Elbow X-Ray Positioning: AP, Lateral, Oblique, and Radial Head Views
The elbow joint is one of the most complex joints in the body — a hinge-and-pivot articulation where three bones (humerus, radius, and ulna) meet. Imaging it requires precise positioning to demonstrate the joint space, surrounding bony architecture, and soft tissue without superimposition that could hide fractures or dislocations.
In this guide, you'll learn the standard elbow radiographic series — AP, lateral, and oblique views — plus the specialized Coyle method for radial head fractures. We'll cover Clark's-recommended centering points, technical factors, evaluation criteria, positioning errors, and ARRT exam tips to help you master this high-yield registry topic.
For other upper extremity positioning guides, check out our hand, wrist, and finger X-ray positioning guide and upper extremity positioning overview.
Anatomy of the Elbow Joint
The elbow joint is actually three articulations within one synovial capsule:
- Humeroulnar joint — trochlea of the humerus articulating with the trochlear notch of the ulna. This is the primary hinge (flexion-extension).
- Humeroradial joint — capitellum of the humerus articulating with the radial head. This also allows forearm rotation (pronation-supination).
- Proximal radioulnar joint — radial head articulating with the radial notch of the ulna. This permits forearm rotation.
Key bony landmarks for positioning:
| Landmark | Location | Palpable? |
|---|---|---|
| Medial epicondyle | Medial prominence of distal humerus | Yes — the "funny bone" area |
| Lateral epicondyle | Lateral prominence of distal humerus | Yes — smaller than medial |
| Olecranon | Posterior prominence of proximal ulna | Yes — the "point" of the elbow |
| Radial head | Lateral, distal to lateral epicondyle | Yes — rotates with supination/pronation |
| Coronoid process | Anterior projection of proximal ulna | No — deep to soft tissue |
The carrying angle (normal cubitus valgus of 10–15°) is important to remember — the forearm angles slightly laterally from the humerus when the arm is extended and supinated. This affects centering on the AP projection.
Standard Elbow Series — Overview
A complete elbow series typically includes three to four projections:
| Projection | Primary Purpose |
|---|---|
| AP Elbow | Overall joint survey, joint space alignment, distal humerus, proximal forearm |
| Lateral Elbow | Fat pad sign (effusion), coronoid process, olecranon, anterior/posterior displacement |
| AP Medial (Internal) Oblique | Coronoid process in profile |
| AP Lateral (External) Oblique | Radial head and neck free of superimposition |
Projection 1: AP Elbow (Clark's)
The AP elbow is the basic survey projection for the elbow joint. It demonstrates the distal humerus, proximal radius and ulna, and the elbow joint space.
Patient Position
- Patient seated alongside the table with the affected arm extended
- Elbow fully extended, palm facing upward (hand supinated)
- Shoulder, elbow, and wrist should be at the same horizontal level — the upper arm, elbow, and forearm should all be in contact with the tabletop
- If the patient cannot fully extend the elbow (common in trauma), perform two separate exposures — one for the distal humerus and one for the proximal forearm
Central Ray (Clark's)
- Perpendicular (0°) to the IR
- Centering point: Midpoint between the humeral epicondyles — approximately 2.5 cm (1 in) distal to the midpoint of the antecubital fossa
Technical Factors
| Parameter | Value |
|---|---|
| IR size | 18 × 24 cm (8 × 10 in) |
| IR orientation | Portrait (lengthwise along forearm) |
| SID | 100 cm (40 in) |
| Grid | No (grid optional for larger patients >10 cm thickness) |
| kVp | 55–65 |
| mAs | 2–5 |
| Breathing | Suspend respiration |
| Shielding | Gonadal (outside primary field) |
Evaluation Criteria
- Elbow joint space open centrally — no overlap of humerus and forearm bones
- Medial and lateral epicondyles visible in profile on respective sides
- Radial head partially superimposed over the ulna (normal for AP)
- Olecranon process seated within the olecranon fossa
- Adequate penetration through the epicondyles
- No rotation — the epicondyles should be symmetric, not tilted
Projection 2: Lateral Elbow (Clark's)
The true lateral elbow is arguably the most important view in the elbow series because it demonstrates the fat pad sign — a key indicator of occult fracture, especially in pediatric patients.
Patient Position
- Patient seated alongside the table
- Elbow flexed 90° — this is the standard degree of flexion; any variation changes the joint relationships
- Hand in true lateral position (thumb pointing upward, forearm midway between pronation and supination)
- Medial aspect of the arm and forearm resting on the table
- Shoulder, elbow, and wrist should be at the same horizontal level
Central Ray (Clark's)
- Perpendicular (0°) to the IR
- Centering point: Lateral epicondyle of the humerus
- CR enters the medial aspect of the elbow and exits the lateral aspect
Technical Factors
| Parameter | Value |
|---|---|
| IR size | 18 × 24 cm (8 × 10 in) |
| IR orientation | Portrait |
| SID | 100 cm (40 in) |
| Grid | No |
| kVp | 55–65 |
| mAs | 2–5 |
| Breathing | Suspend respiration |
The Fat Pad Sign (Sail Sign)
The lateral elbow is the only view where the fat pad sign is reliably visualized. Normal fat pads:
- Anterior fat pad — a thin, triangular lucency visible anterior to the distal humerus. A small anterior fat pad (approximately 1–2 mm wide) is normal.
