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Anterior cruciate ligament (ACL) tear
An ACL tear usually happens during a non-contact pivot or sudden stop, with a pop and rapid swelling from blood in the joint; the Lachman test is the most sensitive examination, and treatment depends on activity level and instability — rehabilitation or surgical reconstruction.
Why it matters in the exam
The classic stem describes a football or basketball player who twisted the knee with the foot planted, heard a pop, could not continue playing, and whose knee swelled within hours. Rapid swelling like this indicates blood in the joint, and the ACL is the most common cause of traumatic haemarthrosis in adults. On examination, the Lachman test at about 20–30 degrees of flexion is the most sensitive, the pivot shift test is the most specific but hard to perform in an awake patient in pain, and the anterior drawer test is less sensitive in the acute phase. MRI confirms the diagnosis and identifies associated injuries to the meniscus, the medial collateral ligament and the cartilage. A Segond fracture — a small avulsion at the lateral edge of the tibia — is a typical indirect sign of an ACL tear. The treatment decision depends on age, activity level, pivoting sport and a feeling of instability: a young athlete who wants to return to pivoting sports, or a patient with recurrent instability, suits reconstruction; a less active patient can do well with rehabilitation alone. Before surgery, full range of motion is restored and swelling reduced. Return to competitive sport is usually not before about 9 months, after strength and function tests. In children with open growth plates, plate-sparing techniques are used. Before return to sport, strength, stability and confidence are tested, because returning too early raises the risk of re-rupture.
Key points
- Mechanism: non-contact pivot or sudden stop, a pop and blood in the joint.
- Lachman is the most sensitive test; the pivot shift is the most specific.
- MRI confirms and finds meniscal and cartilage injury.
- Young athlete in a pivoting sport — reconstruction; less active — rehabilitation.
- Return to competitive sport not before about 9 months.
Summary table
| Test or finding | What is assessed | Meaning |
|---|---|---|
| Lachman test | Anterior shift of the tibia at 20–30 degrees of flexion | The most sensitive test for an ACL tear |
| Pivot shift | Anterior subluxation of the tibia with rotation and valgus load | Very specific; reflects rotational instability |
| Anterior drawer test | Anterior shift at 90 degrees of flexion | Less sensitive in the acute phase |
| Segond fracture | Avulsion at the lateral edge of the tibia on radiograph | Typical indirect sign of an ACL tear |
| MRI | Torn ligament and associated injuries | Confirms diagnosis and plans meniscus and cartilage care |
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Sources
- AAOS Clinical Practice Guideline, 2022 — Management of Anterior Cruciate Ligament Injuries
- Campbell's Operative Orthopaedics, 14th ed. (2021) — Knee Injuries
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.