Biomechanical Study of Work-Related Shoulder Disorder.

摘要

In the United States, there are over 50,000 new cases of atraumatic occupational shoulder injuries reported annually, with an enormous financial impact due to utilization of healthcare services, lost workdays and worker disability costs. Although the etiology of these injuries is clearly multifactorial, based on the available epidemiologic evidence, NIOSH has concluded that there is a direct link between repetitive work activities and occupational shoulder disorders. Furthermore, MRI data have confirmed that a high level of lifetime shoulder elevation represents a significant risk factor for damage to the rotator cuff tendons. While the mechanisms leading from repetitive elevation tasks to shoulder injuries are still under investigation, laboratory research has consistently shown that fatigue resulting from repetitive arm motion leads to deficits in proprioception and kinematics, which could ultimately result in abnormal joint loading and soft tissue injury. However, there is a gap in our knowledge regarding the relationship between these deficits and compromised rotator cuff function, especially in workplace settings. Dental hygienists suffer a high incidence of shoulder pathology that seems to increase with job longevity. It has been hypothesized that occupational injuries could be due to local muscle fatigue caused by repetitive low level work and awkward and constrained working postures. We studied shoulder motion patterns of dental hygienists before, during and after a typical workday. While we did not find any changes in proprioception (the ability to know where their shoulder was in space), we did find that they adapted a more forward leaning posture of their shoulder blade at the end of the workday. Additionally, they used their shoulders at higher elevation angles and with higher speeds than typical office workers for the same duration of work. The rotator cuff muscles are primarily responsible for stabilizing the humeral head at the shoulder joint, helping to prevent unwanted translations that could lead to impingement or dislocations. We studied the effects of a muscle paralysis of some of the rotator cuff muscles with the use of a nerve block (injection of lidocaine). The result showed that there is a compensatory increase in humeral head translation, shoulder blade rotation and deltoid muscle activation due to the nerve block. Taken together, these outcomes suggest that increasing muscular strength and endurance of the shoulder muscles could prevent abnormal shoulder motion patterns. This may be beneficial in preventing overuse injuries in workers that use their shoulders in an elevated posture.