Important morphologic differences have been found between the female and male knee. In addition to these anatomic differences, there are also significant neuromuscular differences between genders. This chapter outlines these differences, as they specifically pertain to the female athlete and risk for injury. The important characteristics of female knee bones, alignment, and soft tissues (ligament, meniscus) are covered first, followed by neuromechanical differences. The majority of research in this topic is directed at anterior cruciate ligament (ACL) tears and patellar instability, and as such, this chapter focuses on these injuries in particular.
When comparing male to female knee osseous anatomy, comparative studies in sports medicine focus on femoral notch morphology, tibial slope, femoral condyle geometry, and patellofemoral articulation.
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.
Femoral Trochlear Groove
It has been theorized that compared to males, females may have lower medial and lateral femoral condyles anteriorly, resulting in a shallower trochlear groove. Yet when correcting for size of the distal femur, these differences may not be significant. In a detailed radiographic study of 400 knees in 1964, Brattstrom found females had, on average, 1.5 mm lower lateral and 1.1 mm lower medial condyles anteriorly. However, when adjusted for size, males and females had similar anterior condylar measurements. Similarly, Poilvache et al. reported an average 1.4 mm lower lateral condyle and a 1.6 mm lower medial condyle for females compared with males. However, these data points were direct, absolute measurements, uncorrected for the size of the distal femur or the patient and thus may have been secondary to smaller overall femora for females compared to males. Varadarajan et al. studied the gender differences in trochlear groove orientation. The proximal portion of the trochlear groove was found to be oriented significantly more medially in females than in males, which was hypothesized to contribute to differences in patellar tracking.
Gender Differences in Coronal and Rotational Alignment and Clinical Implications
The quadriceps angle, or the “Q-angle”, was first described by Brattstrom in 1964. The Q-angle is the angle between the line of pull of the quadriceps (anterior superior iliac spine to mid-patella) and the line connecting the center of the patella with tibial tuberosity. An increased Q-angle is thought to create excessive lateral forces on the patella through a bowstring effect. The literature describing “normal” values of the Q-angle is variable. In general, the average normal Q-angle should fall between 12 and 20 degrees. Some studies have illustrated that values between 8 and 10 degrees for males and up to 15 degrees for females are deemed normal, with higher values indicating potential pathoanatomy. ,
Greater femoral anteversion and/or tibiofemoral angles result in greater Q-angles. Changes in the tibiofemoral angle have a substantially greater impact on the magnitude of the Q-angle compared with femoral anteversion. As such, the Q-angle seems to largely represent a frontal plane alignment measure. As many knee injuries appear to result from a combination of both frontal and transverse plane motions and forces, this may in part explain why the Q-angle has been found to be a poor independent predictor of lower extremity injury risk.
Historically the literature seems in agreement that females have wider Q-angles than males, with mean differences from 3 to 5.8 degrees depending on the resource. , , This difference may be because on average, females have shorter femora and more widely spaced hips than males. , Theoretically, the combination of wider hips and shorter femora would increase the valgus of the lower limbs, thus increasing the Q-angle.
However, when data are corrected for the difference in height between males and females the difference in Q-angle may disappear. After measuring the Q-angle in 69 subjects, Grelsamer et al. found that males and females had similar Q-angles and that despite the gender, shorter individuals had slightly greater Q-angles than those who were taller. This study suggested that the historically proposed slight difference in Q-angles between males and females may simply be explained by the fact that males tend to be taller. In a systematic review of Q-angle measurements, Livingston concluded that “the common belief that females have wider hips than males is not supported by scientific data, nor is the assumption that Q angles are bilaterally symmetric. These outdated assumptions must be replaced by a new approach to the study of the Q angle.”
