The Shoulder Region

Part 2 · The upper limb

Four joints, the rotator cuff, and why stability here is muscular rather than bony

The shoulder is not a joint

Figure 1 · The four joints of the shoulder complex

It is a complex of four joints that must move in a coordinated sequence for the arm to reach overhead. Assess only the glenohumeral joint and you will miss most shoulder pathology.

Two further “pseudo-joints” are worth naming because pathology occurs in them: the subacromial (suprahumeral) space and the bicipital groove.

Learning outcomes

• Describe each of the four joints of the shoulder complex, with articular surfaces, capsule, ligaments and movements.
• Explain the stabilising mechanisms of the glenohumeral joint and rank them.
• Describe the rotator cuff muscles in full and explain their functions individually and as a group.
• Describe scapulohumeral rhythm quantitatively and explain the contribution of each joint to elevation.
• Describe the subacromial space, its contents and the modern understanding of subacromial pain.
• Describe the axilla, its boundaries and contents, and the brachial plexus in outline.
• Explain the mechanism, structures injured, and complications of anterior shoulder dislocation.
• Explain rotator cuff pathology, adhesive capsulitis, AC joint injury and scapular dyskinesis, and their rehabilitation.
• Perform and interpret the principal clinical tests of the shoulder, with their known diagnostic limitations.

The sternoclavicular joint

Clinical. Because the ligaments are so strong, force is usually transmitted past the joint and the clavicle fractures rather than the joint dislocating. When dislocation does occur, posterior dislocation is an emergency — the clavicle can compress the trachea, oesophagus, great vessels or brachial plexus. In adolescents, an apparent dislocation is usually a medial physeal separation (the medial epiphysis fuses at 22–25 years).

The joint’s axial rotation is obligatory for full elevation: fusing or restricting it limits arm elevation to roughly 110°.

The acromioclavicular joint

AC joint injury (Rockwood classification, abbreviated)

Assessment. Localised tenderness over the joint; pain in the high (>120°) painful arc; positive cross-body adduction (scarf) test; and pain on O’Brien’s active compression test. Degenerative AC change is nearly universal on imaging after 40 and correlates poorly with symptoms — treat the patient, not the report.

The glenohumeral joint

Figure 2 · The glenohumeral joint

Articular surfaces

A large hemispherical humeral head (retroverted ~30°, inclined ~130–140°) against a small, shallow, pear-shaped glenoid cavity with ~5° retroversion and slight upward tilt. The surface area ratio is roughly 3:1 or 4:1; only about one-third of the head contacts the glenoid at any time.

The glenoid labrum — a fibrocartilaginous rim — deepens the socket by around 50%, roughly doubles its depth, increases the contact area, acts as the attachment for the capsule and the long head of biceps superiorly and the inferior glenohumeral ligament inferiorly, and creates a suction seal.

Capsule and ligaments

The capsule is lax and voluminous — roughly twice the volume of the humeral head — and is redundant inferiorly (the axillary recess), which is what permits the range. It attaches to the glenoid margin (beyond the labrum) and to the anatomical neck of the humerus, except medially where it extends ~1 cm down the shaft.

Bursae: the subacromial–subdeltoid bursa (the largest in the body, and continuous in most people), the subscapular bursa (communicating with the joint through the foramen of Weitbrecht), the subcoracoid bursa, and the infraspinatus bursa.

Stability: why it is muscular

Static restraints (bony congruence, labrum, capsule, ligaments, negative intra-articular pressure) are important at end range but lax through the mid-range, where most function occurs. Mid-range stability comes from:

• Concavity–compression — the cuff compresses the head into the labrum-deepened concavity; the resulting stabilising force is proportional to the compressive force (Chapter 6).
• Rotator cuff force couples — subscapularis balanced against infraspinatus/teres minor in the transverse plane, and deltoid balanced against the inferior cuff in the coronal plane, converting translation into rotation.
• Scapular positioning — the glenoid must be positioned under the head; scapular muscle failure removes the platform.
• Proprioceptive feedback and reflex muscle activation.
• Adhesion–cohesion and negative intra-articular pressure — small but real; venting the capsule measurably increases inferior translation.

Movements and range

Close-packed position: full abduction with external rotation. Resting position: ~55° abduction, 30° horizontal adduction. Capsular pattern: external rotation > abduction > internal rotation.

The rotator cuff

Figure 3 · The rotator cuff

Four muscles whose tendons blend with the capsule to form a continuous cuff over the head, deficient only anteroinferiorly (the rotator interval, and the site through which most dislocations occur).

