Wrist ligaments : Innervation patterns and ligamento-muscular reflexes

Abstract: Objectives: The principal role of wrist stability is to facilitate the dexterous hand in a multitude of functions. To achieve joint stability, in general, a fine interaction of both static and dynamic elements is of essence. While static joint stability is constituted by the anatomical congruity of joint surfaces and the ligamentous restraints acting to limit joint translations, the dynamic aspects of joint stability chiefly concern sensorimotor control of the muscular forces acting on a joint. Sensorimotor functions require a presence of sensory end organs in the joint, afferent projections from the joint to the spinal cord for fast neuromuscular control, as well as supraspinal integration of sensorimotor information. The wrist joint, with its complex anatomy and ability to facilitate hand functions, is hypothesized to possess innate sensorimotor functions, making it ideal for research on the dynamics of joint neuromuscular control. Methods: By using staining for HTX and the immunohistochemical markers p75, PGP 9.5, S100 and trkB the general morphology, ligament composition and presence of nerves and mechanoreceptors was analyzed in the dorsal radiocarpal (DRC), dorsal intercarpal (DIC), scaphotriquetral (STq), scapholunate interosseous ligament (SLIL), scaphotrapeziotrapezoid (STT), radioscaphoid (RS), scaphocapitate (SC), radioscaphocapitate (RSC), long radiolunate (LRL), short radiolunate (SRL), ulnocarpal (UC), ulnolunate (UL), lunotriquetral interosseous (LTqI), triquetrocapitate (TqC) and triquetrohamate (TqH) wrist ligaments (Papers I-III). Using ultrasound technique, a fine-wire electrode was inserted into the SLIL on 9 healthy volunteers. The ligament was thereafter stimulated and the EMG activity in four forearm muscles recorded while the wrist was kept in isometric flexion, extension, radial, and ulnar deviation (Paper IV). Results: Wrist ligaments have a variable degree of innervation, which is reflected in the general composition of the ligament. The radiovolar ligaments are primarily dense collagenous structures with little or no innervation. The dorsal and triquetral ligaments, on the other hand, have large loose connective tissue regions where nerves, mechanoreceptors and vessels abound. After stimulation of the SLIL, muscle reactions (excitatory or inhibitory) were observed in the forearm muscles at various time intervals. An immediate ligamentomuscular reaction was observed in extensor carpi radialis brevis (ECRB) in flexion and in flexor carpi radialis/ulnaris (FCR/FCU) in extension, radial and ulnar deviation. Later cocontraction reactions, with simultaneous activation of agonist/antagonist muscles, occurred around 150 ms after stimulation of the SLIL. Conclusions: Dense ligaments with sparse innervation are suggested to be primarily mechanically important ligaments. The dorsal and triquetral ligaments are, on the other hand, regarded as sensory important ligaments. These all emanate from the triquetrum and are, thus, able to signal in all wrist positions and motions. The immediate reactions in antagonist muscles are likely to have joint protective functions. Later cocontraction reactions indicate an integrated supraspinal control to stabilize the wrist joint.