Shoulder Anatomy and Biomechanics Quiz

Shoulder: Clinical Anatomy & Biomechanics

• The shoulder complex comprises multiple joints: glenohumeral, acromioclavicular, sternoclavicular, scapulothoracic, and the subacromial space.
• The glenohumeral joint is a ball-and-socket type joint.
• The acromioclavicular joint is a gliding or planar joint.
• The sternoclavicular joint connects the clavicle to the sternum and the first rib.

Anatomical Orientation of the Shoulder Complex

• The scapular plane is at a 35-degree angle from the sagittal plane.
• The humerus has a 30-degree retroversion.
• The clavicle is at a 20-degree angle from the scapular plane.

Stability Functions of Shoulder Girdle

• Provides a stable base for shoulder muscles to generate force.
• Shoulder girdle muscles act as stabilizers.
• Maintains proper force-length relationship.
• Maintains maximal congruence of the shoulder joint.

Mobility Functions of Shoulder Girdle

• Allows for the largest range of motion (ROM) of any complex in the body.
• Shoulder girdle increases ROM with less compromise of stability (scapulo-humeral rhythm) – 4 joints vs. 1 joint.
• Facilitates upper extremity movements by positioning the glenohumeral joint favorably.

Shoulder Girdle Motion

• Elevation/Depression: Movement of the scapula up and down, respectively.
• Protraction/Retraction (Abduction/Adduction): Movement of the scapula forward and backward, respectively.
• Upward/Downward Rotation: Rotational movement of the scapula.
• Upward/Downward Tilt: Tilting of the scapula.

Muscular Considerations

• The muscles that act on the shoulder girdle include the trapezius, levator scapulae, rhomboids, serratus anterior, and pectoralis minor.
• The muscles that act on the shoulder joint include the deltoid, pectoralis major, coracobrachialis, subscapularis, biceps brachii, supraspinatus, infraspinatus, teres minor, latissimus dorsi, and teres major.

Scapulothoracic Muscles

• Elevators: upper trapezius, levator scapulae, rhomboids
• Depressors: lower trapezius, pectoralis minor
• Protractors: serratus anterior, pectoralis minor
• Retractors: mid & lower trapezius, rhomboids, levator scapulae
• Upward Rotators: serratus anterior, upper trapezius
• Downward Rotators: rhomboids, pectoralis minor, levator scapulae

Glenohumeral Joint Muscles

• Abductors: deltoid, supraspinatus
• Adductors: pectoralis major, latissimus dorsi, teres major
• Internal Rotators: pectoralis major, subscapularis, anterior deltoid, latissimus dorsi, and teres major
• External Rotators: infraspinatus, teres minor, posterior deltoid
• Flexors: biceps brachii, coracobrachialis, anterior deltoid
• Extensors: triceps brachii, posterior deltoid, latissimus dorsi, teres major

Specific Joints Structure and Function

• The sternoclavicular joint comprises the clavicle, sternum, and first rib, with various ligaments (sternoclavicular, costoclavicular, interclavicular) and an articular disc.
• The acromioclavicular joint consists of the acromion, clavicle, and an articular disc, with acromioclavicular and coracoclavicular ligaments.
• The scapulothoracic articulation lacks actual joint structure, being a dynamic interaction between the scapula and thoracic cage.

Scapulothoracic Articulation Motion

• Elevation/Depression: Movement of the scapula up and down.
• Retraction/Protraction: Movement of the scapula forward and backward.
• Upward/Downward Rotation: Rotation of the scapula.

Glenohumeral Joint Components

• Humeral head, glenoid fossa, glenoid labrum, rotator cuff muscles, capsuloligamentous complex, coracohumeral ligament, long head of biceps, and subacromial space.

Glenohumeral Ligaments

• Superior glenohumeral ligament (SGHL)
• Middle glenohumeral ligament (MGHL)
• Inferior glenohumeral ligament (IGHL) complex: anterior band, posterior band, axillary pouch

Glenohumeral Joint Motion

• Flexion/Extension: movement of the humerus forward and backward, respectively.
• Abduction/Adduction: movement of the humerus away from and toward the body, respectively.
• Internal/External Rotations: rotation of the humerus internally and externally.

Glenohumeral Joint Stability

• Static restraints: Bony structures, labrum, capsuloligamentous structure, and negative joint pressure.
• Dynamic restraints: Rotator Cuff, Biceps, proprioceptive receptors, and neuromotor coordination

Kinematics and Biomechanics

• Convex-Concave Rules: Describe how bones move and interact in a joint during action. Convex-on-concave rule or concave-on-convex rule.
• Normal Arthrokinematics: Rotation and translation keep the humeral head centered on the glenoid.

Clinical Implications

• Subacromial Space: Avoidance of impingement due to anatomical structure.
• Dynamic Stability Mechanisms: Passive muscle tension during movement, compressive forces from muscle contraction, tightening of passive structures, and redirecting joint forces towards the center of the GH joint.
• Force Couples: Forces generated by muscles for joint movement stability.
• Rotator Cuff Dynamics: Role of the rotator cuff muscles in stabilizing the glenohumeral joint and in producing controlled motion of the humeral head.
• Scapulohumeral Rhythm: Relationship and coordination of scapula and humerus movement in abduction.
• Effects of Poor Scapular Stabilization: Factors and consequences of poor scapular mechanics.