Shoulder is not a joint itself. There are four joints gathering around the shoulder area.
Soft Tissue Mobilisation
So shoulder sits as a cloak over the ribcage and even a stiff rib or a tight back like tightness in LAT Dorsi or thoracolumbar fascia can cause the shoulder pain.
Early diagnosis, correct diagnosis and proper treatment is a must in shoulder pain. A shoulder pain can be due to frozen shoulder, a tear, a pinched structure, arthritis , instability or it can come down from the neck.
Sometimes a stiff shoulder can be treated as a frozen shoulder but the root can be some instability around it which can cause severe tightness of the muscles and hence stiffness of the shoulder.
An impinged shoulder structure if not treated can lead to a complete tear of a tendon or frozen shoulder. A proper release, stabilization and strengthening can give stability to a shoulder and hence full functioning of a shoulder girdle.