Symposium on Surface Replacement Arthroplasty of the Hip. Biomechanics: mutifactorial design choices--an essential compromise?

The main flaws to be overcome in realizing the potential success of the double-cup arthroplasty procedure are failures due to femoral cup loosening, acetabular cup loosening, and femoral-neck fractures. The clinical uncertainties include the selection of a suitable patient with adequate bone stock and the technical difficulties associated with (I) reaming the acetabulum adequately, (2) reaming down onto the neck without violating it, and (3) anchoring the components securely by interdigitation of acrylic cement. The higher frictional torques of the double-cup arthroplasty designs are not a clinical loosening issue--the resulting acetabular cement-bone shear stresses are very low. Computer models of both the femoral and acetabular components predict significant stress shielding of the cancellous bone under metal femoral shells. At the rim of the femoral cup, the stresses are increased by a factor of 3 owing to the stress concentration effect and can rise to a factor of 10 if cystic or osteoporotic changes are present. This finding if confirmed in the three-dimensional models may explain some of the femoral neck fractures. The thin polyethylene acetabular cups may also cause a stress concentration effect on the underlying cement and bone. This may explain the higher incidence of radiographic loosening around the acetabulum in double-cup arthroplasty designs compared with total hip replacements. Metal-backed sockets may reduce cancellous bone stresses and appear advantageous. There is no clinical evidence of unusual wear or wear-related problems. However, new material formulations are now either in use or being planned for the double-cup arthroplasty designs. As yet, there have been no published data on hip simulator wear for the efficacy of any of the current or proposed changes. Biologic fixation appears to be the theme for the 1980s. However, the combination of technology, design instrumentation, and patient selection will be critical in achieving success with this technically difficult procedure.