Functional Anatomy of the Elbow

Original Editor - Ewa Jaraczewska

Top Contributors - Ewa Jaraczewska, Jess Bell and Kim Jackson  

Introduction

The elbow is an "intricate mechanical system." It has three bones - the humerus, radius and ulna - and it is one of the most stable joints. Daily activities rely heavily on this stability. Therefore, any instability caused by injury to the elbow structures can cause pain and significantly impact daily and athletic performance. This article discusses the key anatomical structures of the elbow complex, including the bony structures, articulations, ligaments, muscles, nerves and vascular supply.

Key Terms

Axes: lines around which an object rotates. The rotation axis is a line that passes through the centre of mass. There are three axes of rotation: sagittal passing from posterior to anterior, frontal passing from left to right and vertical passing from inferior to superior. The rotation axes of the foot joints are perpendicular to the cardinal planes. Therefore, motion at these joints results in rotations within three planes. Example: supination involves inversion, internal rotation, and plantarflexion.

Bursae: reduces friction between the moving parts of the body joints. It is a fluid-filled sac. There are four types of bursae: adventitious, subcutaneous, synovial, and sub-muscular.

Capsule: one of the characteristics of the synovial joints. It is a fibrous connective tissue which forms a band that seals the joint space, provides passive and active stability and may even form articular surfaces for the joint. The capsular pattern is "the proportional motion restriction in range of motion during passive exercises due to tightness of the joint capsule."

Closed pack position: the position with the most congruency of the joint surfaces. In this position, joint stability increases. The closed pack position for interphalangeal joints is full extension.

Degrees of freedom: the direction of joint movement or rotation; there is a maximum of six degrees of freedom, including three translations and three rotations.

Ligament: fibrous connective tissue that holds the bones together.

Open (loose) pack position: position with the least joint congruency where joint stability is reduced.

Planes of movement: describe how the body moves. Up and down movements (flexion/extension) occur in the sagittal plane. Sideway movements (abduction/adduction) occur in the frontal plane. The transverse plane movements are rotational (internal and external rotation).

Elbow Structure

Three bones make up the elbow joint: the distal end of the humerus and the proximal ends of the ulna and radius. The ulna and the radius are both forearm bones. Their distal ends form the wrist joint.

The distal end of the humerus includes the medial and lateral columns, medial and lateral epicondyles, and two articular surfaces:

• Medial and lateral columns
  • The distal end of the medial column includes the medial humeral trochlea and the medial epicondyle.
  • The distal end of the lateral column includes the capitulum (also known as the capitellum) and, more proximally, the lateral epicondyle.
• Medial and lateral epicondyles
  • The large medial epicondyle is a bony projection located at the distal end of the medial supracondylar ridge of the humerus. The forearm flexor muscles originate here.
  • The smaller lateral epicondyle is located at the distal end of the lateral supracondylar ridge of the humerus. It is curved forward and provides an attachment point for the radial collateral ligament. The supinator and some forearm extensor muscles originate here.
• Articular surfaces:
  • Medially located trochlea articulates with the ulna.
  • Laterally located capitulum articulates with the radius.

The proximal radius consists of the radial head, neck and tuberosity:

• The radial head is cylindrical and articulates with the capitulum of the humerus. The head rotates within the annular ligament to produce supination and pronation of the forearm.
• The neck and tuberosity support the head and provide attachment points for supinator brevis and biceps brachii.

The proximal ulna:

• The olecranon process articulates with the distal humerus via the trochlea and olecranon fossa.
• The projection of the medial margin is called the sublime tubercle, which serves as an insertion for the ulnar collateral ligament.

Bones, Articulations and Kinematics of the Elbow

Both mobility and stability at the elbow joint are necessary for daily function, sports, and recreational activities. Static and dynamic structures provide elbow stability. The primary static stabilisers of the elbow are the ulnohumeral articulation, the medial collateral ligament and the lateral ulnar collateral ligament. The muscles crossing the elbow joint provide dynamic stability.

