Semimembranosus Anatomy

Origin. Semimembranosus arises from the superolateral facet of the ischial tuberosity, on its lateral and anterior aspect. The conjoint tendon of semitendinosus and the long head of biceps femoris sits posteromedially and semimembranosus anterolaterally. The two footprints are contiguous but distinct, and the semimembranosus footprint extends further anteriorly onto the tuberosity.

The footprint. In cadaveric mapping the semimembranosus footprint is crescentic, 3.1 by 1.1 cm, and lies on the lateral half of the tuberosity; the conjoint footprint, 2.7 by 1.8 cm, sits posteromedially. Being the deep, anterior, lateral part of the origin, it is the hardest to see and to re-anchor in a retracted three-tendon avulsion, and the part closest to the sciatic nerve.

Insertion. The tendon reaches the posteromedial tibia at the level of the joint line and immediately fans out:

1. Anterior (deep) arm. Turns anteriorly and runs deep to the superficial medial collateral ligament (sMCL) to insert on the medial tibia just distal to the joint line, beneath the sMCL. It is the arm most often mistaken for the direct arm at surgery.
2. Direct arm. The strongest and most posterior, inserting into a discrete tubercle on the posteromedial aspect of the medial tibial condyle, just distal and posterior to the joint line. It is the palpable, blunt structure the surgeon finds first through a posteromedial approach.
3. Inferior (distal tibial) expansion. A thin band running distally and anteriorly deep to the sMCL toward the posteromedial tibial crest and the popliteal line, blending with the fascia over popliteus.
4. Oblique popliteal ligament (OPL). A broad, strong lateral reflection that runs superolaterally across the posterior capsule to blend with the lateral posterior capsule, the plantaris origin and the lateral gastrocnemius. It is the largest structure on the posterior aspect of the knee and forms the floor of the popliteal fossa.
5. Capsular arm. Runs proximally and medially into the posterior oblique ligament (POL) and the posteromedial capsule, and reaches the posterior horn of the medial meniscus.

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Nerve and roots. Semimembranosus is supplied by the tibial division of the sciatic nerve, root values L5, S1, S2; some texts include L4 for the most proximal fibres.

Where the branches enter. Branches leave the tibial division in the proximal to mid thigh and enter the muscle on its anterior (deep) surface, the surface facing adductor magnus, not the one facing the surgeon in a posterior approach. There are typically two motor entry points, one to the proximal membranous portion and one to the distal belly, both on the deep surface and generally within the proximal and middle thirds of the muscle.

Why the division matters. All the medial hamstrings and the long head of biceps take the tibial division; only the short head of biceps femoris takes the common peroneal (fibular) division. An electromyographically normal short head with denervated semimembranosus, semitendinosus and biceps long head therefore localises a lesion to the tibial division proximal to the thigh branches, a classic way to separate a proximal sciatic lesion from a peroneal division injury.

Sparing of the hamstrings in a foot drop points the other way, to a common peroneal lesion at the fibular neck rather than a sciatic lesion.

Arterial supply. The dominant supply comes from perforating branches of the profunda femoris artery, usually the first and second perforators to the proximal and middle thirds. The inferior gluteal and medial circumflex femoral arteries supply the tendinous origin at the ischial tuberosity, and branches of the popliteal and superior medial genicular arteries supply the distal tendon and its expansions. Venae comitantes drain to the profunda femoris and popliteal veins.

A segmental muscle. The pedicle pattern is segmental (Mathes-Nahai type IV), which is why semimembranosus is a poor pedicled flap donor compared with gracilis or gastrocnemius: proximal division devascularises the distal belly.

Surgical consequences. The inferior gluteal and medial circumflex branches are the vascular basis for the large haematoma that tracks down the posterior thigh after a complete proximal avulsion, and that haematoma is one route by which the sciatic nerve becomes tethered in scar. Distal release for posteromedial exposure will not devascularise the muscle if the perforators are respected, but division of the tendon proximal to its expansions is unnecessary in almost every case.

In the thigh. Semitendinosus overlies semimembranosus for most of the thigh, with gluteus maximus and the posterior femoral cutaneous nerve above. Deep to it, semimembranosus lies directly on adductor magnus, which with the medial intermuscular septum also lies medial to it. The sciatic nerve is lateral, running distally toward the popliteal fossa, initially just lateral to the semimembranosus origin and then crossing posterior to adductor magnus.

The crossing. Proximally semimembranosus is deep and lateral to semitendinosus. At the knee it lies deep and slightly medial, with semitendinosus crossing superficially to reach the pes anserinus. Candidates routinely get this the wrong way round.

At the knee. Gracilis and, most superficially, sartorius lie medially as the pes anserinus. Laterally lie the medial head of gastrocnemius and, beyond it, the popliteal neurovascular bundle.

The interval that matters. The plane between the semimembranosus tendon anteromedially and the medial head of gastrocnemius posterolaterally is the posteromedial safe interval to the posterior knee, and is exactly where a popliteal (Baker) cyst forms.

Mechanism. Forced hip flexion with the knee extended: water-skiing, hurdling, a slip with the leg going forward, or an awkward split.

Presentation. An audible pop, posterior thigh pain and inability to continue, then within 24-72 hours a large posterior thigh ecchymosis tracking distally. Hip extension power is lost. Later the patient cannot sprint and has buttock pain on sitting, a "cramping" pain from the ischial stump; if the sciatic nerve is tethered there is also a "dead leg" sensation and radiating posterior thigh pain.

Imaging. Radiographs are essential in adolescents to identify a bony apophyseal avulsion, which changes the whole management. MRI is the investigation of choice, with coronal and axial oblique sequences along the hamstring origin; it should report the number of tendons avulsed, the retraction in cm and the relationship of the stump to the sciatic nerve. Ultrasound is useful for dynamic assessment and guided injection but poor at quantifying retraction.

