Knee injuries and ligament tears

• The knee is kept stable by four main ligaments (anterior cruciate or ACL, posterior cruciate or PCL, medial collateral or MCL, lateral collateral or LCL) and two menisci, the fibrocartilage cushions in each compartment.
• A pop, rapid swelling and a knee that gives way suggest an ACL tear; a direct blow to the front of a bent knee suggests a PCL tear.
• MCL tears are the commonest ligament injury and usually heal without surgery; isolated PCL tears are most often treated without surgery too.
• The Lachman test is the most useful bedside test for ACL injury and the posterior drawer test the most useful for PCL injury; valgus and varus stress tests detect collateral ligament injury; MRI confirms and stages most injuries.
• Early ACL reconstruction (within about three months, when surgery is chosen) reduces the risk of further meniscal and cartilage damage; combined ACL and MCL tears often need the MCL treated without surgery while the ACL is reconstructed.
• The classical "unhappy triad" (ACL, MCL and medial meniscus) is now known more often to involve the lateral meniscus, which is the caveat behind the name.

By the end of this page you should be able to:

1. Describe the anatomy and stabilising roles of the cruciate and collateral ligaments and the menisci.
2. Explain how ACL, PCL, MCL and meniscal tears present and why the symptoms occur.
3. Perform and interpret the Lachman, anterior drawer, posterior drawer, valgus stress, varus stress and McMurray tests and state their diagnostic performance.
4. Choose first-line imaging and state the cut-offs that separate partial from complete PCL injury and grade I from grade III MCL injury.
5. Plan management of isolated and combined ACL, PCL, MCL and meniscal injuries using current guideline recommendations, including when to operate and when to rehabilitate.
6. Counsel patients on prognosis, prevention and special considerations for children, athletes and older adults.

The ACL stops the tibia sliding too far forward and resists rotation; the PCL stops it sliding backward. Together the two cruciate ligaments form a cross inside the joint and are bathed in synovial fluid, which is why they tend to heal poorly after complete rupture and are more often reconstructed than repaired. ^[1] The MCL resists valgus (inward-bending) force and the LCL resists varus (outward-bending) force. The menisci are crescent-shaped fibrocartilage pads that share load, absorb shock and stabilise the joint; the lateral meniscus covers about 80% of its compartment, the medial about 60%. ^[9]

Grading of collateral ligament injury follows the American Medical Association system used in the reviews: grade I is a stretched ligament with no loss of integrity; grade II a partial tear; grade III a complete tear with laxity on valgus (for MCL) or varus stress testing. ^[3] For PCL injury, grade I corresponds to 1 to 5 mm of posterior laxity, grade II to 6 to 10 mm, and grade III to more than 10 mm. ^[6]

Knee injuries are common: an estimated 2.29 injuries per 1000 people in the USA, and ligament damage accounts for about 40% of all knee injuries. Of these ligament-specific injuries, roughly 50% involve the individual intra-articular (cruciate) ligaments. ^[1] Cruciate injuries are rarely isolated to the PCL: only 8% of intra-articular ligament injuries involve the PCL, against 92% that involve the ACL. ^[1]

ACL injury happens in cutting, deceleration and landing sports, usually when a planted foot rotates. It is particularly common in football and basketball, and in many high-intensity sports women have a significantly higher risk than men, although men present more often because more men play those sports. ^[1] A tear is often remembered as a pop, with rapid swelling and a knee that gives way. ^[1]

MCL injuries are the commonest ligament injury of the knee. Reported annual incidence is 0.24 to 7.3 per 1,000 people, with a male-to-female ratio of 2 to 1, and up to 60% of skiing knee injuries involve the MCL. ^[3] Ligament injuries of the knee account for roughly 40 percent of all knee injuries, of which MCL injuries are the most frequent. ^[3]

Meniscal tears are the most common pathology of the knee, with a mean annual incidence of 66 per 100,000. ^[9] About 200,000 ACL injuries occur per year in the United States, against only 3% of injured knees having a PCL disruption, and most grade III PCL injuries (79%) occur with other ligament injuries: ACL in 46%, MCL in 31% and posterolateral corner in 62%. ^[5]

The ACL prevents too much forward movement of the tibia and resists rotation of the tibia relative to the femur. The PCL is the stronger and less injury-prone cruciate ligament and is the main restraint to posterior tibial translation. ^[1][5]

