Elbow Anatomy, Stability & Biomechanics

1. Osseous Anatomy

Distal Humerus

• The trochlea — pulley-shaped medially — articulates with the trochlear notch of the ulna
• The capitellum — spherical laterally — articulates with the radial head
• The medial epicondyle — common flexor origin; the ulnar nerve runs in its posterior groove
• The lateral epicondyle — common extensor origin
• Coronoid fossa (anterior): receives coronoid process in flexion
• Radial fossa (anterior): receives radial head in flexion
• Olecranon fossa (posterior): receives olecranon in extension
• The coronoid and olecranon fossae are separated by the thin supratrochlear septum

Proximal Ulna

• The trochlear (greater sigmoid) notch — wraps around the trochlea; the primary stabilizing articulation of the elbow due to its bony congruence
• The olecranon forms the superior wall of the trochlear notch; posterior bony landmark
• The coronoid process (13–18 mm) forms the inferior wall; provides:
  • Anterior buttress against posterior translation
  • Resistance to varus and rotational forces
  • Insertion for the anterior joint capsule, anterior bundle of the medial ulnar collateral ligament (MCL), annular ligament, and brachialis
• The radial notch — lateral surface of the ulna; articulates with the radial head for pronation/supination
• The sublime tubercle — insertion point of the anterior MCL bundle

Proximal Radius

• The radial head — elliptical (not circular), mean diameter ~2 cm; 360° convex articular surface; 240° covered by cartilage; articulates with capitellum proximally and radial notch of ulna circumferentially
• The radial neck — narrow portion just distal to the head
• The radial tuberosity — biceps brachii insertion

Hueter's Line and Triangle

2. Joint Capsule

• Anterior capsule: inserts proximally above the coronoid and radial fossae; distally attaches to the anterior margin of the coronoid process and to the annular ligament
• Posterior capsule: attaches just above the olecranon fossa, extends down along supracondylar columns
• The inner surface is lined by synovial membrane, which does not extend over articular cartilage
• Ligaments are focal thickenings of the joint capsule
• The capsule is thin normally but contributes to stability at the extremes of flexion and extension
• The quadrate ligament lies below the annular ligament, extending from the radial neck to the supinator fossa of the ulna

3. Ligamentous Anatomy

Medial (Ulnar) Collateral Ligament Complex (MCL/UCL)

a. Anterior Bundle (most important)

• Origin: anteroinferior aspect of medial epicondyle
• Insertion: sublime tubercle of the coronoid process
• Contains two bands: anterior band (most important; taut 30–120° flexion) and posterior band (taut 60–120° flexion)
• The primary static stabilizer against valgus stress and internal rotation
• Mean length of the anterior common tendon: ~28 mm

b. Posterior Bundle (Bardinet's Ligament)

• Origin: posterior aspect of medial epicondyle
• Insertion: medial aspect of the olecranon
• Fan-shaped; forms the floor of the cubital tunnel
• Provides secondary stability at extremes of flexion; also resists valgus forces and ulnar rotation

c. Transverse Bundle (Cooper's Ligament)

• Runs between the coronoid process and olecranon
• Contributes minimally to stability as it spans two points on the same bone (ulna)

During elbow flexion, the flexor carpi ulnaris (FCU) aponeurosis tenses while the MCL relaxes and bulges — this decreases cubital tunnel volume and increases pressure within it, explaining ulnar nerve vulnerability in flexion.

Lateral Collateral Ligament Complex (LCL)

a. Radial Collateral Ligament (RCL)

• Origin: lateral epicondyle
• Blends distally with the annular ligament

b. Annular Ligament

• Encircles the radial head, attaching to the anterior and posterior margins of the radial notch of the ulna
• Maintains the radial head within the radial notch, allowing pronation/supination
• The radial head is surrounded by the annular ligament — a sacciform recess forms above it

c. Lateral Ulnar Collateral Ligament (LUCL)

• Origin: lateral epicondyle (posterior to RCL)
• Insertion: crista supinatoris (supinator crest) of the proximal ulna, just distal to the annular ligament
• Wraps around the posterior aspect of the radial neck
• Critical for preventing posterolateral rotatory instability (PLRI)

4. Stability — Static and Dynamic Restraints

"Elbow stability is provided equally by the soft tissue structures surrounding the joint and by the bony articulations of the joint itself." — Rockwood and Green's Fractures in Adults, 10th ed.

