An anterior cruciate ligament (ACL) tear of the knee is not an uncommon injury among young athletes. An ACL tear occurs during sports such as football, soccer, basketball and other sports involving quick acceleration/deceleration movements while attempting to change direction. The running back position in football is of particular high risk for the athlete to sustain an ACL tear because the position requires constant quick, sharp changes of direction to elude tacklers, the common mechanism of ACL injury. Several high profile NFL running backs have seen their careers cut short due to ACL injuries, most notable being Terrell Davis, one of only 5 NFL running backs to rush for more than 2000 yards in one season.

A full tear of the ACL can be a very devastating injury to an athlete because it can happen so quickly, often during what seems to be an innocent but awkward movement. If the ACL is significantly or fully torn, a prolonged absence from sport is required.

Functional Anatomy

The ACL attaches the tibia (lower leg bone) to the femur (upper leg bone). It forms a cross near the middle of the knee joint with the Posterior Collateral Ligament (PCL). Both the ACL and PCL run superior/inferior as well as anterior/posterior. Imagine an X which is tilted somewhat to the horizontal plane. Ligaments act to stabilize a joint during movement and restrict further movement at the end of physiological range. Ligaments also act to restrict unphysiological translational movements exerted on a joint i.e. a football player being tackled from the side of the knee exerting forces to buckle the knee inwards.

Stabilizing Functions of the ACL:

• Restrains anterior translation (force exerted from back to front of the knee)
• Prevents hyperextension
• Secondary stabilizer to valgus stress (knee buckling inward) reinforcing the medial collateral ligament
• Restrains excessive rotation of the tibia on the femur in 0 to 30 degrees of extension

Presentation

The athlete who experiences an ACL tear usually describes sharp, sudden pain during a sharp acceleration/deceleration movement while trying to change direction or landing from a jump. Often, in accompaniment of the sharp pain, the athlete will report their knee "gave out" on them and they fell to the floor. An audible "pop" or "snap" may be heard and rapid swelling begins. Acute hemarthosis, bleeding into the joint, is a strong correlating symptom when diagnosing ACL tears, as blood vessels within the ligament are torn, bleed into the joint and cause rapid swelling.

Objective Signs of ACL Tears

Often a clinician can diagnose an ACL is torn before the physical examination by listening to the athlete describe their symptoms as noted above. However, not all ACL tears present as described in the text book, and it is also important to be able to identify less significant tears. Two tests in particular are valuable tools in diagnosing ACL injury; Lachman's Test and Lateral Pivot Shift Test.

Lachman's Test involves the examiner pulling the tibia forward relative to the femur, with the patient crook lying, while noting laxity and pain reproduction. It is possible to have a false negative during this test if the patient is unable to relax the muscles around the knee to allow the ligament to be put in a position of stretch. Often this is the case as the patient may still be experiencing significant pain and anxiety and can't relax. The Lachman's Test may have to be performed on repeated occasions during treatment to fully evaluate the true extent of the injury.

The Lateral Pivot Shift Test is an aggressive test which is difficult for the examiner to perform. The test reproduces both anterior and rotary forces attempting to cause a subluxation of the tibia on the femur. If the ACL is deficient, the tibia will sublux and the test, surprisingly, is not uncomfortable. Pain may be reproduced if the ACL is not completely torn, but subluxation will not occur.

Conservative Treatment

Conservative treatment usually applies to partial tears. Full tears commonly require surgery. After surgery, physiotherapy treatment begins and follows similar objectives to conservative treatment, only over a more prolonged period of time.

Conservative Treatment Objectives:

• Decrease swelling
• Increase active range of motion
• Increase strength of the muscle which support the knee and lower extremity
• Sport specific exercise
• Gait correction as needed, following altered gait patterns due to pain and gait aids
• Enhance stability using a knee brace, if required

A common complication of ACL tears is wasting of the vastus medialis oblique muscle (VMO). Pain and swelling neurologically inhibits the VMO from being recruited during normal movements. The VMO is one of four muscles which make up the quadriceps muscle. The role of the VMO is to guide the tracking of the patella and enhance the medial stability of the knee. If the VMO is not being neurologically recruited, knee cap tracking becomes altered and may create a new source of knee pain. To prevent VMO wasting and poor recruitment, electrical muscle stimulation is utilized. The patient is given a specific exercise designed to recruit the VMO muscle and during the exercise, an electrical stimulation is applied to the muscle to assure it is contracting normally. The stimulation during the exercise effectively teaches the muscle when it should be recruited. After time, the electrical stimulation is no longer required and the VMO recruitment is restored.

Surgical Treatment

Surgical treatment commonly involves replacing the ruptured ACL with a graft taken from the patella tendon or the hamstring muscle. Physiotherapy commonly follows surgery after the first week. Some surgeons provide their own physiotherapy protocols which outline timelines for treatment progressions. The objectives are similar to conservative treatment. Post surgical treatment objectives are just more pronounced because the patient presents with greater restrictions of movement and greater levels of muscle atrophy than the patient being rehabilitated with a partial tear.

Return to Sport

The reason ACL tears are so devastating, especially to young athletes, is the athlete cannot participate in sports anywhere from 6 months to 1 year following surgery. What began as a simple side step to avoid a would-be tackler, resulted in a laborious journey, possibly involving surgery, prolonged rehabilitation and sitting on the sidelines for the rest of the season.

References:

1. Cross, Mervyn J. (1998). Anterior cruciate ligament injuries: treatment and rehabilitation. In: Encyclopedia of Sports Medicine and Science, T.D. Fahey (Editor). Interned Society for Sports Science: http://sportsci.org. 26 Feb 1998.