When the ACL fails, the tibia shifts forward and rotates, which overloads the menisci and chondral surfaces. This is why a delay of more than about three months between injury and reconstruction, when surgery is indicated, is associated with more severe meniscal and cartilage damage at operation. ^[2] Posterior PCL deficiency changes medial compartment and patellofemoral loading and can lead to degenerative change over time, particularly in the medial compartment and patellofemoral joint. ^[6]

Meniscal tears start with circumferential collagen fibre disruption. The meniscus depends on its peripheral blood supply (the outer 10 to 25% in adults), which explains why tears in the avascular inner zone do not heal, and why repair is restricted to peripheral (red-red or red-white zone) tears. ^[9]

The unhappy triad describes the co-occurrence of ACL, MCL and meniscal injury. Older (O'Donoghue) descriptions emphasised the medial meniscus, because 17 of 22 combined ACL/MCL injuries had a medial meniscus tear; later arthroscopy studies showed a higher prevalence of lateral meniscus injury (32 to 71%), which is the caveat behind the modern definition. ^[4]

• ACL: a non-contact or contact pivoting or deceleration injury; the patient often reports a pop, rapid swelling, inability to continue, and later a giving-way sensation, particularly with twisting. ^[1]
• PCL: a direct blow to the front of the bent knee (dashboard injury), a fall in deep flexion, or hyperextension; posterior knee pain, mild to moderate swelling, and pain on kneeling or descending stairs. ^[5]
• MCL: a valgus force to the knee, often in skiing, rugby or football, with medial pain and swelling; complete tears may be less painful than partial ones. ^[3]
• Meniscal tear: twisting on a loaded knee, joint-line pain, catching or locking (bucket-handle tear), and sometimes delayed swelling. ^[9]
• Combined unhappy triad: immediate swelling with haemarthrosis, pain, impaired muscle control and a sensation of instability. ^[4]

Begin with inspection (alignment, effusion), palpation (joint line, MCL origin and insertion) and range of motion, then neurovascular status. Arrange imaging for patients who meet local NICE-based criteria. ^[1]

Special tests:

• Lachman test (ACL): with the knee slightly flexed, draw the tibia forward; a soft or absent end point is positive. It is generally the most sensitive test for ACL rupture, reported as 90% sensitive and 87.5% specific against arthroscopy in one study. ^[8]
• Anterior drawer test (ACL): with the knee deeply flexed, translate the tibia forwards; positive if excessive translation with a soft end point. Reported sensitivity 88.6% and specificity 87.5% against arthroscopy in the same study. ^{[8] [7]}
• Pivot shift test (ACL): the most specific test for ACL rupture (anterolateral rotatory instability), but the least sensitive, especially when the patient guards in the acute phase. ^[8]
• Posterior drawer test (PCL): the most accurate physical test for PCL injury, with sensitivity 90% and specificity 99%; grade I 1 to 5 mm, grade II 6 to 10 mm, grade III more than 10 mm of posterior translation. ^[6]
• Posterior sag sign and quadriceps active test (PCL): gravity-dependent posterior sag of the tibia, corrected by quadriceps contraction. ^[5]
• Valgus stress test (MCL): valgus force at full extension and at 30 degrees of flexion. Opening only in flexion suggests an isolated MCL tear; opening in full extension suggests a combined injury including cruciate or posteromedial corner. ^[3]
• Varus stress test (LCL and posterolateral corner) at 0 and 30 degrees, with the dial test for external rotation at 30 and 90 degrees. ^[3]
• McMurray and Thessaly tests (meniscus): meniscal tests have poor diagnostic accuracy, particularly in non-specialist settings, so a normal test does not exclude a tear. ^[1]

Severity and joint-line tenderness are best combined: opening of 1 to 5 mm on valgus stress is grade I MCL, 5 to 10 mm grade II, more than 10 mm grade III (Hughston system as summarised in the unhappy triad and MCL reviews; some other series use 3 to 5, 5 to 10 and 10 or more millimetres). ^[3][4]