Bones and Articulations

Bones	Articulations	Characteristics	Key palpation points
Humerus Ulna	Humeroulnar joint	Made up of the trochlear groove on the humerus and the trochlear notch on the ulna. In the literature, this joint is described as a modified hinge joint, with approximately 5 degrees of internal and external rotation at the extremes of flexion and extension.
Humerus Radius	Humeroradial joint	Made up of the capitulum of the humerus and the head of the radius. Due to its dual action in joint flexion/extension and supination/pronation, it is called a hinge/pivot joint.	To palpate the head of the radius, place the patient's forearm in a supinated position. Locate the distal biceps tendon in the cubital fossa. Next, move your finger one thumb width laterally and distally from the biceps tendon, and you will feel the radial head. To confirm your palpation, ask the patient to move from supination to pronation, and you will feel the radial head rotating.
Radius Ulna	Proximal radioulnar joint	Made up of the head of the radius and the radial notch of the ulna (lesser sigmoid cavity). The muscles, bones, and joint capsule provide static and dynamic stabilisation of this joint.	To palpate the radial notch of the ulna, locate the olecranon first. Next, move your fingers gently towards the medial epicondyle. You will feel a soft, round, tubular structure, which is the ulnar nerve on the notch. Firm palpation of this area compresses the ulnar nerve and can produce an unpleasant pinprick sensation which runs down the patient's forearm.

Elbow Kinematics

Sufficient elbow range of motion is necessary for many activities of daily living (ADLs). For example, we need 140 degrees of elbow flexion for dressing and personal hygiene tasks but only 15 degrees of elbow flexion to tie our shoes. In general, most ADLs can be completed with 30 to 130 degrees of elbow flexion, 50 degrees of forearm pronation, and 50 degrees of forearm supination.

Haverstock et al. note that flexion-extension and pronation-supination motions are more frequent in the dominant arm. The dominant arm is also more commonly positioned in pronation. Shoulder abduction can help to compensate for a reduction in elbow pronation in some tasks. However, there is no effective compensation mechanism for a loss of elbow supination.

Remember, individual variation must always be considered when assessing elbow flexion and extension. For example, some individuals may have joint hypermobility, and their elbow may hyperextend by over 10 degrees. Other individuals might have increased muscle bulk (e.g. bodybuilders) and may only achieve 130 degrees of elbow flexion.

Joint	Type of joint	Plane of movement	Motion	Kinematics	Closed pack position	Open pack position
Humeroulnar joint	Hinge joint	Sagittal	Flexion/extension	Flexion: 135 degrees Extension: 0 degrees	Full extension and maximum forearm supination	70 degrees of flexion with 10 degrees of forearm supination
Humeroradial joint	Hinge/pivot joint	Sagittal Transverse	Flexion/extension Pronation/supination	Pronation: 80 degrees Supination: 85 degrees	90 degrees of elbow flexion and 5 degrees of supination	Extension and forearm supination
Proximal radioulnar joint	Synovial joint Note: the proximal radioulnar joint is functionally a pivot joint, allowing for rotation motion between the radius and the ulna (the radius rotates on the ulna)	Transverse	Pronation /supination		5 degrees of forearm supination	70 degrees of flexion and 35 degrees of forearm supination

Elbow Passive Range of Motion Assessment

Elbow range of motion can be affected by age, biological sex and body mass index. In an elbow injury, we can use the uninjured side as a reference for the range of motion assessment.

Elbow flexion

• Patient is sitting with their humerus in a neutral position. An alternative patient position is supine
• Fixate the humerus with one hand and grasp the forearm distally with your other hand
• Passively flex the elbow
• Assess end feel
• Normal range of motion is 135 degrees

Elbow extension

• Patient is sitting with their humerus in a neutral position. An alternative patient position is supine
• Fixate the humerus with one hand and grasp the forearm distally with your other hand
• Passively extend the elbow
• Assess end feel