In a clinical study on lower extremity alignment, Nguyen et al. found significant gender differences in hip anteversion between males and females, along with pelvic angle, genu recurvatum and Q-angle. They concluded that although the reasons contributing to these gender differences are not entirely known, there is evidence to suggest that many of these gender differences are developmental in nature and emerge after puberty. Their finding that females showed greater femoral anteversion than males agreed with that of other studies , ; however, these studies did not conclude this difference to be statistically significant. Thus further research is required to definitively show significant gender differences in lower extremity alignment.
The Q-angle is of importance, as elevated Q-angles may increase the risk for anterior knee pain or patellar dislocation. The larger the angle, the greater the lateralization force on the patella, increasing retropatellar pressure between the lateral facet of the patella and the lateral femoral condyle. A 10% increase in the Q-angle has been shown to increase the stress on the patellofemoral joint by 45%. Continuous compressive forces may give rise to patellofemoral pain syndrome and cause degeneration of the joint. , If the lateral force becomes large enough the patella may potentially sublux or dislocate over the femoral sulcus, with quadriceps activation and knee extension. However, some case-control studies have not supported the hypothesis that larger Q-angle is a risk factor for patellofemoral pain syndrome, as this condition is likely multifactorial ( Fig. 1.3 ).
The Q-angle’s direct relationship with the risk of ACL injury is somewhat controversial because of the multitude of factors that constitute lower limb alignment. For example, if the patella sits laterally, or if the subject stands in an internally rotated position, the value of the Q-angle may be affected. Studies on female athletes have thus found no significant association between Q-angle and ACL injury when comparing injured and noninjured groups. , However, if the Q-angle were interpreted simply as increased valgus alignment it could represent the relation between valgus force and ACL tear. Studies have produced significant ACL strain with pure valgus torque. , Additionally, the knee joint responds to increased lateral axial pressure by increasing the internal and anterior rotation, which increases ACL strain dramatically. In support of this, training programs that reduce knee abduction moments have been shown to reduce risk of injury to the ACL.
Anatomic alignment differences are one of the many factors believed to contribute to the higher incidence of ACL injury in females. An increase in femoral anteversion has been considered a risk for ACL injury because excessive internal rotation at the hip may lead to knee valgus alignment. Femoral rotation also affects the patellofemoral joint. Forces of the femoral trochlea and the peripatellar retinaculum combine to act on the central portion of the patella. With the femur rotated internally, the lateral articular surface of the trochlea encroaches upon the patella’s lateral articular surface and presses it medially. With the femur rotated externally, the medial articular surface of the trochlea impinges upon the patella’s medial articular facet and pushes it laterally. The degree of femoral rotation is important in its potential to alter the biomechanics of the patellofemoral joint. Lee et al. concluded that femoral rotations greater than 20 degrees, often caused by trauma, congenital abnormality or infection of the bone result in severe alterations to the natural biomechanics of the patellofemoral joint.
External tibial rotation has also been associated with possible increased risk of ACL and patellar injury. A gender-specific in vivo study of knee laxity and stiffness suggested that external tibial rotation and abduction were associated with the impingement of the ACL. In this position the ACL would be compressed against the lateral wall of the intercondylar notch, making it more prone to injury. In addition to ACL injury, external tibial rotation has been associated with a variety of patellofemoral dysfunctions, including instability , and patellofemoral pain syndrome. Biomechanical studies have shown that fixing the tibia in 15 degrees of external tibial rotation will significantly increase lateral facet patellofemoral joint contact pressure at all knee flexion angles. To date, no gender differences have been established in tibial torsion ,
Soft Tissue Differences
Anterior Cruciate Ligament Anatomy
It has been well documented that females are more susceptible to ACL tears than males. There are 350,000 ACL injuries in the United States each year, with a disproportionate number of these injuries occurring in female athletes. , Numerous studies have been done to analyze gender differences in ACL characteristics such as cross-sectional area, volume, tensile strength and mechanical properties. While studies have found significant differences in these categories between genders, many conclude that these may more closely be related to differences in weight and height.
