Repeatability and accuracy of a non-invasive method of measuring internal and external rotation of the tibia

The ability to quantify rotational laxity of the knee would increase understanding of functional rotatory instability, identify the best treatment methods for soft tissue injury, and have a role in diagnosis of soft tissue injury. This study aimed to report the reliability, repeatability and precision of a non-invasive adaptation of image-free navigation technology by comparing with a validated invasive system used for computer-assisted surgery.

Twelve cadaveric lower limbs were tested with a commercial image-free navigation system using passive trackers secured by bone screws. They were then tested a non-invasive fabric-strap system. Manual application of torque was used consistent with clinical examination to rotate the tibia to the end of internal rotation and external rotation range. Measurements were taken at 10° intervals from full extension to 90° flexion, and protocol was repeated twice using each system. Intraclass correlation coefficient (ICC) was used to reflect reliability of measurements. At each flexion interval, coefficient of repeatability (CR) was calculated for each system, and limits of agreement (LOA) were used to reflect agreement between the systems.

The results for internal and external rotation were combined throughout flexion: ICC invasive; 0.94 (0.86-0.99), non-invasive; 0.92 (0.7-0.99), CR invasive; 2.4° (1.3-4.8°), non-invasive; 3.5° (1.8-6.6), LOA; 8.2° (4.3-13.5).

Non-invasive optical tracker fixation gives improved agreement with a validated method of measurement compared with devices measuring tibial rotation by foot position. This system gives the added possibility of dynamic, weight-bearing testing in the clinically important range of 0°-30° knee flexion without the need for any limb restraint.