A device for the evaluation of knee rotational-laxity side-to-side difference: Accuracy and repeatability assessment

Abstract

Introduction: When assessing knee ligament injuries, clinicians consider the side-to-side difference in knee rotational-laxity to be an essential information. Although, normally, this difference is assessed through a qualitative physical examination, arthrometers are available that allow a quantitative and standardized assessmentby imposing a preset torque and by measuring the consequent femorotibial rotation. This rotation measurement is affected by uncertainties due to soft tissue deformation and to lack of control of the subject posture and muscle contraction level. Since measurements on right (R) and left (L) knees are carried out in separate tests, relevant results can be affected by different uncertainties that reflect on an inaccurate assessment of side-to-side differences. The aim of the present study was to overcome this limitation by designing and validating a non-invasive bilateral rotational kneearthrometer (BiRKA) that allows the simultaneous assessment of both knees while the subject assumes a symmetrical, controllable, and reproducible posture and minimizes soft tissue artifacts. Materials and methods: The BiRKA consists of ametal frame on which a compound pulley is mounted. A torque, the magnitude of which can be adjusted through a dynamometric lever, is applied to a central pulley and equally transferred to two pulleys the axes of which are aligned with the longitudinal axes of the subject’s tibias. The latter pulleys, which can rotate independently, are connected to the subject shanks through two modified Aircast boots which minimize ankle movement and soft tissue artifacts. The rotation of each pulley [...] is measured by means of an electronic goniometer and an estimate of the difference [...] between the R and L knee rotation is made available. The device allows the generation of both internal and external torques with the patient in either supine or prone position with different knee joint flexion angles. For validation purposes, measurements were carried out with the subject prone on a table with the knees flexed at 30◦. Belts were used to fasten the subject to the table. Accuracy assessment – Two subjects, with no apparent deficit at their knees, underwent simultaneous computed tomography scanning (CT) and BiRKA assessment. The CT scan provided benchmark values of knee rotation and the difference between these values and those provided by the BiRKA was assumed to be an estimate of the inaccuracy of the latter device. Two loading conditions were tested (external torque = 0 and 10 Nm). Repeatability assessment – Three subjects were assessed by the same operator three times in three different days (external torque = 10 Nm). Results: Accuracy – Femorotibial rotation angles were consistently overestimated by the BiRKA and, within each subject, R and L errors were comparable. The [...] values measured using the CT scans and the BiRKA differed by 1.1° for S1 and 3° for S2. Repeatability – The mean absolute deviation of was [...] 1.3°, 0.3°, 1.1° for the three subjects, respectively. Discussion : A non invasive device able to measure the sideto-side difference in knee rotational-laxity has been presented. Preliminary results have shown accuracy and repeatability levels that are adequate to reveal the presence and severity of knee lesions. Due to the presence of soft tissue deformation and undesired movement of adjacent joints, the actual femorotibial rotations were overestimated by the device. However, the inaccuracies that affected R and L knee rotations were similar and cancelled each other thus allowing for a reliable side-to-side difference evaluation.