Lunate–Lunate Facet Ratio and Variance to Predict Articular Gap After Distal Radius Fracture

PurposeIntra-articular gap is a well-defined prognosticator of outcome after distal radius fracture. However, it is often difficult to assess on plain radiographs, particularly in the sagittal plane, as visualized on the lateral view. The purposes of this study were to establish normal values for the lunate–lunate facet ratio (LLFR) and to evaluate whether the LLFR and lunate–lunate facet variance (LLFV) are reliable plain film measures of sagittal articular gap in distal radius fractures compared with sagittal reconstructions of computed tomography (CT) scans.MethodsWe studied 35 uninjured, skeletally mature wrist radiographs to establish the normal LLFR value. The LLFR was defined as the ratio of the maximum width of the lunate (A) to the width of a line spanning the maximum distance from dorsal to volar across the articular surface of the lunate facet, defined by the end point of the radiographically thickened line representing the subchondral bone (B). The LLFV was determined as measurement B minus measurement A. Interobserver and intraobserver reliability for 2 observers were examined by the intraclass correlation coefficient (ICC) for measurements of the uninjured wrists and for the same measurements of 20 intra-articular distal radius fractures. For 12 of the subjects with fractures, the LLFR and LLFV were correlated with the intra-articular gap as measured on CT scan using simple linear regression.ResultsThe mean value of the LLFR in the normal subjects was 1.00 ± 0.04 for observer 1 and 1.00 ± 0.02 for observer 2. There was significant intraobserver reliability for both uninjured (ICC = 0.83, p < .001) and fractured wrists (ICC = 0.87, p < .001), as well as significant interobserver reliability for uninjured (ICC = 0.69, p < .001) and fractured wrists (ICC = 0.64, p = .015). The LLFR was significantly associated with articular gap on sagittal CT reconstructions (R2 = –0.81, p < .01). The LLFV correlated highly with articular gap on sagittal CT reconstructions (R2 = 0.83, p < .01).ConclusionsThe LLFR, measured on the lateral radiograph of the wrist, exhibits high intraobserver and interobserver reliability, with a normal value of 1.0. Both the LLFR and the LLFV are significantly related to articular gap on sagittal CT reconstruction images. The LLFR decreases with increasing intra-articular gap formation. The LLFV allows a direct estimation of articular gap size.Type of study/level of evidenceDiagnostic III.