Complex interplay: anatomy of the shoulder joint and shoulder girdle
The shoulder is the most mobile joint in the human body. The most important bones of the shoulder joint are the scapula, humerus and clavicle. The shoulder consists of four distinct joint components:
- The glenohumeral joint is the main shoulder joint. This ball-and-socket joint is formed by the proximal head of the humerus and a depression in the shoulder blade (the glenoid). It allows many different movements in varying directions.
- Acromioclavicular joint: The acromioclavicular joint connects the collarbone (clavicle) and the shoulder blade (acromion). It helps stabilise the shoulder. It allows several movements, such as raising and lowering the arm above the shoulder girdle.
- Sternoclavicular joint: The sternoclavicular joint is the connection between the collarbone and the sternum. It plays an important role in the transmission of mechanical forces between the upper body and the arms – it also allows e.g. the arms to be lifted above the head.
- There is no joint connection in the conventional sense between the shoulder blade and the chest wall (thoracic wall). Instead, there are a variety of muscles, tendons, and ligaments that stabilise the scapula and control its movements. This movable layer between the shoulder blade and the chest wall (thoracic wall) enables the coordination of complex movements.
Like any joint, the main shoulder joint is surrounded by a joint capsule. It stabilises the shoulder joint and also generates the necessary nutrients for the joint cartilage. The rotator cuff holds the humeral head in place within the shoulder socket when the shoulder moves. The shoulder joint is located in the joint socket (glenoid), while the joint surfaces that rub against each other are covered with cartilage. The cartilage serves as a shock absorber and friction-protection layer. In addition, there are several bursa in the shoulder area, which are important for allowing the joint to move smoothly. They are fluid-filled bags located between bones and soft tissues that help reduce friction and redistribute pressure.
The rotator cuff: a stabilising group of muscles, tendons and ligaments
The rotator cuff guides and stabilises the shoulder joint. It consists of muscles and tendons embracing the head of the humerus, supporting the stability and mobility of the shoulder joint.
The form of the ligaments in the shoulder joint is relatively rudimentary. The better formed a ligament is, the higher the stability it provides.
Instead, joint stability needs to be provided by the rotator cuff guiding the movements of the muscles:
- The supraspinatus muscle originates on the upper rear side of the shoulder blade (supraspinous fossa of the scapula) and extends under the acromion to the lateral humeral head (greater tubercle of the humerus). It raises the arm sideways, especially in the starting phase of the movement when the arms are resting against the body. It also plays a minor role in external rotation of the arm.
- The infraspinatus muscle originates on the lower rear side of the shoulder blade (infraspinous fossa of the scapula) and extends to the greater tubercle of the humeral head, slightly behind the supraspinatus muscle. It is the strongest external rotator of the upper arm.
- The teres minor muscle has its origin on the outside edge of the shoulder blade (lateral margin of the scapula) and extends to the major tubercle as well. Functionally, it is a weak external rotator of the upper arm and also plays a role in pulling the upper arm towards the body.
- The subscapularis muscle originates on the front side of the shoulder blade (subscapular fossa of the scapula) and attaches to the front side of the humeral head (lesser tubercle). When it tenses, it rotates the upper arm inwards (strongest internal rotator of the upper arm) and pulls it towards the body (adductor).
Shoulder injuries, conditions and dysfunctions
Wear, tear and inflammation of the shoulder can cause pain and discomfort such as arthritis (inflammation) of the shoulder joint (omarthrosis), inflammation of the bursa (bursitis), shoulder stiffness (frozen shoulder) or circulatory disorders and calcium deposits in the shoulder tendons (calcific tendinitis). Other shoulder indications:
- Impingement syndrome: Tendon(s) trapped between the humeral head and the acromion
- Omarthrosis: Wear and tear of the shoulder joint due to degradation of cartilage tissue
Blunt trauma such as a fall or accident can cause acute shoulder injuries such as fractures of a bone or bones, as well as injuries to the muscles, tendons and ligaments. Other typical shoulder injuries are:
- Shoulder dislocation: When the humerus pops out of the shoulder joint
- Rotator cuff rupture: a tearing of one or more tendons of the rotator cuff
- Humeral head fracture: A fracture of the head of the humerus
- Acromioclavicular joint separation: Ligaments of the joint get overstretched or even tear entirely
Diagnosis & treatment
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