Start with history, examination and plain radiographs when indicated, using the Ottawa knee rules as adapted in local NICE-based practice to decide who needs an X-ray after injury. ^[1] Stress radiographs objectify laxity: posterior tibial translation under 8 mm indicates a partial PCL tear, 8 to 12 mm a complete isolated PCL tear, and more than 12 mm a combined injury. ^[5] Valgus stress radiography of the knee in 30 degrees of flexion helps grade MCL injury, with side-to-side medial gap differences under 3.2 mm indicating an intact or partially injured superficial MCL, at least 3.2 mm suggesting a complete superficial MCL tear, and more than 9.8 mm suggesting combined MCL and posteromedial corner injury. ^[3]

MRI is the investigation of choice for soft-tissue knee injury. It shows ligament position and grade, meniscal tears and bone bruising, and surgical planning. For ACL, direct signs on MRI have sensitivity 92 to 94% and specificity 95 to 100%, improving with T2-weighted sequences and oblique sagittal views, but MRI is less good at separating complete from partial tears and at detecting chronic tears. ^[7] Acute PCL tears show MRI sensitivity near 100% and specificity near 97%, but chronic PCL injuries may show near-normal MRI in about 28% of cases, so stress radiographs help. ^[5] MCL tears show the expected grade I to III signal changes medially on T2-weighted sequences. ^[3] Meniscal MRI is 91.8% sensitive and 79.9% specific for medial tears and 80.7% sensitive and 85.4% specific for lateral tears; performance falls in ACL-injured knees. ^[10] Ramp lesions and posterior-horn longitudinal tears are detected with only about 65% sensitivity. ^{[10] [4][6]}

Physical examination still matters. In one diagnostic study, the Lachman test had 90% sensitivity and 87.5% specificity, the anterior drawer 88.6% sensitivity and 87.5% specificity, and MRI 98.6% sensitivity and 87.5% specificity against arthroscopy for ACL injury. ^[8] Reported Lachman sensitivity varies widely across meta-analyses, because populations, the timing of examination and use of anaesthesia differ enough to change the expected values. ^[8]

There is no single numerical score that diagnoses an ACL tear; diagnosis follows history, mechanism, Lachman or pivot-shift positivity and MRI. A diagnosis of ACL rupture rests on mechanism, effusion, a positive Lachman or pivot shift, and MRI; end-point quality and side-to-side comparison are more informative than a single millimetre cut-off.

PCL severity uses posterior tibial translation in millimetres, as above. ^[5][6] Meniscal tears are described by type (longitudinal, horizontal, radial, flap, bucket-handle, complex) and location, because treatment follows the pattern: vertical longitudinal tears and bucket-handle tears are often repairable in younger patients, whereas complex or degenerative tears in avascular zones are not. ^[9]

Rest, ice, compression and elevation, analgesia, protection and early mobilisation are standard. Large haemarthrosis may be aspirated to relieve pain and improve early range of motion, although evidence is limited. ^[2] Arrange urgent imaging and an orthopaedic opinion for suspected multi-ligament injury, dislocation, fracture or vascular compromise. ^[1]

Non-operative care with a hinged brace and early range of motion is the mainstay for most isolated MCL injuries, regardless of grade. Grade I injuries may be managed without a brace; grades II and III need a brace for at least 3 and 6 weeks respectively, with weight-bearing as tolerated and early quadriceps activation. ^[3] Surgery is reserved for open injury, medial laxity with the knee in full extension (suggesting posteromedial corner involvement), anteromedial rotatory instability, associated multi-ligament injury, Stener-type lesion (tibial avulsion with interposition of the pes anserine), or chronic symptomatic laxity after at least 6 weeks of conservative care, plus bony conditions such as valgus malalignment or plateau fracture. ^[3]

When an ACL tears at the same time, the MCL is usually treated without surgery while the ACL is reconstructed, with good outcomes; operative MCL treatment may help when there is greater laxity ("floating meniscus") or high-grade injury. ^[2] Multi-ligament injuries needing combined or staged surgery have a prolonged rehabilitation, and the optimal timing remains debated. ^[3]

When surgery is indicated for an acute isolated ACL tear, early reconstruction is preferred because the risk of additional cartilage and meniscal damage starts to increase within 3 months. ^[2] Reconstruction (not repair) is the standard, because it carries a lower revision rate. ^[2] Autograft is favoured over allograft, particularly in young or active patients, to lower graft failure. ^[2] Bone-patellar tendon-bone autograft may reduce graft failure or infection, whereas hamstring may reduce anterior knee and kneeling pain. ^[2] Single and double-bundle techniques give similar measured outcomes. ^[2] Concomitant meniscal tears should be preserved (repaired) when possible to protect the joint. ^[2]

