Association Between the Single Assessment Numeric Evaluation and the Western Ontario and McMaster Universities Osteoarthritis Index

The current study was part of a double-blinded randomized controlled trial evaluating the augmentation of transcutaneous electrical nerve stimulation (TENS) to a traditional 4-week (12-session) TE program.²⁶  All participants performed the same TE but were randomly allocated to 1 of 3 treatment groups that may have included an additional TENS or sham TENS treatment during the TE and 8 hours each day when physically active (TENS + TE [n = 12], sham TENS + TE [n = 12], or TE only [n = 12]).²⁶  The SANE_Function and WOMAC_Dysfunction scores were collected before and after the 4-week TE intervention. Participants removed the TENS or sham TENS before any outcome testing. The Institutional Review Board at the University of Virginia approved the recruiting methods, consenting process, and experimental procedures used in the current study. Sample size for this study was estimated using G*Power Statistical Power Analysis (version 3.1; Heinrich-Heine-Universität Düsseldorf, Germany).²⁷  Previous researchers¹⁵  reported moderate to strong associations (r range = 0.58–0.87) between the SANE_Function and Lysholm scores in patients with ACL injury. Conservatively, we estimated a moderate 2-tailed association would be present between the SANE_Function and WOMAC_Dysfunction outcomes (r = –0.45) and determined that 36 individuals would be needed to detect a significant association with an α of .05 and 1 – β equal to 0.8.

Potential participants were recruited from the university community and from participating orthopaedic surgeons within the University Health System. The inclusion criteria for the current study were consistent with the criteria used to enroll patients in the aforementioned randomized controlled trial.²⁶  All participants had a clinical diagnosis of tibiofemoral OA in at least 1 knee and a Kellgren-Lawrence score between 1 and 4 as assessed with bilateral radiographs obtained within the previous 6 months. Additionally, all participants demonstrated a voluntary quadriceps activation of less than 90%,²⁸  which was assessed isometrically at 70° of knee flexion using the burst-superimposition technique.^26,29  We excluded individuals with a diagnosed heart condition that limited exercise, altered sensation over the anterior knee region, bilateral total knee arthroplasties, or lower body surgery or knee trauma in the previous 6 months. Participants with unilateral knee OA and contralateral total knee arthroplasty were included in the study, but these participants only received targeted TE, TENS, or sham TENS to the limb with knee OA and not to the limb with the total knee arthroplasty. A mandatory 2-week washout period, determined from previously published half-life data, was implemented for all participants who previously had a corticosteroid³⁰  or hyaluronic acid injection.³¹  Participants were asked to not use nonessential pain medication 12 hours before therapy sessions and 24 hours before all testing sessions. Written consent was obtained from the participants before testing.

We used the SANE^21,32  and WOMAC Index¹⁰  scores, which are reliable measures of self-reported function in people with knee injuries or knee OA, respectively. A single investigator (B.P.) provided instructions to each participant on how to complete each outcome measure and explained the differences among rating pain, stiffness, and function for the entire WOMAC index. Participants completed all 3 WOMAC Index subsections (pain, stiffness, and function). The 17-question WOMAC_Dysfunction subsection, in which participants were instructed to rate the degree of difficulty they experienced while performing various activities of daily living, was the primary WOMAC outcome measure for the current study. The WOMAC_Dysfunction subsection has strong reliability (interclass correlation coefficient = 0.90) in patients who demonstrated arthroscopically confirmed knee OA.¹²  We chose the WOMAC_Dysfunction subsection as our main WOMAC outcome measure because we believed that it aligned most clearly with the information that the SANE_Function score sought to capture. For each question, participants indicated their disability as none, slight, moderate, severe, or extreme, which corresponded to scores between 0 and 4, respectively. The WOMAC_Dysfunction was scored between 0 and 68, with a 68 indicating the most physical dysfunction among the examined tasks. Participants completed all 3 traditional WOMAC Index subsections (pain, stiffness, and function). We included 2 additional WOMAC pain questions that assessed pain using the same WOMAC Likert scale and asked, “Please describe the level of pain you have experienced in the last 48 hours for each one of your knees” for both the left and right knees; the WOMAC pain subsection value for the current study was then calculated from a total of 28 possible points (0 = least pain and 28 = most pain). All participants indicated the level of pain in each knee regardless of the presence or severity of bilateral radiographic knee OA. The WOMAC_Stiffness subsection included 2 questions for a total of 8 possible points (0 = least stiffness and 8 = most stiffness). In addition, we included a variable that analyzed all 3 subsections of the WOMAC score together (WOMAC_Aggregate score). We individually normalized each participant's raw score out of 100% or the total possible points in each WOMAC subsection ([raw score for the participant / total possible score] · 100). The SANE_Function was completed after finishing the entire WOMAC Index. Participants were asked, “On a scale of 0 to 100, how would you rate your knee's function, with 100 being normal?” and instructed to write the number on a piece of paper. When completing the 4-week assessment for both outcomes, participants were not allowed to know their baseline SANE_Function or WOMAC_Dysfunction score. Percentage change scores over the 4-week intervention period were calculated for the SANE_Function score as well as for the WOMAC subsections using the following equation (Percentage change = [(4-week value – baseline value) / baseline value] · 100).

