Numerical–Experimental Method For The Validation Of a Controlled Stiffness Femoral Prosthesis

˜ J. A. Simoes
Department of Mechanical Engineering, University of Aveiro, Aveiro, Portugal

Numerical–Experimental Method for the Validation of a Controlled Stiffness Femoral Prosthesis
The aim of this paper is to describe a new numerical–experimental method to determine the stiffness of a conceptual proximal femoral prototype. The methodology consists of the comparison of the numerical andexperimental displacement distributions of the prosthesis loaded as a cantilever beam to validate a design concept: controlled stiffness prosthesis. The manufactured prototype used to test the applicability of the numerical– experimental procedure integrates a stiff metal core bonded to a composite material made of an epoxy resin reinforced with carbon-glass braided pre-forms. The prosthesis withan embedded controlled stiffness concept was obtained by varying the geometry of the core with the composite layer thickness. DOI: 10.1115/1.1375162 Keywords: Femoral Prosthesis, Stiffness, Holography, Displacement, Composite Material

J. Monteiro M. A. Vaz
Department of Mechanical Engineering and Industrial Management, University of Porto, Porto, Portugal

Introduction
The design of a hipprosthesis is a challenging problem that has stimulated many investigators. The femoral component of a hip prosthesis is essentially composed of a stem incorporating a ball and is characterized by its geometry and material s , what is known as stiffness elastic modulus multiplied by the second moment of area . Stiffness is an important design parameter and many authors have addressed its influenceon the prosthesis performance. Stiffness is known to play a key role in the stress shielding phenomena and on the relative interface bone-implant micromotion, the predominant causes for long-term failure of total hip replacements 1 . Stress shielding depends on the design of the intramedullary reconstruction and mainly on the material of the femoral component, which influences the mechanism of loadtransfer from implant to the surrounding bone and can have an alarming effect on a revision procedure. Interface micromotion can lead to a premature failure of the implant and is design and material dependent. Therefore, two important design goals have to be addressed in the design cycle of a hip prosthesis to balance the advantages of flexible and stiff materials concerning stress shielding andmicromotion. These design objectives are strongly affected by the prosthesis material s . Flexible stems provoke less stress shielding in the surrounding bone, but higher proximal interface stresses: cement stresses for cemented designs. Thus, excessive bone resorption is more likely to occur around stiff stems, but proximal interface failure is more likely to occur around flexible stems 1 . Havingthis in mind, a suitable compromise has to be achieved because the femoral component cannot be stiff and flexible at the same time. The design problem is to know how to minimize stress shielding keeping interface stresses and interface micromotions at acceptable levels 1,2 . Conventional proximal femoral prostheses are manufactured with a single constant modulus material. Such prostheses can addresseither the stress shielding effect or the interface stresses, but not both simultaneously. However, with optimized controlled stiffness, it is possible to obtain a compromise between the two conflicting design factors to attenuate the problems described above. There has been substantial active research with composites for total hip replacement applications 3–10 , namely for his prostheses. Thepossibilities of stress shielding reduction, and therefore bone resorption minimization, leading to thigh pain alleviaContributed by the Bioengineering Division for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received by the Bioengineering Division June 26, 2000; revised manuscript received December 13, 2000. Associate Editor: V. K. Goel.

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