Abstract
Observational studies have identified surgical factors that are associated with a reduced risk of mortality after joint replacement. It is not clear whether these are causal or reflect patient selection. Data on the first primary hip (n = 424,156) and knee replacements (n = 469,989) performed for osteoarthritis in the National Joint Registry were analysed. Flexible parametric survival modelling was used to determine if risk factors for mortality in the perioperative period persisted. To explore selection bias, standardised mortality ratios were calculated for all-cause, respiratory and smoking related cancer mortality using population rates. Selection was apparent for hip resurfacing, combined spinal and general anaesthetic and unicondylar knee implants; reduced mortality was observed for many years for both all and other causes of mortality with a waning effect. Mechanical thromboprophylaxis was also suggestive of selection although patients receiving aspirin had sustained reduced mortality, possibly due to to a cardioprotective effect. Posterior approach for hips was ambiguous with a possible causal component. Spinal anaesthesia was suggestive of a causal effect. We are reliant on observational data when it is not feasible to undertake randomised trials. Our approach of looking at long term mortality risks for perioperative interventions provides further insights to differentiate causal interventions from selection. We recommend the use of aspirin chemothromboprophylaxis, the posterior approach and spinal anaesthetic in total hip replacement due to the apparent causal effect on reduced mortality.
Highlights
Total hip replacement (THR), total knee replacement (TKR) and unicompartmental knee replacement (UKR) cause short-term increases in mortality persisting for 90-days in hips[1] and 45-days in knees[2]
We defined the perioperative period as being 90 days as this is the timepoint at which we have previously shown the increased risk of perioperative mortality associated with the surgical intervention returns to baseline1. (ii) If healthier participants were selected for the intervention, this group would have a persistently lower mortality at all time periods, that was unlikely to be explicable by the intervention, though over time mortality risks would converge towards that seen in the general population due to attenuation. (iii) There may be a causal benefit of the intervention but there is selection of which patients receive this
The external comparison for all-cause mortality found a group difference, the biggest difference was in the earliest period after which the Standardised Mortality Ratios (SMRs) were fairly similar for both groups, but slightly lower for the posterior approach (Table 1)
Summary
Total hip replacement (THR), total knee replacement (TKR) and unicompartmental knee replacement (UKR) cause short-term increases in mortality persisting for 90-days in hips[1] and 45-days in knees[2]. Observational studies have identified surgical-related factors associated with decreased mortality. For hip replacement, these included posterior approach, thromboprophylaxis, spinal anaesthetic[1] and resurfacing hip replacement[3]. (ii) If healthier participants were selected for the intervention, this group would have a persistently lower mortality at all time periods, that was unlikely to be explicable by the intervention, though over time mortality risks would converge towards that seen in the general population due to attenuation. It may not be possible to differentiate causality from selection
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