Anatomy of the Extensor Retinaculum of the Upper Extremity
In the upper extremity, the extensor retinaculum is a distal thickening of the antebrachial fascia (fascia antebrachii). The antebrachial fascia forms a dense 'sleeve' that sends septa to the crests of the radius and ulna, as well as the interosseous membrane. Distally on the posterior aspect, it forms the extensor retinaculum.
Beneath the extensor retinaculum on the dorsum of the wrist, there are six osseofibrous compartments. The compartments are numbered from lateral to medial, using Lister's tubercle on the radius as a landmark:
- 1st compartment: tendons of the m. abductor pollicis longus and m. extensor pollicis brevis.
- 2nd compartment: tendons of the mm. extensores carpi radiales longus et brevis.
- 3rd compartment: tendon of the m. extensor pollicis longus, which hooks around Lister's tubercle.
- 4th compartment: tendons of the m. extensor digitorum communis and m. extensor indicis.
- 5th compartment: tendon of the m. extensor digiti minimi.
- 6th compartment: tendon of the m. extensor carpi ulnaris.
The tendons of the m. extensor pollicis brevis and m. abductor pollicis longus bound the 'anatomical snuffbox'.
Additionally, dorsal fascial pockets are located on the dorsum of the hand, and intertendinous connections (connexus intertendinei) run between the tendons of the extensor digitorum.
Topography and Adjacent Structures of the Dorsum of the Hand
The following structures are located near the extensor retinaculum on the radial side of the hand: the cephalic vein (v. cephalica), the superficial branch of the radial nerve (ramus superficialis n. radialis), the radial artery (a. radialis), and its dorsal carpal branch (ramus carpalis dorsalis a. radialis).
Arterial structures of the dorsum of the hand include the radial artery, the dorsal carpal branch of the radial artery, the dorsal metacarpal arteries, and the dorsal digital arteries.
Neural structures include the dorsal digital nerves (nn. digitales dorsales) supplying the digits. The dorsal branch of the ulnar nerve (ramus dorsalis manus n. ulnaris) innervates the medial part of the dorsum of the hand.
Structure of the Retinacula of the Lower Extremity
In the lower extremity, the crural fascia (fascia cruris) distally forms tendon retinacula, including the superior and inferior extensor retinacula.
- Superior extensor retinaculum (retinaculum mm. extensorum superius) is located between the anterior border of the tibia and the lateral malleolus. It is described as a thickening of the fascia in the distal third of the leg.
- Inferior extensor retinaculum (retinaculum mm. extensorum inferius) is Y-shaped, located at the level of the ankle joint, and stretches from the calcaneus to the medial malleolus and the fascia of the dorsum of the foot.
The inferior extensor retinaculum forms fibrous tunnels for the tendons of the m. tibialis anterior, m. extensor hallucis longus, m. extensor digitorum longus, and the m. fibularis tertius (when present).
The tendon of the m. extensor hallucis longus occupies an intermediate position, while the tendon of the m. extensor digitorum longus lies laterally. The neurovascular bundle containing the a. dorsalis pedis, its accompanying veins, and the deep fibular nerve (n. fibularis profundus) runs within the osseofibrous canal of the extensor hallucis longus. This bundle is separated from the ankle joint capsule by a layer of adipose tissue.
Synovial Sheaths and Clinical Significance
The compartments beneath the extensor retinaculum on the dorsum of the wrist are lined by synovial sheaths that minimize tendon friction.
In the hand, the synovial sheath for the tendons of the m. extensor digitorum and m. extensor indicis is triangular in shape, with its base facing distally toward the digits. Proximally, it extends 10 mm above the extensor retinaculum, and distally it ends blindly at the mid-shaft of the metacarpal bones.
In the foot, the synovial sheath of the m. extensor hallucis longus begins 2–3 cm proximal to the intermalleolar line and extends distally to the level of the first tarsometatarsal joint. The synovial sheath of the m. extensor digitorum longus extends proximally 1–2 cm higher than the sheath of the extensor hallucis longus, expanding distally along the diverging tendons to the digits.
Clinical Significance: Inflammation of the synovial sheaths of the 1st wrist compartment leads to de Quervain tenosynovitis.