Flexor Carpi Radialis Anatomy

Origin

• Medial epicondyle of the humerus through the common flexor origin - FCR is the second most radial of the superficial flexor group after pronator teres.
• Additional fibres 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).
• Muscle fibres end on average 8 cm proximal to the wrist (range 6 to 9 cm), so the distal third of the forearm contains pure tendon - the level at which the tendon is harvested or split.

Insertion (three components)

• Base of the second metacarpal - the principal insertion, taking approximately 80 per cent of the tendon.
• Base of the third metacarpal - approximately 20 per cent.
• A small slip to the crest or tuberosity of the trapezium - present in most specimens and the anatomical reason the tendon is intimately applied to the trapezium.

The fibro-osseous tunnel

• The tendon enters a fibro-osseous tunnel at the proximal border of the trapezium.
• The tunnel floor is the roughened groove of the trapezium; the tendon is in direct contact with bone.
• A thick septum separates the FCR tunnel from the carpal canal. The distal border of this septum acts as a pivot point for flexor pollicis longus.
• The synovial sheath extends from origin to insertion of the tendon, so tenosynovitis can extend well proximal to the tunnel.
• Within the tunnel the tendon occupies about 90 per cent of the available cross-sectional space - a functionally unforgiving corridor.

Nerve supply

• Median nerve, root values C6 and C7 (a C8 contribution is described in some texts but C6-C7 is the answer expected).
• Branches arise from the median nerve in the proximal forearm, distal to the pronator teres branches, typically 2 to 4 cm distal to the medial epicondyle.
• Motor entry point: the branches 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.

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 plus 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

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

Pedicle position - practical points

• The radial artery runs 5 to 10 mm radial to the FCR tendon at the level of the distal wrist crease. In the FCR approach it is protected simply by staying within the tendon sheath and retracting the tendon ulnarly.
• The superficial palmar branch of the radial artery 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. Distal dissection to obtain extra tendon length is the moment of maximum arterial risk.

At the wrist, radial to ulnar

• Radial (lateral) to FCR: the radial artery and its venae comitantes, then the brachioradialis tendon and the first extensor compartment.
• Ulnar (medial) to FCR: palmaris longus when present, then the median nerve, then flexor digitorum superficialis, then flexor carpi ulnaris and the ulnar neurovascular bundle.
• Deep to FCR: 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 FCR: skin, subcutaneous fat and the antebrachial fascia; no significant structure other than crossing superficial veins.

The palmar cutaneous branch of the median nerve

• 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.
• The tendon also lies within a few millimetres of the volar rim of the distal radius - the watershed line - which is why plate prominence there produces flexor tendon attrition.

FCR tendinopathy and stenosing tenosynovitis

• Mechanism: the tendon fills roughly 90 per cent of a rigid fibro-osseous tunnel and lies in direct contact with the trapezium. Any reduction in tunnel volume - trapezial or STT osteophytes, thumb basal joint arthritis, a distal radius malunion with volar prominence, a ganglion, or post-traumatic scarring - produces mechanical abrasion and stenosis.
• Presentation: volar radial wrist pain aggravated by gripping, wringing and repetitive wrist flexion. Swelling and crepitus may be felt over the tendon. Frequently mislabelled as a wrist sprain or attributed solely to the thumb base.
• Associations: scaphotrapeziotrapezoid arthritis, thumb carpometacarpal arthritis, scaphoid nonunion with a volar prominence, rheumatoid disease, and prominent volar distal radius hardware.
• 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: chronic abrasion against a trapezial osteophyte, a scaphoid nonunion prominence, or prominent volar plate hardware.
• Presentation is often painless loss of the tendon contour after a period of pain that then settles - the pain stops when the tendon parts.
• 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 pattern. In a transverse volar laceration the expected order of divided structures from radial to ulnar is: 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.
• Non-repair of an isolated FCR division in a low-demand patient is defensible - the functional cost is small - but repair is preferred in manual workers and where palmaris longus is absent.

Associated injury patterns

• Distal radius fracture with volar displacement: the sharp volar cortical spike can lacerate or abrade FCR and FPL.
• Perilunate injury: 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.

Inflammatory disease

• Rheumatoid arthritis produces flexor tenosynovitis that includes the FCR sheath. 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. 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 to the volar distal radius

The default approach for volar plating of distal radius fractures.

Incision. Longitudinal, directly over the palpable FCR tendon, typically 6 to 8 cm long, ending at or just proximal to the wrist crease. Extend 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.
• Retract the FCR 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, and keeps the dissection radial to those structures.
• Incise the floor of the FCR sheath. The flexor pollicis longus muscle belly is then encountered and retracted ulnarly with the rest of the flexor mass.

Deep dissection.

• The pronator quadratus is exposed. Elevate it as an L-shaped flap with 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.

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.
• Always confirm plate position on a true lateral 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 to the scaphoid

Used for scaphoid waist and distal pole fractures, volar bone grafting for nonunion, and correction of humpback deformity.

Incision. 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 and hazards.

• Open the FCR sheath and retract the tendon ulnarly, exactly as for the distal radius approach.
• Identify and ligate the superficial palmar branch of the radial artery, which crosses the distal part of the field.
• Incise the volar wrist capsule along the line of the radioscaphocapitate and long radiolunate ligaments. Do NOT simply transect the radioscaphocapitate ligament 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.
• The scaphoid tubercle and distal pole are then exposed; extending and radially deviating the wrist brings the waist into view.

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.

Closure. Formal capsular repair is mandatory. Failure to close the volar capsule is a recognised cause of post-operative carpal instability.

FCR as an autograft - the standard harvests

Ligament reconstruction with tendon interposition (LRTI) for thumb basal joint arthritis.

• After trapeziectomy, 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 preserves wrist flexion and avoids a secondary donor deficit.
• Harvest is through the same volar incision, usually with a proximal counter-incision to divide the strip at the musculotendinous junction. 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.
• Graft length required is typically 8 to 10 cm, comfortably obtainable from the radial half.

Radial nerve palsy tendon transfer.

• FCR to extensor digitorum communis restores finger extension. FCR is preferred over FCU because FCU is the dominant ulnar deviator and wrist stabiliser during grip.
• The full transfer set: pronator teres to ECRB (wrist extension), FCR to EDC (finger extension), palmaris longus to rerouted EPL (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. Retracting the tendon ulnarly protects the palmar cutaneous branch and the radial artery. This portal gives an unrivalled view of the dorsal scapholunate ligament and the dorsal capsule.