Shoulder joint is classified as a ball and socket joint that is made up of three primary bones that articulates at multiple places to form four joints. The bony architecture of shoulder joint is formed by:

  • Clavicle (the only horizontal S- shaped bone of the body, also known as collar bone)
  • Scapula (that forms the prominence of your shoulder blade)
  • Humerus (the funny bone of the arm)

In simple words, shoulder joint is formed by the bones that connect upper limb to the chest wall for rotational, 3-dimensional movements.

Shoulder joint complex is made up of four primary layers; the components of each layer are:

  • Inner most layer is made up of articulating bones that forms the shoulder joint
  • Second layer is the stabilizing layer that is formed by ligaments that are thick cords of connective tissue that adheres the adjoining bones to prevent injuries and dislocation. Primary ligamentous supports of shoulder joint include; acromioclavicular ligament, conoid ligament, coracoacromial ligament, superior transverse scapular ligament, coracohumeral ligament, transverse ligament of humerus and trapezoid ligament.
  • Third layer is strengthening and supporting layer that is formed by muscles
  • Outer most layer is neurovascular layer that is made up of complex and intricate network of nerves and blood vessels that nourishes the components of shoulder joint.

Components of Shoulder Joint:

Contrary to the common belief, shoulder joint is not a single joint. The three bones of the shoulder joint (listed above) articulates to form four joints, which are:

  1. Glenohumeral Joint: The glenohumeral joint (otherwise referred to as the primary shoulder joint) is formed by the cup-like hollow cavity of the scapula (also known as glenoid cavity) and ball of the humerus bone. The two bones articulates to allow free movement across the shoulder joint. The most mobile joint of the body allow movements like flexion and extension, adduction and abduction, internal and external rotation, as well as medial and lateral rotation.
  2. Acromioclavicular Joint: This joint is formed between acromion process of scapula and lateral end of clavicle bone. AC joint is responsible for further reinforcing Glenohumeral joint for flexibility and extended range of motion (especially in overhead rotation).
  3. Sternoclavicular (SC) joint: This joint is formed between sternum (chest bone) and medial end of clavicle.
  4. Scapulothoracic joint: This joint is formed between upper ribs and scapula.

Supports of the Shoulder joint:

Shoulder joint is a highly mobile joint with a wide range of motion. Additionally, the glenoid cavity of scapula is fairly shallow when compared to the ball of the humerus.In order to prevent injuries like dislocation or fracture of articulating bones, the shoulder joint is strengthened by connective tissue elements; such as:

  • Shoulder capsule: The tough fibrous connective tissue capsule encloses the articulating bones for optimal protection and strength. Shoulder capsule is lined by synovial membrane that secretes a lubricant (also referred to as the synovial fluid) that is responsible for friction-free movement and prevention of wear and tear related damage. A ring of cartilage (known as labrum) also surrounds the glenoid cavity for flexibility.
  • Rotator Cuff Muscles: Strategically aligned rotator cuff muscles strengthen the joints while facilitating effort-less activity. Additionally, antagonizing muscles prevent abnormal joint motion; thereby minimizing the risk of dislocation.
  • Shoulder Bursa:Additionally, subscapular shoulder bursa (anatomically located between the tendon of subscapularis muscle and shoulder capsule) and subdeltoid bursa (situated between deep deltoid muscle and joint capsule) also supports the shoulder joint.

What anatomical changes are observed in frozen shoulder?

Most cases of frozen shoulder are associated with an inciting event (such as an injury or surgery involving the shoulder joint); however, some cases may appear insidiously. The anatomical changes that are characteristic in the early stage of frozen shoulder are:

  • Restricted movement or activity at the Glenohumeral joint due to inflammation or atrophy of major muscles such as deltoid, spinatii and biceps brachi (1)
  • Stiffening or fibrosis of connective tissue that interferes with the circulation and delivery of oxygen/ nutrients to the inflamed tissues (thereby delaying the healing process)
  • Deposition of fibrin exudates in the capsular, muscular and synovial spaces leading to adhesions (hence the name adhesive capsulitis).

Research and clinical data suggests that inflammation and capsulitis significantly alter certain movements across the shoulder joint due to capsular adhesions and loss of synovial fluid; leading to difficulty or restriction of movements like (1):

  • External rotation and abduction
  • Internal rotation and dorsal flexion

Over time, the inflammatory process subsides and the capsular adhesions resolves spontaneously; ultimately restoring pain-free shoulder activity. Although, optimal restoration of functional stability and strength is usually achieved in most cases, a small percentage of frozen shoulder patients develop mild to moderate restriction of shoulder abduction that responds to chiropractic rehabilitation and strength training exercises.

References:

  1. Brand, R. A. (2008). Loss of scapulohumeral motion (frozen shoulder). Clinical Orthopaedics and Related Research®, 466(3), 552-560.