Every year in the United States, more than one million people undergo total joint replacement surgeries. That number only increases as the older adult population grows and as the age at which people have these procedures drops. According to projections, by 2030 some four million Americans will receive new joints annually.
Joint replacement surgery is an effective solution for end-stage arthritis in terms of cost and clinical outcomes, but it’s not perfect. Minor variations in an implant’s placement and fit can make all the difference when it comes to residual joint pain after surgery; joint function; and the lifespan of the new joint.
Surgeons in Yale’s Department of Orthopaedics and Rehabilitation are pioneering computer modeling software and 3D printing to plan for and execute joint replacements with increased precision. The technologies reduce risks for older patients and lead to better outcomes and faster recovery.
3D technology in joint replacement surgery
3D printing, also known as additive manufacturing, is the process of creating a three-dimensional object by layering the material into the desired shape. The printer dispenses liquefied building materials as if they were ink. The method uses less material than so-called subtractive manufacturing, in which an object is cut out of a larger piece of material. The beauty of the technology in orthopaedics is that a 3D printer can create custom implants and patient-specific surgical instrumentation based on imaging of a patient’s affected limb.
“A 3D-printed implant we source from vendors allows us to customize the size of the plastic insert and cut out the smallest amount of the patient’s bone,” said Mary O’Connor, MD, director of the Center for Musculoskeletal Care at Yale School of Medicine and Yale New Haven Health. “In knee replacement surgery, this type of implant is closer to normal anatomy of the original joint.”
Imaging studies also enable surgeons to generate 3D computer-simulated models of patients’ joints, such as a shoulder, and rehearse the surgery virtually with different off-the-shelf implants until they find the perfect one. Based on the virtual surgery, the surgeon can order 3D-printed patient-specific guides and cutting blocks to ensure precision in bone resection and placement of implants.
“In a shoulder replacement, millimeters or a few degrees can make the difference between a replacement that lasts decades and one that only lasts a few years. By generating a patient-specific guide based on a 3D model, we’re able to put the implant in the perfect position at the time of surgery,” said Ken Donohue, MD, assistant professor of orthopaedics and rehabilitation.
In older adults, past injuries compounded by a lifetime of wear and tear can damage an ankle. But with the help of the subtalar joint just under the ankle, it can compensate for quite some time. This compensation has the downside of often causing severe deformities of the ankle joint itself. Eventually though, pain pushes a person to do something about it. By this point, the ankle has become so deformed and misaligned that it can be hard to visualize the original joint surface.
Raymond Walls, MD, assistant professor of orthopaedics and rehabilitation, uses preoperative CT scans and X-rays to determine the ideal alignment for ankle replacement surgery. For many years, ankle fusion was the only option for end-stage ankle arthritis because early implants did not perform well. “A better understanding of ankle kinematics as well as advances in implant technology and design means that current fourth-generation ankle replacements are now on a par with hip and knee implants,” said Walls.
Walls works with Wright Medical Technology, a global medical device company. Wright provides a virtual 3D model of the perfectly aligned ankle with the implants inserted, and a surgical guide that identifies bone cysts and bone spurs that must be addressed to prevent early failure. After Walls approves the plan, he receives custom 3D printed patient-specific guides and a model of the patient’s arthritic joint. During surgery, the surgeon places the guides, which are developed for every patient to fit their unique anatomy, directly on the patient’s bone. This process determines the exact placement of the cutting blocks to remove the precise amount of bone required for the implants. It decreases the risk of implant malalignment, which was previously a common cause of failure.
“We can predict the result we’re going to get preoperatively, and it allows me to use a smaller incision with less soft tissue dissection,” said Walls. This prediction is safer for the patient; makes for a faster surgery; and in general, a faster recovery. A smaller incision also lowers the risk of wound complications and has a better cosmetic appearance once everything has healed.
