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Isoelasticity and total hip arthroplasty.

Important parameters in designing an "isoelastic" prosthesis that transfers stress in a more physiologic manner are explored. The effect of material properties and geometry on the mechanical properties of a femoral component are discussed. The bending "stiffnesses" of cortical bone, metal prosthese, and composite prostheses are compared, and the bending rigidities of prosthesis and bone as they might vary with surgical technique are explained. It is shown that the flexural rigidity of a prosthesis stem as presently designed is close to the flexural rigidity of cortical bone, but 200 to 400 times greater than that of the material it replaces, ie, cancellous bone and fatty marrow.