Elbow Anatomy: Joints, Nerves, and Biomechanics

The elbow is a fundamental joint of the upper limb that allows the hand to be positioned in space, bringing it closer to or further away from the body, and rotating it to orient the palm upwards or downwards. Although often considered a “simple” joint, the elbow is actually a complex of three joints enclosed within a single capsule, traversed by delicate nervous structures, and is the site of extremely common pathologies such as epicondylitis (tennis elbow), epitrochleitis (golfer’s elbow), and ulnar nerve neuropathy.

Understanding elbow anatomy and its biomechanics is essential for correctly interpreting these conditions and for optimizing prevention and rehabilitation strategies.

The Bones of the Elbow

The elbow skeleton comprises three bones—the humerus, radius, and ulna—that form the joint between the upper arm and forearm, enabling flexion, extension, and rotational movements. The elbow is formed by the articulation of three bones: the humerus, ulna, and radius.

Humerus (Distal Extremity)

The distal extremity of the humerus widens into two epicondyles and presents two articular surfaces:

• Humeral trochlea: a medial bony pulley that articulates with the trochlear notch of the ulna. Its spool-like shape with a central crest guides the flexion-extension movement.
• Capitellum: a lateral hemispherical surface that articulates with the radial head.
• Lateral epicondyle: a lateral bony prominence where the extensor muscles of the wrist and fingers originate. It is the site of classic epicondylitis.
• Medial epicondyle (epitrochlea): a larger medial bony prominence where the flexor muscles of the wrist and fingers and the pronator teres originate. It is the site of epitrochleitis.
• Olecranon fossa: a posterior depression that accommodates the olecranon of the ulna in full extension.
• Coronoid fossa: an anterior depression that accommodates the coronoid process of the ulna in full flexion.

Ulna (Proximal Extremity)

The ulna is the medial bone of the forearm. Its proximal extremity is specialized for articulation with the humerus:

• Olecranon: the posterior bony process of the elbow (the “point” of the elbow), which inserts into the olecranon fossa in extension. It is the insertion point of the triceps tendon.
• Trochlear notch (greater sigmoid notch): the large crescent-shaped concavity that wraps around the humeral trochlea, providing intrinsic stability to flexion-extension.
• Coronoid process: an anterior projection that inserts into the coronoid fossa in flexion. It is an important anterior stabilizer.
• Radial notch (lesser sigmoid notch): a lateral concavity where the radial head articulates.

Radius (Proximal Extremity)

The radius is the lateral bone of the forearm. Its proximal extremity presents:

• Radial head (radial capitellum): a bony disc concave superiorly (for articulation with the humeral capitellum) and cylindrical laterally (for rotation within the radial notch of the ulna).
• Radial neck: the narrowing below the head.
• Radial tuberosity (bicipital tuberosity): the insertion point of the biceps brachii tendon.

The Three Joints of the Elbow

The elbow contains three joints enclosed within a single articular capsule and bathed in the same synovial fluid.

1. Humeroulnar Joint

This is the main joint of the elbow, a hinge joint between the humeral trochlea and the trochlear notch of the ulna.

• It allows for flexion (approximately 145°-150°) and extension (0°, with the possibility of physiological hyperextension of 5°-10° in some individuals).
• It is the most stable of the three joints due to the interlocking conformation of the trochlear notch on the trochlea. Osseous congruity provides approximately 50% of the elbow’s varus stability.
• The coronoid process is the primary bony stabilizer against posterior forces.

2. Humeroradial Joint

This is a ball-and-socket joint between the humeral capitellum and the concavity of the radial head.

• It participates in flexion-extension of the elbow.
• It allows for rotation of the radial head (pronation-supination).
• The radial head acts as a secondary stabilizer against valgus forces (after the medial collateral ligament).
• It transmits approximately 60% of forces from the forearm to the humerus.

3. Proximal Radioulnar Joint

This is a pivot joint between the circumference of the radial head and the radial notch of the ulna, stabilized by the annular ligament of the radius.

• It allows for pronation and supination of the forearm: the radial head rotates within the ring formed by the radial notch and the annular ligament.
• It works in synergy with the distal radioulnar joint (at the wrist) to produce complete pronation-supination.

