Digitally subtracted linear tomograms: Three techniques for measuring condylar displacement

Abstract

Using digital subtraction enhancement, we compared the accuracy of three tomographic technique variations for quantifying small changes of condylar position. A phantom made of skeletal material and an adjustable mechanical stage provided 0.1 mm positioning accuracy of the condylar segment. Linear tomographic images were made with the condyle in a centered position and at each of three 1 mm increments of displacement in two of three directions (inferior, and posterior or lateral). Variations for frontal views included arbitrary (+20 degrees) and individualzed (+10 degrees) horizontal correction of the condylar axis. Variations for lateral views included the arbitrary and individualized correction and a fully corrected vertical (+5 degrees) and horizontal (+10 degrees) adjustment. Blinded observers made repeated measurements on randomized subtraction images with a digitizing tablet. Film images were subtracted twice, and each subtraction was read twice by two examiners. The study was repeated with a second series of films treated in an identical manner. Mean values for measurements, standard deviations, and absolute values of differences between measured and expected values were calculated for each subtraction. Analysis of variance identified statistically significant sources of variation in repositioning the phantom and producing exact displacements of the condyle. Average absolute value of differences from expected values were similar for the three techniques and for frontal and lateral orientations and increments of displacement. Typical absolute value of difference for any technique was 0.2 mm for posterior, 0.2 mm for lateral, and 0.1 mm for inferior displacements. Significantly greater accuracy of inferior displacement measures was attributed to greater accuracy of registration of the fossa in its cranio-caudal dimension. Tomography incorporating an arbitrary correction of the condylar axis was not significantly different from individualized horizontal and vertical correction techniques for measuring small changes in condylar position with digital subtraction.