28.1.1 Flexor Carpi Radialis

Etymology: Flexor is derived from the Latin word flexus, meaning “bent” (thus flexor indicates “that which bends,” or “bending”). Carpi is derived from Latin word carpalis and the Greek word karpos, both of which indicate “wrist” (the carpus). Radialis again is derived from the Latin word radii, which means “spoke” (used to describe the radius of the forearm). ^{3 , 4}

Origin: The tendinous portion of the FCR begins approximately 15 cm proximal to the radial styloid, and the muscular portion ends approximately 8 cm from the radial styloid. ^{5 , 6}

Course: The tendon is located on the radial superficial aspect of the forearm. It is ulnar to the radial artery, radial to the median nerve and the tendon of the PL, and superficial to the tendon of the FPL. It enters a fibro-osseous tunnel at the proximal border of the trapezium. In this tunnel, it is bounded radially by the body of the trapezium, palmarly by the trapezial crest and transverse carpal ligament, and ulnarly by a retinacular septum. This septum is continuous with the transverse carpal ligament and separates the tendon from the contents of the carpal tunnel. The tendon is in direct contact with the trapezium and lies in close relation to the distal aspect of the radius, the scaphoid tubercle, the scaphotrapeziotrapezoid (STT) joint, and the carpometacarpal (CMC) joint of the thumb. ⁵ Nigro has divided the fibro-osseous tunnel for the FCR into four sections. These sections, from proximal to distal, are (1) the forearm aponeurosis that encircles the FCR tendon (palmar carpal ligament), (2) the tunnel formed between the radial insertions of the flexor retinaculum and the scaphoid tubercle, (3) the tunnel formed at the trapezial groove, and (4) the insertion of the FCR tendon at the second metacarpal base. ⁷

Insertion: The FCR tendon is inserted at three locations. A small slip inserts into the trapezial crest or tuberosity, 80% of the remaining tendon is inserted on the base of the second metacarpal, and 20% inserts on the base of the third metacarpal. The deep palmar arch is located 2 to 3 mm distal to the insertion of the tendon. ⁵

Characteristics: The FCR tendon is covered with a synovial sheath that extends from the musculotendinous origin till the metacarpal insertion. This sheath is thin proximally and consists only of paratenon. Four to five cm proximal to the radial styloid, the tendon is circumferentially invested by the transverse fibers of the antebrachial fascia and thickens to about 3 mm at the level of the trapezial crest. The FCR tendon gradually changes from a relatively flat and wide configuration in the forearm to an elliptical shape at the wrist. The fibers of the FCR tendon undergo a torsion of approximately 180° as it progresses from proximal to distal. Half of this torsion occurs in the forearm and half in the sheath of the FCR at wrist level. The rotation of its fibers is constant and is usually 180° in the clockwise direction in the right forearm and anticlockwise in the left forearm. ⁸ Simovitch et al have suggested the presence of a putative wrist annular pulley for the FCR tendon in 80% of the cadavers in their study. This pulley was present 1.5 cm proximal to the wrist flexion crease and measured approximately 2.1 × 1.5 cm in size. ⁹ The maximum excursion of the FCR tendon in adults is 4 cm. ¹⁰

Variations: An absence of the FCR has been reported. ¹¹ Other variations of the FCR tendon include slips of attachment to the base of the fourth metacarpal and the tubercle of the scaphoid. The FCR brevis is a small muscle that arises from the palmar surface of the radius between origins of the FPL and the PQ. The tendon of the FCR brevis inserts into base of the second and third metacarpal. ¹²

Clinical correlations: Volar wrist ganglions are the second most commonly seen ganglion in the hand and wrist (18–20%). They arise from the radiocarpal joint or the STT joint and are intimately related to the FCR tendon sheath and the radial artery. A relatively uncommon pathology involving the FCR tendon sheath is FCR tendinitis. This may be primary as a result of tendon irritation within the narrow confines of the trapezial tunnel or secondary in association with scaphoid cysts, STT osteoarthritis (OA), thumb CMC joint OA, or scaphoid fractures. ⁵ Attritional rupture of FCR has been reported in association with STT osteoarthritis. ¹³