Studies have shown the female ACL has inferior mechanical properties than the male ACL, including a lower tensile modulus of elasticity and ultimate failure load. This apparent weakness has been thought to be due to smaller ACL cross-sectional area, smaller ACL volume and fewer collagen fibrils per unit area in female ACLs than in male ACLs. , , Anderson et al. found that even when adjusting for body weight, the female ACL has a smaller volume than the male ACL. Interestingly, their data showed that as height increased among male subjects the size of the ACL increased ( P = .03), but not among the female subjects ( P = .82). Chandrashekar et al. found that ACL size increased in proportion to notch width in males but not in females: female ACLs had smaller volumes and inferior mechanical properties when holding age and anthropometric measurements as covariates. In contrast, Charlton et al. found that when controlling for gender, height and weight, the gender differences in ACL volume became nonsignificant. Finally, Fayad et al. found that while the mean ACL volume was significantly larger in males than in females these differences were no longer statistically significant when adjusting for height. Regression analysis revealed height to be the most significant factor affecting ACL volume.
It is important to note that while the size of the ACL varies among individuals, so does the osseous anatomy. However, some patients may have a large ACL with a small intercondylar space; as previously mentioned, a small NWI has been linked to an increased likelihood of ACL rupture. ,
Females have been observed to have greater general joint laxity than males. In an in vivo study utilizing a Vermont knee laxity device and electromagnetic position sensors to measure anteroposterior, varus-valgus, internal and external laxity, Shultz et al. reported gender differences in laxity during weight-bearing activities. They concluded that the magnitude of difference was fairly consistent across all non-weight-bearing measures, with females having 25%–30% greater motion than males for anterior, varus-valgus and internal-external rotations (posterior laxity measures were not statistically significant in difference). When an axial compressive load of 40% body weight was applied to the knee joint, the total motion decreased in all knees but the magnitude of the difference between genders increased, with females having 50%–60% greater motion during weight bearing than males.
Other studies looking at gender-based differences in tibial external and internal rotations consistently report greater rotational laxity in females than in males, with differences ranging from 3 to 8 degrees. Notably, when considering varus-valgus laxity the differences between genders seemed smaller and, in some cases, not significant. , Compared with males, females have been observed to exhibit greater anterior tibial translation. , Liu et al. speculated the explanation for greater female anterior tibial translation involved hormonal differences, ligament size, the state of physical training, muscle strength, anatomic structure and muscle coordination. In some studies, however, gender differences in anterior laxity have only been exhibited in 50 degrees of flexion, or not at all. ,
Interest in better understanding gender-based differences in knee joint laxity has increased due to the known higher likelihood of ACL tears in females. Anterior tibial translation, internal tibial rotation and valgus tibial rotation all increase the force generated within the ACL. It is postulated that this increased internal laxity contributes to the higher rate of ACL tears in the female population when compared with males.
The medial and lateral menisci are crescent-shaped fibrocartilages, wedge-shaped in cross section, that sit on the rim of the medial and lateral tibial plateaus and conform the femoral and tibial contours. The menisci serve many important biomechanical functions. They decrease contact stresses and increase contact area of the tibiofemoral joint. They also contribute to load transmission, shock absorption, stability, joint lubrication and proprioception. Weight bearing produces axial forces across the knee, which compress the menisci causing a circumferential or “hoop” stress. Few gender-based differences have been established for the meniscus. Stone et al. utilized MRI to determine if meniscal size could be predicted by patient gender, height and weight. They found height had a linear relationship to total tibial plateau and that female patients generally presented with smaller dimensions than males. Additionally, groups with higher BMI presented with significantly larger meniscal dimensions than groups with lower BMI at any given height, thus making BMI a better predictor for meniscal size than height or gender.
