By reviewing the peer-reviewed orthopaedic literature regarding traditional, primary TKA, we tried to determine whether women had worse results than men using various criteria for success or failure. We also sought any evidence to determine whether there are skeletal differences between men’s and women’s knees, and if so, that these differences would be clinically important. Those who propose a need for a female-specific total knee design [6
] should be able to support that proposal with clinical and scientific evidence published with genuine peer review in an evidence-based format without bias. If studies that used traditional, averaged, gender-neutral total knee designs found worse results for women than for men, then the need for female-specific designs might be inferred. However, just the opposite is true. The results from such studies show women achieve results that are at least as good as or, more likely, better than men. Thus, we see, no matter what criteria are used to measure success or failure by the multiple studies about primary TKA (Tables , ), whether it is by implant survival [13
], pain [37
], risk of revision [34
], range of motion [31
], wear-related failures [3
], stiffness [25
], outcomes scores [7
], or satisfaction [33
], when traditional implant designs are used, women achieve essentially equal or even better results than men. This finding refutes the theory that traditional knee prosthetic designs place women at a disadvantage compared with men and that gender-specific designs are needed to address anatomic differences between men and women.
A limitation of our literature search was that we found no study specifically designed to ascertain difference between the results of men and women using traditional total knee designs. Therefore, we had to rely on authors who were gathering data for other reasons to report any male-female differences. Certainly, studies that reported such differences numerically with significant probability values (Table ) were more reliable than those that did not (Table ). However, the fact that our search discovered no studies specifically designed to test for possible male-female differences is instructive. If being female were an obvious risk factor for premature failure or poor outcome, we suspect studies would have been performed to document this risk and its magnitude. Ideally, a long-term study with a large cohort could be designed to specifically answer the question: Are the TKA results worse in females than males? However, the data we have reported seem to leave no question that women’s results are the same or better than those of men.
When the data supporting major anatomic differences in the Q angle between male and female knees are reviewed carefully, it becomes apparent this difference disappears when corrected for the average height difference between men and women. Indeed, Grelsamer et al. [16
] reported the female pelvis is not wider than the male pelvis when measured between the anterior superior iliac spines. Furthermore, in a subject of average height, because the anterior superior iliac spine is so far from the patella, a 4-cm-wider pelvis would only change the Q angle 2°. Conversely, a 5° increase in the Q angle would require almost a 9-cm-wider pelvis. By using a long, more accurate protractor and a consistent, standardized method of measurement, they also reported the Q angle changed 0.2° for each centimeter in height, and the average height difference of 11.5 cm between men and women explained the small 2.3° gender difference in the Q angle. With this information, implant designers might advocate height-specific designs, but not gender- or female-specific designs.
Concerning the claim that the medial and lateral condyles have smaller anterior heights in women compared with men, this difference is explained entirely by size and not gender difference. We found no evidence to support the claim that using implants sized to demographic averages will result in overstuffing of the patellofemoral joint. The concept of overstuffing the patellofemoral joint has been simply and uncritically transferred from the femorotibial joint with no confirmatory studies. Because the capsule and inelastic ligaments secure the femorotibial joint, it is extremely important to balance these ligaments carefully during TKA and avoid a tibial insert that is too large. This will certainly overstuff this joint and lead to a poor result with decreased range of motion. The patellofemoral joint is a totally different articulation. Although the patellar ligament is inelastic, the quadriceps muscles are elastic and stretchable. This explains why the investigation by Bengs and Scott [5
] not only failed to support the claim of overstuffing by Conley et al. [11
], but actually refuted it. More recently, Pierson et al. [27
] reviewed 830 primary TKAs to determine the effects of so-called overstuffing the patellofemoral joint. Their findings did “not support the widely held belief that stuffing of the patellofemoral joint results in adverse outcomes after total knee arthroplasty.”
With reference to the smaller female ML:AP aspect ratio of the distal femur, there is no scientific or clinical evidence to show it has a major clinical impact. Hitt et al. [20
] clearly showed women have a smaller ML:AP aspect ratio on average than men. One reason this difference may not rise to clinical significance is that the differences within groups of men and women are greater than the differences between men and women. That is, the mean difference between groups is almost always smaller than the standard deviations of the measurements within the groups. Because we found women achieve essentially the same or better results after traditional TKA compared with men in 19 studies with more than 120,000 TKAs spanning 18 years, we conclude this smaller female ML:AP aspect ratio has no measurable clinical effect. Other designers [18
] have used the same data presented by Hitt et al. [20
] to improve the sizing options for men and women in the same implant system. They decreased the ML:AP aspect ratio in the smaller sizes and decreased the increment change between sizes, thereby offering more options in the same overall range. They rejected the concept of female-specific total knee designs and in their early short-term study have reported successful results for men and women [18
]. Barrett [4
] studied the need for gender-specific prostheses in TKA, taking into consideration the smaller female ML:AP aspect ratio. He concluded, although some modification of sizing within current total knee systems might be beneficial, the development of separate designs for men and women would likely be costly and unnecessary. Chin et al. [10
] reached the same conclusion after studying intraoperative measurements of male and female distal femurs. They confirmed a 2% to 3% difference in the aspect ratio between men and women and suggested prosthetic manufacturers use these data to produce “…a range of prosthetic sizes that provides more optimum fit across genders…” We agree the availability of more sizes may produce a better anatomic fit for more patients, and this might improve functional or survivorship results; however, that too remains speculative.
The use of female-specific total knee designs poses an interesting ethical and, perhaps, medicolegal dilemma. When obtaining informed consent, should the orthopaedic surgeon disclose to the male patient that he or she plans to use an implant that was designed and intended for use in the female knee? Conversely, should he or she explain to the female patient that there is no scientific or clinical evidence to support the use of this new female-specific total knee prosthetic design?
After a thorough review and analysis of the literature regarding traditional primary TKA, no matter what criteria were used to define success or failure, we found no evidence to support the original hypothesis that women traditionally have worse outcomes than men. On the contrary, women appear to have the same or better results than men. In addition, the average anatomic differences between male and female knees can be explained by the average smaller height and size of women compared with men, not by their gender. The difference in the distal femoral aspect ratio described by Hitt et al. [20
] is apparently too small to be of clinical importance. We conclude the need for female- or gender-specific total knee prosthetic designs is not supported by existing scientific and clinical evidence.