The concept of sex-related differences in knee morphology that has led to sex specificity in total knee replacement has generated a great deal of interest in both the popular press and the orthopaedic literature. Some clinicians and implant designers purport that there are sex-based anatomical differences in the size and shape of the female knee. Because of these differences, they add, women need an implant that is "just for them." Sex-specific knees have been touted as "the next level of sophistication."1 There is, however, legitimate skepticism about the scientific evidence supporting this view; thus, the need for these implants is a matter of debate.
Proponents of this technology have identified three anatomical differences in the female knee: a smaller anterior condyle, an increased quadriceps angle (Q angle), and a reduced medial-lateral to anterior-posterior aspect ratio2,3. The evidence supporting these claims has recently been scrutinized by Merchant et al.4, who concluded that there appear to be no meaningful anatomical differences between male and female knees. Furthermore, they found no evidence that women have worse outcomes than men after traditional total knee arthroplasty, a conclusion that calls into question the need for sex-specific implants. In fact, women do equally well or, in some cases, better than men do4. The ideas regarding the need for a sex-specific implant have largely been introduced into the literature by representatives of the companies most associated with marketing the concept4.
Sex-specific implants have been heavily marketed to surgeons in journal advertisements and direct mailings and to patients through direct-to-consumer advertising in the print and broadcast media. Advertisements stating that a particular implant is "the only knee truly for women" and "the first and only knee that fits the shape and size of a woman" have been seen in a variety of media and appeal strongly to female patients, who, in turn, request the particular knee implants from their surgeons. Direct-to-consumer advertising has an enormous influence on surgeon and patient decision-making in orthopaedics5. The implications for this trend are potentially great and include issues of optimal patient care and cost-containment in a time of diminishing resources. The stakes are very high for the surgeons, patients, and device companies. It is imperative that the claims being made are "scientifically substantiated, accurately presented, and free of false or misleading claims."6
The purpose of our study was to determine if sex-based differences in the shape of the distal part of the femur could be ascertained in a group of male and female cadaveric femora. We hypothesized that we would not find a clinically meaningful difference between the groups.
Sixty-five pairs of embalmed cadaveric femora were obtained from the anatomic gift collection at West Virginia University. Two pairs of knees were unusable and, in six cases, only one femur was available for measurement, leaving fifty-seven pairs and six single femora for measurement. The femora were harvested from thirty-three male cadavera (mean age at the time of death, seventy-one years; range, forty-three to ninety-four years) and thirty female cadavera (mean age at the time of death, seventy-five years; range, fifty-six to ninety-one years). All specimens were from Caucasians with one exception, for which the race was not known. This group of cadaveric specimens was measured and previously reported in 20027. These data have been re-evaluated for this study because the data, gathered with another purpose in mind, are appropriate to address the question of a differing aspect ratio between the sexes.
The specimens were cleaned and prepared by removal of all soft tissue except for the articular cartilage, which was retained. The most prominent parts of the medial and lateral epicondyles were palpated and marked after two or three examiners agreed on the position of the landmarks. A custom-built drill-guide was fixed to the specimen, and a drill bit was slowly passed through the femur. A Steinmann pin that was slightly oversized to ensure a press-fit was tapped through the hole, and the specimen was mounted to a frame that allowed access to each condyle and to the proximal part of the femur (Fig. 1). A custom-built protractor/radius-measuring caliper was affixed to the pin with a locking screw, thus aligning the caliper to a rod that was placed to bisect the greater trochanter (Fig. 2). The line of this rod was called 0°. The device was moved and then fixed by means of a locking pin every 10° from fully anterior (over the ridges of the trochlear groove) to fully posterior (over the posterior aspect of the condyles).
The data representing the distance from the pin to the anterior aspect of the femur and the pin to the posterior aspect of the femur on both the medial and lateral sides (named anterior medial [AM], posterior medial [PM], anterior lateral [AL], and posterior lateral [PL]) as well as the distance from side to side along the pin (the transepicondylar axis [TEA]) were used in this analysis. Values were calculated, as follows, to represent the aspect ratio of each of the specimens:AM+PMTEAAL+PLTEAAL+PMTEAAM+PLTEA
For each aspect ratio, a multivariate regression analysis was carried out: the left and right ratios were used as the multivariate outcome, and the age and sex of the specimens were used as independent variables.
Source of Funding
There was no external funding source for this study.
The only significant difference was the ratio between the lateral condyle and the transepicondylar axis (p = 0.03). The female specimens had a minimally larger ratio of 0.09 on the right and 0.06 on the left. All other ratios were not significantly different (p = 0.05). Age exhibited a significant association in the right AL-PM ratio, with older age being associated with a higher average ratio (p = 0.03).
