Elbow Anatomy

The elbow Anatomy complex is composed of three distinct articulations: the humeroulnar joint, the humeroradial joint, and the proximal radioulnar joint.

The elbow joint serves an important linkage function that enables proper positioning of the hand and the transmission of power from the shoulder to the hand, thus augmenting the versatility and agility of the upper extremity.

Appropriate diagnosis and treatment of elbow problems require a detailed understanding of the normal elbow anatomy.

Elbow Anatomy – Capsule

The anterior joint capsule of the elbow originates from the distal humerus proximal to the radial and coronoid fossa, from where it then inserts distally into the rim of the coronoid and the annular ligament (AL).

Posteriorly, the capsule incorporates the area proximal to the olecranon process; it attaches distally along the articular margin of the sigmoid notch and the proximal aspect of the olecranon fossa.

The joint capsule of the elbow complex is thin but strong, and is reinforced medially and laterally by ligaments.

Anteriorly, the capsule contributes 38% of the resistance to valgus force and 32% of the resistance to varus force in full extension.

The capsule of the joint does not respond well to injury or prolonged immobilization, and often forms thick scar tissue, which may result in flexion contractures of the elbow.

Elbow Anatomy – Joints

The elbow Anatomy consists of 3 joints: Humeroulnar Joint, Humeroradial joint and the Proximal radioulnar joint.

1. Humeroulnar Joint:

The humeroulnar joint (trochlear joint) is a uniaxial hinge joint formed between the incongruent saddle-shaped joint surfaces of the spool-shaped trochlea of the humerus and the trochlear notch of the proximal ulna.

Anteriorly, the humeral trochlear groove is vertical and parallel to the longitudinal groove, while, posteriorly, the groove runs obliquely lateral and distal, forming an acute angle of about 15 degrees with the longitudinal axis of the humerus. This valgus angulation is referred to as the “carrying angle” of the elbow.

The carrying angle serves to direct the ulna laterally during extension and increase the potential for elbow flexion motion, as the offset allows room anteriorly for approximation of the muscles of the arm and the forearm. The carrying angle is approximately 11–14 degrees in males and 13–16 degrees in females.

Three hundred degrees of the articular surface of the trochlea is covered with hyaline cartilage, compared with only 180 degrees on the trochlear notch.

2. Humeroradial Joint:

The humeroradial (radiocapitellar) joint is a uni-axial hinge joint formed between the spherical capitellum of the humerus and the concave fovea of the radial head.

The design of this joint allows the elbow to flex and extend and for the radius to rotate.

The superior surface of the proximal end of the radius is biconcave, while the head of the radius is slightly oval.

The radial tuberosity serves as a site of attachment for the biceps brachii. The humerus widens at the elbow and forms the medial and lateral epicondyles.

3. Proximal Radioulnar Joint:

The radius and the ulna lie side by side, with the radius being the shorter and more lateral of the two forearm bones.

The proximal or superior radioulnar joint is a uniaxial pivot joint. It is formed between the periphery of the convex radial head, and the fibrous osseous ring formed by the concave radial notch of the ulna, which lies distal to the trochlear notch, and the annular ligament.

The annular ligament forms 80% of the articular surface of the proximal radioulnar joint.

The proximal and distal radioulnar joints together form a bicondylar joint. An interosseous membrane, sometimes referred to as the middle radioulnar articulation and located between the radius and the ulna, serves to help distribute forces throughout the forearm, and provide muscle attachment.

Most of the fibers of the interosseous membrane of the forearm are directed away from the radius in an oblique medial and distal direction. Because of the fiber orientation of the interosseous membrane, some of the proximal directed force through the radius is transferred across the membrane to the ulna.

Approximately 8% of the compression force due to bearing weight to the forearm crosses the wrist between the lateral side of the carpus and the radius. The remaining 20% of the compression force passes across the medial side of the carpal bones and the ulna.

Elbow Anatomy – Ligaments

Support for the elbow anatomy complex is provided through strong Ligaments of Elbow Joint :

Medial Ligament Complex:

The ulna collateral ligament (UCL) or medial collateral ligament (MCL) extends from the central two-thirds of the anteroinferior surface of the medial epicondyle to the proximal medial ulna, from just posterior to the axis of the elbow medial epicondyle, to just distal to the tip of the coronoid.

The fan-shaped MCL is the most important ligament in the elbow for providing stability against valgus stress, particularly in the range of 20–130 degrees of flexion and extension, with the humeroradial joint functioning as a secondary stabilizer to valgus loads.

The MCL achieves this stability through almost the total range of flexion and extension due to its eccentric location with respect to the axis of elbow motion.

In full elbow extension, valgus stability of the elbow is provided equally by the MCL, the joint capsule, and the joint interactions.