Over a 28-day period, all participants completed 12 sessions of TE supervised by a licensed physical therapist or a certified athletic trainer.²⁶  Exercises focused on improving quadriceps muscle strength, as quadriceps strength is associated with disability in people with knee OA. We used the daily adjustable progressive resistive exercise system³³  to systematically advance open and closed chain strengthening exercises throughout the 28-day intervention as previously described.²⁶  In addition to strengthening exercises, participants performed balance exercises and gait training during each session. Flexibility limitations were individually managed with stretching, and muscle or joint soreness was managed with cryotherapy.

Means and standard deviations were computed for demographics and outcome measures (Table). Frequencies and percentages were calculated for sex and the Kellgren-Lawrence score of radiographic OA severity (Table). Before conducting the primary analyses, we assessed normality of the SANE_Function at baseline and at the 4-week time point, WOMAC_Dysfunction and WOMAC_Aggregate scores at baseline and 4 weeks, and percentage change in SANE_Function, WOMAC_Dysfunction, and WOMAC_Aggregate over 4 weeks using separate Shapiro-Wilk tests. Spearman (r_s) rank order correlations were used to evaluate all associations because at least 1 nonnormally distributed variable was included in all analyses. We separately evaluated the bivariate associations between SANE_Function and both WOMAC_Dysfunction and WOMAC_Aggregate scores at the baseline and the 4-week time points. Next, we determined the bivariate associations between the changes in SANE_Function, WOMAC_Dysfunction, and WOMAC_Aggregate over the 4-week intervention period. The α level was set a priori at .05 for all analyses. To assess agreement, or if the SANE_Function provided identical results to the WOMAC_Dysfunction and WOMAC_Aggregate scores, we calculated the Lin concordance correlation coefficient (Rc) and corresponding 95% confidence intervals (CIs).³⁴  The definitions regarding the strength of the Rc are arbitrary, with 1.00 representing full agreement and 0 representing no agreement. Therefore, we chose to classify the strength of all associations and agreement statistics for the current study as negligible (0.0–0.3), weak (0.31–0.5), moderate (0.51–0.7), high (0.71– 0.9), or very high (0.9–1.0).³⁵  The Statistical Package for the Social Sciences (version 19.0; IBM Corp, Armonk, NY) was used to assess all bivariate associations.

All initial associations reflected a total of 36 participants with knee OA (Table). Three participants (8.3%) reported a unilateral total knee arthroplasty. We found baseline SANE_Function (W = 0.94; P = .05) and WOMAC_Aggregate scores (W = 0.84; P > .001), as well as 4-week SANE_Function (W = 0.97; P = .001), 4-week WOMAC_Dysfunction (W = 0.92; P = .01), 4-week WOMAC_Aggregate (W = 0.93; P = .03) scores, and change in SANE_Function score (W = 0.33; P < .001) to be nonnormally distributed; therefore, Spearman rank order correlations were used for all analyses. Participants with a higher SANE_Function score demonstrated a lower WOMAC_Dysfunction score (weak association: r_s = –0.44, P = .007; Figure 1) and a lower WOMAC_Aggregate score (weak association: r_s = –0.46, P = .005) at baseline. Similarly, participants with a higher SANE_Function score demonstrated a lower WOMAC_Dysfunction score (moderate association: r_s = −0.69, P < .001; Figure 2) and a lower WOMAC_Aggregate score (moderate association: r_s = –0.65, P < .001) at the 4-week time point (Figure 2). A non–statistically significant and weak association was present between the change in SANE_Function score and the changes in WOMAC_Dysfunction (negligible association: r_s = –0.17, P < .43; Figure 3) and WOMAC_Aggregate (negligible association: r_s = –0.26, P < .13) scores over the 4 weeks of the TE intervention when all participants were considered. After removal of 3 potential outliers who demonstrated a change in SANE_Function score of >130% (visible to the right of the scatterplot in Figure 3), we did not identify an increase in the overall strength of the association between the change in SANE_Function and the changes in WOMAC_Dysfunction (n = 33; negligible association: r_s = –0.21, P < .24; Table, Figure 4) scores or WOMAC_Aggregate (n = 33; negligible association: r_s = –0.19, P < .30) scores over the 4 weeks of the TE intervention. For the entire cohort, agreement was weak between the SANE_Function and WOMAC_Dysfunction scores (Rc = –0.32; 95% CI = –0.54, 0.07), as well as between the SANE_Function and WOMAC_Aggregate scores (Rc = –0.45; 95% CI = –0.68, 0.16) at baseline. We found negligible agreement between the SANE_Function and WOMAC_Dysfunction scores (Rc = –0.21; 95% CI = –0.34, –0.07), as well as between the SANE_Function and WOMAC_Aggregate scores (Rc = –0.12; 95% CI = –0.20, –0.04) at the 4-week time point. There was negligible agreement between the change in the SANE_Function score before and after removal of the outliers, respectively, for the changes in the WOMAC_Dysfunction (n = 36; Rc = –0.11; 95% CI = –0.23, –0.01; n = 33; Rc = –0.19; 95% CI = –0.47, –0.13) and WOMAC_Aggregate (n = 36; Rc = –0.001; 95% CI = –0.001, 0.00; n = 33; Rc = –0.00; 95% CI = –0.002, 0.001) scores.