In shoulder replacement surgery, precision is crucial to the life of the implant. An implant that tilts too far back into the cup-shaped glenoid cavity can lead to early loosening and failure of the new joint. But a surgeon can’t visualize the placement of the original joint in surgery. “During surgery, you can’t see the scapula. You’re looking only at the surface of the glenoid. The relationship between the glenoid—the surface of the cup side of the joint—and the scapula is only known by diligent preoperative planning,” said Donohue.
For Donohue, that planning includes a 3D-printed model of the patient’s scapula. The model provides the surgeon with a unique opportunity to study the patient’s anatomy outside the body. Along with 3D computer modeling software, Donohue uses the model in preoperative planning as well as to run through the surgery in real time in the OR.
“In the operating room, we will hold the model of the patient’s scapula or glenoid, and position the patient-specific guide on that model to make sure that we have the correct orientation,” he explains. “Then we put the patient-specific guide directly on the patient’s scapula to allow us to put the implant in the proper position.”
Improper placement of an implant is a preventable complication of total joint replacement surgery that 3D printing and modeling software can resolve, said Daniel Wiznia, MD, assistant professor of orthopaedics and rehabilitation. Wiznia was trained as a surgeon to use these technologies, along with robotics and computer navigation, and he believes that as future surgeons come through their training, these technologies will become the standard of care.
“Accuracy and precision are far higher when you’re using technology for a total knee replacement,” he said. “So, if you’re using computer navigation, robotics, or 3D-printed instruments, you will have a far more accurate, consistent result. You’re not going to have as many outliers where you might have positioned the implant in a less than perfect position otherwise.”
Even with technological assistance for implant placement, off-the-shelf joints aren’t a perfect fit for everyone. After knee replacement surgery, patients may end up with what’s called “implant overhang,” in which the implant hangs over at least one side of the femur by three millimeters or more. “If there’s improper fit or sizing and the metal component of the knee replacement overhangs the native bone, that can be a source of residual pain,” O’Connor said.
Studies show that some 40% of men and a whopping 68% of women can have some degree of implant overhang. “Women do not do as well with knee replacement surgery as men,” O’Connor said. She addresses overhang and other potential drawbacks of off-the-shelf implants with 3D-printed knee implants. O’Connor works with Conformis, Inc., a Massachusetts-based manufacturer of custom-made joint replacements, to create personalized implants based on CT scans of patients’ legs from hip to ankle. The orthopaedic surgeon is particularly interested in getting this technology to women who could perhaps benefit most from the custom-designed joints.
“I see women in the clinic who share that they feel their orthopaedic surgeon didn’t listen to them regarding their knee pain. At Yale, we really try to listen and understand the impact of their knee pain on their quality of life,” she said. That was the case for Cheryl Elrick, of Clinton, Connecticut, who didn’t feel her previous orthopaedic surgeon was a good match for her. When she learned about a female surgeon who uses custom-made joint implants, she said, “It was a no-brainer.”
“It’s formed to your joint, it’s personalized. The tools, the inserts, are all specifically for you. Why would you do it any other way?” Elrick said. Elrick, who is 67, had lived with debilitating knee pain for 15 years. A registered nurse retired from the neonatal intensive care unit, she is a sought-after babysitter on her block. But less than a year ago, she couldn’t bear to walk across the street to her babysitting job. She had to drive the few hundred feet from her door to the neighbor’s. Now, just six months after her second knee surgery, Elrick is back to babysitting, walking her dog, gardening—all the things she had forgotten how to do without first loading up on ibuprofen. She forgets that she has artificial knee joints.
Elrick’s results may be due to stability that can be more reliably achieved with a custom knee implant. A custom implant, O’Connor said, gets the ligaments in the knee closest to the tension they had with the original joint. Achieving that perfect tension means a more stable joint throughout the range of motion—not just when the leg is held straight.
Elrick has nearly regained full range of motion in both her new knees. “It’s just incredible that I can do it,” she said. “Getting this knee replacement is the best thing I’ve ever done in my life, for sure—well, after getting married and after my two kids. It’s the fourth best thing.”