Introduction: Frozen shoulder affects 2 to 5% of the population. Patients experience pain and limited shoulder range of motion (ROM) for an average of 30 months. A 50% reduction in internal and external rotation as well as shoulder elevation are used as diagnostic criteria. The aim of this study is to find if the structural changes in glenohumeral ligaments and the coracohumeral ligament described in literature can account for this reduction in ROM.
Method: An existing shoulder model build in OpenSim was used for this study, to which ligaments were added. The attachment sites of the ligaments as found in literature were matched to positions on the bone surfaces in the model. The rest lengths of the ligaments was determined so that the glenohumeral ROM described in literature could be achieved with less than 5% strain on the ligaments. An increase in ligament stiffness of 50% and decrease in length of 20% was associated with frozen shoulder. Four additional models were created with different attachment sites that are within the anatomical variance.
Results: Shortening the ligaments by 20% results in an average loss of achievable poses of 72.6% for each plane of elevation. The average loss of internal and external rotation over all planes of elevation is 46.0% and 49.3% respectively, while the loss of shoulder elevation is only 31.8%. The role of stiffness changes on the achievable ROM is limited, where a 50% stiffness increase results in a 5.5% loss of available ROM. The loss of internal rotation across all models ranges from 13.3% to 51.6%, from 44.5% to 61.1% for external rotation and 9.6% to 31.8% for shoulder elevation.
Conclusion: Stiffness changes are not enough to account for the loss of ROM associated with frozen shoulder. Shortening the ligaments by 20% results in a loss of ROM of close to 50%. Combining stiffness and length changes would result in a loss greater than 50%. Certain ligament configurations are more susceptible to length changes, which could make them more prone to developing frozen shoulder.