Mechanics of the shoulder complex and clinical investigation in spinal cord injured individuals

The shoulder joint in spinal cord injured individuals plays a crucial role in providing independence for everyday life. Therefore, any damage or injury to structures of the shoulder should be avoided. Considering musculoskeletal disorders of the shoulder joint in wheelchair users, the pathomechanism begins with a muscular imbalance, that leads to glenohumeral instability, impingement disease, rotator cuff tears, and finally osteoarthritis. This pathomechanism is not just true for wheelchair users, but also for highly loaded shoulders in general, e.g. for throwing athletes. Therefore, well-being of the shoulder girdle musculature and especially of the rotator cuff which is responsible for the active glenohumeral guidance is the most important factor for avoiding the progression of shoulder complaints. Furthermore, only little is known about the incidence of osteoarthritis in the shoulders of wheelchair users and the impact of loading on degenerative changes in the shoulder joint is a controversial debate. The first study mechanically and clinically analyzed manual muscle tests which are used in orthopedics to test the integrity of the muscles. A shoulder model was developed to determine the muscular moment arms and internal moments. Furthermore, the maximal isometric force was measured individually to determine the external moment. This moment was compared to the model’s results and differences between paraplegic individuals and unimpaired controls were detectable. In the second study, the influence of different loading situations (paraplegic vs. quadriplegic individuals) on the morphology of the humeral articular cartilage were determined. To analyze the moment-generating capacity of the shoulder complex between paraplegic individuals (n = 9) and a control group (n = 9), maximum isometric shoulder strength was measured with IsoBex and the corresponding moment was determined, the shoulder integrity was analyzed using the Constant score and clinical tests, and a musculoskeletal model was built for each subject in order to discuss individual intermuscular mechanics. Tests which attempt to isolate the function of the m. supraspinatus were used, as it is the supraspinatus which is most frequently compromised in persons with rotator cuff pathology. Three abduction tests were analyzed with the arm in the neutral position (named Starter test), at 90◦ of scaption, i.e. at 90◦ of scapular abduction (Abd90 test), and at 90◦ of scaption and full internal humeral rotation (the commonly used Jobe test). To compute muscular moment arms and corresponding moments of the supraspinatus, infraspinatus, as well as of the middle and anterior deltoid, MRI shoulder data were segmented and incorporated into a CAD environment for simulation. Using information of muscular contact point, direction of force and center of rotation, moment arms were calculable. The maximum isometric force was computed by implementing tendon properties (cross-sectional area and specific tension at maximum muscular activity) as well as the force-length function of the muscle. Estimation of the error was performed to determine the accuracy of the internal moment arm and moment using the Gaussian error propagation with relative errors smaller than 15%. It was found that manual wheelchair users were not stronger in abduction, although their shoulder muscles are generally more highly loaded. This finding allows the assumption that other factors influenced the result. More shoulder complaints in a randomly selected group of manual wheelchair users than in age-matched controls were observed: the Constant score was lower (88% compared to 98%) and in 7 out of 9 paraplegic subjects shoulder pathologies were clinically diagnosed which are associated with the painful arc (pain during active abduction between 60◦ and 120◦). Therefore, it is speculated that muscular disorders and pain as well as an insufficient fixation in wheelchair users due to their decreased trunk stability reduce the ability to exert a force resulting in a reduced moment. This hypothesis is supported by the model’s results. A trend of higher internal but reduced external moments was found in wheelchair users compared to unimpaired controls in the abducted positions. As moment arms of shoulder muscles were highly dependent on arm position, a best-testsituation for isolated supraspinatus testing considering muscular moment arms and the internal moments was achieved in the Starter position. It was in this position, where mainly the supraspinatus and middle deltoid contributed to the moment, whereas the influence of the infraspinatus and anterior deltoid became relevant when the arm was abducted. A discrepancy between the model’s calculation and the external moment was observed in the Abd90 and Jobe positions. Whereas the internal moment was higher in the abducted positions than in the Starter position, the external moment was not. This difference might be mainly due to the neural activation pattern which was assumed to be maximal in each position. However, activation might decrease with the arm in 90◦ of scaption. Furthermore, the lateral stabilization of the upper body might play a crucial role in order to exert maximal abduction moments if the arm is placed in an abducted position. In the prevention of osteoarthritis, early detection of structural damage to the articular cartilage is necessary. The different level of loading on humeral cartilage thickness was investigated in paraplegic subjects with high loading (n = 11) and in quadriplegic individuals with reduced shoulder loading (n = 8) and compared to unimpaired controls (n = 9). Based on segmented MR images, cartilage thickness was derived from the three-dimensional reconstructed cartilage. For regional evaluation, the reconstructed cartilage was divided into sectors. Re-segmentation reproducibility was excellent with a mean coefficient of variation of 1%. No humeral articular cartilage thinning in any group was detected suggesting that wheelchair users are not predisposed to shoulder cartilage loss. This finding supports the proposed pathomechanism where omarthrosis is not the primary problem in patients with shoulder disorders. Even more, in paraplegic subjects thicker cartilage in the superior region of the humeral head was found, suggesting that cartilage thinning with age is locally reduced in the presence of increased loading. To conclude: musculoskeletal disorders of the shoulder joint in spinal cord injured individuals are primarily related to muscular disorders impeding maximum force exertion, whereas articular cartilage is less affected. The new method for calculation of moment arms and moments using MRI data and a CAD tool for simulation is suitable for any desired shoulder joint configuration and it can easy be transferred to other joints.