Lacertus Fibrosus (Bicipital Aponeurosis)

aponeurosis musculi bicipitis brachii

The lacertus fibrosus (bicipital aponeurosis) is a flat fibrous expansion from the distal biceps tendon that spreads medially over the forearm flexor mass. It is the structure that maintains elbow flexion power after distal biceps tendon rupture from the radial tuberosity — patients retain 70-80% of elbow flexion strength but lose 40-50% of supination strength. The lacertus also compresses the brachial artery and median nerve in the antecubital fossa.

Region: Upper Arm

Biomechanics

Function & Actions

Provides a secondary attachment for the biceps when the main radial tuberosity insertion ruptures; stabilises the ulnar forearm during supination; protects the brachial artery and median nerve in the antecubital fossa

Clinical Relevance

Clinical Notes

The hook test for distal biceps rupture: the examiner hooks a finger under the biceps tendon in the antecubital fossa with the elbow at 90 degrees — in complete rupture no tendon can be hooked (it retracts proximally). The lacertus may remain intact even when the main tendon ruptures, maintaining some flexion power but concealing the proximal tendon retraction.

Pathology

Common Injuries & Conditions

Distal Biceps Tendon Rupture

Radial tuberosity avulsion with intact lacertus preserving partial flexion but losing supination strength managed with surgical repair within 2 weeks.