Posteromedial Corner Knee Injury

The posteromedial corner (PMC) is frequently under-recognised in acute knee trauma. Isolated PMC injury is uncommon: up to 70% of PMC injuries occur as part of a multi-ligament knee injury, so an isolated grade III sMCL tear should always prompt a careful search for an associated ACL tear (the most common), PCL tear or meniscal lesion.

Mechanism. The injury follows a valgus force, external rotation, the two combined with axial load, or high-energy trauma:

• Valgus force - a lateral blow to the knee, or a fixed foot under valgus load (football, rugby)
• External rotation - the foot planted while the body rotates medially (skiing, pivoting sports)
• Combined valgus, external rotation and axial load - the classic mechanism for PMC plus ACL injury
• High-energy trauma - motor vehicle collision or fall from height, with multi-ligament injury

Why it matters. The injury gives medial pain and swelling localised to the sMCL and POL, and limits cutting, pivoting and stair descent. Missed, it leaves persistent anteromedial rotatory instability (AMRI), valgus collapse and accelerated medial compartment osteoarthritis.

The PMC is a layered complex of static and dynamic stabilisers on the medial side of the knee. The sMCL and POL are the primary static stabilisers and divide the work: the sMCL is the primary restraint to valgus at 30° of flexion, and the POL is the primary restraint to internal rotation of the tibia at all flexion angles (the Hughston axis). The deep MCL, a thickening of the medial capsule attached to the meniscus, is a secondary valgus restraint, and the semimembranosus is the main dynamic stabiliser.

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How AMRI arises. In the intact knee the sMCL, POL and semimembranosus together restrain anteromedial subluxation of the tibia. Loss of the sMCL alone produces valgus laxity, and loss of the POL alone internal rotation laxity. Lose both, and combined valgus and external rotation load subluxes the tibial plateau anteromedially: AMRI, the hallmark instability of PMC deficiency.

Why multi-ligament injury is so common. A single blow combining valgus, external rotation and axial load disrupts the ACL, sMCL and POL together, and hyperextension disrupts the PMC, ACL and PCL in sequence from posterior to anterior. The posterolateral corner and the PMC share the semimembranosus and the oblique popliteal ligament. In a high-energy knee dislocation all four corners may be involved.

PMC injuries are classified by the structures torn, by the grade of the sMCL injury and by time from injury. The anatomical pattern dictates the surgical plan: isolated sMCL injury is treated non-operatively, a complete PMC injury needs repair or reconstruction, and PMC plus cruciate injury requires combined ligament surgery.

Tear location. Beyond grade and timing, where the sMCL tears predicts healing. Proximal (femoral-sided) tears heal reliably with bracing because the ends stay apposed. Distal (tibial-sided) tears heal less predictably and are more likely to leave residual valgus laxity.

The Stener-like lesion. The avulsed distal sMCL retracts and flips superficial to the pes anserinus (sartorius, gracilis, semitendinosus), which crosses to the anteromedial tibia. The ligament end no longer contacts its tibial footprint and cannot heal with bracing; the analogy is the thumb UCL Stener lesion, displaced over the adductor aponeurosis. MRI showing the distal sMCL stump lying superficial to the pes tendons is the radiographic clue. A distal tear that is displaced or entrapped, or one with a large bony tibial avulsion, therefore favours acute surgical repair over a non-operative trial.

History. Pain over the medial joint line, the posteromedial corner or the proximal tibia after a valgus or external rotation injury, in contact sport or a motor vehicle collision. The knee gives way into valgus, especially with pivoting. Bruising is localised and medial, and the swelling is often less than the haemarthrosis of an ACL injury.

Examination. Look for medial ecchymosis, effusion, deformity, and watch the gait. Palpate the sMCL at its femoral and tibial attachments, the POL at the adductor tubercle, and the semimembranosus. Range of motion is often preserved unless a displaced meniscus blocks extension. Check the peroneal nerve and the popliteal vessels, always before and after any reduction.

The bedside tests. Valgus stress at 30° is the primary test of the sMCL and grades it. Valgus stress in full extension asks whether more than the sMCL has gone. The anterior drawer in external rotation and the Slocum test look for AMRI.

Opening in full extension. An isolated sMCL injury does not open at 0°, because the cruciates and posterior capsule act as secondary restraints. Valgus opening with the knee straight is therefore never an isolated sMCL injury: it means the PMC and the cruciate ligaments are both injured.

The diagnosis of PMC injury is clinical and MRI-confirmed. Plain radiographs exclude a bony avulsion and a tibial plateau fracture; MRI with corner-specific sequences is the gold standard for surgical planning.

Radiographs. AP, lateral, skyline and tunnel views of the knee. Look for avulsion of the medial femoral epicondyle (sMCL origin) or the adductor tubercle (POL origin), a Segond equivalent, and a tibial plateau fracture. Pellegrini-Stieda ossification along the proximal sMCL is a chronic finding: think chronic MCL insufficiency.

Bilateral valgus stress radiographs quantify side-to-side medial opening in millimetres and are useful in assessing a grade III injury.

