At the conclusion of this activity, participants will be able to summarize the major categories of wrist instability and the current imaging used to evaluate it, identify the major structures responsible for wrist stability and detect abnormalities on imaging which can be seen in the unstable wrist.

Carpal (wrist) stability is dependent on static and dynamic stabilizers. Injury to these structures can result in carpal instability. The wrist is clinically considered unstable if there is symptomatic carpal malalignment, if it is unable to bear physiologic load and is found to be kinematically abnormal1.

Carpal instability encompasses a wide range of pathologies with varying clinical presentations, which can sometimes make clinical diagnosis challenging2. Diagnostic imaging, particularly magnetic resonance (MR) imaging, plays an important role in the evaluation of the patient with suspected carpal instability, and is most successful in doing so when imaging is optimized and the interpreting radiologist is familiar with the complex anatomy and pathologic findings seen on imaging.

MR imaging has been shown to be sensitive and specific for the detection of intrinsic ligament injuries when compared to arthroscsopy3. Because of the inherently small size of the structures evaluated in the wrist joint, MR imaging of the wrist requires high resolution and thin sections (0.6 to 1.2 mm4). The use of dedicated wrist coils and higher field strength MR scanners provides the high signal to noise (SNR) needed to achieve the spatial resolution required to image the small structures in the wrist. Isocenter imaging, as can be achieved when the positioned prone with the arm overhead (“superman position”) is ideal. Routine sequences include anatomic (proton density, T1) and fluid sensitive (fat suppressed proton density, fat suppressed T2, or short tau inversion recovery (STIR)) sequences. MR arthrography may improve the diagnostic performance of MR imaging of ligament pathology5–7.

MR imaging can be used to visualize many of the structures injured in the unstable wrist. This includes the elaborate system of intrinsic and extrinsic ligaments that link the individual carpal bones to each other, and those that link the forearm or hand to the wrist. The scapholunate and lunotriquetral ligaments are the two most important intrinsic ligaments. Some of the extrinsic ligaments routinely identified on MR imaging include the dorsal intercarpal and dorsal radiocarpal ligaments, radioscaphocapitate, long and short radiolunate, radioscapholunate, ulnolunate and ulnotriquetral ligaments.

There are four major patterns of carpal instability: dissociative and non-dissociative instability (CID “carpal instability dissociative) and CIND “carpal instability non-dissociative”), and complex or adaptive instability (CIC “carpal instability complex/combined” and CIA “carpal instability adaptive”). The difference between CID and CIND is that in CID, instability occurs within a carpal row and in CIND instability occurs between carpal rows. When elements of CID and CIND are both present, this is termed complex or combined carpal instability (CIC). Adaptive instability refers to carpal instability that develops when the primary abnormality is extrinsic to the wrist. Distal radial fractures are a common cause of this type of instability. Scapholunate and perilunate instability are two kinds of wrist instability with characteristic imaging findings which we will review during this session.

Scapholunate ligament injury is the most common cause of CID, and scapholunate dissociation is reported to be the most common pattern of wrist instability8. The scapholunate ligament is a c-shaped structure with three histologically distinct zones which can be seen on MR imaging. The dorsal component is the strongest, thickest, and most important for scapholunate stability. It is best seen on axial and coronal images. Additional extrinsic ligaments, including the radioscaphocapitate and short and long radiolunate ligaments on the volar-radial aspect of the wrist, and dorsal intercarpal and dorsal radiocarpal ligaments, provide additional stability. Scapholunate ligament injury treatment depends on the presence of osteoarthritis, the degree of instability, the chronicity of injury, associated injuries and the physical demands of the patient9. Left untreated, chronic scapholunate injury can result a predictable pattern of wrist osteoarthritis known as scapholunate advanced collapse (SLAC wrist).

Perilunate instability is considered CIC, and is a common injury seen by radiologists. It is a result of an injury with begins with injury to the scaphlunate ligament and may end with lunate dislocation. The series of injuries that occur in perilunate instability was described by Mayfield in 1980, and consists of four stages. In Stage I, the scapholunate joint is disrupted via injury to the scapholunate and radioscaphocapitate ligaments. In Stage I, lunocapitate dissociation occurs. In Stage III, the lunotriquetral ligament fails, the lunate loses all connections to the carpus and the carpus dislocates from the lunate, usually in the dorsal direction. In the last stage, Stage IV, the lunate dislocates into the carpal tunnel. Diagnosis is made on plain radiographs, with advanced imaging assisting in the detection of associated fractures and for the assessment of alignment.