Suzie Twiss had been having trouble with her knee for several years. When it finally got to the point where the 59-year-old Prineville woman had a hard time hunting, she decided to do something about it. An MRI showed that the cartilage on the outside of her right knee had worn away, but the other side of the joint was still in good shape. Her orthopedic surgeon gave a her a choice: Get the knee replacement immediately or wait three weeks for a new surgical robot that could shorten the recovery time and perhaps extend the life of the implant.

“He had told me there would be a 30 percent faster recovery,” Twiss said. “So, the amount of time I would be waiting would be used in recovery anyway.”

Twiss decided to wait for the robot. On Sept. 30, she became the first patient at St. Charles Bend to have a partial knee replacement done with a new Mako robotic arm. The device allows the surgeon to plan out the surgery on a computer and then prevents the doctor from accidentally cutting beyond those parameters.

It’s too soon to determine whether the device will lead to better outcomes for knee and hip replacements. But early results show the computerized system improves alignment of joints which doctors say, should improve how long the implants will last.

“The holy grail is we’re trying to put artificial joint implants in people of any age and you can do anything you want and you don’t have to worry about it. It’s going to last you the rest of your life,” said Dr. Blake Nonweiler, an orthopedic surgeon at The Center: Orthopedic & Neurosurgical Care & Research, who replaced Twiss’ knee. “We’re obviously a long way off from that, but the thought is, if we’re putting them in more accurately, they’re going to last longer.”

Preserving anatomy

The Mako robot was developed by Florida-based Mako Surgical in 2006, but was acquired by medical device maker Stryker Corp. in 2013. The device has been approved by the FDA for partial knee and total hip replacements.

Many doctors believe partial knee replacements can offer patients a much better outcome if done correctly. In a total knee replacement, doctors shave off the bone from the ends of the thigh and shin bone, replace them with metal coverings with a plastic insert between them. Generally, both the anterior cruciate ligaments and the posterior cruciate ligaments — commonly called the ACL and PCL — which provide stability to the knee, are removed.

The partial knee replacement, on the other hand, preserves those ligaments, replacing only a portion of the knee where the cartilage has worn away.

“If you take out that disease portion and leave all the rest of the anatomy, those guys come back and say, ‘Hey, my pain is gone and my knee feels like my knee,’” Nonweiler said. “If you really nail the total knee guys down, most patients will say, ‘I’m aware I’ve got an artificial knee. It doesn’t hurt but it’s not the knee I was born with.”

But many doctors have shied away from partial knee replacements because of the difficulty in getting the alignment just right. It’s technically difficult to align the knee properly and just a few degrees of misalignment could cause the joint to fail. It’s one of the reasons why patients are more likely to need a revision surgery with partial knee replacement than with total knees.

Studies have shown that a misalignment of more than three degrees increases the risk that a partial joint replacement will fail.

Traditionally, surgeons have used erector set-type metal rods to judge the alignment and determine where to cut into the bone to place the metal implant.

“You’ve got a little rod, it hangs down, and you kind of eyeball it,” Nonweiler said. “It’s really no different than it was 30 years ago.”

For the past decade, many hospitals have adopted computer navigation to improve alignment and placement of the implant. But the procedure has remained tricky to pull off correctly. So while many surgeons have become proficient at partial knee replacements over time, some doctors with lower volumes gave up doing partial knees altogether, and opted for a total knee replacements instead. But that meant a longer recovery time for patients and a less natural feel to the knee. According to data from the American Joint Replacement Registry, the percentage of knees fixed with partial replacements has dropped for the past four years, from 6.3 percent in 2012 to 4.9 percent in 2015.

The Mako robot may start to reverse that trend. Partial knee replacements fail for three main reasons. The first is that the metal plate on the end of the tibia will loosen. That’s primarily a cementing issue.

The second is that the patient has arthritis in another part of the knee and as it progresses, will need another knee replacement procedure. That’s a patient selection issue.

The third reason is misalignment of the joint and that’s where surgeons say the Mako robot can help.

A study by researchers at the University of Glasgow in Scotland published earlier this year, randomly assigned patients to either robotic or standard partial knee replacements. After three months, patients underwent a CT scan to determine the alignment of the joint along three planes.

From every angle, alignment was more accurate with the robot-assisted surgery than with conventional techniques.

“It’s essentially a tool to help you do a better surgery and the main goal would be to reduce what we call outliers,” said Dr. Thomas Huff, an orthopedic surgeon with Oregon Health & Science University in Portland. “You come across patients with the knee coming out badly, malaligned or unstable or improperly sized. This could provide a number of checks against that.”

Outcomes unclear

What remains unknown is how big a difference the more accurate alignment will make in the long run. A 2014 study compared partial knee replacements with both standard and robotic techniques, but found no significant differences in outcomes between the two groups. Another study enrolled 30 patients who needed partial knee replacements on both knees. Doctors replaced one knee with standard techniques and the other with the robot. Again, there were no differences in outcomes a full year after the surgery.