A split tendon graft can be harvested from the FCR tendon. This is useful in patients who need a short graft but lack the PL. This tendon graft can be harvested by using two to three small transverse incisions. ¹⁴ The split FCR tendon graft with the distal insertion preserved is frequently used in ligamentous reconstruction following excision of the trapezium (LRTI procedure) ¹⁵ and in scapholunate ligament reconstruction (Brunelli or the three ligament tenodesis procedure). ^{16 , 17}

The tendon of the FCR is commonly transferred to the extensor digitorum communis (EDC) in patients with radial nerve palsy. Although the FCU is stronger, the FCR is a better choice for this transfer, having a greater excursion. In addition, in patients who have a low radial nerve palsy (posterior interosseous nerve palsy) and intact radial wrist extensors, preserving the FCU maintains balance between the radial and ulnar deviators of the wrist. A split transfer of the FCR tendinomuscular unit has also been described to provide independent thumb and finger extension. ¹⁸

28.1.2 Palmaris Longus

Etymology: Palmaris is derived from the Latin word palma, which means “pertaining to the palm.” Longus is the Latin for “long.” ^{3 , 4}

Origin: The tendon of the PL begins in the midforearm and has a relatively small musculotendinous portion. A study in a Japanese population estimated that the intramuscular length of the tendon was approximately 0.6 to 1.2 cm. ¹⁹

Course: The PL tendon is initially deep to the antebrachial fascia. In the distal third of the forearm, approximately 5 cm proximal to the distal wrist crease, it passes through an oval opening in the antebrachial fascia to become subcutaneous. ²⁰ The PL tendon is ulnar to the FCR and superficial to the median nerve (▶Fig. 28.1). At the level of the wrist, the PL tendon is superficial to the flexor retinaculum, which is in continuity with the antebrachial fascia (▶Fig. 28.2).

Insertion: Distal to the flexor retinaculum, the PL tendon broadens in a fanlike fashion to merge into the palmar aponeurosis (PA; ▶Fig. 28.2). The PA can be divided into two layers: the superficial one formed by longitudinal fibers (that is in 3 layers), and a deep one, adherent to the skin formed by transverse fibers continuous laterally with the deep fascia of the hand. The PL tendon is in continuity only with the longitudinal fibers of the PA. In addition to the insertion into the PA, fibrous expansions arise from each side of the distal part of the PL tendon that insert into the deep fascia overlying the thenar and hypothenar eminences. The expansions to the thenar eminence were generally thicker. ²⁰

Characteristics: A study in a Japanese population determined that the mean length and width of the PL tendon were 16.6 ± 1.8 cm and 0.4 ± 0.08 cm, respectively. They also determined that the length of the tendon correlated with the length of the forearm and was approximately 50.7 ± 6.5% of forearm length. ¹⁹ This corresponds to earlier reports in Caucasian populations also. ^{20 , 22} The mean cross-sectional area of the PL tendon is 3.1 mm ² , mean volume is 529 mm ³ , and stiffness is 42.0 ± 4.1 N/mm. ²²

Variations: The PL muscle is one of the most variable muscles in the human body, not only in terms of its absence but also in terms of muscle variations and anomalies. Its absence was first reported in 1559 by Colombo in De Re Anatomica Libri and has been the subject of several cadaveric as well as in vivo studies. ²³ The highest reported prevalence of absence of the PL tendon (64%) was reported in the Turkish population. The overall prevalence of the absence of the PL tendon in different Caucasian populations is approximately 22%. In contrast, it is quite low in Black (3%), Asian (4.5%), and Native American (7.1%) populations. There is disagreement in the literature regarding the symmetry of absence and whether absence is more common in women. Given the wide variations between the different ethnic groups, we feel that a general figure for the absence of the PL tendon cannot be quoted. It is, therefore, important for surgeons to observe these variations and be familiar with the values of the ethnic groups they treat or study. ²³

