7. Identify the major joints and associated ligaments in the shoulder region. Explain what kind of movements occur at the joints and other pertinent information.
The acromioclavicular (AC) joint is located at the articulation of the acromial end of the clavicle and acromion of the scapula. It is an example of a plane joint (gliding/sliding of scapula on clavicle).
- Accessory structures:
- Acromioclavicular ligaments are intrinsic ligaments and help to strengthen the joint capsule. There are superior and inferior divisions.
- Coracoclavicular ligaments are extrinsic ligaments and are vital to the organization and support of the AC joint [often considered the strongest ligament of the AC joint].
- Articular disc
- Fibrocartilaginous disc that becomes progressively less complete (particularly in the inferior and center portions) with progressive age.
- Clinical significance: The term shoulder separation is synonymous with acromioclavicular joint injuries.
The glenohumeral (shoulder) joint is the most mobile [ball and socket] joint in the body, and with this high level of mobility, there is decreased stability. Accessory structures play a dominant role in structural stability. The glenohumeral joint is formed by the head of the humerus and glenoid cavity of the scapula. The glenoid cavity is very shallow and accepts only ~1/3rd of the humeral head.
- Accessory structures:
- Rotator cuff tendons are the main source of stability for the glenohumeral joint. These tendons almost completely surround the joint with a notable and clinically important inferior deficiency.
- Long head of biceps brachii tendon attaches to the supraglenoid tubercle of the scapula, which is within the articular capsule, making this tendon intracapsular.
- Glenohumeral ligaments: 3 anterior sets (superior, middle, & inferior); observed on internal portion of articular capsule
- Coracohumeral ligament: extends from coracoid process of scapula to the greater & lesser tubercles of the humerus
- Transverse humeral ligament: connects the greater & lesser tubercles of humerus; the long head of biceps brachii tendon runs between the tubercles [intertubercular sulcus/bicipital groove] and deep to this ligament
- Coraco-acromial ligament: part of the coraco-acromial arch [between coracoid process and acromion of scapula] over the shoulder region and helps prevent superior dislocations of glenohumeral joint
- [Glenoid] labrum
- Fibrocartilaginous lip circumscribing the glenoid cavity, which helps to provide a better fit of the humeral head.
- Subacromial (subdeltoid) bursa: Located deep to the coraco-acromial arch and deltoid m., and superficial to the supraspinatus tendon and joint capsule. It facilitates movement of the supraspinatus tendon deep to the coraco-acromial arch. It does not typically communicate with the joint capsule.
- Subtendinous bursa of subscapularis muscle: Located between the tendon of subscapularis m. and articular capsule. It protects the tendon of subscapularis as it passes across the neck of the scapula. This bursa is unique in that it typically communicates with the articular cavity via an opening of the articular capsule typically located between the superior and middle glenohumeral ligaments.
- Clinical significance:
- Shoulder dislocations occur at the glenohumeral joint. Most dislocations occur inferiorly (due to a weak point in the musculotendinous rotator cuff). Clinically, dislocations are described as anterior (most frequent) or posterior dislocations.
- Rotator cuff injuries are frequent causes of shoulder pain, particularly in cases over repetitive actions of rotator cuff muscles. The supraspinatus tendon (tendonitis) is the most commonly affected area.
- Adhesive capsulitis [frozen shoulder] involves scarring/fibrosis between the articular capsule and surrounding structures, and often associated with other pathologies/injuries of the glenohumeral joint.