Anterior Cruciate Ligament Injuries

Anatomy, Physiology, Biomechanics, and Management

Leon Siegel, BA; Carol Vandenakker-Albanese, MD; David Siegel, MD, MPH


Clin J Sport Med. 2012;22(4):349-355. 

In This Article

Abstract and Introduction


Objective: Anterior cruciate ligament (ACL) injuries are the most common ligament injury in the United States. These injuries can be career ending for athletes and severely disabling for all individuals. Our objectives are to review the epidemiology of these injuries, as well as ACL biomechanics, anatomy, and nonsurgical and surgical management so that generalists as well as sports medicine physicians, orthopedists, and others will have a better understanding of this serious injury as well as choices in its management.
Data Sources: PubMed was used to identify relevant articles. These articles were then used to identify other sources.
Main Results: Anterior cruciate ligament injuries occur more commonly in women than in men due to a variety of anatomical factors. The ACL consists of 2 major bundles, the posterolateral and the anteromedial bundles. Forces transmitted through these bundles vary with knee-joint position. Some patients with ACL injuries may not be candidates for surgery because of serious comorbid medical conditions. However, without surgical repair, the knee generally remains unstable and prone to further injury. There are a variety of surgical decisions that can influence outcomes. Single-bundle versus double-bundle repair, whether to leave the ruptured ACL remnant in the knee, the selection of the graft tissue, graft placement, and whether to use the transtibial, far anteromedial portal, or tibial tunnel–independent technique are choices that must be made.
Conclusions: With a sound knowledge of the anatomy and kinetics of the knee, newer improved surgical techniques have been developed that can restore proper knee function and have allowed many athletes to resume their careers. These new techniques have also limited the disability in nonathletes.


In the United States, anterior cruciate ligament (ACL) injuries total between 100 000 and 200 000 yearly, making this the most common ligament injury.[1–4] This number continues to increase in both the general population and in individuals who play sports. Football players sustain the greatest number of ACL injuries (53% of the total) with skiers and gymnasts also at high risk.[5,6] This is a focused review based on a search of PubMed using the topic headings below that will be of interest to clinicians who take care of patients with ACL injuries. Articles identified through PubMed articles were then used to identify additional sources. A comprehensive review of each area is beyond the scope of this article, and the reader is referred to discussions of specific areas in the references.

Factors That Contribute to Anterior Cruciate Ligament Injuries

Anterior cruciate ligament injury rates tend to be higher for women than for men.[7–9] At the United States Naval academy, in intercollegiate soccer, basketball, and rugby, women had a relative risk of 3.96 of ACL injury compared with men. The National Collegiate Athletic Association Injury Surveillance System (1990–2002) found that the rate of ACL injury, regardless of the mechanism, was significantly higher for female collegiate athletes than for male collegiate athletes in both soccer and basketball.[10] The stronger the quadriceps muscles, the larger and hence stronger the ACL, although it is unclear that in any one individual that an increase in quadriceps size and strength results in an increase in ACL size.[7] Quadriceps muscles, even after adjustments for differences in weight and lean body mass, are larger in male athletes than in female athletes.

There are neuromuscular and biomechanical risk factors associated with ACL injury.[11] Compared with running, there is a significant increase in ACL load during sidestepping and crossover cutting maneuvers.[12] This is the result of a large increase in varus/valgus and internal/external rotation movements. These increased stresses during cutting put the ACL at risk, especially when the knee is at flexion angles between 0 and 40 degrees. Appropriate muscle activation strategies may counter these movements. A project aimed at implementing neuromuscular training for soccer and handball players resulted in increased electromyography activity for the medial hamstring muscles, thereby decreasing the risk of dynamic valgus.[13]

Notch stenosis may contribute to an increase in rates of ACL injury.[14–17] The notch width index (NWI) has been used as a measure of notch stenosis.[14] The NWI is the ratio of the width of the intercondylar notch to the width of the distal femur at the level of the popliteal groove. In a study, the mean NWI for normal knees was 0.2338; for acute ACL injured knees, it was 0.2248; and for those with bilateral ACL injuries, it was 0.1961.[14] Notch stenosis is not felt to be a factor in gender differences in ACL injury.[7] In a study of 895 US Military Academy cadets who were followed up for more than 4 years, small femoral notch width was found to be a significant risk factor for ACL injuries in men and women and body mass index was a risk factor in women.[15] The NWI did not differ significantly between male and female athletes. Notch width has also been found to correlate with ACL and posterior cruciate ligament (PCL) width, and it has been argued that it is the ACL size rather than notch size that is the important risk factor for ACL injury.[17]