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Abstract
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Introduction Previously, in patients with adult spinal deformity, 16 of the non-management items of the SRS-instrument showed a significantly better fit to the theoretical four-factor model (pain, function, self-image, mental health) than did all 20 items. The worst-fitting item per domain was recommended for exclusion (Q17, sick days; Q15, financial difficulties; Q14 personal relationships; Q3, nervous, respectively). Whether the same phenomenon is observed in data from younger (< 20y) patients, for whom the questionnaire was originally designed, is not currently known.
Methods This was a cross-sectional evaluation of the SRS-instrument's factor structure and its equivalence across different language versions in young (< 20y) deformity patients. Confirmatory factor analysis was performed on the 20 non-management items of the questionnaire completed by 3440 adolescents with spinal deformity (2746 English-speaking, 206 Spanish, 223 Italian, and 265 German; 70% female; mean age,14.9 ± 2.2 y), to compare the relative fit of the data to a 20-item single-factor structure, a 20-item 4-factor structure, and a 16-item 4-factor structure. Item loading and the goodness of fit were determined from the root mean square error of approximation (RMSEA) and the comparative fit index (CFI). Equivalence of item-loading was compared across languages.
Results Compared with the 20-item version, the 16-item solution significantly increased the fit (p < 0.001) across all language versions, to achieve good model fit (CFI=0.96, RMSE=0.06; Figure 1). For both 16-item and 20-item models, equivalence across languages was not reached, with some items showing weaker item-loading for some languages, in particular the German-language version.
Conclusion Also in patients with adolescent idiopathic scoliosis, the shorter version of the SRS-instrument shows a better fit to the intended 4-factor structure. The wording of some of the items, and/or their equivalence across language versions, may need to be addressed. Questionnaire completion can be a burden for patients; if a shorter, more structurally valid version is available, its use should be encouraged. This shorter version of the SRS-instrument, with removal of ill-fitting items, should deliver more meaningful information on patient-reported outcomes and may also serve to improve compliance with questionnaire completion.
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