February Answers, Findings and Discussion

Which mensuration line(s) are abnormal?

1. Anterior humeral line
2. Posterior humeral line
3. Radiocapitellar line
4. Shenton’s line

What is the diagnosis?

1. Radial Head Fracture
2. Posterior radial head dislocation/subluxation
3. Anterior radial head dislocation/subluxation
4. Olecranon fracture

Imaging Findings

• Posterior displacement of the radial head relative to the capitellum indicating posterior radial head dislocation/subluxation.
• Angulation of the radial head on the AP projection
• A joint effusion is present with fat pad displacement

What associated injury must be considered with this patient presentation?

1. Ulnar collateral ligament disruption
2. Annular ligament disruption
3. Ulnar Fracture
4. Interosseuous membrane rupture

Discussion

• Isolated radial head dislocation without fracture of the ulna is rare in adults
• The most common dislocation at the elbow involves posterior dislocation of both the radius and the ulna
• The mechanism of posterior radial head dislocation is hyperpronation and axial compression
• The annular ligament must be ruptured to allow posterior radial head dislocation
• The quadrate ligament may also be injured
• Interpretation of elbow radiographs involves assessment of the anterior humeral line and radiocapitellar line
• The anterior humeral line is drawn along the anterior cortex of the distal humerus and should intersect the middle third of the capitellum
• The radiocapitellar line is drawn along the longitudinal axis of the proximal radius and should intersect the center of the capitellum

Normal Elbow Mensuration

Present Case

References

• Pereles DJ, Labropoulos PA. Posterior radial head dislocation without ulnar fracture. Journal of Orthopaedic Trauma 1990;4(4):458-462.
• Greenspan A. Orthopedic Radiology: A Practical Approach 2nd edition. “Upper Limb I: Shoulder Girdle and Elbow” Pg. 5.26-5.43