- Posterior fat pad — ALWAYS abnormal. The posterior fat pad is normally hidden within the olecranon fossa. When an intra-articular fracture causes hemarthrosis, the posterior fat pad is displaced posteriorly where it becomes visible.
📝 ARRT Exam Tip — The Posterior Fat Pad
The presence of a posterior fat pad on the lateral elbow is considered an occult fracture equivalent — even if no fracture line is visible, the elbow is treated as fractured. This is especially common in pediatric patients with radial neck or supracondylar fractures. The anterior fat pad can be present in both traumatic and inflammatory conditions, but a posterior fat pad is virtually always traumatic. This concept appears on every version of the ARRT registry.
Evaluation Criteria
- Humeral condyles superimposed — the single most important check. If the medial and lateral epicondyles appear as separate structures, this is not a true lateral.
- Elbow joint open anteriorly — a V-shaped or "hourglass" lucency between the distal humerus and the coronoid process
- Trochlear notch of the ulna visible in profile
- Coronoid process projected anteriorly, clear of superimposition
- Olecranon process visible posteriorly
- Radial head superimposed over the coronoid process (normal for true lateral)
- Fat pads visible anteriorly (normal) and posteriorly (abnormal if present)
Projection 3: AP Oblique — Medial (Internal) Rotation
This view brings the coronoid process into profile.
Patient Position
- Start from the AP position (elbow extended, hand supinated)
- Rotate the arm medially (internally) 45° — this brings the forearm into pronation
- The medial aspect of the elbow becomes more parallel to the IR
Central Ray
- Perpendicular (0°) to the IR
- Centering: Same as AP — midpoint between epicondyles
Evaluation Criteria
- Coronoid process projected free of superimposition, visible in profile on the ulnar side
- Medial epicondyle visible en face
- Joint space open medially
Projection 4: AP Oblique — Lateral (External) Rotation
This view brings the radial head and neck into profile — essential when radial head fracture is suspected.
Patient Position
- Start from the AP position (elbow extended, hand supinated)
- Rotate the arm laterally (externally) 45° — the hand rotates into full supination or beyond
- The lateral aspect of the elbow becomes more parallel to the IR
Central Ray
- Perpendicular (0°) to the IR
- Centering: Same as AP — midpoint between epicondyles
Evaluation Criteria
- Radial head and neck free of superimposition over the ulna
- Capitellum visible en face
- Joint space visible laterally
Projection 5: Radial Head — Coyle Method (Trauma)
When radial head fracture is suspected but not clearly visible on standard views, the Coyle method (also called the radiocapitellar view) projects the radial head free of the coronoid process.
Patient Position
- Patient seated, elbow flexed 90°
- Forearm in neutral position (thumb up, same as for a lateral)
- The patient does not need to move from the lateral position
Central Ray (Clark's)
- CR angled 45° toward the shoulder (cephalad)
- CR enters the posterior aspect of the elbow, exits the anterior aspect
- Centered to the radial head (approximately 2.5 cm distal to the lateral epicondyle)
Technical Factors
| Parameter | Value |
|---|---|
| IR size | 18 × 24 cm (8 × 10 in) |
| SID | 100 cm (40 in) |
| Grid | No |
| kVp | 55–65 |
| CR angle | 45° cephalad |
Evaluation Criteria
- Radial head projected anteriorly and free of superimposition over the coronoid process
- Capitellum aligned with the radial head
- Open radiocapitellar joint space
Trauma Elbow Series — Modified Techniques
When the patient cannot extend the elbow (suspected fracture, dislocation, or severe pain):
AP in Flexion
- Keep the elbow flexed as much as possible
- Two separate exposures:
- Distal humerus — CR perpendicular to distal humerus, entering the posterior aspect
- Proximal forearm — CR perpendicular to the proximal radius and ulna
- These can be reconstructed for diagnostic assessment
Cross-Table Lateral (Supine Patient)
- Patient supine or semi-recumbent
- Arm abducted and supported on a radiolucent pad
- IR placed vertically against the medial aspect of the elbow
- Horizontal CR perpendicular to the lateral epicondyle
- Adjust IR distance to include the entire elbow joint
🚨 Clinical Pearl — Never Force Extension
If the patient cannot fully extend the elbow, do not force it. Forced extension of a fractured elbow can displace bone fragments, convert a nondisplaced fracture into a displaced one, or worsen a dislocation. In pediatric patients, forced manipulation of a supracondylar fracture risks neurovascular compromise (especially the brachial artery). Use the two-exposure technique (flexed AP) and cross-table lateral instead. This is tested on the ARRT as a patient safety question.
