Biceps Femoris

For Medical Students

Structure and Location

Biceps femoris has two heads. The long head arises from the ischial tuberosity with the other hamstrings. The short head arises from the back of the femur along the linea aspera.

The two join and form a tendon that descends to the fibular head on the outer side of the knee.

The insertion is not a single point. The tendon splits around the lateral collateral ligament, sending a direct arm to the fibular head, an anterior arm forwards, and a broad aponeurosis over the lateral side of the leg.

A bursa sits between the tendon and the collateral ligament where the two meet.

The common peroneal nerve runs along the inner border of this tendon as both descend towards the fibula.

Relations

In front of the tendon at the knee: the lateral collateral ligament, with the bursa between them.

Behind and medial to the tendon: the common peroneal nerve. That relationship is fixed and it is how the nerve is found.

Deep to it: the lateral head of gastrocnemius and the posterolateral capsule.

Superficial to it: the fascia of layer one, continuous with the iliotibial band in front.

Function

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

Through its insertion it is a dynamic stabiliser of the posterolateral corner, the lateral equivalent of semimembranosus.

Its external rotation moment opposes the internal rotation produced by the medial hamstrings, so the two sides balance.

Clinical Relevance

The tendon is palpable and so is its edge. Finding the tendon in a thin knee locates the nerve behind it without an incision.

Because the long and short heads have different nerve supplies, a lesion can weaken one head and spare the other.

The tendon is the landmark for the nerve in every lateral approach, which 2.10.6 and 2.10.7 carry.

MRCS Level

Structure and Location

Name the insertional components: a direct arm to the lateral aspect of the fibular styloid, an anterior arm passing lateral to the collateral ligament to the fibular head, a lateral aponeurosis, and reflected and capsular arms.

The anterior arm forms a space over the collateral ligament. That space is the approach used to expose the ligament's fibular attachment without detaching the tendon.

The long head is supplied by the tibial division of the sciatic nerve and the short head by the common peroneal division. It is the only muscle in the body with a head on each division.

Conventional teaching places the common peroneal nerve about 2 cm posteromedial to the biceps tendon at the level of the fibular head.

Relations

The tendon lies in layer one laterally, superficial to the collateral ligament in layer three.

The nerve runs on its posteromedial border, in the same fascial plane, which is why following the tendon finds the nerve rather than merely approaching it.

The lateral head of gastrocnemius lies deep and medial, with the fabella in its tendon when one is present.

The inferior lateral genicular artery runs forwards deep to the tendon, close to the joint line.

Blood Supply and Innervation

Blood supply to the muscle is from the perforating branches of the profunda femoris and the inferior gluteal artery proximally.

The dual innervation is the point to carry. The short head is the only hamstring supplied by the peroneal division, so a peroneal division lesion weakens knee flexion slightly as well as causing foot drop.

Supply to the insertion is from the lateral inferior genicular artery.

Function

It is the principal dynamic restraint to external rotation and varus on the lateral side, which puts it in the same role as semimembranosus medially.

Through the arms that reach the capsule, contraction tensions the posterolateral structures directly rather than only through the fibula.

Its external rotation moment is the one a dial test has to be performed against, so muscle tone changes the finding. 3.10.7 covers that.

Clinical Relevance

Find the tendon first in any lateral approach and the nerve second. The sequence is the safe one and it is anatomical rather than habitual.

Expect slight flexion weakness with a peroneal division lesion, because the short head goes with it.

Assess the dial test with the patient relaxed, since this muscle produces the movement being measured.

Preserve the anterior arm where the collateral ligament's fibular attachment is being exposed, because the space between them is the approach.

FRCS Level

Relations

The tendon is the fixed reference of the lateral side. Ligaments retract, scar and avulse; this tendon is large, superficial and usually intact even in a knee that has lost everything else.

Its relationship to the nerve does not change with injury, so the landmark survives the circumstances in which it is most needed.

The space between the anterior arm and the collateral ligament is the interval that gives the fibular attachment without detaching anything.

Function

Treat the lateral side as having a dynamic component that a reconstruction does not reproduce, as on the medial side. This muscle tensions the corner and no graft does.

An avulsed biceps insertion removes dynamic restraint and the landmark at once, which makes the nerve harder to find in exactly the injury where it is most often injured.

Its dual innervation means a knee with a peroneal palsy has a weakened lateral dynamic stabiliser as well as a dropped foot.

Clinical Relevance

Use the tendon as the first landmark in a posterolateral approach, and say so in the operation note.

Expect a difficult nerve in a knee with an avulsed biceps insertion, and plan the exposure proximally where the relationship is still intact.

Consider repairing the insertion where it is avulsed, because it is both a stabiliser and the landmark a later operation will need.

Avoid detaching the anterior arm for exposure when the interval beside it will do.