Anatomy of the Elbow Joint

Overview

1. Humeroulnar joint - trochlear notch of ulna articulates with trochlea of humerus (hinge; primary flexion/extension)
2. Humeroradial joint - head of radius articulates with capitulum of humerus (hinge; flexion/extension)
3. Proximal radioulnar joint - head of radius articulates with radial notch of ulna (pivot; pronation/supination)

Bones and Bony Landmarks

Distal Humerus

• Trochlea (medial): pulley-shaped; articulates with the trochlear notch of the ulna; extends anterior to posterior, terminating at the olecranon fossa
• Capitulum (lateral): rounded surface; articulates with the head of the radius
• Medial epicondyle: origin of common flexor muscles; ulnar nerve passes posterior to it
• Lateral epicondyle: origin of common extensor muscles (common extensor tendon)
• Olecranon fossa (posterior): receives olecranon in full extension
• Coronoid fossa (anterior): receives coronoid process in full flexion
• Radial fossa (anterolateral): receives head of radius in full flexion

Proximal Ulna

• Olecranon process: projects posteriorly; inserts triceps tendon; received in olecranon fossa during extension
• Coronoid process: received in coronoid fossa during flexion
• Trochlear notch (semilunar notch): articulates with trochlea of humerus
• Radial notch: articulates with head of radius (proximal radioulnar joint)
• Ulnar tuberosity: attachment for brachialis muscle

Proximal Radius

• Head of radius: disc-shaped; articulates above with capitulum and medially with radial notch of ulna
• Radial tuberosity: attachment for biceps brachii tendon

Bony landmarks: When the elbow is extended, the medial epicondyle, lateral epicondyle, and olecranon tip are in a straight horizontal line (Hueter's line). When the elbow is flexed to 90°, these three points form an equilateral triangle. This relationship is disrupted in fractures and dislocations.

Joint Capsule

Fibrous Membrane

• Anteriorly: attaches to the margins of the coronoid and radial fossae on the humerus, and to the coronoid process and neck of the radius
• Posteriorly: attaches to the margins of the olecranon fossa and olecranon
• Medially and laterally: blends with and is reinforced by the collateral ligaments
• On the lateral side, the free inferior margin passes around the neck of the radius (anterior to posterior), allowing rotation

Synovial Membrane

• Lines the deep surface of the fibrous capsule
• Lines the radial fossa, coronoid fossa, olecranon fossa, and the medial surface of the trochlea
• Separated from the fibrous capsule by fat pads overlying the three fossae (coronoid, olecranon, radial)
• These fat pads are displaced by the brachialis and triceps when the fossae receive bony processes during movement
• The sacciform recess is a pocket of synovial membrane that protrudes inferiorly from the free margin of the capsule - it accommodates radial head rotation during pronation/supination

Ligaments

Medial (Ulnar) Collateral Ligament Complex (UCL)

• Anterior bundle (most important): arises from the inferior margin of the medial epicondyle, inserts at the sublime tubercle of the ulnar coronoid process. It is the primary static stabilizer against valgus stress and internal rotation. Has an anterior band (taut during extension) and a posterior band (taut during flexion)
• Posterior bundle (Bardinet's ligament): from the posterior aspect of the medial epicondyle to the medial olecranon; forms the floor of the cubital tunnel; provides secondary valgus stability at extremes of flexion
• Transverse bundle (Cooper's ligament): connects olecranon to coronoid; minimal stabilizing function

Lateral Collateral Ligament Complex (LCL)

• Radial collateral ligament (RCL): from the lateral epicondyle, blends into the annular ligament anteriorly
• Annular ligament of the radius: strong ring-shaped ligament encircling the radial head; holds the head of radius against the radial notch of the ulna; blends with the fibrous joint capsule (except posteriorly); the deep surface is lined by cartilage for smooth rotation
• Accessory collateral ligament: assists the annular ligament
• Lateral ulnar collateral ligament (LUCL): lateral epicondyle to the supinator crest of the ulna; primary restraint to posterolateral rotatory instability

Muscles Acting on the Elbow Joint

• Brachialis: originates from the anterior distal half of the humerus, inserts onto the coronoid process and ulnar tuberosity - it is the elbow's sole dedicated flexor
• Biceps brachii: long head from supraglenoid tubercle, short head from coracoid process; inserts at radial tuberosity and bicipital aponeurosis; primary supinator
• Triceps brachii: three heads converge on the olecranon; primary extensor
• Anconeus: blends with triceps distally; origin at lateral epicondyle, inserts on lateral olecranon and posterior ulna; secondary extensor and stabilizer against varus forces

Carrying Angle

• Normal: 5-10° in males, 10-15° (up to 20°) in females (valgus)
• Disappears when the forearm is flexed or pronated
• Cubitus valgus: carrying angle increased (>15°); may cause tardy ulnar nerve palsy
• Cubitus varus (gunstock deformity): common sequel to malunited supracondylar fracture in children

Movements and Range

Nerve Supply

• Predominantly: branches of the radial nerve and musculocutaneous nerve
• Additional contributions from branches of the ulnar nerve and median nerve
• The ulnar nerve passes posterior to the medial epicondyle in the cubital tunnel (posterior bundle of UCL forms the floor); it is vulnerable here in fractures and cubitus valgus

Blood Supply

• Collateral branches: anterior and posterior branches of the profunda brachii artery; superior and inferior ulnar collateral arteries
• Recurrent branches: radial recurrent artery (from radial artery), anterior and posterior interosseous recurrent arteries (from ulnar artery), anterior ulnar recurrent and posterior ulnar recurrent arteries
• This network ensures collateral circulation even with arterial injury at the elbow

Elbow Position in Effusion

Clinical Correlations