[A method of measuring the displacement of the distal radioulnar joint on the three-dimensional CT imaging].

Objectives: To establish a clinical method for measuring the displacement of the distal radioulnar joint (DRUJ) precisely irrespective of ulnar variance, and to derive normal population translation references with palmar and dorsal stress.

Methods: Thirty-seven normal distal forearms were scanned with computed tomography using an apparatus designed by Pirela-Cruz. Each extremity was scanned in two positions: maximal ulnar palmar and dorsal stress. The digital imaging and communications in medicine (DICOM) CT images were then imported into Mimics 10.0 for three-dimensional reconstruction. On the DRUJs 3D images, choose the most prominent point of the palmar and dorsal margins of the sigmoid notch and the excavate ulna fovea as the reference points A, B and C. A perpendicular line was then drawn from the point C to a line connecting points A and B with the intersection D. Calculate the ratio of AD/AB and DB/AB. Two observers measured all the DRUJs independently and one repeated the measurements one month later to determine the interobserver and intraobserver reliability.

Results: The mean ratio values of palmar (AD/AB) and dorsal (DB/AB) translation were 0.39 ± 0.07 and 0.37 ± 0.07, and the normal references (x(-) ± 2 s) were from 0.25 to 0.50 and from 0.23 to 0.50, respectively. No significant differences were observed in terms of positions, genders and dominant hands. The intraclass correlation coefficient (ICC) values for interobserver and intraobserver reliability (DB/AB, AD/AB) were 0.84, 0.80, 0.93 and 0.92, respectively.

Conclusions: This new method could accurately measure the displacement of DRUJs with acceptable reliability, even with ulna positive or negative variance. Instability of DRUJ may be indicated when AD/AB is less than 0.25 or BD/AB is less than 0.23.