Implant Design Influences Tibial Post Wear Damage in Posterior-stabilized Knees

Abstract

The tibial post in posterior-stabilized total knees is a potential source of polyethylene wear debris, but the relationship between the shape and location of the tibial post in relation to the tibiofemoral bearing surfaces and the subsequent wear damage patterns remains unknown. We used observations made on retrieved implant components from three contemporary posterior-stabilized knee designs to examine how differences in tibial post design affected wear damage on the post. We examined 113 retrieved Zimmer NexGen(®), 103 Exactech Optetrak(®), and 58 Smith and Nephew Genesis(®) II posterior-stabilized inserts using a subjective scale to grade post damage. All 274 inserts demonstrated wear damage. Total wear scores and scores for wear damage on the anterior post differed among designs: Optetrak(®) 20 ± 4 and 5 ± 1, NexGen(®) 13 ± 4 and 3 ± 1, and Genesis(®) II 8 ± 3 and 1 ± 1, respectively. The Optetrak(®) had predominantly anterior wear damage, the NexGen(®) had more global wear damage, and the Genesis(®) II had predominantly posterior wear damage. Tibial post wear damage and anterior post wear damage were primarily determined by implant design and to a lesser extent by length of implantation and revision diagnosis. Although tibial post wear damage is multifactorial, the primary determinant of wear damage, and specifically anterior wear damage, is implant design. The constraint provided by the posterior-stabilized post-cam contact in modern knee arthroplasties is reflected in the wear damage patterns that occur during in vivo use. Unintended constraint such as anterior impingement should be addressed through design modifications for future posterior-stabilized knee arthroplasties.