There are three distinct components of the MCL: the anterior bundle, the transverse bundle and the posterior bundle.

1. Anterior Bundle:

The anterior bundle of the MCL is the strongest and stiffest of the elbow collateral ligaments, with an average load to failure of 260 newtons (N).

The anterior bundle of the MCL inserts an average of 18 mm distal to the coronoid tip, and is composed of two other components, the anterior band and the posterior band, which perform reciprocal functions:

1. The anterior band of the anterior bundle is the most important component of the ligamentous complex, because it primarily stabilizes the elbow against valgus stress in the ranges of 0–60 degrees of flexion, and becomes a secondary restraint with further flexion. The anterior band is primarily responsible for stability against valgus stresses at 30, 60, and 90 degrees of flexion, making it the most important component in resisting the valgus forces associated with overhead sports activities.
2. The posterior band is taut from 60 degrees to 120 degrees of elbow flexion, and is a secondary restraint to valgus stress at 30 degrees and 90 degrees of flexion, but a primary restraint to valgus at 120 degrees of elbow flexion. The posterior band functions as an equal co-restraint with the anterior band at terminal elbow flexion, and also acts as a primary restraint to passive elbow extension. In the higher degrees of flexion, the posterior band is nearly isometric and is thus functionally important in the overhead athlete in counteracting valgus stresses.

2. Oblique (Transverse) Bundle:

The oblique bundle, also known as Cooper’s ligament is variably present.

It does not cross the elbow joint and comprises fibers running along the medial joint capsule from the tip of the olecranon to the medial ulna, just distal to the coronoid.

The transverse fibers have little role in elbow stability due to the fact that they both originate and insert on the ulna.

3. Posterior Oblique Bundle:

The posterior bundle of the MCL originates from the medial epicondyle and inserts onto the medial margin of the semilunar notch, forming the floor of the cubital tunnel and a thickening of the posterior elbow capsule.

Being thinner and weaker than the anterior bundle, the posterior bundle becomes taut at 60 degrees of elbow flexion, but provides only secondary restraint to valgus stress at flexion beyond 90 degrees.

Lateral Ligament Complex

Unlike the medial ligament complex, the lateral ligamentous complex is less discrete and individual variation is common.

The lateral complex consists of the AL, the lateral radial collateral ligament (LCL), and the lateral ulnar collateral ligament (LUCL).

The LCL courses distally and forms a broad conjoint insertion onto the proximal ulna. The proximal margin of this conjoined insertion begins at the proximal aspect of the radial head, just inferior to the radial notch. From here, it progresses along a rough ridge in line with the supinator crest of the ulna.

The LCL is closely associated with the insertions of the extensor carpi radialis brevis (ECRB) and the supinator, with the latter muscle crossing this ligament complex obliquely from distal to proximal at its ulnar attachment and becoming confluent with the underlying AL and LCL more proximally at its humeral origin.

The LUCL originates from the lateral epicondyle from where it passes over the AL and then begins to blend with it distally where it inserts onto the supinator crest of the ulna. It is unclear as to how much stability is provided by the LUCL, but it may help prevent posterolateral rotary instability.

As a unit, the lateral ligament complex functions to maintain the ulnohumeral and radiohumeral joints in a reduced position when the elbow is loaded in supination.

More specifically:

• The anterior portion of the LCL stabilizes the proximal radioulnar joint during full supination;
• The posterior portion stabilizes the joint during pronation.

Secondary restraints of the lateral elbow consist of the bony articulations, the joint capsule, and the extensor muscles with their fascial bands and intermuscular septa.

Annular Ligament

The AL, which is wider proximally and distally, runs around the radial head from the anterior and the posterior margin of the radial notch, to approximate the radial head to the radial notch and enclose the radial circumference.

The AL forms a band that encircles 80% of the radial head and functions to maintain the relationship between the head of the radius and the humerus and ulna.

The internal circumference of the AL is lined with cartilage to reduce the friction against the radial head during pronation and supination.

The external surface of the ligament receives attachments from the elbow capsule, the radial collateral ligament, and the supinator muscle.

Quadrate Ligament

The quadrate ligament is a short, stout ligament that arises just below the radial notch of the ulna and attaches to the medial surface of the neck of the radius. This ligament lends structural support to the capsule of the proximal radioulnar joint.

Elbow Bursae

There are numerous bursae in the elbow region:

The olecranon bursa is the main bursa of the elbow complex and lies posteriorly between the skin and the olecranon process. Under normal conditions, the bursa does not communicate with the elbow joint, although its superficial location puts it at high risk of injury from direct trauma to the elbow.

Other bursae in the posterior elbow region include the deep intra-tendinous bursa and a deep sub-tendinous bursa, which are present between the triceps tendon and olecranon.

