Snapping / Subluxing Extensor Carpi Ulnaris (ECU)

The sixth compartment. The extensor carpi ulnaris occupies the sixth dorsal compartment, where it lies in a bony groove on the dorsal aspect of the ulnar head. It is held there by its own fibro-osseous subsheath, a structure separate from the overlying extensor retinaculum. The tendon must stay in this groove through forearm rotation, and the subsheath, not the retinaculum, is the primary restraint that keeps it there.

The groove. The depth of the ulnar (ECU) groove varies anatomically. A shallow, flat or convex groove predisposes to recurrent subluxation because the bony restraint is weak.

Mechanism. A forceful or repetitive load in supination, ulnar deviation and wrist flexion can tear or stretch the subsheath. It is classically a sporting injury, from tennis, golf, racquet and bat sports. Chronic or degenerative attenuation and rheumatoid disease can do the same.

What follows. Once the subsheath has failed, the ECU subluxates or dislocates out of its groove, usually toward the volar-ulnar side, during supination, ulnar deviation and flexion, and reduces again on pronation. Each excursion is a painful snap at the dorsoulnar wrist, often visible or palpable. It is an important and under-recognised cause of ulnar-sided wrist pain.

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More than a wrist extensor. The ECU is a dynamic stabiliser of the distal radioulnar joint, most importantly in supination, the position in which the DRUJ is least bony-stable and the ECU and its subsheath resist dorsal-ulnar translation. The floor of the sixth-compartment subsheath blends with the dorsal TFCC/DRUJ capsule, so the structures are anatomically continuous.

Why the injuries travel together. A forceful supination injury can damage the subsheath, the TFCC and the DRUJ together. ECU instability, TFCC tears and DRUJ instability therefore frequently coexist rather than being mutually exclusive, so "exclude TFCC/DRUJ" is better read as "also look for them".

The consequence. Assess and address all of them, because isolated ECU surgery may fail if a TFCC tear or DRUJ instability is missed. Their treatment is covered in those topics.

Provoke the snap. The diagnosis is mainly clinical. Reproduce the instability with the provocative manoeuvre, active supination with ulnar deviation and wrist flexion, watching and palpating the ECU jump out of its groove and relocate on pronation.

The ECU synergy test. With the elbow flexed and the forearm supinated, the patient radially abducts the thumb against resistance, which co-contracts the ECU and reproduces dorsoulnar pain. It applies isolated tension to the ECU without loading the structures inside the joint. It was described to answer one question in chronic dorsal ulnar-sided wrist pain: is the problem extra-articular (the ECU) or intra-articular (TFCC, lunotriquetral, ulnocarpal)?

What it predicted. In the original series of 55 patients with more than four months of dorsal ulnar-sided wrist pain, the test separated the groups cleanly.

How well it performs. Against sonographic ECU abnormality, in 40 wrists with chronic dorsal ulnar-sided pain, the test had a sensitivity of 73.7%, specificity of 85.7%, positive predictive value of 82.4% and negative predictive value of 78.3%. That is useful, and better at ruling in than ruling out, but not decisive on its own.

The diagnostic injection. Pairing the test with a local anaesthetic injection into the ECU sheath converts a localising sign into an answer, and it is the step the original algorithm turns on. Substantial relief supports the ECU as the pain source. Persistent pain, or pain that returns with lunotriquetral ballottement or TFCC compression, points to intra-articular pathology that will not be fixed by treating the tendon.

Why it matters. Used this way, the test and the injection together reduce the need for MRI and diagnostic arthroscopy. Bear in mind that in the original series "confirmed" meant more than 90% relief after a sheath injection that contained steroid, a treatment response rather than an independent reference standard.

Imaging the subluxation. Dynamic ultrasound through supination and pronation is the best way to demonstrate the subluxation itself in real time. MRI shows the disrupted subsheath, the position of the ECU and any tendinopathy.

What a scan cannot tell you. A scan showing the ECU outside its groove is weaker evidence than it looks. In a study comparing 41 symptomatic wrists with 64 asymptomatic wrists scanned for an unrelated synovial cyst, with no ulnar-sided pain, there was no significant relationship between being symptomatic and the degree of instability on MRI, nor with the forearm rotation used during the scan. The authors concluded that ECU instability on MRI is not a specific finding, and that detecting it may have nothing to do with the patient's symptoms.

The practical consequence. Some degree of ECU translation on rotation is common in normal wrists. Imaging confirms a mechanism in a patient whose history and examination already point to the ECU; it does not, on its own, establish the ECU as the source of the pain. Operating on a scan report in a wrist whose symptoms were never reproduced by the provocative manoeuvre is the error this finding warns against.

Differential and co-diagnoses. Look for the other causes of ulnar-sided wrist pain: TFCC tears, DRUJ instability or arthritis, lunotriquetral instability, ulnar impaction and pisotriquetral arthritis. They coexist with ECU instability at least as often as they replace it.

ECU tendinopathy. Tendinopathy or tenosynovitis is overuse inflammation or degeneration of the ECU within its sheath, with no subluxation. It is a separate cause of dorsoulnar pain.

Acute or first presentation. Treat conservatively. Immobilise in a long-arm cast or splint in pronation with slight wrist extension and radial deviation, the position that reduces and rests the ECU, for several weeks, with NSAIDs and activity modification. Many acute cases settle; then rehabilitate progressively.

Tendinopathy without instability. Rest, splinting and a corticosteroid injection within the sheath can help. A steroid injection does not treat instability.

Surgery. Operate for chronic, recurrent or refractory instability. The options are:

• Subsheath repair, for an acute or repairable subsheath
• Subsheath reconstruction, for chronic cases, using a slip of extensor retinaculum, a periosteal flap (Schlesinger), or the dorsal DRUJ capsule, which preserves the extensor retinaculum
• Ulnar groove deepening, which may be added when the groove is shallow

What the reconstruction must restore. Because the subsheath is the primary restraint, the goal of repair or reconstruction is to recreate a competent subsheath floor that holds the tendon in the groove, not merely the retinaculum. Whatever the construct, it must restore that restraint without over-tightening: the tendon must still glide.

Outcome. Reinforcing the repair or reconstruction reduces recurrence and, in athletes, allows return to sport at a few months. The support is thin: the return-to-sport figure is one water-polo player back at four months, and the DRUJ-capsule technique rests on seven patients at short follow-up.