Introduction: Full flexion is a critical performance requirement for patients with total knee replacement (TKR). Different design strategies, such as the post-and-cam, are used to achieve greater femoral rollback during knee flexion. However, substantial damage to the polyethylene tibial post on some posterior cruciate ligament substituting (PS) TKR designs has led to concerns that femoral camtibial post contact will lead to increased insert micromotion and backside wear in modular PS TKR designs. This study evaluated in vivo knee function and polyethylene wear in patients with posterior cruciate ligament retaining (CR) and PS tibial component designs with a full peripheral rim modular locking mechanism.
Methods: Motion Analysis: Thirty two knees with CR (9 knees) and PS (23 knees) tibial inserts participated in fluoroscopic motion analysis during activities of daily living, including stairrise/descent, treadmill gait and maximum kneeling flexion. The metal tibial components used the same full peripheral rim locking mechanism design with the different modular polyethylene articular surfaces. Tibial-femoral contact locations were determined throughout the full range of motion for all activities.
Retrieval Analysis: Polyethylene tibial inserts were retrieved during autopsy and revision surgery after 1 to 74 months. There were 37 CR inserts and 7 PS inserts of the same designs that were evaluated in the motion study. Backside damage on the inserts was assessed on all retrieved inserts using optical microscopy and the damage area and location was measured using digital image analysis.
Results: A relatively posterior position of the femoral component on the tibia was significantly correlated with greater maximum knee flexion. PS TKR had significantly more posterior femoral position and greater maximum flexion than CR TKR. The mean backside damage area was 38%+10% for PS inserts and 45%+15% for CR inserts. Backside surface damage was concentrated near the inserts’ peripheral rim and was dimpled in appearance, consistent with a cast impression of the textured metal baseplate. Scratches and burnishing was infrequently observed. Inserts with the greatest area of backside damage were in-situ for the longest time period.
Discussion: This fluoroscopy-based motion analysis study showed that knees with PS TKR achieve greater maximum flexion than knees with CR TKR. However, retrieved PS inserts did not have larger backside damage areas and the damage pattern location was consistent for both articular geometries. The observed damage morphology suggests that backside damage resulted from axial compression of the polyethylene insert against the textured baseplate rather than micromotion. Previous mechanical tests of this same modular tibial component design have shown that motion between the polyethylene insert and metal baseplate does not increase even after more than six years of in-vivo function.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.