The Ligaments of the Elbow

Medial (Ulnar) Collateral Ligament

The medial collateral ligament (MCL or UCL) is the primary stabilizer against valgus forces (medial opening). It is composed of three bundles:

• Anterior bundle: the most functionally important, extending from the medial epicondyle to the coronoid process. It is taut throughout the entire range of flexion-extension and represents the primary stabilizer against valgus.
• Posterior bundle: from the medial epicondyle to the olecranon. Taut in flexion.
• Transverse bundle: between the coronoid and olecranon, contributes little to stability.

Injury to the anterior bundle of the MCL is a typical injury in throwers (baseball, javelin) subjected to repetitive valgus stress. Surgical reconstruction (Tommy John surgery) is a widely studied topic in sports medicine.

Lateral Collateral Ligament

The lateral ligament complex includes:

• Radial collateral ligament: from the lateral epicondyle to the annular ligament. Contributes to varus stability.
• Lateral ulnar collateral ligament (LUCL): the most important component, extending from the lateral epicondyle to the supinator crest of the ulna. Its injury causes posterolateral rotatory instability (PLRI), a condition in which the radius and ulna sublux posteriorly relative to the humerus during supination and extension.
• Annular ligament of the radius: wraps around the radial head, keeping it in contact with the ulna during pronation-supination. Its laxity in young children is the basis of painful pronation (radial head subluxation).

The Nerves of the Elbow

The elbow is traversed by three main nerves of the upper limb, each vulnerable in specific locations.

Ulnar Nerve and the Cubital Tunnel

The ulnar nerve is the nerve most frequently affected by pathologies at the elbow. It runs behind the medial epicondyle in an osteofibrous tunnel called the cubital tunnel (or epitrochleo-olecranon groove), delimited by:

• Floor: the medial articular capsule and the medial collateral ligament.
• Roof: Osborne’s arcuate ligament (a fascia connecting the two heads of the flexor carpi ulnaris muscle).
• Walls: medial epicondyle (anterior) and olecranon (posterior).

The ulnar nerve is vulnerable in this location for several reasons:

• It is superficial: palpable and “percussible” (the classic “shock” sensation when hitting the elbow, known as the “funny bone”).
• The space of the tunnel reduces by 55% during elbow flexion, increasing pressure on the nerve.
• Repetitive elbow flexion, prolonged compression (resting the elbow on a table), and activities with the elbow flexed for extended periods (phone, computer) can cause cubital tunnel syndrome (ulnar nerve neuropathy).

Symptoms of ulnar nerve compression:

• Tingling and numbness of the 4th and 5th fingers (ring and little finger) and the ulnar border of the hand.
• Weakness of grip and fine motor skills (difficulty manipulating small objects, opening jars).
• In advanced cases, atrophy of the interosseous muscles of the hand and a “claw hand” appearance of the fingers.

Median Nerve

The median nerve crosses the elbow anteriorly, passing medially to the brachial artery and then under the lacertus fibrosus (expansion of the biceps aponeurosis) and between the two heads of the pronator teres.

• It can be compressed in these locations, causing pronator teres syndrome (pain in the proximal forearm, tingling in the thumb, index, and middle fingers).
• Its motor branch, the anterior interosseous nerve, innervates the deep flexors of the thumb, index finger, and the pronator quadratus.

Radial Nerve

The radial nerve crosses the elbow laterally, dividing into a superficial branch (sensory) and a deep branch (posterior interosseous nerve) which passes through the supinator muscle under the arcade of Frohse (fibrous margin of the supinator).

• Compression of the posterior interosseous nerve in the arcade of Frohse causes radial tunnel syndrome, with lateral elbow pain that can mimic epicondylitis (“resistant epicondylitis”).
• Radial nerve palsy causes the inability to extend the wrist and fingers (“wrist drop”).

The Muscles of the Elbow

Elbow Flexors

Biceps brachii

• The best-known muscle of the arm, with two heads of origin (long head: from the supraglenoid tubercle of the scapula; short head: from the coracoid process).
• It inserts onto the radial tuberosity and, via the lacertus fibrosus, onto the forearm fascia.
• It is the most powerful elbow flexor when the forearm is supinated. It is also the most powerful supinator of the forearm (especially with the elbow flexed at 90°).
• Its effectiveness as a flexor decreases in pronation, where the brachialis becomes the dominant flexor.

