ACL sprains - Orthopaedia

Introduction

The anterior cruciate ligament traverses the knee joint from the tibial spines to the medial aspect of the lateral femoral condyle in the notch. The function of the anterior cruciate ligament is primarily to resist anterior tibial subluxation.

Injuries to the anterior cruciate ligament involve traumatic disruption of the normal ligament anatomy, almost always from excessive tensile force.

Pathology

The pathology associated with an anterior cruciate ligament injury includes functional instability (the deficit associated with the absence of the normal anterior cruciate ligament) as well as arthrosis (secondary to the traumatic damage inflicted on the knee during the initial injurious episode).

The anterior cruciate ligament tears when the tibia subluxates a distance greater than the elastic limit of the ligament. As such, there are secondary injuries seen with anterior cruciate ligament disruptions, including tears of the menisci, the collateral ligament (most often medial), as well as an impaction injury to the articular surface (most often the terminal sulcus on the lateral femoral condyle).

Pathophysiology

In the absence of an anterior cruciate ligament, the normal knee mechanics are disrupted. The tibia can subluxate forward. There can be a sense of instability as the tibia moves from a subluxated to a reduced position. In layman's terms, an anterior cruciate ligament deficiency is associated with a "trick knee."

Prevalence

Anterior cruciate ligaments occur, it is estimated, in excess over 100,000 times a year. This injury is thought to be more frequently occurring in women. This may be due to hormonal effects on the strength of the ligament; differences in male, female notch geometry; or on strength differences.

Etiology

The anterior cruciate ligament, by definition, ruptures when there is excess anterior subluxating force on the tibia relative to the femur. The anterior cruciate ligament is thought to experience isolated forces during so called cutting sports. Also, simple consideration of the knee anatomy would indicate that the tibia is also stabilized by muscular force, and whose absence may lead to ligament injury (from loss of protection). Accordingly, one could argue that relative muscular weakness is a cause of anterior cruciate injuries. This is seen in the anecdotal experience of ski injuries. Many of these injuries occur in the afternoon when muscle fatigue has built up.

Presentation

Many people who have ruptured their anterior cruciate ligament will report having literally heard the classic "pop." There is often an acute hemarthrosis. The injury is variably painful. The ligament injury itself is thought not to be painful, but rather pain - even excrutiating pain - may be present depending on the extent of impaction injury at the same time. Instability is not often a present complaint. Instability is, rather, a late finding once the patient has regained motion and attempted to participate in sports again. (Instability as an initial presenting complaint is usually a sign of intra articular cartilage disruption: the instability is from buckling and reflexive quadriceps inhibition.)

Objective physical findings in the setting of an acute anterior cruciate ligament disruption include a large effusion; a positive Lachman test; a positive anterior drawer test, and signs of associated injuries, if present.

Imaging studies acutely are often normal. There may be a "Segund" sign: a linear avulsion off the lateral tibia. MRI is often diagnostic. The novice is cautioned to not mistake hemorrhage within the ligament for the ligament itself. Likewise, an empty notch is a sign of a torn ACL, even though the "torn edges" are not visible. One also must look for signs of bone bruising and impaction injury, along with associated meniscal or collateral ligament injuries.

Prevention

It is thought, but unproven that a strengthening program may reduce the incidence of anterior cruciate ligament tears.

Treatment

Patients without complaints and without instability need no treatment. Some patients are able to control potential tibial subluxation with their own muscular forces, ie they can compensate for the lost ligament. Other patients are unstable only during certain activities, which they may be willing to give up. These patients, too, need no treatment.

All patients considering treatment should undergo a therapeutic regimen to insure full range of motion and strengthening. At the present time anterior cruciate ligament disruptions are thought not to be reparable (The results in the past have been disappointing.) Rather, if surgery is needed, it is in the form of reconstruction. The reconstruction is best performed with biological material - use of artificial ligaments have been disappointing. Details of surgical reconstruction are listed elsewhere.

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