Orthopedists improve outcomes in small-incision UKA with robot assistance

Researchers also found that the improved cut accuracy was associated with better outcomes.

Issue: June 2005

A group of British surgeons and engineers who used robotics in unicompartmental knee arthroplasty said the technology improved accuracy.

Lead investigator Justin P. Cobb, FRCS, of London, who presented findings at the American Academy of Orthopaedic Surgeons 72nd Annual Meeting, said he and his colleagues used the Acrobot System (The Acrobot Co. Ltd.) in 13 patients undergoing unicompartmental knee arthroplasty (UKA). He compared them to 14 other patients who received standard UKA using a minimal incision but without the assistance of a robotic tool to help the surgeon make cuts.

“If we could show a relationship between better accuracy and improved function [while using a robotic tool], I think that would be a pretty persuasive argument for robotics,” he said. “The problem is the information we collected [using robots in the past] hasn’t always been accurate enough.”

Preparation

British orthopedic surgeons used the Acrobot System (The Acrobot Co. Ltd.), which prevented them from cutting outside their preoperative plan.

Cobb and his colleagues used CT scans with coarse slices through hips and ankles, no slices of diaphyseal bone and fine slices of the knee joints. The surgeons also chose the size and orientation of the tibial and femoral implants preoperatively.

After randomizing patients to either the Acrobot group or control group, the investigators transferred the information to the Acrobot in the operating room or printed the information as a surgical aid, the investigators said in their abstract.

In performing the surgery with theAcrobot, “You start by detecting the femoral center and then you clamp the proximal tibia and distal femur so they don’t move and so you know where they are,” Cobb said. “Then, we gather information by touching points on the bone with the robot’s tip — 35 different ones in five zones on the articular surfaces — so it’s a pretty accurate registration.”

A safeguard

The surgeons confirmed the registration before milling the bony surfaces using an electric handpiece at 75,000 rpm mounted on the Acrobot. Only motion within the preoperative plan was permitted by the system’s “active constraint,” Cobb said.

“The software and the hardware stop you from going outside your preop plan. You can see that the last millimeter of cutting area goes from pink to green [on the screen]. That serves as a guide. It’s much easier to see things on a screen than through a small incision.”

Ultimately, this allows for accurate cuts that eliminate the need for cement and enable novel bone-conserving implants, he said.

In the prospective, randomized, double-blind controlled trial, the investigators measured outcomes using CT scans to compare the planned and achieved implant positions in 3-D. According to Cobb, Acrobot-assisted implantation was more accurate than conventional surgical techniques (P=.001), with smaller improvements correlated to less-accurate implantation.

Cobb said the surgeries involving the Acrobot took an additional 16 minutes on average compared to similar UKA procedures performed without the robotic tool but this difference did not reach significance. But the improvements in cutting accuracy and patients’ postop Knee Score results were worth the extra time, he said.

“This study has shown a direct correlation between accuracy and improvement in knee scores at as little as six weeks postoperatively,” the authors wrote. All of the Acrobot cases were implanted with tibio-femoral alignment in the coronal plane within ±2° of the planned position, while only 40% of the conventionally performed cases achieved this level of accuracy. There was also a significant enhancement in American Knee Society Scores at six weeks (P=.004), with the average of the Acrobot cases being double that of the conventional cases and the median being more than three times higher.

Mr. Cobb has a financial interest in a product mentioned in this article.

For more information:

Cobb JP, Henckel J, Richards RR, et al. Robot assistance improves outcomes in small incision unicompartmental knee arthroplasty. #75. Presented at the American Academy of Orthopaedic Surgeons 72nd Annual Meeting. Feb. 23-27, 2005. Washington.