Static Stabilizers

Dynamic Stabilizers

5. Biomechanics

Range of Motion (ROM)

• Flexion/Extension: 0° (full extension) to ~145° flexion; functional arc = 30–130°
• Pronation/Supination: ~85° each direction; functional arc = 50° pronation / 50° supination
• The carrying angle (cubitus valgus): ~10–15° in adults (greater in females)

Joint Reaction Forces

• The elbow experiences high compressive loads during everyday activities
• During activities of daily living: joint forces of 1–3× body weight
• During heavy loading/throwing: forces can exceed 3× body weight
• The humeroradial joint bears approximately 60% of axial load through the elbow in forearm neutral rotation

The Valgus-Extension Overload Mechanism (in throwers)

• Medial tension → microtears of the anterior MCL bundle
• Lateral compression → capitellum–radial head impaction → bone contusions/osteochondral injury
• Posteromedial shear → olecranon–trochlear impaction → posteromedial osteophytes

Carrying Angle and Valgus Alignment

• The normal valgus orientation means most everyday activities exert a varus force on the elbow
• Therefore, residual instability after injury is more commonly due to LCL incompetence (not MCL), since varus loads predominate during activities of daily living

Forearm Rotation Axis

• Pronation-supination occurs about an axis running from the radial head to the distal ulna
• The proximal radioulnar joint (annular ligament + radial notch) and the distal radioulnar joint function as a linked unit

The "Drop Sign"

• After surgical management of elbow dislocation, a drop sign (posterior sag) may indicate LCL laxity
• Often resolves spontaneously with restoration of muscle tone

6. Illustrations

Elbow Capsule and Ligaments — Posterior and Lateral Views

Posterior view (a): ulnar collateral ligament, radial collateral ligament, olecranon fossa, medial and lateral epicondyles. Lateral view (c) with forearm pronated: radial collateral ligament, anular ligament of radius, sacciform recess. — General Anatomy and Musculoskeletal System, THIEME Atlas

UCL Bundles — Medial View

Medial view (b): anterior, posterior, and transverse parts of the ulnar collateral ligament. Anular ligament of radius and coronoid process labeled. Also: Hueter's line (a, extension) and Hueter's triangle (c, flexion posterior view). — THIEME Atlas of Anatomy

CT Arthrography — Normal Elbow Ligaments

Normal elbow CT arthrography: (a) coronal — intact radial collateral ligament, lateral radiocapitellar recess; (b) sagittal — undisplaced annular ligament encircling radial head; (c) axial — posterior recess and smooth chondral surfaces.

MRI Cross-Sections — Elbow Ligament Anatomy

Transverse MRI at three levels: olecranon tuberosity (I), epicondyles (II), proximal radioulnar joint (III). Labeled: lateral (8) and medial (9) collateral ligaments, annular ligament (11), articular cavities, and muscular anatomy.

Valgus Stress Mechanism (Throwing)

Valgus-extension overload progression: (A) normal MCL; (B) medial tension → MCL microtears; (C) valgus overload + lateral capitellum–radial head impaction; (D) partial MCL tear + posteromedial osteophytes from olecranon shearing.

Elbow Flexion Biomechanics — ROM

Elbow flexion kinematics: 120° flexion angle shown on 3D skeletal upper limb model. Clinically relevant for ROM assessment, physical therapy, and orthotic design.

Summary Table: Stability Contributions

• Rockwood and Green's Fractures in Adults, 10th ed. 2025
• Imaging Anatomy: Text and Atlas Vol. 3, Bones, Joints, Vessels and Nerves (Thieme)
• General Anatomy and Musculoskeletal System, THIEME Atlas of Anatomy