The Anterior Cruciate Ligament Enigma

Injury Mechanisms and Prevention

Abstract

The reasons for the higher frequency of anterior cruciate ligament injuries in women are largely conjecture. These injuries may result from direct contact or, more frequently, from no direct contact to the knee during activities that most athletes consider routine to their sport. This implies that there are intrinsic factors that lead to anterior cruciate ligament rupture. For the anterior cruciate ligament to tear, there must be excess anterior tibial translation or rotation of the femur on the tibia. In the former case, the tibia can move anteriorly during quadriceps activation that is not counterbalanced by hamstring activation. Patients describe their injury as occurring when landing, stopping, or when planting to change directions. The knee typically was near full extension. Mechanically, the angle of the patellar tendon and tibial shaft increases as the knee approaches full extension. This gives a mechanical advantage to the quadriceps. During cutting maneuvers, athletes tend to cut with a knee near extension (0°-20°) when the quadriceps are active and the hamstrings are neither very active nor at a knee flexion angle that offers much of a mechanical advantage. In performing cutting and landing maneuvers, women tend to perform the activities more erect; that is, with their knees and hips closer to extension. One possible factor to help reduce the frequency of anterior cruciate ligament injuries in women may be in proper instruction for performing cutting and landing maneuvers which will lower their center of gravity thereby denying the quadriceps the opportunity to shift the tibia anteriorly.

One of the most commonly damaged ligaments of the knee is the anterior cruciate ligament. Significant strides have been made in appreciating the structure and function of the ligament and progress in surgical technique and rehabilitation has lead to reconstruction being a relatively common procedure. Most published studies are directed toward the basic mechanics of the ligament, method of surgical repair, and rehabilitation.^{3,7,8,9,10,12,16-20} Unfortunately, little is known about the actual mechanism of injury during sport activity.

One sport that has been studied in some detail is alpine (downhill) skiing.^4,5 Three common causes of anterior cruciate ligament injury have been discussed. One mode of ligament failure is referred to as the "phantomfoot" mechanism.⁵ This occurs when a skier falls backward with the knee flexed and internally rotated. The combination of a strong quadriceps contraction (to maintain balance) with a rigid boot that fails to release may be lead to an anterior cruciate ligament disruption.⁶ As a result, the ligament disruption occurs during a hard landing when the skier is off-balance. In the second mechanism, when the skier lands on the ski's tail, the stiff posterior shell of the boot combined with a strong quadriceps contraction to maintain balance slides the tibia anteriorly leading to an anterior drawer maneuver. The third mechanism, valgus rotation, seems to be more common in men who participate in downhill or cross-country skiing. This occurs when the inside (medial) edge of the front of the ski gets caught in the snow. The leg is abducted and externally rotated while the skier is driven forward by momentum. High-speed skiing and poorly kept ski slopes also are factors that contribute to this type of injury.

Regrettably, little attention has been directed to the mechanism of anterior cruciate ligament injuries in other sports. Sudden deceleration, an abrupt change of direction, and a fixed foot have been described as key elements of an anterior cruciate ligament injury.⁶ Mechanisms of injury were reported in 23 athletes.¹³ Seventy percent of the injuries were noncontact injuries and 30% involved direct contact to the knee. The patients who could fully describe the injury estimated knee flexion angle to be between full extension and 20° with some tibial internal rotation. The prominent mechanism involved a slightly flexed knee and internal rotation of the tibia at foot-strike.

Anterior cruciate ligament injury factors have been divided into intrinsic and extrinsic factors.¹ Intrinsic factors include a narrow intercondylar notch, a weak anterior cruciate ligament, generalized overall joint laxity, and lower extremity malalignment. Extrinsic factors include abnormal quadriceps and hamstring interactions, altered neuromuscular control, shoe to surface interface, the playing surface, and the athlete's playing style.

Interviews and Videotapes

A focus of the authors' current research has been on the lower limb kinetics and kinematics while the subject performs athletic maneuvers that seem to be implicated in anterior cruciate ligament injuries. Although the specific activities leading to an anterior cruciate ligament tear in subjects who ski have been described, the medical community is only just beginning to get a better understanding of the specific activities and movement patterns that seem to be injurious to the ligament.

