The detection of subclinical corneal ectasia is of utmost importance for the corneal refractive surgeon. Keratoconus is typically diagnosed by clinical examination along with corneal tomography. However, in the early stages of keratoconus, corneal tomographic imaging can be normal or have only subtle, easily missed abnormalities. Subclinical keratoconus is the leading risk factor for iatrogenic ectasia after refractive surgery. As such, the addition of other objective tests in conjunction with tomography and clinical examination may better detect these early changes.
The measurement of corneal biomechanics may become an essential component of the preoperative assessment for refractive surgery. Corneal hysteresis is a biomechanical property of the cornea that describes the deformation of tissue under force. There are currently two devices available that measure corneal hysteresis: the Ocular Response Analyzer (Reichert) and the Corvis ST (Oculus). These devices measure the inherent biomechanical integrity of the cornea based on its deformation and rebound in response to a brief puff of air.
At our center, we use the Ocular Response Analyzer with the Keratoconus Match Index (KMI) software for screening refractive surgery candidates. Corneal hysteresis is compared with a normative database of normal and keratoconic patients, and a percentile risk score is generated via the KMI software. In fact, at Weill Cornell Medicine, we are currently conducting a study to measure the diagnostic agreement and predictability of keratoconic and normal eyes using corneal biomechanics with tomographic images. These new technologies have great potential for widespread adoption within ophthalmology, as they may better identify patients at risk for postrefractive ectasia.
Understanding corneal biomechanics can be an extremely useful tool in diagnosing patients at risk for developing corneal ectasia. This modality, in conjunction with tomography and clinical examination, can be especially helpful in the screening of potential refractive surgery patients when tomography can still be normal.