High-flexion designs benefit stiffest knees most

By: Peter Pollack

By Peter Pollack

Using a high-flexion knee replacement rather than a standard design can result in a small but statistically significant increase in range of motion, and may offer the greatest benefit to patients with the stiffest knees, according to the paper, “Does Implant Design Affect Knee Flexion? A Simultaneous Bilateral TKA Randomized Controlled Trial,” which was presented by R. David Heekin, MD, yesterday.

“Several high-flexion (HF) knee designs require resection of an increased magnitude of bone from the posterior femoral condyle,” explained Douglas A. Dennis, MD, the lead author on the study. “In general, you don’t want to resect extra bone unless you know it’s going to be worth it. When you consider patients with life expectancies of 25 years or more, it’s important to know whether high-flexion designs provide better function.”

As identical as possible
The study compared standard and HF components from the same implant design system, used the same patellar components and tibial trays, and had a bilateral design, in which patients served as their own controls. Each patient received one standard knee and one HF knee implanted during the same surgical procedure. Preoperative and postoperative measurements were performed by a restricted number of blinded individuals who were specifically trained for participation in the study.

All patients and physical therapists were provided with preprogrammed personal digital assistants (PDAs) for data collection. Data from the PDAs were downloaded to a central database for analysis.

Overall, 93 patients underwent bilateral total knee arthroplasty (TKA) at eight centers in the United States. One patient withdrew, two patients had revision TKAs secondary to infection, eight patients lacked primary end-point data, and one patient had a protocol violation, leaving 81 patients (162 knees) for analysis.

The average age of the patients was 60.8 years; 99 percent were diagnosed with osteoarthritis, 66 percent were female, and the average body mass index (BMI) was 31.6.

Stiffer knees show greater improvement
At 12 months after bilateral surgery, researchers found that outcomes for knees that received high-flexion designs were statistically significantly in superiority to standard knees in several areas (
Table 1). Knees implanted with an HF design demonstrated higher flexion overall, but magnitudes were small, and patients with less than 120 degrees of preoperative flexion displayed the greatest gains at 12-month follow-up.

“My initial opinion was probably commensurate with most doctors … that you want to put an HF design in a patient with a trim leg and initial high flexion,” said

Dr. Dennis. “But the patients who actually gained the most were the ones who had the least motion preoperatively.”

Higher flexion equals higher satisfaction
As this study was limited to a single knee design, Dr. Dennis was quick to point out that the results may not translate to other total knee platforms. He also believes longer-term studies would be of value.

“Younger patients want higher functional performance—at 45, you want your knee to do a lot more than your grandma’s knee does. Patient satisfaction correlates with flexion, so we need to continue to study which implant design and patient factors make a difference,” he said.

“HF designs need to be carefully followed over the long term to see if the increased flexion exacts a price later. The more the patient can flex the knee, the higher the loads that are placed on the implant, so extremely high flexion could potentially be detrimental in the long term. The primary areas to investigate would be a potential for premature prosthetic loosening and premature polyethylene wear,” said Dr. Dennis. NOW

Jeff Murphy, MS, a DePuy representative, was also a co-author. Both Drs. Dennis and Heekin report ties to DePuy.

Peter Pollack is a staff writer for AAOS Now. He can be reached at