Tears of the anterior cruciate ligament (ACL) often result from sports-related injuries and significantly compromise the stability of the knee joint. In particularly to anterior translation and rotational forces and also disrupts the proprioceptive function of the knee. Patients often require surgical reconstruction of the ligament to enable them to return to playing pivoting sports and prevent further instability and damage to the knee, the main risk being meniscal tears and chondral injuries. The significance of this injury and ligament function is highlighted by the approximate 60,000-175000 ACL reconstruction surgeries performed annually in the United states.
Structure and Function
The complicated anatomy of the knee includes the interaction of three bones, 4 major ligaments, menisci and the surrounding musculature. This hinge joint is an articulation between the femur, tibia and patella. Stabilizing and firmly connecting these bones externally are two major ligaments, the medial collateral (MCL) and lateral collateral ligament (LCL). Internally, joint stabilization can be attributed to the ACL and posterior cruciate ligament (PCL). These ligaments are the primary restraints to anterior and posterior (particularly at 90 degrees knee flexion) translation of the tibia on the femur respectively. Acting as shock absorbers are the two fibrocartilaginous menisci, each located laterally and medially on the surface of the tibia. It is important to appreciate the different morphology of the medial and lateral tibial plateaus. The medial plateau is concave and thus congruent with the medial femoral condyle whereas the lateral plateau is convex making it less congruent. The lateral meniscus, therefore, has an important role in increasing the congruence of the lateral compartment and its absence results in rapid degenerative changes. Finally, two major muscle groups surround the knee joint, the quadriceps and the hamstring muscles. The quadriceps consists of four muscles, and is responsible for knee extension. The hamstrings consist of three muscles and generates flexion at the knee.
The ACL originates on the posterormedial aspect of the lateral femoral condyle and inserts into the intercondyloid eminence of the tibia, blending with the anterior horn of the medial meniscus. It is approximately 3 cm long and 1 cm in diameter. It's composition is 90% type 1 collagen and 10% type three collagen and elastin. Two bundles make up the ACL, the anteromedial bundle, which is taut in flexion and the posterolateral bundle, taut during extension. Additionally, mechanoreceptor nerve fibres lay within the ACL from the tibial nerve and aid with knee proprioception.
The main function of the ACL is primary restraint, 85%, to anterior tibial translation. Secondarily it restricts internal tibial rotation along with varus or valgus angulation at full extension. It is slightly weaker than the PCL with an approximate tensile strength of 2200N-2500N and half as strong as the MCL.
ACL injuries are most often sports-related. Blows to the knee created through contact during sports such as football or martial arts can cause excessive stretching leading to injuries and potential ACL tears. However, the majority of ACL injuries, 72% as reported by Krosshaug et al and 65% reported by Agel et al, arise from non-contact activities. Movements causing hyperextension or quick pivoting and internal rotation to the knee place extreme stress on the ligament. Skiing, basketball, soccer, volleyball, wrestling and gymnastics are common sports where these movements occur quite readily. Non-athletes can also sustain these injuries, simply through a misguided step or fall mimicking those injury prone actions. As with other body parts, injury is more likely to occur with aging.
It has been reported that women are more likely than men to experience an ACL injury. The reasons for this finding are still to be determined but possible causes have been proposed. The evidence suggests that this may be due to differences in skeletal morphology, proprioceptive function, reaction time and may be associated with hormone levels.
Injuries are caused by hyperextension, excessive pivoting or side-to-side bending of the knee. As the knee extends 100 past its normal extension, the tibia is forced forward in relation to the femur. This produces enormous stress on the ACL. Landing flat footed or sudden deceleration on an extended knee are typical situations where this occurs. Cutting movements, that involve a quick and sudden change in direction coupled with rapid deceleration and acceleration, generates the extreme internal rotation at the knee, again excessively stretching the ACL.
Patients presenting with ACL ruptures usually describe a classical mechanism as discussed above, including describing a ‘pop’ and have a large hemarthrosis that usually arises within 4 hours of the injury. Those with acute injuries have severe pain, may be unable to weight-bear and have severely restricted range-of-motion. Instability is unusual at this early stage. It is important to note that ACL ruptures account for approximately 70% of acute hemarthroses with other causes including intra-articular fractures, osteochondral injuries, peripheral meniscal tears, patella dislocation and patellar sleeve fractures. They may also present with associated injuries to the knee: meniscal tears (especially medial); collateral ligament tears; osteochondral injuries; posterolateral corner tears.
Those presenting after the initial inflammatory phase has resolved may describe a sense of instability and giving way and feel unable to ‘trust’ the knee especially with pivoting activities. They may also have associated joint line pain and swelling due to meniscal tears (especially lateral) and chondral damage due to recurrent subluxation.