Non-operative ACL care is reasonable for low-demand or older patients with a stable knee, but reconstruction should be discussed for active patients, especially with meniscal or chondral injury or persistent instability. ^[2] Prophylactic knee bracing is not recommended to prevent ACL injury. ^[2] Neuromuscular training programmes (plyometrics, strengthening, agility, feedback) can reduce primary ACL injuries in athletes in high-risk sports, with moderate-level evidence. ^[2]

Rehabilitation after ACL reconstruction follows staged milestones: early range of motion and quadriceps control, then strength and neuromuscular training; return to sport is usually considered from about 9 months, with functional testing such as hop testing used as one factor among several. ^[2]

Isolated grade I and II PCL injuries are managed without surgery: the knee is immobilised in extension for 2 to 4 weeks, then quadriceps strengthening with hamstring inhibition, with return to sport only when the knee is stable and quadriceps strength is about 90% of the other side, usually not before 3 months. ^[5] Grade III lesions (more than 10 mm posterior laxity) may still be treated without surgery first, but if instability persists reconstruction should be considered. ^[5] Surgery is indicated for grade III isolated tears with pain or instability that fail conservative care, combined ligament injuries, and avulsion fractures from the tibial insertion. ^[6] Reconstruction options (single or double bundle, transtibial or tibial inlay, autograft or allograft) have not shown definitive superiority in clinical studies. ^[5][6] After reconstruction, protect the graft in extension for 3 to 6 weeks and full loading is delayed, with return to sport not before 6 to 9 months. ^{[5] [4]}

Rehabilitation phases for a conservatively treated isolated PCL tear: phase I is the first 6 weeks (partial weight-bearing, brace in extension, quadriceps strengthening), phase II 6 to 12 weeks (full range of motion and proprioception), phase III 13 to 18 weeks (running and sport-specific training), with return to sport at 4 to 6 months. ^[6]

For degenerative meniscal tears in middle-aged or older adults, a trial of supervised exercise and weight loss is a reasonable first step: in a randomised trial, 90 middle-aged patients with non-traumatic medial meniscal tears improved with exercise, with no added benefit from surgery at eight weeks or six months; one third later had surgery but reached the same levels. ^[9] A multicentre randomised trial of 351 patients over 45 with a meniscal tear and osteoarthritis found no difference in pain or function between partial meniscectomy and physical therapy alone at 12 months, although 35% of the therapy group later crossed over. ^[9]

Partial meniscectomy helps symptomatic tears that are not repairable and preserves the peripheral rim; about 90% of patients report good or excellent short-term results. ^[9] Meniscal repair (inside-out, all-inside or outside-in) is suitable for younger patients with reducible, peripheral, vertical/longitudinal or bucket-handle tears; it succeeds in about 80% at 2 years but requires protected weight-bearing in a brace for 4 to 6 weeks. ^[9] Repair heals better within 8 weeks of injury and better when combined with ACL reconstruction, presumably because of bleeding and growth factors from drilling. ^[9] Meniscal preservation during ACL reconstruction is encouraged to protect long-term joint health. ^[2]

• ACL and meniscal: delaying indicated ACL reconstruction beyond about 3 months increases the risk of meniscal and cartilage damage at operation. ^[2] Recurrent instability, graft failure/re-rupture, arthrofibrosis, infection and donor-site morbidity are the main surgical risks. ^[2]
• ACL-related osteoarthritis: most long-term studies either favour or dispute progression after reconstruction, and many patients with meniscectomy show radiographic degeneration by 14 years. ^[9]
• PCL: residual laxity (more than 4 mm posterior translation), flexion loss after prolonged immobilisation, neurovascular injury during posterior approaches, and medial/patellofemoral arthrosis from chronic deficiency. ^[5][6]
• MCL: arthrofibrosis, Stener lesions, posteromedial corner injury; combined injuries raise the risk of ACL graft failure if the MCL remains lax. ^[2][3]
• Meniscus: repair failure and re-operation; meniscectomy increases long-term osteoarthritis risk, more so on the lateral side. ^[9]
• Unhappy triad: about half of patients develop osteoarthritis over 10 to 20 years after these combined injuries. ^[4]

