[Three-dimensional CT study of the carpus under pronation-supination constraints]

Abstract

By studying 3D imaging of the wrist under pronation-supination strain, we found that the simple comparison of a series of two corresponding cuts may provide a great deal of useful information on how the carpus transmits the longitudinal torque from the forearm to the hand. A special wooden trestle was made to fix the subject in the CT scanner in a permanent effort of pronation or supination. In the first group of scans, this effort was said to be "free" because the hand was simply maintained in a fixed window without any muscular contraction, except pronation or supination muscles. In the second group of scans, this effort was said to be "constrained" because the hand gripped a fixed bar with contraction of the flexor muscles. The thickness of the cuts was 1.2 millimeters and they were separated by 1.5 millimeters. Four levels were specially studied: the lower radio-ulnar joint (LRUJ), the proximal row of the carpus, the distal one and the metacarpal bases. Many elementary movements occur in the carpus in constrained supination: the triquetrum "supinates" (7 degrees), the scaphoid flattens and "pronates" (2 degrees) around the capitum the ridges of the carpal anterior concavity approximate (3 mm). In constrained pronation, the anterior concavity of the carpus flattens emphasizing the role of the anterior retinaculum. The LRUJ is very unstable: in free pronation, the ulnar head moves dorsally, firmly pressing the posterior part of the sigmoid notch, responsible for fracture of a postero-medial fragment in Colles fracture. The quadratus pronatus is a very important muscle to coapt this joint. We propose the "screwing (or unscrewing) test" in the diagnosis of arthrosis or instability of the LRUJ. We define the notion of "rotational shift" to appreciate the quality of the pronation/supination torque transmission. In constrained pronation/supination, this rotational shift is 5 degrees in the radio-carpal joint. This is very important to appreciate the quality of the wrist prosthesis. In free pronation/supination, the rotational shift is 45 degrees between radius and metacarpal bases. In constrained pronation/supination, it becomes 10 degrees. The wrist ligaments are unable to resist the wrist rotational shift and favor the torque transmission. The tendinous caging of the wrist is the main factor for maintaining rigidity of the carpus and transmitting the torque as muscles are contracted. The wrist can be compared with a fluid drive clutch, whose pedal is muscular contraction.