Flexor Retinaculum — Definition, Attachments and Slips
Definition
The flexor retinaculum (transverse carpal ligament) is a strong, thick, rectangular fibrous band formed by thickening of the deep fascia in front of the carpal bones.
It bridges the anterior (palmar) concavity of the carpus, converting it into an osseofibrous tunnel called the carpal tunnel, through which the long flexor tendons of the digits and the median nerve pass.
Dimensions and Orientation
- Rectangular in shape
- Approximately 2–3 cm wide (transversely) and 3 cm long (proximodistally)
- Directed transversely across the wrist
Bony Pillars
The flexor retinaculum is attached to four bony prominences — two on each side — which serve as the four corners (pillars) of the carpal arch:
| Side | Proximal Attachment | Distal Attachment |
|---|---|---|
| Medial | Pisiform bone | Hook (hamulus) of hamate |
| Lateral | Tubercle of scaphoid | Crest (ridge) of trapezium |
Slips of the Flexor Retinaculum
On each side, the flexor retinaculum gives off an additional slip:
Medial Slip (Superficial — Volar Carpal Ligament)
- A superficial slip on the medial side
- Attached to the pisiform bone proximally
- This slip is more superficial than the main retinaculum
- The ulnar nerve and ulnar artery pass deep to this slip (but superficial to the main retinaculum) through Guyon’s canal (ulnar canal)
Lateral Slip (Deep)
- A deep slip on the lateral side
- Attached to the medial lip of the groove of the trapezium
- Converts the groove on the trapezium into a separate osseofibrous tunnel for the tendon of flexor carpi radialis
- The tendon of FCR therefore passes in its own tunnel within the lateral part of the flexor retinaculum — separate from the carpal tunnel
Summary
| Structure | Relationship to Retinaculum |
|---|---|
| Carpal tunnel | Deep to main flexor retinaculum |
| FCR tunnel | Lateral slip of retinaculum (separate canal) |
| Guyon’s canal (ulnar canal) | Deep to medial superficial slip (volar carpal ligament) |
Additional Attachments
The flexor retinaculum gives origin to muscles of the thenar and hypothenar eminences:
- Thenar side (lateral): Abductor pollicis brevis, flexor pollicis brevis (superficial head), opponens pollicis
- Hypothenar side (medial): Abductor digiti minimi, flexor digiti minimi, opponens digiti minimi
The palmaris longus tendon continues distally as the palmar aponeurosis, which blends with the anterior surface of the retinaculum.