Boden and Garrett interviewed 65 males with 72 anterior cruciate ligament injuries and 25 females with 28 injuries (mean age at time of injury was 26 years; range, 14-48 years) using a standardized questionnaire to determine what common characteristics might be found. (Boden BP, Garrett WE: Mechanisms of injuries to the anterior cruciate ligament. Presented at the American College of Sports Medicine, Cincinnati, May 1996). No patients with skiing injuries were included. A physician administered a questionnaire that focused on lower extremity position at the time of injury. The questions requested information regarding contact versus noncontact, foot and tibia position, knee flexion, direction of knee collapse, and direction that the body was twisting. Hamstring flexibility and knee recurvatum were measured. In addition to the questionnaires, videotapes of 28 additional patients with anterior cruciate ligament injuries were obtained from Division I National Collegiate Athletic Association programs. The diagnosis was confirmed clinically (Lachman's test) and arthroscopically. In addition, the authors had access to videotapes of other athletes at the time of their injury. These videotapes were reviewed and compared with the information acquired from the patients.

The authors found that contact was involved only 29% of the time, typical with other reports.^1,6,14 The most common activities were basketball (25%), football (21%), and soccer (21%). The level of sports participation was fairly equally distributed across levels (recreational, 41%; varsity, 34%; and intramural, 23%).

Approximately 3/4 of the patients heard a popping sound, and all but three had to be removed from play at the time of their injury. The knee angle averaged 21°, but there was a wide range of knee flexion angles (range, −10-110°). If the few patients with hyperextension injuries are excluded, the average knee flexion angle at the time of injury changed little (to 24°). At the time of injury, 34 patients were decelerating, 30 patients were landing (another form of deceleration), 13 patients were accelerating, and four patients were falling backward. Nineteen patients could not recall when they were injured.

Twenty-one athletes thought that their injury was caused by the interface between the shoe and the surface. Of these 21 athletes, 15 thought their cleats became fixed to the ground. The other six thought their shoes did not allow them to pivot. Good hamstring flexibility may allow the tibia added room for an anterior drawer induced by activity. Thus, the authors were interested in the relative flexibility of the patients. Only three subjects were unable to touch the ground with their fingers, knuckles, or palms suggesting good hamstring flexibility.

The interviews showed that numerous different mechanisms with two broad classifications garnered the majority of injuries. For example, in 39 injuries, tibial rotation was implicated: the subject's body was twisting in a direction opposite from the tibial rotation. Thirty-one of the injuries involved landing (varus, valgus, or hyperextension).

The videotapes of 22 other patients were studied for comparison with the descriptions from the interviews with patients. From viewing the videotapes, it was found that two of three injuries resulted from no direct contact to the knee during sports activities. In these injuries, the athletes were in close proximity to an opposing player. Although there was not direct contact with the knee, frequently there was some contact that may have thrown the athlete off balance. The position of the leg before the patient collapsed was near footstrike with the knee flexed between 10° and 30°. A single-leg landing in valgus led to four injuries. The other injuries that did not result from direct contact with the knee occurred during deceleration or pivoting.

Is the Extensor Mechanism Implicated?

The interviews and videotapes provided insight into the circumstances surrounding injuries to the anterior cruciate ligament that did not result from direct contact with the knee. The next step was to see whether the quadriceps had the necessary capabilities to exert an anterior drawer sufficient to tear the ligament.

It has been reported that the quadriceps is capable of loading the anterior cruciate ligament throughout the full motion of the knee.^8,9 More importantly, one must understand the forces on the anterior cruciate ligament that the quadriceps is capable of producing. In most cases, these studies were done on cadavers in an "open chain" setting.^{2,11,15,20,21} To estimate these forces, the patellar tendon to tibial shaft angle must be determined. These studies typically showed that the patellar tendon to tibial shaft angle is inversely proportional to knee flexion angle.

These data, although being very well-defined, may not be applicable to the athlete who performs cutting or landing maneuvers. First there is the obvious difficulty in generalizing from the cadaver to the performing athlete. Second, from the review of the videotapes and patients with anterior cruciate ligament injury, these injuries do not seem to occur in an open chain setting. Therefore, an estimate of the in vivo forces on the anterior cruciate ligament during closed chain activities is lacking.

