Reply : Calculating equivalent K readings

We appreciate Holladay’s comments about our paper and his contribution to our discussion with respect to the agreement between automated K and the equivalent K readings at 4.5 mm. Our study investigated virgin corneas prior to cataract surgery. In the introduction, we stated that “[t]he Scheimpflug device… has found a place in the preoperative biometry of eyes with cataract that have previously had refractive surgery.” These were carefully chosen words, since the choice of lens power for post-refractive-surgery patients with cataract still represents a significant challenge for the cataract surgeon. Scheimpflug imaging has certainly increased the options available with the advent of software such as the BESSt formula1 and the Holladay equivalent K readings, but as correctly stated by Holladay et al.2 in their paper describing the equivalent K readings, the historical method should always be calculated for comparison if preoperative refractive data are available and patients should be counseled prior to surgery regarding the risk for a secondary procedure to optimize their refraction. Most cataract surgery, however, is performed in patients who have not had prior refractive surgery. As stated in our paper, with improvements over the years in axial length measurement techniques, keratometry is an important source of potential biometry error. The Pentacam (Oculus) is able to measure many more points on the cornea than a conventional keratometer and can also image the posterior curvature. The hope for cataract surgeons is that, in the future, this technology may become applicable to routine cataract surgery, increasing the accuracy of biometry and reducing the risk for refractive surprises. The equivalent K is a helpful innovation as it allows Scheimpflug K values to be substituted into conventional IOL power prediction formulas. Ultimately, prediction formulas may be modified to incorporate more corneal parameters, as a result of developments in corneal imaging such as Scheimpflug. Anecdotally, we would comment that in the mostly elderly cataract population from which our study sample was derived, we sometimes had difficulty obtaining results from the Pentacam with the “OK” quality statement, even when measurements were repeated (although all the data included in the study were OK). This may partly relate to the relatively long time required to acquire the images (approximately 2 seconds for our device), and it has been suggested this may be relevant even when the OK quality specification is obtained.3 As the technology evolves, this is likely to produce increased accuracy for the mathematical algorithms based on the Scheimpflug measurements.