Humerus
Long bone; part of the appendicular skeleton within the upper limb. · Located in the brachium (upper arm). It is positioned between the shoulder (superiorly) and the elbow (inferiorly). It is oriented vertically with the head facing medially and superiorly.
Also known as: Upper Arm Bone; Brachium
Classification
Long bone; part of the appendicular skeleton within the upper limb.
Location
Located in the brachium (upper arm). It is positioned between the shoulder (superiorly) and the elbow (inferiorly). It is oriented vertically with the head facing medially and superiorly.
Origin
Endochondral ossification. It originates from one primary center for the shaft (appearing in the 8th week of fetal life) and seven secondary centers (head, greater tubercle, lesser tubercle, capitulum, trochlea, and both epicondyles) that fuse during adolescence.
Dimensions
In adult humans, the humerus typically ranges from 30 to 35 cm in length. The proximal head is roughly one-third of a sphere. The diameter of the shaft is greatest at the proximal end and narrows toward the middle before flaring distally into the epicondyles.
Description
The humerus is the largest and longest bone of the upper limb. It acts as a mechanical lever for the movements of the arm and serves as a critical bridge between the highly mobile shoulder girdle and the specialized forearm. Its complex articulations allow for both the wide range of circumduction at the shoulder and the precise hinge motions of the elbow.
Key Features
Hemispherical proximal head, bicipital groove, V-shaped deltoid tuberosity, and the distal 'hourglass' shaped trochlea and rounded capitulum.
Anatomical Description
The humerus consists of a proximal head, a cylindrical shaft, and a complex distal end. The proximal end features a hemispherical head, greater and lesser tubercles, and the intertubercular (bicipital) groove. The shaft (body) houses the deltoid tuberosity and radial groove. The distal end is characterized by the condyle, which includes the trochlea (medial) and capitulum (lateral), along with the olecranon fossa posteriorly and coronoid and radial fossae anteriorly.
Dimensions & Proportions
In adult humans, the humerus typically ranges from 30 to 35 cm in length. The proximal head is roughly one-third of a sphere. The diameter of the shaft is greatest at the proximal end and narrows toward the middle before flaring distally into the epicondyles.
Surface Features
Smooth articular hyaline cartilage covers the head and distal condyles. The bicipital groove has distinct lateral and medial lips. The deltoid tuberosity provides a roughened V-shaped area on the lateral mid-shaft. Nutrient foramina are typically located on the anteromedial surface of the shaft near the middle.
Articulations
Proximally, it articulates with the glenoid cavity of the scapula (glenohumeral joint). Distally, the capitulum articulates with the head of the radius, and the trochlea articulates with the trochlear notch of the ulna (humeroradial and humeroulnar joints of the elbow).
Muscle Attachments
The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) attach to the tubercles. The pectoralis major, latissimus dorsi, and teres major insert into the bicipital groove area. The deltoid attaches to its respective tuberosity, and the brachialis and triceps brachii utilize large portions of the shaft.
Blood Supply
The primary blood supply is via the nutrient artery (branch of the brachial artery). The proximal end is supplied by the anterior and posterior circumflex humeral arteries; the distal end receives blood from the profunda brachii and collateral branches around the elbow.
Developmental Origin
Endochondral ossification. It originates from one primary center for the shaft (appearing in the 8th week of fetal life) and seven secondary centers (head, greater tubercle, lesser tubercle, capitulum, trochlea, and both epicondyles) that fuse during adolescence.
Clinical Significance
Common fractures occur at the surgical neck (proximal) and the mid-shaft (potentially involving the radial nerve). Supracondylar fractures are common in children. Dislocation of the glenohumeral joint is a frequent clinical presentation.
Age & Sex Determination
Epiphyseal fusion of the proximal head occurs between ages 18-21. Sexual dimorphism is assessed via vertical head diameter and maximum length; male humeri are generally longer and have more robust muscle attachment sites.
Forensic Importance
Used primarily for stature estimation using Trotter and Gleser formulas. It is also useful for determining the minimum number of individuals (MNI) in fragmented remains due to its high preservation rate.
Comparative Anatomy
In quadrupeds, the humerus is often more robust and shorter relative to total body size. Non-human primates exhibit varying degrees of torsion in the shaft. The presence of an entepicondylar foramen is common in many mammals (e.g., felines) but rare in humans.
Evolutionary History
The humerus evolved from the proximal element of the sarcopterygian (lobe-finned fish) pectoral fin. Over time, it transitioned from a horizontal orientation in early tetrapods to a more vertical, weight-bearing, or mobile orientation in mammals.
Imaging Characteristics
On X-ray, the cortex appears thick and radiopaque. The trabecular pattern in the proximal head (Ward's triangle equivalent) can indicate bone density. The relationships of the fat pads around the distal humerus are critical for detecting occult elbow fractures.
Pathological Conditions
Osteoarthritis of the glenohumeral joint, osteosarcoma (often found in the proximal metaphysis), and Paget's disease. Osteoporosis significantly increases the risk of proximal humeral fractures in the elderly.
Surgical Relevance
Surgical approaches include the deltopectoral approach for shoulder arthroplasty and the posterior approach for fixing mid-shaft fractures. Anterolateral approaches are used for distal humeral plates.
Similar Bones
The femur is the most similar bone but is significantly larger, has a distinct neck, and a globular head with a fovea capitis. The tibia is triangular in cross-section and lacks a proximal ball-and-socket head.
Interesting Facts
The 'funny bone' sensation is not caused by the bone itself, but by the ulnar nerve being compressed against the medial epicondyle. The humerus is the only bone in the upper arm.
Photography Tips
Capture anterior and posterior views to show the tubercles and olecranon fossa respectively. Use a side-profile to highlight the deltoid tuberosity and distal anterior/posterior curvature.