Brachialis

• A deep muscle, beneath the biceps, from the anterior surface of the humerus to the coronoid process of the ulna.
• It is the pure flexor of the elbow (acts independently of pronation-supination position) and the primary flexor for heavy loads.

Brachioradialis

• A forearm muscle originating from the lateral margin of the humerus and inserting onto the radial styloid process.
• Most effective elbow flexor in a neutral position of pronation-supination (“hammer” position).

Elbow Extensors

Triceps brachii

• The primary and almost sole elbow extensor. It has three heads: long (from the scapula, biarticular), lateral, and medial (from the humerus).
• It inserts onto the olecranon of the ulna.
• The medial head is the most consistently active component during extension; the lateral and long heads are recruited for greater efforts.

Anconeus

• A small posterior triangular muscle, from the lateral epicondyle to the olecranon.
• Assists extension and stabilizes the humeroulnar joint.

Lateral Epicondylar Muscles (Extensors)

The muscles originating from the lateral epicondyle form the extensor mass of the forearm:

• Extensor carpi radialis longus (ECRL)
• Extensor carpi radialis brevis (ECRB): the tendon most frequently involved in epicondylitis
• Extensor digitorum communis
• Extensor digiti minimi
• Extensor carpi ulnaris
• Supinator

Epicondylitis (tennis elbow) is an overload tendinopathy of the common extensor origin, particularly the ECRB, caused by repetitive microtrauma during gripping movements, wrist extension, and pronation-supination.

Medial Epicondylar Muscles (Flexors)

The muscles originating from the medial epicondyle (epitrochlea) form the flexor mass:

• Pronator teres
• Flexor carpi radialis
• Palmaris longus (absent in 15% of the population)
• Flexor digitorum superficialis
• Flexor carpi ulnaris

Epitrochleitis (golfer’s elbow) is a tendinopathy of the common flexor origin.

Elbow Biomechanics

Flexion and Extension

The flexion-extension axis of the elbow is not perfectly transverse but slightly oblique, passing through the center of the trochlea and capitellum. This orientation produces a physiological carrying angle when the elbow is in full extension:

• Men: approximately 10°-15° of valgus.
• Women: approximately 15°-20° of valgus.

This angle disappears in flexion and reappears in extension, allowing the arm to carry objects without hitting the hip.

Morrey’s functional arc (30°-130° of flexion and 50° of pronation + 50° of supination) is the minimum range necessary to perform 90% of daily living activities. This parameter is fundamental in post-traumatic and post-surgical rehabilitation.

Pronation and Supination

Pronation-supination is the rotatory movement of the forearm around its longitudinal axis:

• Supination (approximately 80°-90°): rotation that brings the palm upwards. Motor muscles: biceps brachii (the most powerful, especially with the elbow flexed at 90°) and supinator.
• Pronation (approximately 70°-80°): rotation that brings the palm downwards. Motor muscles: pronator teres and pronator quadratus.

The mechanism of pronation-supination involves two joints:

• Proximal radioulnar (at the elbow): the radial head rotates within the ring formed by the radial notch of the ulna and the annular ligament.
• Distal radioulnar (at the wrist): the distal extremity of the radius rotates around the head of the ulna.

During pronation, the radius crosses the ulna; during supination, the two bones are parallel. The ulna remains substantially fixed while the radius rotates around it.

Elbow Stability

Elbow stability is ensured by a multilevel system:

Primary stabilizers:

• Humeroulnar joint: provides intrinsic osseous stability.
• Medial collateral ligament (anterior bundle): primary stabilizer against valgus.
• Lateral ulnar collateral ligament: primary stabilizer against posterolateral rotatory instability.

Secondary stabilizers:

• Radial head: secondary stabilizer against valgus (becomes primary if the MCL is injured).
• Anterior capsule: limits hyperextension and distraction.
• Common origin of extensors and flexors: dynamic medial and lateral stabilizers.

Forces on the Elbow

The forces traversing the elbow during daily activities are considerable:

Elbow Synovial Bursae

The elbow has several synovial bursae, of which the most clinically relevant is the olecranon bursa, located between the skin and the olecranon. Its inflammation (elbow bursitis) is common in those who repeatedly rest their elbows on hard surfaces.