MRI. Indicated in all suspected PMC injuries, especially when multi-ligament involvement is suspected. Each plane has its structures:

• Coronal - the sMCL origin, midsubstance and insertion: periligamentous oedema, fibre discontinuity, a wavy or stripped ligament
• Axial - the POL at the adductor tubercle, the semimembranosus tendon and its 5-6 arms, the posteromedial capsule
• Sagittal - the cruciate ligaments, menisci and posterior capsule

A bone marrow oedema pattern of medial femoral condyle and lateral tibial plateau bruising suggests combined ACL and PMC injury.

Before surgery. CT assesses bony avulsions and tibial plateau depression and measures fragment size for ORIF planning. Stress radiographs under anaesthesia quantify valgus opening and external rotation before surgical planning. A CT angiogram or ankle-brachial index is mandatory in any multi-ligament or high-energy knee injury to exclude popliteal artery injury.

The decision. Grade, pattern and timing decide treatment, weighed against the patient's demands. An isolated low-grade sMCL injury heals well and is braced. An isolated grade III injury is braced in the low-demand patient and repaired acutely in the high-demand one. The combined injury depends on whether it is an ACL tear with a medial injury that can be left to heal, or a multi-ligament knee in which every injured ligament is addressed.

Isolated grade I-II. The goals are to protect the healing sMCL, restore range of motion and prevent stiffness. Hinged bracing does not need to lock the knee in extension full-time: protected early motion (0-90°) prevents arthrofibrosis and is the modern standard. The sMCL heals by secondary intention and benefits from controlled motion to align its collagen fibres.

Isolated grade III. The goal is to restore valgus stability while protecting the POL, and outcomes are generally good with non-operative care in the absence of multi-ligament injury. The sMCL has excellent healing potential even in complete tears, and bracing for 6-8 weeks may restore valgus stability in low-demand patients. The choice lies between three routes:

• Bracing trial - for sedentary or low-demand patients, comorbidities precluding surgery, or patient preference. Hinged brace for 6-8 weeks, protected weight-bearing for 4 weeks, progressive ROM, then functional rehabilitation; 80-90% return to function in well-selected patients
• Acute repair, under 3 weeks - for a bony avulsion, the high-demand athlete, a failed bracing trial, a displaced distal (Stener-like) tear, or a multi-ligament pattern
• Chronic reconstruction, over 6 weeks - for failed bracing, persistent valgus laxity, AMRI, or the high-demand patient

ACL with a grade III MCL. The everyday combined injury, an ACL tear with a grade III sMCL/PMC tear, is distinct from the gross knee-dislocation pattern and has a characteristic staged approach that examiners look for. Because the medial side heals well, the usual sequence is to treat the MCL first in a hinged brace, regain full range of motion and let the medial side heal, then reconstruct the ACL at around 6 weeks once the knee is supple. Reconstructing the ACL acutely alongside a healing medial side risks arthrofibrosis and stiffness, and many grade III MCLs that are clinically stable at 6 weeks need no medial surgery at all.

The principle is to restore the central pivot (ACL) but let a healing medial side declare itself first. The exceptions that justify acute medial repair or reconstruction at the time of ACL surgery are:

• Persistent gross valgus laxity after a brace trial
• A distal or displaced (Stener-like) tear
• A large bony avulsion
• Valgus opening in full extension
• A true multi-ligament or dislocation pattern

PMC plus cruciate in the multi-ligament knee. Here the goal is to restore every injured ligament for a functional, stable knee. The cruciate is the cornerstone and PMC repair or reconstruction is added to it. Acute injuries (under 3 weeks) have all ligaments addressed in one setting; chronic injuries are staged if there is severe stiffness or fixed deformity. Neglecting the PMC component is a common cause of residual AMRI after ACL reconstruction.

Acute repair (under 3 weeks). Through a longitudinal incision over the medial femoral epicondyle, extended distally along the sMCL:

1. Identify the sMCL origin and insertion, the dMCL, the POL at the adductor tubercle and the semimembranosus expansion
2. Reattach a proximal (femoral) avulsion with 2-3 suture anchors at the medial epicondyle
3. Fix a distal (tibial) avulsion with a staple or screw-and-washer
4. Tighten the POL by advancing it onto the adductor tubercle with suture anchors
5. Re-tension the 5-6 insertional arms of the semimembranosus if torn

After repair: hinged brace for 6 weeks, protected weight-bearing, progressive ROM, and return to sport at 6-9 months.

Chronic reconstruction (over 6 weeks). Both the sMCL and the POL are reconstructed:

1. sMCL reconstruction with hamstring autograft or allograft through anatomic femoral and tibial tunnels
2. POL reconstruction with a separate limb of graft, advanced onto the adductor tubercle
3. Posteromedial capsule plication to address AMRI
4. Augmentation with an internal brace or suture tape reinforcement, which is increasingly used

Multi-ligament reconstruction. The cruciates go first because they set the central pivot.

Combined ACL plus PMC reconstruction gives 80-90% good-to-excellent results in the hands of an experienced knee surgeon. The 3-week threshold from injury is the one to remember: outcomes decline significantly after it.

Best prognosis. Acute presentation under 3 weeks, an isolated injury with no associated cruciate, a compliant patient, and hamstring autograft or primary repair.

Poor prognosis. Chronic injury over 6 weeks, a multi-ligament pattern or knee dislocation, severe chondral damage seen at arthroscopy, prior surgery, smoking, diabetes and obesity.