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]

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]

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]