Female-Specific Knee Implants May Not Improve Outcomes

CHICAGO, Illinois — A narrower female-specific prosthesis (Gender Solutions, Zimmer Inc) might not be better than a standard unisex implant in women who undergo total knee arthroplasty, a new study finds.

Investigators report that improvement in knee pain and function scores, range of motion, patellar tracking, and rates of lateral release procedures during arthroplasty were similar for the artificial knee joint designed to better fit a woman's knee shape and for standard older models.

"If the gender-specific knee doesn't provide additional benefit, do we need to incur the extra cost?" asked principal investigator Alexander Sah, MD, an orthopedic surgeon from Fremont, California.

Still, Dr. Sah and his colleague, John Dearborn, MD, also from Fremont, found that the female-specific knee arthroplasty was associated with less need to undergo a second surgical procedure to manipulate the knee.

Dr. Sah presented the findings here at the American Academy of Orthopaedic Surgeons 2013 Annual Meeting.

The investigators retrospectively evaluated medical records and radiographs from women who underwent primary knee replacement at a single center before and after May 2006, when the US Food and Drug Administration approved the female-specific device.

The new prosthesis uses flex femoral components, but is narrower and thinner than other high-flex designs.

The researchers report 771 knee replacements performed from 2002 to 2006, , before the female-specific device became commercially available (the before group), and 1957 performed from 2006 to 2011 (the after group).

In the after group, female-specific prostheses were used in 77.4% of knees, flex implants were used in 20.6%, and standard implants were used in 1.9%.

In the before group, flex implants were used in 78.2% of knees and standard original implants were used in 21.8%. Height was similar in both groups, but women were slightly older in the after group than in the before group (mean age, 69.9 vs 67.1 years; P < .02), Dr. Sah reported.

Patellar tracking identified on radiographic evaluation was similar in the 2 groups, but there was less mediolateral component overhang in the after group. There was no statistically significant difference between the groups in improvement in the range of motion score on the Knee Society system, he noted.

Scores for knee pain and function were also similar in the 2 groups, except that function going up and down stairs was significantly better in the after group.

It is not clear why stair function was better in the after group. It is possible that the female-specific devices produce subtle changes in other areas of knee function that the scoring system does not detect, Dr. Sah explained.

The only other clinical outcome that was better in the after group was the rate of knee manipulation, under anesthesia, performed postoperatively.

Table. Outcomes for Primary Total Knee Replacements

Outcome	Before Group (n = 771)	After Group (n = 1957)	P Value
Femoral component overhang (%)	18.0	1.0	<.005
Stair function score	39.0	43.9	<.030
Postoperative knee manipulation (%)	8.2	6.1	<.010

Manipulation under general anesthesia "is a marker of knee stiffness" and involves a second surgery that patients want to avoid, session moderator Jeffrey Geller, MD, told Medscape Medical News. A lower manipulation rate is therefore an important benefit, said Dr. Geller, who is from Columbia University in New York City.

He added that the narrower female-specific knee replacement also provides surgeons with more sizing options. This, Dr. Geller said, could decrease the complication rate in both women and men who need a narrower knee implant.

"Having more sizing options, whether the patient is male or female, can only improve outcomes," Dr. Geller said.

Before 2006, the most frequent sizes used by surgeons were E (54%) and D (24%), according to Dr. Sah. After 2006, the most common size remained E (51%), but next most common was size F (38%).

The investigators suggest that surgeons feel more comfortable using a longer but narrower, thinner implant to avoid femoral component overhang.

Dr. Sah and Dr. Dearborn report receiving research funding from Zimmer Inc. Dr. Geller has disclosed no relevant financial relationships.

American Academy of Orthopaedic Surgeons (AAOS) 2013 Annual Meeting: Paper 351. Presented March 21, 2013.