Medial Collateral Ligament Injuries

The MCL is the most common knee ligament injury and is usually treated without an operation: over 90% heal without surgery, and even a complete tear usually heals with bracing and rehabilitation. Functional bracing and early range of motion are the key.

Why it heals. The ligament is extra-articular and has a good blood supply, so its healing capacity is excellent. That is what separates it from the ACL and PCL.

Who. Males more than females, in the contact sports (football, rugby, hockey) and in skiing, where the combined ACL and MCL injury is common. Injuries are often combined.

Mechanism. Valgus stress is the most common mechanism: a contact blow to the lateral side of the knee, or a non-contact cutting or pivoting movement. External rotation may also injure the ligament, and the same valgus mechanism commonly produces a combined ACL and MCL injury.

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Three static stabilisers. LaPrade's quantitative anatomy describes the medial side as three primary static stabilisers: the superficial MCL (sMCL), the deep MCL (dMCL) and the posterior oblique ligament (POL).

Superficial MCL. The primary static restraint to valgus and a key secondary restraint to external and internal tibial rotation. Its femoral attachment is a small depression just proximal and posterior to the medial epicondyle, on average a few millimetres from it, not several centimetres proximal to the joint. The ligament averages 9 to 10 cm in length and has two tibial attachments: a proximal soft-tissue attachment to the meniscus and a distal, firm bony attachment roughly 6 cm below the joint line, deep to the pes anserinus.

Deep MCL. A capsular thickening with meniscofemoral and meniscotibial components that firmly anchors the body of the medial meniscus. It contributes mainly to anteromedial rotatory restraint.

Posterior oblique ligament. A fan-shaped condensation posterior to the sMCL that blends with the semimembranosus. It is the principal restraint to valgus and rotation near full extension and the key structure in posteromedial corner (PMC) injury.

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Biomechanics. In the classic Grood and Warren sectioning data the sMCL provides the large majority of valgus restraint at around 25 to 30 degrees of flexion. The proximal division of the sMCL and the POL also restrain external and internal tibial rotation. Their functional lengths change with knee position: the POL tightens in extension, while the sMCL provides its dominant valgus restraint in mid-flexion.

Why the test is done at 30 degrees. At about 30 degrees of flexion the cruciates and the posteromedial capsule are relatively relaxed, so valgus laxity there isolates the sMCL. At 0 degrees the POL, the posteromedial capsule and the cruciates are recruited, so valgus laxity in full extension signals a more extensive injury, involving the PMC and/or a cruciate, not an isolated MCL.

Warren's three layers. The medial side is dissected in three layers, and understanding them matters for surgical exposures:

• Layer 1: sartorius fascia and the investing fascia
• Layer 2: superficial MCL and posterior oblique ligament
• Layer 3: joint capsule, deep MCL and posteromedial capsule

The mnemonic SDP (Superficial, Deep to superficial, Proper capsule) orders them. The pes tendons, sartorius, gracilis and semitendinosus, are the superficial structures of the exposure: the sartorius fascia belongs to layer 1, and the pes lies superficial to the sMCL at its distal tibial attachment.

Grading. Injuries are graded on valgus stress at 30 degrees, by the amount of opening and whether there is an endpoint. Compare with the other knee, because absolute values vary between individuals.

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Location. The femoral origin is the most common site and may avulse bone; mid-substance tears are less common. A tear at the tibial insertion can become a Stener equivalent, the sMCL retracting to lie over the pes anserinus. Location affects healing and may influence the surgical approach.

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Associated injuries. Always assess for them.

• MCL plus ACL: very common. Address the ACL; the MCL usually heals.
• MCL plus PCL: less common and more severe.
• Multiligament injury: a dislocation pattern.

The unhappy triad. O'Donoghue's triad is classically described as ACL, MCL and medial meniscus, but in practice lateral meniscus tears co-occur with ACL injury more often than medial ones. It is a common exam trap.

History. A valgus blow, usually in contact, with medial knee pain at the time of injury and swelling that is often localised to the medial side. Ask about giving way under valgus load, and about the symptoms of the injuries that travel with it: a pop suggests the ACL, locking suggests the meniscus.

