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 comprises the humerus and two forearm bones: radius and ulna. The relationship between joint articulation and soft tissue makes the elbow one of the most stable joints. Daily activities rely heavily on this joint stability. Hence, elbow instability due to damage to the bone joint surface and the ligament structure can cause pain and severe impairment in daily and athletic performances.

This article discusses the key anatomical structures of the elbow complex, including the bony structures, articulations, ligaments, muscles, nerves and the 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 a 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 which form the wrist joint at their distal end.

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

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

• The radial head is cylindrical and articulates with the capitellum 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 points of attachment for the supinator brevis and biceps brachii

The proximal ulna

• Olecranon process articulates with distal humerus via 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

Mobility and stability of the elbow joint are necessary for daily functions, sports, and recreational activities. Static stabilisers provide elbow stability via ulnohumeral articulation, the medial collateral ligament and the lateral ulnar collateral ligament. The joint capsule and the medial and lateral collateral ligaments provide 50% elbow stability. The remaining stabilisation is associated with the bony structure of the joint and dynamic stabilisers, including all muscles crossing the elbow joint and providing joint compressive forces.

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, the 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 capitulation of the humerus and the head of the radius. Due to its dual action in joint flexion/extension and supination/pronation 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 can feel the radial head. To confirm your palpation, ask the patient to rotate the arm from supination and pronation, and you can feel the radial head rotating.
Radius Ulna	Proximal radioulnar joint	Made up of the head of a radius and the radial notch of the ulna (lesser sigmoid cavity). It is supplied the muscles, bones, and joint capsule to provide static and dynamic stabilisation of the joint.	To palpate the radial notch of the ulna locate the olecranon first. Next, move your fingers gently towards the medial epicondyle. The soft, round, tubular structure you can palpate is the ulnar nerve on the notch. Firm palpation can produce an unpleasant pinprick sensation running down the forearm due to the ulnar nerve compression.

Elbow Kinematics

The variation between individuals must be considered when assessing elbow flexion and extension. It includes individuals with joint hyperlaxity who may hyperextend by over 10 degrees or bodybuilders who flex only up to 130 degrees due to their muscle bulk. According to Morrey et al., activities of daily living, including dressing and hygiene, require 140 degrees of elbow flexion, while 15 degrees of flexion is needed to tie a shoe. In general, most daily living activities can be completed with 30 to 130 degrees of available elbow flexion, 50 degrees of forearm pronation, and 50 degrees of forearm supination.For daily task performances, these ranges are required in a dominant upper extremity as the dominant forearm is in pronation. Reduction in elbow pronation range of motion can be compensated by shoulder abduction in some tasks. However, there is no effective compensation mechanism for loss of elbow supination.

Joint	Type of joint	Plane of movement	Motion	Kinematics	Closed pack position	Open pack position
Humeroulnar joint	Hinge joint	Saggital	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	Pivot joint	Saggital 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 Note: it is functionally a pivot joint, allowing for rotation motion between the radius and the ulna (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, sex and Body Mass Index. In an elbow injury, the results obtained on the uninjured side can serve as a reference for the elbow range of motion assessment.

Elbow flexion

• The patient is in a sitting position with the humerus held in a neutral position. Alternative patient's position: supine
• Fixate the humerus with one hand
• Grab the forearm distally
• Perform elbow flexion
• Assess the end feel
• Norm: 135 degrees

Elbow extension

• The patient is in a sitting position with the humerus held in a neutral position. Alternative patient's position: supine
• Fixate the humerus with one hand
• Grab the distal forearm with the other hand
• Perform elbow extension
• Assess the end feel

Elbow supination

• The patient is in a sitting position with the humerus held in a neutral position. Alternative patient's position: supine
• Flex the patient's forearm to 90 degrees
• Support the elbow with one hand
• Grasp the patient's distal forearm
• Place your palm over the palm of the patient's hand
• Perform supination
• Assess the end feel

Elbow pronation

• The patient is in a sitting position with the humerus held in a neutral position. Alternative patient's position: supine
• Flex the patient's forearm to 90 degrees
• Support the elbow with one hand
• Grasp the patient's distal forearm
• Place your palm over the palm of the patient's hand
• Perform pronation
• Assess the end feel

Elbow Bursae

• The 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
• Intratendinous bursa
• 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