Isolated MCL tears usually recover well and most patients can be managed without surgery. ^[3] Most athletes with isolated PCL tears return to sport after conservative rehabilitation, and subjective outcome often does not match the degree of laxity. ^[6] About 80% of meniscal repairs survive at 2 years, while about two-thirds of patients achieve lasting relief after exercise-based non-operative care for degenerative meniscal tears. ^[9] ACL reconstruction, when indicated, restores stability in most patients, but graft survival is better with autograft, and return-to-sport criteria (time, strength and functional tests) are used together. ^[2] Chronic PCL deficiency worsens objective knee function compared with isolated ACL injury, partly from altered loading. ^[6]

Screening is not a single blood test or scan; it is risk identification and modification. The AAOS guidance advises injury-prevention training programmes for athletes in high-risk sports (grade: moderate evidence) because they reduce primary ACL injury; programme content includes strengthening, plyometrics and sport-specific agility. ^[2] Prophylactic knee braces are not a preferred way to prevent ACL injury. ^[2] For MCL, most acute tears happen in skiing and contact sports and teaching safer landing and cutting technique matters. Early ACL reconstruction when indicated, and timely repair of meniscal tears, are themselves preventive, because a functioning ACL protects the meniscus and cartilage. ^[2]

Children and adolescents: ACL injury in children usually occurs with sport, often with meniscal and MCL injury, and growth-plate considerations shape treatment (physeal-sparing techniques). Meniscal tears under the age of 10 are uncommon except with discoid meniscus. ^[9] Conservative or surgical decisions, timing and bracing should be made with paediatric orthopaedic input, and prevention programmes are especially relevant during the 11 to 17 year age range when ACL risk rises in both sexes. ^[2]

Older adults: degenerative meniscal tears without mechanical symptoms should usually start with therapy and weight loss, because randomised trials show similar function at 12 months and late surgery still benefits non-responders. ^[9] Arthroscopic partial meniscectomy gives better results when there is little pre-existing cartilage damage. ^[9]

Women and athletes: women in pivoting sports carry a higher ACL risk; strength, landing and cutting programmes are recommended. Pregnancy is not specifically addressed in the cited sources, and any imaging or surgical plan should be individualised with obstetric and anaesthetic input.

Immunocompromised or medically complex patients: the cited sources do not address this directly; management is tailored to comorbidity and surgical risk.

A second useful comparison is between the injury types themselves:

• The ACL resists forward and rotational tibial movement; the PCL resists backward movement; the MCL resists valgus and the menisci share load and stabilise.
• ACL injury typically follows a pivot or deceleration with a pop and rapid haemarthrosis; PCL injury follows a dashboard blow or hyperextension; MCL injury follows a valgus blow.
• The Lachman test is the most useful bedside test for ACL injury; the posterior drawer test is the most useful for PCL injury; valgus and varus stress tests grade collateral injury.
• MRI is highly accurate for acute ACL (92 to 94% sensitivity, 95 to 100% specificity) and acute PCL (near 100% and near 97%), and it grades MCL and meniscal injury; meniscal MRI is 91.8%/79.9% (medial) and 80.7%/85.4% (lateral). ^[7][5][10]
• Most isolated MCL and isolated grade I/II PCL injuries heal with brace and physiotherapy; surgery is reserved for specific indications such as Stener lesions, grade III/combined PCL injury, or failed conservative care. ^[3][5][6]
• When surgery is chosen for an acute ACL tear, reconstruction is preferred to repair and autograft to allograft, and operating within about 3 months reduces further meniscal and cartilage damage. ^[2]
• Combined ACL/MCL tears usually need ACL reconstruction with non-operative MCL treatment; multi-ligament knee injuries need careful staged management. ^[2]
• The classical unhappy triad (ACL, MCL, medial meniscus) more often involves the lateral meniscus in modern series. ^[4]
• Meniscal tears in older adults without mechanical symptoms should first be treated with structured exercise; partial meniscectomy gives good short-term results in suitable tears and repair suits reducible peripheral tears, especially in younger patients. ^[9]
• Prevention of primary ACL injury relies on neuromuscular training programmes; prophylactic bracing is not helpful. ^[2]
• Children, athletes and older adults need tailored treatment paths, and delay of indicated surgery is not harmless. ^[2]

Knee osteoarthritis, fractures of the tibial plateau, patellar dislocation, ACL rehabilitation protocols, meniscal root tears, posterolateral corner injury, return-to-sport testing and neuromuscular injury-prevention programmes.