The shoulder is a very complex joint, as it consists of several joints that act together to ensure the large range of movement of the arm. Other parts of the shoulder consist of three bones (bone of the upper arm, shoulder blade, and collarbone), three muscle groups, as well as numerous tendons and ligaments for the joint capsule.
In a frozen shoulder, for example, the capsule and the synovial membrane of the shoulder joint are affected. In concrete terms, it involves a hardening and shrinkage of the connective tissue structures that form the joint capsule.
In comparison to the other joints, the shoulder can be dislocated relatively easily. The joint's socket is small in relation to the head of the humerus. Although this makes the joint enormously mobile, it also makes it unstable.