Abstract
Introduction
Unicompartmental knee replacement (UKR) offers advantages over total knee replacement but has higher revision rates particularly for aseptic loosening. Cementless UKR was introduced in an attempt to address this. We used National Joint Registry (NJR) data to compare the 10-year results of cemented and cementless mobile bearing UKR whilst matching for important patient, implant and surgical factors. We also explored the influence of caseload on outcome.
Methods
We performed a retrospective observational study using NJR data on 30,814 cemented and 9,708 cementless mobile bearing UKR implanted between 2004 and 2016. Logistic regression was utilised to calculate propensity scores allowing for matching of cemented and cementless groups for various patient, implant and surgical confounders, including surgeon's caseload, using a one to one ratio. 14,814 UKRs (7407 cemented and 7407 cementless) were propensity score matched. Outcomes studied were revision, defined as removal, addition or exchange of a component, and reasons for revision. Implant survival was compared using Cox regression models and groups were stratified according to surgeon caseload.
Results
Based on raw unmatched data the 10 year survival for cementless and cemented UKR were 89% (95% CI 88%–90%) and 93% (CI 90%–96%), with cementless having a lower revision rate (Hazard ratio (HR)=0.59 (CI 0.52–0.68, p<0.001). However, there were differences between the cohorts in many potential confounding factors particularly surgeons caseload: Surgeons using cementless had a higher caseloads than those using cemented and for both cohorts the revision rate decreased with increasing caseload.
Following matching, all potential confounders were well balanced and the 10-year survival for cementless and cemented were 90% (CI 88%–92%) and 93% (95% CI 90–96%) with cementless having a lower revision rate (HR 0.76; CI 0.64–0.91; p=0.003). This was due to rate of revision for aseptic loosening more than halving (p<0.001) in the cementless (n=31, 0.4%) compared to cemented (n=74, 1.0%) and the rate of revision for pain decreasing (p=0.03) in the cementless (n=34, 0.5%) compared to the cemented (n=55, 0.7%). However, the rate of peri-prosthetic fracture increased significantly (p=0.01) in the cementless (n=19, 0.3%) compared to the cemented (n=7, 0.1%).
Following matching the decrease in revision rate with the cementless was similar for low (<10 cases/year; HR 0.74), medium (10–30 cases/year; HR 0.79) and high (>10 cases/year; HR 0.79) caseload surgeons. The 10- year survival for cementless and cemented were for low caseload 87% & 82%, medium caseload 94% & 92% and high caseload 98% & 94% respectively.
Conclusions
This is the first study to compare the 10-year survival of the cementless and cemented mobile bearing UKR. We have demonstrated that the cementless device has a 24% reduced risk of revision and that this was independent of surgeon caseload and other important patient, surgical and implant confounders. This improvement was due to the rate of revision for aseptic loosening and pain halving. However, there was a small increase in rate of periprosthetic fracture.
The results of both cemented and cementless UKR improved with increasing surgeon caseload. Low volume surgeons have poor results with both cemented and cementless UKR so should consider either stopping doing UKR or doing more. Medium and high volume surgeons should consider using the cementless. High volume surgeons using the cementless had particularly good results with a 10-year survival of 98%.
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