Is ROSA Always Reliable? High Accuracy but Notable Deviations in Medial Tibial Resection and Femoral Component Sizing in Mechanically-Aligned Total Knee Arthroplasty

For millions of people suffering from the grinding pain of worn-out knees, total knee replacement is a standard path back to a normal life. The surgery involves removing the damaged ends of the leg bones and capping them with smooth metal and plastic parts. For the new joint to work without pain or instability, these parts must be positioned with extreme precision. If the surgeon cuts the bone even a little too much or too little, or if the metal pieces are the wrong size, the knee can feel loose, stiff, or painful. In recent years, surgeons have begun using robotic assistants to help them make these cuts. These machines do not perform the surgery themselves; instead, they act as high-tech guides, using cameras and sensors to map the patient's unique leg shape and suggest exactly where to cut and how thick the new parts should be. The hope is that this technology removes the guesswork, ensuring every knee is aligned perfectly.

A team of researchers in South Korea decided to test whether this robotic guidance system, known as ROSA, truly delivers on that promise of perfect precision. They looked back at the records of 123 patients who had undergone their first knee replacement using this specific robot between 2022 and 2024. The surgeons followed a strict plan to align the legs straight, a method called mechanical alignment, which aims to make the leg stand as a straight line from hip to ankle. After the operations were finished, the researchers did something unusual: they measured the actual thickness of the bone that had been removed and the exact size of the metal parts that were implanted. They then compared these real-world measurements against the numbers the robot had predicted and displayed during the surgery. It was a direct check to see if the machine's digital map matched the physical reality of the bone.

The results showed that the robot is remarkably good at some things but less reliable at others. When it came to lining up the leg, the robot was highly accurate. Before the surgery, the average angle of the patients' legs showed a significant bend, but one year after the operation, the legs were nearly straight, with an average angle of just over one degree. The robot also did an excellent job of balancing the gaps between the bones when the knee was bent or straight, ensuring the new joint would not feel too tight or too loose on either side. The bone cuts on the thigh bone and the outer side of the shin bone matched the robot's predictions almost perfectly, with differences so small they were likely just measurement noise.

However, the study uncovered two specific areas where the robot's guidance drifted from reality. On the inner side of the shin bone, the robot consistently suggested removing more bone than was actually needed. The system predicted a cut of nearly four millimeters, but the surgeons ended up removing only about two and a half millimeters. This happened because the robot, which relies on a probe to touch the bone surface, likely touched the hard bone underneath the worn cartilage rather than the cartilage itself, leading it to think more bone needed to be taken off. Similarly, the robot tended to overestimate the size of the metal cap needed for the thigh bone. While the system suggested a size that was roughly one size larger than what the surgeons actually needed, the surgeons were able to correct this by using a manual measuring tool to verify the fit before finalizing the implant.

Despite these specific discrepancies, the patients did very well. By the end of the two-year follow-up, their knees were moving much better than before the surgery. The stiffness that had kept them from fully bending their knees was largely gone, and their ability to walk and function improved significantly. The researchers concluded that while the robotic system is a powerful and reliable tool for achieving straight alignment and balanced joints, it is not infallible. Surgeons should not blindly trust the machine's numbers for every single measurement. Instead, they should use the robot as a guide but double-check the thickness of the bone cut on the inner shin and the size of the thigh implant with their own tools. In this way, the technology can be used to its full potential, combining the precision of the machine with the critical judgment of the human surgeon.