Discussion
osteoarthritis, knee DVT, VTE, TKA, THA
OSTEOARTHRITIS, SECONDARY
Despite the low symptomatic VTE rate seen in the aspirin group on unadjusted analysis (1.6%), there was no significant association between prophylaxis type and VTE after adjusting for confounders. The multivariable analysis suggested that the low VTE rate in the aspirin group was due to a lower baseline VTE risk in these patients (lower BMI, lower previous VTE rate and shorter time to mobilization).The strengths of this study include data collection across multiple high-volume institutions, increasing the generalizability of the results and reflecting real-world practice, and the inclusion of parameters that have been omitted from previous studies such as duration of prophylaxis and time to mobilization. Further, all primary and secondary outcomes were verified through contact with treating clinicians and hospitals.This study also has several limitations. First, the study was likely underpowered to detect a difference for the primary outcome, given the low incidence of VTE within each prophylaxis group. Reliable conclusions cannot be drawn regarding the effectiveness of aspirin from this study due to the low included sample size compared to existing published randomized and observational studies. Second, there was considerable heterogeneity of prophylaxis protocols among participating institutions which limited the ability to perform a direct comparison between each form of prophylaxis. Third, its retrospective, observational nature increased the risk of selection bias. It is likely that aspirin was prescribed to patients with a lower baseline VTE risk in this study and that LMWH and DOACs were prescribed to higher risk patients.Selection bias may also be a contributing factor to the low rates of VTE reported with aspirin use compared to other forms of prophylaxis in other observational and registry-based studies [Although several of the previous observational studies attempted to adjust for potential confounders with multivariable analyses, these have produced inconsistent results [The effect of early mobilization as a confounder has not been included in the majority of observational studies investigating the efficacy of aspirin for VTE prophylaxis following THA and TKA. Early mobilization has long been associated with lower VTE rates [The results from this study and previous observational studies highlight the limitations associated with relying on retrospective evidence regarding the efficacy and safety of aspirin when used for VTE prophylaxis for all patients following THA or TKA. It is likely that patients with a low baseline risk of VTE were more likely to be allocated to aspirin groups (as occurred in our study), leading to lower symptomatic VTE rates compared to other forms of prophylaxis (selection bias). This may have been the result of risk stratification protocols implemented by individual surgeons at participating hospitals, which have been proposed as an alternate strategy for VTE prophylaxis rather than prescribing one agent for all patients [Balancing all confounding factors is more reliable through randomization. Only three randomized trials have investigated the use of aspirin for symptomatic VTE prophylaxis following THA and TKA within the last 20 years. The largest study (n = 9711), published in 2022, demonstrated that enoxaparin was superior to aspirin in preventing all symptomatic VTE (including below knee DVT, above knee DVT and PE) within 90 days following THA or TKA performed for a diagnosis of osteoarthritis [Evidence from observational studies has led to an increase in the use of aspirin use over the last decade [