Palpation. Tenderness along the course of the ligament places the injury at the femoral origin, the mid-substance or the tibial insertion.

Valgus stress testing. With the patient supine and the knee at 30 degrees of flexion, stabilise the thigh and apply a valgus force at the ankle, comparing the opening and the endpoint with the other side. Repeat at 0 degrees: laxity in full extension indicates posteromedial corner injury.

The rest of the knee. Always perform a Lachman and a pivot shift, because combined injuries are common, and examine the meniscus with McMurray's test and joint-line palpation.

The pattern. An isolated sMCL injury produces straight-plane valgus laxity. When the posteromedial corner (PMC), the posterior oblique ligament, the posteromedial capsule and the semimembranosus expansions, is torn alongside the sMCL, the result is anteromedial rotatory instability (AMRI): the medial tibial plateau rotates anteriorly as the joint opens medially.

How to detect it.

• Valgus laxity at 0 degrees: the POL and posteromedial capsule are the restraints in extension, so opening here, and not just at 30 degrees, signals PMC involvement rather than an isolated sMCL injury
• Anteromedial drawer (Slocum test): an anterior drawer performed with the tibia in roughly 15 degrees of external rotation; increased anteromedial translation indicates AMRI
• MRI confirms injury to the POL, the posteromedial capsule and the semimembranosus

Why it matters. Unlike the isolated sMCL, the PMC heals poorly and is a genuine surgical target. Combined sMCL and POL disruption, and any knee with valgus laxity in full extension, should be evaluated for PMC repair or reconstruction rather than assumed to heal in a brace, especially alongside an ACL reconstruction. Missed AMRI is a recognised cause of failed ACL reconstruction through residual rotatory laxity.

The physis fails first. In a child or adolescent with open physes, the valgus injury that would tear the MCL in an adult more often fails through the distal femoral physis, which is weaker than the collateral ligament. A Salter-Harris fracture of the distal femur can therefore masquerade as an MCL sprain, and apparent valgus laxity on stress testing may be opening through the growth plate rather than through the ligament.

What to do. In a skeletally immature patient with a valgus injury and medial tenderness or instability, scrutinise the physis and obtain plain radiographs; if the examination suggests instability, valgus stress radiographs distinguish physeal opening from true ligamentous laxity. True paediatric MCL injuries do occur and, as in adults, heal well, but the physeal fracture must be excluded first.

Why the label matters. A distal femoral physeal fracture is an anatomical-reduction problem with a real risk of growth arrest and angular deformity, since the distal femoral physis contributes the most growth of any lower-limb physis. Calling it a ligament sprain is a significant error.

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Radiographs. The standard first step to rule out bony injury; they are usually normal in a pure MCL injury. Look for an avulsion fragment and for the associated fractures of the tibial plateau and femur. In the chronic knee, look for the Pellegrini-Stieda lesion: calcification at the femoral origin of the MCL, representing a chronic injury in which the haematoma has calcified. It is visible on plain films and may be asymptomatic.

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Stress radiographs can document valgus laxity, measured as the side-to-side difference in degrees or millimetres of opening. They are useful in chronic cases, for medicolegal purposes and in research, and are not commonly required in clinical practice.

MRI. Sensitivity for MCL injury is excellent. It shows oedema surrounding the ligament, thickening or discontinuity, and the level of the tear (femoral, mid-substance or tibial), together with the associated injuries: ACL, meniscus, cartilage and bone bruise. It is not always required for an isolated MCL injury, but it helps define a Grade III tear and the injuries that accompany it.

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Algorithm

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Acutely. RICE and a brace in extension while the knee is examined for associated injuries. Consider MRI if the tear is Grade III or an associated injury is suspected.

Isolated injury. Treatment is non-operative and graded, with weight bearing as tolerated and early physiotherapy for Grades I and II. Healing rates are excellent, and most Grade III tears heal.

