Brief report: validation of a system for automated measurement of knee laxity

Objective. To determine the accuracy and repeatability of an automated quantitative fluoroscopic imaging system for measuring knee laxity. Design. Cadaveric validation study. Background. Current methods of measuring anterior-posterior laxity lack sufficient accuracy and repeatability. A commercially developed fluoroscopic software package, capable of measuring laxity, required validation. Methods. Five human cadaveric knees were used. A constant force of 130 N was applied anteriorly and posteriorly in turn to the tibia of each knee with the femur fixed in 30degrees and 90degrees of flexion. Quantitative fluoroscopic measurements of anterior-posterior laxity were determined using image analysis software. Fluoroscopic results were compared to the true anterior-posterior displacements of the tibia, which were simultaneously recorded using linear transducers directly attached to the cadaveric specimens. Results. The quantitative fluoroscopic method underestimated laxity by an average of 0.40 mm with a root mean square error of 0.49 mm. The 95% confidence intervals for anterior and posterior laxity error were calculated to be -0.99 to 0.25 mm and -0.89 to 0.03 mm, respectively, where a negative error represents an underestimation. Conclusions. The quantitative fluoroscopic method offers a dramatic improvement in accuracy over current laxity measurement techniques and acceptable repeatability for assessing ligament damage.