Introduction

The humerus is the largest bone of the upper extremity and defines the human brachium (arm). It articulates proximally with the glenoid via the glenohumeral (GH) joint and distally with the radius and ulna at the elbow joint. The most proximal portion of the humerus is the head of the humerus, which forms a ball and socket joint with the glenoid cavity on the scapula.[1] Just inferior to the head of the humerus is the anatomical neck of the humerus, which divides the head of the humerus from the greater and lesser tubercles. The anatomical neck of the humerus is the residual epiphyseal plate. An intertubercular groove appears proximally, which demarcates the two tubercles vertically. Following the tubercles is the surgical neck of the humerus, a site commonly susceptible to fractures.

Continuing distally is the cylindrical-shaped shaft of the humerus, which contains a deltoid tubercle on its lateral aspect and a radial groove on its posterior aspect (also referred to as the spiral groove).[2] At the distal portion of the humerus, there exists a widening of the bone that forms the medial and lateral epicondyles. The distal portion of the humerus ends with an area referred to as the condyle, which is composed of the trochlea, capitulum, olecranon, coronoid and radial fossae.[3]  On the anterior lateral surface of the condyle is the lateral capitulum, which articulates with the head of the radius bone, and on the anterior medial surface of the condyle is the trochlea, which articulates the trochlear notch of the ulna bone. The coronoid fossa is located superior to the trochlea and accommodates the coronoid process of the ulna and superior to the capitulum on the anterior surface of the condyle, which is the radial fossa which receives with the head of the radius, both upon flexion of the elbow joint. On the posterior surface of the condyle is the olecranon fossa, which articulates with the olecranon of the ulnar bone upon flexion of the elbow joint.[4]

Structure and Function

The humeral head articulates with the glenoid fossa of the scapula and forms the glenohumeral joint, a synovial ball and socket joint.[5] This joint allows movement along multiple planes, including internal and external rotation, abduction and adduction, flexion and extension, and is principally determined by activation of the rotator cuff muscles (teres minor, subscapularis, supraspinatus, infraspinatus) pectoralis major and deltoid. The glenohumeral joint contains multiple synovial bursae that allow frictionless mobility, including the subacromial, subdeltoid, subcoracoid, and coracobrachial bursae.[6]  The coracoacromial and acromioclavicular ligaments stabilize the GH joint; these prevent proximal migration of the humerus.[7]

Articulation of the capitellum and trochlea of the humerus with the head of the radius bone and trochlear notch of the ulna forms the elbow joint, a synovial hinge joint. This joint is stabilized by the ulnar (medial) collateral ligament and radial (lateral) collateral ligament complexes.[8] The ulnar collateral ligament (UCL) has three components:

• Anterior oblique band: the strongest and most significant stabilizer to valgus stress forces
  • Courses from the medial epicondyle of the humerus to the sublime tubercle at the proximal ulna
• Posterior oblique band: tightest in flexion; the posterior component of the UCL demonstrates the most significant change in tension from flexion to extension range of motion (ROM) at the elbow.
• Transverse ligament: contributes minimal, if any, to elbow joint stability.

An olecranon bursa exists here to reduce friction during movement as well. This joint allows only extension and flexion. Elbow movement is principally the result of the biceps brachii, coracobrachialis, and triceps muscles.[9] One should note that in anatomical position, the humerus and antebrachial bones do not perfectly align, and instead form what is known as a carrying angle. In males, this angle varies from 5 to 10 degrees, and in females may reach up to 18 degrees.[10] This angle allows clearing of the hips, and an excessive lateral deviation is referred to as cubitus valgus, with excessive medial deviation referred to as cubitus varus.

