The flexor retinaculum is a strong band that is attached on the radial side to the tubercle of the scaphoid and ridge of the trapezium and on the ulnar side to the pisiform and hook of the hamate. Note that these four bony points are all palpable in the living hand and that the pisiform in the only carpal bone to give attachments to both the flexor and extensor retinacula. There is a further attachment on the radial side, where a deep partition bridges the groove on the trapezium. The fibro-osseous tunnel so formed transmits the tendon of flexor carpi radialis and its synovial sheath.
The muscles of the thenar and hypothenar eminences arise from the retinaculum and several structures pass across it. The tendon of palmaris longus is fused to the midline of the retinaculum as it passes distally to expand into the palmar aponeurosis. The ulnar nerve lies on the retinaculum alongside the pisiform bone, with the ulnar artery on its radial side. Both are bridged over by a slender band of tissue officially called the superficial part of the retinaculum, forming a small canal (of Guyon) that may occasionally cause compression of the nerve. The nerve divides into a superficial (cutaneous) and a deep (muscular) branch at the distal border of the retinaculum; the deep branch is accompanied by a small deep branch of the artery. On the radial side of the palmaris longus tendon the palmar branch of the median nerve and the superficial palmar branch of the radial artery cross the retinaculum. The radial side of the retinaculum is pierced proximally by the tendon of flexor carpi radialis over the scaphoid; further distally the tendon lies deeply in its fibro-osseous tunnel on the trapezium.