Female athletes have a substantially increased risk of ACL rupture than their male counterparts, when playing the same sports. Although the medical literature in the past three or four decades is replete with articles on ACL injury, its incidence and gender prevalence have been studied only recently. In a 1985 study, 95% of ACL injuries occurred in patients between the ages of 16 and 45 years, and almost three-quarters of these were sports-related. The annual incidence of ACL injury in this age group has been calculated at one in 1,750, with the 15 to 25 year old age group incurring more than half of these injuries. Thus, the risk for an athlete between the ages of 15 and 25 may be closer to one in a thousand. For female athletes, it is estimated that 1 in 100 will sustain an ACL tear in high school, and 1 in 10 in college.

Earlier studies of the incidence of ACL injury noticed a male predominance of approximately two-to-one, which can be attributed to low female sports participation rates at the time. In the study by Daniel et al., only 19% of the patients were female.¹ As the majority of ACL injuries occur in sports, one would expect incidence by gender to reflect the amount of each gender’s participation in sports, especially sports placing them at risk for this type of injury. As female sports participation has increased, owing substantially to Title IX and its enforcement, female ACL injuries have also increased. Comparison between males and females by sport has definitely shown an increased risk for ACL rupture among female athletes.

Adjusted for equal numbers of participants, it was found that female high school basketball players required knee surgery more than three times as often as male basketball players. When ACL injury was studied specifically, Arendt and Dick found a 2.4 times increased risk for female soccer players and 4.1 times higher for basketball. In other studies, team handball showed a 5-fold increased risk, and a U.S. Naval Academy study showed a 2.44-fold increased risk when data were combined for several sports.^, In competitive alpine skiing, Robert Johnson’s Burlington, Vermont group found that competitive female skiers appear to injure their ACL’s at a rate 3.1-fold higher than competitive males and re-injure their reconstructed ACL’s more than twice as often. This gender disparity is not seen in recreational alpine skiers, however.

These findings are a cause for significant concern. Although ACL reconstruction is a well-developed procedure, satisfactory knee stability is not universally achieved, and the initial ACL injury is often accompanied by meniscal damage. Partial menisectomy, usually required to some degree in such a situation, is well known to result in increased contact stress between femoral condylar and tibial plateau articular cartilage with weightbearing. This leads to earlier onset of osteoarthritis. In the absence of ACL reconstruction, subsequent instability episodes are likely to cause further meniscal injury, as well as articular cartilage shear injury, thus worsening the situation.

In addition, ACL reconstruction and rehabilitation is an arduous process, requiring many months and likely resulting in substantial changes to an athlete’s playing season. In this clinic, it has been noticed that athletes who have witnessed a friend going through the process of ACL reconstruction appear much more upset upon learning that they have ruptured their ACL.

Prevention of ACL injuries in female athletes rests upon developing an adequate understanding of the reason for the gender disparity. A number of factors have been investigated: phase of the menstrual cycle, footwear and playing surface, intercondylar notch width and biomechanics. The effect of prophylactic knee bracing has also been studied.

Regarding hormonal effects, estrogen and progesterone receptors have been found in the human ACL, raising the possibility that the ACL may undergo changes throughout the menstrual cycle. However, studies of the relationship between risk of ACL injury and phase of the menstrual cycle have been conflicting. Part of the disagreement between studies may be related to the degree of oral contraceptive use among athletes, and other studies fail to distinguish between ACL ruptures and other types of injuries. It is clear that further studies are needed.

Studies of the effect of intercondylar notch width have been plagued by apparent difficulties in obtaining accurate and reproducible measurements. Even intra-operative measurements seem to share this problem. When radiographs are used for the measurement, rotation can significantly affect the result. Reproducibility between knees in the same subject can be obtained when a jig is used to hold the leg in an exact position while the x-ray is taken, but not if the jig is not utilized. The one study utilizing such a jig showed no difference in notch width index (NWI, notch width divided by) between males and females, although another study with more than three times as many subjects did show females to have a smaller NWI.

Although hormonal effects and NWI have been popular topics in the past, with regard to ACL injuries in females, nowadays they appear to be given only passing notice. Whether or not these factors actually play a significant role, one might take the view that the NWI is a non-modifiable risk factor, and that few, if any, female athletes would avoid practice or competition during a particular phase of their menstrual cycle.

Prophylactic knee bracing (bracing a knee that has not been previously injured) has shown mixed results. Some college athletic programs have braced all athletes in a particular sport or playing certain positions, believing that they are preventing serious knee injuries. However, controlled studies have either shown no difference between braced and unbraced players, or position-dependent differences.^, Some studies have found a higher injury rate in braced players, including ACL injuries and ankle and foot injuries. Thus, the benefits of bracing are highly questionable, and they may actually cause harm. Until research emerges showing a clear advantage to bracing, and it looks as if this will not occur, prophylactic bracing should be avoided.

