Association between activity limitations and pain in patients scheduled for total knee arthroplasty

This paper reports the functional and activity limitations among persons scheduled for TKA, stratified by preoperative pain level. About one-fifth of subjects in the cohort reported Low WOMAC Pain scores (0–25) prior to surgery. However, these subjects frequently reported functional limitations such as limping or poor range of motion, as well as limitations with the high demand activities measured by the KOOS Sport and Recreational Activity scale, such as twisting, squatting, and kneeling.

Persons scheduled for TKA have been found to express goals of performing more than just low demand activities following surgery, such as returning to sports or gardening [23, 33]. A report by Noble and colleagues found that persons scheduled for TKA tend to place high importance on biomechanically-demanding activities such as kneeling and squatting, and that satisfaction with surgery is associated with their ability to return to the activities that they deem most important [18]. Our results corroborate these and other similar findings that TKA recipients may consider improvement in the ability to engage in more demanding activities such as kneeling or gardening as being important to their decisions to undergo TKA [22, 23, 33].

Despite evidence that many TKA recipients place high importance on demanding activities, data that support TKA recipients being able to engage in high demand activities such as kneeling, squatting, and twisting after surgery are limited [19, 22, 34]. Roos and colleagues evaluated the ability of subjects to perform the activities on the KOOS Sport and Recreational Activity subscale both pre-operatively and six months post-TKA and found that TKA provided only modest increases in the number of subjects who reported being able to squat, run, jump, and twist, and decreases in the number who reported being able to kneel [22]. Additionally, Weiss and colleagues found that patients often regard kneeling and gardening as some of the most important but also some of the most difficult activities to perform following TKA [23]. The increasing numbers of patients with low WOMAC Pain electing TKA highlights a need for more research that uses post-TKA data to evaluate the benefits of surgery specifically for patients with low WOMAC Pain. If patients are motivated to undergo TKA not by limitations in low demand activities but by the desire to return to more demanding activities, more attention should be paid to outcomes for patients with low preoperative pain that are related to performing these high demand activities. Moreover, persons with low WOMAC Pain who opt to undergo TKA may benefit from additional discussions with their surgeon regarding expectations of returning to such activities. Thorough discussions about managing expectations before indicating TKA may help to alleviate concerns about patients with low pain prior to surgery expecting improvements in high demand activities.

Additionally, future research on appropriateness criteria for TKA should account for patients who report low WOMAC Pain but who may seek surgery as a way to return to the activities that they deem important for their quality of life. Previous work on developing appropriateness criteria for TKA has included factors such as age, preoperative pain and function, and radiographic findings. Escobar and colleagues created criteria for TKA based on the RAND/UCLA Appropriateness Method, where a panel of experts rated cases as inappropriate, inconclusive, or appropriate [3]. The resulting criteria deemed patients with mild or moderate symptoms inappropriate or uncertain candidates for TKA regardless of age or radiographic severity, where moderate symptoms were defined as pain when walking on level surfaces and having some limitation in daily activities. Using Escobar’s criteria, Riddle and colleagues deemed over half of 175 TKA recipients in the Osteoarthritis Initiative (OAI) to be inappropriate or inconclusive TKA candidates [3, 5]. Hawker and colleagues used a cutoff of 39 points on the combined WOMAC Pain and Function scales (out of 100 points, 100 worst) to identify patients who had OA symptoms severe enough for TKA [25]. An evaluation conducted by Ghomrawi and colleagues found poor agreement between the criteria used by Escobar and Hawker, demonstrating a critical need for consistent and relevant appropriateness standards for TKA [24].

Fifty-five percent of SPARKS participants had WOMAC Pain below 40 points, and would likely not be considered appropriate TKA candidates based on several proposed appropriateness criteria [3, 25]. These data are consistent with the assessment by Riddle and colleagues that deemed only 44 % of 175 patients in the OAI to be appropriate TKA recipients based on Escobar’s criteria [3, 5]. The substantial number of TKAs in our patient sample and in the OAI that would likely be considered inappropriate based on Escobar’s criteria highlights the mismatch between these criteria developed almost fifteen years ago and current practice [3].

