Femoral overhang greater than 3 mm increases odds of painful TKR

KOHALA COAST, Hawaii — Total knee arthroplasties in which the femoral component overhangs the femur by 3 mm or more increases the probability that they will be painful, according to an orthopedic investigator.

“Having an overhang of more than 3 mm in any zone increased the risk of having any pain by 80%,” Ormonde M. Mahoney, MD, said at Orthopedics Today Hawaii 2009, held here. Gender and age were not major factors in the correlation.

“We think this is a significant issue, and even though we are not talking about revision-causing pain, this is limiting function,” he said. “This is something that we should be aware of and something we should be harping on with the manufacturers.”

Mahoney presented data from a series of 437 consecutive knees using the same device. All the surgeries were performed by a single surgeon between 2005 and 2006 using a high-flexion posterior stabilized device. All patients received the same rehabilitation.

Overall there were more women than men, but they were all similar in their preoperative and postoperative status in terms of alignment and pain between the genders.

During each of the surgeries, Mahoney and colleagues measured the overhang of the femoral component by using a ruler in 10 zones after permanent fixation.

Next, they assessed their principle function through chart reviews, interviews and examinations. They then correlated the data with the overhang measurements that were made intraoperatively to assess any effects the overhang might have had on knee pain in flexion.

“As far as the prevalence of overhang is concerned, as the components got bigger, the overhang became more prevalent, regardless of sex. This is a design issue in terms of the aspect ratio of modern total knee replacements,” he said.

Reference:
Mahoney OM. Gender vs. component fit. Presented at Orthopedics Today Hawaii 2009. Jan. 11-14, 2009. Kohala Coast, Hawaii.