In this prospective study of a large cohort of patients with primary THA, we found that after adjusting for important predictors, patients with OA had significantly better functional outcomes compared to those with RA/inflammatory arthritis at two-year follow-up. Results were notable for overall ADL limitation and key ADLs. Pain outcomes were not significantly different for OA versus RA. We also found that patients with AVN experienced better ADL outcome at two years compared to RA patients. In contrast, patients with AVN reported worse pain outcomes at five years compared to those with RA. Several findings in our study are of interest and merit further discussion.
One of the main findings of our study was that patients with OA had significantly better functional outcomes compared to those with RA at the two-year follow-up, as indicated by lower odds of overall moderate-severe ADL limitation. These findings were confirmed for patients younger and older than 65 years. To our knowledge, this is the first study to have analyzed the association of diagnosis with functional outcomes adjusted for known correlates of outcome in a large sample of patients at both the two- and five-year follow-up after primary THA. On the other hand, we found no significant differences in risk of moderate-severe pain between OA and RA in multivariable-adjusted models. The greater ADL limitation noted in RA patients compared to OA patients may be due to the systemic inflammatory processes in RA versus OA , a higher risk of postoperative dislocation after primary THA in RA versus OA patients , and/or a more polyarticular disease in RA compared to OA. As stated previously, ‘A single THR (total hip replacement) apparently solves the main problem of most OA patients, but only one of a number of joint problems for most RA patients’ . It is important to note that in exploratory analyses, several associations for key ADLs that were significant in univariate models were no longer significant in multivariable-adjusted models, indicating that these associations were not independent of the covariates adjusted in the multivariable analyses. However, results were significant for rising from a chair and climbing stairs in multivariable-adjusted models.
Previous studies that have examined functional outcomes in THA patients have reported contradictory findings. In a study of 381 patients, pain and function (walking scores) at two years post-THA did not differ by the underlying diagnosis . On the other hand, some studies have reported worse outcomes in patients with RA compared to OA. In their study of 97 patients, Borstlap et al. reported that patients with OA experienced better functional outcomes one year after THA compared to those with RA . A study of 106 RA patients by Creighton et al. suggested that RA was associated with less optimal functional improvement at 10 years as compared to other diagnoses (OA, and so on) in their other studies . Kirwan et al. studied 293 patients who underwent total hip or total knee replacement at 2.5 years and found greater improvements in pain and function in OA compared to RA . Hawker et al. examined the predictors of successful joint arthroplasty outcome in a cohort of 233 patients with either hip or knee joint replacement and found that RA was associated with 0.33 odds of good outcome compared to OA, based on WOMAC total score improvements, a pain and function composite . Two of the four positive studies combined knee and hip arthroplasty patients, and studies show that outcomes from these two procedures differ , as do the underlying diagnoses [32, 33], which makes the interpretation of these studies difficult for THA populations. Previous studies had small sample sizes, heterogeneous populations, unadjusted analyses, historical controls and contradictory findings. Our findings from a multivariable-adjusted analysis from a large total joint registry provide clarity and add to the current knowledge. Our findings suggest that functional outcomes were better in OA versus RA and that there were no significant differences in pain outcomes two years and five years after THA.
Another important study finding was the lack of difference in pain outcomes in RA versus OA patients after THA. This is reassuring given that the most common symptom leading to THA is severe, refractory hip joint pain. This implies that patients with RA can be reassured that their pain outcomes will be similar to the majority of patients with OA undergoing THA.
Another interesting finding from our study is that patients with AVN were less likely to have moderate-severe ADL limitations and more likely to report moderate-severe pain compared to patients with RA. To our knowledge, this is a new finding and adds to the literature. A recent systematic review of outcomes of THA in patients with AVN found that most studies had a low evidence level of III and IV and provided data only related to revision rates , indicating a lack of studies of PROs in this patient population. Our finding of worse pain outcomes in AVN patients compared to RA patients might indicate the difference in pathophysiology of the two conditions (AVN versus RA) and polyarticular and bilateral involvement with RA  compared to AVN. In addition, documented high rates of early complications and reoperations in AVN patients (17% and 11%, respectively, at an average follow-up of 20 months)  may also explain better ADL outcomes in AVN versus RA.
The practical implications of our study are several-fold. First our study highlights the importance of studying both pain and functional outcomes in arthroplasty patients, as discussed previously . Although these domains are somewhat interrelated, they can be discordant due to different slopes of recovery post-arthroplasty; for example, at three months post-THA, patients are likely to report significant improvement in pain, but function may be the same as it was preoperatively due to continuing recovery and rehabilitation. Similarly, other lower extremity joint involvement and back problems impact pain and function differently, that is, the impact on function may be tremendous, but there may be no impact on index hip pain. Our study further demonstrates that the underlying diagnosis impacts pain and function after primary THA differently. This new information can help surgeons inform their patients preoperatively during the informed consent process with regard to expected outcomes after primary THA, based on their underlying diagnosis. Given the longevity of the implant and the elective nature of the surgery, a better insight into why certain diagnoses are associated with worse outcome can help to improve these outcomes even further, if modifiable intermediate factors can be identified. This will also lead to even more informed patients and reduction of unsatisfactory outcome after primary THA, a highly successful surgery.
Our study has several limitations and strengths. Survey non-response may have introduced some bias, and the direction of this bias is unclear. Our response rates are similar to the average 60% response rate reported for large surveys of this size ; however, the five-year estimates should be interpreted with caution. Due to a cohort study design, residual confounding is possible, despite inclusion of multiple clinical and demographic variables. There may be some misclassification of operative diagnosis due to similarity of gross findings at surgery between RA and AVN and because classification criteria are not used in clinical orthopedic practice for RA and are not available for AVN. However, the surgeon incorporates history, examination and medication use in making the diagnosis that should provide good accuracy. Misclassification bias may have biased our results towards null. Generalizability is always a challenge, but the similarity of our cohort to other hip arthroplasty cohorts indicates that results may be generalizable to other settings [6, 7, 38]. Study strengths include a follow-up at two time-points, large sample size to allow adequate power, prospective data collection by dedicated Total Joint Registry staff and multivariable-adjusted analyses that adjusted for other factors known/likely to be associated with the outcomes.