The word "humerus" is derived fromLate Latinhumerus, fromLatinumerus, meaning upper arm, shoulder, and is linguistically related toGothicams (shoulder) andGreekōmos.[1]
Theupper orproximal extremity of the humerus consists of the bone's large rounded head joined to the body by a constricted portion called the neck, and two eminences, the greater and lesser tubercles.
The head (caput humeri), is nearly hemispherical in form. It is directed upward, medialward, and a little backward, and articulates with theglenoid cavity of the scapula to form theglenohumeral joint (shoulder joint). The circumference of its articular surface is slightly constricted and is termed the anatomical neck, in contradistinction to a constriction below the tubercles called the surgical neck which is frequently the seat of fracture. Fracture of the anatomical neck rarely occurs.[2]The diameter of the humeral head is generally larger in men than in women.
The anatomical neck (collum anatomicum) is obliquely directed, forming an obtuse angle with the body. It is most prominent in the lower half of its circumference, while in the upper half, it is represented by a narrow groove separating the head from the tubercles. The line separating the head from the rest of the upper end is called theanatomical neck. It affords attachment to the articular capsule of the shoulder-joint, and is perforated by numerous vascularforamens. Fracture of the anatomical neck rarely occurs.[2]
The anatomical neck of the humerus is an indentation distal to the head of the humerus on which the articular capsule attaches.
The greater tubercle (tuberculum majus; greater tuberosity) is a large, posteriorly placed projection that is placed laterally. The greater tubercle is wheresupraspinatus,infraspinatus andteres minor muscles are attached. The crest of the greater tubercle forms the lateral lip of the bicipital groove and is the site for insertion ofpectoralis major.
The greater tubercle is just lateral to the anatomical neck. Its upper surface is rounded and marked by three flat impressions: the highest of these gives insertion to thesupraspinatus muscle; the middle to theinfraspinatus muscle; the lowest one, and the body of the bone for about 2.5 cm. below it, to theteres minor muscle. The lateral surface of the greater tubercle is convex, rough, and continuous with the lateral surface of the body.[2]
Greater tubercle of the right humerusAttachments on the greater tubercle
The lesser tubercle (tuberculum minus; lesser tuberosity) is smaller, anterolaterally placed to the head of the humerus. The lesser tubercle provides insertion tosubscapularis muscle. Both these tubercles are found in the proximal part of the shaft. The crest of the lesser tubercle forms the medial lip of the bicipital groove and is the site for insertion ofteres major andlatissimus dorsi muscles.
The lesser tuberosity, is more prominent than the greater: it is situated in front, and is directed medialward and forward. Above and in front it presents an impression for the insertion of the tendon of thesubscapularis muscle.[2]
Lesser tubercle of the right humerusInsertion of subscapularis muscle on the lesser tubercle
The tubercles are separated from each other by a deep groove, thebicipital groove (intertubercular groove; bicipital sulcus), which lodges the long tendon of thebiceps brachii muscle and transmits a branch of theanterior humeral circumflex artery to the shoulder-joint. It runs obliquely downward, and ends near the junction of the upper with the middle third of the bone. In the fresh state its upper part is covered with a thin layer of cartilage, lined by a prolongation of thesynovial membrane of the shoulder-joint; its lower portion gives insertion to the tendon of thelatissimus dorsi muscle. It is deep and narrow above, and becomes shallow and a little broader as it descends. Its lips are called, respectively, the crests of the greater and lesser tubercles (bicipital ridges), and form the upper parts of the anterior and medial borders of the body of the bone.[2]
Thebody orshaft of the humerus is triangular to cylindrical in cut section and is compressed anteroposteriorly. It has 3 surfaces, namely:
Anterolateral surface: the area between the lateral border of the humerus to the line drawn as a continuation of the crest of the greater tubercle. The antero-lateral surface is directed lateralward above, where it is smooth, rounded, and covered by thedeltoid muscle; forward and lateralward below, where it is slightly concave from above downward, and gives origin to part of thebrachialis. About the middle of this surface is a rough, rectangular elevation, thedeltoid tuberosity for the insertion of thedeltoid muscle; below this is theradial sulcus, directed obliquely from behind, forward, and downward, and transmitting theradial nerve andprofunda artery.
Anteromedial surface: the area between the medial border of the humerus to the line drawn as a continuation of the crest of the greater tubercle. The antero-medial surface, less extensive than the antero-lateral, is directed medialward above, forward and medialward below; its upper part is narrow, and forms the floor of theintertubercular groove which gives insertion to the tendon of thelatissimus dorsi muscle; its middle part is slightly rough for the attachment of some of the fibers of the tendon of insertion of thecoracobrachialis muscle; its lower part is smooth, concave from above downward, and gives origin to thebrachialis muscle.
