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The elbow is a complex synovial joint which allows for significant motion and function primarily in the form of extension and flexion, however, the elbow joint also facilitates motion in the hand in the form of supination and pronation of the forearm. The main stability of the elbow joint results from the osseous articulations of the trochlea of the humerus and the ulnar olecranon. The olecranon resembles a scoop or a wrench in which a landmark referred to as the trochlear notch, fits around the trochlea of the humerus and serves as the primary pivot point during flexion and extension of the elbow joint. The trochlear notch wraps around the humerus almost 180 degrees, while the trochlea of the humerus is wide with a central groove, this allows for a tight conforming of the two structures, adding to the stability. After the osseous articulations of the humerus and ulna (ulnohumeral articulation), the rest of the major stability components of the elbow joint comes from two strong ligaments in the form of the medial collateral ligament (MCL) and the lateral collateral ligament (LCL). Both the LCL and the MCL form into the joint capsule to provide further stability.[rx]
Anatomy of The Elbow
The elbow joint, although non-weight bearing, may be the most complex joint in the human body. The elbow is a synovial hinge joint made up of articulations of mainly the distal humerus and the proximal ulna. However, articulations exist between the proximal radius and the humerus as well as the proximal radius and ulna. The three articulations are referred to as the ulnohumeral, radiohumeral, and proximal radioulnar joints respectively. Many of the muscles of both the upper and lower arm either cross or attach to at least one component of the elbow joint making it no surprise that after the shoulder, the elbow joint is the second most commonly injured joint in sports-related injuries.[rx]
The Medial collateral ligament is a triangular-shaped ligament made up of three smaller ligaments named the anterior oblique ligament, posterior oblique ligament, and the transverse ligament, also sometimes called Cooper’s ligament. The anterior oblique ligament originates from the anterior inferior surface of the medial epicondyle of the humerus and inserts onto the coronoid process of the ulna. The anterior oblique ligament is the strongest and most important medial stabilizer of the elbow joint and protects against excessive valgus forces. The posterior oblique ligament also originates from the medial epicondyle but as its name suggests, runs posteriorly to insert onto the olecranon of the ulna. The transverse ligament passes between the previous two, running from the coronoid process and the tip of the olecranon.[rx]
The lateral collateral ligament is also composed of three parts; the annular ligament, the lateral radial collateral ligament, and the lateral ulnar collateral ligament. The three components of the LCL provide stability during varus stresses applied on the elbow and posterolateral rotational stability. The lateral ulnar collateral ligament is the predominant stabilizer in posterolateral rotational structure as it crosses over the inferior aspect of the radial head, originating from the lateral epicondyle of the humerus and inserting at the supinator crest of the ulna. The annular ligament stabilizes the proximal radioulnar joint and originates and inserts at the sigmoid notch of the ulna as it wraps around the neck of the radius.[rx]
The cubital tunnel is a clinically significant space allowing the passage of the ulnar nerve. The roof of the cubital tunnel is created by the arcuate (Osborne’s) ligament running from the medial epicondyle to the olecranon of the ulna. The floor of the cubital tunnel is made up of the MCL, while the anterior, posterior, and lateral borders are comprised of the medial epicondyle, medial head of the triceps brachii, and the olecranon respectively.
As a synovial joint, the elbow has a synovial membrane as well as a joint capsule which are separated from each other by fat pads. These fat pads are located superficial to areas of stress. These areas include the olecranon, coronoid and radial fossas. During flexion and extension, these fat pads are pulled away by attachments to the brachialis and the triceps brachii to allow space for bony processes. A lubricating bursa, namely the olecranon bursa, facilitates triceps action. This bursa serves as a lubricating component between the olecranon process of the ulna and the triceps tendon of insertion.[rx]
Blood Supply of The Elbow
The elbow joint serves as a crossing for most of the neurovasculature in the upper extremity. The superficial nature of most of these structures allows a provider to visualize many of these structures using ultrasound. The brachial artery is one of the continuations of the axillary artery and is the source of all the main arteries feeding into the elbow joint. The branches of the brachial artery include a deep brachial artery which courses posteriorly to give rise to a few other small arteries which ultimately anastomose with the radial artery. The majority of the blood flow travels through the brachial artery distally and branches into the radial and ulnar arteries.
The major lymphatic structures around the elbow joint are the deep and superficial cubital lymph nodes, and the epitrochlear and supratrochlear lymph nodes, the latter comprising up to five nodes lying superior to the medial epicondyle. Lymph drains up the arm and passes through the deep brachial lymph nodes, eventually ending up in the axillary lymph nodes.[rx]
Nerves
The nerves that cross the elbow joint predominately innervate structures of the forearm and hand. One of the major nerves responsible for acting on the elbow joint is the musculocutaneous nerve. This nerve never crosses the elbow joint itself, but rather innervates both the biceps brachii, the muscle that accounts for the majority of supination as well as some elbow flexion, and the brachialis muscle, which is responsible for elbow flexion.
The radial nerve starts medially in the proximal humerus, then spirals posteriorly and laterally into the spiral groove. The radial nerve crosses the elbow joint as it courses laterally and enters the groove for the radial nerve in the distal humerus. The radial nerve innervates five muscles that affect the elbow joint including the brachialis (lateral portion), the anconeus, the supinator muscle, brachioradialis, and triceps brachii.
The ulnar nerve stays in the medial arm and travels from the anterior compartment to the posterior compartment where it enters into the cubital tunnel and the groove for the ulnar nerve. The ulnar nerve innervates muscles in the forearm and hand.
The median nerve traverses the elbow joint anteriorly and continues distally to innervate the muscles in the forearm and hand.
Also spanning the elbow joint is the lateral antebrachial cutaneous nerve and the medial antebrachial cutaneous nerve derived from the musculocutaneous and radial nerves respectively. These two nerves supply sensory information for the skin of the forearm.[rx]
Muscles of The Elbow
Many muscles cross over and attach around the elbow joint. These muscles are responsible for the secondary stabilization of the joint. The majority of the muscles originating from the elbow joint provide very little motion at the elbow joint itself, but rather act as flexors and extensors of the wrist, hand, and digits. Contribution to elbow stability comes in the form of protection against varus and valgus forces. Muscles that protect against valgus forces by initiating a varus force include the flexor digitorum superficialis, flexor carpi ulnaris, flexor carpi radialis, and the pronator teres. Muscles that contribute to varus stability by initiating a valgus force include extensor digitorum communis, extensor carpi radialis brevis and longus, anconeus, and extensor carpi ulnaris.
Muscles which act primarily on the elbow joint include those responsible for flexion, including biceps brachii, brachioradialis, and brachialis. The biceps brachii has a modest impact on elbow flexion and acts as the primary muscle responsible for supination. The biceps brachii is especially unique as it has two distal attachments at the elbow joint, a more laterally oriented tendon inserting on the proximal radius, and a medially oriented aponeurosis which continues into the fascia of the proximal forearm. Extension of the elbow joint is accomplished almost entirely from contraction of the triceps brachii with very modest action from the anconeus muscle.[rx][rx]
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