Humerus: Structure, Function & Diseases

Humerus is the upper arm bone, one of the strongest bones of the upper extremities. Nerves and blood vessels run along the humerus, and numerous muscles have their sinewy attachments here. Despite its tremendous stability, fractures of the humerus are not uncommon.

What is the humerus?

Humerus or Os humeri (bone of the humerus) is the Latin name for the upper arm bone. It is one of the strongest tubular bones in the human body and is the longest bone in the arm. A tubular bone is – as the name suggests – built like a tube. It has a kind of canal inside it, in which bone marrow is located. On the outside, it is covered by periosteum (bone skin).

Anatomy and structure

The humerus belongs to the tubular or long bones (ossa longa). It consists of three parts, the upper and lower ends (epiphyses) and the shaft (diaphysis). At the upper end is the head of the bone (caput humeri). It is hemispherical in shape and lies in the socket of the shoulder joint. Next to the head are two bony prominences, a larger one (tuberculum majus) and a smaller one (tuberculum minus). The large muscles of the upper arm attach to these two bony regions. Adjacent to the head, in the direction of the shaft, is the neck (collum) of the humerus. Two regions are distinguished here. The collum anatomicum separates the head anatomically from the shaft and serves as the attachment point for the shoulder joint capsule. The term collum chirurgicum refers to the site that is particularly susceptible to fracture. The shaft of the humerus has three surfaces: anterior lateral, anterior medial, and posterior. This also creates three edges. Both the surfaces and the edges serve as attachment points for muscles. Along the length of the shaft is a groove in which the radial nerve and an artery of the arm pass. The lower end of the humerus has two bony prominences on the sides, the epicondyles, between which the cartilaginous roll is located.

Function and tasks

The humerus connects the shoulder to the forearm. The head of the humerus (caput humeri), together with the clavicle (collarbone) and the scapula (shoulder blade), forms the shoulder joint, one of the most complex joints in the human body. In the caudal direction, i.e. downward, the humerus joins the ulna and radius to form the elbow joint. This in turn consists of three individual joints. The humeroulnar joint is formed with the humeral roll (trochlea humeri) and the ulna, and the humeroradial joint is formed with the head of the humerus (capitulum humeri) and the head of the radius (caput radii). The third part of the elbow joint does not involve the humerus; only the ulna and radius meet here. There are numerous attachment sites for muscles on the humerus. The shoulder muscles run from the scapula to the top of the humerus. Part of this musculature is the so-called rotator cuff. It stabilizes the joint and holds the head of the humerus in the glenoid cavity. It also allows the arm to rotate, inward and outward, and to abduct. Other muscle attachments are located on the shaft of the humerus and at the lower end. Since the humerus connects the shoulder and forearm, it also enables the movements of the forearm through the interaction of the various muscles.

Diseases and complaints

While the humerus is a very strong bone and can withstand some stress, it is not a strong bone. However, it can fracture if a larger or sudden force is applied. A common injury is the proximal humerus fracture. This is a fracture at the upper part of the bone that is close to the body. It usually affects older people who have osteoporosis, a disease in which the bone substance loses stability. This injury is also called an indicator fracture, which means that the fracture is an indication of the disease osteoporosis. Most often, proximal humerus fracture affects older women. During a fall, the arm is instinctively extended to break the fall, putting all the force of the weight on the humerus. It is not uncommon for this to result in a compression fracture at the collum surgicum, but fractures are also possible directly at the humeral head or at the bony prominences. The distal humerus fracture, on the other hand, takes place at the lower end of the bone. A distinction is made whether only the humerus or also parts of the elbow joint are fractured. If the shaft is affected by a fracture, physicians call this a humeral shaft fracture.These fractures occur when extreme force is applied to the arm from the side, either by a blow or, in a traffic accident, by a side impact. This type of injury often also involves nerves, vessels, muscles or tendons that attach to or run along the bone. In cases of complex damage to the humerus and its surrounding structures, prompt treatment is absolutely necessary. Comminuted or torsion fractures require surgical treatment because they must be set up and fixed. In addition to fractures, tennis elbow (epicondylitis humeri radialis) is one of the most common disorders of the humerus. In this case, the tendon insertion of the extensor muscles of the forearm located on the humerus becomes inflamed. Golfer’s elbow (epicondylitis humeri ulnaris), on the other hand, which occurs much less frequently, develops when the tendons of the flexor muscles become inflamed. Both diseases manifest themselves with pain in the elbow area, which can radiate into the upper arm or forearm. When the upper end of the humerus slips out of the shoulder joint, this is called dislocation of the shoulder (shoulder luxation). This often happens during sports or accidents.