Posterior surface: the area between the medial and lateral borders. The posterior surface appears somewhat twisted, so that its upper part is directed a little medialward, its lower part backward and a little lateralward. Nearly the whole of this surface is covered by the lateral and medial heads of the Triceps brachii, the former arising above, the latter below the radial sulcus.
Its three borders are:
Anterior: the anterior border runs from the front of the greater tubercle above to thecoronoid fossa below, separating the antero-medial from the antero-lateral surface. Its upper part is a prominent ridge, the crest of the greater tubercle; it serves for the insertion of the tendon of thepectoralis major muscle. About its center it forms the anterior boundary of thedeltoid tuberosity, on which thedeltoid muscle attaches; below, it is smooth and rounded, affording attachment to thebrachialis muscle.
Lateral: the lateral border runs from the back part of the greater tubercle to the lateralepicondyle, and separates the anterolateral from the posterior surface. Its upper half is rounded and indistinctly marked, serving for the attachment of the lower part of the insertion of theteres minor muscle, and below this giving origin to the lateral head of thetriceps brachii muscle; its center is traversed by a broad but shallow oblique depression, thespiral groove (musculospiral groove). Theradial nerve runs in the spiral groove. Its lower part forms a prominent, rough margin, a little curved from backward, forward thelateral supracondylar ridge, which presents an anterior lip for the origin of thebrachioradialis muscle two-thirds above, andextensor carpi radialis longus muscle one-third below, a posterior lip for thetriceps brachii muscle, and an intermediate ridge for the attachment of thelateral intermuscular septum.
Medial: the medial border extends from the lesser tubercle to themedial epicondyle. Its upper third consists of a prominent ridge, the crest of the lesser tubercle, which gives insertion to the tendon of theteres major muscle. About its center is a slight impression for the insertion of thecoracobrachialis muscle, and just below this is the entrance of the nutrient canal, directed downward; sometimes there is a second nutrient canal at the commencement of theradial sulcus. The inferior third of this border is raised into a slight ridge, themedial supracondylar ridge, which became very prominent below; it presents an anterior lip for the origins of thebrachialis muscle and thepronator teres muscle, a posterior lip for the medial head of thetriceps brachii muscle, and an intermediate ridge for the attachment of themedial intermuscular septum.
Anterior view of the humerus showing borders and surfacesPosterior view of the humerus showing the medial and lateral borders and the posterior surface of the humerus
The deltoid tuberosity is a roughened surface on the lateral surface of the shaft of the humerus and acts as the site of insertion ofdeltoideus muscle. The posterorsuperior part of the shaft has a crest, beginning just below the surgical neck of the humerus and extends till the superior tip of the deltoid tuberosity. This is where the lateral head oftriceps brachii is attached.
Deltoid tuberosity of the humerus
The radial sulcus, also known as the spiral groove, is found on the posterior surface of the shaft and is a shallow oblique groove through which the radial nerve passes along with deep vessels. This is located posteroinferior to the deltoid tuberosity. The inferior boundary of the spiral groove is continuous distally with the lateral border of the shaft.
Radial groove continuing as the lateral border of shaft of the humerus
The nutrient foramen of the humerus is located in the anteromedial surface of the humerus. The nutrient arteries enter the humerus through this foramen.
Thedistal orlower extremity of the humerus is flattened from before backward, and curved slightly forward; it ends below in a broad, articular surface, which is divided into two parts by a slight ridge. Projecting on either side are the lateral and medialepicondyles.
The articular surface extends a little lower than the epicondyles, and is curved slightly forward; its medial extremity occupies a lower level than the lateral. Thelateral portion of this surface consists of a smooth, rounded eminence, named thecapitulum of the humerus; it articulates with the cup-shaped depression on the head of the radius, and is limited to the front and lower part of the bone.
Above the front part of the trochlea is a small depression, thecoronoid fossa, which receives thecoronoid process of the ulna duringflexion of the forearm.
Above the back part of the trochlea is a deep triangular depression, theolecranon fossa, in which the summit of theolecranon is received inextension of the forearm.
Olecranon fossa of the humerus
The coronoid fossa is the medial hollow part on the anterior surface of the distal humerus. The coronoid fossa is smaller than the olecranon fossa and receives the coronoid process of the ulna during maximum flexion of the elbow.
Coronoid fossa of the humerus
Above the front part of thecapitulum is a slight depression, theradial fossa, which receives the anterior border of the head of the radius, when the forearm is flexed.
Radial fossa of the humerus
These fossæ are separated from one another by a thin, transparent lamina of bone, which is sometimes perforated by asupratrochlear foramen; they are lined in the fresh state by the synovial membrane of theelbow-joint, and their margins afford attachment to the anterior and posterior ligaments of this articulation.
Thecapitulum is a rounded eminence forming the lateral part of the distal humerus. The head of the radius articulates with the capitulum.
