Turning to topography

Using preoperative corneal topography or tomography enhances the security and accuracy of biometry and should be considered as a potentially valuable addition to all cataract surgery procedures and not just for implanting toric lenses, according to Steve Arshinoff MD. “Using topography or tomography allows greater certainty of the corneal contribution to biometry and enables us to achieve the 90% of patients within 0.5D of the targeted refraction, which Warren Hill has claimed is achievable,” Dr Arshinoff said at the World Ophthalmology Congress 2020 Virtual. Topography defines the anterior curvature of the cornea with technology based on the ground-breaking work by Antonio Placido in the 19th Century, said Dr Arshinoff. Devices based on this concept include hand-held keratoscopes and a range of Placido disc topographers (Atlas, Nidek, OPD, Schwind Peramis, etc). There is the also the scanning slit technique (Orbscan) capable of measuring both the anterior and posterior surfaces of the cornea. Scheimpflug tomography (Oculus Pentacam, Ziemer Galilei, Schwind Sirius) renders 3D images of the cornea and is better equipped than topographers to detect the posterior cornea, keratoconus and early ectasia, he added. Accurate biometry requires at least two measurements by different devices of axial length and corneal power (Ks), Dr Arshinoff explained. The rationale for using topography is that while only about 10% of corneas are found to be irregular, identifying this significant minority is not possible unless you use equipment adapted to the task. “In 90% of cases the IOLMaster or Lenstar will be accurate enough so that topography can be ignored. However not all corneas are perfect and using topography or tomography allows us to detect the amount and type of LASIK that a patient has had, an irregular bowtie pattern or other corneal irregularities,” he said. Dr Arshinoff said that he has been performing preoperative corneal topography in every patient since 1990. This enables him to identify patients with irregular corneas or those who had undergone corneal surgery in the past and to prepare specific treatment plans in order to obtain better outcomes. However, he believes that tomography is now preferable to topography because it additionally accurately assesses the posterior surface of the cornea. “If you are still using topography, I advise to estimate 0.5D against-the-rule total corneal power above the value shown by the topographer. My technique is to leave all patients with 0.5D with-the-rule astigmatism postoperatively because that decreases with age and patients are happy because they feel that they get better as time passes,” he said.

Tags: tomography