Anteriorly, the bicipitoradial bursa separates the biceps tendon from the radial tuberosity.

Along the medial and lateral aspects of the elbow are the subcutaneous medial epicondylar bursa and the subcutaneous lateral epicondylar bursa.

Elbow Muscles

The muscles inserting on the proximal radius and ulna act to flex or extend the elbow and pronate or supinate the forearm. Many of the prime movers of the wrist and hand originate from the humeral epicondyles and the proximal radius and ulna.

Elbow Flexor and Supinator Group

The biceps brachii, brachialis, and brachioradialis are the primary elbow flexors. The relative position (pronated, supinated, or neutral) of the forearm determines which of the muscles provides the primary contribution to the movement. The biceps brachii is the prime elbow flexor when the forearm is supinated; when the forearm is pronated, the brachialis is the prime flexor.

The contribution of the biceps brachii to forearm supination increases with the amount of elbow flexion until approximately 90 degrees where the maximum length–tension relationship is reached. When the forearm is in its neutral position (radial side upward), the brachioradialis is the primary elbow flexor.

The supinator is assisted by the biceps brachii during forceful supination. The brachioradialis contributes to both pronation and supination when the forearm is at the end of the opposite motion (i.e., the brachioradialis contributes to pronation when the forearm is fully supinated).

The lateral bulk of the forearm muscles is formed by the extensor carpi radialis longus, extensor carpi radialis brevis, extensor carpi ulnaris, and extensor digitorum communis muscles.

Elbow Extensor and Pronator Group

The triceps brachii and anconeus extend the elbow, with the anconeus stabilizing the ulna during pronation and supination of the forearm. The primary pronator muscles of the forearm are the pronator teres, arising from just above the medial epicondyle of the humerus and inserting on the anterolateral aspect of the radius, and the pronator quadratus, located on the distal forearm running obliquely from the medial aspect of the ulna to the radius. The remaining medial bulk of the proximal forearm is formed by the flexor carpi radialis, palmaris longus, flexor digitorum superficialis, and flexor carpi ulnaris muscles.

Cubital Tunnel

The cubital tunnel is a fibro-osseous canal, that the ulnar nerve passes through.

The floor of the Cubital Tunnel is formed by the MCL, whereas the roof is formed by an aponeurosis, the arcuate ligament, or Osborne’s band, which extends from the medial epicondyle to the olecranon and arises from the origin of the two heads of the FCU.

The medial head of the triceps constitutes the posterior border of the Cubital Tunnel, and its anterior and lateral borders are formed by the medial epicondyle and olecranon, respectively.

The volume of the cubital tunnel is greatest with the elbow held in extension. As the elbow is brought into full flexion, there is a 55% decrease in canal volume.

A few other factors have been associated with a decrease in the size of the cubital tunnel, these include:

1. Space-occupying lesions,
2. Osteoarthritis,
3. Rheumatoid arthritis,
4. Heterotopic bone formation,
5. Trauma to the nerve.
6. Patients with systemic conditions such as diabetes mellitus, hypothyroidism, alcoholism, and renal failure also may have a predisposition.
7. Bulging of the MCL has also been described as a factor.

Cubital Fossa

The cubital fossa represents a triangular space, or depression that is located over the anterior surface of the elbow joint, and which serves as an “entry way” to the forearm, or antebrachium.

The boundaries of the cubital fossa are:

• Lateral: Brachioradialis and ECRL muscles;
• Medial: Pronator teres muscle;
• Proximal: An imaginary line that passes through the humeral condyles;
• Floor: Brachialis muscle.

The contents of the cubital fossa include:

1. The tendon of the biceps brachii, which lies as the central structure in the cubital fossa;
2. The median nerve, which runs along the lateral edge of the pronator teres muscle;
3. The brachial artery, which enters the cubital fossa just lateral to the median nerve and just medial to the biceps brachii tendon;
4. The radial nerve (not shown), which runs along the medial edge of the brachioradialis and ECRL muscles and is vulnerable to injury here;
5. The median cubital or intermediate cubital cutaneous vein, which crosses the surface of the cubital fossa.

Nerves

The nerves around the elbow joint are:

Ulnar Nerve (C8–T1):

At the level of the elbow, the ulnar nerve passes posterior to the medial epicondyle, where it enters the cubital tunnel.

Ulnar nerve compression in the cubital tunnel is a common entrapment neuropathy of the upper extremity, second only to carpal tunnel syndrome (CTS).

After leaving the cubital tunnel, the ulnar nerve passes between the two heads of the FCU origin and traverses the deep flexor–pronator aponeurosis.

Median Nerve (C5–T1):

As the median nerve passes through the cubital fossa, the anterior interosseous nerve (AIN) branches off it, as it passes through the two heads of the pronator teres muscle.