Capitulum on the lateral side and trochlea on the medial side of the humerus
Thetrochlea is spool-shaped medial portion of the distal humerus and articulates with the ulna.
The epicondyles are continuous above with the supracondylar ridges.
Thelateral epicondyle is a small, tuberculated eminence, curved a little forward, and giving attachment to the radial collateral ligament of the elbow-joint, and to a tendon common to the origin of thesupinator and some of the extensor muscles.
Themedial epicondyle, larger and more prominent than the lateral, is directed a little backward; it gives attachment to the ulnar collateral ligament of the elbow-joint, to thepronator teres, and to a common tendon of origin of some of the flexor muscles of the forearm; theulnar nerve runs in a groove on the back of this epicondyle.
Medial and lateral epicondyles of the humerusMedial and lateral supracondylar ridges of the humerus
The medial supracondylar crest forms the sharp medial border of the distal humerus continuing superiorly from the medial epicondyle. The lateral supracondylar crest forms the sharp lateral border of the distal humerus continuing superiorly from the lateral epicondyle.[3]
The medial portion of the articular surface is named thetrochlea, and presents a deep depression between two well-marked borders; it is convex from before backward, concave from side to side, and occupies the anterior, lower, and posterior parts of the extremity.
Thelateral border separates it from the groove which articulates with the margin of the head of the radius.
Themedial border is thicker, of greater length, and consequently more prominent, than the lateral.
The grooved portion of the articular surface fits accurately within thesemilunar notch of the ulna; it is broader and deeper on the posterior than on the anterior aspect of the bone, and is inclined obliquely downward and forward toward the medial side.
At the shoulder, the head of the humerus articulates with the glenoid fossa of thescapula. More distally, at the elbow, the capitulum of the humerus articulates with the head of theradius, and thetrochlea of the humerus articulates with the trochlear notch of theulna.
Theaxillary nerve is located at the proximal end, against the shoulder girdle. Dislocation of the humerus'sglenohumeral joint has the potential to injure the axillary nerve or theaxillary artery. Signs and symptoms of this dislocation include a loss of the normal shoulder contour and a palpable depression under the acromion.
Theradial nerve follows the humerus closely. At the midshaft of the humerus, the radial nerve travels from the posterior to the anterior aspect of the bone in thespiral groove. A fracture of the humerus in this region can result in radial nerve injury.
Theulnar nerve lies at the distal end of the humerus near the elbow. When struck, it can cause a distinct tingling sensation, and sometimes a significant amount of pain. It is sometimes popularly referred to as 'the funny bone', possibly due to this sensation (a "funny" feeling), as well as the fact that the bone's name is a homophone of 'humorous'.[4] It lies posterior to the medial epicondyle, and is easily damaged in elbow injuries.[citation needed]
Thedeltoid originates on the lateral third of theclavicle,acromion and the crest of thespine of the scapula. It is inserted on thedeltoid tuberosity of the humerus and has several actions including abduction, extension, and circumduction of the shoulder. Thesupraspinatus also originates on the spine of the scapula. It inserts on the greater tubercle of the humerus, and assists in abduction of the shoulder.
Thepectoralis major,teres major, andlatissimus dorsi insert at theintertubercular groove of the humerus. They work to adduct and medially, or internally, rotate the humerus.
Theinfraspinatus andteres minor insert on the greater tubercle, and work to laterally, or externally, rotate the humerus. In contrast, thesubscapularis muscle inserts onto the lesser tubercle and works to medially, or internally, rotate the humerus.
The four muscles of supraspinatus, infraspinatus, teres minor and subscapularis form a musculo-ligamentous girdle called therotator cuff. This cuff stabilizes the very mobile but inherently unstableglenohumeral joint. The other muscles are used as counterbalances for the actions of lifting/pulling and pressing/pushing.
Primitive fossils ofamphibians had little, if any, shaft connecting the upper and lower extremities, making their limbs very short. In most livingtetrapods, however, the humerus has a similar form to that of humans. In manyreptiles and somemammals (where it is the primitive state), the lower extremity includes a large opening called theentepicondylar foramen to allow the passage of nerves and blood vessels.[5]
During embryonic development, the humerus is one of the first structures to ossify, beginning with the firstossification center in the shaft of the bone. Ossification of the humerus occurs predictably in the embryo and fetus, and is therefore used as a fetal biometric measurement when determining gestational age of a fetus. At birth, the neonatal humerus is only ossified in the shaft. The epiphyses are cartilaginous at birth.[6] The medial humeral head develops an ossification center around 4 months of age and the greater tuberosity around 10 months of age. These ossification centers begin to fuse at 3 years of age. The process of ossification is complete by 13 years of age, though theepiphyseal plate (growth plate) persists until skeletal maturity, usually around 17 years of age.[7]
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