The single most important cuff concept: the cuff’s primary role is not to move the arm but to hold the humeral head centred on the glenoid so that the large muscles can move it. Deltoid’s line of pull in early abduction is almost vertical, and would translate the head superiorly into the acromion; the inferior cuff generates an opposing inferior and compressive force so that the resultant is a rotation, not a translation. Cuff failure allows superior migration of the humeral head, visible radiographically as a reduced acromiohumeral interval (<7 mm), and eventually cuff tear arthropathy.

Other muscles of the region

Scapulohumeral rhythm

Figure 4 · How the joint and the scapula share the work

Full arm elevation to 180° requires the coordinated contribution of all four joints.

Accompanying obligatory movements:

• Clavicular elevation at the sternoclavicular joint (~30–35°), then posterior axial rotation of the clavicle (~30–50°) as the coracoclavicular ligament tightens.
• Acromioclavicular rotation (~5–8° early, ~10–20° late).
• Scapular posterior tilt and external rotation, which are as important as upward rotation and are the components most often lost in shoulder pain.
• Glenohumeral external rotation, which is obligatory above ~90° of abduction: it clears the greater tubercle from beneath the coracoacromial arch and slackens the inferior capsule. Without it, elevation stops at approximately 120°.

Clinically: ask a patient to elevate with the arm held in internal rotation and watch it stop early. That is a demonstration, not a trick, and it explains why restoring external rotation is a prerequisite for restoring elevation.

Scapular dyskinesis is best described by pattern (inferior angle prominence, medial border prominence, excessive superior translation) and is best treated by addressing what causes it — pain, stiffness (especially pectoralis minor and posterior capsule), and weakness or mistiming of the serratus–trapezius force couple. It is common in asymptomatic athletes, so its presence alone is not a diagnosis.

The subacromial space

Roof: the coracoacromial arch — acromion, coracoacromial ligament, coracoid process, and the undersurface of the acromioclavicular joint. Floor: the humeral head and greater tubercle. Contents: the supraspinatus tendon, the upper part of infraspinatus, the subacromial–subdeltoid bursa, the long head of biceps tendon, and the superior capsule.

The space measures ~7–14 mm with the arm at the side and narrows during elevation, particularly between 60° and 120° — the painful arc.

From “impingement” to “subacromial pain syndrome”

The Neer model — that a hooked acromion mechanically abrades the cuff, causing progressive bursitis, tendinitis and tearing, treatable by acromioplasty — dominated shoulder practice for thirty years. It has not survived scrutiny:

• Subacromial decompression surgery performs no better than placebo surgery in high-quality randomised trials (CSAW, 2018; FIMPACT, 2018), and both perform similarly to exercise therapy.
• Acromial morphology correlates weakly with cuff pathology, and hooking may be a consequence of cuff dysfunction rather than a cause.
• Cuff tears are found in large proportions of asymptomatic people — around 20–25% overall, rising above 50% after age 65 — so a tear on imaging does not establish the source of pain.

The current framing is subacromial (or rotator cuff related) shoulder pain: a clinical syndrome with contributions from tendon pathology, load intolerance, muscle dysfunction, scapular and postural factors, and central pain mechanisms. The primary treatment is progressive, graded exercise, with corticosteroid injection offering short-term relief and surgery reserved for the minority.

This is worth teaching explicitly because the older model is still widely taught, and it leads clinicians toward passive treatment and unnecessary surgery.

The axilla

Figure 5 · The axilla and the brachial plexus

A pyramidal space between the upper limb and the thoracic wall — the neurovascular gateway.

Contents: the axillary artery and vein, the cords and branches of the brachial plexus, axillary lymph nodes (five groups: pectoral/anterior, subscapular/posterior, humeral/lateral, central, apical), the long thoracic and thoracodorsal nerves on the walls, the intercostobrachial nerve (T2), the axillary tail of the breast, and fat.

Clinical. The axillary nodes drain the breast, upper limb and adjacent trunk — hence axillary clearance in breast cancer, and hence the physiotherapist’s role in post-surgical shoulder range and lymphoedema management. The long thoracic nerve is vulnerable during axillary dissection (producing winging) and the intercostobrachial nerve is routinely sacrificed, producing medial arm numbness that patients should be warned about.

The axillary artery and its three parts

Divided by pectoralis minor: the first part has one branch (superior thoracic), the second two (thoracoacromial, lateral thoracic), and the third three (subscapular — the largest, anterior and posterior circumflex humeral).