Positioning Errors: Quick Reference
| Error | Likely Cause | Correction |
|---|---|---|
| Joint space not open (AP) | Elbow not fully extended or CR off-center | Ensure full extension; center to midpoint between epicondyles |
| Epicondyles not symmetric (AP) | Patient rotated | Both shoulders should be at same horizontal level |
| Condyles not superimposed (lateral) | Elbow not in true lateral | Check hand position — thumb should point straight up |
| Radial head obscured (lateral) | Elbow less than 90° flexion | Flex to exactly 90°; use positioning sponge |
| Coronoid process not visible (oblique) | Insufficient rotation | Rotate at least 45° from AP |
| Olecranon cut off (lateral) | CR too far anterior | Recentre to lateral epicondyle |
| Elbow underexposed (AP) | kVp too low for epicondyle thickness | Increase to 60–65 kVp for average adult |
| Motion blur | Patient discomfort | Use immobilization; increase mA, decrease time |
| Radial head obscured (Coyle method) | CR angle incorrect | Angle exactly 45° cephalad toward shoulder |
| Fat pad sign obscured (lateral) | Overexposed or rotated lateral | Check technique and rotation |
ARRT Exam Prep: Elbow Positioning High-Yield Facts
The elbow is a staple topic on the ARRT registry. Here are the most frequently tested concepts:
- AP elbow centering — midpoint between the humeral epicondyles (not the cubital fossa or radial head)
- Lateral elbow flexion — 90° — this is the standard; the ARRT will test this exact degree
- Lateral evaluation check — humeral condyles must be superimposed — if they appear separate, the view is not a true lateral
- Posterior fat pad — always abnormal — indicates occult fracture until proven otherwise (especially pediatric supracondylar fracture)
- Oblique views — medial (internal) 45° demonstrates the coronoid process; lateral (external) 45° demonstrates the radial head and neck
- Coyle method — 45° cephalad CR with the elbow flexed 90° — specific for radial head fractures
- Trauma protocol — never force extension — use cross-table lateral and flexed AP instead
- Elbow series — minimum 3 views (AP, lateral, at least one oblique); 4 views (both obliques) is standard in many departments
- Pediatric differences — distal humeral epiphyses ossify at different ages. The CRITOE mnemonic (Capitellum, Radial head, Internal/Trochlea, Olecranon, External epicondyle) helps remember ossification order.
- IR size — standard 18 × 24 cm (8 × 10 in) for all elbow projections
📝 ARRT Exam Tip — Elbow Oblique Anatomy
A classic ARRT question: "Which oblique projection demonstrates the coronoid process in profile?" The answer: the AP medial (internal) oblique — the arm is rotated medially 45° from the AP position. The lateral (external) oblique, by contrast, demonstrates the radial head and neck. Remember: medial = coronoid, lateral = radial head. For more upper extremity practice, see our hand and wrist positioning guide and forearm X-ray positioning guide.
Clinical Pearls: Tips from the Techs
- Palpate the epicondyles — they're easy to feel even on a swollen elbow. Run your fingers down the distal humerus until you feel the bony prominences on each side. Center exactly halfway between them.
- Use the "thumb test" for the lateral — if the patient's thumb is pointing straight up (the "hitchhiker" position), the elbow is in a true lateral. If the thumb points medially or laterally, the elbow is rotated.
- Low-dose technique for pediatrics — drop the kVp to 50–55 for pediatric elbows. The posterior fat pad is your most sensitive indicator of occult fracture in children — always look for it on the lateral view.
- The "fat pad" needs a true lateral — if the elbow is rotated even slightly, the fat pads may not be visible or can be misinterpreted. Make the lateral elbow the most carefully positioned view of the series.
- Watch for the "figure-8" sign on the lateral — the normal anterior fat pad appears as a thin, triangular lucency anterior to the distal humeral shaft. If it looks excessively large or there's a posterior fat pad, suspect fracture.
- Document your trauma technique — when using the cross-table lateral or flexed AP, note on the requisition that the patient could not extend the elbow. The radiologist needs this clinical context to properly interpret the images.
Summary
| Projection | Patient Position | CR | Centering Point | Primary Purpose |
|---|---|---|---|---|
| AP Elbow | Seated, elbow extended, hand supinated | 0° (perpendicular) | Midpoint between epicondyles | Joint survey, alignment |
| Lateral Elbow | Seated, elbow 90° flexed, true lateral hand | 0° (perpendicular) | Lateral epicondyle | Fat pad sign, coronoid, olecranon |
| AP Medial Oblique | From AP, arm medially rotated 45° | 0° (perpendicular) | Midpoint between epicondyles | Coronoid process in profile |
| AP Lateral Oblique | From AP, arm laterally rotated 45° | 0° (perpendicular) | Midpoint between epicondyles | Radial head and neck |
| Coyle Method | Elbow 90° flexed, neutral forearm | 45° cephalad | Radial head | Radial head fracture |
📝 Key Takeaway — Master the Lateral First
The true lateral elbow is the most important and the most technically demanding projection in the elbow series. Once you can consistently get the epicondyles superimposed and the joint space open anteriorly, the oblique views and Coyle method are straightforward extensions. On the ARRT, expect at least one question about the lateral elbow evaluation criteria — specifically the superimposition of humeral condyles as the check for a true lateral. For more upper extremity practice, see our hand and wrist positioning guide or the upper extremity overview.