The Kinematics and Clinical Implications of the Dart-Throwing Motion

INTRODUCTION

The vision of carpal kinematics might be obscured by the relatively rigid adherence to the orthogonal sagittal and coronal planes of wrist motion, when in fact most activities of daily living rarely use these planes of motion. Most activities are performed using an oblique wrist motion in the radiodorsal to ulnopalmar direction like that involved in throwing darts ( Fig. 4-1 ). Thus, such movements are often referred to as “dart-throwing motion” (DTM) or “dart thrower’s motion.” However, relatively few studies have comprehensively examined DTM, and the definition and terminology of this functional oblique motion remain obscure. Therefore, the Wrist Biomechanics Committee of the IFSSH (International Federation of the Society for Surgery of the Hand) comprehensively analyzed DTM from the viewpoint of anatomy, anthropology, and biomechanics to emphasize its importance and published its conclusions as a report in 2007. The purpose of this chapter is to explain the main points of the kinematics of DTM and to further explore the clinical implications of DTM.

KINEMATICS OF THE DART-THROWING MOTION

The DTM plane can be defined as that in which functional oblique wrist motion proceeds, specifically from radial extension (radial deviation-extension) to ulnar flexion (ulnar deviation-flexion) ( Fig. 4-2 ).

The oblique plane of DTM depends on several anatomic factors. The DTM uses the midcarpal joint to a considerable degree. Among anatomic factors in the midcarpal joint, the scaphotrapeziotrapezoidal (STT) joint is a key factor that stabilizes and controls DTM.

The distal scaphoid surface contains an obliquely oriented ridge, the orientation of which guides STT motion in a semiconstrained fashion ( Fig. 4-3 ). Any motion at the STT joint mostly proceeds along the oblique direction of DTM. The two major ligaments connecting the distal scaphoid to the distal row do not originate in the most palmar apex of the scaphoid tuberosity. The scaphocapitate ligament inserts on the medial side of the scaphoid tuberosity, whereas the STT ligament inserts on the anterolateral side of the scaphoid tuberosity ( Fig. 4-4 ). The line that connects these two insertion sites is perpendicular to the oblique-sagittal ridge of the STT joint, indicating that they function as collateral ligaments during monoaxial articulation. A DTM might be the most stable and controlled wrist motion and is explained by the anatomy and kinematics of the STT joint.

The first three-dimensional (3-D) kinematics of the STT joint was initially investigated in cadavers. These studies showed that STT motion proceeded from the radiodorsal to the ulnopalmar direction during both wrist flexion-extension and radioulnar deviation and that the direction is almost parallel with the plane of the trapeziotrapezoid articulation. Recent 3-D motion analysis technologies ( Fig. 4-5 ) have enabled 3-D quantitation of wrist oblique motion in vivo, and detailed information of the functional oblique motion has been obtained. Isolated STT motion during wrist flexion-extension, radioulnar deviation motion, and DTM are very similar, and the direction is almost parallel with the plane of DTM ( Fig. 4-6 ). The axis runs obliquely from the radiopalmar aspect of the scaphoid tuberosity to the ulnodorsal aspect of the hamate ( Figs. 4-6 and 4-7 ). These findings indicated that the STT joint is essentially uniaxial and that it moves in a DTM plane of motion.

The essential plane of overall midcarpal motion was also apparently the DTM plane, whereas the directions of motion of the lunocapitate and triquetrohamate joints somewhat differ among wrist flexion-extension, radioulnar deviation motion, and DTM. Regardless of the types of wrist motion, all animations of the midcarpal motion showed that most of the joint surfaces that contact the midcarpal joint form an imaginary “ovoid” with a major axis that runs obliquely in the radiopalmar to ulnodorsal direction ( Fig. 4-8 ). Thus, the shape of the midcarpal joint appears to allow this joint to rotate smoothly and congruently in an oblique plane of DTM around the major axis of the ovoid, which runs obliquely in a radiopalmar to ulnodorsal direction. In an oblique view from the ulnodorsal side of the wrist where the axis of the midcarpal ovoid is perpendicular to the picture (see Fig. 4-8 D), the outlines of the surfaces of the STT, lunocapitate, and triquetrohamate joints assume a C shape that is a part of a concentric circle. This view corresponds to the semisupinated x-ray view ( Fig. 4-9 ), in which the center of the C appears to coincide with the major axis of the ovoid. The C shape of the outlines of the midcarpal joint surface remain constant during DTM in the semisupinated view.

Based on these findings, a model was created to explain the basic kinematics of the midcarpal joint ( Fig. 4-10 ). When a joint is cylindrical, it is easy to imagine that the joint is uniaxial ( Fig. 4-10 A). Even when the shape is as complex as that of the midcarpal joint in which all cross-sections of the joint surfaces are concentric circles, the joint can still be uniaxial ( Fig. 4-10 B). I postulate that this type of device is furnished obliquely in the human midcarpal joint ( Fig. 4-10 C). Strictly speaking, however, this theory is not always true because there are intercarpal motions between the proximal row bones.