The AIN supplies the motor innervation to the index and middle flexor digitorum profundus (FDP), the flexor pollicis longus (FPL), and the pronator quadratus.

Radial Nerve (C5–T1):

At a point approximately 10–12 cm proximal to the elbow joint, the radial nerve passes from the posterior compartment of the arm by piercing the lateral intermuscular septum.

The nerve travels in the anterior forearm between the brachialis muscle and the biceps tendon medially, and the brachioradialis, ECRL, and ECRB muscles laterally.

Within an area approximately 3 cm proximal or distal to the elbow joint, the radial nerve branches into a deep mixed nerve, the posterior interosseous nerve (PIN), and a superficial sensory branch.

After dividing, the two terminal divisions usually follow the same course, sharing a single epineurium for several centimeters, before the superficial radial nerve moves anteriorly to lie on the undersurface of the brachioradialis, and the deep branch travels posteriorly to enter the radial tunnel/supinator canal, distal to the origin of the ECRB, at the level of the radiohumeral joint.

Entering the canal, the deep branch supplies the ECRB then passes deep to the superficial head of the supinator, where the arcade of Frohse can impinge on the nerve. The nerve continues between the two heads of the supinator and innervates this muscle during its passage to the posterolateral aspect of the radius.

On emerging from the supinator, a motor division (supplying the abductor pollicis longus, extensor pollicis brevis, extensor indicis proprius, and extensor pollicis longus muscles) and a mixed lateral branch (supplying the extensor carpi ulnaris, extensor digitorum communis, and extensor digiti minimi muscles) are recognized.

The lateral branch continues along the posterior-radial border of the radius to the wrist as the sensory branch of the PIN, which innervates the posterior (dorsal) capsule of the wrist and intercarpal joints.

The Radial Tunnel/Supinator Canal:

The radial tunnel lies on the anterior aspect of the radius and is approximately three to four fingerbreadths long, beginning just proximal to the radiohumeral joint and ending at the site where the PIN passes deep to the superficial part of the supinator muscle.

The lateral wall of the tunnel is formed by the brachioradialis, ECRL, and ECRB. These muscles cross over the nerve to form the anterior wall of the radial tunnel as well, while the floor of the tunnel is formed by the capsule of the humeroradial joint, and the medial wall is composed of the brachialis and biceps tendon.

Vascular Supply

The vascular supply to the elbow includes the brachial artery, the radial and ulnar arteries, the middle and radial collateral artery laterally, and the superior and inferior ulnar collateral arteries.

Elbow Joint Biomechanics

Biomechanically, the elbow predominantly functions as an important central link in the upper extremity kinetic chain, allowing for the generation and transfer of forces that occur in the upper extremity.

The common stresses that occur at the elbow include:

• Valgus stress, which results in medial tension and lateral compression loading;
• Varus stress, which results in lateral tension loading;
• Rxtension stress;
• Multiple combinations of these stresses.

The Elbow Joint motions include flexion and extension, and forearm pronation and supination.

Humeroulnar Joint:

The motions that occur at the humeroulnar joint involve impure flexion and extension, which are primarily the result of rotation of the concave trochlear notch of the ulna about the convex trochlea of the humerus. From a sagittal section, the firm mechanical link between the trochlear and the trochlear notch, however, limits the motion to essentially a sagittal plane.

Humeroradial Joint:

Any motion at the elbow and forearm complex involves motion at the humeroradial joint. Thus, any limitation of motion at the humeroradial joint can disrupt both flexion and extension, and pronation and supination.

During flexion and extension of the elbow, the humeroradial joint follows the pathway dictated by the anatomy of the ulnohumeral joint to which it is firmly attached by the annular and interosseous ligaments.

At rest in full extension, little if any physical contact exists at the humeroradial joint.

During active flexion, however, muscle contraction pulls the radial fovea against the capitulum.

Some supination and pronation also occur at this joint due to a spinning of the radial head.

Although the humeroradial joint provides minimal structural stability to the elbow complex, it does provide an important bony resistance against a valgus force.

Proximal Radioulnar Joint:

At the proximal radioulnar joint, one degree of freedom exists, permitting pronation and supination. Both the fascia and the musculature of the forearm depend on the integrity of the interosseous radioulnar relationship for their mechanical efficiency.

The proximal radioulnar joint is structurally an ovoid, with very little swing available to it due to its ligamentous constraints. As a consequence, its movement is confined to accommodating the osteokinematic spin of the radius.

As the radial head forms the convex partner, there is a tendency for the radial head to move posterolaterally during pronation and anteromedially during supination, but these movements are strongly curtailed by the annular and interosseous ligaments.

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