Hearing a "popping" sound, followed by immediate and severe swelling combined with instability are three symptoms highly suggestive of an ACL tear. If these symptoms are experienced, continued activity on the damaged knee leads to further consequences, such as tearing of menisci and cartilage. May also present with a locked knee (inability to fully extend the knee) – this may be due to a bucket handle tear of the medial meniscus or a bulky ACL stump and requires urgent arthroscopic surgery to remove the mechanical block and restore the range-of-motion (ROM) of the knee.
An accurate diagnosis begins with patient history, in particularly the mechanism of injury as this is usually classical for an ACL injury followed by a clinical examination. The three most useful tests to diagnose an ACL injury include the Lachman test, the anterior draw and pivot shift.
The Lachman Test is the gold standard and most reliable examination with almost 100% sensitivity. The patient's knee is flexed 200-300, as the examiner places their hands on the femur and the other on the tibia. Examining tibial displacement, the tibia is anteriorly translated. Any displacement greater then 2mm suggests an ACL tear. Comparison with the uninvolved knee is critical. Instruments to quantify exact laxity are the KT-1000 and KT-2000. Determining exact laxity can help differentiate between a partial or complete tear.
A second test is the Anterior Drawer Test. With the knee flexed 900, similar anterior force is applied, again focusing on displacement of the tibia. The Anterior Drawer Test is the least reliable especially as it is possible to record a false negative result due to quadriceps contraction in an un-relaxed patient with pain or a false positive result if there is an associated PCL injury.
The pivot-shift test can be performed with the knee fully extended and applying internal rotation to the tibia with a valgus force at the knee to cause anterolateral subluxation of the lateral tibial plateau. The iliotibial band (ITB) acts as a knee extensor in this position so that when the knee is flexed to 200-300, the tibia is reduced by the action of the ITB as it is now a knee flexor and the examiner feels a “clunk” as this occurs. When performed correctly in a relaxed patient, the specificity is almost 100% although the sensitivity is lower. The test requires an intact MCL and ITB, and may be negative if the ACL stump has healed to the PCL, but should not be performed in patients with a partial MCL injury in the acute phase due to the possibility of damaging it further. It is the most useful test in clinical practice as it often reproduces the patient’s own feeling of instability and can guide the surgeon as to whether to offer surgical reconstruction in borderline cases and can be performed in the acute phase if performed carefully having gained the patient’s trust.
ACL injuries rarely occur in isolation. Often other ligaments, menisci, articular cartilage and bone can be damaged. In specific relation to ACL tears, MCL injuries are common. Other ligament damage occurs less often, but must be examined for as unresolved damage can affect the success rates of the ACL reconstruction. Specific tests for other ligaments are beyond the scope of this article, but can be found at: http://emedicine.medscape.com/article/89442-differential.
Additionally, meniscal tears are common as a result of anatomical association with the ACL. An ACL, MCL and meniscal tear simultaneously is known as a terrible triad and occurs mostly due to a lateral force to the knee. Again specific diagnostic tests for meniscus are beyond the scope of this article, but can be found: The basic science of Human Knee Menisci- Structure, Composition and Function, by Alice J. S. Fox, Sports health, A Multidisciplinary Approach.
Standard radiological imaging includes anterior-posterior and lateral views. Forty-five degree weight-bearing posterior-anterior views can be performed to assess associated arthritis. Additionally, merchant and tunnel views can be performed. Soft tissue damage will not be shown with basic radiology, but indirect fractures and loose bodies associated with an ACL tear can be present. Three fractures with strong association are Segond or lateral capsular avulsion fracture, a tibial plateau fracture and lateral notch fracture (more chronic than acute).
MRI has become the modality of choice for imaging as it has a 90-98% sensitivity rate for detecting ACL tears. ACL tears are best seen in the saggital view. MRI also has the advantage of being able to detect associated meniscal tears and chondral injuries.
Risk Factors and Prevention
There are both external and internal risk factors for sustaining this injury. As stated earlier, participating in high-risk sports involving contact such as football or wrestling increases one's risk. Also, participation in quick lateral movement athletics such as basketball, soccer and skiing puts one also at risk. Repeated studies have documented that women are more prone than men to obtain an ACL injury.
Internal risk factors are less conclusive but a smaller femoral intercondyler notch and slower neuromuscular control related to biomechanics.
Prevention is being targeted towards specific training therapies for improving neuromuscular control and biomechanics. However, evidence is still very limited. For in depth discussion on internal risk factors, Dr. Boyi Dai provides an extensive systematic review titled: Prevention of ACL injury Parts 1 and 2.
