Abstract
Complications of patellofemoral arthroplasty often occur soon after implantation and, as well as other factors, can be due to the design of the implant or its surgical positioning. A number of studies have previously considered the wear of ultra-high-molecular-weight polyethylene patellae following suboptimal implantation; however, studies have primarily been carried out under a limited number of degrees of freedom. The aim of this study was to develop a protocol to assess the wear of patellae under a malaligned condition in a six-axis patellofemoral joint simulator. The malalignment protocol hindered the tracking of the patella centrally in the trochlear groove and imparted a constant 5° external rotation (tilt) on the patella button. Following 3 million cycles of wear simulation, this condition had no influence on the wear of ultra-high-molecular-weight polyethylene patellae aged for 4 years compared to well-positioned non-aged implants (p > 0.05). However, under the malaligned condition, ultra-high-molecular-weight polyethylene patellae aged 8–10 years after unpacking (following sterilisation by gamma irradiation in an inert atmosphere) and worn ultra-high-molecular-weight polyethylene components also aged 4 years after unpacking (following the same sterilisation process) exhibited a high rate of wear. Fatigue failure due to elevated contact stress led to delamination of the ultra-high-molecular-weight polyethylene and in some cases complete failure of the patellae. The results suggest that suboptimal tracking of the patella in the trochlear groove and tilt of the patella button could have a significant effect on the wear of ultra-high-molecular-weight polyethylene and could lead to implant failure.
Highlights
Through the 1970s (35%) and 1980s (68%), there was an increase in the popularity of replacing the patella during total knee replacement (TKR).[1]
There was evidence of subsurface cracking on all the Group 2 ultra-highmolecular-weight polyethylene (UHMWPE) patella buttons; this was more prominent in the round dome implants where there was delamination of the UHMWPE (Figure 5)
For two of the Group 3 implants, the test was stopped after 1.5 million cycles (MC); the mean wear rate of the patella buttons could not be accurately measured and was in excess of 2000 mm3/MC, and they have been excluded from Table 2 and the statistical analysis
Summary
Through the 1970s (35%) and 1980s (68%), there was an increase in the popularity of replacing the patella during total knee replacement (TKR).[1] there is still debate as to whether the patella should be resurfaced routinely[2] during TKR or whether a more selective approach should be taken.[3] While there have been shown to be functional benefits of resurfacing the patella,[4] and a reduction in the reoperation rate due to patellofemoral joint (PFJ) problems,[5] failure of the patella button can occur due to loosening, fracture, infection, instability, mal-tracking, wear and overstuffing.[3,6,7,8]. Along with the patella resurfacing carried out during TKR, approximately 10,000 unicompartmental PFJ replacements are carried out in England and Wales annually.[6] These implants are often used as a conservative approach in younger patients. The National Joint Registry (NJR) reports a relatively high revision rate of unicompartmental PFJ implants compared to hip and Institute of Medical and Biological Engineering, University of Leeds, Leeds, UK
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