Based on the mechanism that manipulates professional flight simulators, a new robot could help surgeons realign the largest bone in the body.
Robots in the operating room are on the rise. Led by the Da Vinci surgical robot, these machines improve surgeons’ dexterity in such intricate tasks as separating tumors from the voice box or removing the prostate. But Mohammad Abedin-Nasab, a professor of biomedical engineering at Rowan University, is working on a different kind of surgery. His robot, called Robossis, could help surgeons perform the tricky operation of realigning a broken femur, the bone between the knee and the hip.
When a patient arrives at the hospital with a broken femur, surgeons must straighten and stabilize the largest bone in the body. Sizeable muscles surround the femur, resisting and obstructing a surgeon’s ability to adjust the broken bone. Typically, coaxing the bone fragments back into a straight line requires hundreds of Newtons of force.
This physically demanding operation also calls for considerable accuracy. A misaligned fracture may leave a patient with knee pain or arthritis from a bone fixed in place at the wrong rotation. In the worst cases, patients might need a second surgery to correct the bone’s alignment. To align the bone properly, surgeons typically use X-Rays or fluoroscopy to check the bone’s position visually. But they usually need dozens of X-Rays, or around two minutes of fluoroscopy, and each image exposes the patient and the operating room staff to radiation.
An ideal solution would help surgeons align the broken bone and minimize imaging during and after surgery. So Abedin-Nasab and his students are creating a two-part technology: a robot to manipulate bone fragments and image-processing software to guide alignment without repeated radioimaging.
In addition to preventing misalignment and reducing radiation, Abedin-Nasab hopes to reduce the time of surgery dramatically. This would improve patient outcomes by lowering blood loss and infection risk while helping hospitals and surgeons lower costs.
The Robossis robot consists of two C-rings connected to one another by three long actuators. Both C-rings fit over the leg. One ring is stationary while the other is mobile, and each ring attaches to the leg bone using a series of pins. In an operation, a surgeon would guide both rings over a patient’s leg, and pin each ring to one fragment of the broken femur. He or she would then adjust the three actuators independently to position the mobile ring with in three-dimensional space. This would enable the surgeon to pull, align, and set the bone for healing.
The idea for the device grew out of the Gough-Stewart platform, the mechanism that professional flight simulators use to impart three-dimensional motion to the platform containing the pilot and training cockpit. The platform’s surface rests on six piston-style legs, each capable of single-axis motion along its length. As the spidery legs lengthen and shorten in various combinations, the platform pitches, rolls, and yaws with six degrees of freedom.
Other researchers have tried to extend the platform’s flexibility for large-scale surgical tasks like bone setting, but so far, it has been difficult to achieve the right combination of force, mobility, and precision. When Abedin-Nassab looked at the Gough-Stewart platform, he realized that while it readily manipulated objects above a surface, its many legs limit the available motion below that surface. “If you reduce the number of legs, you will be much more flexible in moving the main platform,” he said, referring to motion possible between the Gough-Stewart platform and its base.
His design turns the Gough-Stewart platform on its side and uses three legs rather than six to manipulate the position of the moveable C-ring. Since using only three piston-style legs gave him only three degrees of freedom, he added an active, controllable rotary joint to each leg to maintain capabilities in six directions. Abedin-Nasab did not start working on the robot with an application in mind. As his work progressed, however, he began visiting surgeries to see how he could put it to use.
Femur surgeries jumped out at him as one area where the robot could make an impact. The first femur operation he shadowed involved a two-year-old boy who had broken his leg falling off a Ferris wheel. The surgeon told him that the surgery would probably be easier than most, yet it still took two hours and required the help of multiple nurses. Reducing even a fraction of the labor, time, and radiation for this procedure would improve the operation dramatically, he decided.
With a mechanism design and a targeted application, Abedin-Nasab and his students began prototyping. The first version of Robossis featured two C-shaped rings connected by three legs, with both a rotary and a linear actuator controlling the motion of each leg. They tested the device by aligning model femur bones tethered to thick elastic bands simulating thigh muscles. The first prototype could overcome 200 N of resistance from the elastic bands, but only with motion errors.
After investigation, it became clear that most of the error arose from a single source: the universal joint between the leg and the moving ring. Machining tighter joints brought the new prototype’s maximum motion error down to one percent, so that it mapped a specified trajectory to within about a millimeter. The second prototype’s rings now span three-quarters of a circle, enabling Abedin-Nassab to position the legs farther apart from each other for more stability.
To complement the physical device, Abedin-Nassab and his team are developing software to plan a path for bone positioning, informing a surgeon’s choices. The software starts after surgeons take two orthogonal X-rays of the broken bone. For patients with a single broken bone, the software uses an image of the unbroken femur in the other leg as a template to determine optimal alignment of the fracture.
For more complex fractures or fractures of both legs, the software will compare the images of the broken bones with a library of bone shapes to find a template to guide reconstruction. The software will also identify landmarks, such as the center of the hip’s ball-and-socket joint and the direction of the femur shaft, and use them to calculate how the robot should rotate and move the broken fragments to achieve the proper alignment.
Abedin-Nasab and his students continue to improve the robot. They have interviewed dozens of patients who have had broken femurs, and they hope to get input from surgeons as well. Next, they will tackle cadaver testing of the mechanism, and improve the ability of the software to assist physicians with aligning bone fragments. Eventually, they hope to develop a fully functioning device that receives FDA approval.
Abedin-Nasab points out that Robossis is not intended to replace surgeons. Its role will be to assist surgeons, who will still need to insert the nail that joins bone fragments. Yet he is hopeful that one day, this technology will join the ranks of robots making surgeries faster and safer for patients and surgeons alike.
Menaka Wilhelm is an independent technical writer.
Read more about Robotics on AABME.org.