Correlations between the ring segment thicknesses and the refractive, keratometric, and corneal aberrometric changes were also investigated. As described in the Methods section, a specific criterion was used for defining the concept of superior and inferior ring segments, to simplify the statistical analysis: A superior segment was any segment with a geometric center located on the superior half of the cornea and an inferior segment was any segment with a geometric center located on the inferior half of the cornea. In addition, a nonimplanted ring segment (superior or inferior) was regarded as an implant with a thickness of 0. Several statistically significant weak and moderate correlations were found between the thickness of the superior and inferior ring segments, and some clinical changes (
Table 3). For example, these thicknesses correlated inversely with the change in mean keratometry and positively with the change in the RMS value corresponding to the corneal higher order residual aberrations. As mentioned earlier, a nearly linear relationship between the degree of central corneal flattening and ring thickness was found in normal corneas.
25,26 Besides these moderate relationships, we found that some clinical changes also correlated significantly with some preoperative conditions, as the magnitude of the spherocylindrical error or the corneal curvature. Therefore, it seems clear that some factors influence the visual and refractive outcomes achieved with the KeraRing segments. In other words, this process cannot be represented by means of a simple linear model with two variables. The effect achieved with each KeraRing segment is a multifactorial process depending on the ocular preoperative conditions and on the thickness of the implant. It should be remembered that all segments were implanted according to the same surgical criteria (inner and outer diameters of 4.8 and 5.7 mm, respectively, and ring placement at ∼80% of the depth of the cornea). The diameter and the depth of the implant are also factors in the final effect achieved with the ring segments,
24 but these factors have not been modified in the present study. It should be considered that corneal changes induced by the ICRS must be in relation to the structural properties of the collagen framework in the corneal stroma. The stroma accounts for 90% of corneal thickness, and evidently its mechanical properties define, for the most part, the mechanical properties of the whole corneal structure. In the normal cornea, there is a preferred orientation of collagen lamellae along the horizontal and vertical directions, but this trend is maintained to within approximately 1 mm from the limbus, where a circular or tangential disposition of fibrils occurs.
34 However, this well-organized lamellar structure is lost when the corneal tissue degenerates, as happens in keratoconus.
27 The regular orthogonal arrangement of the collagen fibrils is destroyed within the apical scar of the keratoconus.
27 Therefore, the ICRS effect in keratoconus seems to be a more complicated phenomenon that needs a more complex mathematical model. The current investigation was conducted to define an approach to devising such a model through multiple linear regression analysis. A more accurate model should be defined in the future, considering clinical parameters and also accurate measurements of the structural and mechanical properties of the corneal tissue.