The elbow joint is created by three bones – one from the upper arm and two from the lower arm. Its construction allows it to act like a hinge or to rotate via the shoulder joint and crossover of the lower two bones. The elbow is a synovial joint and consists of the tendons, ligaments and other components that these joints have in common.
Bones meeting at the elbow
The upper bone extending from the shoulder to the elbow is called the humerus. The bony edges of the elbow that can be felt under the skin are the medial and lateral epicondyles of the humerus. The two lower bones extending from the elbow to the wrist (i.e. the forearm) are the radius and ulna (see Southern California Orthopedic Institute for an illustration of the relative location). The ulna is the larger of the two forearm bones, positioned to the inside of the joint and forming a cup shape for the humerus to fit into. In the one synovial cavity there are three articulations. An articulation is the meeting of bones at a joint. The humerus articulates with each of the two bones of the forearm, and the ulna and radius articulate with each other, which is what provides some rotation to the joint.
Ligaments of the elbow
Ligaments are the connective tissues that hold bones together. There are three primary ligaments in the elbow. The main supporting ligament is the ulnar collateral ligament (also called the medial collateral ligament) connecting the ulna and humerus on the inside of the elbow. It includes two bands – the anterior band connecting the coronoid process of the ulna and medial epicondyle of the humerus and the posterior band connecting the olecranon process of the ulna and medial epicondyle of the humerus.
The radial collateral ligament (also called the lateral collateral ligament) attaches to the lateral epicondyle of the humerus and another ligament, the annular ligament, providing support to the elbow joint capsule. The annular ligament is actually a fibrous band that encircles the head of the radius to maintain contact with the humerus. It is attached to the trochlear notch of the ulna.
Muscles and tendons of the elbow
Many muscles cross the elbow joint, allowing the elbow to bend and the arm to move. Tendons are connective tissues that connect the muscles to the bones. The biceps and triceps start at the shoulder area and cover the entire upper arm, attaching to the radius and ulna, respectively, via the biceps brachii tendon and triceps brachii tendon. Muscles that overlap these at the lower end of the humerus, attaching to the ulna and radius, are the brachialis and brachioradialis.
Originating even closer to the elbow, at the medial epicondyle of the humerus, is the pronator teres, which attaches to the outer surface of the radius approximately one-third down the forearm. Originating at the lateral epicondyle of the humerus is the extensor carpi radialis brevis, which runs the length of the forearm to the hand. Small muscles supporting the elbow also include the anconeus and supinator, as well as the pronator quadratus, which is attached at the lower forearm to the hand but assists in elbow function.
Where the muscles cross the elbow there are two bursas, or synovial sacs, which make up synovial joint capsules. One is subcutaneous, the other is subtendinous. Both occur at the olecranon process (see a presentation from Newham University Hospital for visuals).
Innervation and blood supply
The muscles are controlled by nerves. The main nerves serving the muscles that cross the elbow are the median nerve, radial nerve and ulnar nerve. The ulnar nerve is responsible for the “funny bone” feeling when you smack your elbow. The tissues also require a blood supply, which is delivered by the brachial artery.
The anatomy of the elbow is quite complex, though it is sometimes considered a more simple joint in the body by only bringing together two body parts – the upper and lower arm. However, its most basic function requires a number of anatomical components.