Kinesiology Ch.13: Shoulder Joint

Introduction to Shoulder Anatomy and Joint Characteristics

• Joint Type and Classification:

  • The shoulder is classified as a synovial joint, containing synovial fluid within the joint cavity.

  • It is a ball and socket joint characterized by three degrees of freedom.

  • Mobility vs. Stability: The shoulder possesses significantly more mobility than stability. This high level of functionality and movement variety results in a joint that is less stable than simpler joints like the elbow.

  • Clinical Prevalence: In orthopedic repairs, the knee is the most commonly repaired joint, followed by the shoulder as the second most common. In geriatric populations (e.g., nursing homes), knee replacements are more frequent than shoulder replacements.

• Key Articulation:

  • The head of the humerus articulates with the glenoid fossa of the scapula.

  • The glenoid fossa is described as a teardrop-shaped depression.

  • Labrum: This is a ring of fibrocartilage attached to the edge of the glenoid fossa. It functions to deepen the fossa and provide enhanced stability to the glenohumeral connection.

Planes of Motion and Ranges of Motion (ROM)

• Planes and Axes:

  • Flexion and Extension: Occur in the sagittal plane.

  • Abduction and Adduction: Occur in the frontal plane. Mnemonic: "Add" means adding the limb to the body (moving closer); "Abduct" is like an alien taking you away (moving away).

  • Scaption: The natural way the arm is raised, occurring at approximately a $45^{\circ}$ angle.

  • Circumduction: A combination of motions. In clinical settings, pendulums are a common early exercise for shoulder replacement patients, which involve a form of circumduction (often in a gravity-reduced or gravity-assisted environment).

• Ideal Ranges of Motion (Estimates):

  • Shoulder Flexion: $0$ to $180$ units.

  • Shoulder Extension: $0$ to $60$ units.

  • Abduction: Typically measured up to $180$ units.

  • Medial/Lateral Rotation: Important to measure accurately by following the zero point on the goniometer to avoid measuring from the wrong side.

Arthrokinematics: Joint Surface Motion

• Surface Relationship: The surface area of the head of the humerus is greater than the surface area of the glenoid fossa.

• Geometry:

  • The head of the humerus is convex (bulges outward).

  • The glenoid fossa is concave (hollowed inward).

• Motion Types: Because the surfaces are unequal, the joint must perform three types of motion:

  1. Roll

  2. Spin

  3. Glide

  • The humeral head glides within the fossa during flexion and extension.

Shoulder Girdle Dynamics

• Coordination: The shoulder joint is not fixed; movements of the humerus are accompanied by specific actions of the shoulder girdle (scapula and clavicle).

  • Flexion: Involves upward rotation and protraction of the shoulder girdle.

  • Extension: Involves downward rotation, retraction, and scapular tilt (where the scapula pulls away from the torso).

  • Pathology - Scapular Winging: A condition where the inferior angle of the scapula protrudes from the back, resembling wings.

Soft Tissue Structures: Bursae, Fascia, and Ligaments

• Bursae: These are fluid-filled sacs that provide cushioning and reduce friction around the joint.

  • Subacromial Bursa: Located between the deltoid muscle and the joint capsule, and between the acromion and the coracoacromial ligament.

  • Clinical Note: Bursitis (inflammation of the bursa) is extremely painful and can severely limit movement.

• Fascia: A fibrous sheet covering muscles.

  • Analogy: It is described as being like a rubbery sheet or being inside a "balloon" (used for proprioception and pressure). If this sheet is tight due to inactivity, aggressive stretching is required.

  • Thoracolumbar Fascia: Attaches to the spinous processes (the visible bulges on the spine). It connects the latissimus dorsi to the iliac crest and sacrum.

• Bony Landmarks and Ligaments:

  • Styloid Process: Located on the radius and ulna; commonly fractured during falls.

  • Bicipital Groove (Intertubercular Groove): A groove on the humerus that houses the long head of the biceps brachii tendon.

  • Glenohumeral Ligaments (Superior, Middle, Inferior): Reinforce the joint capsule.

  • Coracohumeral Ligament: Runs from the coracoid process to the greater and lesser tubercles.

  • Transverse Humeral Ligament: Spans the bicipital groove.

The Brachial Plexus and Innervation

• Brachial Plexus: A complex network of nerves ranging from $C5$ to $T1$.

