Sensory innervation of the subregions of the scapholunate interosseous ligament in relation to their structural composition.

PURPOSE: To investigate the different types of innervation and the presence of mechanoreceptors in relation to the structural composition of the scapholunate interosseous ligament and to correlate the findings with the known mechanical properties of the ligament subregions. METHODS: Six fresh cadaveric scapholunate interosseous ligaments were divided into their 3 subregions: dorsal, palmar, and proximal. The microscopic features were investigated with use of a standard hematoxylin-eosin stain and immunostains for S-100, neurofilaments, neuron-specific enolase, protein gene product 9.5, CD31, and smooth muscle actin. The connective tissue structural composition and the presence of blood vessels and neural structures (myelinated and unmyelinated nerve fibers and mechanoreceptors) were investigated. The macroscopic anatomic details were also noted. RESULTS: The palmar subregion consists of structured, densely collagenized tissue at the core, surrounded by looser connective tissue. Myelinated nerve fibers forming fascicles accompany the interspersed blood vessels inside the ligament substance. Their concentration is greater in the proximal part of the palmar subregion, reaching a distance of approximately 150 mum from the ligament free surface. The dorsal subregion has similar structure to the palmar one, but the fibrous tissue ratio and density are higher. The proximal subregion consists of chondroid matrix and of loose connective tissue at its core. The radio-scapholunate ligament insertion is noted at the palmar aspect of the proximal subregion. Pacinian and other sensory corpuscles were found mostly at the palmar and proximal subregions. CONCLUSIONS: The scapholunate interosseous ligament is a richly innervated ligament that contributes to carpal proprioception, a fundamental element of dynamic wrist stability. The palmar subregion, apart from its major mechanical role, contains the greatest amount of the neural structures and mechanoreceptors. The dorsal subregion, with densely packed collagen fibers and limited innervation, functions mainly to constrain the scaphoid-lunate relative motion.

Evidence of wrist proprioceptive reflexes elicited after stimulation of the scapholunate interosseous ligament.

PURPOSE: Recent publications on the sensory innervation of wrist ligaments have challenged our understanding of ligaments as mere passive restraints in wrist stability. Mechanoreceptors in ligaments have a role in signaling joint perturbations, in which the afferent information is believed to influence periarticular muscles. The scapholunate interosseous ligament is one of the most richly innervated ligaments in the wrist. The purpose of our study was to investigate the possible existence of a wrist proprioceptive reflex, by which afferent information elicited in the scapholunate interosseous ligament was hypothesized to influence the muscles moving the wrist joint. METHODS: Nine volunteers (4 women and 5 men; mean age, 26 years; range, 21-28 years) participated in this study. Using ultrasound guidance, a fine-wire electrode was inserted into the dorsal scapholunate interosseous ligament and stimulated with four 1-ms pulses at 200 Hz. Electromyographic activities in extensor carpi radialis brevis, extensor carpi ulnaris, flexor carpi radialis, and flexor carpi ulnaris muscles were recorded using surface electrodes with the wrist actively positioned in isometric extension, flexion, and radial and ulnar deviation. The average EMGs from 30 consecutive stimulations were rectified and analyzed using the Student's t-test to compare the prestimulus (t(1)) and poststimulus (t(2)) EMG activities. RESULTS: Statistically significant changes in poststimulus EMG activity (t(1)- t(2)) were observed at various time intervals. Within 20 ms, an excitation was seen in the flexor carpi radialis and flexor carpi ulnaris in extension, radial and ulnar deviation, and in extensor carpi radialis brevis in flexion. Co-contractions between agonist and antagonist muscles were observed, with peaks around 150 ms after stimulus. CONCLUSIONS: We present evidence of wrist ligamento-muscular reactions. The early-onset reactions may serve in a joint-protective manner, and later co-contractions indicate a supraspinal control of wrist neuromuscular stability. These findings contribute new information to the physiologic functions of the wrist joint, which may further our understanding of dynamic wrist stability and serve as a foundation for future studies on proprioceptive dysfunctions after wrist ligament injuries.