Date of Graduation

1997

Document Type

Dissertation/Thesis

Abstract

Loosening of total joint replacements continues to be a major mode of failure in cemented total joint arthroplasty. Clinical investigations report failure of the stem-cement interface after total hip replacement may be an important factor in loosening of hip prostheses. There are basically two design philosophies to reduce the risk of cemented stem failure: promote bonding (i.e. pre-coat, rough surface) and promote debonding (i.e. polished surface). The latter approach is examined in this study. The influence of stem-cement debonding and the viscoelasticity of the bone cement was investigated using a smooth tapered collarless stem. It has been hypothesized that the tapered stem, upon debonding, permits mechanical 'taper-lock' between stem and cement mantle due to increased radial and hoop stresses. It was also hypothesized that cement stresses would undergo creep due to viscoelastic behavior causing subsidence of the stem, and thereby, promoting 'taper-lock' stability. The objectives of this research were to: (1) Evaluate the above hypothesis with a 3D finite element model that has non-linear interface conditions, unsupported distal tip and viscoelastic behavior of bone cement and (2) Develop a technique to perform shape optimization of the proposed implant. 3D FE models of a human femur and the proposed hip implant were developed using the cross-sectional data obtained through CT scans. Bone cement was modeled as a viscoelastic material using experimental data and the stem-cement interface was modeled as smooth {dollar}(\\mu = 0),{dollar} friction {dollar}(\\mu = 0.22){dollar} and bonded {dollar}(\\mu = \\infty ).{dollar} Debonding of the stem-cement interface increased radial and hoop stresses. Shear stresses decreased, more so with decreasing coefficient of friction. Cement stresses were significantly reduced. Cement stress distribution and distal subsidence of the stem due to creep indicated 'taper-lock'. Shape optimization of the implant was performed with bonded stem-cement interface condition. The results of this investigation will help in designing a prosthesis that will extend the longevity of cemented total hip replacements.

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