What does the ACL do and how is it injured?
The anterior cruciate ligament (ACL) is one of the two cruciate ligaments that stabilise the knee. It arises from the anteromedial part of the tibial intercondylar region and runs posterolaterally to attach to the medial aspect of the lateral femoral condyle. It is about 32 mm long and 7 to 12 mm wide, is made of type I collagen (about 90%) and type III collagen, and receives its blood supply mainly from the middle geniculate artery; its nerve supply comes from the posterior articular nerve, a branch of the tibial nerve.
It has two bundles. The anteromedial bundle is tightest in flexion and provides most of the resistance to anterior tibial translation (about 85% of the stability), while the posterolateral bundle is tightest in extension and contributes mainly to rotational stability. Together with the PCL it forms an 'X' that stops the tibia sliding too far forward or backward on the femur.
Most ACL tears happen in athletes through a non-contact pivoting injury: the knee is slightly flexed and in valgus while the tibia translates forward. A direct blow to the lateral knee can also tear it. Skiers, soccer players and basketball players are most at risk for non-contact injuries; football players for contact injuries. Female athletes are at higher risk, partly because they tend to be quadriceps-dominant on deceleration and land with more knee valgus.
How does an ACL tear present?
The typical story is a sports injury with a sudden change of direction, abrupt stopping or an awkward landing. Most patients feel or hear a sudden 'pop' with deep knee pain, and about 70% develop immediate swelling because of haemarthrosis. Other complaints are the knee giving way, difficulty walking and reduced movement. The patient may walk with a quadriceps avoidance gait.
Associated injuries are common. The lateral meniscus is injured in over half of acute ACL tears, whereas the medial meniscus is more often involved in chronic ACL deficiency, which also leads to chondral damage and complex meniscal tears (for example bucket-handle tears). The MCL, LCL and posterolateral corner may also be injured. The classic combination is the unhappy triad — see the collateral ligament section below.
How is the Lachman test done and interpreted?
The Lachman test is regarded as the best clinical test for the ACL and is a variant of the anterior drawer test. StatPearls describes it as the most sensitive test for ACL rupture, with a reported sensitivity of 95% and specificity of 94% in one chapter and 87% and 93% in another — so quote it as 'the most sensitive and best overall test', not as a fixed number.
- Patient lies supine with the injured knee flexed 20 to 30 degrees and slightly externally rotated (this relaxes the iliotibial band).
- One hand stabilises the distal femur; the other grasps the proximal tibia.
- Apply an anterior force to the tibia to sublux it forward on the femur.
- Positive = excess anterior translation compared with the normal knee and a soft or absent endpoint. A firm (hard) endpoint means the ACL is intact.
- Always examine the normal knee first for comparison — baseline laxity differs between people.
| Grade | Anterior translation (ACL chapter) | Anterior translation (Lachman chapter) | Meaning |
|---|---|---|---|
| 1 | 3 to 5 mm | 0 to 5 mm | Mild laxity |
| 2 | 5 to 10 mm | 6 to 10 mm | Moderate laxity |
| 3 | More than 10 mm | 11 to 15 mm | Severe laxity; suspect associated MCL or meniscal injury |
Translation of 5 mm or more than the normal limb generally indicates ACL rupture, and more than 2 mm side-to-side difference is considered a positive test. Variants include the modified Lachman (examiner's knee placed under the patient's thigh as an anchor) and the prone Lachman, used for comfort. Examine soon after injury if possible — a very swollen, guarded knee reduces accuracy.
What are the anterior drawer, pivot shift and lever sign tests?
The anterior drawer test is done supine with the knee flexed to 90 degrees and the foot planted. The examiner grips the proximal tibia with both hands and pulls it forward; excess translation compared with the other knee is positive. StatPearls quotes a sensitivity of 92% and specificity of 91% in chronic injuries but not acute ones — in an acute swollen knee, hamstring spasm and guarding make it unreliable.
The pivot shift test reproduces the 'giving way' episode of an ACL-deficient knee. The tibia is internally rotated with a valgus stress while the knee is brought from extension into flexion. The anteriorly subluxed tibia reduces with a clunk at about 20 to 30 degrees of flexion as the iliotibial band pulls it back. It needs an intact ITB and MCL and no flexion contracture. It is highly specific (98%) but insensitive (24%) because pain and guarding stop patients cooperating.
The lever sign places a fulcrum (for example the examiner's fist) under the proximal calf of a supine patient while a downward force is applied to the distal thigh; the heel rises off the couch if the ACL is intact and stays down if it is ruptured. The KT-1000 arthrometer quantifies anterior laxity with the knee in slight flexion and 10 to 30 degrees of external rotation.
| Test | Knee position | Positive finding | Remember |
|---|---|---|---|
| Lachman | 20–30° flexion, supine | Increased anterior translation, soft or absent endpoint | Most sensitive; best for acute tears |
| Anterior drawer | 90° flexion, foot planted | Excess anterior tibial translation | Reliable in chronic, not acute, injury |
| Pivot shift | Extension to flexion, valgus plus internal rotation | Subluxed tibia reduces with a clunk at 20–30° | Specificity 98%, sensitivity 24% |
| Lever sign | Supine, fulcrum under calf | Heel stays on the couch | Intact ACL lifts the heel |
How are the PCL, MCL and the unhappy triad tested?