Embryology

As one of the many long bones within the appendicular skeleton, the humerus develops via endochondral ossification.[11] This process is characterized by the replacement of a cartilage template by bone. Initially, a relatively tiny cartilage model is laid down by mesenchymal cells that form cartilage secreting chondrocytes. Second, in the center of the cartilage template (ossification center), there is chondrocyte hypertrophy and secretion factors such as alkaline phosphatase to promote calcification of that cartilage. This action forms a blockade of nutrients and causes chondrocyte death. However, before dying, these cells also secrete vascular endothelial growth factor (VEGF) to support angiogenesis towards the interior of this calcifying cartilage. Meanwhile, under the influence of Indian hedgehog homolog (IHH) protein, cells outside in the perichondrium differentiate into osteoblasts and form a layer of bone surrounding the center of cartilage known as the bony collar.[11] 

As time progresses, a central area of dead chondrocytes develops, with little pieces of calcified cartilage left behind, a shell of bone around the center, and the vascular supply going around the interior. Vascular supply brings into the interior mesenchymal cells that differentiate into more osteoblasts and monocytes that form osteoclasts. In the center of the initial cartilage template, bone is resorbed on the inside and deposited on the outside, creating the growth in width of a hollow entity (forming a marrow). Simultaneously, the proximal and distal ends of the cartilage template have chondrocytes that continue to proliferate as secondary ossification centers. This activity allows cartilage growth at the end of the bones, providing vertically expansive capabilities. Ultimately, the areas where bone meets cartilage forms the epiphyseal plate (a linear zone of cartilage), where bone replaces cartilage continuously until bone growth completely fuses and stops around puberty. In the humerus specifically, there are eight ossification centers, at the head of the humerus, humeral shaft, greater and lesser tubercles, medial and lateral epicondyles, trochlea, and olecranon. 

The ossification of the shaft occurs at eight weeks gestation, with the humeral head ossification occurring at birth/after birth. The greater tubercle's ossification takes place during the first year of life and the lesser tubercle ossification within the first six years of life.[12][13] By the time of adolescence, all proximal ossification centers of the humerus have fully fused with the shaft. The distal ossifications at the condyle and trochlea and olecranon take place between early and later adolescence and fuse with the shaft of the humerus by late adolescence.[14]

Blood Supply and Lymphatics

The primary blood supply of the proximal humerus comes from anastomoses between the anterior and posterior circumflex humeral arteries.[15] These are branches of the distal third of the axillary artery.  Recent studies suggest the posterior humeral circumflex artery to be the primary source of blood supply to the humeral head.[16] The terminal division of the anterior humeral circumflex artery is the arcuate artery, which supplies the majority of the greater tuberosity.  

The axillary artery continues to become the brachial artery, which, along with one of its branches, the profunda brachii artery, will give off peri-osteal arteries to provide the blood supply for the rest humerus and its attached muscles.[17] The inner portions of the humerus are vascularized by nutrient arteries that also branch off from the brachial artery in the vicinity of the middle of the humerus.[18]

Nerves

The axillary nerve, formed from the posterior cord of the brachial plexus, wraps around the surgical neck of the humerus, and provides innervation to the deltoid and rotator cuff muscles, specifically the teres minor.[19] Innervation of the anterior portion of the brachium is supplied by the musculocutaneous nerve, which forms as a division of the lateral cord of the brachial plexus. This nerve pierces through the coracobrachialis muscle and travels between the biceps brachii and coracobrachialis, ultimately finding its termination as the lateral cutaneous nerve of the forearm. The radial nerve from the posterior cord of the brachial plexus courses through the spiral groove of the humerus, posterior to the brachial artery, and anterior to the long head of the triceps.[20] It innervates the posterior muscles of the arm, forearm, and overlying skin. The radial nerve is also responsible for the innervation of the lateral and medial epicondyle of the humerus.[21] One should note that the median nerve and ulnar nerve likewise come off the brachial plexus and travel down along the brachium, however, do not supply innervation to this region.

Muscles

The humerus serves as the origin and insertion site of many upper limb muscles that divide into the following distinctions: scapulohumeral muscles, anterior compartment muscles, posterior compartment muscles.

