Comprehensive Osteology Notes on the Humerus

General Features and Morphological Classification

  • The humerus is the bone of the arm (superior extremity).
  • As a typical long bone, it exhibits the following general morphological characteristics:
    • Its length is significantly greater than its width or breadth.
    • It possesses two expanded ends (an upper end and a lower end) separated by an intervening shaft.
    • It functions to transmit body weight.
    • It ossifies via intracartilaginous (endochondral) ossification.
    • It is vertically positioned within the body.

Anatomical Position and Side Determination

  • Side determination and anatomical alignment rely on three primary anatomical landmarks:
    • Head: Positioned superiorly, facing upward, backward, and medially.
    • Intertubercular Sulcus (Bicipital Groove): Positioned anteriorly, facing directly forward.
    • Olecranon Fossa: Positioned at the inferior end, facing posteriorly.
  • Standard Descriptive Statement for Side Determination:
    • "This is the humerus of the left side. The spheroidal head is directed upward, backward, and medially. The intertubercular sulcus (bicipital groove) is directed anteriorly. The olecranon fossa lies inferiorly and is directed posteriorly."
  • Once these three criteria are established, the specific side (left or right) is unequivocally identified (determined as the left humerus in this demonstration).

Osteology of the Upper End

  • Head of the Humerus:
    • Articular structure forming approximately 13\frac{1}{3} of a sphere.
    • Articulates directly with the glenoid cavity of the scapula to form the shoulder joint.
    • The shoulder joint represents a ball-and-socket variety of synovial joint.
    • Covered by hyaline cartilage, which exhibits maximum thickness at the center and thins toward the periphery.
  • Tubercles of the Humerus:
    • Lesser Tubercle: Features a distinct muscular impression providing insertion for the subscapularis muscle.
    • Greater Tubercle: Features three distinct surfaces:
    • Lateral surface: Forms the anatomical rounded contour of the shoulder; covered by the deltoid muscle and separated from it by the subacromial bursa.
    • Superior and Posterior surfaces: Present three distinct muscular impressions arranged sequentially from top to bottom:
      1. Supraspinatus muscle insertion
      2. Infraspinatus muscle insertion
      3. Teres minor muscle insertion
  • The Rotator Cuff:
    • Consists of four intrinsic muscles inserting surrounding the humeral tubercles:
    • Subscapularis: Inserts onto the lesser tubercle.
    • Supraspinatus: Inserts onto the highest impression of the greater tubercle.
    • Infraspinatus: Inserts onto the middle impression of the greater tubercle.
    • Teres minor: Inserts onto the lowest impression of the greater tubercle.
    • Functional role: Musculotendinous envelope providing essential structural stability to the glenohumeral (shoulder) joint.
  • Anatomical Necks of the Humerus:
    • Anatomical Neck:
    • Constricted groove immediately circumscribing the articular head.
    • Provides site of attachment for the capsular ligament of the shoulder joint.
    • Surgical Neck:
    • Constricted area located directly below the tubercles, demarcating the junction between the upper end and the cylindrical shaft.
    • Morphological Neck:
    • Demarcates the epiphyseal line separating the upper epiphysis from the diaphysis.
    • Present exclusively in growing bones and becomes obliterated in the fully formed adult humerus.
    • Situated approximately 0.5 cm0.5\,\text{cm} superior to the surgical neck.
  • Capsular Ligament Attachments and Deficiencies:
    • The capsular ligament does not maintain a continuous attachment along the anatomical neck, demonstrating deficiencies in two specific areas:
    • Superior deficiency at the intertubercular sulcus: Deficient to permit passage of the long head tendon of biceps brachii and the ascending branch of the anterior circumflex humeral artery.
    • Inferior extension: Extends 1 cm1\,\text{cm} downward beyond the anatomical neck, encroaching onto the medial aspect of the surgical neck.
  • Clinical Correlation — Surgical Neck Fracture:
    • Neurovascular Relations: Posteriorly, the surgical neck is in intimate contact with the axillary nerve and the posterior circumflex humeral vessels.
    • Pathology: Fracture at the surgical neck risks transaction or compression injury to the axillary nerve.
    • Clinical Manifestations:
    • Paralysis of the deltoid muscle (due to loss of axillary nerve supply).
    • Inability to abduct the shoulder joint properly.
    • Loss of the normal rounded contour of the shoulder, resulting in a flattened shoulder appearance.

