The Joints of the Shoulder Girdle
The shoulder and arm bone are joined together by ligaments. The joints allow the shoulder to move your arm up and down, in circles, in front, and toward the back. You may think of the shoulder as one joint, but there are three in the pectoral girdle: sternoclavicular, acromioclavicular, and glenohumeral.
The sternoclavicular joint
The sternoclavicular joint is formed by the articulation of the sternal end of the clavicle with the manubrium of the sternum. It’s a strong joint, but it also allows mobility of the pectoral girdle.
The sternoclavicular joint is a synovial joint that has an articular disc and is surrounded by the joint capsule. The sternoclavicular joint is held together by a few ligaments:
Anterior and posterior sternoclavicular ligaments: These ligaments reinforce the joint in the front and back.
Interclavicular ligament: This ligament runs between the sternal ends of the two clavicles across the tops of the joints.
Costoclavicular ligament: This ligament attaches the bottom of the sternal end of the clavicle to the 1st rib.
The acromioclavicular joint
The acromioclavicular joint is a synovial joint formed by the articulation of the acromial end of the clavicle with the acromion of the scapula. The acromioclavicular joint allows the scapula to rotate on the clavicle.
The acromioclavicular joint includes a wedge-shaped articular disc and is surrounded by a fibrous joint capsule. It’s stabilized by the coracoclavicular ligament, which is composed of the conoid ligament and the trapezoid ligament and extends from the coracoid process of the scapula to the acromial end of the clavicle. The conoid ligament runs from the coracoid process to the conoid tubercle on the clavicle, and the trapezoid ligament runs from the coracoid process to the inferior part of the clavicle.
The glenohumeral joint
The glenohumeral joint joins the head of the humerus to the glenoid cavity. It’s a ball-and-socket joint that allows for a wide range of movement for the arm.
Because the glenohumeral joint is a ball-and-socket joint, you can adduct, abduct, extend your arm backward, or flex your arm forward. You can also rotate your arm medially or laterally and circumduct the arm .
Imagine your arm hanging down by your side with your elbow bent (so your forearm is parallel to the floor). If you move your forearm and hand toward the midline of your body, you’re medially rotating your arm in the glenohumeral joint. If you move your forearm and hand out away from your body in the opposite direction, then you’re laterally rotating the arm at the glenohumeral joint.
The glenoid cavity is lined with a ring-shaped piece of cartilage called the labrum, which forms a lip that helps the head of the humerus fit into the cavity a little more securely. Three ligaments help stabilize the joint:
Coracohumeral ligament: This ligament connects the base of the coracoid process to the front of the greater tubercle of the humerus.
Transverse humeral ligament: This ligament runs between the greater and lesser tubercles and forms a canal with the intertubercular sulcus (bicipital groove). The bicipital tendon runs through this canal.
Coracoacromial ligament: This ligament runs between the acromion and the coracoid process. This structure is also called the coracoacromial arch.
These ligaments can’t stabilize the glenohumeral joint alone. Additional support is provided by the muscles of the rotator cuff.
The following two bursae form cushions between the tendons and bones of the glenohumeral joint:
Subacromial bursa: Also called the subdeltoid bursa, this bursa is found between the acromion, the coracoacromial ligament, and the deltoid muscle and the supraspinatus muscle.
Subscapular bursa: This bursa is between the scapula and the tendon of the subscapularis muscle.