Flexor Carpi Radialis Anatomy

Origin. From the medial epicondyle of the humerus through the common flexor origin, where FCR is the second most radial of the superficial flexor group after pronator teres. Additional fibres arise from the deep antebrachial fascia and from the intermuscular septa it shares with pronator teres radially and palmaris longus ulnarly. The origin lies superficial to the ulnar collateral ligament of the elbow, which is why medial epicondyle debridement and UCL reconstruction both traverse this muscle group.

Muscle belly and musculotendinous junction. The musculotendinous junction begins on average 15 cm proximal to the radiocarpal joint (range 12 to 17 cm), and the muscle fibres end on average 8 cm proximal to the wrist (range 6 to 9 cm). The distal third of the forearm therefore contains pure tendon, which is the level at which the tendon is harvested or split. Flexor carpi ulnaris, by contrast, is muscular almost to the pisiform, so FCR is easier to harvest as pure tendon while an FCU harvest carries muscle.

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Insertion. Three sites. A small slip goes first to the crest or tuberosity of the trapezium, present in most specimens and the anatomical reason the tendon is intimately applied to that bone. Of the remainder, approximately 80 per cent inserts on the base of the second metacarpal, the principal insertion, and approximately 20 per cent on the base of the third.

The fibro-osseous tunnel. The tendon enters its tunnel at the proximal border of the trapezium. The floor is the roughened groove of the trapezium, so the tendon is in direct contact with bone, and a thick septum separates the tunnel from the carpal canal; the distal border of that septum acts as a pivot point for flexor pollicis longus. Within the tunnel the tendon occupies about 90 per cent of the available cross-sectional space, a far more crowded corridor than the carpal tunnel and a functionally unforgiving one, which is why small osteophytes cause disproportionate stenosis. The synovial sheath runs the whole length of the tendon, from the musculotendinous junction to the insertion, so tenosynovitis can extend well proximal to the tunnel.

Nerve supply. The median nerve, root values C6 and C7; a C8 contribution is described in some texts, but C6-C7 is the answer expected. The branches arise in the proximal forearm, distal to the pronator teres branches and typically 2 to 4 cm distal to the medial epicondyle, and enter the deep (dorsal) surface of the proximal third of the muscle belly. Dissection distal to the proximal third is therefore safe for the nerve supply. Flexor carpi ulnaris, for comparison, is ulnar nerve C7-C8: FCR carries the more proximal root pattern, which is useful in root-level localisation.

Order of median innervation in the forearm. The median nerve supplies the superficial flexor group in a predictable sequence, which examiners use to localise lesions:

1. Pronator teres
2. Flexor carpi radialis
3. Palmaris longus
4. Flexor digitorum superficialis

The anterior interosseous nerve then takes flexor pollicis longus, the radial half of flexor digitorum profundus and pronator quadratus.

Localising value. Weak wrist flexion with radial deviation and a normal pronator teres suggests a lesion distal to the pronator branches, the classic pronator syndrome level. FCR weakness with preserved thumb interphalangeal flexion excludes an isolated anterior interosseous lesion, because FCR is a main-trunk median branch while FPL is an AIN branch.

Arterial supply. The proximal third receives muscular branches from the ulnar artery and the anterior interosseous artery, with a contribution from the inferior ulnar collateral system at the common flexor origin. The middle third is fed by direct radial artery muscular branches; the radial artery lies immediately radial to the muscle belly and then to the tendon. The distal tendon is a relatively hypovascular segment supplied by vessels entering from the radial artery and by diffusion within the synovial sheath, and the intratunnel segment over the trapezium is the watershed: the site of degenerative tendinopathy and of the rare spontaneous rupture.

The radial artery and its branches. At the distal wrist crease the radial artery runs 5 to 10 mm radial to the FCR tendon, and in the FCR approach it is protected simply by staying within the tendon sheath. The superficial palmar branch leaves the parent vessel just proximal to the radial styloid and crosses the operative field in the extended distal (scaphoid) exposure; it may be ligated with impunity but must be recognised. The deep palmar arch lies only 2 to 3 mm distal to the FCR insertion at the second metacarpal base, so distal dissection to obtain extra tendon length is the moment of maximum arterial risk.

At the wrist, radial to ulnar. Radial to FCR lie the radial artery and its venae comitantes, then the brachioradialis tendon and the first extensor compartment. Ulnar to it lie palmaris longus when present, then the median nerve, deep and ulnar to palmaris longus, then flexor digitorum superficialis, then flexor carpi ulnaris and the ulnar neurovascular bundle. Deep to the tendon is flexor pollicis longus, which pivots at the distal end of the FCR septum, then pronator quadratus and the volar cortex of the distal radius; superficial to it are only skin, subcutaneous fat, the antebrachial fascia and crossing superficial veins.

The palmar cutaneous branch of the median nerve. It arises from the radial side of the median nerve approximately 5 to 8 cm proximal to the wrist crease, descends in the interval between FCR and palmaris longus, pierces the antebrachial fascia near the crease and supplies the skin of the thenar eminence and central palm. It is the nerve at risk in every FCR-based approach, and the reason the tendon is retracted ulnarly with the sheath intact rather than dissected free from its ulnar side.

