[Significance of computerized tomography in the diagnosis of post-traumatic proximal carpal instability]

Proximal carpal instability is a painful condition characterized by early or late loss of radioulnar joint (RUJ) congruence not affecting the normal bone alignment of the two carpal rows. The joint incongruence or (incomplete) dislocation which leads to proximal instability is caused by many traumatic and nontraumatic events. The diagnosis of (incomplete) dislocation of the distal RUJ may be extremely difficult to make at conventional radiography because such injuries can be seen only when the lateral joint projection is perfect; otherwise the diagnosis is not reliable. CT is the only imaging tool diagnosing the grade of distal RUJ congruence independent of the examination technique.

We studied the radiocarpal complex conditions leading to proximal instability at the Rizzoli Orthopedic Institute from December, 1995, through December, 1997. In all, 389 cases were seen, 376 from trauma and 13 of nontraumatic origin. Each injury was studied with conventional radiography, CT, and MRI. Radiography was performed in two projections, namely the posteroanterior one with hand extension and the lateral one with the forearm in neutral position and the elbow bent at 90 degrees; the projections were repeated whenever a cast brace was applied. Unenhanced CT was performed for comparison with the patient prone and the forearm and wrist in prone and neutral position, as well as with the patient, forearm and wrist supine. Three criteria of electronic image processing were adopted for the RUJ studies in the 3 projections: radioulnar lines, congruence, and epicenter. MRI was always performed after conventional radiography and CT. Only the involved radiocarpal region was studied; coronal, axial and sagittal images were acquired with T2-weighted GE and T1-weighted SE sequences.

Proximal instability was found in 17 of 389 patients; it was early in 13 and late in 4 of them. The comparison of radiographic and CT results showed that the former method is unreliable, with 53% false negatives. Pain, a cast brace, congenital or acquired deformities of distal radius and ulna and patient mispositioning by the radiology technician can change the rotation of the forearm, wrist and hand and make a perfect laterolateral projection in neutral position unfeasible, which affects the radiographic diagnosis. Conversely, CT showed its extreme efficacy in assessing the distal RUJ congruence with no false negatives independent of the RUJ rotation and of instability type and grade.

Conventional radiography is a poorly reliable tool for the diagnosis of joint incongruence and its grade. In contrast, CT can diagnose a RUJ (incomplete) dislocation easily and unquestionably, thanks to its axial capabilities, even when adequate radiographic studies would be unfeasible. If the anteroposterior projection of the radiocarpal complex shows a congenital or acquired deformity of distal radius, the lateral projection can be skipped and a CT scan in prone, neutral and supine position performed. The 3 CT criteria quantify incongruence type and grade, and also demonstrate the position of maximum incongruence and its decrease by position. The comparative study of the radiocarpal region makes CT a very useful and valuable tool in congenital instability because its allows the assessment of contralateral radioulnar congruence too. MRI is very useful in the diagnosis of injury or degeneration of the fibrocartilage complex, namely in patients with no bone changes at conventional radiography.