Frequently Asked Questions (FAQ)

How many joints does the elbow have?
The elbow contains three joints enclosed within a single capsule: the humeroulnar (flexion-extension), the humeroradial (flexion-extension and rotation), and the proximal radioulnar (pronation-supination).

What is the cubital tunnel?
The cubital tunnel is the osteofibrous canal located behind the medial epicondyle of the elbow through which the ulnar nerve passes. It is the second most common site of nerve compression in the upper limb (after the carpal tunnel). Elbow flexion reduces the space in the tunnel and can compress the nerve.

Why do you feel a “shock” when you hit your elbow?
The shock sensation is caused by percussion of the ulnar nerve in the epitrochleo-olecranon groove, where the nerve is superficial and vulnerable. The nerve impulse generated propagates along the nerve to the 4th and 5th fingers.

What is Morrey’s functional arc?
It is the minimum range of motion necessary to perform 90% of daily living activities: 30°-130° of flexion-extension and 50° of pronation + 50° of supination. It is a key parameter in elbow rehabilitation.

What is the carrying angle?
It is the physiological valgus angle of the elbow visible in full extension (10°-15° in men, 15°-20° in women). It allows objects to be carried at the side without the forearm hitting the body. It changes with distal humerus fractures.

Why is epicondylitis so common?
Epicondylitis affects the origin of the extensor muscles on the lateral epicondyle, particularly the extensor carpi radialis brevis (ECRB). It is caused by repetitive microtrauma during activities requiring grip and wrist movements. Poor vascularization of the tendinous origin slows healing.

Conclusion

The anatomy of the elbow reveals a joint of remarkable complexity, where three joints cooperate within a single capsule to produce the essential movements of the upper limb: flexion-extension and pronation-supination. The proximity of delicate nervous structures—particularly the ulnar nerve in the cubital tunnel—and the constant mechanical stress on the epicondylar tendons make the elbow vulnerable to pathologies that, although common, require accurate anatomical understanding to be treated effectively.

In case of elbow pain, limited movement, tingling in the fingers, grip weakness, or swelling, it is advisable to consult your doctor or physical therapist.

Frequently Asked Questions

What are the primary anatomical components that form the elbow joint?

The elbow is a complex articulation of three bones: the humerus, ulna, and radius. These bones form three distinct joints—the humeroulnar, humeroradial, and proximal radioulnar joints—all enclosed within a single capsule and supported by crucial ligaments.

Why is a comprehensive understanding of elbow anatomy considered essential in medical practice?

Grasping the intricate anatomy and biomechanics of the elbow is fundamental for accurately interpreting various conditions affecting this joint. This knowledge is critical for developing effective prevention strategies and optimizing rehabilitation protocols for common pathologies.

What are the main types of movements performed by the elbow?

The elbow joint primarily enables flexion and extension, which are movements that bring the hand closer to or further away from the body. Furthermore, the proximal radioulnar joint, an integral part of the elbow complex, facilitates pronation and supination, allowing for forearm rotation.

Which specific anatomical areas of the elbow are frequently implicated in common painful conditions?

Common elbow pains often originate from specific bony prominences, such as the lateral and medial epicondyles, where muscles attach. Conditions like epicondylitis (tennis elbow) and epitrochleitis (golfer’s elbow) are directly associated with these regions, and delicate nervous structures like the ulnar nerve are also susceptible to pathology within the elbow.

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Sources and Scientific References

1. Schoenfeld BJ et al. (2016). Longer Interset Rest Periods Enhance Muscle Strength and Hypertrophy in Resistance-Trained Men. J Strength Cond Res. 30:1805-12. DOI | PubMed
2. Redler LH et al. (2019). Treatment of Adhesive Capsulitis of the Shoulder. J Am Acad Orthop Surg. 27:e544-e554. DOI | PubMed
3. Schoenfeld BJ et al. (2019). Resistance Training Volume Enhances Muscle Hypertrophy but Not Strength in Trained Men. Med Sci Sports Exerc. 51:94-103. DOI | PubMed
4. Bonczar M et al. (2023). Treatment Options for Tennis Elbow – An Umbrella Review. Folia Med Cracov. 63:31-58. DOI | PubMed
5. Schoenfeld BJ et al. (2015). Effects of Low- vs. High-Load Resistance Training on Muscle Strength and Hypertrophy in Well-Trained Men. J Strength Cond Res. 29:2954-63. DOI | PubMed