• Grade I: functional brace, early range of motion, return to sport at 1-2 weeks
• Grade II: hinged brace for 4-6 weeks, progressive range of motion and strengthening, return to sport at 4-6 weeks
• Grade III: hinged brace for 6-8 weeks, protected weight bearing initially, supervised rehabilitation, return to sport at 8-12 weeks

Combined injury. Reconstruct the ACL if indicated; the MCL usually heals during the ACL recovery. The exception is a Grade III tear with valgus laxity at 0 degrees, which may need MCL surgery at the same time.

Surgery. Uncommon for isolated MCL injuries.

• Primary repair: Stener equivalent with the MCL trapped, bony avulsion with displacement, multiligament injury
• Reconstruction: chronic instability despite rehabilitation, failed conservative treatment, and combined with other ligament surgery if the knee is Grade III at 0 degrees

Repair. An acute femoral avulsion is reattached with suture anchors or bone tunnels to restore the origin; an acute tibial avulsion is fixed with a screw and soft-tissue washer; an acute mid-substance tear is repaired primarily with non-absorbable sutures. Augmentation with a graft may be added for severe injuries.

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Reconstruction. The graft is a hamstring autograft (semitendinosus) or an allograft (Achilles or tibialis anterior), placed through isometric femoral and tibial tunnels and fixed with interference screws or suture buttons. An anatomic reconstruction restores valgus stability.

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Combined ACL and MCL surgery. Most Grade III MCL tears heal with ACL reconstruction alone, and the MCL is addressed surgically if the knee is valgus-lax at 0 degrees. The ACL is reconstructed first; the MCL is then repaired or augmented if needed and tensioned with the knee in slight flexion. Postoperative rehabilitation is modified to protect both.

Stiffness is the enemy. Early range of motion is what prevents it, and even Grade III injuries do better in a hinged brace that allows motion than in a cast.

Conservative versus surgical rehabilitation. The conservatively treated knee progresses faster, with early motion encouraged; after reconstruction the knee is more protected initially but reaches a similar final timeline. Both pathways aim for full range of motion and strength before return to sport.

By grade. Grade I injuries have excellent outcomes with minimal long-term sequelae. Grade II injuries do well, and a brace during sport is helpful initially. Most isolated Grade III injuries heal with bracing; some chronic laxity may persist but is often asymptomatic.

Combined injuries. Outcomes depend on addressing all the pathology. ACL reconstruction with the MCL left to heal is the typical course.

Side-by-Side Guidance (where emphasis differs)

High- vs Limited-Resource Practice Variation

• Well-resourced settings: ready access to MRI and hinged functional bracing, supervised physiotherapy and graded return-to-sport testing; specialist reconstruction available for the minority who need it.
• Limited-resource settings: diagnosis relies more on careful clinical valgus stress testing (which is sufficient for most isolated injuries); improvised or non-hinged bracing and home-based rehab are used; MRI and reconstruction are reserved for combined/multiligament injury. Because isolated MCL injury heals well without surgery, clinical-examination-led conservative care travels well across resource settings.

Timing of ACL reconstruction in the combined injury. Most surgeons allow the MCL to recover full extension and resolve acute valgus laxity before ACL reconstruction, typically a few weeks, citing the arthrofibrosis risk of very early combined surgery. The optimal window remains debated, and some advocate earlier reconstruction once motion is restored.

Repair versus reconstruction in the acute high-grade injury. For acute distal (tibial-sided, Stener-equivalent) sMCL avulsions and complete posteromedial corner disruption, primary repair has historically had higher failure rates than for lateral-side injuries. There is growing interest in repair with suture augmentation versus primary reconstruction, but high-level comparative data are lacking.

Is the isolated Grade III tear always conservative? Landmark series support non-operative care even for complete isolated tears, yet a subset develops chronic valgus or anteromedial rotatory instability. Identifying which complete tears will fail bracing, for example those that are distally based or involve the POL as well, is unresolved.

Stress radiography and its cut-offs. Side-to-side valgus opening on stress radiographs helps grade severity, but the published cut-offs distinguishing isolated sMCL from combined sMCL and POL injury vary between studies, limiting any single agreed threshold.