The lateral head of the triceps brachii arises from the posterior surface of the humerus above and lateral to the radial groove, with a flat tendinous proximal attachment that covers a broad area from the greater tubercle to the radial groove. The lateral head is the most superficial of the three heads and its proximal tendon is palpable on the posterior lateral humerus. The radial nerve and the profunda brachii artery pass through the spiral groove in the interval between the lateral and medial heads.
The lateral head of triceps proximal tendon and its fibres are disrupted in Saturday night palsy and Holstein-Lewis fractures of the distal humerus where the radial nerve is compressed or trapped in the spiral groove at the lateral head muscle boundary. Distinguishing a lateral head triceps injury from a full triceps tendon injury requires MRI to delineate which component is torn. The lateral head also provides a useful posterior approach plane between it and the long head during approaches to the posterior humerus.
Direct impact or eccentric overload injures the lateral triceps head at its broad posterior humeral origin, producing posterolateral upper arm pain reproduced by resisted elbow extension, managed conservatively with progressive loading rehabilitation.
The radial nerve in the spiral groove passes between the lateral and medial triceps heads; compression at this level from a humeral fracture or sustained pressure produces radial nerve palsy with wrist drop and loss of finger extension, with the lateral head tendon anatomy relevant to surgical exploration and nerve decompression.
