AI-Powered Knee Surgery Delivers Better Results for Patients With Bowed Legs

A groundbreaking clinical trial shows that using AI to customize knee implants based on patients’ natural leg curvature leads to superior outcomes—especially for the one in three people with bowed legs.

​​​​​​​2025 Knee Society Award: The John N. Insall Award: Functional Versus Mechanical Alignment in Total Knee Arthroplasty: A Randomized Controlled Trial. ​​​​​​​Image Credit: sasirin pamai / Shutterstock

Patients whose legs are naturally curved are responding well to a novel form of knee-replacement surgery that uses AI to find the best alignment of the new components for their body.

The research, published in the Journal of Arthroplasty, involved patients who were randomly assigned to receive either conventional knee replacement surgery or the new 'functionally aligned' knee surgery.

"The results are positive for patients whose legs are naturally bowed," says lead author Associate Professor Simon Young of Waipapa Taumata Rau, University of Auckland.

Young is also an orthopaedic surgeon at Health New Zealand Te Whatu Ora Waitematā. He operates at the Elective Surgery Centre, part of Auckland's North Shore hospital campus. In 2017, they started using a robot for knee operations, which allows greater accuracy.

Traditionally, knee-replacement surgery is not universally successful, with around one in five patients not fully satisfied with their knee following the procedure.

A number of factors may contribute, but one reason could be that they had naturally bowed legs.

Young saw the potential to use the surgical robot to compare imaging of the patient's knee with thousands of possible options for placement of the replacement knee, allowing the surgeon to select the best match for that person's natural knee alignment.

Conventional surgery straightens the knee, whereas the patient's knee may never have been perfectly aligned with the leg bones to begin with.

"We developed an artificial intelligence (AI) algorithm to use with the robot to help surgeons position the knee components in a way that better matches each patient's natural anatomy," says Young, an associate professor in the University's Department of Orthopaedics.

Young says around thirty percent of the population has bowed legs, either naturally or because of conditions such as arthritis, which causes the knees to wear out.

"If you look around on a football pitch, you will see a lot of people who are young, fit, and healthy, who have naturally quite bowed legs. If, when they get older, they get arthritis and you put the new knee in straight, it will be in a position it has never been in their lives.

"For these people, usual knee replacements that assume the leg is straight may not work as well as ones that are functionally aligned."

The new AI software takes the patient's knee alignment and surrounding soft tissue and allows the surgeon to test different options – digitally.

"When we are in the operating room, we're virtually positioning the components, and then we consider that patient's native alignment and soft tissue tension. The computer model goes through and analyses the 20,000-25,000 potential positions and ranks them according to what would be optimal for that patient.

"We then we choose what we think is the best option."

Young and colleagues randomly assigned 244 knee-replacement patients to traditional or AI-assisted alignment for the study. They then followed the patients for two years, assessing their recovery with X-rays and questionnaires.

Overall, both groups had good outcomes and were happy with their knee replacements.

However, patients with more bowed legs naturally reported better results with the newer functionally aligned knees.

Based on the study, Young recommends surgeons consider a patient's natural leg shape when planning knee replacement surgery.

Young has developed an app for orthopaedic surgeons in New Zealand, Australia, and Asia who wish to use functional alignment.

The study won the prestigious John N. Insall, MD award from The Knee Society in the US. This meant that the paper that described the study was submitted to the Journal of Arthroplasty, and Young received US$1,000.

Young traveled to San Diego on March 14 to attend the society's annual meeting, where he received the award.

Young says research is ongoing to refine and expand the tool and ensure the best possible outcomes for all patients.