Upper Extremities
19 Elbow
Anatomy & Pathology
Anatomy Overview
The elbow joint is a complex synovial hinge joint formed by articulations among three bones:
- Humerus (distal): includes the capitulum (lateral), trochlea (medial), medial and lateral epicondyles, coronoid and olecranon fossae.
- Ulna (medial forearm): includes the olecranon and coronoid processes, trochlear notch, and radial notch.
- Radius (lateral forearm): includes the radial head, neck, and tuberosity.
Associated fat pads—anterior, posterior, and supinator—are radiographically significant indicators of joint effusion or occult fracture.
Common Pathology
- Fractures: radial head/neck, olecranon, coronoid process, distal humerus, supracondylar fractures in pediatrics.
- Joint injuries: dislocations or subluxations, especially radial head dislocations.
- Pathologic signs: visible posterior fat pad = effusion or occult fracture.
- Degenerative change: osteoarthritis or rheumatoid involvement of the humeroulnar articulation.
Projections
Elbow – AP Projection
CR Location & Positioning
- SID: 40 inches (102 cm)
- Patient position: Seated at table, shoulder, elbow, and wrist in the same plane (entire limb parallel to tabletop).
- Adjustments: Fully extend the elbow, supinate the hand so the forearm is true AP. Lean laterally if needed until humeral epicondyles and anterior surface of elbow are parallel to IR.
- CR: Perpendicular to the elbow joint; centered midway between epicondyles.
- Pt. Instructions: Suspend breathing; remain still.
- Exposure: Moderate kVp (60–70) with adequate contrast to visualize both bony and soft-tissue detail; ensure soft-tissue visibility for fat-pad evaluation.
Evaluation Criteria
- Coverage: Distal humerus, proximal forearm, and open elbow joint centered to CR.
- Rotation checks: Epicondyles seen in profile (no rotation); radial head, neck, and tuberosity slightly superimposed over proximal ulna; open humeroradial and humeroulnar joint spaces.
- Motion checks: Sharp cortical margins and visible trabecular pattern confirm no motion.
- Technique checks: Even exposure across both bone densities; soft tissue and fat pads visible; no artifacts.
- Clinical aim: Evaluate for fracture/dislocation alignment, joint space narrowing, or pathology indicated by fat-pad displacement.
Elbow – Lateral Projection (Lateromedial)
CR Location & Positioning
- SID: 40 inches (102 cm)
- Patient position: Seated with shoulder, elbow, and wrist in same horizontal plane.
- Adjustments: Flex elbow 90°; forearm and humerus resting on same plane; hand in true lateral position (thumb up). Ensure epicondyles perpendicular to IR. Elevate wrist slightly if forearm is muscular to maintain true lateral alignment.
- CR: Perpendicular to the elbow joint; centered midway between epicondyles.
- Pt. Instructions: Suspend breathing; remain still.
- Exposure: Moderate kVp emphasizing soft-tissue detail to visualize fat pads; exposure must allow differentiation of anterior and posterior fat pads.
Evaluation Criteria
- Coverage: Distal humerus and proximal forearm fully included with elbow flexed 90°.
- Rotation checks:
- Epicondyles superimposed
- Radial head partially superimposing coronoid process
- Olecranon process in profile
- Radial tuberosity facing anteriorly
- Motion checks: Crisp cortical outlines; soft tissue not blurred.
- Technique checks: Visualization of anterior and supinator fat pads (posterior should not be visible unless pathology present); proper contrast for soft-tissue assessment.
- Clinical aim: Demonstrate alignment, displacement, effusion, or occult fractures.
Elbow – AP Oblique Projection (Medial Rotation)
CR Location & Positioning
- SID: 40 inches (102 cm)
- Patient position: Seated at table with arm extended and in contact with tabletop.
- Adjustments: From AP, pronate hand and medially rotate the arm until anterior surface of elbow forms a 45° angle to IR.
- CR: Perpendicular to the elbow joint, midway between epicondyles.
- Pt. Instructions: Hold still; suspend breathing.
- Exposure: Standard kVp range; adjust for forearm thickness; maintain uniform contrast.
Evaluation Criteria
- Coverage: Distal humerus and proximal forearm with open elbow joint centered to CR.
