Our study group described the concept of relational thickness, which considers the single-point metrics central corneal thickness (CCT) and TP with the PPI [13, 14, 18]. The combinations of pachymetric metrics TP and PPI were used in a simple ratio formula to provide a single metric that better describes corneal thickness. The ART (“Ambrósio Relational Thickness”) can be calculated for the minimal (ART-Min), average (ART Ave), and maximal (ART Max) progression indices. These parameters are obtained by dividing the thinnest pachymetric value by the respective pachymetric progression index [18].

We published a study that enrolled 46 eyes diagnosed as mild to moderate keratoconus and 364 normal patients, which were evaluated by the Pentacam Comprehensive Eye Scanner (OCULUS, Wetzlar, Germany) [17]. Annular pachymetric distribution (CTSP and PTI) has been used successfully and validated statistically for the diagnosis of keratoconus, with higher accuracy than single-point CCT and TP. Significant differences were found for all positions of the CTSP from normal eyes and keratoconus (p < 0.01), in which keratoconus had much lower (thinner) values. It was estimated that keratoconic corneas were, in average, 27.3 μm thinner than normal corneas. There were also significant differences for all positions of the PTI from normal eyes and keratoconus (p < 0.0001), in which keratoconus had much higher increase. It was noticeable that normal corneas have a more homogeneous increase than in keratoconus [17].

Another study enrolled 113 individual eyes randomly selected from 113 normal patients and 44 eyes of 44 patients with keratoconus [18]. The Pentacam Scanner was used to obtain CCT, TP, PPI Min, PPI Max, PPI Ave, ART Max, and ATR Ave values. Statistically significant differences were noted between normal and keratoconic eyes for all parameters (p < 0.001), except for horizontal position of TP (p = 0.79). As shown in Table 8.1, the best parameters were ART Ave and ART Max with areas under the ROC curves of 0.987 and 0.983, respectively. Pachymetric progression indices and ART parameters had a greater area under the curve than TP and CCT (p < 0.001) [18].

Based on the presented data and other studies, tomographic-derived pachymetric parameters provided a higher accuracy to differentiate normal and keratoconic corneas than single-point pachymetric measurements [17, 18, 21].

It is worth mentioning that screening ectasia risk should go beyond the detection of keratoconus; it is critical to consider studies that include mild or subclinical forms of ectasia [22]. Regarding this issue, one of the most important groups is composed by the eyes with relatively normal topography from patients with keratoconus detected in the fellow eye, referred as Forme Fruste Keratoconus (FFKC) [13, 23, 24].

Tables 8.2 and 8.3 provide the cutoff values and ROC curves details of the most effective parameters from the Pentacam for distinguishing normal from ectatic corneas [14]. We included parameters derived from the anterior curvature of the cornea (K Max as the point of maximum curvature), back elevation at the thinnest point using a best fit sphere (PostElev Thinnest BFS), and a best fit toric ellipsoid (PostElev Thinnest BFTE); ART Ave and Max; and Belin Ambrósio Final Deviation Value (BAD D) . The Belin-Ambrósio Enhance Ectasia Display (Figs. 8.6 and 8.7) is a comprehensive display that enables a global view of the tomographic structure of the cornea through combination of the enhanced elevation approach, described by Belin, pachymetric, and curvature data [13]. The BAD considers the deviations of normality values for different parameters, so that a value of zero represents the average of the normal population and one represents the value is one SD toward the disease (ectasia) value. A final ‘D’ is calculated based on a regression analysis that weights differently the parameters [13, 25].