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.

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. Normal range: 64-81 degrees (average approximately 75 degrees). Used to assess varus/valgus alignment after supracondylar fracture reduction. A decrease in the Baumann angle compared to the contralateral side suggests varus malreduction (cubitus varus or 'gunstock deformity').

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.

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.

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

Advanced imaging guides critical management decisions for elbow pathology.