Aim: The carpus is an intricate series of intercalated segments that are controlled by specific ligamentous constraints. During radial deviation (RD), the lateral carpal column shortens but the apparent differential rotation between the scaphoid, lunate and trapezium is not well explained by current theories. This project aims to demonstrate the 3D excursion of the various wrist components and identify those rules that guide and control such motions.

Materials and Methods: By animating 3D CT scans of the normal and abnormal wrist in various positions of coronal and sagittal deviation, the dynamic intercarpal relationships can be demonstrated, ligamentous constraints inferred, and reconstructive options identified and assessed. This involved the development of specific surface rendering software that created a true 3D model (within a graphics animation environment) of the carpus in various positions of sagittal as well as coronal deviation. The intercarpal isometric points that correspond to known ligaments were identified, and motion of the bones plotted through space.

Results: During ulnar to RD, the trapezium, which is firmly attached to the scaphoid, supinates around the foreshortening lateral column. Further, the axial rotation of the scaphoid in combination with its “scaphoid” shape produces an increase in the apparent scaphoid flexion in the sagittal plane. The scaphoid acts as a rotating link between the lunate and trapezium, and lunate stability is explained by the obligate translation combined with the obliquity of the (volar) radio-lunate ligament. “Virtual” scapho-trapezial, radio-capitate and radio-lunate ligaments are evident, however, the scapho-lunate connection is less rigid.

Discussion: An understanding of the fixed constraints (isometric points) and motion rules and patterns within the carpus allows for injury characterisation and the development of more logical reconstructive interventions that attempt to replicate normal kinetics. Specific motion rules of the carpus have been established allowing virtual reconstructive surgery on normal and pathological wrists.

Ref: www.madacademy.com.au/tla, www.truelifeanatomy.com