Vibration-induced muscle injury in the hand - experimental and clinical studies

Sammanfattning: Exposure to vibration is known to cause vasospastic white fingers and/or sensorineural symtoms such as numbness and tingling in fingers and/or musculoskeletal disorders such as reduction in muscle force, fatigue and reduced endurance. Classifications for the vasospastic and sensorineural symptoms are established. The general purpose of this thesis was to test the hypothesis that vibration exposure causes muscle injury. In an experimental model, muscle biopsies from hind paws of rats demonstrated that short-term vibration exposure caused damage to the muscles directly exposed to vibration stimuli. Degenerative changes were observed after two and regenerative activity after five days of exposure. Morphological changes, indicated by increased percentage of fibres with internal myonuclei and increased muscle fibre cross-sectional area, were differentiated between muscle fibre types and also related to the physical characteristics of the vibration. The level of tissue displacement was demonstrated to be a crucial factor for the severity of muscle damage. Biopsies from a human hand muscle, the abductor pollicis brevis (APB) muscle, in 20 patients suffering from hand-arm vibration syndrome (HAVS), and thus subjected to long-term vibration exposure from hand-held vibrating tools, demonstrated several morphological changes reflecting damage to both muscle fibres and motor nerves. These findings can well explain the commonly observed loss of hand strength despite well-preserved muscle volume in these patients. In two populations, in total 102 vibration-exposed workers with and without the classified symptoms of HAVS, measurements of muscle strength in grip (mainly extrinsics) and in various hand muscles (intrinsics) revealed greater loss in intrinsic hand muscles than in extrinsic muscles. However, this greater loss of intrinsic muscle strength was not demonstrated to be determinant for subjective hand weakness but might be of importance for the impaired dexterity often noted. The greatest strength reduction was demonstrated for radial abduction of the index finger. Therefore, the first dorsal interosseus (IOD I) muscle is suggested as an important and reliable indicator of intrinsic muscle dysfunction and recommended to be tested in long-term vibration exposed individuals. A linear dose-response relationship between duration of exposure and morphological abnormalities as well as impairment of muscle function could not be demonstrated. The results from these studies clearly indicate the incompleteness of the present classifications for vibration-induced symptoms and emphasise the need to also include symptoms of muscular origin.