OBJECTIVE: To determine if prone leg length measurements for inequality are valid to estimate standing X-ray measured differences.

DESIGN: Leg length inequality (LLI) was measured, in millimeters, with each patient prone and with a standing X-ray, by an experienced chiropractor. Correlation between the two was calculated, and dependent t test performed.

SETTING: Private chiropractic practice.

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

RESULTS: Correlation between the two variables was 0.71. Standard error of estimation was 5.4 mm. In 54% of subjects, the prone measurement was within 3 mm of the X-ray LLI; in 12%, however, opposite legs were identified as being "shorter" between the two methods. In 76% of patients, prone measurements were within 6 mm of X-ray, but there was 12-mm difference between the two measurement methods in 8% of the comparisons.

CONCLUSIONS: Despite positive correlation, prone leg length measurements for inequality are not entirely valid estimates of standing X-ray differences. Large differences between prone and X-ray measurements in some cases indicate that one should be cautious when using the prone method alone to estimate leg length discrepancy. Additional research is needed to determine the causes of measurement differences between the two methods. Other methods for estimating standing leg length differential must be developed and evaluated for validity.

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