Artificial shoulder. Coloured X-ray of the back of the right shoulder of a male patient, showing a prosthetic shoulder joint (yellow). The prosthesis is anchored in the humerus (upper arm bone) and covers its head. It is made to fit into the shoulder socket and rotate freely. The humerus articulates with the clavicle (collar bone, the uppermost bone at left) and the scapula (shoulder blade, below clavicle) to form the shoulder joint. Joints may need to be replaced due to injury or disease, such as arthritis.