Mechanisms of Noncontact Anterior Cruciate Ligament Injuries

Published on 11/04/2015 by admin

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Chapter 2 Mechanisms of Noncontact Anterior Cruciate Ligament Injuries

As in the prevention of other injuries in sports, understanding injury mechanisms is a key component of preventing noncontact anterior cruciate ligament (ACL) injuries.¹ The research effort to determine risk factors of sustaining noncontact ACL injuries is increasing as the concerns of increased incidents and cost for treatment, as well as serious consequences of noncontact ACL injuries, are growing. Prospective cohort studies are commonly used in epidemiological research designs for determining risk factors of injuries and diseases² and are being used to determine risk factors of sustaining noncontact ACL injuries.³ The results of epidemiological studies with cohort designs, however, are descriptive in nature and lack cause-and-effect relationship between identified risk factors and the injury.² Without a good understanding of the injury mechanisms, the risk factors of sustaining noncontact ACL injuries identified from epidemiological studies could be misinterpreted and could lead to the selection of nonoptimal injury prevention programs.

Injuries of the ACL frequently occur in athletic movements such as stopping or quickly changing directions. These kinds of movements often are awkward and off-balance maneuvers. Video analysis often shows a hard landing with the knee near full extension in these movements as the athlete experienced a sensation of the knee collapsing into a valgus position. The quadriceps muscles are likely to be the major source of the anterior shear force that causes the rupture of the ACL in these movements. However, we have not been accustomed to considering the fact that our own muscles can create injuries. Although a valgus moment applied to the knee can create enough deformation to cause an injury of the ACL, few noncontact ACL injuries involve serious injuries to the medial collateral ligament (MCL) that would occur if the knee sustained sufficient valgus moment loading to injure the ACL. This chapter will examine biomechanical studies relating to ACL injury and explore strains induced by the quadriceps muscles near full knee extension and by valgus moment loading.

Mechanically, ACL injury occurs when an excessive tension force is applied on the ACL. A noncontact ACL injury occurs when a person self-generates great forces or moments at the knee that applied excessive loading on the ACL. An understanding of the mechanisms of ACL loading during active human movements, therefore, is crucial for understanding the mechanisms of noncontact ACL injuries and risk factors of sustaining noncontact ACL injuries. Berns et al ⁴ investigated the effects of combined knee loading on ACL strain on 13 cadaver knees. The strain of the anteromedial (AM) bundle of the ACL was recorded using liquid mercury strain gauges at 0 and 30 degrees knee flexion. The results of this study showed that anterior shear force on the proximal end of the tibia was the primary determinant of the strain in the AM bundle of the ACL, whereas neither pure knee internal-external rotation moment nor pure knee valgus-varus moment had significant effects on the strain of the AM bundle of the ACL. The results of this study further showed that anterior shear force at the proximal end of the tibia combined with a knee valgus moment resulted in a significantly greater strain in the AM bundle of the ACL than did the anterior shear force at the proximal end of the tibia alone.

Markolf et al ⁵ also investigated effects of anterior shear force at the proximal end of the tibia and knee valgus, varus, internal rotation, and external rotation moments on the ACL loading of cadaver knees. A 100N anterior shear force and 10-Nm knee valgus, varus, internal rotation, and external rotation moments were added to cadaver knees. The ACL loading was recorded as the knee was extended from 90 degrees flexion to 5 degrees hyperextension. The results of this study showed that an anterior shear force on the tibia generated significant ACL loading, whereas the knee valgus, varus, and internal rotation moments also generated significant ACL loading only when the ACL was loaded by the anterior shear force at the proximal end of the tibia. The results of this study further showed that the ACL loading due to the anterior shear force combined with either a valgus or a varus moment to the knee was greater than that due to the anterior shear force alone, whereas the ACL loading due to the anterior shear force combined with a knee external rotation moment was lower than that due to anterior shear force alone. The knee valgus and external rotation moment loading are elements of dynamic valgus that many current ACL injury prevention programs are trying to avoid.³ The results of the study by Markolf et al⁵ also showed that ACL loading due to the combined knee varus and internal rotation moment loading was greater than that due to either knee varus moment loading or internal rotation moment loading alone and that the ACL loading due to combined knee valgus and external rotation moment loading was lower than that due to either knee valgus or external rotation moment loading alone. Finally, the results of this study showed that the ACL loading due to the anterior shear force and knee valgus, varus, and internal rotation moments increased as the knee flexion angle decreased.