Forearm supination

• Patient is sitting with their humerus in a neutral position. An alternative patient position is supine
• Flex the patient's forearm to 90 degrees
• Support the patient's elbow with one hand and grasp their distal forearm
• Place your palm over the palm of the patient's hand
• Passively supinate the forearm
• An alternative option: place the palm of your hand over the palmar side of the patient's radius and bring their forearm into supination
• Assess end feel

Forearm pronation

• Patient is sitting with their humerus in a neutral position. An alternative patient position is supine
• Flex the patient's forearm to 90 degrees
• Support the patient's elbow with one hand and grasp their distal forearm
• Place your palm over the palm of the patient's hand
• Passively pronate the forearm
• An alternative option: place your hand over the dorsal side of the patient's radius and bring their forearm into pronation
• Assess end feel

Elbow Bursae

The elbow has the following bursae:

• Olecranon bursa
  • the main bursa of the elbow complex
  • located posteriorly between the skin and the olecranon process
  • in a healthy elbow, there is no communication between the bursa and the elbow joint
  • susceptible to injury from direct trauma to the elbow due to its superficial location
  • inflammation (bursitis) can produce swelling around the elbow and pain with elbow movement
• Intratendinous bursa: between the tip of the olecranon and deep to the triceps brachii tendon
• Subtendinous bursa: between the triceps tendon and olecranon
• Bicipitoradial bursa: separates the biceps tendon from the radial tuberosity
• Subcutaneous medial epicondylar bursa
• Subcutaneous lateral epicondylar bursa
• Subanconeus bursa

Elbow Joint Capsule

The capsule of the elbow is connected to the annular ligament.

• The anterior capsule extends between the coronoid and radial fossae, the edge of the coronoid process, and the annular ligament.
• The posterior capsule extends between the olecranon fossa, along the medial and lateral articular margins of the greater sigmoid notch, and becomes an extension of the annular ligament.
• There are inconsistent findings regarding the contribution of the joint capsule to the passive stabilisation of the elbow.

Ligaments of the Elbow

Key ligaments	Origin	Insertion	Action/role	Key palpation points
Ulnar collateral ligament or medial collateral ligament Anterior bundle: anterior, central and posterior bands Posterior bundle Transverse segment	Anteroinferior aspect of the medial epicondyle	Proximal aspect of the ulna	Provides medial stability to the elbow joint. The anterior band of the anterior bundle is taut throughout the arc of motion and is a primary static stabiliser of the elbow. The posterior bundle is taut only in flexion as it attaches posterior to the sagittal axis of rotation. The transverse segment has the least contribution to elbow stability. The ulnar collateral ligament commonly gets stressed in sports with repetitive overhead arm use or throwing.	Position the patient's forearm in supination. When the elbow is in flexion and external rotation, the sharp bony eminence of the medial epicondyle can be palpated on the the medial aspect of the elbow. Locate the medial epicondyle by finding the olecranon process - this is the sharp point of the elbow, and it is the most palpable structure of the elbow. You can palpate the medial epicondyle directly medial to the olecranon process. The attachment point for the common flexor tendons is slightly distal to this landmark. You can confirm your palpation with wrist flexion. The medial collateral ligament cannot be directly palpated.
Radial collateral ligament or lateral collateral ligament	Lateral epicondyle	Annular ligament	Provides lateral stability to the elbow joint	To palpate the lateral epicondyle, position the patient's forearm in pronation. The lateral epicondyle is the bony ridge structure on the lateral aspect of the elbow joint. It can be palpated lateral to the olecranon process. The attachment point for the common extensor tendon is located distal to the lateral epicondyle.
Annular ligament	Anterior margin of the lesser sigmoid notch	Posterior margin of the lesser sigmoid notch	Encircles 80% of the radial head. Maintains the relationship between the head of the radius and the humerus and ulna. Reduces friction against the radial head during pronation and supination.	It is palpable ONLY in the presence of pathologic changes in the ligament or the radial head.
Quadrate ligament	Radial notch of the ulna	Medial surface of the neck of the radius	Provides structural support to the capsule of the proximal radioulnar joint.

