Mechanical engineer endeavors to improve hand surgeries

Sutures have been the primary way to connect muscles, tendons, or any biological tissue for 30,000 years. This fundamental method of sewing together living body parts has served humankind well, but Ravi Balasubramanian sees room for improvement. Through a new research project called REHand (for Re-Engineering the Hand) he is designing a mechanical implant that provides an alternative to the suture for attaching muscles to tendons in certain applications such as tendon transfer surgeries on patients with hand injuries. 

conventional suture

Figure 1: In a conventional suture procedure, the fingers do not close in completely on the object because of coupled finger movement.

Balasubramanian’s mechanical system

Figure 2:  Balasubramanian’s mechanical system features a hierarchy of interconnected pulleys and cables. The fingers close in completely around the object due to adaptive movement enabled by the pulley system.

Balasubramanian, an assistant professor of mechanical engineering, directs the robotics and human control systems lab at Oregon State, which uses robotic technology to study the human body and simultaneously draws inspiration from the human body for designing robotic systems. The cross-disciplinary nature of his work brings together surgeons, bio-material experts, and statisticians, facilitating his research to advance tendon transfer surgery.

Traditional tendon transfer surgery uses sutures to re-route a tendon from one non-functioning muscle to another. Some capability is restored this way, but not as much as Balasubramanian envisions. The human hand has nearly 40 muscles controlled by three nerves, so the loss of just one nerve can severely limit finger dexterity and lead to weak grasps. Patients can also suffer from “coupled” movements where fingers flex or extend together. “You lose one nerve, you can’t flex the fingers, which means you can’t hold a cup. You can’t hold a spoon. You can’t hold a pen. You can’t do anything, really,” he said.

Balasubramanian engineered a more efficient and responsive solution for patients with hand disabilities: an implant consisting of a hierarchy of mechanical pulleys and cables that naturally and passively move inside the human forearm as needed to enable normal hand function. A primary pulley implanted above the wrist would connect to a forearm muscle and slide and spin when the muscle is flexed. Two smaller pulleys with cables connected to the primary pulley and directly to the fingers would then slide and rotate in response, creating an interconnected system that allows for individual finger dexterity and a better grasp.

Balasubramanian compares his responsive pulley system to the way a motor controls the wheels of a car. If all four wheels moved at the same speed while making a turn, the wheels would slip and reduce the driver’s control — similar to how a suture patient has coupled finger movement and therefore limited control. But because cars have a differential system, each wheel moves at the appropriate speed to safely make the turn. Likewise, Balasubramanian’s system allows the fingers to operate individually and adapt when grasping objects and performing everyday tasks.

Although each pulley is very small, it can withstand up to 300 newtons — about 67 pounds of force. The system has no motor or sensors and, once implanted, will be invisible to the patient.

While the technology is in the prototype stage, Balasubramanian is conducting surgical trials on cadavers in Seattle. An advantage of the cadaver tests is they focus on how the implant works and how different materials will perform, without worry of biological tissue rejecting a foreign object.

Once testing goes to clinical trials with live subjects, the pulley system will need to be bio-compatible. Partnering with the University of Washington School of Medicine should help in finding patients, since it is one of the busiest trauma centers in the country. Also, nearly 20,000 tendon transfer surgeries are performed each year in the United States alone.

Kadee Mardula and Ravi BalasubramanianKadee Mardula, a graduate of the mechanical engineering master’s program, worked closely with Balasubramanian during cadaver trials. She took the lead in performing the first round of surgeries, which brought together her interests in anatomy, biology, and engineering.

“This interdisciplinary project is a wonderful mix for me. It gives everyone a new outlook on the problem and new ways to answer it, since we are each using our expertise in robotics, surgery, and biology,” she said.

After tests on 13 cadavers, Balasubramanian has seen some encouraging results. “One benefit we’re seeing is that the fingers will wrap around an object in a nicer manner to create better grasps,” he explained. “The second is that with the pulleys, the force required from the muscles could be significantly smaller than what is required with the suture. It’s less stressful on a patient who is already suffering from hand disability.”

Balasubramanian’s pioneering work earned him an Outstanding Researcher Award from the National Center for Simulation in Rehabilitation Research at the National Institutes of Health. Balasubramanian sees broad potential for his pulley system. People with hand injuries, palsies, brain trauma, or congenital defects could benefit. His design isn’t limited to the hand, but could be implanted wherever tendons connect to muscle.

— Abby P. Metzger

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