The average results on the implant published by the inventor team differed markedly from the outcome published in independent clinical studies or shown by worldwide national arthroplasty registry data. They also exceeded the maximum deviation of individual departments (due to factors such as personal expertise and patient selection) from the national mean that has been registered for hip and knee prostheses in countries such as Sweden and Denmark.
The cause of this divergence can only be an issue for speculation. Irrespective of the reasons for these deviations, however, the average surgeon should be aware of the fact that the outcome published by the inventing center appears to be hardly reproducible in average patient care and other institutions. Thus, the published results of this group are only of limited value for decision making by other users since they cannot expect to be able to reproduce such excellent results.
On average, the revision rate data of the Oxford team have been 2.7 times lower than the revision rates quoted in the independent literature, and 4.4 times lower than the results from worldwide registry data. This means that on average, publications involving revisions from this group only match 23% of the revisions documented in worldwide registers, and 37% of the revisions published in independent studies.
Even though one third of the clinical studies (7/22) have originated from the inventor group, these papers account for one half of all cases published worldwide. Owing to the longer follow-up periods, the value for observed component years reached 65%.
While multiple mentions of the same patients in different studies unfortunately cannot be excluded in a literature analysis, this does not affect the impact publications have on experts and decision-makers. As a rule, major studies covering longer periods of follow-up are assigned superior value.
The implant development team in Oxford has therefore been clearly overrepresented in the clinical literature, which—along with the discrepancy regarding clinical outcome—has had a statistically significant influence on all the published results. In a conventional analysis of the clinical literature, this influence therefore represents a confounder that could also affect assessment of the product by stakeholders.
By contrast, the independent clinical literature puts forward revision rates that are 1.6 times lower than the comparative figures in registries. These differences can, however, be plausibly explained by factors such as higher surgical expertise. In general, studies without participation of the developers of the implant can be said to have good reproducibility.
The variation in results is clearly less in registries of different countries than it is in the clinical literature. This applies to both annual reports and registry-based journal publications. Apart from the larger numbers of cases, it is probably the minimization of confounding factors, which basically cannot be excluded in sample-based studies, that accounts for this effect.
Moreover, the impact of a single group on the results is automatically limited by the wider scope of data collection. For the assessment of outcome results on orthopedic implants, registry data are therefore superior to clinical studies. The potential influence of national circumstances can be quantified and narrowed by comparing data from different countries.
Registry data can be used as a benchmark in the assessment of clinical studies, particularly when it comes to evaluation of whether relevant bias factors could possibly have an influence on the outcome. Thus, registry data can provide a valuable contribution to the assessment of outcome data.
Regardless of the confounding factors detected in the clinical literature, registry data on the Oxford unicompartmental knee prosthesis indicate similar performance of this implant in comparison with other well-performing products for unicompartmental knee arthroplasty.