Muscles of the Elbow

The muscles of the elbow apply a compressive load to the elbow joint when they contract. They, therefore, act as dynamic stabilisers of the elbow joint. They can be grouped as follows: elbow extensors located posteriorly, elbow flexors located anteriorly, supinators positioned laterally and pronators positioned medially in relation to the joint axis.

The following sections include muscles according to their role in moving the elbow. Other functions of these muscles unrelated to elbow motion are not included (e.g. biceps brachii flexes the shoulder and helps to stabilise the head of the humerus in the glenoid cavity, but this section focuses on its role in elbow flexion and supination).

Elbow Flexors

Muscle	Origin	Insertion	Innervation	Action
Pronator teres	Medial epicondyle of the humerus via the common flexor tendon, and the coronoid process of the ulna	Lateral surface of the radial shaft	Median nerve (C6-C7)	Elbow flexion Elbow pronation
Brachialis	Shaft of the humerus	Coronoid process and tuberosity of the ulna	Musculocutaneous nerve (a medial portion of the muscle) Radial nerve (lateral portion of the muscle)	Pure elbow flexor
Brachioradialis	Lateral supracondylar ridge	Styloid process of the radius	Radial nerve (C5-C6)	Elbow flexor when the forearm is neutral.Involved in supination and pronation depending on the position of the forearm.
Biceps brachii: Short head (SH); Long head (LH)	SH: coracoid process of the scapula; LH: supraglenoid tubercle of the scapula	SH/LH: radial tuberosity	Musculocutaneous nerve	Flexes the elbow joint, particularly when the forearm is supinated. Assists with forearm supination.

Elbow Extensors

Muscle	Origin	Insertion	Innervation	Action
Anconeus	Lateral epicondyle	Shaft and olecranon of the ulna	Radial nerve (C6-C8)	Assists in elbow extension
Triceps brachii Long head (LH); Lateral head (LTH); Medial head (MH)	LH: infraglenoid tubercle of the scapula; LTH: lateral surface of the humeral shaft; MH: medial surface of the humeral shaft	LH/LTH/MH: olecranon of the ulna	Radial nerve: LH: C7 LTH: C6 MH: C8	Primary extensor of the elbow

Forearm Pronators

Muscle	Origin	Insertion	Innervation	Action
Pronator teres	Medial epicondyle of the humerus via the common flexor tendon, and the coronoid process of the ulna	Lateral surface of the radial shaft	Median nerve (C6-C7)	Elbow flexion; Forearm pronation
Pronator quadratus	Oblique ridge of the ulnar shaft	Anterior surface of the radial shaft	Anterior interosseous nerve (terminal motor branch of median nerve)	Pulls the distal end of the radius over the ulna, which results in the pronation of the radioulnar joint
Brachioradialis	Lateral supracondylar ridge	Styloid process of the radius	Radial nerve (C5-C6)	Elbow flexor when the forearm is neutral. Involved in pronation to neutral when the forearm is in supination.

Forearm Supinators

Muscle	Origin	Insertion	Innervation	Action
Supinator	Supinator crest of the ulna, radial collateral ligament, annular ligament and the lateral epicondyle of the humerus	Lateral surface of the radial shaft	Radial nerve (C5-C6)	Prime supinator of the forearm
Brachioradialis	Lateral supracondylar ridge	Styloid process of the radius	Radial nerve (C5-C6) (via a branch called the posterior interosseous nerve)	Elbow flexor when the forearm is neutral. Assists in supinating the forearm to neutral when the forearm is in pronation.
Biceps brachii: Short head (SH); Long head (LH)	SH: coracoid process of the scapula; LH: supraglenoid tubercle of the scapula	SH/LH: radial tuberosity	Musculocutaneous nerve	Flexes the elbow joint, particularly when the forearm is supinated. Assists with forearm supination

Innervation of the Elbow

The nerve supply to the elbow stems from the brachial plexus that originates in the shoulder. The brachial plexus is a network of nerves formed from the ventral rami of nerve roots C5 to T1. From proximal to distal, the brachial plexus is organised by roots, trunks, divisions, and cords.

