Flexor Carpi Radialis (FCR) Tendinitis

Its own canal. The FCR does not run in the carpal tunnel. It has its own fibro-osseous canal, the "FCR tunnel": a groove in the trapezium roofed by a fibrous septum, a split of the flexor retinaculum that attaches to the trapezial ridge and the scaphoid, which separates it from the carpal tunnel contents.

A tight fit with a bend in it. The tendon occupies most of the canal and angulates about 30 degrees around the trapezial ridge before it inserts, which concentrates friction and compression there. Its course runs against the trapezial ridge and the scaphoid tuberosity, immediately adjacent to the trapeziometacarpal and scaphotrapeziotrapezoid (STT) joints. That confined course makes it prone to friction tendinopathy and explains its close association with basal-thumb and STT arthritis.

Insertion and action. The FCR inserts primarily on the base of the second metacarpal, with slips to the third metacarpal base and the trapezial tubercle. It does not insert on the pisiform; that is flexor carpi ulnaris. It flexes and radially deviates the wrist and assists pronation, which is why resisted flexion and radial deviation provoke the pain.

Surgical relations. The neurovascular neighbours are what matter at operation.

• The radial artery lies just radial to the FCR, a useful landmark
• The palmar cutaneous branch of the median nerve runs between FCR and palmaris longus, and is at risk in a volar (FCR) approach
• The median nerve is ulnar and deep
• The superficial radial nerve is dorsal

Tendinopathy. FCR tendinitis is usually a degenerative overuse tendinopathy rather than a true acute inflammation. Overuse and repetitive loading provoke it, and it may follow local trauma.

Attrition from the joints next door. Osteophytes from STT and trapeziometacarpal osteoarthritis, and the trapezial ridge itself, project into the tight canal and mechanically abrade the gliding tendon. The result is chronic attrition tendinopathy and, ultimately, attritional rupture. It is directly analogous to the extensor attrition ruptures, EPL over Lister's tubercle or the extensors over the caput ulnae in the Vaughan-Jackson pattern, except that here the FCR frays over trapezial and STT osteophytes.

Causes of rupture.

1. Attrition over STT or trapezial osteophytes: spontaneous, in basal-thumb OA, and rare
2. A complication of trapeziometacarpal suspension arthroplasty, the abductor pollicis longus "hammock" wrap
3. Rarely, acute laceration or trauma

After suspension arthroplasty. FCR tendinopathy, and even rupture, is a recognised complication of trapeziometacarpal suspension arthroplasty, in which tissue is wrapped around the FCR for suspension. After the APL hammock procedure it is common rather than merely recognised. In the Nakai series of 21 hands followed for more than a year, a third developed FCR tendinopathy (7 of 21, 33.3%): tendinitis in 3 (14.3%) and frank rupture in 4 (19.0%).

How far to trust those numbers. With 21 hands, each rupture moves the figure by about 5 percentage points, so the percentages carry wide uncertainty, and they apply to the APL-around-FCR technique rather than to suspension arthroplasty in general. Manual workers had tendinopathy in 50% against 18.2% of non-manual workers, an odds ratio of 4.5, but this did not reach statistical significance (p=0.18). It is a reasonable basis for counselling and postoperative restriction, not an established risk factor.

Symptoms and signs. Volar-radial wrist pain, with tenderness over the FCR tendon at and just proximal to the wrist crease, where the tendon enters its tunnel. The pain is worse on resisted wrist flexion and radial deviation and on direct palpation of the tendon.

Look next door. Because the FCR runs right beside the trapeziometacarpal and STT joints, volar-radial wrist pain attributed to the tendon is frequently driven by, or accompanied by, basal-thumb or STT osteoarthritis. The differential must be worked through and the source localised by examination and imaging.

Radial-sided wrist ganglia complete the list. The neighbouring diagnoses have their own pages: the joint that most often drives the pain attributed to this tendon, and whose surgery generates the complication above, is thumb carpometacarpal arthritis; the dorso-radial mimic with the positive Finkelstein test is de Quervain's tenosynovitis.

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Radiographs, including a basal-thumb/STT view, assess the adjacent joints for osteoarthritis. They can also show calcific tendinosis at the FCR, a recognised cause of FCR-region wrist pain that typically resolves with conservative treatment.

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Ultrasound or MRI characterises the tendon itself: tendinopathy, tenosynovitis, partial tear or rupture.

Diagnostic injection. Examination, radiographs and, if needed, an injection into the FCR sheath localise the source. The injection can both confirm the source and treat it.

Non-operative first. Activity modification or rest, a wrist splint (sometimes including the thumb), non-steroidal anti-inflammatories and physiotherapy, with a corticosteroid injection into the FCR tunnel or sheath for refractory symptoms. This is effective for most FCR tendinitis.

Treat the joint as well as the tendon. Do not treat the FCR in isolation. Where basal-thumb or STT osteoarthritis coexists, address it as indicated, including at the time of tunnel release.

Surgery for refractory symptoms. Release or decompression of the FCR fibro-osseous tunnel, with debridement of the diseased tendon.

Rupture. Flexor carpi ulnaris and palmaris longus provide most wrist flexion, so an isolated FCR rupture is frequently well tolerated, and a spontaneous rupture in a low-demand patient may be managed conservatively. A rupture after suspension arthroplasty, which destabilises the reconstruction, and a symptomatic or high-demand rupture require surgery. In the Nakai series all four ruptures did.