Construct validity of lumbar extension measures in McKenzie's derangement syndrome.

The McKenzie treatment model advocates extension-based treatments for sub-groups of low back pain (LBP) patients and an improvement in extension range is seen as a positive outcome. The treatment model states that patients who fit the McKenzie derangement classification respond faster than other patients. The validity of this treatment model and of the clinical measures of extension has not yet been established. Fifty patients with LBP were classified as derangement (n=40) or non-derangement (n=10) based on a McKenzie assessment and then treated with extension procedures. Lumbar extension was measured in two positions, standing and prone, with three methods, inclinometer, Schober and finger tip to floor, on Day 1 and Day 5 of treatment. Patients completed a global perceived effect (GPE) scale on Day 5. Construct validity was tested, by comparing extension improvement and the GPE scores between the two groups. Responsiveness of the six extension measures was calculated. All patients gained extension range however the derangement group had significantly higher GPE scores and greater improvement in extension range. The modified Schober method in standing was the most responsive method for measuring lumbar extension. The results of this study support the measurement of lumbar extension, for patients, treated with extension procedures and provides evidence for the construct validity of one aspect of the McKenzie treatment model. The modified Schober method is the preferred protocol for a clinical setting.