Shoulder Complex and Brachial Plexus Review

Flashcards covering key anatomical structures, biomechanics, muscle actions, kinesiology, joint mechanisms, and nerve pathways of the shoulder complex and brachial plexus.

What is the main job of the shoulder complex?

To position your hand in space, giving the hand a huge range for reaching, pushing, pulling and manipulating objects.

Why is the shoulder more injury-prone than the hip?

It has a shallow socket and prioritises mobility over bony stability.

What are the three joints of the shoulder complex?

Sternoclavicular joint (SCJ), acromioclavicular joint (ACJ), and glenohumeral joint (GHJ).

What bones make up the shoulder region?

Clavicle, scapula, proximal humerus and manubrium of the sternum.

What is the pectoral girdle?

A clavicle and scapula together.

Why is the clavicle important?

It acts like a strut, holding the scapula and arm away from the trunk.

What travels through the costoclavicular space?

Subclavian artery, subclavian vein and brachial plexus.

Why is the costoclavicular space important?

It can become a site of nerve or blood-vessel compression.

What are the normal resting scapula landmarks and position?

Root of spine: $T2\text{--}T3$. Inferior angle: $T6\text{--}T7$. The scapula sits about $30^\circ$ internally rotated from the frontal plane.

Where does the humeral head point in relaxed standing?

Posteriorly, medially and superiorly.

Why do clavicle fractures usually occur in the middle third?

It is a structural weak point where the curve and shape of the clavicle change.

What happens after a midshaft clavicle fracture?

The lateral fragment and scapula drop and move medially because the clavicle can no longer hold the shoulder out to the side.

What type of joint is the SCJ?

Synovial saddle joint.

What articulates at the SCJ?

Medial clavicle with clavicular notch of manubrium and first costal cartilage.

Why does the SCJ have poor bony stability?

The large medial clavicle sits on a small, shallow sternum surface.

Why is SCJ stability very important?

It is the only bony connection between the upper limb and axial skeleton.

What structures stabilise the SCJ?

Anterior/posterior SC ligaments, interclavicular ligament, costoclavicular ligament and intra-articular disc.

What is the role of the costoclavicular ligament?

It strongly stabilises the SCJ and forms the pivot for clavicle movement.

What does the SCJ intra-articular disc do?

Deepens the joint, absorbs shock and spreads forces from the arm to the trunk.

What type of joint is the ACJ?

Synovial plane joint.

What articulates at the ACJ?

Lateral clavicle with the medial aspect of the acromion.

What movement occurs at the ACJ?

Small glides and rotations of the scapula relative to the clavicle.

Why does the ACJ rely on ligaments?

Its joint surfaces are small and flat, so there is little bony stability.

Which ligaments stabilise the ACJ?

Superior and inferior AC ligaments, plus conoid and trapezoid ligaments.

What do the superior and inferior AC ligaments resist?

Anterior-posterior displacement of the clavicle on the scapula.

What do conoid and trapezoid ligaments resist?

Vertical separation and rotation between clavicle and scapula.

Where does the conoid ligament attach?

Coracoid process to conoid tubercle of clavicle.

Where does the trapezoid ligament attach?

Coracoid process to trapezoid line of clavicle.

Why does a severe AC injury create a step deformity?

When conoid/trapezoid tear, the lateral clavicle is no longer pulled down by the weight of the arm; upper trapezius elevates it.

What is the common mechanism of ACJ injury?

Falling directly onto the point of the shoulder.

Which ligaments are injured first in an AC injury?

The capsular AC ligaments; more force can then tear conoid and trapezoid.

What happens during clavicle elevation?

The lateral end of the clavicle moves superiorly.

What happens during clavicle depression?

The lateral end moves inferiorly.

What happens during clavicle protraction?

The lateral end moves anteriorly.

What happens during clavicle retraction?

The lateral end moves posteriorly.

Why does the clavicle upwardly rotate during arm elevation?

Scapular upward rotation tensions the conoid ligament, which rotates the anterior clavicle upward and posteriorly.

What happens during scapular elevation and depression?

Elevation = moves superiorly; depression = moves inferiorly on the thorax.

What happens during scapular protraction and retraction?

Protraction = medial border moves away from spine. Retraction = medial border moves toward spine.

What characterizes scapular upward rotation?

Glenoid turns upward and inferior angle moves away from the spine.

When does scapular upward rotation occur?

During arm flexion and abduction.

What characterizes scapular downward rotation?

Glenoid turns downward and inferior angle moves toward the spine.

When does scapular downward rotation occur?

Lowering arm from elevation, arm adduction/extension from elevation, or reaching behind your back.

What is scapular internal rotation?

Glenoid turns anteromedially.

What is scapular external rotation?

Glenoid turns posterolaterally.

What usually happens as the scapula protracts?

It tends to internally rotate.

What usually happens as the scapula retracts?

It tends to externally rotate.

What occurs during scapular anterior tilt?

Acromion moves anteriorly and inferior angle moves posteriorly.

