Gender-specific sizing for knee implants: Do the data confirm the need?

J. David Blaha, MD, answers 4 Questions about the data behind the claims and physician and industry responsibilities in promoting devices.

This month I have asked J. David Blaha, MD, to share his insight into gender-specific considerations in knee replacement with us. His opinions are based on his research and interpretation of the orthopedic literature. This issue is just one example of the many concerns that need to be considered in terms of the rising costs of implants and controlling health care costs. Does a new technology always justify the additional expense and can its impact on patient outcomes be documented? Here he provides us with his informed perspective on these topics.

— Douglas W. Jackson, MD
Chief Medical Editor

Douglas W. Jackson, MD: Frequently, my patients ask me if I use the gender-specific knee. They want to be certain they will get one that fits women. Is there any specific evidence to support the need for sex-specific sizing?

J. David Blaha, MD: Orthopedic surgeons who have evaluated the available evidence for sex-specific knee implant sizing have concluded that sex-specific implants do not appear to be necessary. Scientifically, to assert that such “gender” implants were necessary, evidence would need to be clear that statistically significant differences occur in the size and shape of the knee between men and women, and that the results of total knee replacement in women are inferior to those in men based on those differences.

J. David Blaha

For clinical success, a total joint prosthesis must fit the bones of the knee onto which it is implanted. Unquestionably, the size of the knee bones is, on average, smaller in women than in men. However, many men fall into the smaller size range, just as many women fall into the larger size range. This suggests that the stature of the individual, not the sex, is the important factor.

Arguably, small differences occur in the shape of a woman’s knee compared with that of a man. However, the reported results of careful measurement studies suggest that the differences are very small and likely are not clinically significant.

Most importantly, reported total knee replacement clinical outcomes for women are equal to or better than those for men when traditional, non-sex-specific, implants are used. Therefore, no evidence conclusively points to a clinical need for a sex-specific implant over an implant that is sized correctly for the individual patient.

Jackson: What then is the basis for sex-specific sizing as expressed by proponents?

Blaha: The proponents of sex-specific implants cite three anatomic differences in the female knee to justify the need for separate implants.

The first difference is a reduced anterior-posterior to medial-lateral ratio, known as the aspect ratio, of a woman’s knee compared with that of a man’s knee. If this difference were to exist, a femoral prosthesis appropriate for one aspect ratio may not fit the cut surfaces of a femur with a difference ratio. Proponents of sex-specific implants suggest that this aspect ratio mismatch could lead to problems with the arthroplasty.

The second difference is a less prominent anterior part of the knee. If this difference were to exist, the patellofemoral part of the knee articulation could be inappropriately large, or overstuffed. Proponents of sex-specific implants suggest that a change in this part of the femoral prosthesis could lead to functional problems with the arthroplasty.

The third difference is an increased Q-angle in a woman’s knee, which creates a difference in the direction of pull of the quadriceps muscle relative to the knee. If this difference were to exist, the pull of the quadriceps muscle would not be in line with the trochlear groove of the femoral component if a sex-specific implant were not used. Proponents of these designs suggest that a functional problem could result from such a mismatch.

Jackson: What have your investigations and data revealed in this area?

Blaha: We used data gathered from 63 cadavers (33 men and 30 women) in which we measured and calculated the aspect ratio of the knees. We found virtually no difference in the aspect ratio of the knees except for one measurement that was minimally larger in the women, not smaller as has been suggested, compared with the men. We concluded that no clinically significant difference occurred in the shape of the knee based on the sex of the cadaver.

Jackson: The direct-to-consumer marketing on this issue has been quite successful. What are your thoughts on the responsibilities of companies and physicians doing work on new designs for knee replacement?

Blaha: As physicians and surgeons, orthopedists are charged to do what is best for their patients while keeping the costs of medical care low. We use evidence-based medicine to balance these sometimes conflicting responsibilities.

Patients are becoming more and more sophisticated in making health care choices. The Internet has provided an unprecedented source of information for the general public. Often patients arrive in the office with pages abstracted from online journals and from which they have gleaned a firm idea of their need for treatment.

Other direct-to-consumer marketing in electronic and print media adds to patients’ knowledge and opinions. Unfortunately, patients often assume that inaccurate or misleading information from these sources is fact. A patient may be completely convinced of their need for a given procedure or implant and cannot be deterred from their opinion, which was derived from a convincing Web site or television advertisement. The procedure or implant may have markedly increased cost with no proven benefit. When a patient arrives with such information, the surgeon is faced with a dilemma: do what the patient wants, or do what is best according to evidence-based principles.

If the information that patients receive from direct-to-consumer marketing were required to be evidence-based, then they would be better informed and the surgeon’s job of discussing options would be considerably easier.

J. David Blaha, MD, professor, Department of Orthopaedic Surgery, University of Michigan, can be reached at 1500 E. Medical Center Drive, Ann Arbor, MI 48109-0328; 734-647-9961; e-mail: jdblaha@med.umich.edu.