Global mechanical consequences of reduced cement/bone coupling rigidity in proximal femoral arthroplasty: a three-dimensional finite element analysis.

Component loosening in total hip arthroplasty is often accompanied by substantial bony remodelling, associated initially with reduced stiffness of the cancellous bone bordering the cement, and eventually with the formation and proliferation of a compliant fibrous membrane at the bone/cement interface. An anatomically based three-dimensional finite element model has been developed to explore the salient stress changes occurring with progressive degradation of the stiffness of the cancellous bone in a thin zone bordering the cement. This border zone, modelled as a distinct linearly elastic and isotropic material layer, assumed a geometry and a range of mechanical properties inferred from eventual membrane thickness apparent in recent animal studies of component loosening. The major variables considered were: stiffness (elastic modulus) and compressibility (Poisson ratio) of the border zone, stiffness changes in the outlying cancellous bone, resultant hip contact force, and trochanteric muscle loadings. The results for the limiting case of a non-degraded border zone compared reasonably with previous studies of femoral reconstructions having rigid bone-to-cement attachment. Progressive decay of border zone stiffness produced complex changes in load transmission. Foremost among these were a generalized increase in stress levels (especially of transverse-plane tension) in the proximo-lateral cancellous bone, and a corresponding generalized decrease in stress levels in the proximo-medial cancellous bone. There were also large bending moment increases in the prosthesis and its cement mantle, especially at mid-stem. At almost all sites, the critical stress levels were those developed for peak stance-phase loading, rather than for the lower loads (and different resultant contact force directions) occurring elsewhere in the gait cycle. The elevated proximo-lateral cancellous bone stresses occurring with eventual membrane development, consistent with localized bony hypertrophy seen in recent animal studies, may be a response to hoop stresses occurring during pistoning of the tapered cement mantle.