Measuring corneal biomechanical parameters may be a helpful supplementary diagnostic tool for keratoconus, the progressive corneal ectatic disease associated with corneal thinning and abnormal corneal surface bulging. Evidence has shown that certain corneal biomechanical features can discriminate keratoconic corneas from normal ones and detect the disease in its early stages. Now, researchers have observed that initial signs of ectasia are principally detectable on the back corneal surface before becoming present on the front.

One hundred keratoconus eyes from 91 patients were included in the cross-sectional study. Of these, 47 eyes had abnormal elevation values on only the posterior corneal surface (group 1), while the remaining 53 had or bulging on both the anterior and posterior corneal surfaces (group 2). Corneal biomechanics were assessed using Corvis ST and Ocular Response Analyzer. Differences were detected differences in all corneal shape parameters between the two groups. Group 1 was found to have much stiffer corneas and more asymmetric shape parameters. 

Other findings from the study include:

• Corneal hysteresis and corneal resistance factor were 1mmHg higher in group 1.

• 97.9% of cases in group 1 had grades of 0 and 1 (meaning “normal” and “suspect” by topographic keratoconus classification), while this value was just 7.6% for group 2. 

• The best fit sphere radius was significantly steeper in group 2.

• The epithelial thickness at the center and the thinness point of the cornea were significantly thinner in group 2.
  

Although early signs of ectasia may be visible on the back corneal surface, these parameters shouldn’t be used as a diagnostic method on their own, as the epithelium can smooth irregularities and mask the disease. “Once the disease has advanced to the point where the anterior stromal changes have overwhelmed the ability of the epithelium to compensate, abnormal front elevation is the result,” the researchers wrote in their study. “The epithelium is thinner when both surfaces show abnormal elevation than when only the back surface is involved. This is consistent with epithelial thinning as a response to increasing curvature of the anterior stroma as keratoconus progresses.”