To try to fill this void, sagittal plane radiographs were obtained of nine healthy, young males at 5°, 30°, 45° and 60° knee flexion while the subjects were bearing approximately 50% of their body weight (Noonan TM, Garrett WE: unpublished data from the Duke Orthopaedic Research Laboratory, Durham, NC, 1997). The patellar tendon and tibia vectors for each angle then were determined from the radiographs. The angle between these two vectors was defined as the patellar tendon to tibial shaft angle. Noonan and Garrett showed an inverse linear relationship between resultant force vector and knee flexion angle; that is, the force vector increases as knee flexion angle decreases. From these measurements, the shear force on the tibia by the patellar tendon could be estimated using data from prior work.^2,11,15,20 It was estimated that the resultant force on the patellar tendon at footstrike, when the quadriceps are contracting, eccentrically could exceed 5000 N. During running and cutting maneuvers, these loads can exceed 2000 N, which is near the anterior cruciate ligament failure load previously published.²³ These data are limited to men and knee angles between 15° and 60° knee flexion. It is unknown at this time whether these relationship are the same for women, or people of a wider age range, at a wider range of knee flexion angles. In addition, there was wide variability between the subjects which suggested it may be possible to determine who is at risk based on the patellar tendon to tibial shaft angle.

Motor Control of Athletic Maneuvers

Based on the interviews and review of videotapes, the authors have an idea of the type of maneuvers that put the anterior cruciate ligament at risk. The unpublished work of Noonan and Garrett shows that there is sufficient force within the quadriceps to cause and anterior drawer that could exceed the rupture force of the anterior cruciate ligament (Noonan TM, Garret, WE: unpublished data from the Duke Orthopaedic Research Laboratory, Durham, NC, 1997). The current authors were curious how the quadriceps and hamstrings interact while subjects perform risky activities.

Malinzak et al reported the kinematics of running and cutting in male and female athletes. (Malinak RA, Colby SM, Kirkendall DT, Garrett Jr WE: Electromyographic and 3-dimensional kinematic analysis of cutting maneuvers in men and women: Implications for anterior cruciate ligament injury. Presented at the American Academy of Orthopaedic Surgeons, Anaheim, CA, Feb 1999). Motion capture was used to determine knee, hip, and ankle flexion angles and knee varus and valgus. Electromyography was used to record the activity of the quadriceps, medial hamstrings, and lateral hamstrings. The data on muscle activation were reported as relative activation (quadriceps to hamstring ratio). The subject would run along a runway and perform a side-cut or a cross-cut.

The data showed that women performed the cutting maneuvers in a more erect posture with less knee and hip flexion and more knee valgus. Thus, the knee was in a position that favored a quadriceps-induced anterior drawer. The females also had greater relative quadriceps activation. The women seemed to perform cutting maneuvers in a manner that placed the anterior cruciate ligament at risk for an injury.

If women do perform these actions with a more erect posture, might coaching women to play in a more crouched position reduce the incidence of anterior cruciate ligament injuries? An insightful project was presented in 1989 (Griffis ND, Vequist SW, Yearout KM, et al: Injury prevention of the anterior cruciate ligament. Presented at the American Orthopaedic Society for Sports Medicine, Traverse City, MI, June 1989). National Collegiate Athletic Association Division 1 female basketball players were trained to perform cuts in a three-step pattern in which the knee was flexed and the feet were kept under the hips. Anterior cruciate ligament injuries were reduced by 89% during a 2-year period.

From the interviews and videotapes, it was shown that in many cases there was another player nearby at the time of injury. Although this situation is not reproducible in the laboratory, it is possible that just before a cut or jump stop that the player is slightly thrown off balance. When attempting to perform the act they intended to do, a change in the planned movement must take place and the adjustment can favor the quadriceps activation while the hip and knee are in positions that place the anterior cruciate ligament at risk. The forces on the ligament during this slightly unplanned maneuver may be sufficient to tear the anterior cruciate ligament.

It is impossible to say that any one mechanism is responsible for anterior cruciate ligament injuries. The multitude of intrinsic and extrinsic factors that come into play make focusing on one variable difficult. However, given that most injuries to this ligament do not result from direct contact with the knee and that there are predictable movement patterns that, when slightly disrupted, place the anterior cruciate ligament at risk, it is possible that preventive strategies might be developed.