Many variations in morphology of the PL tendon have been reported, including tendon multiplicity and anomalous insertions into antebrachial fascia, thenar fascia, carpal bones, or the FCU tendon. ²⁴

Clinical correlations: The tendon of the PL is the most frequently used source of a tendon graft. A single PL tendon graft can be used to reconstruct a single FDP tendon from its insertion till the palm. If a longer graft is required (till the forearm) or multiple fingers need to be reconstructed, one must consider other grafts such as the plantaris or fascia lata. The PL tendon has also been used for reconstruction of collateral ligaments of the metacarpophalangeal (MCP) and interphalangeal (IP) joints and for correction of the swan neck deformity.

The PL has been used as a motor to provide palmar abduction in patients with low median nerve palsy. The tendon of PL is harvested along with a strip of the PA and transferred to the insertion of the abductor pollicis brevis. This tendon transfer, also known as the Camitz transfer, is especially valuable in low demand patients with severe long-standing carpal tunnel syndrome. ²⁵ The PL has also been used as a motor to restore thumb extension by a transfer to the extensor pollicis longus in patients with radial nerve palsy. ²⁶

A number of clinical maneuvers have been described in literature to determine the presence of the PL tendon preoperatively. We prefer the use of the resisted wrist flexion test described by Mishra in determining the presence of the PL. In this test, the examiner passively hyperextends the MCP joints of the fingers to make the PA taut. The patient is then asked to attempt active flexion of the wrist. The tendon of the PL can then be clearly visualized. ²⁷

28.1.3 Flexor Carpi Ulnaris

Etymology: Flexor is derived from the Latin word flexus, meaning “bent” (thus flexor indicates “that which bends,” or “bending”). Carpi is derived from Latin word carpalis and Greek word karpos, both of which indicate “wrist” (the carpus). Ulnaris is derived from the Latin word ulna, which means “elbow.” ^{3 , 4}

Origin: The tendon of the FCU is formed in the distal third of the muscle along the anterolateral border. It is quite thick and unlike the FCR has muscle fibers inserting into it almost till the level of its insertion.

Course: The tendon of the FCU is quite short, and in the distal half of the forearm, the ulnar artery and nerve pass deep and radial to the FCU tendon (▶Fig. 28.1).

Insertion: The FCU tendon inserts mainly onto the pisiform, with extensions onto the hook of hamate and the base of the fifth metacarpal via the pisohamate and pisometacarpal ligaments. In addition, some fibers insert into the flexor retinaculum and the bases of the third and fourth metacarpals. The pisiform is believed to be a sesamoid bone that lies within the FCU tendon.

Characteristics: The FCU tendon is approximately 47 ± 4.7 mm in length and has a cross-sectional area of 27.4 ± 3.6 mm ² . ²⁸ Unlike the tendons of the FCR and the digital flexors, the FCU does not have a tendon sheath. ²⁹ In addition to its function as a flexor and ulnar deviator of the wrist, the tendon of the FCU plays a role in stabilizing the wrist; in the strong power grip, such as when holding a hammer; and in stabilizing the pisiform. When a subject is asked to abduct the small finger against resistance, the FCU synergistically contracts to stabilize the pisiform, and this in turn stabilizes the origin of the abductor digiti minimi. This can be used to test the function of the FCU by palpating the FCU tendon while asking the subject to abduct the small finger against resistance. The maximum excursion of the FCU tendon in adults is 3.3 cm. ¹⁰

Variations: Many variations of the insertion of the FCU tendon have been described. In addition to the multiple insertions described earlier, it may have extensions to the metacarpals of the small, ring, or long fingers or to the capsule of the CMC joints. A split FCU tendon with the ulnar nerve passing between the split has also been described. ³⁰

Clinical correlations: There have been reports of patients with FCU tendinopathy. These patients present with pain about 3 to 4 cm proximal to the pisiform and histology shows features of tendinosis (degeneration) and not tendinitis (inflammation). ³¹ The FCU has been used as a motor in tendon transfers for radial and median nerve palsies. A split FCU transfer with independent innervation based on both heads of the FCU has also been described. ³² The FCU is designed optimally for force generation and less for excursion. Therefore, the FCR may represent a better option for tendon transfer.

