Abstract
Many total knee replacements (TKR) are designed with more conforming articular geometry to increase the femoral contact area and decrease surface stresses. These designs are supported by studies suggesting that implants with coronally flat articular surfaces are vulnerable to medial-lateral lift-off and edge-loading on the polyethylene insert. However, few retrieved inserts from contemporary TKR’s have shown wear consistent with this loading mechanism. This study presents wear measurements from 37 consecutively retrieved polyethylene inserts of the same PCL-retaining design with coronally flat-on-flat articulations. If substantial edge-loading occurred in-vivo, it was hypothesise that wear would be located closer to the medial or lateral edge of the articular surface with a high incidence of delamination.
Inserts were retrieved at autopsy (n=12) after 41 (15–74) months in-situ and at revision TKR (n=25) after 26 (1–71) months in-situ. Reason for revision was infection (28%), patellar component complications (24%), loosening (24%), patellar resurfacing (20%), and supra-condylar fracture (4%). Articular damage was measured using light microscopy and digitising the circumference of each damage region on calibrated images. Surface deformation was measured relative to unused control inserts using a hand-held digitising stylus.
Wear patterns were not significantly different between autopsy or revision retrievals (ANOVA, p> 0.05). Articular wear covered 48%+16% and 47%+14% of the medial and lateral surfaces, respectively. The most frequent wear modes were burnishing and scratching. Delamination occurred on 4(11%) inserts, but involved < 2% of the articular surface. Wear patterns were internally rotated and centrally located. Not one insert had a wear area centroid located in the medial or lateral third of the articular surface. Surface deformations were greatest in the inserts’ central region and the linear deformation rate decreased with time.
Concerns of high contact stresses associated with edge-loading were unsupported by these retrievals. Condylar lift-off, if it occurs, does not appear to substantially impact polyethylene damage in coronally flat-on-flat articulations.
The abstracts were prepared by Nico Verdoschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, Universitair Medisch Centrum, Orthopaedie / CSS1, Huispost 800, Postbus 9101, 6500 HB Nijmegen, Th. Craanenlaan 7, 6525 GH Nijmegen, The Netherlands.