Repair. The steps, in order:

1. Position prone, with the hip extended and the knee flexed 30-45 degrees to relax the hamstrings and reduce repair tension.
2. Incise transversely in the gluteal crease for acute repairs, with a longitudinal or L-shaped extension for retracted chronic cases.
3. Expose by identifying the inferior border of gluteus maximus and retracting it superiorly, then the posterior femoral cutaneous nerve, then the sciatic nerve laterally before any lateral mobilisation. Work from medial to lateral and place every lateral retractor under direct vision.
4. Fix with suture anchors into the decorticated tuberosity, restoring the semimembranosus footprint anterolaterally and the conjoint footprint posteromedially. Anatomical restoration of both footprints separately is the goal, not a single mass repair.

The chronic case. In a chronic retracted avulsion the sciatic nerve is frequently adherent to the scarred tendon stump, and must be identified and neurolysed before the stump is mobilised.

The injury. A grade III medial-sided injury is rarely isolated. It is a valgus and external rotation injury that frequently accompanies an ACL or PCL tear, and the structures fail in sequence: the sMCL, then the deep MCL and meniscotibial ligament, then the POL and posteromedial capsule, and finally the semimembranosus expansions.

Anteromedial rotatory instability. It is defined by anterior subluxation and external rotation of the medial tibial plateau with an anteromedial drawer: with the corner incompetent, a valgus and external rotation force lets the medial plateau rotate anteriorly around an intact posterolateral pivot. Being rotatory rather than translational, it will not be captured by a simple valgus stress test at 30 degrees, and it is the reason a grade III medial injury with a positive anteromedial drawer will not settle with a brace alone.

The ACL. The PMC is a secondary restraint to anterior translation. An unrecognised or unaddressed PMC injury increases ACL graft force and is a recognised contributor to early graft failure.

Acute repair. Within 3 weeks, the sMCL and POL are repaired anatomically and the semimembranosus is advanced to retension the corner. Hughston's original description of POL repair used exactly this principle, and the classical Hughston posteromedial capsular reconstruction advances the POL and the semimembranosus insertion anteriorly and proximally.

Chronic reconstruction. The sMCL and POL are reconstructed with graft, restoring the femoral isometric points, with semimembranosus recruited as a dynamic tensioner rather than a graft source.

Tensioning. Tension the POL in extension and the sMCL at 20-30 degrees. Over-tensioning the POL in flexion produces a stiff knee that cannot flex.

Two bursae. The semimembranosus bursa lies between the distal semimembranosus tendon and the medial tibial condyle. The gastrocnemio-semimembranosus bursa lies between the tendon and the medial head of gastrocnemius. The two lie in the same plane and share drainage with the joint, the anatomical basis for the popliteal cyst.

Semimembranosus bursitis. Posteromedial knee pain, worse on resisted knee flexion and on deep flexion, with tenderness posterior to the sMCL and just distal to the joint line. The pes anserine bursa, by contrast, is anteromedial, roughly 4-5 cm distal to the medial joint line, deep to the pes tendons and superficial to the sMCL, and is tender well below and in front of the semimembranosus insertion. Treatment is activity modification, eccentric loading and image-guided corticosteroid injection where symptoms persist; excision is rarely required.

The Baker cyst. It is a distension of the gastrocnemio-semimembranosus bursa, which communicates with the knee through a valvular slit in the posteromedial capsule between the semimembranosus tendon and the medial head of gastrocnemius. The slit acts as a check valve: joint fluid enters during knee flexion and quadriceps contraction and cannot readily return, so the cyst enlarges with any cause of effusion.

Adults and children. In adults a Baker cyst is a symptom, not a diagnosis. Look for the driver, a medial meniscal posterior horn tear, osteoarthritis or inflammatory arthropathy; treat the joint and the cyst usually resolves. In children the cyst is typically primary and non-communicating and resolves spontaneously, and excision is almost never indicated.

Imaging and rupture. Ultrasound confirms a fluid collection with a neck passing between the semimembranosus tendon and medial gastrocnemius; MRI additionally shows the intra-articular driver. Rupture produces calf pain, swelling and a crescent sign of ecchymosis around the malleoli, the classic mimic of deep vein thrombosis.

Proximal hamstring tendinopathy. Long-distance runners, sprinters and middle-aged recreational athletes present with deep buttock pain at the ischial tuberosity, worse on sitting on hard surfaces, uphill running and accelerating. The lesion is an insertional tendinopathy of the conjoint tendon and semimembranosus origin, often with adjacent ischial bursitis and, in some, sciatic nerve irritation from the adjacent inflamed tissue (deep gluteal syndrome overlap).

Treating it. Load management and progressive isometric then heavy slow resistance loading in progressively lengthened positions. Aggressive static stretching compresses the tendon against the tuberosity and is avoided. Corticosteroid injection provides only short-term relief and is used sparingly.

Distal semimembranosus tendinopathy. Posteromedial knee pain in runners and footballers, tender over the tibial insertion and worse on resisted flexion with internal rotation. In a runner with posteromedial knee pain, tenderness at the insertion is distal semimembranosus tendinopathy until proved otherwise. MRI shows high signal and thickening at the direct arm insertion with a reactive marrow oedema pattern in the posteromedial tibial condyle.

Not a meniscal tear. It is frequently misdiagnosed as a medial meniscal tear. The discriminators are the site of tenderness, posterior and distal to the joint line rather than on it, and a negative McMurray.

Muscle strain. Semimembranosus strains are less common than biceps femoris strains but take longer to recover, particularly those involving the long proximal free tendon. The MRI prognostic factors are involvement of the free tendon, the cranio-caudal length of oedema and the cross-sectional area involved.