Shoulder Joint Anatomy Flashcards
What is the glenohumeral joint?
The glenohumeral, or shoulder, joint is a synovial joint that attaches the upper limb to the axial skeleton. It is a ball-and-socket joint, formed between the glenoid fossa of scapula (gleno-) and the head of humerus (-humeral).
Is the shoulder joint the most mobile joint in the body?
Yes
What is the joint stabilised by?
This shoulder function comes at the cost of stability however, as the bony surfaces offer little support. Instead the surrounding shoulder muscles and ligamentous structures offer the joint security; the capsule, ligaments and tendons of the rotator cuff muscles. Because of this mobility-stability compromise, the shoulder joint is one of the most frequently injured joints of the body.
Describe the articulating surfaces of the shoulder joint
The shoulder joint is formed by the articulating surface of the head of the humerus with the glenoid cavity of the scapula forming the glenohumeral joint
The articular surfaces are covered in hyaline cartilage
What is the glenoid fossa?
What is the glenoid labrum, what does it do?
The glenoid fossa is a shallow pear-shaped pit on the superolateral angle of scapula. The concavity of the fossa is less acute than the convexity of the humeral head, meaning that the articular surfaces are not fully congruent.
Congruency is increased somewhat by the presence of a glenoid labrum, a fibrocartilaginous ring that attaches to the margins of the fossa. The labrum acts to deepen the glenoid fossa slightly, it is triangular in shape and thicker anteriorly than inferiorly.
The surface of the humeral head is three to four times larger than the surface of glenoid fossa, meaning that only a third of the humeral head is ever in contact with the fossa and labrum.
This incongruent bony anatomy allows for the wide range of movement available at the shoulder joint but is also the reason for the lack of joint stability. Instead, joint security is provided entirely by the soft tissue structures; the fibrous capsule, ligaments, shoulder muscles and their tendons.
What is the joint capsule of the shoulder?
The shoulder joint is encircled by a loose fibrous capsule. It extends from the scapula to the humerus, enclosing the joint on all sides. The internal surface of the capsule is lined by a synovial membrane.
Where does the capsule attach from?
On the humerus, the capsule attaches to its anatomical neck. Extending only at its medial margin, where the fibers protrude by around 1 cm. On the scapula, the capsule has two lines of attachments. The first is on its anterior and inferior sides where the capsule inserts into the scapular neck, posterior to the glenoid labrum. The second is on its superior and posterior aspects, where the capsular fibers blend directly with the glenoid labrum. Here the capsule arches over the supraglenoid tubercle and it’s long head of biceps brachii muscle attachment, thus making these intra-articular structures.
Does the capsule remain lax?
The capsule remains lax to allow for mobility of the upper limb. It relies on ligaments and muscle tendons to provide reinforcement. The anterior capsule is thickened by the three glenohumeral ligaments while the tendons of the rotator cuff muscles spread over the capsule blending with its external surface. These tendons form a continuous covering called the rotator capsule. It is comprised of the supraspinatus superiorly, infraspinatus and teres minor posteriorly, subscapularis anteriorly and the long head of triceps brachii inferiorly.
What are the 2 weak spots of the joint capsule?
Two weak spots exist in this reinforced capsule. The first is the rotator interval, an area of unreinforced capsule that exists between the subscapularis and supraspinatus tendons. The second is the inferior capsular aspect, this is the point where the capsule is the weakest. The loose inferior capsule forms a fold when the arm is in the anatomical position. It becomes stretched, and least supported, when the arm is abducted.
What are the two openings to the capsule?
Between the greater and lesser tubercles of humerus, through which the tendon of the long head of biceps brachii passes.
Between the superior and middle glenohumeral ligaments, via which the subscapular bursa communicates with the glenohumeral joint cavity.
Describe the bursae of the shoulder joint
To reduce friction in the shoulder joint, several synovial bursae are present. A bursa is a synovial fluid filled sac, which acts as a cushion between tendons and other joint structures.
The bursae that are important clinically are:
Subacromial – located deep to the deltoid and acromion, and superficial to the supraspinatus tendon and joint capsule. The subacromial bursa reduces friction beneath the deltoid, promoting free motion of the rotator cuff tendons.
Subscapular – located between the subscapularis tendon and the scapula. It reduces wear and tear on the tendon during movement at the shoulder joint.
What are the 5 ligaments of the shoulder?
In the shoulder joint, the ligaments play a key role in stabilising the bony structures.
Glenohumeral ligaments (superior, middle and inferior) – the joint capsule is formed by this group of ligaments connecting the humerus to the glenoid fossa. They are the main source of stability for the shoulder, holding it in place and preventing it from dislocating anteriorly. They act to stabilise the anterior aspect of the joint.
Coracohumeral ligament – attaches the base of the coracoid process to the greater tubercle of the humerus. It supports the superior part of the joint capsule.
Transverse humeral ligament – spans the distance between the two tubercles of the humerus. It holds the tendon of the long head of the biceps in the intertubercular groove.]
Coraco–clavicular ligament – composed of the trapezoid and conoid ligaments and runs from the clavicle to the coracoid process of the scapula. They work alongside the acromioclavicular ligament to maintain the alignment of the clavicle in relation to the scapula. They have significant strength but large forces (e.g. after a high energy fall) can rupture these ligaments as part of an acromio-clavicular joint (ACJ) injury. In severe ACJ injury, the coraco-clavicular ligaments may require surgical repair.
The other major ligament is the coracoacromial ligament. Running between the acromion and coracoid process of the scapula it forms the coraco-acromial arch. This structure overlies the shoulder joint, preventing superior displacement of the humeral head.
What is the glenohumeral joint innervated by?
The glenohumeral joint is innervated by the subscapular nerve (C5-C6), a branch of the posterior cord of brachial plexus. The joint capsule is supplied from several sources;
Suprascapular nerve supplies the posterior and superior aspects
Axillary nerve innervates the anteroinferior part of the capsule
Lateral pectoral nerve supplies the anterosuperior part and the rotator capsule
Describe the blood supply to the shoulder joint
Blood supply to the shoulder joint comes from the anterior and posterior circumflex humeral, circumflex scapular and suprascapular arteries.
Describe movements of the shoulder joint
The glenohumeral joint has a greater range of movement (RoM) than any other body joint. Being a ball-and-socket joint, it allows movements in three degrees of freedom (average maximum glenohumeral active RoM is shown in brackets);
Flexion (110°) - extension (60°)
Abduction (120°) - adduction (0°)
Internal rotation (90°) - external rotation (90°)
Combination of these movements gives circumduction.
Activities of the arm rely on movement from not only the glenohumeral joint but also the scapulothoracic joint (acromioclavicular, sternoclavicular and scapulothoracic articulations). Together these joints can change the position of the glenoid fossa, relative to the chest wall. Thus repositioning the glenohumeral joint, and upper limb, within space. This provides for a greater range of motion available within the greater shoulder complex;
Flexion (180°) - extension (90°)
Abduction (180°) - adduction (30°)
Internal rotation (90°) - External rotation (90°)
The close-packed position of the glenohumeral joint is abduction and external rotation, while open packed (resting) position is abduction (40-50°) with horizontal adduction (30°). The joints’ capsular pattern is external rotation, followed by abduction, internal rotation and flexion. The additional accessory movements of spin, roll and slide (glide) are also available within the glenohumeral joint.
