A substantial percentage of persons (24% – 39%) in our study had clinically important improvements in WOMAC physical function 30 months after initial assessment. People who had clinically important improvement had a lower BMI, faster walking speeds, and fewer depressive symptoms across all three definitions of MCII unadjusted for other risk factors. After mutual adjustment for other risk factors, people who improved walked faster and did not have radiographic evidence of knee OA at baseline compared with those who did not improve.
The MCII allows one to estimate how many people had clinically meaningful improvement, and represents the smallest improvement in score which can be regarded as important. Limiting investigation of longitudinal changes to group level analysis, such as mean change and standard deviation summary statistics, may create a perception that subjects’ functional status is fixed. For instance, Botha-Scheepers and colleagues recently reported little change in functional limitations in a cohort of 115 people with symptomatic knee and hip OA over 2 years as evidenced by a mean increase of 2.2 and standard deviation of 12.7 in WOMAC physical function(29
). We found similar mean change in WOMAC physical function (mean=0.7, standard deviation of 9.8), however our evaluation of change at the level of the individual revealed a large percentage of people with substantial improvement. Improvements in functional limitation have been reported elsewhere. Most recently, Aysis and Dieppe found that 107 (19.6%) of 545 subjects with functional limitation at baseline had improvement when measured 8 years later, though these changes were not necessarily measured at a level of clinically meaningful improvement(14
Our study findings reveal that people with ROA had at least a 40% reduction in odds of clinically important improvement in function across all three definitions of MCII than those without ROA. Several studies support the notion that ROA influences changes in function. Roos and colleagues found that the presence of tibiofemoral OA was predictive of decline in sport and recreation activities 4 to 10 years later(30
), and Davis and coauthors reported people with ROA at baseline were more likely to report difficulty with mobility related activities 10 years later than those without ROA(31
). While some studies did not find an association between ROA status and function(32
), several reasons exist which may have contributed to this association in our study. First, we had ample power and heterogeneity of age to detect this association. We included 1801 people who were at least 50 years of age. Second, our primary outcome was clinically meaningful improvement in function, which was not used in previous studies(32
). Lastly, our study took knee radiographs with a standing fixed-flexion body position which has been shown to have high test-retest reliability(34
). Other studies used a full extended position of the knee(32
), which has been shown to be less reliable and accurate with estimating the severity of radiographic changes in the knee than a standing fixed-flexion body position(35
Walking speed over 20 meters was also found to be associated with meaningful improvement in function across all three definitions of MCII. This is consistent with previous studies which show that slow walking speed in older adults is associated with a variety of adverse outcomes including incident functional limitation(18
), hospital admission(36
), and mortality(37
). Our findings extend walking speed as a marker of meaningful improvement in younger adults over the age of 50 with or at high risk of knee OA. The speed of walking can be considered an estimate of walking ability. Given that the WOMAC physical function subscale measures self reported difficulty with walking and several tasks for which walking is prerequisite, we expected faster walking speeds to be associated of clinically important improvement in function.
Certainly it is plausible that interventions which took place over 30 months may be responsible for subsequent meaningful improvements in function. However, our cohort had mild to moderate limitations in function at baseline, as evidenced by a mean WOMAC physical function score was 18.7. Hence most study subjects would not have been referred for physical rehabilitation. For medications, we found persons taking prescription medication or those who had a steroid injection by the baseline examination to be less likely to have meaningful improvements in function in the unadjusted analysis. It is likely that these individuals had greater functional involvement and were hence less likely to improve. Thus, the association of persons starting to take medications over the 30 month period with meaningful improvement in function would be confounded by indication(40
There are some limitations in our study. First, we employed cutoff values for MCII from pervious studies that used patient anchored definitions of meaningful improvements, and not other anchoring methods such as clinician or consensus cutoffs. Second, Gill and colleagues have recently suggested that fluctuations between states of ability and inability are much higher when outcomes are measured monthly compared to longer assessment intervals(41
). Since we calculated change in WOMAC physical function using only two reference points, baseline and 30 months, it is possible the proportion of those with transient meaningful improvement on a monthly basis may be even more common than we estimated over 30 months. Future studies should employ repeated measures within shorter time intervals to investigate the cumulative frequency of meaningful improvement and time course of fluctuations in function. Third, we measured the construct of function using a self report instrument, and lower rates of improvement have been reported for performance based measures compared to self report measures(33
). Future study should incorporate both self-report and performance-based outcomes to better measure the construct of function. Fourth, we only used one measure, VAS, to estimate the construct of knee pain, which may underestimate the ability of knee pain to predict meaningful improvements in function. We were reluctant to use the WOMAC pain score as a modifiable factor due to its high correlation with the WOMAC physical function score(42
). Fifth, potential bias may exist in our estimate of 24–39% of subjects achieving MCII. We excluded those with new total joint replacements, and included those who had or were at high risk of symptomatic knee OA. Also, it is important to note that the percentage of those with meaningful improvement will naturally be higher using the MCII 17% cut-point compared with MCII 26%, given that less change is needed for meaningful improvement. Sixth, we arbitrarily selected a WOMAC physical function cutoff of 4/68 to represent those with at least a minimal amount of functional limitation. We have analyzed the data using other cutoffs (range 3–6) and found similar percentages of recovery across all methods of calculating MCII. Lastly, we did not differentiate between persons who had one versus two painful knees, which could have an effect on meaningful improvement in function. Future research should investigate if persons with one painful knee are more likely to have meaningful improvement in function compared with those with two painful knees.
Nonetheless, our study has two important clinically relevant conclusions. First, meaningful improvement is common among those with generally mild to moderate self reported limitations in function who have or are at high risk for knee over a two and a half year period. Our study found a robust percentage of people to have these improvements irrespective of the method used to estimate improvement. We emphasize that our definition of meaningful improvement excluded those with unchanged or worsening WOMAC physical function scores over 30 months. Second, people without radiographic evidence of knee OA and those with fast walking speeds are more likely to have improvements than those with ROA and slower walking speeds. Providers may want to consider these risk factors when determining who may benefit from therapeutic intervention.