Knee Anatomy and Biomechanics

Femoral Notch Width

Gender differences in femoral notch anatomy has been a topic of research because of the proposition that notch morphology relates to ACL injury risk, yet its significance remains unclear. It is possible that a small notch indicates a small ACL making it more susceptible to injury or that a narrower notch impinges the ACL resulting in shear force leading to tears. It is also possible a smaller ratio of notch width to bicondylar width (notch width index, or NWI) leads to ACL injury, yet some studies have found this risk to be nonexistent. ^,

In a study using axial plane magnetic resonance imaging (MRI) and three-dimensional analysis to measure femoral notch anatomy, Charlton et al. collected measurements of femoral notch volume and femoral bicondylar width, considering patient gender, height, and weight. Compared with males, females had a statistically smaller femoral notch volume, but this difference was primarily related to weight and height. Comparing male and female femoral bicondylar width, they found a statistically significant difference between males and females, with males having a larger bicondylar width (difference, 4.4 mm; P = .001). This study excluded subjects with a history of previous knee injury or surgery, which may have introduced a selection bias, creating a cohort of subjects possessing certain anatomic or physiologic differences from those prone to injury. Additionally, they found that notch width did not directly increase with height, as did femoral bicondylar width, which led the authors to question the use of the NWI as a standardizing tool.

Shelbourne et al. measured the intercondylar notch width intraoperatively and performed radiographic measurements on 714 ACL-deficient patients. In agreement with prior studies, they found that females, on average, had smaller notches than males. They also found that the NWI changed with height because the femoral condylar width increased more than the notch width in taller subjects. They critiqued the use of NWI, as it was assumed that both the notch width and the femoral bicondylar width increase similarly with increasing height. Consequently, they recommended comparison of different subjects with absolute notch width measurements rather than NWI. In a later study by Anderson et al. utilizing MRI measurements in high-school students, notch width was found to increase with height for male subjects, but not for females. The NWI was stable with increasing height in male players, but it reduced with increasing height in female players. This study suggested that, in contrast to males, as females grow taller, the size of the femoral notch does not increase with the absolute width of the femoral condyles. This discrepancy may cause intercondylar notch stenosis and lead to an increased risk of ACL tear ( Fig. 1.1 ).

In summary, differences in femoral notch volume and bicondylar width between males and females may mostly be due to height and weight rather than gender specifically. However, the aforementioned disproportionate growth between femoral notch width and bicondylar width in growing females, resulting in a lower NWI with increased height, may be a significant contributor to ACL injury risk from notch stenosis. This phenomenon has not been seen in males, who have a much more proportionate growth of both notch and bicondylar widths. Further research between genders considering age, race and body type is needed to support this theory.

Tibial Plateau Slope

Several studies suggest that females have a steeper angle to the anterior to posterior slope of the tibial plateau than males. ^, This increased slope is thought to increase the risk of ACL rupture because of the relationship between the slope of the tibial plateau and the anterior tibial translation of the knee. With steeper tibial slopes, the application of large compressive joint loads exposes greater magnitudes of anteriorly directed force on the proximal part of the tibia, and this may lead to an increased risk of ACL injury as the ACL attempts to provide restraint to this anteriorly directed force. Several studies have suggested that higher tibial slope increases the risk for ACL tears, especially in females.

Hohmann et al. compared males and females who suffered ACL injuries and found that females had a significantly greater posterior tibial slope, which was thought to place the female knee at an increased risk for a pivot shift injury. Utilizing MRI, Hashemi et al. measured the medial, lateral and coronal slopes of the tibial plateau in 33 female and 22 male patients. This study found the mean medial and lateral tibial slopes for the female subjects were significantly greater than those for the male subjects (medial: 5.9 degrees compared with 3.7 degrees; P = .01; lateral: 7.0 degrees compared with 5.4 degrees; P = .02) ( Fig. 1.2 ). Weinberg et al. utilized 545 bilateral cadaver specimens to establish normative values of medial and lateral posterior tibial slope and to determine differences in genders, age and race. Tibial slope measurements were taken with radiographic measurement and digital laser-derived three-dimensional analysis. The mean medial and lateral tibial slopes were greater in females than those in males, and regression analysis confirmed gender to be an independent predictor for increased slope.

In opposition, Karimi et al. used MRI to measure posterior tibial slope in an Iranian population and found no significant correlation between genders. Given contradictory evidence, there may be factors other than gender that contribute to tibial slope, such as age, body mass index (BMI), or ethnicity and these should be considered in future research.