膝の脱ジェンダー化：性差の問題点の過剰な強調

課題

2007年には、全人工膝関節置換術（TKA）が世界中で推定50万件実施され、患者の約2/3が女性であった（Kurtz et al., 2011; Blunt et al., 2008）。1990年代に入り、女性の健康研究が注目されるようになると、メーカーは「性別に応じた」膝を製造し始め、女性に直接販売するようになった。これは医療の質の向上につながっているのだろうか？

性差の問題点の過剰な強調

一般的に、女性に特化した人工関節が女性のTKAの結果を向上させるという証拠はない（Jacobs et al., 2007）。性差を強調しすぎるのは問題であり、特に企業が臨床的な優位性を示すことなく、女性専用の膝を直接女性患者に販売する場合には問題である。このような過度の強調は、実際には身長のほうが性別よりも形態的な判断材料として重視されるべき場合においても、患者が膝インプラントを選択する際に、性別にもとづく選択肢に過度に依存してしまう可能性がある。また、膝の形態は同性間でも異なるため、「女性用」の膝が、一部の女性には合わないが一部の男性には適合している場合もある（Blaha et al., 2009）。

ジェンダード・イノベーション：

1. 他の変数との関連で生物学的な性差を検討する。性別に特化した義肢のデザインにはまだ議論の余地があるが、性別と他の重要な変数（身長、民族、身体組成など）がどのように交差するかを分析することは、ジェンダード・イノベーションにつながる（Bellemans et al., 2010）。このような研究は、違いに対する認識を高め、白人男性が標準とされる医学モデルの「中立性」に疑問を投げかけるものである。他の重要な変数と関連付けて生物学的な性差の分析を行うことで、研究の質と患者の安全を確保することができる。

The Challenge

Total knee arthroplasty (TKA) is a common procedure—with women a majority of patients (Kurtz et al., 2011). Prior to 2006, prostheses used in knee replacement were unisex, modeled off the knees of both women and men, although women were typically underrepresented in these studies (42-62%) compared to their representation among TKA recipients overall (66%) (Mahfouz et al., 2007; Mahfouz et al., 2006; Hitt et al., 2003; Chin et al., 2002). Unisex TKA implants were not “one size fits all”; surgeons had access to implants of varying sizes and femoral shapes, and fit TKA candidates primarily based on measurements of the anterior-posterior dimension of the knee (Fricka et al., 2009).

Zimmer, Inc., performed a meta-analysis of sex differences in knee anatomy and released a report concluding that women have, on average, greater Q-angles (Figure 1), a less prominent anterior condyle, and a reduced medial-lateral to anterior-posterior aspect ratio (Figures 2-3) (Conley et al., 2007).

Studying Sex Differences in the Knee

A number of differences in female and male knee anatomy have been identified, but does designing for these differences provide the best clinical outcomes for women and men?

Gendered Innovation 1: Examining Sex within the Context of other Variables

Studying sex differences in the knee marks a gendered innovation because it raises questions about diversity and challenges medical models which are often male by default. The relevance of sex differences in knee anatomy, however, must be explored in the context of other variables, especially height and ethnicity (see Method).

Method: Intersectional Approaches

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.

View General Method

 

Height
Sex may appear to be the most important variable in implant choice until height is considered. Specifically, research shows that that two of the anatomical sex differences identified above (greater Q-angle and lesser anterior condylar height in women) disappear when corrected for standing height (Grelsamer et al., 2005). The third anatomical difference (reduced medial-lateral: anterior-posterior aspect ratio) has not been shown to be clinically significant (Merchant et al., 2008). An anatomical study of 60 cadavers revealed that the male linear knee joint dimensions are on average greater than female dimensions, but that sex differences disappear when female and male cadavers are matched by femur length, which predicts knee anatomy more accurately than sex (Dargel et al., 2010). This suggests that height is more important than sex in determining the size of knee implant a patient should receive (see Term: Overemphasizing Sex Differences as a Problem).

Race and Ethnicity
Most U.S. FDA-approved knee implants were modeled on research done in Caucasians (van den Heever et al., 2011). This raises several questions: Do other populations have specific biological traits or cultural behaviors that need to be taken into consideration when designing knee implants (MacDonald et al., 2008)? Would creating race- or ethnicity-specific knees improve patient outcomes? What effect would the proliferation of knee implant models have on the cost of care, and would race/ethnicity-specific knees create the risk that prosthetics companies, clinicians, or patients would overemphasize race/ethnicity as has occurred with regard to sex?

It is important to keep in mind that differentiating the effects of race and ethnicity from the effects of socioeconomic status, cultural and physical environment, geographic location, access to healthcare, etc. may be difficult or impossible. Further, race and ethnicity are often used inconsistently.

Most reported ethnic differences in knee morphology relate to Asians versus Caucasians. Cadaveric studies have suggested that Chinese, Koreans, and Singaporeans have smaller knees on average than Caucasians, and an intra-operative study in Taiwan also found that Asian knees tend to be smaller than Caucasian knees. Researchers have reported differences in knee shape (rather than overall size) which remain when patient height is controlled for (Chin et al., 2011; Ho et al., 2006).

Cultural considerations may also lead to important innovations in prosthesis design: for example, high flexion may be important to TKA patients in Asian countries, where getting up from the floor or sitting cross-legged may be more common than in Western countries (Bin et al., 2007). However, knee designs which allow higher flexion have not been shown to produce superior TKA outcomes in Asian populations (Kim et al., 2010)

Conclusions

Design must be done with diverse populations in mind. Analyzing sex in knee anatomy, prosthesis design, and outcomes research is important—but sex is not the only variable that should be considered. Critical evaluation of different populations and their needs is important –if research demonstrates that better prosthetic fit results in better clinical outcomes, then it might be appropriate to bring to market a spectrum of knees based on sex, ethnicity, height, and other significant “fit” factors. Because of the complex relationships between knee replacement and lifestyle, environmental, and health factors, such as race and ethnicity, nationality, socioeconomic status, diet, exercise, etc., selecting the best prosthesis for a patient requires careful attention.

Next Steps

More basic research needs to be done in this area. Key questions include:

1. Do female-specific implants improve women's outcomes relative to a unisex knee? One prospective study with 146 women showed no difference (Johnson et al., 2011). Many studies designed to evaluate outcomes have been performed retrospectively, and the American Academy of Orthopedic Surgeons has described how this limits the level of evidence provided (Jacobs et al., 2007).

2. How do the outcomes of TKA with unisex prostheses compare in women and men? A 2003 report commissioned by the U.S. National Institutes of Health concluded that no evidence pointed to sex as a strong predictor of outcome, but that this conclusion was "tempered by the limitations of many of the designs of the studies included in the analysis" (Kane et al., 2003).

3. What are the financial relationships between prosthesis developers, researchers, consultants, physicians, and universities, and how might these affect the objectivity of outcome studies (Gelberman et al., 2010)? The International Committee of Medical Journal Editors requires researchers to declare any associations with "commercial entities that provided support for the work reported" as well as with "entities that could be viewed as having an interest in the general area of the submitted manuscript." However, such policies are not universally or uniformly implemented (Drazen et al., 2010).