Review of preoperative tomographic and biomechanical corneal assessment for refractive surgery

Screening prior to laser refractive surgery is important to detect abnormal preoperative topography, which is a significant risk factor for developing postoperative iatrogenic corneal ectasia. Pentacam Scheimpflug corneal tomography is effective in screening for ectasia, especially with the Belin/Ambrosio Enhanced Ectasia Display (BAD). However, Pentacam tomography only images corneal shape and it does not evaluate the dynamic biomechanical properties of the cornea. Corneal Visualization Scheimpflug Technology (Corvis ST) provides information on dynamic corneal biomechanical properties and supplements the data provided by Pentacam tomography. The Corvis Biomechanical Index (CBI) uses a combination of biomechanical parameters obtained from Corvis ST to screen for ectasia. Both Pentacam tomography and Corvis ST data are combined to give the Tomographical Biomechanical Index (TBI) which uses machine learning to provide a more accurate screening for early ectasia or keratoconus. The recommended cutoff value for CBI to differentiate normal versus keratoconic eyes is 0.5, with higher values indicating keratoconus. Out of the three indicators - BAD final D, CBI and TBI, TBI had the highest area under the receiver operating characteristic curve for diagnosing ectasia, with high sensitivity and specificity. Corvis ST is a useful tool which complements Pentacam tomographic results to enhance the diagnosis of clinical keratoconus, subclinical keratoconus, and forme fruste keratoconus. This is especially useful to determine the suitability for refractive surgery in patients.