Imaging the Elbow

Key Alignment Lines and Angles

Anterior humeral line: Drawn along the anterior cortex of the distal humerus on the TRUE lateral view. This line should pass through the MIDDLE THIRD of the capitellum. If the line passes through the anterior third or misses the capitellum entirely, this indicates posterior displacement of the capitellum relative to the humeral shaft — consistent with a posteriorly displaced supracondylar fracture. This is the most important line for assessing supracondylar fracture reduction.

But it is far less reliable in normal children than its reputation suggests. Herman and colleagues measured it on 60 normal paediatric elbows, read twice by each of three observers (DOI): it passed through the middle third in only 52%, the anterior third in 31% and the posterior third in 18%, with only moderate-to-substantial reliability. Under four years of age it fell nearly equally in the anterior or middle third; in older children it reached the middle third in 62%. The practical consequence is over-diagnosis — an anterior-third line in a toddler is often normal, so do not use it alone to declare a fracture or to condemn an otherwise acceptable reduction.

Radiocapitellar line: A line drawn through the centre of the radial shaft and proximal radius should pass through the centre of the capitellum on ALL views (AP, lateral, and oblique). If this line does not bisect the capitellum on ANY view, the radial head is dislocated. This is the key to diagnosing Monteggia fracture-dislocation: an ulnar shaft fracture with an associated radial head dislocation (the ulnar fracture may be subtle, and the dislocation may only be apparent on one view).

Baumann angle: Measured on the AP view between the lateral condylar physis and the long axis of the humeral shaft. Do not use it as an absolute number. Two healthy-child series put the MEAN at essentially the same place - 71 degrees plus or minus 3.2 in 56 children (PMID 36550703) and 71.5 degrees in 148 elbows (PMID 42024825) - but the second reports the observed range across normal elbows as 54 to 90 degrees, and interobserver reliability is only moderate to good (ICC 0.71-0.78). The two studies differ not because they disagree but because one reports the spread of a mean and the other the spread of individuals: a tight "normal 70-75" quotes the former as if it were the latter, and will label plenty of normal elbows abnormal.

So the angle is a COMPARATIVE measurement, not a threshold. Measure the injured elbow against the uninjured one; a difference greater than about 5 degrees indicates varus malreduction (cubitus varus, the 'gunstock deformity'). If the contralateral elbow is unavailable, treat the number as weak evidence and lean on the anterior humeral line and column alignment instead.

Carrying angle: Measured on the AP view between the long axis of the humerus and the long axis of the forearm (ulna). Normal: 5-15 degrees of valgus (slightly higher in females). Increased carrying angle (cubitus valgus) is associated with malunited lateral condyle fractures and may cause delayed ulnar nerve palsy.

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Paediatric Elbow Imaging Pitfalls

The paediatric elbow is one of the most challenging radiographic assessments due to the sequential appearance and fusion of six ossification centres. Key pitfalls and their clinical significance:

Medial epicondyle avulsion: The medial epicondyle (I in CRITOE, appears at age 5) can be avulsed during elbow dislocation and become trapped within the joint. The KEY pitfall: the trapped medial epicondyle fragment may mimic the trochlear ossification centre. How to avoid: the trochlea (T in CRITOE) CANNOT appear before the internal (medial) epicondyle — if you see what looks like a trochlear ossification centre but the medial epicondyle is absent, the 'trochlea' is actually a trapped medial epicondyle fragment. This requires surgical removal.

Supracondylar fracture assessment in children: (1) Assess the anterior humeral line on the lateral view (should bisect the middle third of the capitellum). (2) Look for the posterior fat pad sign (most sensitive indicator of occult fracture). (3) Check Baumann angle on AP for varus/valgus alignment. (4) Assess for associated vascular injury (pulseless, pale hand) and nerve injury (anterior interosseous nerve: loss of index finger DIP flexion and thumb IP flexion).

Comparison views: In uncertain cases, comparison radiographs of the contralateral elbow are invaluable. The ossification centres should be symmetric. Any asymmetry suggests either fracture or normal variant (but asymmetry of a known normal variant would be bilateral).

Lateral condyle fracture: Can be subtle on radiographs — a small flake of bone off the lateral condyle with an associated joint effusion (fat pad signs). The true fracture is much larger than the visible bone fragment because the fracture extends through unossified cartilage. MRI or arthrography can demonstrate the true extent.

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Additional Radiographic Views

AP and true lateral are the minimum, but targeted extra views resolve specific questions:

• Radiocapitellar (Greenspan) view — a modified lateral with the X-ray tube angled ~45 degrees cephalad (toward the shoulder). It projects the radial head and capitellum free of the overlapping coronoid/proximal ulna, and is the most useful view for a subtle radial head/neck fracture suggested only by a fat pad sign.
• Internal and external oblique views — the internal (medial) oblique best shows the coronoid, trochlea and medial epicondyle; the external (lateral) oblique best shows the radial head, capitellum and lateral epicondyle.
• Axial (cubital tunnel) view — profiles the olecranon and the ulnar groove; used for cubital-tunnel/osteophyte assessment.
• Stress views — valgus/varus stress radiographs (or dynamic ultrasound) assess collateral-ligament competence.

Mason Classification of Radial Head Fractures

The radial head is the commonest adult elbow fracture and the commonest occult fracture behind a positive fat pad sign, so its imaging-based classification is high-yield.

Assess the radiocapitellar joint for a mechanical block (the indication to fix a Type II), and obtain CT for comminuted (Type III) or dislocation-associated (Type IV) fractures to map fragment size and plan fixation versus replacement.

Fat pad signs are the most sensitive radiographic indicator of elbow fracture.

Advanced imaging guides critical management decisions for elbow pathology.