We hypothesized that, in musculoskeletal RCTs using PROMs, higher baseline scores and the clustering of follow-up scores near the upper bound (i.e., ceiling effect) compress variability and attenuate measurable between-group differences, thereby lowering the likelihood of observing a statistically significant effect. We therefore examined how score distributions at pre-treatment and follow-up influence the likelihood of detecting between-group differences.

We conducted a meta-epidemiologic study of RCTs, published between 2015 and 2024, that compared treatment effects on musculoskeletal disorders between two study groups using PROMs. The observed distributions of the PROM scores at baseline and follow-up were collected from the included studies. All PROM scores were rescaled to 0-100 with higher scores indicating better health. The likelihood of observing a statistically significant difference in PROM scores between the study groups was examined by calculating the score difference required to achieve a p-value < 0.05.

A total of 255 RCTs were included. PROM scores improved from baseline to follow-up in most studies (98%), with a mean change of +28 points. The correlation coefficient between the mean baseline score and mean score change was -0.66 (95% CI -0.72 - -0.59) indicating that higher baseline scores were associated with lower score change. Additionally, there was a moderate correlation between the mean and SD of PROM scores at follow-up (-0.39; 95% CI -0.48 - -0.28). The mean likelihood of detecting a difference was 65% (SD 11%) at baseline and 65% (SD 11%) at follow-up. The likelihood reached the 80% benchmark in only 8.5% and 8.1% of the studies at baseline and follow-up, respectively.

The concentration of PROM score distributions towards the high end of the scale, especially when higher baseline scores are present, diminishes the likelihood of detecting significant differences between study groups, particularly at follow-up assessments in studies analyzing musculoskeletal complaints. This underscores the importance of critically evaluating the conclusions drawn from these studies.