Shoulder Complex Anatomy Quiz

Shoulder Complex Overview

• Comprises four joints: Sternoclavicular (SC), Acromioclavicular (AC), Glenohumeral (GH), and Scapulothoracic (ST).
• Essential for linking upper extremity to the trunk and facilitating extensive arm mobility.
• Provides stability necessary for fine and gross motor tasks at the elbow, wrist, and hand.

Scapula Anatomy and Positioning

• Features three angles: inferior, superior, and lateral; three borders: medial, lateral, and superior.
• Resting position aligns: superior angle with thoracic vertebra 2 (T2), spine of the scapula with T3, inferior angle with T7.
• Slight upward rotation (5º-10º) in the frontal plane; anterior tilt of the scapula noted.

Plane of the Scapula

• Positioned at a 30º to 45º angle to the frontal plane for stress reduction during daily activities.
• Scapular motion in this plane is crucial for shoulder joint stability and function.

Humerus Alignment

• Humerus has a 135º angle of inclination; retroversion (30º-45º) aligns it with the scapular plane.
• These anatomical features enhance stability of the GH joint.

Clavicle Features

• Curved S-shaped structure resting at a 20º angle posterior to the frontal plane and slightly elevated.

Sternoclavicular Joint (SC)

• Only true articulation connecting upper extremity to axial skeleton, classified as a diarthrodial saddle joint.
• Allows three degrees of freedom: elevation/depression, protraction/retraction, and posterior rotation.
• Costoclavicular ligament primarily stabilizes SC joint, limiting motion except for depression.

Acromioclavicular Joint (AC)

• Plane synovial joint offering three degrees of freedom, important for maintaining scapuloclavicular relationships.
• Key ligaments for stability include the coracoclavicular ligament and superior/inferior AC ligaments.

Scapulothoracic Joint (ST)

• Functions as a joint without traditional anatomical structure, pivotal for scapular motion during arm activity.
• Movement correlates to scapular elevation/depression, protraction/retraction, and upward/downward rotation.

Glenohumeral Joint (GH)

• Most mobile joint in the body, functioning as a diarthrodial ball-and-socket joint with a large humeral head and smaller glenoid fossa.
• Only 25%-33% of the humeral head articulates with the glenoid, maximizing mobility but sacrificing stability.

GH Joint Stability

• Glenoid labrum deepens the fossa, increasing stability and reducing humeral head translation.
• Capsuloligamentous complex allows high mobility while stabilizing the joint through ligaments like the coracohumeral and glenohumeral ligaments.

Movement Mechanics of the GH Joint

• Offers three degrees of freedom: flexion/extension, abduction/adduction, and internal/external rotation.
• Critical to maintain arthrokinematic principles during movements to prevent impingement and optimize function.

Scapulohumeral Rhythm

• Coordinated movements across all four shoulder joints enhance overall shoulder complex functionality.
• Follows a 2:1 ratio of GH movement to ST upward rotation for optimum motion and stability.

Muscle Function in Shoulder Dynamics

• Axioscapular muscles (trapezius, serratus anterior, levator scapula, rhomboids, pectoralis minor) play essential roles in stabilizing the scapula and supporting shoulder movements.
• Trapezius functions include elevation and upward rotation, while serratus anterior aids in scapular abduction and stabilization on the thoracic cage.

Clinical Notes

• Underactive or overactive muscular dynamics can lead to dysfunction, such as scapular winging or elevated shoulder posture.
• Rehabilitation and strengthening exercises should emphasize maintaining proper movement mechanics and muscle balance.### Shoulder Complex Overview
• True weakness can result from other joint limitations, such as glenohumeral (GH) internal rotation range of motion (ROM) affecting scapular positioning.
• Tightness may lead to protracted scapulae, impacting functional movements like pushing, throwing, and reaching.

Upward Rotation Force Couple

• Involves synergistic action of trapezius and serratus anterior for scapular upward rotation.
• Trapezius acts in three segments:
  • Upper: Elevates distal clavicle and acromion.
  • Middle: Adducts scapula.
  • Lower: Depresses medial spine of scapula.
• Serratus anterior elevates and abducts the scapula.

Axioscapular Muscles

Levator Scapula

• Function: Elevates and downwardly rotates scapula; facilitates lateral neck flexion and cervical spine extension.
• Innervation: Dorsal Scapular Nerve.
• Overactive in downwardly rotated and elevated scapula scenarios.

Rhomboids

• Function: Scapular adduction (retraction) and stabilization.
• Innervation: Dorsal Scapular Nerve.
• Overactive when scapula is adducted and downwardly rotated; underactive when shoulders are rounded.

Pectoralis Minor

• Actions: Anterior tilt, depression, and downward rotation of scapula.
• Innervation: Medial and Lateral Pectoral Nerves.
• Overactivity can risk impingement through increased internal rotation of the scapula during movement.

Elevation and Retraction Force Couples

• Elevation: Involves upper trapezius and levator scapula working together.
• Retraction: Middle trapezius, lower trapezius, and rhomboids coordinate to achieve pure retraction motion.

Axiohumeral Muscles

• Major muscles: Latissimus Dorsi and Pectoralis Major, responsible for gross motor movements of the GH joint.

Latissimus Dorsi

• Actions: Extension, adduction, and internal rotation of humerus; aids in climbing.
• Innervation: Thoracodorsal Nerve.
• Overactivity can limit shoulder flexion and external rotation.

Pectoralis Major

• Actions: Flexion, adduction, internal rotation, and horizontal adduction of humerus.
• Innervation: Medial and Lateral Pectoral Nerves.
• Dysfunction leads to reduced strength in various motions.

Shoulder Girdle Movements

• Depression: Involves lower trapezius, pec minor, and latissimus dorsi.
• Protraction: Engages serratus anterior and pectoralis major.

Scapulohumeral Muscles

• Function: Provide motion and dynamic stabilization to GC joint. Key muscles include deltoid, supraspinatus, infraspinatus, teres minor, subscapularis, teres major, and coracobrachialis.

Deltoid

• Actions: Abduction, flexion, horizontal adduction, and internal rotation.
• Innervation: Axillary Nerve.
• Dysfunction results in compensatory movements or limited ROM.

Supraspinatus

• Key for initiating abduction in the scapular plane; also stabilizes GH joint.
• Innervation: Suprascapular Nerve.

Infraspinatus and Teres Minor

• Work synergistically for external rotation and horizontal abduction of the humerus.

Subscapularis

• Primary mover for internal rotation of humerus; stabilizes GH joint.
• Innervation: Upper and Lower Subscapular Nerves.

Rotator Cuff Functions

• Critical for humeral head stability during arm movements, counteracting superior translations resulting from deltoid action.

Dynamic Stability and Kinematics

• Each muscle contributes specific actions, with synergistic efforts stabilizing the humerus and maintaining shoulder integrity.

Functional Motion Dynamics

• Internal Rotation: Primarily driven by subscapularis and assisted by latissimus dorsi and pectoralis major.
• External Rotation: Infraspinatus, teres minor, and posterior deltoid are the main movers.

Relationship of Shoulder to Spine

• Coordination between shoulder girdle and spine is essential for optimal movement; poor posture in one often affects the other.