A Passive Implant That Scales Muscle Force in Knee-replacement Surgery

Diminished musculoskeletal function, such as reduced joint strength or reduced range of motion, result from a variety of causes such as old age, muscle or nerve trauma, chemotherapy, or even surgery [1]–[3]. While limb function can be partially restored with a significant amount of physical therapy, oftentimes full limb function never fully returns and has lifealtering consequences. Inspired by robotics design technology, we are exploring the development of passive implants to reverse the problem of weak-joint function. Specifically, we are developing passive implants in the form of engineered mechanisms such as pulleys, gears, and levers that will scale up muscle force in order to increase joint strength or scale up muscle excursion in order to increase joint range of motion. The use of such devices offer an advantage because the devices can be custom designed based on the patient’s desired postsurgery function. In this paper, we present preliminary results from developing an implant to be used in conjunction with kneereplacement surgery. Knee replacement surgery is performed in nearly 600,000 patients a year in the U.S. alone [4]. The surgery replaces the biological knee joint with a single degree of freedom joint that mimics the knee’s kinematics [5]. However, following the surgery, knee joint strength typically decreases by 30% for a variety of reasons, such as soft tissue disruption, implant pitting and fatigue, disturbance of the quadriceps mechanism (moment arms), tibiofemoral capsular dislocation, even though range of motion is unaffected [6]– [10]. This loss of knee strength affects the activities of daily living such as stair climbing and chair rising. We show through simulation that using a passive engineering mechanism in the form of a pulley between the quadriceps muscle and patella restores knee joint strength while sacrificing some knee range of motion (Figure 1). Losing some range of motion in this surgery is acceptable since only about 105◦ flexion is required at the knee joint for most activities of daily living [11], [12] and usually up to 160◦ of flexion is available after knee replacement surgery [12].