KCN eyes reportedly have different cornea-related parameters compared with normal eyes, including corneal curvature, corneal thickness, corneal endothelial density, corneal hysteresis, and corneal resistance factor [8, 24, 25]. Clinical manifestations and signs of keratoconus are different at different stages, and mild keratoconus is usually difficult to detect with slit-lamp biomicroscopy. Fleischer’s ring and Vogt’s striae can be seen on the corneal inferior or center as the disease progresses. Vogt’s striae or stress lines are typical clinical signs of progressive KCN . We observed that diopter, visual acuity, and corneal characteristics were significantly altered when Vogt’s striae appeared. However, associations corneal morphological parameters, mechanisms of the formation of Vogt’s striae, and where the striae first appeared remain ambiguous.
Our study found that KCN eyes with Vogt’s striae had significantly higher absolute values of sphere, cylinder, and spherical equivalents than eyes without Vogt’s striae, and UCVA and BCVA were significantly lower in KCN eyes with Vogt’s striae than in eyes without Vogt’s striae. This is consistent with the finding of previous research . Sedaghat et al.  reported that morphological parameters (K1, K2, and Kmax) measured by Pentacam HR were higher in KCN eyes with Vogt’s striae than in eyes without Vogt’s striae. In the present study, we evaluated additional morphological parameters (i.e., CCT, TCT, ACD, ACE, and PCE, in addition to the standard parameters of K1, K2, Km, and Kmax). In addition to an increase in the anterior corneal curvature, results indicated significant increases in ACE, PCE, and ACD, and significant decreases in CCT and TCT in eyes with Vogt’s striae. Thus, we can conclude that the occurrence of Vogt’s striae is positively correlated with the severity of KCN. In the absence of corneal topography, keratoconus severity can be assessed by observing the presence of Vogt’s striae under slit-lamp biomicroscopy during clinical examination.
Vogt’s striae may affect the densitometry of the cornea as well as morphological parameters. To the best of our knowledge, few studies have investigated associations between corneal densitometry and Vogt’s striae in KCN eyes. As demonstrated in a previous study, several factors (i.e., the size and arrangement of collagen fibrils) could influence corneal transparency and densitometry . The disarrangement of the corneal collagen structure in KCN patients  may lead to differences in corneal densitometry compared with that in healthy individuals [28, 29].
In the current study, we found that corneal densitometry values of the anterior 0–2 and 2–6 mm and the intermediate 0–2 and 2–6 mm were significantly higher, and that of the posterior 0–2 mm was significantly lower in KCN eyes with Vogt’s striae than in KCN eyes without Vogt’s striae. This is similar to the results observed by Shen et al.  in a study on corneal densitometry in KCN. Alternations in corneal densitometry are related to the degree of changes in the structure of corneal collagen fiber . In previous studies, Uçakhan et al.  and Ghosh et al.  reported that the presence of Vogt’s striae in the anterior stroma of KCN eyes is usually present in moderate and severe KCN. Erie et al.  and Hollingsworth et al.  reported a decrease in keratocyte density, with 19% reduction in the anterior stroma and 10% reduction in the posterior stroma. We hypothesize that KCN eyes with Vogt’s striae present with less epithelial cell density, less stromal keratocyte density in the anterior and intermediate stroma, greater stromal degeneration, and fewer intact corneal collagen fibrils. The abnormal separation of collagen fibrils and wound healing reactions in stromal lamellae may lead to increased light scattering of corneal tissue . Hence, the corneal densitometry values of the anterior and intermediate corneal stroma are higher than those of the posterior stroma in KCN eyes with Vogt’s striae (Fig. 2). Since the cone of KCN is mainly located in the central area as well as under the temporal area of the cornea , corneal densitometry changes within the 0–6 mm area of the cornea are more obvious.
As demonstrated in previous studies [22, 23, 26, 37], UCVA and BCVA were significantly lower in KCN eyes with Vogt’s striae than in eyes without Vogt’s striae, and corneal morphological parameters are clearly affected in KCN eyes with Vogt’s striae. These results are consistent with our findings. We evaluated possible correlations between corneal densitometry results and corneal morphological parameters in KCN eyes with and without Vogt’s striae. In KCN eyes with Vogt’s striae, we found that corneal densitometry values in the anterior 0–2 and 2–6 mm and the total cornea (2–6 mm) were positively correlated with K1, K2, Km, Kmax, ACE, and PCE and negatively correlated with CCT and TCT. The severity of KCN is positively correlated with anterior surface curvature and the elevation of anterior and posterior surfaces and is negatively correlated with corneal thickness . Therefore, corneal densitometry values within 0–6 mm of the anterior corneal layer can reflect the severity of KCN. We speculate that misalignment of corneal collagen and the degeneration of fibronectin in the extracellular matrix occur in the anterior corneal stroma when Vogt’s striae appear in KCN eyes. Corneal densitometry values in the central area of the cornea can be used to evaluate the efficacy and impact of corneal collagen cross-linking surgery for KCN .
In KCN eyes without Vogt’s striae, we found that only K2, Km, and Kmax correlated with the densitometry values of the anterior 0–2 mm and intermediate 0–2 mm. There was no correlation between K1 and corneal densitometry values at any location. Since the orientation of most stromal bands usually occurs in the steepest Sim K axis of the cornea , the corneal ultrastructure of the flat corneal axis is less altered in eyes with mild KCN. We hypothesize that corneal densitometry in the central area may change in the early stages of KCN. Therefore, in the evaluation and diagnosis of early KCN, corneal densitometry in the central region can be used as an auxiliary diagnostic indicator.
We were not able to determine a correlation between central corneal densitometry and ACD in KCN eyes with and without Vogt’s striae. However, we found negative correlations between ACD and peripheral corneal densitometry in all patients. We postulate that those alternations in ACD may be associated with changes in corneal morphology, as described by other authors .
One limitation of our study is that participants were not grouped by corneal curvature; this may have led to potential bias. However, the main objective of the current study was to investigate changes in corneal densitometry in KCN eyes with Vogt’s striae and to explore regions of alternations of corneal collagen fiber that occur with the progression of KCN. Therefore, curvature has little effect on our study. Another limitation of our study is that the examination of corneal densitometry in our study was not performed concurrently. We will thus more rigidly control inspection time to reduce errors in future investigations.