Humerus. Shoulder joint

Humerus

The humerus is the long bone of the upper arm, extending from the shoulder to the elbow. It is critical for the function and mobility of the upper limb.

Anatomy of the Humerus

  1. Proximal End:

    • Head: Spherical structure that articulates with the glenoid cavity of the scapula to form the shoulder joint.

    • Anatomical Neck: Narrow area just below the head; attachment site for the joint capsule.

    • Surgical Neck: Common fracture site located below the tubercles.

    • Greater Tubercle: Lateral prominence; attachment site for supraspinatus, infraspinatus, and teres minor muscles.

    • Lesser Tubercle: Medial prominence; attachment site for the subscapularis muscle.

    • Intertubercular Sulcus (Bicipital Groove): Groove between the tubercles; passage for the long head of the biceps tendon.

  2. Shaft:

    • Cylindrical proximally and more triangular distally.

    • Deltoid Tuberosity: Lateral roughened area for the attachment of the deltoid muscle.

    • Radial Groove: On the posterior aspect; houses the radial nerve and profunda brachii artery.

  3. Distal End:

    • Capitulum: Articulates with the head of the radius.

    • Trochlea: Articulates with the trochlear notch of the ulna.

    • Medial Epicondyle: Attachment for flexor muscles of the forearm.

    • Lateral Epicondyle: Attachment for extensor muscles of the forearm.

    • Olecranon Fossa: Posterior depression for the olecranon of the ulna during elbow extension.

Shoulder Joint (Glenohumeral Joint)

The shoulder joint is a synovial ball-and-socket joint, providing the greatest range of motion of any joint in the body.

Articulating Surfaces

  • Head of the Humerus: Convex and fits into the glenoid cavity of the scapula.

  • Glenoid Cavity: Shallow depression on the scapula, deepened by the glenoid labrum (fibrocartilaginous rim).

Joint Capsule and Ligaments

  1. Joint Capsule:

    • Thin and loose, allowing significant mobility.

    • Reinforced by ligaments and rotator cuff muscles.

  2. Ligaments:

    • Glenohumeral Ligaments: Superior, middle, and inferior bands strengthen the anterior capsule.

    • Coracohumeral Ligament: Extends from the coracoid process to the humeral head; reinforces the superior part of the capsule.

    • Transverse Humeral Ligament: Holds the long head of the biceps tendon within the intertubercular sulcus.

    • Coracoacromial Ligament: Forms the coracoacromial arch, preventing superior displacement of the humeral head.

Movements

  • Flexion and Extension.

  • Abduction and Adduction.

  • Medial (Internal) and Lateral (External) Rotation.

  • Circumduction (combination of all movements).

Muscles Acting on the Shoulder Joint

  1. Rotator Cuff Muscles: Stabilize the joint and control movements.

    • Supraspinatus: Abduction.

    • Infraspinatus: External rotation.

    • Teres Minor: External rotation.

    • Subscapularis: Internal rotation.

  2. Deltoid: Major abductor of the shoulder.

  3. Pectoralis Major: Flexion, adduction, and internal rotation.

  4. Latissimus Dorsi and Teres Major: Extension and adduction.

Stability of the Joint

  • The shoulder joint is inherently unstable due to the shallow glenoid cavity.

  • Stability is provided by:

    • Static Factors: Labrum, ligaments, and joint capsule.

    • Dynamic Factors: Rotator cuff muscles and surrounding musculature.

Clinical Relevance

  1. Shoulder Dislocation:

    • Common due to the joint's mobility and shallow socket.

    • Anterior dislocation is the most frequent type.

  2. Rotator Cuff Injuries:

    • Result from overuse or trauma.

    • Commonly involves the supraspinatus tendon.

  3. Frozen Shoulder (Adhesive Capsulitis):

    • Stiffness and pain due to inflammation or fibrosis.

  4. Fractures of the Humerus:

    • Surgical Neck Fracture: Affects axillary nerve.

    • Midshaft Fracture: May damage the radial nerve.