What is Ulnar Variance
Ulnar variance is the radiographic measurement of the relative length difference between the distal articular surfaces of the radius and ulna at the wrist joint. 1
Definition and Measurement
Ulnar variance represents the distance between the contiguous articular surfaces of the distal radiocarpal and ulnocarpal joints, measured on standardized posteroanterior (PA) wrist radiographs 2
The measurement is expressed in millimeters and can be positive (ulna longer than radius), neutral (equal length), or negative (ulna shorter than radius) 3, 4
Radiographs are necessary for accurate measurement of ulnar variance, as emphasized by the American College of Radiology 1
Clinical Significance
Both positive and negative ulnar variance have been implicated in a variety of wrist disorders, including ulnar impaction syndrome, triangular fibrocartilage complex (TFCC) tears, and Kienböck disease 5
Standard radiographic projections allow assessment of alignment including ulnar variance, joint spaces, impaction syndromes, and static instability, according to the American College of Radiology 1
Measurement Techniques and Accuracy
The lateral wrist radiograph technique correlates most highly with directly measured osseous ulnar variance, with an average measurement difference of only 0.06 mm from anatomic dissection 5
Coronal MRI best reflects true ulnar variance when compared to direct caliper measurements, with an average bias of 0.49 mm when excluding cartilage 3
All imaging modalities (PA radiograph, CT, MRI) tend to underestimate the magnitude of true ulnar variance by approximately 0.5-1.0 mm, which should be considered when planning surgical procedures that alter relative radius and ulna lengths 3
Technical Considerations
Wrist and forearm position significantly affects ulnar variance measurements: supination increases negative ulnar variance, while pronation decreases it 2
Standardized positioning is essential: the wrist should be in neutral rotation with PA and lateral views performed in neutral position to ensure reproducible measurements 1, 2
No radiographic measurement technique can accurately account for articular cartilage thickness at the lunate facet of the radius or distal ulnar head, which varies unpredictably between individuals 5
Normal Variation
Ulnar variance differs significantly by gender: females have greater positive ulnar variance (median 0.85 mm) compared to males (median 0.4 mm) 4
No significant differences exist between right and left sides within the same individual, or with aging across decades 4
The entire ulnar head configuration is affected by ulnar variance, including ulnar head diameter, ulnar seat inclination, and the distance of the ulnar head peak to the distal radioulnar joint 6