The purposes of this study were to assess the repeatability of measuring meniscus extrusion using ultrasound, to measure the amount of medial meniscus extrusion using ultrasound, and to analyze knee kinematics of both intact knees and injured knees (complete posterior root tear) under various loading conditions using a combination of the robotic testing system and ultrasound.
Ten fresh frozen porcine knees were used. Intra- and inter-observer ultrasound agreement was assessed by calculating intra-class correlation coefficients (ICC).
A varus torque up to 5 Nm, an external torque up to 5 Nm, and an axial load up to 150N were individually applied to both intact knees and injured knee at 30°, 45°, 60°, and 90° of the knee flexion. Ultrasound examination was performed during each of the previous external loads and the amount of meniscus extrusion was measured.
ICC demonstrated excellent intra- and inter-observer agreement in meniscus extrusion measurements. The amount of medial meniscus extrusion of intact knees under the unloaded condition was the smallest (1.2±0.6 mm) at 30°. When a varus torque was applied to the injured knee, meniscus extrusion was the largest (6.1±0.8 mm) at 90°. The difference in meniscus extrusion compared to the unloaded condition was largest in response to the varus torque at all knee flexion angles. A varus torque resulted in a significantly larger change in meniscus extrusion than an external torque at all flexion angles (all p<0.01) and an axial load at 30° (p=0.014).
Meniscus extrusion measurement using standardized ultrasound in combination with a robotic testing system has a high degree of repeatability. A complete posterior root tear changed knee kinematics, especially varus rotation, leading to an increase in medial meniscus extrusion. Regardless of meniscus injury status, meniscus extrusion increased with increased knee flexion angle.