Response to “Functional Gains Using Radial and Combined Shockwave Therapy in the Management of Achilles Tendinopathy”

Dear Editor, In their recently published article, Robinson et al. compared the functional outcomes and safety of radial shockwave therapy (R-SWT) versus combined shockwave therapy (C-SWT) in patients with Achilles tendinopathy (AT) that is refractory to exercise therapy (1Robinson DM Tan CO Tenforde AS. Functional gains using radial and combined shockwave therapy in the management of Achilles tendinopathy.J Foot Ankle Surg. 2022; 61: 99-103Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar). We appreciate the authors’ clear description of the differences between the mechanism of action of R-SWT and C-SWT. The authors explicitly acknowledge that the study lacked a control group, was of limited sample size, and that there was no mean difference in the Victorian Institute of Sport Assessment-Achilles (VISA-A) score changes between those treated with C-SWT compared to R-SWT. However, authors do highlight that a higher proportion of C-SWT achieved the minimal clinically important difference (MCID) than the R-SWT group. We would like to address the usage of the MCID for the VISA-A as a point of consideration. The MCID is designed to aid in interpreting whether a change score is meaningful to patients. It is not clear how the authors decided upon an MCID of 12 points for mid-portion AT for use in their study. They cite one small study (n = 15) that calculated an MCID for mid-portion AT of 16 points after 12 weeks of treatment (2McCormack J Underwood F Slaven E Cappaert T. The minimum clinically important difference on the VISA-A and lefs for patients with insertional Achilles tendinopathy.Int J Sports Phys Ther. 2015; 10: 639-644PubMed Google Scholar). They also cite a narrative review (3Murphy M Rio E Debenham J Docking S Travers M Gibson W. Evaluating the progress of mid-portion Achilles tendinopathy during rehabilitation: a review of outcome measures for self-reported pain and function.Int J Sports Phys Ther. 2018; 13: 283-292Crossref PubMed Google Scholar) which refers back to the first study (2McCormack J Underwood F Slaven E Cappaert T. The minimum clinically important difference on the VISA-A and lefs for patients with insertional Achilles tendinopathy.Int J Sports Phys Ther. 2015; 10: 639-644PubMed Google Scholar). We are curious what the results of the present study would have shown had the authors used the MCID of 16 that they had cited. Of interest, we found a recent study (n = 61) that calculated an MCID of 14 points after 12 weeks of treatment (4Lagas IF van der Vlist AC van Oosterom RF van Veldhoven PLJ Reijman M Verhaar JAN de Vos R Victorian Institute of Sport Assessment-Achilles (VISA-A) Questionnaire – minimal clinically important difference for active people with midportion Achilles tendinopathy: a prospective cohort study.J Orthop Sports Phys Ther. 2021; 51: 510-516Crossref PubMed Scopus (9) Google Scholar). This study was likely not available at the time the authors were preparing the present manuscript. However, this does highlight how much variability exists in MCID calculations in the literature, and calls into question how useful these metrics are in helping to interpret research findings. The authors conclude “Our findings suggest combined radial and focused shockwave therapy may provide more predictable functional gains for treatment of Achilles tendinopathy compared to radial shockwave therapy.” Given that the authors found very small and nonsignificant differences in mean VISA-A scores between the C-SWT and R-SWT groups, and that the only positive findings were derived from an MCID value that lacks rigor and robustness, we find the conclusion to be somewhat optimistic. Even if a more robust MCID existed for the VISA-A in this population, using MCID to do a responder analysis should be done as a secondary analysis, and not as a main outcome or main focus when interpreting study results (5European Medicines AgencyGuideline on Multiplicity Issues in Clinical Trials, Committee for Human Medicinal Products. 12. European Medicines Agency, London2017Google Scholar). Although the concept of MCID is designed to aid interpretation of patient-reported outcomes, applying MCID thresholds to evaluate treatment effectiveness may overestimate their success rates (6Roos EM Boyle E Frobell RB Lohmander LS Ingelsrud LH. It is good to feel better, but better to feel good: whether a patient finds treatment ‘successful’ or not depends on the questions researchers ask.Br J Sports Med. 2019; 53: 1474-1478Crossref PubMed Scopus (23) Google Scholar). In conclusion, we consider the quality improvement report of Robinson et al. as an informative preliminary study that lays a foundation for future research. We would encourage the authors to base their conclusions more heavily on the between-group differences in change scores, and interpret the responder analysis with greater caution given the arbitrary choice of MCID threshold and recommended use of this statistic when comparing treatment groups. We would be grateful if the authors could clarify their choice of MCID and provide any further insights into how best to determine when differences between groups might be considered meaningful.