The wrist is the articulation between the distal forearm bones (radius and ulna) and eight carpal bones. The most common carpal instability is between the scaphoid and lunate, termed scapholunate instability. Complete scapholunate dissociation results in a gap between the scaphoid and lunate on posterior–anterior (PA) radiographic projections (scapholunate gap). On lateral radiographs, the scaphoid assumes a flexed posture and the lunate extends, resulting in increased scapholunate and radiolunate angles. The second most common carpal instability patters is between the triquetrum and lunate, termed lunotriquetral instability. Ulnar translocation of the carpus, also known as carpal translation, refers to medial displacement of the entire carpus on the forearm (radius and ulna) and is the most common radiocarpal instability. In the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), Sixth Edition, carpal instabilities are rated using Table 15-3, Wrist Regional Grid; the impairment class is determined using Table 15-9, Clinical Studies Adjustment: Upper Extremities. Measurements from plain radiographs are used in Table 15-9 to characterize the instability as mild, moderate, or severe; these characterizations, in turn, determine the class. The assessment of carpal instability using radiographic measurements has limitations, particularly that the radiographic parameters used to grade impairment are imprecise; angular measurements between individual bones are particularly variable, and carpal bone axes can be difficult to determine due to bony overlap on plain radiographs.

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