OBJECTIVE: To assess the validity and reliability of prone and supine measurements of leg length inequality and to determine the potential use of measurements at the iliac crests and patient demographics as predictors to estimate standing leg length differential.

DESIGN: Repeated prone and supine measurements of leg length inequality were made by an experienced chiropractor and compared with iliac crest and femur head measurements made on X-rays of standing patients. Multiple regression analysis was performed.

SETTING: Private chiropractic practice.

PARTICIPANTS: The first 50 new patients with low back pain that were X-rayed were included in the study.

RESULTS: Intraexaminer reliability was excellent for the prone measurements. The supine tests were less reliable. The prone measurements were highly correlated with the standing X-ray femur head measurement. The supine measurements were poorly correlated. Measurements of deficiency at the iliac crests on X-ray were most highly correlated with X-ray measurements of discrepancy. In multiple regression, the prone measurements and duration of problem were the only significant predictors of standing X-ray difference.

CONCLUSIONS: In this study, crest measurements were made on X-ray; the degree of accuracy with which millimeter differences can be measured clinically on patients is unknown. In a few cases, the supine measurements were more accurate than the prone; however, the supine test validity was poor when compared with the standing X-ray measurements, and reliability was less than expected. Supine measurements should not be used to estimate standing leg length discrepancy in new low back pain patients but perhaps can be used in other clinically meaningful ways. Intraexaminer reliability of the prone measurements was higher, but further investigations need to focus on interexaminer reliability. The prone measurement as a predictor holds promise, but new measurement tools must be developed.

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