Virtual Bowman's topography software provides a new noninvasive way to look at crosslinking outcomes.
This article was reviewed by Gairik Kundu, MBBS, MS
A novel method of noninvasively evaluating the outcomes after crosslinking is under development. The software that can be plugged into existing topographers will become commercially available in the near future.
“A decade after crosslinking was developed, the question remains about what actually determines the true outcomes of crosslinking; is it flattening or the demarcation line? We have numerous topographers available, but what are we missing?” asked Gairik Kundu, MBBS, MS, a fellow, Department of Cornea and Refractive Surgery, Narayana Nethralaya, Bangalore, Karnataka, India.
An important consideration regarding topographers is how perfect the technology is and what are their limitations. Some devices are based on Scheimpflug and others on Placido. Placido devices can image the anterior but not the posterior corneal surface; however, the technology it is affected by changes in corneal irregularities and alterations of the ocular surface. The Scheimpflug camera does not have sufficient resolution to delink the epithelium from the surface of Bowman’s layer, according to Dr. Kundu.
“It is important for us to understand that the current topographers cannot dissociate the epithelium,” he said
Another extremely important factor to consider after crosslinking is that the epithelium might be a “masquerader,” Dr. Kundu suggested.
“Is the epithelium the confounder? Is this the factor that we are not actually looking at?” he asked.
The epithelium, he pointed out, can change after crosslinking. The curvature at the air/epithelium interface will differ from that at the epithelium/Bowman’s interface.
“Knowing this, are we seeing the actual outcomes?” Dr. Kundu asked.
Not all outcomes after crosslinking are necessarily the desired ones. This can happen as the result of progression. This is the point at which surgeons must question what is being measured and what should be measured.
“Are we looking at the air/epithelium interface?” Dr. Kundu asked. “In most cases, this is the masquerader that actually masks the changes that are taking place beneath.”
Dr. Kundu and colleagues conducted a study to assess the true outcomes after crosslinking. The method that he proposed to accomplish this is by using a virtual de-epithelialization technique that he and his colleagues developed, i.e., a noninvasive way of delinking the epithelium from the Bowman’s surface beneath.
The basis for the study was the recognition that the previous work described in the literature required an invasive procedure to evaluate Bowman’s layer. The epithelium had to be removed intraoperatively before the topography could be assessed.
The investigators analyzed 40 eyes of 40 patients with keratoconus, all of whom had undergone crosslinking. The Pentacam and anterior-segment optical coherence tomography (OCT) data were collected preoperatively and postoperatively.
They used the Optovue spectral-domain OCT and obtained eight radial two-dimensional frames using MATLAB software, which facilitated viewing of the air/epithelium and the epithelium/Bowman’s interface.
“These raw elevation points were then translated into cartesian points and we determined the tangential and axial curvature data,” Dr. Kundu explained. “We then performed a ray tracing method and Zernike analysis to calculate the aberrations. This is how we arrived at the air/epithelium and the epithelium/Bowman’s interface.”