The flexor retinaculum is a fibrous connective tissue band that forms the anterior roof of the carpal tunnel. Many experts consider the flexor retinaculum synonymous with the transverse carpal ligament and the annular ligament; for this discussion, they will be regarded as the same structure. The flexor retinaculum protects nine of the forearm flexor tendons and median nerve as they pass through the carpal tunnel.
If this tissue becomes inflamed, swollen, or fibrotic, the median nerve can become irritated or compressed, leading to the formation of a carpal tunnel syndrome. This is very common in repetitive motion injuries such as those experienced by computer operators, secretaries, and factory jobs that many repetitive motions of the wrist. Diseases such as diabetes mellitus can narrow the carpal tunnel, causing the formation of a carpal tunnel syndrome, which can be quite painful.
Structure and Function
The flexor retinaculum begins when the longitudinally-oriented fibers of the antebrachial fascial fibers of the forearm abruptly thicken and change direction to a transverse orientation. The flexor retinaculum thickens proximally to distally, ranging in thickness from 1.5 mm to 6.0 mm. and is about 3 centimeters long and about 2.5 centimeters wide. The flexor retinaculum attaches laterally and medially to the carpal bones. Specifically, the flexor retinaculum attaches to the scaphoid tuberosity and trapezial ridge on the radial side and the pisiform, and the hook of hamate on the ulnar side. The flexor retinaculum forms the carpal tunnel's volar (anterior) boundary, while the carpal bones form the dorsal boundary. Contained below the flexor retinaculum within the carpal tunnel are the four flexor digitorum superficialis tendons, the four flexor digitorum profundus tendons, the single flexor pollicis tendon, as well as the median nerve.
The median nerve is subject to compression as it passes through the carpal tunnel. The median nerve has a number of branches in the wrist and hand. The first and most important branch - the recurrent branch of the median nerve - arises after the median nerve has passed through the carpal tunnel. The recurrent branch of the median nerve innervates the muscles of the thenar eminence. These are the abductor pollicis brevis, the flexor pollicis brevis, and the opponens pollicis. It is worth remembering that the motions of the thumb are different from that of the rest of the body due to 90-degree rotation at the carpometacarpal joint. Thus, one flexes the arm at the shoulder in the sagittal plane, and one flexes the forearm at the elbow in the sagittal plane. The same is true of the fingers. However, one extends the thumb in the coronal plane and flexes it in the same plane. One abducts the thumb in the sagittal plane, and one adducts it in the sagittal plane. Unless one understands the distinction of the actions at the wrist, the names make no sense.
The anterior surface of the flexor retinaculum gives rise to some muscles of the thenar and hypothenar eminences. The tendon of the palmaris longus adheres to it, although it should be noted that this muscle is absent in approximately 14 percent of the population. The upper margin continues in the palmar carpal ligament, and the lower margin merges with the palmar aponeurosis.
Its principal function is to serve as a pulley for the carpal flexor muscles and stabilize the carpal system's morphology.
Its constitution is mainly of collagen fibers, whose embryological origin is the mesodermal leaflet (hypaxial myotome).
Blood Supply and Lymphatics
The ulnar artery is one of the major blood vessels in the upper extremity and is close to the flexor retinaculum. The ulnar artery is a continuation of the brachial artery and lies beneath the flexor carpi ulnaris muscle. It lies on top of the flexor retinaculum lateral to the ulnar nerve. The ulnar nerve passes through Guyon's canal, but the artery does not. The ulnar artery supplies the medial aspect of the forearm and forms the superficial palmar arch providing blood to the hand. In some patients, a branch of the ulnar artery passes over the retinaculum.
The flexor retinaculum is best known for sheltering the median nerve in the carpal tunnel. The median nerve originates from the cervical spinal cord roots C5-C7 and C8-T1. These cervical roots merge to form the medial and lateral cords of the brachial plexus, ultimately forming the median nerve. The median nerve then courses down the anterior arm, forearm, and eventually into the hand. While traveling through the forearm between the flexor digitorum superficialis and flexor digitorum profundus, the median nerve passes deep to the flexor retinaculum and through the carpal tunnel. In addition to the median nerve, the ulnar nerve is near the flexor retinaculum in Guyon's canal, which is formed by the hook of the hamate bone and the pisiform bone. The canal is roofed over by the pisohamate ligament.
Fracture of the hook of the hamate can damage the ulnar nerve, causing loss of function of the abductor digiti minimi, flexor digiti minimi, and opponens digiti minimi. In addition, the palmar and dorsal interossei will be denervated. Guyon's canal is located superficial to the flexor retinaculum on its medial edge. A clinically important test for loss of the opponens pollicis and opponens digiti minimi is to touch the pads of the little finger (oppopnens digiti minimi) and thumb (opponens pollicis).
The ulnar nerve has suffered damage if the patient cannot oppose the little finger. If one cannot oppose the thumb, the median nerve or its recurrent branch has been damaged. A good test for the dorsal interossei is to hold the patient's fingers and have the patient attempt to abduct them. Loss of strength of the dorsal interossei is caused by a lesion of the ulnar nerve. Because it is sheltered in the forearm, the ulnar nerve is usually subject to damage only as it passes around the medial epicondyle or in Guyon's canal, or in deep puncture wounds of the hand.
