150 Chapter 8 Strengths and limitations This study has several strengths as we adhered to the CHAMP statement for analysis and reporting of the results and, to our knowledge, we performed the largest cohort study in AT patients. Nonetheless, this study has certain limitations that must be acknowledged. There was a substantial proportion of missing outcome data during the follow-up period. This issue arises due to the observational and longitudinal design of the study, in which the follow-up questionnaires were integrated into routine care. While we did not identify between-group differences in responders versus non-responders, the large proportion of missing outcome data warrants caution when interpreting the results. Additionally, the potential for unmeasured confounding must be considered as a limitation. While we adjusted for several known confounders, including age, sex, BMI, AAS, duration of symptoms, and baseline VISA-A score, there may be other unmeasured variables that could influence the outcomes. The nature of observational studies inherently limits our ability to control for all possible confounding factors. Thus, although the findings are statistically significant and clinically relevant, they should be interpreted as hypothesisgenerating rather than definitive.39 To provide a more comprehensive understanding of the results, we conducted a sensitivity analysis to explore the robustness of the findings. The sensitivity analysis revealed a clinically relevant difference, yet did not demonstrate statistical significance. One possible explanation for this discrepancy could be the substantial dispersion observed in the VISA-A scores (Supplementary File 1). The wide range of scores suggests considerable variability among the AT patients, which could impact the statistical significance of the findings. We did not observe any difference in treatment satisfaction between patients with low and high SES which may question the robustness of the findings of the VISA-A questionnaire. Recent studies have highlighted some shortcomings of the VISA-A, particularly concerning its content validity50-52. It is also unknown whether reduced health literacy influences the ability to complete the VISA-A, as shown in other PROMs used in musculoskeletal care.53 This is especially relevant in our study design. However, the VISA-A continues to demonstrate sufficient reliability and responsiveness30,51. Next to this, the VISA-A has been cross culturally adapted in numerous languages (including the Dutch version of the current study).54 We feel that the use of the VISA-A in our study is justified and that our study's findings and conclusions remain valid, but they should be considered along with the criticism on the psychometric properties of the VISA-A. Another limitation is that we did not obtain data on treatment adherence and guidance of physiotherapists during treatment. This hinders a comprehensive understanding of the factors influencing the outcomes of patients with low SES and impedes a thorough analysis of the barriers they may face in receiving appropriate care.
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