Evaluation of contact stress in metal-backed patellar replacements. A predictor of survivorship.

Significant wear-through and dissociation failures of metal-backed, point and line contact patellar replacements are associated with excessive contact stresses on the polyethylene-bearing surfaces. Analytical mathematical contact-stress analysis was used to evaluate various patellar component geometries under loading conditions consistent with walking, stair descent, and deep knee bends, respectively. Typical point- and line-contact patellar-surface geometries exceeded the manufacturer's recommended maximum permissible compressive stress level of 10 MPa by a factor greater than three, which also exceeds the yield stress of ultra-high, molecular-weight polyethylene. A metal-backed, rotating-bearing, area contact geometry patellar replacement maintained safe contact stress levels at less than half of the maximum permissible compressive stress level. These contact stress analyses predict early fatigue failure of all-polyethylene or metal-backed, point- and line-contact patellar replacements, while predicting long-term survival of area-contact, rotating-patellar replacements. Clinical evaluation of these various implants support the conclusions of these analyses. Contact stress analysis should precede any clinical use of patellar implants to avoid predictable failure mechanisms.