Background Shoulder immobilization can induce adhesion of the joint,capsular contracture or lead to the condition of frozen shoulder.However,little is known about the histological effects of immobilization on the shoulder joint.This study aimed to explore the effect of immobility on the subscapular bursa (SSB) and the joint capsular content,including the distribution of types Ⅰ and Ⅲ collagen,within an immobilized rat shoulder.Methods Forty-six Sprague-Dawley rats were randomly divided into one control group (n=6) and four immobilization groups (n=10 in each group),in which the left shoulders were immobilized with plaster for 1,2,3 and 4 weeks.At the end of each time point,2 rats from each group were euthanized and shoulders prepared for serial histological observations of the glenohumeral joints,as well as picrosirius red and immunohistochemical observation of type Ⅲ collagen.Histological sections of the remaining rat shoulders were used for the immunohistochemical detection of the capsular content of types Ⅰ and Ⅲ collagen.Results The hyperplastic synovium of the anterior capsule obstructed the communication between the SSB and the glenohumeral joint cavity at 2 and 3 weeks.The adhesion of the SSB appeared at 3 and 4 weeks.The quantitative and qualitative results showed that the capsular contents of types Ⅰ and Ⅲ collagen progressively increased at 2,3 and 4 weeks,and that type Ⅲ collagen was distributed extensively within the joint capsule at 2 and 3 weeks.Conclusion Immobilization of the rat shoulder induced synovial hyperplasia of the joint capsule,adhesion of the SSB and an increase of the capsular content of types Ⅰ and Ⅲ collagen.