Flexor Retinaculum of the Upper Extremity (Wrist)
In the upper limb, the retinaculum mm. flexorum is the distal derivative of the forearm fascia (fascia antebrachii). This fascia forms a dense 'cuff' around the forearm, and in the wrist region, it forms the palmar carpal ligament, also known as the transverse carpal ligament.
- Function: The retinaculum forms the roof of the carpal tunnel.
- Contents: Passing beneath the retinaculum are the tendons of the m. flexor digitorum superficialis et profundus and the tendon of the m. flexor pollicis longus. These tendons are enclosed within synovial sheaths: the common flexor sheath and the radial bursa.
- Topography: In the proximal middle compartment of the palm and within the carpal tunnel, the flexor tendons lie closely against the retinaculum and the palmar aponeurosis. Consequently, the subaponeurotic space of the palm typically does not communicate directly with the potential space of Pirogov-Parona. The blind end of the radial synovial sheath lies 2 cm proximal to the retinaculum mm. flexorum, while the proximal end of the common flexor sheath extends 3–4 cm proximal to it.
Flexor Retinaculum of the Lower Extremity (Ankle)
In the lower limb, the corresponding structure is located in the medial retromalleolar region (regio retromalleolaris medialis). It is a distal thickening of the deep fascia of the leg (fascia cruris), which contributes to intermuscular septa.
- Location: The ligament spans between the medial malleolus of the tibia and the calcaneus.
- Function: It forms the tarsal tunnels (canales tarsales) for the tendons of the tibialis posterior, flexor digitorum longus, and flexor hallucis longus muscles. Additionally, the retinaculum serves as a proximal attachment site for the m. abductor hallucis.
- Structures passing beneath: Passing beneath the retinaculum are the synovial sheaths enclosing the tendons of the tibialis posterior (m. tibialis posterior), flexor digitorum longus (m. flexor digitorum longus), and flexor hallucis longus (m. flexor hallucis longus).
- Vessels and Nerves: Accompanying the tendons is the neurovascular bundle, which includes the posterior tibial artery (a. tibialis posterior) and the tibial nerve (n. tibialis). In this region, calcanean branches (rr. calcanei) branch off from the tibial neurovascular bundle.
Clinical Significance
Compression of the posterior tibial artery (a. tibialis posterior) and tibial nerve behind the medial malleolus beneath the retinaculum mm. flexorum leads to tarsal tunnel syndrome.