MEASUREMENT OF TIBIAL DISPLACEMENT DURING THE ANTERIOR DRAWER TEST, USING A MAGNETIC TRACKING DEVICE.

Abstract

Introduction: The anterior drawer test for anterior cruciate ligament (ACL) deficiency, requires a subjective assessment of joint movement, as the tibia is pulled forward. The aim of this study was to objectively quantify this movement using a magnetic tracking device.

Materials and Methods: Ten patients aged 24 to 44 years were assessed as having unilateral ACL deficiency with conventional clinical tests. These patients were then re-assessed using a magnetic tracking device (Polhemus Fastrak). Patients had magnetic sensors attached around their femurs and tibias using elasticated Velcro straps. The Anterior Drawer test was then performed with the patient lying within range of the system’s magnetic source. The test was performed three times on the normal and injured knees of each patient, using a spring balance to apply a standard 20 lb (=89 N) force. During the tests, sensor position and orientation data was collected with an accuracy better than 1 mm and 1 degree, respectively. The data was sampled at 10Hz and stored on a computer for post-test analysis. This analysis deduced the tibial displacement resulting from each anterior drawer.

Results: During the anterior drawer test the supine patient’s knee is in 90 degrees flexion, with the foot planted on the examination couch. As the tibia is pulled anteriorly, it rotates upwards from the foot and the femur experiences a corresponding rotation from the hip. These complex coupled movements are best quantified in terms of absolute displacement of the tibia from the femur. In the normal knees, the mean displacement of the tibia from the femur was 4.2 mm (SD=1.6). In comparison the ACL deficient knees had a mean displacement of 6.3 mm (SD=2.9). This is 50 % more. A paired t test of this data showed a highly significant difference, with P = 0.005.

Conclusion: This study has quantified the movement produced during the Anterior Draw test for ACL deficiency. The tracker’s lightweight sensors caused minimal disturbance to the established clinical test. The system therefore provides objective measurement data to augment the clinicians subjective assessment.