The RICE (rest, ice, compression, elevation) regime is used for all patients in the acute phase after ACL injury, the aim of which is to reduce the pain and swelling, restore ROM and begin strengthening of the knee. The decision as whether to offer surgery or not depends on several factors which include the age of the patient, whether they have symptoms of instability and a positive pivot-shift, and their individual goals including returning to fast pivoting sports. Patients also need to be counselled regarding the significant rehabilitation period required following a reconstruction when making a decision, as non-compliance is associated with a significantly poorer outcome. When associated with other ligament injuries including the LCL, PCL, MCL (if a multi-ligament injury) and posterolateral corner, all ligaments are usually repaired or reconstructed. If an ACL rupture is combined with a grade 2 or 3 MCL injury, the MCL is usually treated with bracing and physiotherapy first for a six week period, after which the patient is re-assessed to ensure that the MCL has healed and to demonstrate any instability that may necessitate ACL reconstruction.
Non-surgical treatment of an ACL rupture includes a rehabilitation program designed to strengthen the surrounding muscles focussing specifically on hamstring strengthening which can dynamically prevent anterior subluxation of the tibia. The program also includes proprioceptive and sport-specific exercises to restore function. Occasionally, the patient may experience delayed instability that may only become apparent when reaching the more advanced stages of the program and may then be a candidate for ligament reconstruction.
Direct repairs of the ACL were previously attempted but proved ineffective. Reconstruction of the ligament is now the gold standard surgical treatment. Most cases are done arthroscopically with autografts more commonly used than allografts. The types of autografts used are hamstring tendons, patella tendon and quadriceps tendon that can be either ipsilateral or contralateral (reserved for revision cases or for multi-ligament injuries). Single and double bundle reconstruction techniques can be performed. Single bundle reconstruction is the traditional choice and includes anatomical restoration of the anteromedial bundle. This is usually performed using a four-strand hamstring graft (semitendinosus and gracilis folded over) or a bone-patella tendon-bone graft from the central third of the tendon (BPTB) depending upon surgeon preference. Evidence has shown the incorporation of bone from the BPTB graft has faster healing and improved tensile strength with a force of 90N compared to 80N in hamstring tendon. More recently, double bundle reconstructions have been performed to more closely resemble normal anatomy by also reconstructing the posterolateral part. Initial studies of this method have demonstrated increased overall stability with better restoration of internal rotational stability although the evidence is still currently limited. For both procedures, tunnels are drilled in the proximal tibia and distal femur, with best results if tunnels are as anatomically close to the original ACL insertions.
Rehabilitation is a critical factor in achieving a stable and well-functioning knee after ACL reconstruction. It generally takes around 2-3 months for graft incorporation into the bone. A brace may be worn 1-2 weeks post-surgery while initial healing takes place. A basic outline of rehab is as follows. Within the first month, passive ROM isometric exercises can begin as soon as 1 week post surgery. At the second and third month, the patient can start closed followed by open kinetic chain exercises. At the 4th-5th month mark, sport-specific training can begin and at 6-9 months the patient can return to full sports with 6 months being the earliest that this should be attempted ensuring that the patient has achieved the required goals prior to this.
ACL reconstruction has a reported success rate of over 90%1. Possible failures and complications can arise such as deep venous thrombosis, infection, graft impingement and graft rejection. Most importantly, placement of the graft insertion in particularly the femoral tunnel is the most underlying cause for ACL reconstruction failure.
Recent literature has demonstrated that there is no significant difference in outcomes based on graft choice. A systematic review by Timothy Foster, et al discussing allograft versus autograft concluded no superior outcome arises from either graft choice. Additionally, a 10- year follow-up study by Leo Pinczewski, et al comparing patella tendon versus hamstring tendon demonstrated no significant difference in functional outcomes. However, the article preferred hamstring reconstructions due to less donor site symptoms and decreased rates of osteoarthritis.
Osteoarthritis is reported as a consequence with ACL reconstruction with a rate of 37% prevalence at 20 years beyond surgery. Newer longitudinal studies such as that by Britt Oiestad, et al, demonstrate a significant higher prevalence of osteoarthritis in patients with combined knee injuries at time of ACL reconstruction.
Outcomes in regards to returning to competitive sports are generally good. In a study by Shelbourne, et al of ACL reconstructions in high-school athletes, 95% recovered to participate in competitive sports. A reported 84% of females and 81% of males were able to compete at the same, if not higher level then before.
Anterior Cruciate Ligament, Anterior Tibial translation, Reconstruction, Osteoarthritis
- Understand the underlying anatomy and mechanism of ACL injury
- Being able to identify and diagnose the injury
- Understand the options for treatment including deciding on the appropriate treatment for the individual patient.