What is the clinical significance of the anterior humeral line on a lateral elbow radiograph?

Clinical Significance of the Anterior Humeral Line on Lateral Elbow Radiography

The anterior humeral line (AHL) is a critical radiographic tool for detecting supracondylar fractures and assessing sagittal plane alignment in pediatric elbows, but its reliability varies significantly with age—in children under 5 years, the AHL passes through the anterior third of the capitellum in up to 30% of normal elbows, making it less reliable for excluding pathology in this age group. 1, 2

Primary Clinical Application

The AHL serves as a screening tool for occult fractures when joint effusion is present on lateral elbow radiographs. In the setting of acute elbow trauma, the presence of a joint effusion (posterior and anterior fat pad elevation) combined with clinical context can imply an occult elbow fracture, and the AHL helps assess for associated displacement. 3

Age-Dependent Reliability

Children ≥5 Years of Age

• The AHL passes through the middle third of the capitellum in 100% of normal elbows in children 5 years and older. 1
• In older children (4-9 years), the AHL intersects the middle third in 62% of cases. 2
• If the AHL does not pass through the middle third in children ≥5 years, pathology should be suspected. 1

Children <5 Years of Age

• In children under 5 years, the AHL demonstrates significant variability, passing through the anterior third in 25% of normal elbows. 1
• In children under 2 years, the AHL lies in the anterior third in 30% of normal cases. 1
• In children under 4 years, the AHL passes nearly equally through either the anterior or middle third of the capitellum. 2
• This age-related variability must be considered when using the AHL to assess for injury in young children. 2

Universal Reliability Marker

Regardless of age, the AHL should always touch the ossific nucleus of the capitellum in 100% of normal elbows. 1

• If the AHL does not touch the capitellum at all, it is appropriate to look for pathology regardless of patient age. 1
• This represents the most reliable application of the AHL across all pediatric age groups. 1

Technical Considerations for Accurate Measurement

Impact of Radiographic Rotation

• Rotational variations in elbow positioning significantly affect AHL measurements, particularly when using the distal humerus as a reference point. 4
• Drawing the AHL along the humeral shaft provides more consistent measurements than using the distal humerus, with better tolerance to rotational variations. 4
• When the shaft is used as a guide, the intraclass correlation coefficient is 0.81 compared to 0.14 when using the distal humerus. 4

Measurement Reliability

• Overall intra-rater and inter-rater reliability for AHL measurements are moderate to substantial. 2
• The AHL should be drawn starting from the anterior cortex of the humeral shaft rather than the distal humerus for surgical decision-making. 4

Clinical Decision-Making Algorithm

Step 1: Initial Assessment

• Obtain standard lateral elbow radiographs as the initial imaging study for acute elbow pain. 3
• Assess for joint effusion (fat pad signs) which suggests occult fracture. 3

Step 2: AHL Evaluation

• Draw the AHL along the anterior cortex of the humeral shaft (not distal humerus). 4
• If the AHL does not touch the capitellum → pathology is present regardless of age. 1

Step 3: Age-Specific Interpretation

• In children ≥5 years: If AHL falls outside the middle third → suspect pathology. 1
• In children <5 years: AHL position is less reliable; if outside middle third, correlate with clinical findings and consider repeat radiographs in 10-14 days or CT without contrast. 3, 1

Step 4: Follow-up Imaging

• If radiographs are normal or indeterminate with high clinical suspicion for fracture, repeat radiographs in 10-14 days or CT without IV contrast are equivalent appropriate next steps. 3

Common Pitfalls

• Failing to account for age-related variability: The AHL is significantly less reliable in children under 5 years, where it normally passes through the anterior third in up to 30% of cases. 1, 2
• Using the distal humerus as a reference: This technique is more susceptible to rotational artifact and provides inconsistent measurements. 4
• Over-relying on middle-third intersection: While useful in older children, this criterion has 31% false-positive rate overall when considering all ages. 2
• Ignoring the universal rule: The most reliable finding is that the AHL must touch the capitellum—failure to do so always warrants investigation for pathology. 1