In this study, we describe one of the largest U.S. cohort of TKA patients followed for patient-reported pain outcomes. We found that a higher BMI was not associated with worse pain outcomes 2- and 5-years after primary or revision TKR. Higher comorbidity was associated with a greater likelihood of moderate-severe pain at 2-years. This study also confirmed our previous findings of association of female gender and younger age with moderate-severe pain at both 2- and 5-years after primary TKA [25
]. We found that older patients were less likely to report moderate-severe pain 2- and 5-years after revision TKA. Other factors associated with moderate-severe pain following knee arthroplasty implant fixation (uncemented or hybrid) for primary TKA, the underlying diagnosis for revision TKA and greater distance from the medical center, for both. Uncemented knees have a higher rates of aseptic loosening [40
] and revision than cemented knees [41
], which may explain the higher prevalence of moderate or severe pain in patients receiving non-cemented implants.
One of the most remarkable finding in our study is that obesity was not associated with any higher prevalence of moderate-severe pain in either primary or revision TKA patients at 2- or 5-year follow-up. Previously published literature regarding BMI and TKA outcomes is contradictory. Most previous studies have examined the association of BMI with summary scores (not pain), mostly using the Knee Society Scale (KSS) total and objective/subjective scores, with most reporting lack of association [7
], while few were positive [3
]. All previous studies had <600 patients. The limited sample size in previous studies precluded use of WHO-recommended BMI categories. Most studies collapsed many BMI categories due to small number of poor outcomes. We are aware of only one small study of 67 TKA patients that examined association of obesity with pain outcomes [17
]. BMI of 35-39.9 was significantly associated with pain scores (p=0.049) in multivariable-adjusted models that included age, gender and comorbidity [17
]. Our study results are in contrast to this previous study, likely due to differences in patient population (67 years, 50% female vs. 75 years, 80% female), sample size (>800 patients for each of the four cohorts vs. 67 patients), covariates in multivariable model (income, distance, implant fixation vs. intra-operative, surgical, in-patient and postoperative clinical variables), or the type of regression analyses (model including all covariates vs. step-wise regression).
The large study sample size provided us with adequate number of patients in each BMI category (vs. 8 patients in previous study with BMI 35-39.9) to examine the link between presence and severity of obesity and pain. Our results and interpretations did not change depending on how the BMI was examined as a predictor in the analyses (i.e continuous or categorical). Due to a large sample size, negative findings are very unlikely due to type II error (i.e. missing an effect, when one exists due to lack of power). Our study adds to the post-primary TKA literature by showing that obesity is not associated with any higher risk of moderate-severe pain during short-intermediate-term follow-up post-TKA. To our knowledge, the findings of lack of BMI association with pain outcomes 2- and 5-years after revision TKA are new and add to existing knowledge of pain predictors. Several previous studies of smaller sample size have also reported similar HRQoL outcomes in patients with obese and non-obese patients undergoing TKA. This implies that TKA can (and should) be offered to patients across the range of BMI without concern for significant variation of pain relief.
It is possible that patients with higher BMI have lower level of activity and therefore despite greater forces across the joint, less frequent weight-bearing occurs. Our findings should reassure patients and surgeons that obesity is not by itself a risk for poor pain outcomes after primary or revision TKA. In conjunction with our recent finding of association of BMI≥40 with higher activity limitation 2- and 5-years after revision TKA from this cohort [43
], this implies that higher BMI has different impact on pain versus function outcomes.
Another important observation from this study is the positive association between comorbidity and pain outcomes at 2-year post-primary TKA. Our findings agree with some studies [5
] and are in contrast to studies that found no association [9
]. There were no meaningful differences between follow-up durations between the studies with positive association (1-2 years) versus those finding no association (0.5-5 years). A higher comorbidity is associated with poorer post-operative outcomes [30
]. Higher prevalence of moderate-severe pain at 2-year post-TKA in patients with more comorbidities may be due to higher complication rates, higher risk of pain with comorbidities such as diabetes and/or lower tolerance to pain. Future studies need to examine whether specific diseases, disease severity at baseline or change in severity during follow-up predicts pain and HRQoL outcomes. If optimization of comorbidity can lead to a better outcome, this may be indicated before an elective procedure like arthroplasty.
We found that older patients reported less moderate-severe pain after primary TKA compared to younger patients. This is in contrast to many studies [10
], but in agreement with others [6
] and our previous observation [25
]. In most previous studies, age has been examined mostly as a continuous variable, which assumes that the increase in risk is the same across each year of increased age across the entire age spectrum. It is also difficult to interpret higher risk per 1-year increase in age. Two of the positive studies categorized age and reported that patients ≥75 years had better pain outcomes at 1-year [20
] and <60 years worse pain outcomes at 5-years post-primary TKA [19
]. We confirm these findings in a much larger cohort and extend them to other older age groups.
To our knowledge age associations have not been reported in detail in previous studies of revision TKA. Our findings in revision TKA add to the current literature: patients aged 61-70 years had better pain outcome at 2- and 5-years; 71-80 year-olds had better pain outcome at 2-years; and those older than 80 years had better pain at 2-years post-revision TKA. In fact, age had a consistent strong relationship with moderate-severe pain at both time-points in both primary and revision TKA. This may be due to lower activity level, better pain-coping skills and/or more self-efficacy in older patients.
It is not surprising that moderate-severe pain was found to be more prevalent after revision than after primary TKA in our study. This is similar to previously reported better HRQoL in patients with primary versus revision total hip arthroplasty [46
]. Our study extends these findings to patients with TKAs.
Our study has several strengths and limitations. We report on the largest cohort of patients followed up to 5-years for clinical outcome and pain to date, (to our knowledge). As a result we have robust estimates of association allowing for the control of many important clinical and socio-demographic confounders/covariates. On the other hand, we were unable to control for pre-operative knee pain severity as was data available in only half of the sample, which could have lead to selection bias [15
] We also were unable to control for the presence of anxiety/depression. Being a tertiary referral center, we did not think these would be accurately or completely captured in our records. Both preop pain and anxiety/depression are considered important predictors of post-operative pain and HRQoL and would be valuable additions to future studies.. The response rate for 2-year follow-up of 57-65%, although not perfect, is similar to that reported in large surveys of this size [47
], and may even be considered very good for a clinical follow-up, considering this is for every patient operated over 12-years
. Our estimates may be somewhat biased due to non-response; however, since non-responders were more likely to be female, and younger (associated with poorer pain outcome), our estimates are conservative and the differences would at least be as large or larger had all eligible patients responded. Despite a large sample size, the number of responders was lower at 5-years (than 2-year), making the 5-year estimates less precise. Our findings need to be confirmed in other large patient cohorts.
In conclusion, we found that obesity was not associated with worse pain outcomes after primary or revision TKA. Three-fold more patients report moderate-severe pain after revision compared to patients with primary TKA over the first five years after the surgery. Higher comorbidity predicted worse pain outcome after primary TKA. Female gender and younger age predicted worse pain outcome after primary TKA and younger age after revision TKA. A better understanding of patient- or disease-related factors that impact post-arthroplasty pain can help us better inform patients before surgery and in the case of modifiable predictors, assist efforts to target interventions or preventive programs to improve these outcomes. Further studies are needed to better understand these relationships.