What occurs during scapular posterior tilt?

Acromion moves posteriorly and inferior angle moves anteriorly.

Why does the humerus laterally rotate during abduction?

It moves the greater tubercle posteriorly, helping prevent it from hitting the coracoacromial arch.

What is the difference between shoulder shrugging and full arm elevation?

Shrugging is scapula/clavicle elevation. Full elevation means lifting the arm and requires SCJ, ACJ and GHJ movement.

What happens to the clavicle during full upper-limb elevation?

It slightly elevates, retracts and upwardly rotates.

What happens to the scapula during full upper-limb elevation?

Upward rotation, posterior tilt and external rotation.

What happens to the humerus during full upper-limb elevation?

Abduction with external rotation.

How much elevation comes from GHJ versus scapula?

Roughly $\frac{2}{3}$ GHJ and $\frac{1}{3}$ scapular upward rotation.

Do GHJ, SCJ and ACJ move one after the other during arm elevation?

No. They contribute together through most of the movement.

What are the three shoulder muscle groups?

Axioscapular/axioclavicular, scapulohumeral and axiohumeral.

What do axioscapular muscles do?

Move and stabilise the scapula/clavicle.

What do scapulohumeral muscles do?

Move and stabilise the humerus at the GHJ.

Why does scapular stability matter?

The humeral muscles need a stable scapular base to work efficiently.

How do you work out a muscle's action?

Look at its line of action relative to the joint axis.

What is torque?

Turning force: muscle force $\times$ moment arm.

What is a moment arm?

The perpendicular distance between a joint axis and a muscle's line of action.

Why does a larger moment arm matter?

It lets the same muscle force create more torque.

What is PCSA?

Physiological cross-sectional area; a larger PCSA usually means a muscle can create more force.

What is joint compression?

Force pushing joint surfaces together; it improves stability.

What is shear force at a joint?

Force sliding one joint surface on another; it can be destabilising.

What is the origin of superior trapezius?

Superior nuchal line and ligamentum nuchae.

What is the insertion of superior trapezius?

Posterior border of lateral one-third of clavicle.

What is the innervation of superior trapezius?

Spinal accessory nerve (CN XI), with $C3\text{--}C4$ proprioception.

What is the origin of middle trapezius?

$C7\text{--}T1$ spinous processes.

What is the insertion of middle trapezius?

Acromion and spine of scapula.

What is the origin of lower trapezius?

Lower thoracic spinous processes.

What is the insertion of lower trapezius?

Medial end/root of spine of scapula.

What action do all parts of trapezius share?

Scapular retraction.

Why do all trapezius parts retract the scapula?

Their pull has a medial component toward the vertebral column.

What are the actions of superior trapezius?

Scapular/clavicle elevation, retraction and upward rotation.

Why does superior trapezius elevate the scapula?

It pulls upward on the lateral clavicle.

What are the actions of middle trapezius?

Strong scapular retraction; assists upward rotation.

What are the actions of lower trapezius?

Scapular depression, retraction and assistance with upward rotation.

Why is lower trapezius important during upward rotation?

It stabilises the scapular rotation centre and prevents excessive elevation.

What is the origin of serratus anterior?

Lateral surfaces of ribs $1\text{--}9$.

What is the insertion of serratus anterior?

Anterior surface of medial border of scapula.

What is the innervation of serratus anterior?

Long thoracic nerve, $C5\text{--}C7$.

What is the main action of serratus anterior?

Scapular protraction.

Why is serratus anterior the main protractor?

Its fibres run mostly horizontally and pull the scapula around the thorax.

How does serratus anterior upwardly rotate the scapula?

Its lower fibres pull the inferior angle laterally.

Why is serratus anterior the strongest upward rotator?

It is large and has a long upward-rotation moment arm.

What else does serratus anterior do?

Holds the medial border of scapula against the rib cage.

What muscles create scapular upward rotation during arm elevation?

Serratus anterior plus trapezius.

What muscles help produce scapular posterior tilt?

Serratus anterior and lower trapezius.

What muscles help produce scapular external rotation?

Serratus anterior and middle trapezius.

Why is scapular upward rotation important during arm elevation?

It keeps deltoid at a useful length, gives GHJ a stable base and preserves subacromial space.

Why would deltoid struggle without scapular rotation?

It would become too shortened to keep producing strong abduction.

What happens with serratus anterior weakness during elevation?

Downward rotation, anterior tilt and internal rotation of the scapula.

Why does serratus weakness cause winging?

The medial border is no longer held against the ribs.

When is serratus winging easiest to see?

With weight through the arm, such as a push-up, plank or pushing a heavy door.

What is the origin of levator scapulae?

Posterior tubercles of $C1\text{--}C4$ transverse processes.

What is the insertion of levator scapulae?

Superior medial border of scapula.

What is the innervation of levator scapulae?

Dorsal scapular nerve and $C3\text{--}C4$.

What are the origin and insertion of rhomboid minor?

$C7\text{--}T1$ spinous processes to medial end of scapular spine.