Most attention is now focused on issues of neuromuscular control and biomechanics. Perhaps surprisingly, the quadriceps are capable of rupturing the ACL by the force of contraction alone, and it is thought that this mechanism plays a significant role in non-contact ACL ruptures. At knee angles approaching extension, quadriceps contraction exerts an anterior force on the tibia, owing to the angle that the patellar tendon makes with the tibia. This appears to be most pronounced at 20 degrees of flexion and disappears at angles greater than 50 degrees. Other investigators have found the anterior sheer load on the tibia during running and cutting to be greater than 2000 N, which approaches the tensile strength of the ACL, and perhaps as high as 5000 N with eccentric quadriceps contraction during heel strike. Videotape analysis of athletes injuring their ACL’s confirms that the athlete tends to be in a position of knee extension, usually with some degree of valgus, and with internal rotation of the femur on the tibia. This rotational position tightens the ACL by causing the femoral condyles to ride up on the tibial spine.

Female athletes appear to put their ACL’s at risk by performing many maneuvers with more knee extension than their male counterparts. Added to this is the finding that trained female athletes are more "quadriceps-dominant" than untrained females or both trained and untrained males. When male and female subjects were challenged with an abrupt anterior tibial translation force, the trained females responded by firing their quadriceps, whereas all others fired their hamstrings. Hamstring firing would be the more appropriate response, since this muscle group acts as a dynamic stabilizer to anterior tibial translation.

Hip weakness also plays a role. Absorption of energy while landing from a jump is shared by the ankle plantarflexors, quadriceps, hamstrings and hip extensors. Calculated work performed by three major muscle groups during landing from jumps of varying heights are shown in Table 1. The hip extensors clearly play nearly as much as a role as the quadriceps. Weakness in any one of these links in the kinetic chain forces other muscle groups to incur a higher load. If the hip extensors are weak, not only is more force transmitted through the knee extensor mechanism, but the athlete lands in a more upright stance to prevent having to utilize the hip extensors. This combination of landing in an upright stance and increased force transmission through the knee extensor mechanism can be disastrous for the ACL.

Table 1. Work in Joules/kg by muscle groups in landing from a height¹⁰

Footnotes

1. Daniel DM, Stone ML, Sachs R, Malcom L. Instrumented measurement of anterior knee laxity in patients with acute anterior cruciate ligament disruption. Am J Sports Med. 1985,13(6):401.

2. Garrick JG, Requa RK. Anterior cruciate ligament injuries in men and women: how common are they? In Prevention of Noncontact ACL Injuries, Griffin LY, ed., American Academy of Orthopaedic Surgeons, Rosemont, IL, 2001.

3. Chandy TA, Grana WA. Secondary school athletic injury in boys and girls: A three-year comparison. Phys Sportsmed. 1985,13(3):106.

4. Arendt E, Dick R. Knee injury patterns among men and women in collegiate basketball and soccer: NCAA data and review of literature. Am J Sports Med. 1995,23(6):694.

5. Myklebust G, Maehlum S, Holm I, Bahr R. A prospective cohort study of anterior cruciate ligament injuries in elite Norwegian team handball. Scand J Med Sci Sports. 1998,8(3):149.

6. Gwinn DE, Wilckens JH, McDevitt ER, Ross G, Kao TC. Relative gender incidence of anterior cruciate ligament injury at a military service academy (abstract). 66^th Annual Meeting Proceedings, American Academy of Orthopaedic Surgeons, 1999, 117.

7. Gwinn DE, Wilckens JH, McDevitt ER, Ross G, Kao TC. The relative incidence of anterior cruciate ligament injury in men and women at the United States Naval Academy. Am J Sports Med. 2000;28(1):98.

8. Stevenson H, Webster J, Johnson R, Reynnon B. Gender differences in knee injury epidemiology among competitive alpine ski racers. Iowa Orthop J. 1998,18:64.

9. LaPrade RF, Burnett QM II, Femoral intercondylar notch stenosis and correlation to anterior cruciate ligament injuries: A prospective study. Am J Sports Med. 1994,22(2):198.

10. Hewson GF Jr, Mendini RA, Wang JB. Prophylactic knee bracing in college football. Am J Sports Med. 1986;14(4):262.

11. Albright JP, Powell JW, Smith W, Martindale A, Crowley E, Monroe J, Miller R, Connolly J, Hill BA, Miller D, et al. Medial collateral ligament knee sprains in college football: Effectiveness of preventive braces. Am J Sports Med. 1994;22(1):12.

12. Albright JP, Powell JW, Smith W, Martindale A, Crowley E, Monroe J, Miller R, Connolly J, Hill BA, Miller D, et al. Medial collateral ligament knee sprains in college football: Brace wear preference and injury risk. Am J Sports Med. 1994;22(1):2.

13. Rovere GD, Haupt HA, Yates CS. Prophylactic knee bracing in college football. Am J Sports Med. 1987;15(2):111.

14. Grace TG, Skipper BJ, Newberry JC, Nelson MA, Sweetser ER, Rothman ML. Prophylactic knee braces and injury to the lower extremity. J Bone Joint Surg Am. 1988;70(3):422.

15. Beynnon BD, Howe, JG, Pope MH, Johnson RJ, Fleming BC. The measurement of anterior cruciate ligament strain in vivo. Int Orthop. 1992; 16:1.

16. Zhang S-N, Bates BT, Dufek JS. Contributions of lower extremity joints to energy dissipation during landings. Med Sci Sports Exercise. 2000; 32(4)812.

17. Caraffa A, Cerulli G, Projetti M, Aisa G, Rizzo A. Prevention of anterior cruciate ligament injuries in soccer: A prospective controlled study of proprioceptive training. Knee Surg Sports Traumatol Arthrosc. 1996;4:19.