The ulnar nerve and artery lie deep and radial to the FCU tendon. The ulnar nerve can be blocked by infiltrating local anesthetic agent deep to the palpable FCU tendon. In order to obtain a complete block of the ulnar nerve, the dorsal branch of the ulnar nerve needs to be blocked by injecting a wheal of anesthetic around the ulnar styloid.

28.1.4 Flexor Digitorum Superficialis/Sublimis

Etymology: Flexor is derived from the Latin word flexus meaning “bent” (thus flexor indicates “that which bends,” or bending). Digitorum is derived from the Latin word digitus or digitorum, indicating the digits. Superficialis denotes its superficial location in the forearm. Sublimis is again derived from Latin, meaning “superficial.”

Origin: The FDS has two heads of origin—a proximal humeroulnar head and a distal radial head. The median nerve and ulnar artery pass below the muscular arch formed by the two heads of the FDS. The humeroulnar head lies in a deeper plane. It has a complex digastric anatomy with a large flat common tendon that connects a single proximal muscle belly to two or three separate distal muscles that give rise to the tendons to the index, ring, and small fingers. The tendon to the index and small fingers arises completely from the distal muscle bellies, whereas the tendon to the ring finger arises partly from distal muscle belly and partly from the humeroulnar head. ³³ The radial head lies in a superficial plane and gives rise to the tendon to the long finger. Although the FDS is often thought of as four independent muscles, only the long finger FDS has truly independent function. The superficialis tendons to the index, ring, and small fingers have a common proximal muscle belly, act as a conjoined unit, and do not have completely independent actions.

Course: The FDS tendons to the ring and long fingers are superficial and central, whereas the tendons to the index and small fingers are deep and located radially and ulnarly, respectively (▶Fig. 28.3). This arrangement of tendons is maintained in the forearm and in the carpal tunnel. Once the tendons exit from the carpal tunnel, they diverge toward the respective fingers. In the forearm, these tendons are deep to the PL, FCR, PT, and radial artery and are superficial to the FDP, FPL, ulnar artery, and median nerve. In the palm, the tendons of the FDS are deep to the superficial palmar arch and the digital branches of the median and ulnar nerves (▶Fig. 28.4) and are superficial to the tendons of the FDP, along with the lumbricals and the deep palmar arch. ³⁴

At the level of the MCP joint, the FDS tendon changes from a relatively oval configuration to a flattened tendon. This divides into two slips over the proximal third of the proximal phalanx to form an interval for the passage of the tendon of the FDP—the “bifurca” (▶Fig. 28.5). The two slips of the FDS rotate 180° with the radial slip moving in a clockwise direction and the ulnar slip in an anticlockwise direction (▶Fig. 28.6). The slips of the FDS encircle the FDP tendon as they pass from proximal to distal. They are initially palmar to the FDP tendon, then become lateral, and finally end up dorsal to the FDP tendon (▶Fig. 28.7). As the two slips approach each other dorsal to the FDP tendon at the level of the neck of the proximal phalanx, they divide again into a radial and an ulnar band. The radial band of the radial slip and the ulnar band of the ulnar slip continue straight ahead (linear bands), whereas the ulnar band of the radial slip and the radial band of the ulnar slip decussate in an X pattern behind the FDP tendon (▶Fig. 28.8) forming the chiasm of Camper. ³⁵ The chiasma can be variable in terms of anatomy and morphology. ³⁶

Insertion: These crossing bands join with the linear bands to form the triangular insertion of the FDS tendon into the lateral crests on the palmar aspect of the shaft of mid-middle phalanx (▶Fig. 28.8), lying on either side of the FDP tendon (▶Fig. 28.7).