Osteology of the Lower End

  • Articular Features:
    • Capitulum (Lateral):
    • Convex articular surface articulating with the concave upper surface of the radial head.
    • Contributes to the radiohumeral portion of the elbow joint.
    • Trochlea (Medial):
    • Pulley-shaped articular surface articulating with the trochlear notch of the ulna.
    • Contributes to the humeroulnar portion of the elbow joint.
    • Elbow Joint Classification: Functions structurally and functionally as a hinge variety of synovial joint.
  • Non-Articular Features and Fossae:
    • Coronoid Fossa: Depressed area located anteriorly above the trochlea; receives the anterior border of the ulnar coronoid process during full forearm flexion.
    • Radial Fossa: Depressed area located anteriorly above the capitulum; receives the rim of the radial head during full forearm flexion.
    • Olecranon Fossa: Deep depression located posteriorly above the trochlea; receives the apex of the ulnar olecranon process during full forearm extension.
    • Medial Epicondyle: Prominent medial projection acting as the common flexor origin for all superficial flexor muscles of the forearm.
    • Features a shallow groove on its posterior surface through which the ulnar nerve courses.
    • Lateral Epicondyle: Prominent lateral projection acting as the common extensor origin for all superficial extensor muscles of the forearm.

Osteology of the Shaft

  • Borders:
    • Anterior Border:
    • Extends from the anterior portion of the greater tubercle (lateral lip of the intertubercular sulcus) down to the midpoint between the radial and coronoid fossae.
    • Medial Border:
    • Extends from the medial lip of the intertubercular sulcus down to the medial epicondyle.
    • Medial Supracondylar Ridge: Sharp ridge formed by the lower portion of the medial border that provides attachment to the medial intermuscular septum.
    • Pierced by three anatomical structures:
      1. Ulnar nerve
      2. Superior ulnar collateral artery
      3. Posterior branch of the inferior ulnar collateral artery
    • Lateral Border:
    • Extends from the posterior margin of the greater tubercle down to the tip of the lateral epicondyle.
    • Lateral Supracondylar Ridge: Sharp ridge formed by the lower portion of the lateral border that provides attachment to the lateral intermuscular septum.
    • Pierced by two anatomical structures:
      1. Radial nerve
      2. Profunda brachii artery
    • Muscular origins along the ridge:
      • Upper two-thirds: Gives origin to brachioradialis muscle.
      • Lower one-third: Gives origin to extensor carpi radialis longus muscle.
  • Surfaces:
    • Anteromedial Surface:
    • Bound between the anterior and medial borders.
    • Middle portion: Demonstrates a rough impression for the insertion of coracobrachialis muscle.
    • Lower half: Shared with the anterolateral surface, giving origin to brachialis muscle.
    • Anterolateral Surface:
    • Bound between the anterior and lateral borders.
    • Middle portion: Features a prominent V-shaped rough area termed the deltoid tuberosity, which receives the insertion of the deltoid muscle.
    • Lower half: Shared with the anteromedial surface, giving origin to brachialis muscle.
    • Posterior Surface:
    • Bound between the medial and lateral borders.
    • Spiral (Radial) Groove: Oblique shallow depression transmitting the radial nerve and the profunda brachii artery.
    • Above the spiral groove: Features an oblique ridge giving origin to the lateral head of triceps brachii.
    • Below the spiral groove: Surface area giving origin to the medial head of triceps brachii.

Peripheral Nerve Relations

  • Axillary Nerve: Direct relationship to the posterior surface of the surgical neck.
  • Radial Nerve: Direct relationship to the radial (spiral) groove located on the posterior surface of the shaft.
  • Ulnar Nerve: Direct relationship to the posterior surface of the medial epicondyle.

Patterns of Ossification

  • Ossification Process: Occurs via intracartilaginous (endochondral) ossification.
  • Primary Ossification Center:
    • Shaft (Diaphysis): Formed from 11 primary ossification center.
  • Secondary Ossification Centers:
    • Upper End (Epiphysis): Formed from 33 secondary ossification centers.
    • Lower End (Epiphysis): Formed from 44 secondary ossification centers.

Quick Revision Summary

  • Anatomical Orientation Criteria:
    • Head directed upward, backward, and medially.
    • Intertubercular sulcus directed anteriorly.
    • Olecranon fossa directed posteriorly.
  • Joint Formations:
    • Shoulder joint = Ball & socket joint.
    • Elbow joint = Hinge joint.
  • Rotator Cuff Components:
    • Subscapularis
    • Supraspinatus
    • Infraspinatus
    • Teres minor
  • Three Necks:
    • Anatomical neck
    • Surgical neck
    • Morphological neck
  • Three Directly Related Nerves:
    • Axillary nerve (at surgical neck)
    • Radial nerve (at spiral groove)
    • Ulnar nerve (at medial epicondyle)
  • Ossification Center Count:
    • Shaft: 11 primary center
    • Upper end: 33 secondary centers
    • Lower end: 44 secondary centers