Deep to the tendon distally. The scaphoid tubercle, the scaphotrapeziotrapezoid (STT) joint, the trapezium and the thumb carpometacarpal joint all lie within a few millimetres of the tendon, as does the volar rim of the distal radius at the watershed line.

FCR tendinopathy and stenosing tenosynovitis. Because the tendon nearly fills a rigid fibro-osseous tunnel and lies in direct contact with the trapezium, any reduction in tunnel volume produces mechanical abrasion and stenosis. The causes and the conditions that travel with it:

• Trapezial or STT osteophytes and scaphotrapeziotrapezoid arthritis
• Thumb basal (carpometacarpal) joint arthritis
• A distal radius malunion with volar prominence, or prominent volar distal radius hardware
• A scaphoid nonunion with a volar prominence
• A ganglion or post-traumatic scarring
• Rheumatoid disease

Presentation. Volar radial wrist pain aggravated by gripping, wringing and repetitive wrist flexion; swelling and crepitus may be felt over the tendon. It is frequently mislabelled as a wrist sprain or attributed solely to the thumb base.

Imaging. Ultrasound shows sheath fluid, tendon thickening and hypoechoic change, and is dynamic. MRI demonstrates peritendinous fluid, tendon signal change and the causative spur or arthritic joint. Plain radiographs including a Robert view and an STT view identify the culprit osteophyte.

Management ladder. Activity modification with a thumb-spica or wrist splint, then an image-guided injection into the sheath, not into the tendon, and finally surgical release of the tunnel roof with excision of the offending osteophyte. Where basal joint arthritis is the driver, treating the joint often resolves the tendon problem.

FCR rupture. Uncommon and almost always attritional rather than traumatic, from chronic abrasion against a trapezial osteophyte, a scaphoid nonunion prominence or prominent volar plate hardware. Presentation is often a painless loss of the tendon contour after a period of pain that then settles, because the pain stops when the tendon parts. The functional deficit is modest and reconstruction is rarely required, but the underlying bony cause must be addressed or the neighbouring tendons follow.

Volar wrist laceration. FCR is the radial-most tendon in the spaghetti wrist. In a transverse volar laceration the expected order of divided structures from radial to ulnar is the radial artery, FCR, palmar cutaneous branch, palmaris longus, median nerve, FDS slips, FDP slips, FCU, then the ulnar artery and nerve. Because FCR is superficial and radial, an isolated FCR laceration with intact median nerve function is common in glass injuries.

Repair. A core suture with a circumferential epitendinous suture; the tendon is broad and flat and tolerates a four-strand repair well. Immobilise the wrist in slight flexion for 3 to 4 weeks, then mobilise. Not repairing an isolated FCR division in a low-demand patient is defensible, since the functional cost is small, but repair is preferred in manual workers and where palmaris longus is absent.

Associated injury patterns.

• A distal radius fracture with volar displacement: the sharp volar cortical spike can lacerate or abrade FCR and FPL
• Perilunate injury: the volar capsular disruption occurs in the plane deep to FCR; the tendon itself is usually intact, but its sheath is the route of surgical access
• Trapezial ridge fracture: directly abrades the FCR tendon within its tunnel and is a recognised cause of post-traumatic FCR tendinopathy; look for it on a carpal tunnel view or CT when radiographs seem normal

Rheumatoid arthritis produces flexor tenosynovitis that includes the FCR sheath, and because the sheath extends from the musculotendinous junction to the insertion, swelling may be palpable well proximal to the wrist. Attritional rupture over carpal bone spurs is the rheumatoid pattern, the same mechanism that causes the Mannerfelt lesion of flexor pollicis longus over a scaphoid osteophyte.

Crystal arthropathy. Calcific deposits within the FCR sheath cause acute, intensely painful, red and hot volar radial wrist swelling that mimics infection. Radiographs show amorphous calcification; the condition is self-limiting and treated with anti-inflammatories.

Ganglia and masses. Volar radial wrist ganglia arise from the radiocarpal or STT joint and sit immediately deep or adjacent to FCR, often adherent to the radial artery, so excision demands meticulous protection of the artery and the palmar cutaneous branch. An FCR sheath ganglion moves with the tendon and is distinguished from a joint ganglion by dynamic ultrasound.

Anatomical variants. Flexor carpi radialis brevis is an accessory muscle arising from the distal volar radius and inserting on the second or third metacarpal base or the trapezium. It is present in a small percentage of wrists, lies deep in the pronator quadratus plane, and can be mistaken for aberrant tissue during the volar approach or recruited as an additional graft. Slips of insertion vary; a dominant third metacarpal insertion is described and matters when planning distal harvest.

The FCR (modified Henry) approach. The default approach for volar plating of distal radius fractures. The incision is longitudinal, directly over the palpable FCR tendon, typically 6 to 8 cm long, ending at or just proximal to the wrist crease; it is extended distally with an oblique limb across the crease only if carpal exposure is needed.

