Normal Ulnar Angle Size on Radiography | Elbow
The ulnar angle is a geometric measurement derived from the lateral radiograph of the elbow, reflecting the angular relationship of the ulna at the trochlear notch. Accurate assessment of this angle is important for evaluating elbow joint congruence, detecting subtle malalignment, and guiding clinical decision-making in both acute trauma and chronic conditions.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | 72-99¡ |
Clinical Significance
The normal ulnar angle ranges from 72° to 99° on lateral projection. Values outside this range may indicate osseous deformity, growth disturbance, or post-traumatic malalignment of the ulna relative to the distal humerus. Deviation can alter joint mechanics, predisposing patients to accelerated articular wear or instability.
When the ulnar angle is abnormal, the clinician should consider a range of underlying etiologies. Pitfalls include patient positioning errors on the lateral view — even slight rotation can falsely alter the measured angle — so strict lateral positioning is essential before drawing clinical conclusions.
- Post-traumatic deformity (e.g., malunited olecranon or coronoid fracture)
- Cubitus varus or valgus deformity
- Developmental dysplasia of the elbow
- Degenerative joint disease with osteophyte formation
- Inflammatory arthropathy causing bony erosion and malalignment
Reference: Yochum TR, Rowe LJ. Essentials of Skeletal Radiology. (2005).
Imaging Notes
The ulnar angle is measured exclusively on the lateral radiograph of the elbow. To ensure accuracy, the elbow should be positioned in true lateral projection with the joint flexed to approximately 90° and the forearm parallel to the imaging table. The angle is constructed from lines drawn along the long axis of the ulnar shaft and the articular surface of the trochlear notch.
Radiographic technique should minimize rotation, as even minor obliquity will introduce measurement error. If standard positioning is not achievable due to patient discomfort or trauma, this limitation should be noted in the report and measurements interpreted with appropriate caution.