Scapulohumeral muscles: The deltoid muscle, which defines the shoulder contour of the upper limb, originates at three locations: the acromion of the scapula, the spine of the scapula and clavicle, with each part inserting into the deltoid tuberosity of the humerus. The deltoid muscle allows internal/external rotation and abduction/adduction of the humerus.[22] The pectoralis major muscle originates at the clavicle, manubrium, sternum body, and true ribs and inserts into the intertubercular sulcus of the humerus. It allows adduction, flexion, extension, and medial rotation of the humerus.[23] Four muscles form the rotator cuff; the subscapularis, supraspinatus, infraspinatus, and teres minor. The subscapularis muscle originates from the subscapular fossa of the scapula and inserts onto the lesser tubercle of the humerus, facilitating internal rotation of the humerus. The supraspinatus muscle originates in the supraspinous fossa of the scapula and inserts into the greater tubercle of the humerus, facilitating the abduction of the humerus. The infraspinatus muscle originates in the infraspinous fossa and scapula spine and inserts into the greater tubercle of the humerus as well and allows external rotation of the humerus. The teres major originates the inferior angle of the scapula and inserts into the lesser tuberosity of the humerus, allowing internal rotation and adduction. The teres minor originates on the lateral border of the scapula and inserts into the greater tubercle and allows external rotation.[24][25][26][27]

Anterior compartment muscles: The biceps brachii muscle has a long head and short head but does not have an actual origin or insertion sites on the humerus.[28] However, there is a transverse humeral ligament which projects from the lesser tubercle to the greater tubercle of the humerus and converts the humeral intertubercular groove into a canal which the tendon of the biceps brachii long head travels through on its way from its origination on the supraglenoid tubercle of the scapula to the radius.  The coracobrachialis originates on the coracoid process of the scapula and inserts onto the humerus and its medial surface, allowing flexion and internal rotation.[29] The brachialis muscle originates on the anterior surface of the distal humerus and inserts onto the ulna for flexion of the forearm.[30]

Posterior compartment muscles: The triceps brachii muscle contains three heads, with the medial head originating on the posterior aspect of the humerus, inferior to the spiral groove, and the lateral head originating on the posterior surface, both insert into the olecranon process of the ulna and allow extension of the forearm at the elbow joint.[31] Of note, the humerus along with the long head of the triceps, teres major and teres minor, form a quadrangular space through which the posterior circumflex artery and vein and radian nerve travel.[32]

Surgical Considerations

Elderly patients who fall on their shoulders or land on their outstretched arm can sustain proximal humeral fractures.[33] This injury presents with shoulder pain and immobility of the extremity. Multiple fragmentations of these proximal fractures can also lead to post-traumatic osteonecrosis, which can cause long-term morbidity.[34] The majority of proximal humerus fractures receive non-operative treatment. However, elderly patients with severe displacement can benefit from operative intervention, as these patients have limited bone remodeling and growth. Internal fixation via smooth wires, threaded wires, cannulated screws and intramedullary nailing are interventional choices.[35]. A reverse shoulder arthroplasty is also an option for proximal humerus fracture in the elderly, glenohumeral dislocations, rotator cuff tears, or joint disease.[36] Shoulder arthroplasty can also be indicated for shoulder osteoarthritis, inflammatory arthritis.[37] Anterior dislocation of the glenohumeral joint is common among young and active patients.[38] The greater tuberosity of the humerus is susceptible to displacement fractures, which can affect the rotator cuff muscles and also cause subacromial impingement. Surgical intervention is usually required with displacements greater than 3 mm.[39]

Children ages three and younger can sustain transphyseal fractures of the distal humerus after a fall or trauma.[40] On radiologic imaging, the humerus and forearm bone will not align according to the standard carrying angle and will have a noticeable cubitus varus. Surgical intervention should take place with closed reduction and percutaneous pinning techniques. Common sequelae include osteonecrosis of the condyles and growth arrest. Surgical intervention usually includes lateral closing wedge osteotomy, dome osteotomy, or multi-planar osteotomy.[41]