Data from the measured cadaveric femora are presented as ratios of the condylar measurements and the transepicondylar width and grouped by the sex of the specimen (Table I).
In our study, we did not find a clinically significant difference in the aspect ratio of the male and female cadaveric femora that were measured. The measurement method for this study is accurate enough that the study is adequately powered to reach statistically valid conclusions. All of the cadaveric femora that were measured were from Caucasian donors, so the conclusions obtained from this study may not be appropriate for other groups. The only significant difference in these measurements suggests that the size of the lateral part of the knee (AL + PL) is relatively larger in women, a finding that does not support the suggestion that women have a smaller lateral part of the trochlear groove.
There are clear differences between men and women. In some circumstances, these differences require different treatments for the two sexes. In orthopaedics, there are some well-known differences between men and women. For example, with regard to osteoarthritis of the knee, female patients have greater disability but seek out total knee replacement in fewer numbers than male patients do8,9. Another difference is the increased prevalence of anterior cruciate ligament tears in women10. There is a legitimate question as to how far these differences extend. It is reasonable to ask the question, "Are there differences in the fundamental anatomy and biomechanics of the knee that require special implants for women?"
A number of anatomical differences between male and female knees have been proposed. Females reportedly have a smaller anterior condyle, an increased Q angle, and a reduced medial-lateral to anterior-posterior aspect ratio1,2. These differences are used to support the need for a sex-specific knee that caters to the "female anatomy." However, we found no meaningful anatomical difference between the aspect ratio of male and female knees in our study population. This conclusion is supported by other studies in which the data have been examined for the purpose of determining the need for separate implants.
Merchant et al.4 recently analyzed the data presented by Conley et al.2 and Booth1 and reviewed by the American Academy of Orthopaedic Surgeons (AAOS) Gender-Specific Knee Replacement Work Group11. They found that the claims about the existence of anatomic differences between the sexes are nonexistent or exaggerated. Furthermore, and perhaps more importantly, there is no evidence in the literature that women have worse outcomes than men have after traditional total knee arthroplasty4,10. In fact, studies show that women do equally well or better than men do after the procedure4. Bellemans et al.12 measured computed tomographic scans and concluded that both morphotype (i.e., ectomorph, mesomorph, and endomorph) and sex were predictive of the shape of the distal part of the femur. Barnes et al.13, using virtual surgical cuts on three-dimensional models from computed tomography images, found a contribution of race to the shape of the cut surfaces, with less contribution of sex. On the basis of the results of our study and those of others in the literature, we conclude that our null hypothesis cannot be rejected and that sex is only a minor predictor of the shape of the distal part of the femur.
Despite the lack of scientific evidence of the need for an implant made specifically for women, some implant manufacturers have pursued the concept and are aggressively marketing it to surgeons and patients14. Merchant et al.4 point out the ethical and potential legal implications of the unsupported claims that have been made by implant manufacturers and their spokespersons. One example occurs when obtaining informed consent from a male patient who is about to undergo implantation of a device that has been designed and intended for female patients. Likewise, should surgeons explain to female patients that there is no evidence to support the use of a female-specific implant design4?
Direct-to-consumer advertising of orthopaedic devices is an emerging area of concern5. Although this marketing may provide some benefit by providing information about options to patients, its purpose is to sell products and the advertisement usually only tells part of the story. Advertisers appeal to emotions or use other marketing tactics to influence the medical decision-making process. With regard to sex-specific implants, female patients are told that women are different and are encouraged to believe that there is a special knee implant that has been designed just for them. However, they are not told that the existence of differences in the knee is subject to debate or that there is no evidence that traditional implants are inappropriate for women. Patient expectations become altered by the advertising. This, in turn, affects the exchange between the doctor and patient and may have an adverse impact on the patient's health care. It may also lead to the overuse of costly devices when the real benefit of using those devices is still unknown. This type of advertising, we believe, is misleading and contrary to the AAOS guidelines for direct-to-consumer advertising6.
Surgeons are now charged to practice evidence-based medicine. When patients are exposed to persuasive advertising about unproven technology, surgeons are placed in a difficult situation. Because medical care is expensive and it is recognized that systems-based practice demands efficient use of resources, we believe that implant manufacturers should be held to the higher standard of "evidence-based marketing." The marketing and advertising methods that might be appropriate for nonprescription weight-loss and "joint-health" products are not the same as those that are appropriate for orthopaedic implants, which permanently alter a patient's anatomy. The results of this study and others clearly illustrate the need for implant manufacturers to embrace "evidence-based marketing" so that surgeons can truly practice evidence-based medicine. 