The brachial plexus in outline

Roots C5–T1 → trunks (upper C5–6, middle C7, lower C8–T1) → divisions (anterior and posterior) → cords (lateral, posterior, medial, named for their relation to the axillary artery) → branches.

Mnemonic: Really Tired? Drink Coffee Before. The plexus is treated in detail in Chapter 24; for now know:

Erb’s palsy (C5–6, upper trunk — birth traction or a fall on the shoulder): loss of deltoid, supraspinatus, infraspinatus, biceps, brachialis, brachioradialis and supinator → the arm hangs adducted, internally rotated, elbow extended, forearm pronated — the “waiter’s tip” posture. Klumpke’s palsy (C8–T1, lower trunk — traction with the arm above the head): intrinsic hand muscle paralysis → claw hand; may include Horner’s syndrome if T1 sympathetic fibres are involved.

The quadrangular and triangular spaces

Clinical conditions

Anterior glenohumeral dislocation

~95–97% of shoulder dislocations. Mechanism: abduction, extension and external rotation — the arm forced back and out, as in a tackle or a fall on an outstretched abducted arm.

Clinical picture:

• the arm held in slight abduction and external rotation, unable to internally rotate
• a squared-off shoulder with loss of the deltoid contour
• a palpable gap beneath the acromion. Recurrence risk is strongly age-dependent — well over 70–90% in patients under 20, falling to below 20% over 40

Posterior dislocation is rare (~2–4%) but classically missed: it follows seizures, electric shock and posterior blows, and the arm is held in internal rotation with no external rotation possible. The AP radiograph can look almost normal — the “light bulb sign” — so an axillary or scapular-Y view is mandatory.

Inferior dislocation (luxatio erecta) presents with the arm fixed overhead and has a high rate of neurovascular injury.

Rotator cuff pathology

A spectrum from reactive tendinopathy through partial-thickness to full-thickness tearing, with age the dominant risk factor. Supraspinatus is most commonly involved.

Assessment uses a battery rather than a single test, because individual special tests have poor diagnostic accuracy in isolation:

Management is progressive loading — isometrics initially where irritable, then graded cuff and scapular strengthening with restoration of range and control — with surgery reserved for acute traumatic tears in younger patients, and for those who fail a genuine course of rehabilitation.

Adhesive capsulitis (frozen shoulder)

A fibrotic contracture of the capsule, particularly the rotator interval and coracohumeral ligament, with reduced joint volume.

• Defining feature: passive range is as restricted as active range, in the capsular pattern (external rotation lost first and most). This is what distinguishes it from cuff pathology, in which passive range is preserved.
• Phases: freezing (painful, 2–9 months), frozen (stiff, less painful, 4–12 months), thawing (6–24 months). Total course typically 1–3 years; a proportion retain some restriction.
• Associations: diabetes mellitus (up to five-fold risk, and more resistant), thyroid disease, Dupuytren’s disease, cardiac disease, and immobilisation after any shoulder injury or surgery.
• Management is phase-dependent: pain control and gentle range in the freezing phase (aggressive stretching worsens it); progressive stretching and mobilisation in the frozen phase; corticosteroid injection has good short-term evidence, particularly early; hydrodilatation, manipulation under anaesthesia and arthroscopic release for the resistant minority.

Nerve injuries around the shoulder

Where students consistently go wrong

• Treating the shoulder as one joint. Four joints, plus the subacromial space.
• Calling supraspinatus a prime mover. It initiates and, more importantly, stabilises; deltoid does most of the abduction work.
• Forgetting obligatory external rotation above 90°. Without it, elevation stops at ~120°.
• Applying the 2:1 rhythm to the first 30°. The setting phase is highly variable.
• Confusing teres major with teres minor. Minor is cuff (axillary nerve, external rotator); major is not (lower subscapular, internal rotator).
• Teaching the Neer impingement model as settled. Decompression surgery is no better than placebo.
• Equating imaging findings with symptoms. Asymptomatic cuff tears and AC degeneration are extremely common.
• Missing a posterior dislocation. Fixed internal rotation after a seizure — get the axillary view.
• Not documenting axillary nerve function before reduction. Medico-legally essential.
• Stretching aggressively in the freezing phase of adhesive capsulitis. It prolongs the condition.

Check yourself

15 questions on this chapter. Tap one to see the answer and the reasoning.

Quick review

Further reading

Chapter 10 of 24 · Human Anatomy · Physiotherapist India Next: Chapter 11 — Arm, Elbow and Forearm: compartments, the cubital fossa, pronation and supination, and the two epicondyles.

Reviewed by the Physiotherapist India Team. · Human Anatomy contents