Validity of compressive leg checking in measuring artificial leg-length inequality

To determine the accuracy of instrumented prone compressive leg checking.

Repeated measures (n = 26) on single subjects (n = 3).

Chiropractic college research clinic.

A pair of surgical boots were modified to permit continuous measurement of leg-length inequality (LLI). Multiple prone leg-check observations of a blinded examiner on 3 subjects were tested against artificial LLI that was created by randomly inserting 0 to 6 1.6-mm shims in either boot. Accuracy was assessed both within observations (observed versus artificial LLI) and between observations (observed versus artificial changes in LLI). The intraclass correlation coefficient (ICC), Lin's concordance correlation coefficient (CCC), Bland-Altman limits of agreement, and linear regression statistics were obtained to determine the reliability and validity of compressive leg checking compared to a reference standard.

For each shim condition, test-retest reliability was excellent (ICC =.85 and CCC = 0.95). The 95% confidence interval for the limits of agreement for observed versus artificial change in LLI was -5.44 to 5.67. The observed and artificial LLI shared 87% of their variation within observations (n = 78) and 88% between observations (n = 75). The mean examiner error was 1.72 mm and 2.01 mm, respectively.

Compressive leg checking seems highly accurate, detecting artificial changes in leg length +/-1.87 mm, and thus possesses concurrent validity assessed against artificial LLI. Pre-leg-check and post-leg-check differences should exceed 3.74 mm to be confident a real change has occurred. It is unknown whether compressive leg checking is clinically relevant.