Arthroscopic proximal row carpectomy

Biomechanics and kinematics

A proximal row carpectomy (PRC) significantly alters the radiocarpal kinematics. Blankenhorn et al. found that following a PRC, wrist flexion and extension were accomplished by capitate rotation. In radioulnar deviation, capitate motion changed from predominantly midcarpal rotation in the intact wrist to a combination of rotation and translation at the radiocarpal joint. Overall flexion decreased 28%, extension decreased 30%, radial deviation decreased 40%, and ulnar deviation decreased 12%. Motion at the radiocarpal joint during flexion and extension after a PRC, however, was greater compared with motion at the radiocarpal and midcarpal joints of the intact wrist. Radial deviation was limited because of impingement between the trapezoid and the radial styloid.

Hogan et al. examined radiocarpal loading following a PRC in 7 cadaver wrists. The contact area increased 37% in the lunate fossa and the average contact pressure increased 57%. The location of the contact moved radially 5.5 mm. With wrist motion between 40 degrees of extension and 20 degrees of flexion, the volar-dorsal excursion of the lunate fossa contact point increased 108%. They postulated that the increased radiocarpal excursion with wrist motion might explain the low incidence of radiocapitate arthritis in patients who have had a PRC. This was echoed by Tang et al. They compared the contact biomechanics of the intact wrist with PRC wrist in 6 cadaver wrists. In the intact wrist, scaphoid contact pressure averaged 1.4 megapascals (MPa), and lunate contact pressure averaged 1.3 MPa. Scaphoid contact in the intact wrist moved dorsal and ulnar in flexion, and volar and radial in extension. Lunate contact moved dorsal in flexion. The contact pressure after a PRC was 3.8 times that of the intact wrist, and the contact area was approximately 26% that of the intact wrist. After a PRC, the capitate contact (7.5 mm) translated more than did the scaphoid contact (5.6 mm) and had about equal translation to that of the lunate (7.3 mm). This provided quantitative support of the theory that translational motion of the PRC may explain its good clinical outcomes. At the radiocarpal joint, the radius of curvature of the capitate head is approximately two-thirds of the radius of curvature of the lunate fossa. Over time, the capitate appears to conform better to the lunate fossa.

Diagnosis

The diagnosis of SLAC or SNAC wrist arthritis is made by history, physical examination, and radiographs. The wrist examination often reveals a joint effusion, dorsal-radial wrist swelling, and tenderness over the radioscaphoid joint. There may or may not be a positive scaphoid shift test. Chronic synovitis over the snuffbox may be misdiagnosed as a ganglion cyst. Wrist motion may be decreased, depending on the stage of degeneration. The definitive diagnosis is made radiographically. Standard posteroanterior, oblique, and lateral views should be performed. Marked changes as seen in SLAC and SNAC are easily identified. An AP grip view and radioulnar deviation views can magnify any SL diastasis. An MRI and/or CT scan may be useful to evaluate any midcarpal joint changes and DISI deformity, and to determine whether there is a styloid pattern or dorsal lip pattern of impingement.