• Musculocutaneous nerve: the terminal branch of the lateral cord of the brachial plexus
• Median nerve: emerges from the lateral and medial cords of the brachial plexus
• Radial nerve: arises from the posterior cord of the brachial plexus
• Ulnar nerve: continues from the medial cord of the brachial plexus within the axilla region. It does not innervate any muscles that move the elbow.

Nerve	Origin	Branches	Motor fibres	Sensory fibres
Median nerve	Terminal branch of the medial cord of the brachial plexus (nerve roots C6-T1 - can contain fibres from C5 in some individuals)	Anterior interosseous nerve; Palmar cutaneous branch; Motor branch in the hand	Flexors (anterior) compartment of the forearm; pronator teres, thenar and intrinsic hand muscles	The skin over the thenar eminence, the palmar aspect of the thumb, index, middle finger and radial half of the ring finger
Musculocutaneous nerve	Terminal branch of the lateral cord of the brachial plexus (nerve roots C5-C7)	Motor branches; Articular branches; Lateral cutaneous nerve of the forearm	Flexors (anterior) compartment of the forearm; coracobrachialis, biceps brachii, and brachialis muscles	Cutaneous innervation of the lateral forearm
Radial nerve	Posterior cord of brachial plexus (C5-T1)	Posterior brachial cutaneous nerve, inferior lateral brachial cutaneous nerve, posterior antebrachial cutaneous nerve, muscular branches, deep branch of radial nerve, superficial branch of radial nerve	Triceps brachii Anconeus Brachioradialis	Central and posterior aspects of the forearm
Anterior interosseous nerve	Median nerve	Terminal branches around the wrist joint	Pronator quadratus	Mainly a motor nerve. May supply some distal sensory branches

Vascular Supply of the Elbow

The branches from the brachial artery supply the elbow. This artery is located near the skin's surface. It is used to measure blood pressure and can be easily damaged during arm fractures.

Artery	Origin	Branches	Supply
Deep brachial artery (profunda brachii artery)	Brachial artery	Nutrient arteries of humerus, deltoid branch, middle collateral artery, radial collateral artery	Triceps brachii, anconeus, brachialis, brachioradialis muscles; Lateral intermuscular septum; Radial nerve.
Superior ulnar collateral artery		Anterior and posterior ulnar recurrent arteries Periarticular arterial anastomosis of the elbow	Triceps brachii; Elbow joint
Inferior ulnar collateral artery		Anterior ulnar recurrent artery (anastomoses) Branch to superior ulnar collateral artery	Biceps brachii and brachialis
Ulnar artery		Anterior and posterior ulnar recurrent arteries Common interosseous artery, palmar carpal arch, superficial palmar arch, and dorsal carpal branch	Elbow joint, medial and central forearm muscles, median and ulnar nerves, and common flexor sheath
Radial recurrent artery		Muscular branches	Elbow joint, brachialis, brachioradialis, and other forearm muscles

Clinical Relevance

1. Medial epicondyle tendinopathy (also known as golfer's elbow) causes medial elbow and proximal forearm pain with activities that require wrist flexion and forearm pronation. Clinical examination reveals tenderness from the medial epicondyle to the pronator teres and flexor carpi radial muscles. Degenerative changes at the origin of the flexor tendons due to micro-trauma and overload can lead to the development of this condition.
2. Pain on the lateral side of the elbow can be associated with lateral epicondyle tendinopathy, also known as tennis elbow.
3. Rupture of the biceps brachii tendon results in a Popeye sign.

Resources

• Li K, Zhang J, Liu X, Zhang M. Estimation of continuous elbow joint movement based on human physiological structure. Biomed Eng Online. 2019 Mar 20;18(1):31.
• Gates DH, Walters LS, Cowley J, Wilken JM, Resnik L. Range of motion requirements for upper-limb activities of daily living. Am J Occup Ther. 2016 Jan-Feb;70(1):7001350010p1-7001350010p10.