Relative survival

[eribul]

I would like to know if the risk of dying in 90 days was comparable to the risk if the patient had not had a THR? Ie, is there an additional risk in having a THR or is the risk the same? For example, in a 99 year old, their risk of dying may also be almost 9% just by the fact they are 99 years old. Can the authors provide comparisons to normal mortality rates? The authors have already stated (line 232) that THA was not regarded as an intervention.

[eribul]

Review: We agree that this is undeniably a very important and relevant issue. We can not simply compare the number of deaths within 90 days for patients with THA and without, however, since that would require a control group with patients in need of elective THA but for whom such procedure is not undertaken at a certain date comparable to the date of surgery. What we can do, however, is to study relative survival, a concept otherwise most commonly used in cancer epidemiology. This is a comparison of cohort survival, related to the survival of the general population of the same age, sex and year of birth. Such study was previously performed on the same cohort by Cnudde et al. (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6263594/). They found that the relative one year survival for patients with THA due to osteoarthritis, was 101 %. This means that our patients are likely to have a better survival than the general population (due to "selection bias", hence that only relatively healthy patients undergo elective THA). If we take a closer look at only the cemented cases during the initial 90 days, however, as done by Cnudde in figure 23 of his PhD thesis expanding the topic of the published paper (https://gupea.ub.gu.se/bitstream/2077/54531/6/gupea_2077_54531_6.pdf), we can see that the relative survival is below 100 % during this specific period. This combined, is a strong indication that the patients under study are not likely to die in general, but that the THA surgery itself elevates this risk during a short time following surgery. We think this is a strong reason to believe that most deaths occurring within 90 days are in fact caused by THA-related issues. This is equally true for the 99-year olds, since those patients do not constitute a random sample from their age group, but are relatively healthy with a foreseen advantage of THA outweighing the risk of near-by mortality (they would otherwise not have undertaken such elective surgery).

We have now expended the discussion on line 290 to include this reasoning.