The PCL is about 1.3 to 2 times as thick and roughly twice as strong as the ACL, so it is injured less often. It tears when an extreme anterior force hits the proximal tibia of a flexed knee — the classic dashboard injury in a road traffic accident — or after a fall onto a flexed knee. The most accurate test for PCL integrity is the posterior drawer test: supine, hip flexed to 45 degrees and knee to 90 degrees, with a posterior force applied to the proximal tibia. The sag test is positive when the tibia sags backward relative to the femur compared with the other side.
For the MCL, valgus stress testing is the best direct test: supine, hip abducted, knee at 30 degrees of flexion, ankle pushed laterally while a valgus force is applied at the knee, then repeated in full extension. Grade 1 means pain with little or no opening, grade 2 some opening with a firm endpoint and grade 3 marked opening with no endpoint. MCL injuries are the most common knee ligament injury, and about 60% of skiing knee injuries involve the MCL.
| Ligament | Typical mechanism | Key test | Abnormal finding |
|---|---|---|---|
| ACL | Non-contact pivot, valgus | Lachman (anterior drawer, pivot shift) | Anterior tibial translation |
| PCL | Dashboard injury, fall on flexed knee | Posterior drawer, sag test | Posterior tibial translation or sag |
| MCL | Valgus blow to the lateral knee | Valgus stress at 30° and 0° | Medial joint opening |
How is a meniscal tear tested (McMurray, Thessaly, Apley)?
Meniscal tears cause pain over the anteromedial or anterolateral joint line, with locking, clicking, catching or inability to fully extend the knee. Joint line tenderness examined with the knee at 90 degrees is about 83% sensitive and 83% specific. The special tests below are each less accurate than MRI.
| Test | How it is done | Sensitivity | Specificity |
|---|---|---|---|
| McMurray | Supine; knee passively extended from full flexion to 90° while held in full external or internal rotation | 61% | 84% |
| Thessaly | Standing on one leg, squat to 20° flexion and rotate the knee | 75% | 87% |
| Apley compression | Prone, knee flexed to 90°, axial compression with rotation | Under 20% | 80–90% |
Blood supply decides healing. The peripheral third (red zone) is supplied by the medial and lateral genicular arteries and is well vascularised; the inner two-thirds (white zone) is avascular and nourished by diffusion from synovial fluid. Tears confined to the outer third and degenerative tears may be given a 4 to 6 week trial of relative rest and physiotherapy; if surgery is needed, meniscal repair is preferred to meniscectomy because removing meniscal tissue accelerates osteoarthritis.
Which imaging findings point to an ACL tear?
An ACL tear can be diagnosed clinically, but MRI is the primary imaging modality: StatPearls quotes a sensitivity of 86% and specificity of 95% in one passage, and much higher figures (97% and 100%) in another, so remember simply that MRI is the confirmatory test and also shows associated injuries. Arthroscopy distinguishes complete from partial and chronic tears. Plain radiographs are generally non-contributory to the diagnosis but exclude fractures.
- Bone bruising (marrow oedema) is seen in over half of ACL tears, characteristically in the middle third of the lateral femoral condyle and the posterior third of the lateral tibial plateau.
- Segond fracture — an avulsion fracture of the anterolateral proximal tibia at the attachment of the lateral capsular ligaments (anterolateral ligament); a secondary sign of ACL injury.
- Anterior tibial translation greater than 7 mm on a lateral view, and a tibial spine avulsion in younger patients.
- Fluid against the lateral notch wall and discontinuous ligament fibres on MRI.
How is an ACL tear managed and which graft is used?
Acute care follows RICE (rest, ice, compression, elevation) with analgesia. Treatment is individualised by age, activity level and associated injuries. Non-operative management — about 12 weeks of supervised physiotherapy — suits low-demand patients, those who avoid cutting and pivoting sports, and partial tears, but repeated giving way risks further meniscal and cartilage damage. ACL reconstruction is indicated for complete tears in young or high-demand patients (including those over 40 who are active), children, and partial tears with functional instability. Patients should regain full range of motion before surgery to lower the risk of postoperative arthrofibrosis, and meniscal tears are usually dealt with at the same sitting.
| Graft | Strength / feature | Main drawback |
|---|---|---|
| Bone-patellar tendon-bone (BPTB) autograft | Considered the gold standard; bone-to-bone healing; failure load 2600 N (native ACL 1725 N) | Anterior knee pain on kneeling (10–30%); patellar fracture or tendon rupture |
| Quadrupled hamstring autograft | Very common in primary surgery; failure load 4000 N; small incision, no anterior knee pain | Hamstring weakness; saphenous nerve branch paraesthesia |
| Allograft | No harvest-site problem | Infection and immune concerns; used in revision or by preference |
How is this tested in NEET PG and INI-CET?
- Most sensitive test for ACL rupture → Lachman. Most specific → pivot shift. PCL → posterior drawer (most accurate) and sag sign.
- Drawer test at 90 degrees — anterior translation = ACL; posterior translation = PCL.
- Unhappy triad → ACL + MCL + medial meniscus; dashboard injury → PCL.
- Bone bruise on the lateral femoral condyle and posterior lateral tibial plateau, and a Segond fracture, → ACL tear.
- Gold standard graft → BPTB autograft; commonest complication → anterior knee pain.
- Meniscal tests: McMurray, Thessaly and Apley; the red zone heals, the white zone does not.