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Correlation of Ulnar Variance on MRI and Plain Radiographs
Natalie Marie Egge, MD; Hartley Sirkis, MD; Lacey McIntosh, DO; Wenyun Yang, MS; Marci Jones, MD; University of Massachusetts
University of Massachusetts, Shrewsbury, MA, USA

Introduction:

Several methods for measuring ulnar variance exist. The gold standard technique used on PA radiographs is the method of perpendiculars. Measurement using coronal MRI has been described but accuracy of ulnar variance measurements between imaging modalities has yet to be determined. A neutral rotation radiograph is described to standardize the forearm rotation. However, when an MRI is performed, no standard position is used to control for rotation. Additionally, one cannot determine the forearm rotation of the patient at the time the image was taken by examining the MRI or radiograph. The purpose of this study is to investigate the correlation of ulnar variance measurements on coronal MRI and PA radiograph.

Methods:

Neutral rotation PA radiographs and coronal MRI scans were retrospectively reviewed on 48 patients. Four reviewers, including one orthopedic hand attending, an orthopedic resident, a radiology attending, and a radiology resident measured the ulnar variance by previously described methods. On the PA radiograph, the ulnar variance was determined using the method of perpendiculars. For MRI, the ulnar variance was measured as the difference between a line at the level of the lunate fossa cartilage surface and a line at the level of the ulnar head cartilage surface. Correlation between x-ray and MRI was statistically determined using a correlation coefficient. Power analysis performed following a pilot study showed adequate power with a minimum of 30 patients. Inter-observer reliability was assessed using a kappa-statistic measure of agreement.

Results:

Correlation between x-ray and MRI was best with negative ulnar variance measuring less than -1, demonstrating a 0.738 correlation coefficient. For ulnar variance measuring between -1 and 1, the correlation coefficient between x-ray and MRI was 0.251. For ulnar positive patients measuring greater than +1 the correlation coefficient was 0.423. The inter-observer reliability was good between the four reviewers, with a combined kappa value of 0.694.

Conclusion:

Higher correlation is observed between x-ray and MRI measurements when more pronounced ulnar variance is observed. This correlation decreases when patients have ulnar variance measuring close to neutral. Most specifically, greater negative ulnar variance shows the best correlation between imaging studies. Inter-observer reliability between the two types of imaging is also reliable. This study demonstrates that there is only a moderate correlation of MRI measurement of ulnar variance with neutral rotation PA radiographic measurement. While MRI can be a valuable tool for surgical decision-making, its utility for measurement of ulnar variance is limited.


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