The post hoc analyses were conducted without the 3 outliers who demonstrated changes in the SANE_Function score of >130%. An increase in SANE_Function score was significantly but weakly associated with a decrease in WOMAC_Pain score (r_s = –0.36, P = .04) and nonsignificantly weakly associated with a decrease in WOMAC_Stiffness score (r_s = –0.3, P = .1) over the 4-week TE intervention period.

The main findings of our study were that SANE_Function and lesser WOMAC_Dysfunction scores demonstrated moderate and weak associations, respectively, before and immediately after a 4-week TE program in individuals with knee OA (Figures 1 and 2). Similar weak and moderate associations, respectively, were evident between the SANE_Function and WOMAC_Aggregate scores before and immediately after a 4-week TE program suggesting that the SANE_Function score had a comparable association with WOMAC_Aggregate and WOMAC_Dysfunction scores. The negative direction of the association between the SANE_Function and WOMAC_Dysfunction scores reflects the nature of the 2 instruments. A score of 100% on the SANE_Function indicates the best possible participant function, whereas a normalized 100% on the WOMAC_Dysfunction demonstrates the worst possible function. It is interesting that a decrease in the WOMAC_Dysfunction score over the 4-week TE intervention was not associated with an increase in the SANE_Function score in the same participants. We found a weak association between a decrease in WOMAC_Pain score and an increase in SANE_Function score over the 4 weeks, suggesting that a change in the SANE_Function score may be influenced by a change in pain when performing activities that compose the WOMAC_Dysfunction score. It is possible that a change in overall function, as measured by the SANE_Function score, was influenced to some extent by a change in pain after a TE intervention. A change in the WOMAC_Stiffness score did not significantly influence the SANE_Function score. Additionally, we observed only negligible to weak agreement between SANE_Function and WOMAC_Dysfunction scores before and immediately after a 4-week TE program, as well as negligible agreement for the change in SANE_Function and WOMAC_Dysfunction scores over the same time, further indicating that information gleaned from these 2 PROs was not interchangeable.

In our study, the magnitudes of the associations between SANE_Function and WOMAC_Dysfunction scores in individuals with knee OA were smaller than those shown previously between the SANE and other PROs, such as the Lysholm score¹⁵  and the IKDC Index,²¹  that have been assessed in patients with acute knee injury or surgery. The strength of the association at the 4-week time point (r_s = –0.69) in our participants with knee OA was similar to associations between the SANE and IKDC scores at 12 months after knee injury (r = 0.65)²¹  as well as between the SANE and Lysholm scores at 24 months after ACL reconstruction (r = 0.65).¹⁵  Although the SANE_Function has not previously been compared with other PROs in patients with knee OA, the 4-week associations are consistent with those between the SANE and PROs in patients who may demonstrate persistent or chronic disability at 12 months and 24 months after acute knee injury.^15,21 

Contrary to our hypothesis, an improvement in SANE_Function was not associated with a decrease in WOMAC_Dysfunction (Figures 3 and 4). Additionally, an improvement in SANE_Function was only weakly associated with a decrease in WOMAC_Pain (r_s = –0.36) or WOMAC_Stiffness (r_s = –0.30). Therefore, even though the SANE_Function score may be a reliable measure of self-reported function, individuals may express a change in overall function (SANE_Function score) differently than a change in dysfunction based on specific activities of daily living as measured by the WOMAC_Dysfunction score. On average, after the outliers were removed, participants in our study demonstrated greater changes in the SANE_Function score than in the WOMAC_Dysfunction score (Table). We cannot conclude from our analyses that the change in function evaluated with the SANE_Function score was systematically overestimated compared with the WOMAC_Dysfunction score. Yet our data suggest that individuals with knee OA described a change in function differently if they were asked a single global question (SANE_Function) compared with a series of questions related to tasks that typically cause pain in patients with knee OA (WOMAC_Dysfunction). Individuals may experience significant disability during many of the activities that are included in the WOMAC_Dysfunction and therefore decrease their exposure to these activities during daily life. It is possible that the improvements individuals demonstrate in the SANE_Function may be specific to a limited number of activities they continue to pursue in daily life. Subsequently, individuals may continue to indicate high levels of disability when specifically asked on the WOMAC about activities they may not have pursued recently due to a previous history of dysfunction.

