Development of a Servo Hydraulic Test Bench and a Leg/Knee Surrogate for Knee Braces Dynamic Stiffness Evaluation

INTRODUCTION. Knee shows the highest injury incidence both in competitive and recreational modern alpine skiing. Ski industry efforts are oriented towards the identification of protective strategies involving intervention at the binding [1] or other protective devices such as knee braces [2]. In addition to studies regarding proprioception [3], this work addresses the quantitative comparison of knee brace stiffness under realistic conditions. METHODS. Analysis of World Cup knee accidents [4] allowed estimating knee I/E rotation and Ab/Adduction critical angle/speed values at different Flexion angles. Three rotational knee DOFs were included in the bench design, to produce a cyclic/dynamic imposed movement to leg rotation (1), ab/adduction (2) at different knee flexions (3) (Figure 1.a). A rotational servo hydraulic actuator (+/-130°, max 800°/s) and linear actuator (200mm) were introduced to drive the distal extremity of the tibia. A 6-axis load cell was applied to the proximal extremity of the thigh: the knee flexion angle was fixed at different angles. A 3 DOF instrumented cardan joint was applied to the knee joint of the leg surrogate (Figure 1.c): soft tissue was made of polyethylene foam (104G/30) after scanning the right leg of an elite downhill skier. Three knee braces were preliminary tested (Figure 1.b): two conventional knee braces (CKB1,2) and an inflatable prototype (SKBP2), tested at various pressures (Figure 1.d). RESULTS. The cyclic loading at each flexion angle with decoupled and coupled torsion/abduction conditions were obtained. Torque/rotation plots, (Figure 1.d) showed differences in stiffness and hysteresis of knee braces and evident effect of air pressure on stiffness. CKBs showed torsional stiffness of 0.1 Nm/° and Ab/Ad stiffness of 0.04 Nm/°: SKB at 0.8 bar showed about double stiffness values. DISCUSSION AND CONCLUSIONS. The bench allows a quantitative conventional comparison of knee braces under combined flexion/abduction/rotation angles: this is a progress towards knee braces standard test methods. Limitations regarding the biofidelity of skin surrogate and lack of knee drawer compliance will be overcome by further improvements of the leg surrogate, including a sensorized anatomical knee replica with artificial ligaments. REFERENCES. [1] Senner, V. et al. (2013), Sports Engineering. doi: 10.1007/s12283-013-0138-7. [2] Supej, M. et al. (2017), British Journal of Sports Medicine, 51(1). doi: 10.1136/bjsports-2016-096502. [3] Sugimoto D., et al., J Sport Rehabil. 2016 May;25(2):190-4. [4] Bere, T. et al. (2013), American Journal of Sports Medicine, 41(5), 1067–1073.