Abstract
Introduction
Backside wear of polyethylene (PE) inlays in fixed-bearing total knee replacement (TKR) generates high number of wear debris, but is poorly studied in modern plants with improved locking mechanisms.
Aim of study
Retrieval analysis of PE inlays from contemporary fixed bearing TKRs - to evaluate the relationship between backside wear and liner locking mechanism and material type and roughness of the tibial tray.
Methods
MATERIAL
We included five types of implants, revised after min. 12 months (14–71): three models with a peripheral locking rim and two models with a dove-tail locking mechanism. Altogether this study included 15 inlays were removed from TKRs with CoCr alloy tray with a roughened surface and a peripheral locking lip liner (Stryker Triathlon, Ra 5,61 µm), 9 from CoCr trays with peripheral locking lip and untreated surface (Aesculap Search, Ra 0,81 µm), 13 from Ti alloy trays with peripheral locking lip and untreated surface (DePuy PFC Sigma 0,61 µm), 11 from Ti alloy trays with untreated surface and dovetail locking mechanism (Zimmer NexGen, 0,34 µm), and 9 from iplants with a Ti alloy tibial tray with mirror polished surface and dovetail locking mechanism (Smitn&Nephew Genesis II, 0,11 µm).
METHODS
Wear of bearing surface and back side of retrieved inlays was examined in 10 sectors under a light microscope. Seven modes of wear were analysed and quantified according to the Hood scale: surface deformation, pitting, embedded third bodies, pitting, scratching, burnishing (polishing), abrasion and delamination. Damage of inlays caused by backside wear was also evaluated using scanning electron microscopy (SEM). Roughness of tibial tray was evaluated using a contact profilometer.
Results
We found no differences between wear scores on the articulating surface in all group, they did not correlate with backside wear scores in all groups as well. Compared to all other groups, backside wear scores were significantly higher in implants with untreated Ti alloy tibial tray (P<0,001 Wilcoxon test). Lowest wear rates were found in implants from both Ti and CoCr alloys and peripheral locking rim. Interestingly there was no difference between wear of implants with polished and untreated surface (Fig. 1). SEM analysis demonstrated different wear modes in implants with dovetail mechanism and peripheral rim. The first group demonstrated signs of gross rotational instability, with severe abrasion with an arch-shaped pattern and delaminated PE (Fig 2). In one design we observed severe extrusion of PE into screw holes of the tibial tray. Inlays from trays with peripheral rim presented two types of wear: flattening of machining marks or protrusion of the material caused by the rough surface (Fig 3).
Conclusions
This study demonstrates that backside wear is still a problem in modern TKR. Our findings suggest that it is predominantly affected by type of locking mechanism (with peripheral rim performing better), to a lesser extent by surface roughness of the tibial component, while material type does not seem to play an important role.
This study was funded by a grant from the National Science Centre nr 2012/05/D/NZ5/01840
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