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. 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.Introduction
Methods
A tibial insert with choices in posterior slope, size, and thickness is proposed to improve ligament balancing in total knee arthroplasty. However, increasing slope, or the angle between the distal and proximal insert surfaces, will redistribute ultra-high molecular weight poly-ethylene (UHMWPE) thickness in the sagittal plane, potentially affecting wear. This study used in-vitro testing to compare UHMWPE wear for a standard cruciate-retaining (CR) tibial insert (STD) and a corresponding 6° sloped insert (SLP). Our hypothe sis was that slope variation would have little effect on wear. Two of each style inserts were tested on an Instron-Stanmore knee simulator with a force-control regime. The gait cycle and other settings followed ISO 14243-1 &
2, except for the reference position, which was posteriorly shifted 6 mm to simulate the worst-case scenario. The STD insert was tilted 6° more than the SLP to level the articular surfaces. Wear was gravimetrically measured at intervals according to strict protocol. No statistical difference (p=0.36) was found between wear for the STD (9.5 ±1.8 mg/Mc) and SLP (11.4 ±0.5 mg/Mc) inserts. The overall wear rate measured was higher than previously published rates using implants similar to the STD inserts. This may relate to the shift in the reference position and the 6° slope, leading to increased shear loads. This is the first time the effect of tibial insert slope on wear has been evaluated in-vitro. When limited to 6°, wear testing suggests that al tering the tibial insert slope will have a minor effect on UHMWPE wear.
