress shielding is perhaps one of the most critical complications that occur after total hip arthroplasty (THA). Insertion of the prosthesis permanently alters the stress distribution in the human femur. Stress shielding occurs in those areas in which a bone experiences a reduction of stress. Essentially, the prosthesis "shields" the stress from the bone by transmitting the force through the implant rather than the bone, which can lead to a decrease in bone mineral density due to the reduction of stress or failure of the implant itself. Seven test femurs were instrumented with strain gages to measure the strain at three different levels. Two loading conditions were applied to represent the forces exerted on the femur when a human is in a natural standing position. The loading conditions were applied to the intact femur: when the prosthesis was inserted into the femur with a press-fit; and when hydroxyapatite bone cement was injected around the implant in the proximal region. All femurs exhibited stress shielding effects after surgically inserting the modular implants. The experimental data confirmed that the greatest amount of stress shielding occurred primarily in the proximal and mid-stem regions. The strains at the junction between the modular body and stem were relatively low. The experimental data also confirmed that with the addition of BoneSource® Hydroxyapatite cement the strain in the proximal and mid-stem regions significantly increased, but stress shielding still occurred. The strains at the junction between the body and stem also decreased.

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