To compare the 15-cm Schober test with the 10-cm Schober test and the intermalleolar distance (IMD) with the internal hip rotation (IHR).
A cross-sectional study was conducted among both healthy individuals and patients with axial spondyloarthritis (SpA).The spinal and hip mobility tests were assessed in all subjects. Agreement between the different tests was assessed with correlation coefficients, Bland and Altman agreement, and by cumulative probability plots. The component scores for lumbar flexion in the Bath Ankylosing Spondylitis Metrology Index (BASMI) with use of either the 10-cm Schober test or the 15-cm Schober test were compared.
A total of 393 healthy individuals (51% males; mean age 44 years) and 90 patients with axial SpA (53% males; mean age 49 years) were included. A strong correlation was found between the 10-cm and 15-cm Schober test, in both healthy individuals (r = 0.89) and in patients (r = 0.93). The 10-cm Schober test was systematically lower than the 15-cm test with a mean ± SD difference of 1.5 ± 0.8 cm in patients, resulting in a higher total mean ± SD BASMI score of 0.4 ± 0.2 points. The correlation between IMD and IRH was weak, both in healthy individuals (r = 0.29) and in patients (r = 0.40).
The 10-cm and 15-cm Schober tests are highly correlated but, as expected, the 10-cm method is systematically lower than the 15-cm method. Therefore, these measures cannot be interchangeably used in the BASMI. The IMD and IHR are not correlated, indicating that they measure different aspects of hip mobility. Therefore, the IHR cannot be used as a substitute for the IMD.