Furthermore, we were unable to discern whether the SANE_Function or WOMAC_Dysfunction score was a more valid measure of the change in overall patient-reported function. Our analysis was meant to evaluate if these instruments were related before and after a 4-week TE intervention and if the self-reported response to the TE intervention could be measured similarly with both instruments. Compared with the WOMAC_Dysfunction score, the SANE_Function score is less burdensome for the patient to complete and for the clinician to administer. Therefore, a strong association between the SANE_Function and WOMAC_Dysfunction scores would have provided evidence to clinicians that the former could be used to evaluate similar information about physical function and disability as traditionally assessed with the latter. However, our study suggests that the SANE_Function and the WOMAC_Dysfunction cannot be used interchangeably to evaluate the self-reported functional capacity of individuals with knee OA. Our data indicate that these 2 measures may be assessing different aspects of self-reported disability or that the SANE_Function may not be a valid measure of self-reported disability in individuals with knee OA. Yet whether a change in the SANE_Function score can be useful for predicting other important outcomes related to knee OA remains unknown. For example, we did not determine whether a change in the SANE_Function or WOMAC_Dysfunction score is better able to identify the level of patient satisfaction with the intervention, tolerability of the intervention, knee-related confidence, or likelihood of requiring knee arthroplasty.³⁶ 

Given the study design, we were unable to determine if clinicians should administer the SANE_Function. We can conclude only that the SANE_Function is not measuring aspects of self-reported function similar to those measured by the WOMAC_Dysfunction. We were also unable to determine if a change in either of these PROs (SANE_Function or WOMAC_Dysfunction) was associated with a change in the physiological progression of OA. Further, the SANE_Function does not allow a clinician to determine which activities cause the most disability to the patient or, in the case of a change in SANE_Function score, which activities caused an overall change in patient function after an intervention. Therefore, due to the nature of the SANE_Function, clinicians may need to follow up with additional questions to fully understand the cause of self-reported disability. It is possible that the order of testing influenced the results of our study, as the WOMAC scores were collected before the SANE score in all participants. Although a potential order bias was consistent at baseline and the 4-week posttest, thereby affecting the change scores equally, future investigators should randomize the order of testing in case completing 1 PRO influences how participants respond to questions on another PRO. Our study did not exclude participants based on level of patient-reported functionality at the beginning of the study, which is evident from our large range for both the SANE_Function (range = 10%–98%) and WOMAC_Dysfunction (range = 23.53%–81.18%) scores at baseline. It is possible that SANE_Function and WOMAC_Dysfunction scores may be associated differently based on the magnitude of functionality in patients with knee OA. We tested a relatively small sample of individuals (N = 36). Future researchers should evaluate larger samples to assess the utility of the SANE_Function in different subsets of individuals with knee OA based on level of patient-reported functionality or radiographic knee OA severity. The current study was part of a project that assessed the ability of conventional strength training and TENS to improve muscle function compared with sham TENS with conventional strength training and conventional strength training alone³⁷ ; we did not find differences in the WOMAC Index or WOMAC subscales between groups³⁷  and, therefore, evaluated all participants together in this study. We lacked a large enough sample to identify differences in associations among patients who underwent specific interventions, yet it is possible that participants whose function improved more over the 4-week intervention would demonstrate different associations between the SANE and WOMAC scores compared with those who had less improvement. Further research is necessary to determine the utility of the SANE_Function in the clinical setting for patients with knee OA.

In conclusion, we found that the SANE_Function and WOMAC_Dysfunction scores demonstrated moderate and low associations, respectively, before and immediately after a 4-week TE program in individuals with knee OA. Changes in the SANE_Function and WOMAC_Dysfunction scores over the 4-week TE intervention period were not associated. Agreement was negligible to weak between the SANE_Function and WOMAC_Dysfunction scores, respectively, before and immediately after a 4-week TE program, as well as negligible for the change in SANE_Function and WOMAC_Dysfunction scores over the 4-week TE program. Therefore, the SANE_Function and WOMAC_Dysfunction should not be used interchangeably to determine a therapeutic response to an intervention.

Partially funded by the Orthopaedic Section of the American Physical Therapy Association and the National Athletic Trainers' Association Research & Education Foundation.