Characteristics: The FDS is the prime flexor of the proximal interphalangeal (PIP) joint of the fingers. It also contributes to flexion at the wrist and the MCP joint. When making a fist, it has a slight adduction component and brings the fingers together. The small finger FDS is also believed to have a minor opposing action at the CMC joint. ³⁰ There are also differences in strength and available excursion between the FDS tendons to the four fingers. The long finger FDS is 75% stronger than the ring or the index fingers, while the small finger FDS is 50% weaker than the index or ring finger FDS. ³³ The maximum excursion of the FDS tendon in adults is 6.4 cm. ¹⁰

Variations: Most variations of the FDS involve the muscle belly. They include accessory muscle slips that connect the muscle to other forearm flexors, absence of the radial head, and the presence of anomalous muscles in the palm that can result in carpal tunnel syndrome. ³⁰ The muscle belly and/or the tendon to the small finger may be absent. The prevalence of absence of FDS to the small finger can vary from 6.5% in Asian populations up to 21% in Caucasian populations. ³⁷

Clinical correlations: When evaluating an injured hand for the presence of a flexor tendon injury, one must differentiate between an injury to the FDS and the FDP. The FDP can be easily evaluated by checking flexion of the distal interphalangeal joint (DIP), as it is the only flexor of that joint. Testing for injury to the FDS is more complex, because the PIP joint is flexed both by the FDS and the FDP. Therefore, one needs to check the function of the FDS while blocking the action of the FDP. The standard test for the FDS takes advantage of the fact that the FDP tendons to the long, ring, and small fingers share a common muscle belly and lack independent function. The finger being tested is allowed to flex while the examiner blocks the action of the FDP tendon by preventing flexion of the DIP joint of the other two fingers. The standard test is not reliable for the index finger, because the index finger FDP has an independent muscle belly. In addition, the action of the FDS to the small finger may be dependent on the FDS to the ring finger, and they may need to be tested together. ³⁸ We prefer to use the test described by Mishra to evaluate the FDS. ³⁹ In this test, the subject is asked to press the fingertip pulp of all the fingers together against the proximal part of the palm, such that the DIP joint is kept extended. If the FDS is acting, the DIP joint remains in a position of extension to hypertension while the MCP and PIP joints are fully flexed. If the FDS of any of the fingers is injured or absent, the DIP joint flexes. This test works on the principle that the FDP can flex the PIP joint only after it has flexed the DIP joint. If the DIP joint is maintained in extension, PIP joint flexion is purely a function of the FDS.

The FDS tendon is often used is a motor for tendon transfers, because it is believed to have an independent function, making it easy to retrain, and its function at the donor finger can be taken over by the FDP. However as previously mentioned, only the FDS tendon to the middle finger is truly independent. The index and small finger FDS tendons are closely linked as they arise from a common proximal muscle. They have independence only of their distal fibers. If one of these tendons is transferred to the dorsal side of the forearm (nonsynergistic transfers), only the distal fibers would transfer, as the proximal muscle belly would need to simultaneously be a flexor and an extensor. In addition, the index finger FDS is necessary for pulp pinch with the thumb and the small finger FDS is quite slender and often absent. For these reasons, the FDS tendons to the long or ring finger are preferred for tendon transfers. The FDS to the long finger is most suited for nonsynergistic transfers. One must also be aware of the morbidity associated with these transfers. The loss of FDS can result in a swan neck deformity in mobile hands from loss of the volar restraint and a PIP joint flexion contracture from tenodesis of the stump of the divided FDS. The loss of FDS of the middle finger will result in inability to perform a chuck grip (pulp-to-pulp pinch between the index finger, middle finger, and thumb), and the loss of the ring finger FDS may result in a decrease in grip strength. The use of the ring finger FDS for a transfer to the dorsum of the forearm or hand may require division of the band of muscle fibers that often connects it to the digastric tendon in the midforearm. ^{33 , 40}