Semimembranosus

For Medical Students

Structure and Location

Semimembranosus is the deepest of the hamstrings on the medial side. It arises from the ischial tuberosity and runs down to the back of the knee, where its tendon does something unusual: it ends in five places rather than one.

The direct arm is the main insertion, to a tubercle on the posteromedial tibia just below the joint line.

The anterior arm, also called the pars reflexa, turns forwards under the superficial collateral ligament to reach the tibia beneath it.

The oblique popliteal ligament is the third. It leaves the tendon and runs upwards and laterally across the back of the knee to the capsule, which makes a ligament out of a tendon.

The fourth is an expansion into the posterior oblique ligament, and the fifth runs distally over the popliteus muscle as the popliteal arm.

Relations

In front of the tendon: the posteromedial capsule and the posterior horn of the medial meniscus, which the tendon reaches through an attachment.

Behind it: semitendinosus, which is the more superficial of the two and the one that can be felt as a cord.

Laterally: the medial head of gastrocnemius, with the semimembranosus bursa between them.

Deep to the whole group: the popliteal vessels.

Function

As a muscle it flexes the knee and rotates the tibia internally.

Through its five arms it tensions the posteromedial corner when it contracts, so the corner is tight when the muscle is working and slack when it is not.

Its attachment to the posterior horn of the medial meniscus pulls the meniscus backwards as the knee flexes, which keeps it out of the way of the advancing femoral condyle.

Clinical Relevance

The muscle is the reason a posteromedial corner injury is partly a muscle problem. A corner with intact ligaments and a wasted semimembranosus is not a working corner.

Because one of its arms runs under the superficial collateral ligament, an injury to that ligament's proximal tibial attachment can involve the tendon too.

The semimembranosus bursa lies between the tendon and the medial head of gastrocnemius, and a swelling there is a bursa rather than a joint problem.

MRCS Level

Structure and Location

Name the five arms in a viva: direct, anterior or pars reflexa, oblique popliteal ligament, posterior oblique expansion, and popliteal. Descriptions differ in how they group them, so say which scheme is being used.

The direct arm attaches to a tubercle on the posteromedial tibia. Conventional teaching places it about 10 mm below the joint line, which makes it a reliable landmark when the soft tissue is disturbed.

The anterior arm passes beneath the superficial collateral ligament and attaches to the tibia under it, which is the anatomical basis for that ligament's proximal attachment being to soft tissue rather than bone.

The oblique popliteal ligament is the largest structure on the back of the knee and the main restraint to hyperextension. It is an arm of this tendon by origin and a capsular ligament by function.

Relations

The tendon lies between the medial head of gastrocnemius laterally and semitendinosus superficially, with a bursa at each of those interfaces.

Its attachment to the posterior horn of the medial meniscus is direct and short, which is why the meniscus follows the muscle.

The posteromedial capsule, the posterior oblique ligament and this tendon form a single functional unit. 2.9.4 treats them together.

The saphenous nerve lies anterior to the group and is not usually at risk from a posteromedial exposure that stays behind the collateral ligament.

Blood Supply and Innervation

Blood supply to the muscle is from the perforating branches of the profunda femoris, and to the tendon's insertion from the medial genicular arteries.

Nerve supply is from the tibial division of the sciatic nerve, which separates it from the short head of biceps and matters when a sciatic lesion is being localised.

The capsular arms carry articular nerve fibres, so the tendon contributes to position sense as well as to movement.

Function

Contraction tightens the posterior oblique ligament, the posteromedial capsule and the oblique popliteal ligament at once. One muscle tensions three ligaments.

It resists anterior translation of the tibia as a secondary structure, which is part of why hamstring tone matters in a cruciate deficient knee.

Its internal rotation moment opposes the external rotation that a posterolateral corner injury produces, so the medial and lateral dynamic stabilisers balance each other. 2.10.5 covers the lateral side.

Clinical Relevance

Assess the hamstrings when a posteromedial injury is being staged. The static anatomy is only part of what holds that corner.

Expect the posterior horn of the medial meniscus to sit differently in a knee where this muscle is not working, because its position is partly muscular.

Identify the direct arm's tubercle as a landmark in a posteromedial approach. It is bone and it does not move.

Consider a bursa before a joint when a posteromedial swelling is found, since two bursae sit at this tendon.

FRCS Level

Relations

The tendon is the organising structure of the posteromedial corner. Every other component either attaches to it, arises from it, or is tensioned by it.

Its arms cross three planes: under the collateral ligament, across the capsule and up the back of the joint. An approach that divides one arm has changed the behaviour of structures it has not touched.

The direct arm's tubercle is the fixed reference on the posteromedial tibia, and the soft tissue around it is not.

Function

Treat the corner as dynamic before static. The ligaments set the limit and this muscle sets the tension at which that limit is reached.

Its secondary restraint to anterior translation is the anatomical basis for hamstring work in a cruciate deficient knee, and that connection is anatomy rather than rehabilitation doctrine. Section 8 carries the programmes.

In a multiligament knee the tendon is often the one intact medial structure, so what it can still tension decides how the knee behaves between injury and surgery.

Clinical Relevance

Preserve the direct arm wherever the approach allows. It is the anchor the rest of the corner is described from.

Assess hamstring function before accepting that a corner is competent, because the ligaments alone do not make it so.

Plan posteromedial fixation from the tubercle rather than from the joint line, since the tubercle is bone and the line is a soft tissue estimate.

Consider which arms an exposure divides and say so in the record. An arm divided changes the tension on structures left intact.