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 joint capsule contribution 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	The anteroinferior aspect of the medial epicondyle	Proximal aspect of the ulna	The anterior band of the anterior bundle is taut throughout the arc of motion and is a primary static stabilizer 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.	Patient's forearm is positioned supinated. The medial wide end of the humerus is the medial condyle. With elbow flexion and external rotation, the sharp bony eminence on the medial aspect of the elbow is the medial epicondyle. Slightly distal to this landmark, there is a point of attachment for the common flexor tendons. Wrist flexion will confirm your palpation. You can locate the medial epicondyle by finding the most palpable structure of the elbow - the sharp point of the elbow, which is the olecranon process. Directly medial to the olecranon process, you can palpate the bony projection-the medial epicondyle. The medial collateral ligament alone cannot be palpated.
Radial collateral ligament or lateral collateral ligament	Lateral epicondyle	The annular ligament	Provides stabilisation for the radial head	To palpate the lateral epicondyle, the patient's forearm is pronated. The lateral epicondyle is the bony ridge structure on the lateral aspect of the elbow joint. Distal to the lateral epicondyle, the attachment point for the common extensor tendons is located. The lateral epicondyle can be palpated lateral to the olecranon process.
Annular ligament	The anterior margin of the lesser sigmoid notch	The 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 the friction against the radial head during pronation and supination	It is palpable ONLY in the presence of pathologic changes in the ligament or in the radial head
Quadrate ligament	Radial notch of the ulna	The medial surface of the neck of the radius	Provides structural support to the capsule of the proximal radioulnar joint

Muscles of the Elbow

Muscles are dynamic stabilisers of the elbow joint through compressive load applied to the joint when muscles contract. They can be grouped into elbow extensors located posteriorly, elbow flexors at the front of the joint, the supinators positioned laterally and pronators medially in relation to the joint axis.

The following sections will include muscles according to their role in moving the elbow. Other functions of these muscles not related to the 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).

Elbow Flexors

Muscle	Origin	Insertion	Innervation	Action
Pronator teres	Medial epicondyle of the humerus via the common flexor tendon The coronoid process of the ulna	The lateral surface of the radial shaft	Median nerve (C6-C7)	Elbow flexion Elbow pronation
Brachialis	The shaft of the humerus	The 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	The lateral supracondylar ridge	The 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: the coracoid process of the scapula LH: the 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	The 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

Elbow Pronators

Muscle	Origin	Insertion	Innervation	Action
Pronator teres	Medial epicondyle of the humerus via the common flexor tendon The coronoid process of the ulna	The lateral surface of the radial shaft	Median nerve (C6-C7)	Elbow flexion Elbow pronation
Pronator quadratus	The oblique ridge of the ulnar shaft	The anterior surface of the radial shaft.	The anterior interosseous nerve	Pulls the distal end of the radius over the ulna, which results in the pronation of the radioulnar joint.
Brachioradialis	The lateral supracondylar ridge	The 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

Elbow Supinators

Muscle	Origin	Insertion	Innervation	Action
Supinator	Supinator crest of the ulna The radial collateral ligament The annular ligament The lateral epicondyle of the humerus	Lateral surface of the radial shaft	Radial nerve (C5-C6)	Prime supinator of the forearm
Brachioradialis	The lateral supracondylar ridge	The styloid process of the radius	Radial nerve (C5-C6)	Elbow flexor when the forearm is neutral Assist in supinating the forearm to neutral when the forearm is in pronation
Biceps brachii: Short head(SH) Long head(LH)	SH: the coracoid process of the scapula LH: the 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 originated 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	Brunches	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	Flexor (anterior) compartment of the forearm: pronator teres and pronator quadratus muscles Thenar and intrinsic hand muscles	The skin over thenar eminence, 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 The lateral cutaneous nerve of the forearm	Flexor (anterior) compartment of the forearm: the 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. It is located near the skin's surface. The brachial artery measures 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 and brachialis muscles
Ulnar artery		Anterior and posterior ulnar recurrent arteries Common interosseous artery, palmar carpal arch, superficial palmar arch, and dorsal carpal branch	The elbow joint, medial and central forearm muscles, median and ulnar nerves, and common flexor sheath
Radial recurrent artery		The muscular branches	The elbow joint, brachialis, brachioradialis, and other forearm muscles.

Clinical Relevance

1. Medial epicondylopathy (golfer's elbow) presents with medial elbow and proximal forearm pain with activities requiring 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 elbow's lateral side can be a symptom consistent with lateral tendinopathy or the tennis elbow.
3. Popeye's deformity is "a pronounced bulging muscle in the distal aspect of the biceps region of the arm."

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.
• Special Tests for the Elbow Exam https://www.sportsmedreview.com/blog/special-tests-elbow-exam/
• 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.