ACL - female anterior cruciate ligament injuries review

ACL, anterior cruciate ligament injuries in women - the tip of the iceberg

The anterior cruciate ligament (ACL) is the most commonly damaged ligament of the knee and accounts for up to 50% of documented ligamentous knee injuries. anterior cruciate ligament injury rates are four to eight times higher in women than men1. Female athletes who take part in sports involving jumping and 'cutting' such as soccer, basketball, volleyball and gymnastics are particularly at risk 2. This review will consider the basic anatomy and mechanism of injury of anterior cruciate ligament injuries with particular emphasis on the epidemic of injuries in female athletes that has recently come to light. Later articles will detail the prevention of such injuries and their optimal management.
The anterior cruciate ligament is an intra-articular structure that traverses the knee joint attaching the tibia to the femur. It comprises multiple longitudinal fascicles that insert proximally on the medial aspect of the lateral femoral condyle and distally on the anterior tibial plateau. These fascicles fan out as they approach their tibial insertion.
The anterior cruciate ligament prevents hyperextension (overstraightening) of the knee and is the primary restraint to anterior displacement of the tibia. The intact anterior cruciate ligament is a secondary stabiliser to varus and valgus (side to side) stress at the knee, and also plays a role in limiting internal rotation of the knee.
Unlike the medial collateral and posterior cruciate ligaments, the anterior cruciate ligament does not have the capacity to heal. Once injured, it does not reconstitute as a functional entity and surgical repair has proved less efficacious than reconstruction using other tissues.

Why has the injury rate increased?
The cause of the increased anterior cruciate ligament injury rate in females is unclear and is most likely to be a complex interplay between multiple variables. Speculation on the possible aetiology of anterior cruciate ligament injuries in women has centred on anatomical differences, joint laxity, physiological differences between men and women, and the effects of hormones and training techniques. Investigators have not agreed on the causal factors for anterior cruciate ligament injury but have started to profile the type of athlete who is at risk 3.
The primary factors involved in the high incidence of female anterior cruciate ligament injury include:
» the effects of the wide female pelvis
» a narrow femoral intercondylar notch
» physiological laxity of the ACL with hormonal interplay
» muscle reaction time disparity

Wide female pelvis. The wider female pelvis exaggerates the angle between the femur and the tibia at the knee. The resulting increased inward pressure on the knee with external rotation of the tibia may place excessive stress on the anterior cruciate ligament 4.
Narrow intercondylar notch. The intercondylar notch of the femur through which the anterior cruciate ligament has to pass is generally smaller in women than it is in men4. This difference may not be statistically significant5,6, since there is a wide overlap of notch sizes between the sexes. It is postulated that cutting and jumping movements in patients with narrow femoral notches may weaken and fray the anterior cruciate ligament. Shelbourne et al have shown that patients with narrow notches (<15mm) have a higher incidence of tears in
their contralateral ACL5. After reconstruction with a 10mm autograft, the incidence of graft rupture appears the same for men and women.
It is unlikely that the increased incidence of anterior cruciate ligament tears in females compared to males in the same sports can be attributed to notch width alone6, but this factor undoubtedly plays a part. Graft impingement from regrowth of the notch is also recognised as a relevant factor that can result in late graft failure in the anterior cruciate ligament reconstructed knee7. No attempt has been made to prophylactically enlarge the femoral notch, but the measurement of notch size helps to predict the risk of anterior cruciate ligament injury.
Lax anterior cruciate ligament and hormonal interplay. Receptors for both oestrogen and progesterone have been identified on the anterior cruciate ligament. These hormones are believed to make the female anterior cruciate ligament more lax and susceptible to overstretching8. Furthermore, recent research suggests that women are more susceptible to anterior cruciate ligament injuries during the ovulatory phase of the menstrual cycle rather than the follicular phase1. anterior cruciate ligament fibroblast proliferation and type I procollagen synthesis vary in a dose dependent manner with oestradiol concentrations 9. Clinically, alterations in anterior cruciate ligament cellular metabolism caused by oestrogen fluctuations in a menstruating athlete may render the anterior cruciate ligament more susceptible to injury at this time.
Muscle reaction time disparity. Female athletes typically have less strength in their leg muscles and slower muscle reaction times than males. Strong fast-reacting hamstrings are needed to optimise the chance of keeping the anterior cruciate ligament intact - for example, by keeping the tibia in place during landing from sudden jumps and stops. Even if the hamstrings are strong, if they react slowly they may be unable to protect the anterior cruciate ligament in time to avoid injury10. A low level of hamstring activity and low angle of knee flexion at foot strike and during eccentric contraction, coupled with forces generated by the quadriceps muscles at the knee, could produce significant anterior displacement of the tibia, which may play a role in anterior cruciate ligament injury 11.

