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Total Knee Sizing

Gender differentiated knee implant design is a new trend in Orthopaedics. Several large companies have marketed men and women have different bony anatomy that requires specialized knee prostheses. Throughout the marketplace it is believed there are three main disparities between men and women relating to implant design: women have a greater trochlear groove angle, a narrower distal femur, and are more prone to anterior overstuffing.

There are several studies demonstrating an anatomic difference between men and women.^{65, 66, 67} However, few of these studies have taken into account the physical stature of individuals. Those studies that have examined other factors besides gender have found patient size is more of a determinant of implant size than gender. It is important to note that although over 6,000 total knee arthroplasty studies have been performed, none demonstrate a clinical issue with overhang of the femoral component.⁶⁸

Research has shown the Q-angle is not different due to gender, but is actually dependent on the height of the individual.⁶⁹ If a man and a woman of equal height were measured, their Q-angles would be the same. Studies demonstrating a gender difference have not recognized men are on average taller than women. The studies simply separate the samples by gender; not stature. Other studies have shown there is no difference in the morphologies of the trochlear grooves of male and female fetuses. Furthermore, these morphologies do not change in adulthood.^70,71

One of the most referenced studies on gender sizing was performed by Kirby Hitt, MD.⁶⁶ Distal femora were compared to the dimensions of several knee systems (Figure 1). It was determined that most knee components were on average 4.9mm too wide for the female population. However, this study did not examine other factors such as patient height. Figure 2 shows the Hitt data incorporated with the dimensions of several other systems including the ADVANCE® Primary, ADVANCE STATURE™ Femoral Components, Zimmer® Gender Solutions™ (GSK), and Stryker® Triathlon® Knees.⁷⁰

Due to stature differences, it is common for the female patella to be thinner than the male. Once again, research has found this is not due to gender, but to the differences in average height between men and women.⁷² A correlation has been identified between height and increased kneecap thickness. Therefore women may not be more prone to anterior overstuffing than men. To resist overstuffing, the ADVANCE STATURE™ Femoral Component features a reduced anterior flange and replicates the constant sagittal radius of the normal trochlear groove (Figure 3). These features work in conjunction to reduce stress on the extensor mechanism.

Anterior overstuffing is further avoided through a lack of anterior sliding due to the anterior stability of the ADVANCE® Medial-Pivot and Double-High Knees⁷³ (Figures 4a, 4b). Femoral components with a j-curve and traditional articulation experience anterior sliding (termed paradoxical motion) in flexion. This anterior sliding can place excess stress on the extensor mechanism of the knee.

ADVANCE STATURE™ femoral components are designed to accommodate those male or female femora with a larger A/P dimension than M/L. Due to the finding in the Hitt study, the M/L dimension has been reduced by 5mm from the standard ADVANCE® Knee. The components utilize the ADVANCE® 3.6° trochlear groove angle that replicates the sulcus morphology of both males and females¹⁵ (Figure 5). This trochlear groove has demonstrated excellent clinical results regardless of gender.⁷³