Intercondylar fractures at the distal humerus are often treated surgically as well with olecranon osteotomy.[42] Post-traumatic supracondylar humeral fractures can lead to avascular necrosis as there is an interruption of the trochlear blood supply. Patients may present asymptomatically at the time of the injury but can develop pain or loss of motion due to necrosis within six months.[43]

Clinical Significance

Radial Nerve Injury

Radial nerve palsy is among the most common peripheral nerve injuries following a humeral fracture.[44] Typically, treatment includes observation, unless nerve recovery (as demonstrated on EMG/NCS studies) does not occur within 3 to 6 months. Advocates for early exploration of the radial nerve cite delayed repair and significant internal loss of patient function as deterrents for late exploration. Trauma or fracture at the radial groove (mid-shaft) may cause radial nerve injury.

Conditions of the Shoulder

Conditions such as calcific tendinitis of the rotator cuff and adhesive capsulitis of the shoulder (i.e., frozen shoulder syndrome) are relatively common conditions with controversial and/or multifactorial etiologies.[45] Treatment consists of rest and exercise, with little need for operative intervention. However, surgical management of frozen shoulder syndrome can occur via infiltration brisement under general anesthesia.

Metastatic Disease

Metastatic bone disease causes destructive bone lesions and significant localized pain, most often in the humerus. Lesions can increase the risk of a fracture within the humerus.[46] In patients with lesions less than 50% of the cortex, treatment is done via external beam irradiation. However, with destructive lesions involving more than half the cortex, treatment consists of intramedullary nailing with postoperative external beam irradiation. Bone resection/reconstruction may be indicated in the instance where the disease persists.

Supracondylar Fractures of the Distal Humerus 

This fracture is the most common elbow fracture in the pediatric population. The fracture location is superior to the medial and lateral condyles and epicondyles. Supracondylar fractures of the distal humerus by the elbow joint can compromise nerve and vasculature depending on its displacement. Anterior displacement puts the median nerve and brachial artery at risk. Posterior displacement puts the radial artery at risk. Palpation of distal pulses and capillary refill should be part of the initial workup to survey for preserved blood supply. Anteroposterior and lateral radiographs are necessary for accurate diagnosis and treatment. Nondisplaced fractures are treated initially with a posterior splint and eventual casting. Displaced fractures are reduced, and pinned percutaneously. Malunion, compartment syndrome, and neurovascular complications are morbidities related to this fracture.[47] 

Other/Miscellaneous Conditions

Hematologic, infectious, genetic, and neurologic disorders may cause humerus varus.[48] In this condition, the medial portion of the proximal humerus will fail to develop correctly, and the lateral portion will develop more rapidly. This condition creates a varus rotation of the humerus causing decreased arm abduction and limited flexion at the shoulder joint. However, functional impairment is not common. Surgical intervention involves a valgus osteotomy of the humerus using a plate screw fixation.

Disappearing bone disease is a rare musculoskeletal condition marked by bone resorption, lack of bone formation, and lack of vascular proliferation. This disorder has severe quality of life implications.[49] Similarly, Gorham-Stout disease in the humerus is characterized by resorption of the osseous matrix with a lack of bone formation.[50] The etiology of the disease is unknown; however, lymphovascular malformation in the bone is present.  Common symptoms include aching pain, progressive weakness, and subsequent fractures. Treatment is challenging and involves a combination of surgical intervention, medication, and radiotherapy.

Charcot arthropathy is a rare disorder characterized by debilitating joint destruction. The disease can lead to bone and soft tissue loss around the humerus and can alter muscular control. Treatment involves shoulder arthroplasty.[51]

Osteochondrosis of the humerus is associated with Panner disease and osteochondritis dissecans. The two diseases present similarly.[52] Panner disease involves avascular necrosis of the capitulum, and onset is typically between ages 7 and 10, younger than the onset of osteochondritis dissecans. Panner disease generally is not treated with operative intervention.[53]