Shoulder abduction

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Here is a thorough overview of shoulder abduction based on authoritative anatomy and orthopaedics textbooks.

Shoulder Abduction

Shoulder abduction is the movement of the arm away from the body in the frontal (coronal) plane, from 0° at rest to approximately 180° at full elevation. It is one of six movements at the glenohumeral joint (along with flexion, extension, adduction, medial rotation, lateral rotation, and circumduction).

Fig. 7.4 - Gray's Anatomy for Students: Movements of the arm at the glenohumeral joint, showing abduction, adduction, flexion, extension, circumduction, and rotations.

Joints Involved

Full shoulder abduction requires coordinated motion at two joints:

Glenohumeral joint - the ball-and-socket joint where the humeral head articulates with the glenoid fossa of the scapula. It provides the primary arc of motion.
Scapulothoracic "joint" - the functional articulation where the scapula slides and rotates on the thoracic wall. As elevation increases above 90°, the proportion of scapulothoracic motion relative to glenohumeral motion increases. This coordinated ratio is called the scapulohumeral rhythm; disturbance of it leads to impaired biomechanics and secondary impingement. - Rheumatology, 2-Volume Set (Elsevier), p. 700

The sternoclavicular and acromioclavicular joints also contribute to the full arc by allowing clavicular and scapular elevation.

Muscles and their Roles

Primary abductors

Muscle	Role	Nerve Supply
Supraspinatus	Initiates abduction from 0-10° to 15° - "kicks off" the movement	Suprascapular nerve (C5, C6)
Deltoid (middle fibers)	Takes over from ~15° onward and drives the majority of the arc	Axillary nerve (C5, C6)

The supraspinatus must initiate abduction because, at 0°, the deltoid has poor mechanical advantage - its line of pull is nearly vertical and would compress the humeral head into the glenoid rather than raise the arm. Once the supraspinatus moves the arm into the early range, the deltoid gets its mechanical advantage and continues. - Gray's Anatomy for Students, p. 954

A clinical point: if the supraspinatus tendon is torn, the patient cannot initiate abduction. However, by leaning and tilting the trunk (lowering and tilting the glenohumeral joint inferiorly), the deltoid can be brought into a mechanically advantageous position and abduction can still occur, though with a characteristic "trick movement." Full abduction is possible even with deltoid paralysis, driven by supraspinatus, because of this complementary relationship. - Campbell's Operative Orthopaedics, 15th Ed.

Scapular stabilizers / force couples

For the glenohumeral abductors to work effectively, the scapula must be stabilized and upwardly rotated:

Trapezius (upper and lower fibers) - upward rotation of scapula
Serratus anterior - forward/lateral displacement and upward rotation of scapula; responsible for forward elevation after shoulder arthrodesis - Campbell's Operative Orthopaedics
Levator scapulae and rhomboids also contribute to scapular control

Rotator cuff (SITS muscles)

The four rotator cuff muscles (Supraspinatus, Infraspinatus, Teres minor, Subscapularis) send tendinous expansions into the joint capsule and act to compress the humeral head into the glenoid - the "concavity-compression" mechanism. This keeps the humeral head centered during abduction, preventing it from riding up under the coracoacromial arch.

Subacromial Space During Abduction

As the arm abducts, the supraspinatus tendon and the greater tubercle of the humerus pass beneath the coracoacromial arch (formed by the acromion, coracoacromial ligament, and coracoid process). The subacromial bursa (and its communicating subdeltoid bursa) sit in this space and allow frictionless gliding during movement.

Shoulder anatomy showing the coracoacromial arch, subacromial bursa, and supraspinatus

THIEME Atlas of Anatomy - General Anatomy: The coracoacromial arch (top) and the position of the subacromial and subdeltoid bursae between the arch and supraspinatus (bottom).

If this space is reduced (due to bony spurs, rotator cuff thickening, or bursal inflammation), subacromial impingement occurs - producing a characteristic painful arc of abduction, typically between 60° and 120°.

Shoulder joint showing ligamentous/musculotendinous attachments, bursae, and surrounding structures

THIEME Atlas - Ligamentous and musculotendinous attachments about the shoulder, including the subacromial and subdeltoid bursae.

Nerve Supply and Segmental Innervation

Deltoid - axillary nerve (C5, C6)
Supraspinatus - suprascapular nerve (C5, C6)
The key nerve root for shoulder abduction is C5; an isolated C5 radiculopathy or upper brachial plexus (Erb-Duchenne) injury will impair abduction. - Textbook of Family Medicine 9e; Neuroanatomy through Clinical Cases 3rd Ed.
Upper brachial plexus lesions (C5-C6) also paralyze biceps, brachioradialis, and brachialis, producing the classic "waiter's tip" posture.

Range of Motion

0-15°: supraspinatus dominant
15-90°: deltoid dominant, glenohumeral motion predominates
90-180°: increasing scapulothoracic contribution (upward rotation of scapula); the humerus also must externally rotate to clear the greater tubercle from under the acromion above 90°

Normal full abduction: 0-180° (approximately 120° at the glenohumeral joint + 60° from scapular rotation).

Clinical Testing

Abduction against resistance tests C5 (and axillary/suprascapular nerve integrity)
Painful arc (60-120°): subacromial impingement or rotator cuff tendinopathy
Inability to initiate abduction: suggests supraspinatus tear
Active > Passive restriction: rotator cuff pathology (pain on active but not passive abduction)
Winging of the scapula during abduction: serratus anterior (long thoracic nerve) or trapezius (spinal accessory nerve) dysfunction disrupts the scapulohumeral rhythm

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