Non-contact sports are worse
ACL injuries in women have until recently been underdiagnosed. This is partly because of the perception that this was primarily a male injury, but also partly because the extent of the knee injury is initially underestimated in many men and women. Contrary to popular belief, anterior cruciate ligament injuries are more commonly seen after non-contact rather than contact sports. An understanding of the mechanisms involved is important as this gives a clue as to associated injuries, and is occasionally pathognemonic of anterior cruciate ligament injury on the history alone. Patients often describe one of the following modes of injury:

Non-contact injuries
Deceleration injuries. These may be seen in basketball or football players who decelerate to change direction. Increased quadriceps contraction induces an anterior force on the proximal tibia. If the tibia (lower leg) is internally rotated at the time, the anterior cruciate ligament is at risk.
Flexion/valgus/external rotation. As the knee is bent and twisted out (for example if a football or rugby boot is stuck in the turf, or a ski binding does not release the boot in a twisting fall), the medial knee ligaments are initially injured, and then the anterior cruciate ligament gives way. These injuries are often associated with medial meniscal tears - the so-called unhappy triad of O'Donoghue.
Hyperextension. This may be seen in basketball and volleyball players who land awkwardly, and in gymnasts during the dismount. It is the mechanism of injury in some footballers who cannot control their landing after going up for a header.
Quadriceps active tear. In skiers the sudden pull of the quadriceps with the knee bent can occasionally tear the anterior cruciate ligament.

Contact injuries
A direct blow to the knee or shin can also lead to an anterior cruciate ligament tear. These injuries are usually associated with damage to other structures within the knee.
In over 50% of cases, the patient will hear a 'pop' or feel tearing within the knee. The patient is usually unable to continue playing sport and the knee swells rapidly. The swelling is due to a haemarthrosis (bleeding into the joint) and studies have shown that 80% of patients attending accident departments with acute haemarthroses have sustained anterior cruciate ligament injuries. Of these, 60% have concomitant knee pathology at presentation, most commonly a lateral meniscal tear12.

Most need reconstruction
The initial treatment of an anterior cruciate ligament injury is based on reducing pain and swelling and getting the knee joint moving normally. The debate as to the need and timing of reconstructive surgery is ongoing. Conservative treatment involves the re-education of the quadriceps and hamstring muscles with an emphasis placed on the hamstrings as they can restrict the amount of forward tibial translation on the femur. Many less active patients manage their day to day activity after
a rehabilitation programme. Difficulties arise when the athlete tries to return to his / her sports but finds that the knee gives way. Athletes who continue sports in spite of recurrent giving way have a very high rate of meniscal and chondral (joint surface) injuries.
In the majority of sporting individuals who wish to return to full activity, some form of reconstruction is required. These will be discussed in a future issue. The results of operative reconstruction obviously vary but the majority of studies suggest that 80 to 90% of patients can return to their pre-injury sporting activities. Most studies after reconstruction have been male dominated. Barber-Westin et al compared results of anterior cruciate ligament reconstruction between sexes and concluded that they were similar and that sex alone cannot be used as a selection criterion for anterior cruciate ligament reconstruction15.

Alex Watson and
Fares Haddad


References
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2. American Journal of Sports Medicine, vol 23, pp 694-701, 1995
3. Clinical Orthopaedics & Related Research, vol 372, pp 50-63, 2000
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8. American Journal of Sports Medicine, vol 24, pp 427-436, 1996
9. Clinical Orthopaedics & Related Research, vol 366, pp 229-238, 1999
10. Peak Performance, vol 140, pp 1-6, 2000
11. American Journal of Sports Medicine, vol 28 (2), pp 234-240, 2000
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13. Journal of Bone and Joint Surgery, vol 65A, pp 154-163, 1983
14. Clinical Journal of Sport Medicine, vol 9 (2), pp 63-69, 1999
15. American Journal of Sports Medicine, vol 25 (4), pp 514-526, 1997