Part of that could reflect the scoring systems used to evaluate patient outcomes after knee surgery, measuring factors such as pain, stiffness or functional limitation.

But while those include questions on things such as being able to walk stairs or getting in and out of car, they may not be sensitive enough to factor the advantages of robotic surgery over conventional surgery.

“How about your return to function? Are you doing everything you want to do or are you not doing some stuff because of your knee?” Nonweiler said. “I don’t anticipate ever doing another partial knee replacement without it.”

Nor do doctors know whether the better alignment will help the implants last longer. Results to date have been mixed.

A 2013 study reported on 620 patients who underwent robotic assisted partial knee replacement and found that only 1.1 percent of the implants had failed within an average of 2 years. While that represents a short time frame, the low failure rate had doctors enthusiastic about the potential lifespan of the implants.

On the other hand, a 2015 study found a three-year revision rate of 5.8 percent, which was not much different than the rates for conventional techniques.

“It hasn’t been around 20 or 30 years, so we really don’t know,” Nonweiler said. “We’re on mile two of the marathon.”

Helping the surgeon

The Mako robot doesn’t operate on its own. After planning the surgery on the computer, the surgeon holds the large robotic arm, and pulls a trigger to deploy a small burr that shaves away the predetermined segments of bone. If the doctor strays more than a millimeter off the plan, the robot shuts down, preventing excess bone removal or damage to nearby ligaments. In essence, it doesn’t allow the surgeon to color outside the lines.

The system can also be used for hip replacements, swapping out the burr for a cutting tool that resembles a spherical cheese grater.

“It’s a little bit more personalized, because everybody has a little bit different anatomy,” said Dr. James Hall, a surgeon with The Center who is using the Mako for hip replacements. “We’re good at doing it now, but not to the accuracy it will allow us.”

The Mako robot may make the most difference in places where surgeons just don’t have enough candidates for partial knee replacements to become proficient at the operation. A recent study found that high volume surgeons had a partial knee revision rate of less than 1 percent. Surgeons who performed eight to 12 partial knee replacements per year had a revision rate of 5 percent while surgeons who performed fewer than eight partial knees per year had a revision rate of 6 to 8 percent.

“Most of us who do partial knees will never do a huge volume of them,” said Dr. Tim Bollom, an orthopedic surgeon at The Center. “I think the advantage (of the robot) to patients is a very low variability in results. If you only do a certain number a year, which most surgeons in the United States do, this is a much safer reliable way for patients to preserve their joints.”

There are some small added risks for patients, including the additional exposure to radiation from the CT scan needed to create the computer model of the joint. The Mako system also requires the placement of tracking pins in the bones to allow the computer to track the knee in real time during the surgery. Some patients have experienced fractures at the drilling site for the pins.

For surgeons, the surgery takes a little bit longer than traditional knee surgery.

“We’re probably all in our learning curve right now. It takes twice as long under tourniquet for me to do a partial knee using a robot,” said Dr. Erin Finter, an orthopedic surgeon with Desert Orthopedics in Bend. “Hopefully when that learning curve gets down, the time to operate will go down too.”

The Mako robot system costs about $1.2 million per unit, with additional costs for maintenance and supplies. But that cost is primarily incurred by the hospital. Hospitals and doctors are paid a fixed amount for knee surgery regardless of what equipment is being used or how long the patient must stay in the hospital to recover.

Hall said some insurance companies have balked at paying for the CT scans needed to create the computer model beforehand. But because the device is FDA approved, most insurance companies will pay their standard rate for knee or hip replacements.

Jen Oulman, senior director for surgical services at St. Charles Health System, said other facilities in the Pacific Northwest who use the Mako robot have experienced a decline in the length of stay after surgery. Those savings could offset the costs of the robot.

Oulman said the hospital is tracking changes in both the length of stay after knee surgery and its revision rates to determine whether the robot is worth the cost.

“There are robots in the I-5 corridor, Seattle, Spokane, and we want to be able to provide the technology to our patients so they don’t have to go far away to get it,” she said.

There may also be a marketing advantage for the hospital, allowing them to keep more of their knee surgery patients in Bend, rather than lose them to other hospitals in the region with a robot.

“It’s absolutely going to be a marketing tool,” said Huff, the OHSU orthopedic surgeon. “I would place a wager that at St. Charles you will see the number of partial knees go up a lot.”

Twiss remains happy with the results of her surgery.

“I would do it again in a minute,” she said. “It was definitely worth it.”

She went home the same day and returned to work after a week. Now she can’t wait to get back to hunting after missing the past season because of her knee.

“I would have had to sit in one spot, and that’s not my idea of fun,” she said. “But I sure plan on it next year.”

— Reporter: 541-633-2162, mhawryluk@bendbulletin.com

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