Fig. 9. De Quervain's tenosynovitis. Coronal fat-suppressed T2-weighted image demonstrates tendon sheath effusion, peritendinous edema, and tendon thickening of the first extensor compartment (arrowheads).
. MRI can be useful in mapping the course of the superficial radial nerve before surgery and also in identifying causes for postsurgical failure.
The intersection syndrome is characterized by pain and swelling approximately 4 to 8 cm proximal to Lister's tubercle of the radius where the first extensor compartment tendons cross over the second extensor compartment tendons . It is a peritendinosis of the second extensor compartment tendons that is most likely secondary to chronic overuse and friction and is commonly associated with sports-related activities, such as rowing, canoeing, playing racket sports, horseback riding, and skiing . Tendon sheath stenosis can also cause entrapment of the extensor carpi radialis longus and extensor carpi radialis brevis tendons.
Intersection syndrome may be overlooked clinically, so MRI can play a significant role in its diagnosis [53,54]. MR findings reflect peritendinosis and include thickening and interstitial fluid concentrically surrounding the second and the first extensor compartment tendons and tendon sheaths, beginning at the point of crossover and extending proximally. Fluid-sensitive sequences are most useful to demonstrate these changes. Associated reactive tenosynovial effusion can be observed .
Chronic friction at Lister's tubercle can produce extensor pollicis longus te-nosynovitis and rupture (Fig. 10). Predisposing factors include degenerative or posttraumatic spurring of Lister's tubercle, synovitis associated with rheumatoid arthritis, and systemic lupus erythematosus. Anatomic variants such as an accessory extensor pollicis longus muscle within the third extensor compartment  can also predispose to the condition. Nondisplaced or minimally displaced distal radial fractures can produce tears of the extensor pollicis longus (EPL) tendon, typically distal to the extensor retinaculum [56,57] where the
Fig. 10. Tenosynovitis of the extensor pollicis longus, extensor carpi ulnaris brevis, and extensor carpi ulnaris longus tendons associated with postfracture deformity of the distal radius. (A) Axial T2-weighted image at the level of the proximal carpal row demonstrates prominent tenosynovial effusion (asterisk) involving the extensor carpi radialis longus (ecrl), extensor carpi radialis brevis (ecrb), and extensor pollicis longus (epl) tendons. (B) Sagittal T1-weighted image demonstrates a healed fracture of the distal radius with a large dorsal spur.
tendon has decreased vascularity. The rupture usually develops between 3 weeks and 3 months following injury [58,59].
MRI characteristics of EPL tenosynovitis include thickening of the EPL tendon sheath and fluid surrounding the tendon. Stenosing tenosynovitis, noted in chronic cases, is displayed by the presence of low-signal scar tissue about the tendon sheath. Heterogeneity of the tendon consistent with either tendinosis or partial rupture may be seen, but care should be taken not to confuse normal increased signal related to magic-angle effect with tendon disease. In cases of rupture, MRI is useful for locating the retracted tendon margins and for assessing the size of the tendon gap. In acute cases, fluid will help to outline the torn edges. However, chronic tears will often have scar tissue filling the tendon gap, obscuring the torn ends.
Extensor digitorum, extensor indicis, and extensor digiti minimi are susceptible to lacerations because of their superficial position. The ruptures may also be closed injuries, however. MRI can be useful in making the diagnosis of tendon lacerations, because they can be missed clinically when the laceration occurs proximal to the interconnecting junctura. Function of the middle and little fingers' extensor tendons may be preserved through ring finger interconnecting junctura contributing to the distal ends of these tendons when lacerated. Primary surgical repair is mandatory in this type of injury. Digital extensor tendon injuries at the wrist and distal forearm are usually related to deep lacerations and are associated with significant proximal tendon retraction [60,61].
Tenosynovitis of the extensor carpi ulnaris within the sixth compartment is common. Instability of the tendon is frequently associated with the tenosynovitis. Ulnar subluxation of the tendon during supination and volar flexion can occur secondary to disruption of the ulnar aspect of the extensor carpi ulnaris (ECU) tunnel. Causes for dislocation of the ECU tendon include recurrent instability, distal radial fractures with disruption of the extensor retinaculum, and posttraumatic distal radioulnar subluxation.
The MRI characteristics of tenosynovitis of the ECU tendon include thickening of the tendon sheath, tenosynovial effusion, and peritendinous inflammatory changes (Fig. 11). Peritendinous low signal intensity and irregular tendon margins may be seen with chronic stenosing tenosynovitis. Partial tears secondary to chronic attrition of the tendon in patients who have underlying instability or chronic stenosing tenosynovitis  demonstrate heterogeneity and splits within the tendon. Closed, nonrheumatoid ruptures of the ECU tendon are rare and have been described in tennis and hockey players [63,64].
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