Additionally, the substantial number of patients with low WOMAC Pain scheduled for TKA suggests that the WOMAC Pain scale is an insufficient measure of TKA appropriateness, as has been previously described [35–37]. Researchers have attempted to use other measures such as the KOOS Pain and Function subscales to aid in the assessment of TKA appropriateness, but the KOOS Sport and Recreational Activity subscale has not been explored in this capacity [35]. The use of computerized adaptive testing may be a potential option for overcoming the limitations of the WOMAC for measuring a wide range of activity and function limitations for persons considering TKA. For example, PROMIS computerized adaptive testing has been used to measure self-reported physical function in patients with arthritis and in orthopedic trauma patients [38, 39].

In the development of appropriateness criteria for TKA, it is important to recognize that pain may not be the primary focus for patients. There may be other factors besides pain on the WOMAC Pain items, such as BMI, that contribute to limitations in the high demand activities measured by the KOOS Sport and Recreational Activity subscale. In the development of appropriateness criteria for TKA, it is important to recognize that pain may not be the primary focus for patients. More work is needed to develop appropriateness criteria that account for the interplay between pain and other variables such as demographic characteristics and activity limitations. The relevance of our hypothesis that TKA recipients who report low levels of pain with low demand activities will be limited in high demand activities lies in fact that WOMAC Pain relies largely (3 out 5 items) on sedentary activities and therefore could miss the disability of the increasingly active population of TKA candidates.

We found that participants with Low Pain prior to surgery did not differ from those with Medium or High Pain with regard to the average number of steps they walked every day (Table 2). This finding is similar to that of White and colleagues, who reported that knee pain severity did not impact walking behaviors in a cohort with or at risk for knee OA [40]. Lo and colleagues also recently showed that WOMAC Pain scores did not predict physical activity levels among OAI participants with or without knee OA [41]. It is somewhat paradoxical that participants who report low WOMAC Pain and therefore experience less pain when standing or walking on flat surfaces do not walk more than those with more pain. This conveys discordance between potential capacity and performance that is often observed in knee OA cohorts, where participants who can walk without pain nonetheless choose not to [40, 41]. It is also possible that participants with low WOMAC Pain do not walk more than those with more pain because they have modified their activity to be in less pain. Our findings were not affected by missing data, since completing the baseline assessment was a key inclusion criterion for the study.

The results of this study should be viewed within the context of several limitations. The study population was recruited as a part of randomized controlled clinical trial, which introduces inherent selection bias, and the participants were recruited from a single study center. Additionally, because the study sample was obtained from a randomized controlled trial of a behavioral intervention for physical activity following TKA, subjects with low pain may have been more willing to participate. This selection bias may have enriched the proportion of subjects in our sample with low WOMAC Pain, allowing us to examine their characteristics more carefully. Individuals with severe mobility limitations were excluded from the study, and thus our sample may be more active than other TKA cohorts. This study did not include radiographs, and thus we were unable to determine the radiographic severity of subjects’ knee OA, which could have influenced decisions to pursue TKA. Knee range of motion was obtained using self-report; however, participant-reported knee range of motion has been shown to match measured range of motion in a similar population with knee OA [31]. This analysis also uses single items from multi-item scales (the KOOS and the WOMAC), which have unknown validity and may compromise the reliability of the results. Our questionnaire did not give participants the option to indicate that they did not perform the high demand activities measured by the KOOS, which may have led some participants to report “extreme” difficulty with activities that they do not perform. Functional limitation items were selected based on expert opinion and were not extensively validated. In addition, we did not collect data related to motivation for TKA. Future studies should directly measure patient motivations for undergoing TKA and how satisfied they are with surgery in order to better understand why patients with low pain on low demand activities undergo TKA.