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In Vivo Deterioration of Baseplate Locking Mechanism in Modular Knee Implants

Smain Lounici, MS, Anand Rao, BS, Gerard A. Engh, MD

Background
Wear debris of modular knee components causes osteolysis after total knee arthroplasty (TKA). Although several studies have reported micromotion of locking mechanisms, none have investigated locking mechanism performance in vivo.

Hypothesizing that locking mechanism stability would deteriorate in vivo, we evaluated 38 modular tibial trays with a variety of snap-fit and tongue-and-groove designs.

Methods
This involved 9 unused components as controls, 17 failed components retrieved at revision TKA, and 12 well-functioning components harvested postmortem. Average time in situ for retrieved implants was 53 months (range: 6-144). In the revision group, average patient age at surgery was 63 (range: 38-80) and average weight 190 pounds (range: 115-320).

We recorded motion at the tibial insert-tray interface by measuring anteroposterior (AP) and mediolateral (ML) displacements with an extensometer under 100N load. The micromotion index (M.I.), a 2-dimensional vector representing total motion in the transverse plane, characterized locking mechanism stability.

Results

• Implants retrieved from revisions had the greatest micromotion index (555µm + 165µm)
• Well-functioning components retrieved postmortem exhibited lower motion (420µm + 48µm)
• Control implants had the least micromotion (70µm + 39µm)
• The controls had a significantly lower average micromotion index than the revision and postmortem groups (p < 0.001)
• no statistical difference existed between the micromotion index of the revision and postmortem-retrieved groups (p=0.115).

Conclusion:
This study shows that the stability of locking mechanisms deteriorates in vivo, increasing motion at the insert-baseplate interface. This constitutes a major source of wear debris. Manufacturers must strive to improve TKA locking mechanisms to ultimately decrease wear and wear-related complications, such as osteolysis.

Hip Research