It is important to analyze sex differences before ruling them out. Many additional factors also influence outcomes in TKA, including age, body composition, comorbidities, diagnosis, preoperative knee mobility, ethnicity, sex, and surgeon or hospital volume. Problems can emerge if sex is inaccurately identified as the key difference, without taking intersections between sex and other variables into consideration.
Overemphasizing sex differences is a problem, especially when companies market female-specific knees directly to surgeons and to women patients despite the lack of evidence of clinical advantages (Johnson et al., 2011). Note that Zimmer calls their knee “Gender Solutions,” implying that their product addresses not only biological but also cultural differences between women and men. Zimmer’s marketing campaign emphasizes gender as the basis of prosthesis choice. However, other factors (such as a patient’s stature, ethnicity, or a surgeon’s experience installing a particular prosthesis) have been demonstrated to be important to outcome (Sampath et al., 2009; Bellemans et al., 2010).
Sex must be analyzed, but overemphasizing sex to the exclusion of other factors is also a problem. First, overemphasizing sex may alter women’s medical decisions and outcome expectations, leading them to choose a more costly prosthesis. Moreover, surgeons using an unfamiliar implant to satisfy patient requests may have worse patient outcomes (Sampath et al., 2009). Second, a “female knee” may be a poor fit for some women and a good fit for some men, and physicians have expressed concern that a male patient may object to receiving an implant “designed for women” even if it offered the best fit for him (Blaha et al., 2009).
Direct-to-consumer advertising of orthopedic devices is more common in the U.S. than Europe and can adversely affect healthcare delivery (Bozic et al., 2007). The American Academy of Orthopedic Surgeons (AAOS) has stated that responsible direct-to-consumer advertising must provide information that is “scientifically substantiated, accurately presented, and free of false or misleading claims” (AAOS, 2009). Although anatomical difference between female and male knee anatomy may be found, clinical trials have not been able to show a difference in clinical outcome between standard and gender specific implant designs (Johnson et al., 2011).
This case study presents a cautionary tale. While studying sex differences is important, overemphasizing sex to the exclusion of other intersecting factors can lead to problematic outcomes.
In 2007, an estimated 500,000 total knee arthroplasty (TKA) procedures were performed worldwide—about two-thirds in women. In the 1990s, with increased attention to women's health research, manufacturers began producing "gender-specific" knees, and marketing them directly to women. Does this lead to better healthcare quality?
Sex may appear to be the most important variable in choosing a knee implant until height is considered. Specifically, research shows that that two anatomical sex differences (greater Q-angle and lesser anterior condylar height in women) disappear when corrected for standing height. This suggests that height may be more important than sex in determining the knee implant a patient should receive.
It is important to analyze sex differences before ruling them out. Many additional factors, however, influence outcomes in TKA, including age, body composition, comorbidities, preoperative knee mobility, ethnicity, and surgeon or hospital volume.