The Stable Central Column Theory of Carpal Mechanics—Midcarpal Stability Explained and Why Chimps Can't Throw Darts

Introduction The concept of the stable central column theory of carpal mechanics based on computationally derived isometric constraints has been previously presented, and was published in May 2014 (JHS-Eu May 2014). This theory characterizes wrist mechanics as a stable central carpal column with a lateral column stabilizer, a medial column translation restraint, and physiological pronation of the midcarpal axis.

The primary function of the wrist is to maintain a stable central metacarpal axis (2nd and 3rd) against which the more mobile thenar and hypothenar rays can act. The stability of the critical central column is achieved by its connection to the lateral column (scaphoid) as a two gear four-bar linkage.

The trapezoid-shaped trapezoid places the trapezium anterior to the transverse plane of the radius and ulna, and thus rotates the principal axis of the central column to correspond to that used in the “dart thrower's motion.” This created a coronal axis at the radiocarpal joint and an oblique axis at the midcarpal joint.

This paper presents the application of the theory to clinical cases as well as cross species comparison, in an attempt to explain midcarpal stability and the dart-throwing motion.

Materials and Method The study was performed in two parts. (1) Clinical case examples who presented with midcarpal instability characterized by a midcarpal catch-up clunk were studied using dynamic 3D-spatial carpal bone analysis, and a reconstruction was performed based on the theory that midcarpal instability is due to a loss of the stability between the trapezium and scaphoid. (2) By comparing cross-species isometric constraints and carpal-bone morphology, the axis of the midcarpal joint was identified and the carpal motion of the human and chimpanzee characterized, with a theoretical proposal regarding carpal motion and function of Homo floresiensis.

Results Four patients have undergone lateral column reconstruction to address midcarpal instability. In each patient, the medial column alignment and stability were improved, although complete anatomical correction was not possible in every case due to secondary stretching of medial interosseous ligamentous structures. Analysis of trapezoid morphology and isometric constraints of the lateral column demonstrated the origin of the more coronal midcarpal axis of the chimpanzee and explains the reason for the cross species difference with respect to the ability to throw darts.

Conclusion Midcarpal instability is due to a lateral column disruption which prevents the smooth transition from a flexed proximal row to an extended proximal row as the wrist moves from radial to ulnar deviation. The loss of a strong isometric connection to the volar trapezium creates a delayed extension of the scaphoid, thus delaying normal physiological proximal-row extension until an obligate and sudden lunate and proximal-row exertion correction, most likely initiated by the triquetrum as it glides up the volar aspect of the hamate. This loss of this carpal rhythm was corrected by restoring the lateral column integrity.

This, however, is unimportant when compared with the most important bone in evolution—the trapezoid. Without a “trapezoid shaped trapezoid,”dart (or spear) throwing, tool use, and the offensive or defensive use of a spear is impaired, as would be the survival of the species—unless one was stuck on a small island with no competitors.

References

References

1 Sandow MJ, Fisher TJ, Howard CQ, Papas S. Unifying model of carpal mechanics based on computationally derived isometric constraints and rules-based motion - the stable central column theory. J Hand Surg Eur Vol 2014;39(4):353–363