Bagherzadeh Cham et al. Prosth Orthot Int 2014;38: 310–315

Dear Editor,

We write with regard to the recent publication by Bagherzadeh Cham et al. ¹ to highlight a number of methodological issues that reduce our confidence in the authors’ conclusion that a rocker shoe can reduce pain, disability, and activity limitation in people with rheumatoid arthritis (RA).

We are concerned that the Foot Function Index (FFI) scores are unusually low when compared to similar investigations.^2–4 We suggest two sources of error with the scoring of this instrument. First, the authors describe scoring the visual analogue scales (VAS) of the FFI from 0 to 10, ¹ citing the work by Budiman-Mak et al. ⁵ in support of their approach. Budiman-Mak et al. ⁵ describe that the FFI is scored by dividing each VAS into 10 equal segments and assigning a number ranging from 0 to 9 to each segment. ⁵ This difference in scoring would account for a 10% error, equivalent to the minimum clinically important difference for the FFI. ⁶ Second, we expect the authors have not multiplied scores by 100 to eliminate the decimal point, ⁵ thus explaining why the FFI values are about one-tenth that reported in similar publications.^2–4

Given the aim of the study was “… to evaluate the effect of heel-to-toe rocker shoe on pain, disability and activity limitation in RA patients immediately, 7 and 30 days after their first visit,” ¹ we were surprised that FFI subscale scores were also compared, over time, in groups based on the region of pain (i.e. forefoot, rearfoot, and ankle pain groups). Unfortunately, these groups were not independent; that is, participants were included in more than one group. Nearly all participants (16/17) had forefoot pain and were grouped accordingly. Many of these participants (10/17) were also included in the ankle pain group (10/17) and a rearfoot pain group (6/17). Grouping participants this way biases our understanding of how the FFI scores change over time in people with pain in different regions, because many of the same FFI scores were included in more than one group. A better design would ensure the independence of groups by forming them based on each participant’s most painful region, as an illustrative example. In this way, the most painful region could be included as an independent variable in a more sophisticated inferential analysis that would separate the effect of the painful region from that of time.

A more sophisticated analysis would also be warranted, given the relationship between the dependent variables. If the assumptions of a multiple analysis of variance (MANOVA) were met, this analysis could control for the influence that changes in the FFI subscale scores have on each other and the FFI total score. With a more robust statistical technique, we wonder whether the differences observed in the FFI disability or FFI activity limitation subscales would be statistically significant.

We were disappointed that complete details of the inferential analysis were not reported, such as the type of post hoc test used, if any. More complete reporting should include the F ratio, degrees of freedom, p values, and effect size, as well as the results of the post hoc comparisons. With these additional details, we hope to affirm our assessment of the descriptive data (Table 4) that indicates the FFI scores declined significantly between the initial and 7-day visits, but not between the 7-day and 30-day visits. This has important implications for the accuracy of the discussion which implies that changes in the FFI scores occurred over the 7- and 30-day periods: “… use of high-top rocker shoes with wide toe box significantly decreased pain, disability and activity limitation … in the RA patient with foot problems after 7 and 30 days follow-up.” ¹

Our final concern is that the clinical implications and conclusions do not fairly acknowledge that participants were in the early stage of the disease process, and therefore, atypical of people living with RA who would usually be prescribed rocker shoes. Subjects in the study were all female, relatively young (47.2 ± 8.1 years), without fixed deformities, in remission (disease activity score 28: 1.7 ± 1.0), and had short disease duration (7.88 ± 7.2 years). The representativeness of the sample is an important limitation to acknowledge, so the findings of this work can be fairly applied to people in the early stage of the disease, not those with the advanced deformities who would normally receive such footwear and rocker soles.

We encourage Bagherzadeh Cham and colleagues to rectify the errors with FFI scoring and to republish their data with more complete reporting of the results, including post hoc analysis. We hope the findings of this investigation can then contribute to the growing body of evidence about the effect of rocker shoes on foot pain, disability, and activity limitation in people with RA.