• Key Nerves:

  • Axillary Nerve: Branches from $C5$ and $C6$. Innervates the deltoid and teres minor.

  • Musculocutaneous Nerve: Innervates the biceps brachii, brachialis, and coracobrachialis.

  • Radial Nerve: Innervates the triceps brachii. Midhumeral fractures can often lead to radial nerve injury.

Detailed Muscle Anatomy

• The Deltoid Muscle: All three portions insert at the deltoid tuberosity (a rough patch on the humerus).

  • Anterior Deltoid: Originates on the lateral third of the clavicle. Actions: Abduction, flexion, medial rotation, horizontal adduction.

  • Middle Deltoid: Originates from the acromion process. Action: Abduction.

  • Posterior Deltoid: Originates from the spine of the scapula. Actions: Abduction, extension, lateral rotation, horizontal abduction.

• Biceps Brachii: Has two heads.

  • Long Head: Sits in the intertubercular (bicipital) groove. Originates at the supraglenoid tubercle.

  • Short Head: Originates at the coracoid process.

  • Insertion: Radial tuberosity. Actions: Shoulder flexion, elbow flexion, forearm supination. Innervation: Musculocutaneous nerve.

• Triceps Brachii: Has three heads (Long, Lateral, Medial).

  • Insertion: Olecranon process of the ulna. Actions: Shoulder and elbow extension. Innervation: Radial nerve.

• Pectoralis Major:

  • Clavicular Portion: Originates from the clavicle.

  • Sternal Portion: Originates from the sternum and costal cartilage of the first $6$ ribs.

  • Insertion: Lateral lip of the bicipital groove.

• Latissimus Dorsi: A massive muscle covering the mid and lower back.

  • Origin: Thoracolumbar fascia, $T7$ through $L5$, iliac crest, and sacrum.

  • Insertion: Floor of the bicipital groove. Actions: Extension, hyperextension, medial rotation, adduction.

• Teres Major: Called the "little helper" of the latissimus dorsi. Originates near the inferior angle of the scapula and inserts on the medial lip of the bicipital groove.

The Rotator Cuff (SITS Muscles)

• The rotator cuff forms a functional cuff around the head of the humerus to maintain its position in the glenoid fossa. If one is injured, the others must compensate.

  1. Supraspinatus: Sits in the supraspinous fossa (above the scapular spine). Action: Abduction and stabilization.

  2. Infraspinatus: Sits in the infraspinous fossa (below the spine). Action: Lateral rotation, horizontal abduction.

  3. Teres Minor: Originates on the axillary border. Action: Lateral rotation, horizontal abduction.

  4. Subscapularis: Located on the anterior surface of the scapula (between the scapula and the ribs). Very difficult to palpate. Action: Medial rotation.

Functional Concepts and Biomechanics

• Force Couples: Muscles like the deltoid and the rotator cuff work together to produce movement. The deltoid pulls upward (abduction), while the rotator cuff stabilizes the head of the humerus.

• Kinetic Chains:

  • Closed Kinetic Chain: Occurs when the distal segment is fixed. Example: Ambulation with crutches. The hands and arms bear weight on the crutch handles, meaning the distal area is not free to move.

  • Terminology: In clinical documentation, "ambulate" is preferred over "walked" (e.g., "Patient ambulated from edge of bed to bathroom with standby assist").

Shoulder Pathologies

• Subluxation: Partial dislocation. Common in stroke patients. Must be handled delicately; wrapping a subluxed arm to an exercise machine without realizing the severity (e.g., a $2$ or $3$ finger width subluxation) is an ethical and safety concern.

• Dislocation: Total loss of joint integrity. Dislocation causes micro-traumas to ligaments and tendons, and the joint is rarely as strong as it was pre-injury.

• Impingement Syndrome: Compression between the acromial arch and the humeral head. Can be treated with manual traction to create space. Often caused by loss of scapulohumeral rhythm due to muscle weakness or atrophy.

• Adhesive Capsulitis (Frozen Shoulder): Inflammation of the joint capsule resulting in severe loss of range of motion. In extreme cases, doctors may perform a manipulation under anesthesia to break up scar tissue.

• Tendonal Issues: Tendonitis frequently involves the long head of the biceps. Rotator cuff tears require isolating the specific muscle (SITS) involved for proper treatment.