Quantifier variables of the back surface deformity obtained with a noninvasive structured light method: evaluation of their usefulness in idiopathic scoliosis diagnosis

Abstract

New noninvasive techniques, amongst them structured light methods, have been applied to study rachis deformities, providing a way to evaluate external back deformities in the three planes of space. These methods are aimed at reducing the number of radiographic examinations necessary to diagnose and follow-up patients with scoliosis. By projecting a grid over the patient's back, the corresponding software for image treatment provides a topography of the back in a color or gray scale. Visual inspection of back topographic images using this method immediately provides information about back deformity, but it is important to determine quantifier variables of the deformity to establish diagnostic criteria. In this paper, two topographic variables [deformity in the axial plane index (DAPI) and posterior trunk symmetry index (POTSI)] that quantify deformity in two different planes are analyzed. Although other authors have reported the POTSI variable, the DAPI variable proposed in this paper is innovative. The upper normality limit of these variables in a nonpathological group was determined. These two variables have different and complementary diagnostic characteristics, therefore we devised a combined diagnostic criterion: cases with normal DAPI and POTSI (DAPI < or = 3.9% and POTSI < or = 27.5%) were diagnosed as nonpathologic, but cases with high DAPI or POTSI were diagnosed as pathologic. When we used this criterion to analyze all the cases in the sample (56 nonpathologic and 30 with idiopathic scoliosis), we obtained 76.6% sensitivity, 91% specificity, and a positive predictive value of 82%. The interobserver, intraobserver, and interassay variability were studied by determining the variation coefficient. There was good correlation between topographic variables (DAPI and POTSI) and clinical variables (Cobb's angle and vertebral rotation angle).

Figures

Fig. 1

Scoliotic patients: radiographic parameters. The Ponseti classification was used to establish the type of curve. Cobb’s angle was categorized into four groups following the Scoliosis Research Society’s recommendations. The vertebral rotation angle was categorized into three groups following the Pedriolle–Vidal method

Fig. 2

Method for obtaining topographical images. A screen, B camera, C computer, D projector. In the calibration process the screen (A) is moved into two different positions and an image is taken of the projected grid in both positions. The patient is situated in front of the screen in the second position and an image of the grid projected over his/her back is taken

Fig. 3

Back surface topographies in color or gray scale. The points with the same color or gray scale correspond to points with same depth. Visual inspection shows the differences between normal and pathologic backs

Fig. 4

Calculation of the POTSI variable; addition of frontal asymmetry index (FAI) and height asymmetry index (HAI). Points 1–8 are anatomical points. I: Distance between points 1 and 2; A: y-component of I distance and reference line for the rest of measures; B–E are, respectively, the distance from points 5–8 to line A; F–H are, respectively, the y-component of the distance between points 3–4, 5–6, and 7–8

Fig. 5

Calculation of the DAPI variable; addition of the difference of depths of the symmetrical points at the level of the scapulae and waist. Points 1, 2, and 9–14 are anatomical points. I: Distance between points 1 and 2, and reference line for the rest of measures; 10′ and 11′ symmetrical points of 10 and 11, following the line 9–10 and 11–12, respectively. The symmetrical point is found for the most prominent of each couple, 9 or 10 in one case and 11 or 12 in the other. Points 13 and 14 must have equal prominence if the patient is positioned correctly if not the prominence of points 9–12 is automatically recalculated (sub-index c)

Fig. 6

Back topographic reconstruction. The values of the POTSI and DAPI variables appear together with the topographic image

Fig. 7

Graph of the normal and pathologic cases according to their DAPI and POTSI values; value limits are marked by the dotted line. The bottom left square shows a normal diagnosis (normal DAPI and POTSI values) and the rest of the squares show a pathologic diagnosis (pathologic DAPI and/or POTSI values)

Fig. 8

Cobb’s angle values versus DAPI and POTSI values. Good correlation is shown between variables (ar=0.668y, br=0.706)