The proprioception and neuromuscular stability of the basal thumb joint

Sammanfattning: OBJECTIVES The basal thumb joint, or the first carpometacarpal joint (CMC1) is an enigmatic construction. The concavo-convex shape of the CMC1 and wide range of movement, allows for both precision and power grips and is particularly susceptible to the development of osteoarthritis. Previous theories regarding the development of CMC1 osteoarthritis have focused on the role of gender, excessive joint load and ligament laxity as causative factors. An emerging theory is the role of proprioception and sensorimotor functions that act to maintain adequate joint equilibrium. This concept constitutes the basis of this thesis with the intent on examining both healthy and osteoarthritic joints. Using microscopic, biomechanical and neurophysiological methods, we address the hypothesis that CMC1 holds proprioceptive and neuromuscular properties that are integral for proper thumb function. METHODS Patients with CMC1 osteoarthritis that were treated surgically with complete trapeziectomy and suspension arthroplasty were included. Two CMC1 ligaments were collected from these subjects: the anterior oblique ligament (AOL) and the dorsal radial ligament (DRL). Following immunofluorescent triple staining techniques using markers p75, PGP9.5 and DAPI, the ligamentous innervation and mechanoreceptor populations were analyzed (study I-II). In study III, biomechanical assessment of the role of isometric muscle load on CMC1 was performed by a FASTRAK® motion tracking device. In study IV, two intrinsic (the first dorsal interosseous muscle; DI, abductor pollicis brevis; APB,) and two extrinsic muscles (extensor pollicis longus; EPL, abductor pollicis longus, APL) were examined in healthy subjects using electromyographic (EMG) analysis during functional thumb tasks. Post-stimulus reactions following stimulation of the DRL were analyzed for each muscle and each thumb position. RESULTS The CMC1 ligaments investigated displayed a varying degree of innervation. AOL presented with little innervation and was mainly composed of incoherent connective tissue and scarce collagen fibers, whereas DRL presented with abundant innervation and well-organized collagen (study I). Unclassifiable mechanoreceptors were the most frequent type of receptor found in AOL and DRL. No significant difference in the innervation between the proximal and distal portions of these osteoarthritic ligaments was identified (study II). The first dorsal interosseous muscle (FDI or DI) produced the highest level of distal migration and the least dorsoradial translation of the base of the first metacarpal. APL was identified as the main destabilizer as it increases dorsoradial misalignment (study III). Following stimulation of the DRL, significant (p<0.05) post-stimulus changes were found in all four muscles (DI, APB, APL, EPL) and positions tested. A mass inhibitory reaction was observed during tip pinch whilst key pinch produced rapid co-contractions. During palmar pinch a rapid inhibitory response was evoked in antagonistic muscles (study IV). CONCLUSIONS Ligaments from osteoarthritic patients displayed alterations in distribution and type of mechanoreceptors as compared to previous studies on normal joints. The results reinforce DRL’s proprioceptive and stabilizing role for the CMC1 joint. Isometric load of CMC1 indicates that DI promotes joint congruency by reducing dorsoradial translation, whereas APL acts in an opposing manner as a destabilizing force. Ligamento-muscular reflexes were observed following the stimulation of DRL. Thus, the dorsoradial ligament, in addition to being the primary static stabilizer of the joint, also possesses proprioceptive qualities. The post-stimulus reactions detected in DI, APB, APL and EPL are indicative of protective ligamento-muscular pathways, which facilitate neuromuscular functions and the maintenance of joint stability. CLINICAL RELEVANCE Patients with CMC1 osteoarthritis constitute a large group. Their symptoms are of varying severity, where some can be addressed within the scope of primary care units, whilst others are in need of operative treatment. Patients with CMC1 osteoarthritis often present with impaired neuromuscular functions, reduced range of motion, weakness, joint and ligamentous pathology as well as pain, which affect their daily activities. Understanding the proprioceptive and neuromuscular characteristics of the basal thumb joint is therefore essential in deciphering the complex pathophysiology of the basal thumb joint.