Studies have shown that the contraction of the quadriceps muscle group applies an anterior shear force to the tibia through the patella tendon. This shear force may increase the load on the ACL when the knee flexion angle is less than 30 degrees. In this position, if the hamstring muscles do not apply sufficient posterior shear force as counter the ACL is at risk of tear. , Wojtys and Huston found that with maximum contraction of the knee musculature females reduced anterior tibial translation by 217%, whereas males reduced translation by 473%. In terms of injury to the knee, they proposed that these gender differences in strength and the hamstring-to-quadriceps (H/Q) ratio suggest that the ACL in females may be subjected to higher strain than the ACL in males during sports activities. In a review on female and male peak torque H/Q ratios, Hewett et al. found that gender differences in isokinetic H/Q ratios were not observed at slower angular velocities. They did find, however, that at high knee flexion/extension angular velocities (seen during sports activities) there were significant gender differences in the H/Q ratios. Unlike males, females did not show an increased H/Q torque ratio at velocities approaching those of functional activities. They also found with isokinetic dynamometer measurements that male athletes demonstrated significantly greater hamstring peak torque with increasing maturity, while females’ peak hamstring torque remained stable with increasing maturity. Hewett proposed that the decreased H/Q ratio of female athletes relative to males could possibly be related to imbalances in neuromusculature with the onset of puberty and hypothesized that this may be a contributing factor in the increased risk of ACL injury in postpubertal female athletes.
Many studies on neuromuscular control agree that females tend to be more “quadriceps dominant” compared to males. , The lateral pull of a dominant quadriceps muscle, in addition to an increased Q-angle, may contribute to the higher risk of patellar dislocation seen in females over males. Analyzing the epidemiology and natural history of acute patellar dislocation, Fithian et al. reported that between the ages of 10 and 17 years, the risk of primary dislocation is about 33% higher in girls than boys.
In addition to this, ACL tears in young athletes are associated with poor neuromuscular control leading to altered lower limb biomechanics including increased knee valgus and foot pronation angles, decreased hip and knee flexion, and hip abduction during cutting and landing. , Female athletes commonly demonstrate imbalances in strength, timing of activation and recruitment patterns of the lower extremity muscles. These gender differences in neuromuscular control of the knee appear to become prominent with the onset of puberty, as studies on younger adolescents have detected no significant difference in neuromuscular control with various athletic tasks until the age of 14 years. In a study including postpubertal females, Myer et al. found that risk factors for ACL tears in females (increased knee valgus and foot pronation angles, decreased hip and knee flexion, and hip abduction during cutting and landing) were exacerbated as they matured and peaked following the postpubertal stage of development.
It has been shown that implementation of neuromuscular training (NMT) programs that alter knee biomechanics and reduce abduction knee motion during the landing phase of a jump appear to reduce ACL injury risk in youth athletes. These findings have been paramount in designing effective NMT in female athletes. There may be a potential window of opportunity to decrease ACL injury risk in young female athletes if NMT is implemented prior to the onset of puberty. After a 6-week NMT intervention inclusive of strength training, plyometric training and balance training, teenage female basketball, volleyball and soccer athletes have been found to significantly increase their knee flexion angle and decrease internal knee valgus in response to a drop vertical jump task. The effects of NMT in preventing the incidence of ACL injury have been extensively studied in adolescent basketball, soccer and volleyball athletes. , , NMT programs have been shown to be effective in improving lower limb alignment on a drop-jump test, increasing hamstring strength, increasing knee flexion angles on landing and reducing deleterious moments and ground reaction forces. The reduction of injurious forces on the knee after NMT suggests that risk factors such as lower H/Q ratios and uncoordinated muscle recruitment are modifiable.
The male and female knee anatomy and biomechanics are generally quite similar. However, there are subtle differences that may account for a slightly higher risk of specific injuries in females compared with males, such as ACL tears and/or patellar dislocations. Neuromuscular patterns may also play a role in injury risk and could potentially represent modifiable risk factors. It should be noted that many studies on gender differences have concluded that other factors such as patient height, weight and even ethnicity may have substantial impacts on knee anatomy and neuromuscular patterns. Future research on female and male knee anatomy and biomechanics should attempt to control for these factors when searching for gender-specific differences.