Abstract
Introduction
A tibial insert with choices in size, thickness, and posterior slope is proposed to improve ligament balancing in total knee arthroplasty. However, increasing posterior slope, or the angle between the distal and proximal insert surfaces, will redistribute ultra-high molecular weight polyethylene (UHMWPE) thickness in the sagittal plane, potentially affecting wear. This study used in-vitro testing to compare wear for a standard cruciate-retaining tibial insert (STD) and a corresponding 6° sloped insert (SLP), both manufactured from direct-compression molded (DCM) UHMWPE. Our hypothesis was slope variation would have no significant effect on wear.
Methods
Two of each insert (STD and SLP) were tested on an Instron-Stanmore knee simulator with a force-control regime. The gait cycle and other settings followed ISO 14243-1 and -2, except for reference positions. The STD insert was tilted 6° more than the SLP insert to level the articular surfaces. Wear was gravimetrically measured at intervals according to strict protocol.
Results
No statistical difference (p=0.36) in wear rates was found for the STD (9.5 ±1.8 mg/Mc)) and SLP (11.4 ±0.5 mg/Mc) inserts.
Discussion
The overall wear rate measured was higher than previously published rates for implants similar to the STD inserts. This may result from increased shear loads due to the shift in reference position and 6° slope. This is the first time the effect of tibial insert slope on wear has been evaluated in-vitro. For inserts made from DCM UHMWPE with a slope limited to 6°, this test suggests altering tibial insert slope has an insignificant effect on wear.
