The scapulothoracic “joint” is made up of the scapula (shoulder blade) and its articulation with the ribcage. In between these structures are 2 muscles, namely the subscapularis (aka one of the rotator cuff muscles) and the serratus anterior. This “joint” is highly dependent on the integrity of the acromioclavicular joint and the sternoclavicular joint. The word joint is in quotations because the scapulothoracic articulation is not a true joint due to it not having the typical characteristics of a joint.
Joints are typically classified structurally into 3 categories. Those categories are the fibrous, cartilaginous, and synovial joint. Picture the sutures in the skull or the union between teeth and its socket when thinking about a fibrous joint. Fibrous joints are connected and fixed by dense/thick connective tissue. Cartilaginous joint are connected almost entirely by cartilage. Examples of this are the growth plates of long bones and the discs between the vertebrae. Lastly, the synovial joint joins 2 bones that are held together by a joint capsule, which is filled with synovial fluid acting as a lubricant. This type of joint is the most common in the body and what people typically picture when they think of a joint. For example, the hips, knees, shoulders, and knuckles are all synovial joints. Since the shoulder blade is not connected to the ribcage by cartilage or dense connective tissue, nor joined together by a capsule filled with synovial fluid, it is not a true joint. Not to say this articulation is unimportant.
As mentioned earlier, the acromioclavicular (AC) joint and the sternoclavicular (SC) joint are closely related with the scapulothoracic articulation because the shoulder blade attaches to the collarbone on the lateral end of the shoulder (acromioclavicular joint) and the collarbone attaches to the breastbone, which becomes the sternoclavicular joint. So any motion, good or bad, in the shoulder blade will affect both joints that are connected to the rest of the body. The main function of the scapulothoracic articulation is to center the socket part of the shoulder joint to the ball of the arm during elevation of the arm. It also adds range to the arm when reaching overhead. For every 2 degrees of arm movement the shoulder blade will also move 1 degree, which increases the total range the arm can reach overhead. This is called scapulohumeral rhythm. As you can see, a lot of joints are involved when reaching overhead.
The shoulder blade also is home to multiple muscle attachments. There are 18 muscles (if you separate the biceps) that have attachments to the shoulder blade so dysfunction in any of these muscles can cause impaired movement of the scapulothoracic articulation, the shoulder joint, the AC joint, and the SC joint. This will ultimately throw off the scapulohumeral rhythm. If this happens, it can lead to shoulder dysfunctions such as impingement syndrome, rotator muscle injury, decreased range of motion to the shoulder, and muscle pain from overuse.
As you can see, even though the scapulothoracic “joint” is not a true joint by definition, it is very important for the health of the shoulder. A skilled physical therapist will be able to assess your posture, resting position of the shoulder blade, and quality of movement of the shoulder blade on the ribcage. If you have shoulder pain or want to prevent shoulder pain, don’t overlook the scapulothoracic “joint!”