The body is made up of areas of regional interdependence, meaning that movement and function of one joint can be affected by a joint located elsewhere in the body. The shoulder, for example, is made up of a complex of 4 joints: the sternoclavicular, acromioclavicular, glenohumeral and scapulothoracic joints. It is the most mobile complex within the body, allowing us to interact with our world from multiple positions with a kinematic redundancy that is unparalleled by the rest of our body. If a negative change occurs with any of the aforementioned joints, the ability to effectively move our shoulder is greatly reduced, allowing for a greater likelihood of shoulder pain and altered function.
The one joint people focus on the least is the scapulothoracic joint, which is the interaction of the scapula (shoulder blade) and the thoracic rib cage. This joint is different, compared to the other shoulder complex joints, in that the thoracic spine and associated rib cage offers the scapula its surface upon which to move with no restraint except surrounding musculature. The thoracic spine and associated ribs need to be mobile to accommodate for the movement of the scapula, which moves as the arm is raised up in order to avoid potential shoulder impingement at the ball and socket joint (i.e. glenohumeral joint). If these thoracic spine and rib movements do not occur appropriately as the arm is raised overhead, then the other shoulder complex movements become affected, increasing the likelihood of future shoulder impingement and pain.
This can be easily demonstrated by sitting in an exaggerated slumped position with your shoulders rounded forward and attempt to raise both of your arms overhead. You will notice it is uncomfortable in the shoulder and that something impedes your ability to fully reach overhead. If you correct your posture and repeat the same motion, by sitting upright through the thoracic spine, the arms will move more freely and is more comfortable.
Furthermore, thoracic spine and shoulder joint mobility is also related to the proper functioning of several unifying muscles. Poor thoracic mobility, through slouched posture for example, affects the ability of many of these muscles to assist coordinated arm movements, and increases likelihood for shoulder injury. Patterns of muscle dysfunction have been shown in numerous studies to be associated with shoulder pain, and that improvement of muscle function can occur with improved thoracic and rib mobility and posturing.
As you can see, pain and stiffness in the thoracic spine can cause problems in more regions than just its own, which is why addressing it before it turns chronic can help prevent future injuries. Your physical therapist is qualified to evaluate your thoracic spine pain and assess its influence on other areas, such as the shoulder complex, and help come up with a plan of care appropriate for your condition.