Fleming et al ⁶ studied the effects of weight bearing and tibia external loading on ACL strain. They implanted a differential variable reluctance transducer to the AM bundle of the ACL of 11 subjects. ACL strains were measured in vivo when a subject’s leg was attached to a knee loading fixture that allowed independent application of anterior-posterior shear force, valgus-varus moments, and internal-external rotation moments to the tibia and simulation of weight-bearing conditions. The anterior shear force was applied on the proximal end of the tibia from 0N to 130N in 10-N increments. The valgus-varus moments were applied to the knee from −10 Nm to 10 Nm in 1-Nm increments. The internal-external rotation moments were applied to the knee from −9 Nm to 9 Nm in 1-Nm increments. The knee flexion angle was fixed at 20 degrees during the test. The results of this study showed that ACL strain significantly increased as the anterior shear force at the proximal end of the tibia and the knee internal rotation moment increased, whereas knee valgus-varus and external rotation moments had little effects on ACL strain under the weight-bearing condition.

The previously mentioned studies consistently showed that the anterior shear force at the proximal end of the tibia is a major contributor to ACL loading, whereas the knee valgus, varus, and internal rotation moments may increase ACL loading when an anterior shear force at the proximal end of the tibia is applied. According to these ACL loading mechanisms, a small knee flexion angle, a strong quadriceps muscle contraction, or a great posterior ground reaction force can increase ACL loading.

Quadriceps muscles are the major contributor to the anterior shear force at the proximal end of the tibia through the patella tendon. DeMorat et al ⁷ demonstrated that a 4500-N quadriceps muscle force could create ACL injuries at 20 degrees knee flexion. Eleven cadaver knee specimens were fixed to a knee simulator and loaded with 4500-N quadriceps muscle force. Quadriceps muscle contraction tests at 400 N (Q-400 tests) and KT-1000 tests were performed before and after the 4500-N quadriceps muscle force loading. Tibia anterior translations were recorded during the Q-400 and KT-1000 tests. All cadaver knee specimens were dissected after all tests to determine the ACL injury states. Six of the 11 specimens had confirmed ACL injuries (three complete ACL tears and three partial tears). All specimens showed increased tibia anterior translation in Q-400 and KT-1000 tests. The result of this study also showed that quadriceps muscle contraction caused not only tibia anterior translation but also tibia internal rotation.

Decreasing knee flexion angle increases the anterior shear force at the proximal end of the tibia by increasing the patella tendon–tibia shaft angle. With a given quadriceps muscle force, the anterior shear force at the proximal end of the tibia is determined by the patella tendon–tibia shaft angle, defined as the angle between the patella tendon and the longitudinal axis of the tibia.⁸ With a given quadriceps muscle force, the greater the patella tendon–tibia shaft angle, the greater the anterior shear force on the tibia. Nunley et al⁸ studied the relationship between the patella tendon–tibia shaft angle and knee flexion angle with weight bearing. Ten male and 10 female university students without known history of lower extremity injuries were recruited as the subjects. Sagittal plane x-ray films were taken for each subject at 0, 15, 30, 45, 60, 75, and 90 degrees knee flexion, bearing 50% of body weight. Patella tendon–tibia shaft angles were measured from the x-ray films. Regression analyses were performed to determine the relationship between patella tendon–tibia shaft angle and knee flexion angle and to compare the relationship between genders. The results showed that the patella tendon–tibia shaft angle was a function of the knee flexion angle, with the patella tendon–tibia shaft angle increasing as the knee flexion angle decreased, and that on average the patella tendon–tibia shaft angle was 4 degrees greater in females than in males. The relationship between the patella tendon–tibia shaft angle and knee flexion angle obtained by Nunley et al⁸ was consistent with those from other studies on the patella tendon–tibia shaft angle under non–weight-bearing conditions.⁹^–¹¹

Decreasing the knee flexion angle also increases ACL loading by increasing the ACL elevation angle and deviation angle, defined as the angle between the longitudinal axis of the ACL and the tibia plateau and the angle between the projection of the longitudinal axis of the ACL on the tibia plateau and the posterior direction of the tibia, respectively.¹² The resultant force along the longitudinal axis of the ACL equals the anterior shear force on the ACL divided by the cosines of the ACL elevation and deviation angles. The greater the ACL elevation and deviation angles, the greater the ACL loading with a given anterior shear force on the ACL. Li et al¹²

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