There are several muscles associated with the flexor retinaculum. Situated deep to the flexor retinaculum are the four tendons of the flexor digitorum superficialis. The flexor digitorum superficialis, which is in the anterior compartment of the forearm, originates at the medial epicondyle of the humerus as well as parts of the radius and ulna. The flexor digitorum superficialis inserts on the base of the second to fifth middle phalanges. The median nerve innervates the flexor digitorum superficialis, which protects the median nerve in the forearm because it passes deep to the flexor digitorum superficialis. The median nerve gives rise to the anterior interosseus nerve, which innervates the lateral half of the flexor digitorum profundus (first and second fingers).
The main action of the flexor digitorum superficialis is flexion of the phalanges, primarily at the proximal interphalangeal joints. Deep to the flexor digitorum superficialis is the flexor digitorum profundus. The origin and insertion are the proximal anteromedial surface of the ulna and the distal interphalangeal joints of the phalanges, respectively. The flexor digitorum profundus is innervated medially by the ulnar nerve, which sends tendons to insert on the distal phalanx of the third and fourth fingers.
The main action of the flexor digitorum profundus is to flex the distal phalanges at the distal interphalangeal joints. It has a secondary action to flex the proximal interphalangeal joints, metacarpal phalangeal joints, and wrist. Below the flexor retinaculum and lateral to the flexor digitorum superficialis lies the flexor pollicis longus. The origin of the flexor pollicis longus is the middle half of the volar (anterior) radius, and the insertion is the base of the distal phalanx of the thumb. The innervation of the flexor pollicis longus is a branch of the median nerve, the anterior interosseous nerve.
One clinical test of the flexor pollicis longus (anterior interosseus nerve) and the adductor pollicis (ulnar nerve) is to have the patient attempt to hold a piece of paper between the thumb and first finger. If the patient cannot do so successfully, the problem lies either with the ulnar nerve (adductor pollicis) or the anterior interosseus nerve). A tip as to which is involved: watch the thumb as the patient fails to hold the paper. The thumb will flex at the distal phalanx if the problem is with the adductor pollicis (ulnar nerve). If the difficulty involves the anterior interosseus nerve/flexor pollicis longus), the thumb will not flex at the distal phalanx. This information can guide the physician in further investigating the cause of the patient's lesion.
The main anatomical variations concerning the flexor retinaculum involve the median nerve. As previously stated, the median nerve enters the carpal tunnel deep to the flexor retinaculum. A bifid median nerve occurs in some humans, which occurs when the median nerve splits proximally to the flexor retinaculum instead of distally, like normal anatomy. Several studies have demonstrated a wide range of variations in the motor branch of the median nerve. These different courses may explain why patients present differently in cases of severe compression of the median nerve.
If conservative management for carpal tunnel does not work, surgical options are available. If the flexor retinaculum is the cause of a patient's carpal tunnel symptoms, surgical release is often necessary. This procedure releases pressure on the median nerve and can often relieve or eliminate the patient's symptoms. Like any surgery, there are risks involved; knowing the anatomy is key to avoiding damaging any surrounding structures. Because so many vital structures surround the flexor retinaculum, one must be aware of where each tendon, artery, and nerve are relative to the flexor retinaculum.
If the patient suffers from systemic diseases, the recurrence rate after surgery is very high.
The flexor retinaculum can entrap and compress the median nerve causing the condition of carpal tunnel. Carpal tunnel is the most common compression neuropathy in the upper extremity. Several causes for compression of the median nerve range from repetitive motions, space-occupying lesions, or certain health conditions like diabetes mellitus or hypothyroidism. Clinically patients may present with numbness and tingling in the radial three fingers. Often symptoms are worse at night. Patients may have difficulty holding objects and regularly report dropping items. Clinically patients frequently will have thenar atrophy and weakness. Evaluating sensations in the fingers may show decreased sensation in the radial three fingers.
Compression of the flexor retinaculum and, therefore, the median nerve may reproduce the patient's symptoms. A classic physical exam maneuver used to diagnose carpal tunnel is the Tinel test. The Tinel test is performed by repeatedly tapping on the flexor retinaculum and seeing if it reproduces the patient's symptoms. Treatments to consider include wrist splinting, work-related ergonomic modification, NSAIDs, steroid injections, and surgical management.
When the flexor retinaculum compresses the underlying median nerve, it undergoes a thickening, reducing its elastic capacity. According to a recent study, there is a difference in size between males and females. In women, the retinaculum is smaller; in the female sex, carpal tunnel syndrome has a higher prevalence.
In carpal tunnel syndrome, in the early stages of the disorder, the proliferation of myofibroblasts seems to play an important role, which not only increases inflammatory responses but reduces tissue elasticity.
(Click Image to Enlarge)
The image shows the transverse carpal ligament cut transversely to remove tension from the underlying nerve.
Contributed by Bruno Bordoni, PhD
Sebastin SJ,Puhaindran ME,Lim AY,Lim IJ,Bee WH, The prevalence of absence of the palmaris longus--a study in a Chinese population and a review of the literature. Journal of hand surgery (Edinburgh, Scotland). 2005 Oct; [PubMed PMID: 16006020]