TBL4 - Shoulder Joint Flashcards
What is the glenohumeral joint?
1) The glenohumeral joint, or shoulder joint, is a multiaxial synovial ball and socket joint and involves articulation between the glenoid cavity (fossa) of the scapula (shoulder blade) and the head of the humerus (upper arm bone)
2) The glenoid cavity only accepts a third of the humeral head
3) Due to the very limited interface of the humerus and scapula, it is the most mobile joint of the human body
What makes the glenoid cavity deeper for a less superficial connection between the head of the humerus and the scapula?
The glenoid labrum, concentric rings of Type I collagen fibers, attaches to the rim of the glenoid cavity to slightly but effectively deepen it
What is the glenohumeral fibrous capsule? What helps to reinforce the capsule?
1) The shoulder joint has a loose capsule, composed of loose connective tissue, that surrounds the glenohumeral joint, that is lax inferiorly and therefore is at risk of dislocation inferiorly
2) A number of bursae in the capsule aid mobility
3) The glenohumeral ligaments are three fibrous bands made up of dense connective tissue that reinforce the anterior part of the joint capsule
4) Note: The long head of the biceps brachii muscle travels inside the capsule to attach to the supraglenoid tubercle of the scapula
What is the function of the rotator cuff muscles?
The prime function of these muscles and the musculotendinous rotator cuff is to hold the relatively large head of the humerus in the much smaller and shallow glenoid cavity of the scapula
How would loss of rotator cuff muscle tone affect the glenohumeral joint?
1) The absence of muscle tone in an unconscious patient may allow joints to be dislocated as he or she is being lifted or positioned
2) When a muscle is denervated (loses its nerve supply), it becomes paralyzed (flaccid, lacking both its tonus and its ability to contract phasically on demand or reflexively)
3) In the absence of a muscle’s normal tonus, that of opposing (antagonist) muscle(s) may cause a limb to assume an abnormal resting position
4) In addition, the denervated muscle will become fibrotic and lose its elasticity, also contributing to the abnormal position at rest
What is the coracoid process?
1) The coracoid process is a small hook-like structure on the lateral edge of the superior anterior (anterolateral) portion of the scapula
2) Pointing laterally forward (towards the glenohumeral joint), it, together with the acromion, serves to stabilize the shoulder joint
3) It is palpable in the deltopectoral groove between the deltoid and pectoralis major muscles
What are the scapular spine, acromion, & acromioclavicular joint?
1) The spine of the scapula or scapular spine is a prominent plate of bone, which crosses obliquely the medial four-fifths of the scapula at its upper part
2) The acromion is a bony process on the scapula (shoulder blade). Together with the coracoid process it extends laterally over the shoulder joint
3) The acromion is a continuation of the scapular spine, and hooks over anteriorly. It articulates with the lateral end of the clavicle (collar bone) to form the acromioclavicular joint (superior to the glenohumeral joint)
4) The posterior surface of the scapula is unevenly divided by the thick bony spine that continues laterally as the flat expanded acromion
5) Note: The medial end of the clavicle attaches to the sternum
How does the acromioclavicular (AC) joint allow for a greater freedom of limb motion?
1) The AC joint enables the clavicle to act as a strut (rigid support) suspending the scapula and allowing it to slide along the posterior thoracic wall
2) By keeping the upper limb away from the thoracic wall, the strut further maximizes the limb’s freedom of motion
What is the coraco-acromial arch and what is its function?
1) The coraco-acromial ligament interconnects the coracoid process and acromion of the scapula to form the strong coraco-acromial arch (Coracoid process + coraco-acromial ligament + acromion = coraco-acromial arch)
2) The arch overlies the humeral head and prevents its superior displacement from the glenoid cavity
What does the acromioclavicular ligament do? What does the coracoclavicular ligament do in addition to the acromioclavicular ligament?
1) The thin acromioclavicular ligament reinforces the superior aspect of the AC joint capsule
2) Envision the stronger coracoclavicular ligament prevents dislocation of the AC joint thereby insuring normal strut function of the clavicle
Why do forceful superior thrusts of the humerus typically fail to dislocate the glenohumeral joint but fracture the humeral shaft or clavicle?
1) Remember the coraco-acromial ligament forms a protective arch that overlies the humeral head, preventing its superior displacement from the glenoid cavity
2) The coracoacromial arch is so strong that a forceful superior thrust of the humerus will not fracture it; the humeral shaft or clavicle fractures first
Why do movements of the medial and lateral fragments of a fractured clavicle result in dropping of the shoulder?
1) Because of the subcutaneous position of the clavicle, the end of the superiorly directed fragment is prominent—readily palpable and/or apparent
2) The trapezius muscle is unable to hold the lateral fragment up owing to the weight of the upper limb; thus, the shoulder drops
3) In addition to being depressed, the lateral fragment of the clavicle may be pulled medially by the adductor muscles of the arm, such as the pectoralis major. Overriding of the bone fragments shortens the clavicle
What is a greenstick fracture?
1) A fracture of the clavicle is often incomplete in younger children—that is, it is a greenstick fracture, in which one side of a bone is broken and the other is bent
2) This fracture was so named because the parts of the bone do not separate; the bone resembles a tree branch (greenstick) that has been sharply bent but not disconnected
When does dislocation of the acromioclavicular joint occur and why does physical examination of the injury lead to its description as a “shoulder separation”?
1) Although its extrinsic coracoclavicular ligament is strong, the AC joint itself is weak and easily injured by a direct blow
2) Dislocation of the AC joint can result from a hard fall on the shoulder or on the outstretched upper limb
3) An AC joint dislocation, often called a “shoulder separation,” is severe when both the AC and coracoclavicular ligaments are torn
4) When the coracoclavicular ligament tears, the shoulder separates from the clavicle and falls because of the weight of the upper limb
5) Rupture of the coracoclavicular ligament
allows the fibrous layer of the joint capsule to be torn so that the acromion can pass inferior to the acromial end of the
clavicle
6) Dislocation of the AC joint makes the acromion more prominent, and the clavicle may move superior to this process
Why do most dislocations of the humeral head occur in an inferior direction, and why are such dislocations commonly described clinically as anterior dislocations?
1) Because of its freedom of movement and instability, the glenohumeral joint is commonly dislocated by direct or indirect injury
2) Because the presence of the coraco acromial arch and support of the rotator cuff are effective in preventing upward dislocation, most dislocations of the humeral head occur in the downward (inferior) direction
3) However, they are described clinically as anterior or (more rarely) posterior dislocations, indicating whether the humeral head has descended anterior or posterior to the infraglenoid tubercle and long head of the triceps
4) The head ends up lying anterior or posterior to the glenoid cavity
