ACL INJURY

MAKING THE DIAGNOSIS The history In most cases the diagnosis can be made virtually on the history alone. The majority involve non-contact injuries in which an individual undertakes a change of direction, such as when 'cutting' past an opponent, which goes wrong or if footwear gets caught in the ground. Different sports can produce an ACL tear from uncon- trolled jumps/landing. Skiing, of course, is a common cause of significant torsion to the knee often related to skis that don't detach during a twisting force.

The majority of patients will describe a popping sensation within the joint and even be able to describe that the tibia and femur move on one another. This phenomenon occurs laterally and it is not uncommon that the early pain from the injury is felt postero-laterally. This is due to overstretching of the soft tissues in this region. In most patients there is swelling of some degree which occurs within the first hour or so of injury. This is a manifestation of bleeding into the joint. This is not necessarily major. It is the time of onset that is more important than the amount of swelling - a reactive effusion, such as that from a meniscal tear, occurs slowly over hours. From this history alone, one should be very aware of the possibility of an anterior cruciate ligament rupture. If an X-ray taken subsequently is normal, suspi- cion would be even higher.

Examination findings Examination of the acute ACL rupture is usually limited by discomfort. Nevertheless, patience and gentle handling can usually provide vital confirmatory signs of the injury. Most patients will have some swelling in the joint and tenderness at the postero- lateral corner. Associated injuries to other structures, of course, may produce tenderness elsewhere. With an acutely injured knee it is usually not possible to bend it to 900

to assess for drawer tests.

Far more useful, however, is to undertake the anterior drawer with the knee at around 300

knee flexion ie. the Lachman test.

The other ligaments should be checked carefully, as should be the distal neurovascular status in case the ACL tear is part of a knee dislocation. Most ACL ruptures occur simply with a subluxation of the joint, but if dislocation has occurred, which usually means a minimum of severe disruption of at least two and often three ligaments, the nerves and blood vessels are at risk of damage.

Imaging Plain X-rays are important to rule out a significant fracture. In some cases of ACL rupture a small flake of bone pulled off from the lateral tibia by the lateral capsule (Ségond fracture) is present (Figure 1). On the antero-posterior view X-ray the fragment is seen sitting lateral to the tibia. It must not be confused with a pull-off fracture of the fibular head.

X-rays are also useful for the potential ACL injury involving pull-off of a fragment of bone from its attachment to the tibia. This typically occurs in the adolescent or young adult. This is an important diagnosis to make since early re-attachment of the bony avulsion fragment can restore the ACL. Unfortunately, many cases don't have as good an outcome as one would think since, as well as the bony damage, the ligament itself has been injured in the process.

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Figure 1: Ségond fracture- see arrows – on X-ray left and MRI right

MRI scanning is a reliable way of diagnosing ACL rupture (Figure 2) and for assessing the other structures in the joint. The quality of scans and radiologists of course does vary. With the current scanners available and the expertise of skilled radiologists however rarely can the diagnosis be genuinely in doubt. Although MRI scans are often reported showing a partial injury to the ACL, this is actually rarely the case. In the majority of cases, the ligament is actually completely ruptured. Partial ACL ruptures do occur (often in children/adolescents) but they are genuinely very rare.

If the diagnosis is clear on clinical grounds then an MRI is not mandatory.

ASSOCIATED INJURIES WITH ACL RUPTURE Not surprisingly with the major injury that ACL rupture represents, other structures within the knee joint can be damaged. The classic 'unhappy Triad of O'Donaghue' (a combination of medial meniscal and medial collateral ligament tears with an ACL rupture) is actually very rarely associated with a fresh ACL rupture. It was described years ago when it was normal for ACL ruptures to present at a late stage after the original injury. In late presentations medial meniscal tears tend to appear. It is far more common for the lateral meniscus to be torn than the medial in an acute ACL rupture due to the lateral subluxation of the joint that typically occurs.

Figure 2: An MRI showing the typical appearance of the ACL remnant (white arrows) lying in a flatter less inclined position in the intercondylar notch having torn proximally.

Associated medial collateral ligament sprains are not uncommon particularly in contact sports such as rugby, and also in skiing.

An MRI scan after ACL rupture will typically show bone bruising in the femur and posterior tibia in the lateral compartment of the joint. A disruption of the 'postero-lateral corner' of the knee (lateral collateral ligament, popliteus, posterolateral capsule, and importantly the popliteo-fibular ligament) to a level that requires surgery is rare. Nevertheless this is an injury that should not be missed since, if untreated, will compromise the result of any ACL reconstruction subsequently undertaken.

ACUTE MANAGEMENT Early management of ACL rupture should focus on restoration of full extension as soon as possible, and reducing the inflammatory

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