Superficial dissection. Incise the radial wall of the FCR tendon sheath and retract the tendon ulnarward. This is the key manoeuvre: it interposes the tendon between the operative field and the palmar cutaneous branch of the median nerve and the median nerve itself, keeps the dissection radial to those structures and keeps the radial artery safely radial, whereas radial retraction exposes the artery and gains nothing. Then incise the floor of the sheath; the flexor pollicis longus muscle belly is encountered and retracted ulnarly with the rest of the flexor mass.

Deep dissection. Elevate the exposed pronator quadratus as an L-shaped flap, the vertical limb along the radial border of the radius and the horizontal limb along its distal edge, preserving a cuff for later repair. The radial artery lies radial and is protected by intact soft tissue. The brachioradialis insertion may be released to correct radial-sided deformity, which also relaxes the deforming force on the radial styloid fragment.

The distal limit: the watershed line. The watershed line is the most volar transverse ridge of the distal radius, just proximal to the volar rim, where the flexor tendons lie closest to bone. Plates placed distal to or directly on the volar rim (Soong grade 2) are associated with flexor tendon attrition and rupture, with flexor pollicis longus rupturing first because it lies most radial and most directly on the plate, and the index profundus after it. Always confirm plate position on a true lateral before leaving theatre, and consider early removal of a prominent plate in a symptomatic patient.

Closure. Repair pronator quadratus over the plate where the tissue allows. Do not repair the FCR sheath tightly: a snug repair reproduces the stenosis of FCR tendinopathy.

The volar (Russe) FCR approach. Used for scaphoid waist and distal pole fractures, volar bone grafting for nonunion and correction of humpback deformity. The incision runs along the FCR tendon, curving radially at the wrist crease over the scaphoid tubercle and extending toward the thumb metacarpal base in a lazy-S.

Steps.

1. Open the FCR sheath and retract the tendon ulnarly, exactly as for the distal radius approach
2. Identify and ligate the superficial palmar branch of the radial artery, which crosses the distal part of the field
3. Incise the volar wrist capsule along the line of the radioscaphocapitate and long radiolunate ligaments
4. Expose the scaphoid tubercle and distal pole; extending and radially deviating the wrist brings the waist into view

The radioscaphocapitate ligament. Do not simply transect it transversely. It is the volar tether of the scaphoid, and unrepaired division permits ulnar carpal translation; split it in line with its fibres, or divide and formally repair it at closure. Formal capsular repair is mandatory, and failure to close the volar capsule is a recognised cause of post-operative carpal instability.

Why volar rather than dorsal. The volar approach preserves the dorsal ridge blood supply on which the proximal pole depends, and it directly addresses the volar comminution and humpback (flexion) deformity typical of waist nonunions by allowing a volar wedge graft. The dorsal approach is preferred for proximal pole fractures and for percutaneous fixation of non-displaced waist fractures, where central screw placement is easier.

Ligament reconstruction with tendon interposition (LRTI). After trapeziectomy for thumb basal joint arthritis, the radial half of the FCR tendon is raised, left attached distally at the second metacarpal base and passed through a drill hole in the base of the thumb metacarpal from volar-ulnar to dorsal-radial. This reconstructs the palmar (volar) oblique or beak ligament and suspends the thumb metacarpal; the remaining tendon is coiled as an interposition anchovy in the trapezial space. Only the radial half is taken, leaving the ulnar half to preserve wrist flexion and avoid a secondary donor deficit.

The harvest. Through the same volar incision, usually with a proximal counter-incision to divide the strip at the musculotendinous junction under direct vision rather than by blind distal traction. The strip is delivered distally; never drag it distally with force, which can avulse the metacarpal insertion.

Eaton-Littler volar ligament reconstruction. For symptomatic thumb carpometacarpal instability without established arthritis (Eaton stage I to II), a slip of FCR is passed through a drill hole in the thumb metacarpal base and around abductor pollicis longus to recreate the volar beak ligament.

Scapholunate reconstruction. The modified Brunelli / three-ligament tenodesis uses a distally based strip of FCR passed through a bone tunnel in the scaphoid from volar-distal to dorsal-proximal, then secured to the lunate and to the dorsal radius, reconstructing the scapholunate, dorsal intercarpal and radiocarpal restraints. The graft length required is typically 8 to 10 cm, comfortably obtainable from the radial half.

Radial nerve palsy transfer. FCR to extensor digitorum communis restores finger extension. FCR is chosen over FCU in most modern practice because FCU is the principal ulnar deviator and the key wrist stabiliser during power grip, so taking it costs more function than taking FCR. The full transfer set is pronator teres to ECRB for wrist extension, FCR to EDC for finger extension and palmaris longus to rerouted EPL for thumb extension; where palmaris longus is absent, the ring finger superficialis substitutes for thumb extension.

Volar wrist arthroscopy. The volar radial portal is established through the FCR sheath, passing between the radioscaphocapitate and long radiolunate ligaments, with the tendon retracted ulnarly to protect the palmar cutaneous branch and the radial artery. It gives an unrivalled view of the dorsal scapholunate ligament and the dorsal capsule.