- Rotation checks:
- Coronoid process in profile
- Elongated medial epicondyle
- Ulna partially superimposed by radial head and neck
- Olecranon process within its fossa
- Motion checks: Sharp bone edges, no motion blur.
- Technique checks: Appropriate brightness/contrast to differentiate coronoid and surrounding structures.
- Clinical aim: Evaluate coronoid process and trochlear articulation; assess for fractures or osteophyte formation on the ulna.
Elbow – AP Oblique Projection (Lateral Rotation)
CR Location & Positioning
- SID: 40 inches (102 cm)
- Patient position: Seated at table, arm extended and supinated.
- Adjustments: From AP, laterally rotate hand and arm until posterior surface of elbow is 45° to IR; first and second digits contact table.
- CR: Perpendicular to the elbow joint, centered midway between epicondyles.
- Pt. Instructions: Suspend breathing; remain still.
- Exposure: Similar to AP; optimize contrast for visualization of radial head and capitulum.
Evaluation Criteria
- Coverage: Distal humerus and proximal forearm with open elbow joint.
- Rotation checks:
- Radial head, neck, and tuberosity free of superimposition by ulna
- Elongated lateral epicondyle
- Capitulum in profile
- Motion checks: Sharp bone and soft-tissue margins.
- Technique checks: Appropriate brightness and contrast across joint; no artifacts.
- Clinical aim: Demonstrate radial head and capitulum relationships; identify radial head or neck fractures.
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Hows & Whys of Elbow Radiography
Anatomy
- Which bone articulates most directly with the humerus at the elbow?
The ulna articulates with the trochlea of the humerus to form the primary hinge motion of the elbow. - Which structure of the radius articulates with the humerus?
The radial head articulates with the capitulum of the humerus. - What type of joint is the elbow?
A synovial hinge joint permitting flexion and extension. - What type of joint is the proximal radioulnar articulation?
A pivot joint allowing pronation and supination of the forearm. - What radiographic sign can indicate an occult elbow fracture?
Visualization of the posterior fat pad on a lateral projection.
Positioning
- Why must the shoulder, elbow, and wrist be in the same plane for the AP projection?
To prevent joint distortion and ensure the elbow joint is open and properly centered. - Why is the hand supinated for the AP projection?
Supination prevents the radius and ulna from crossing, giving a true AP view. - Why is the elbow flexed 90° for the lateral projection?
It shows the olecranon process in profile and keeps the posterior fat pad depressed, avoiding false indications of pathology. - Why is the hand placed in true lateral with thumb up for the lateral projection?
This ensures the forearm is in a true lateral position, producing superimposed epicondyles. - Why is the arm rotated medially for the medial oblique projection?
To visualize the coronoid process free of superimposition and profile the trochlea. - Why is the arm rotated laterally for the lateral oblique projection?
To separate the radial head, neck, and tuberosity from the ulna for clearer evaluation.
Technique & Image Evaluation
- How can you tell that the AP elbow is correctly positioned?
Epicondyles appear in profile and the joint space is open, confirming the limb was in the same plane. - How can you tell that the AP projection was not truly supinated?
The radius crosses over the ulna proximally, indicating pronation. - How can you tell the lateral projection is correctly positioned?
Epicondyles are superimposed, the radial head overlaps the coronoid process, and the olecranon is in profile. - How can you confirm correct medial oblique positioning?
The coronoid process is seen in profile with the trochlea; the medial epicondyle appears elongated. - How can you confirm correct lateral oblique positioning?
The radial head and neck are completely free of ulnar superimposition and the lateral epicondyle appears elongated. - How can you tell if motion occurred?
Loss of cortical sharpness or soft-tissue definition, especially at the fat pads. - How can you tell exposure factors were appropriate?
Bony cortices and fat pads visible without overexposure; consistent brightness across both humeral and forearm portions.
Clinical Applications
- Which projection best demonstrates the radial head and neck?
The AP lateral oblique projection. - Which projection best demonstrates the coronoid process in profile?
The AP medial oblique projection. - Why are fat pads important diagnostically?
Displacement of the posterior fat pad or distortion of the anterior fat pad suggests joint effusion or occult fracture. - Which projection provides the best assessment of anterior or posterior fracture displacement?
The lateral projection. - Why is it important to visualize both distal humerus and proximal forearm on each elbow image?
To ensure alignment and exclude combined injuries involving the humerus or radius/ulna.