Analysis of total corneal astigmatism with a rotating Scheimpflug camera in keratoconus

Abstract

Background To analyze mean corneal powers and astigmatisms on anterior, posterior, and total cornea in patients with keratoconus as calculated according to various keratometric measurements using a Scheimpflug camera. Methods We examined the left eyes of 64 patients (41 males and 23 females; mean age 29.94 ± 6.63 years) with keratoconus. We measured simulated K (Sim-K), posterior K, true net power (TNP) and four types of total corneal refractive powers (TCRP). We then used the obtained values to analyze mean K, and corneal astigmatism. TCRP were measured at 2.0 ~ 5.0 mm. Results Mean corneal powers from Sim K, posterior K, and TNP were 49.12 ± 3.99, − 7.39 ± 0.79, and 47.78 ± 4.09 diopters, respectively. For TCRP centered on the pupil, mean K tended to decrease with measurement area (all p < 0.01). While, both mean K and astigmatism measured using TCRP centered on the apex decreased with measurement area (all p < 0.001). TCRP centered on the apex were greater than those centered on the pupil for mean K values calculated using TCRP (all p < 0.001). The proportion of WTR was greatest on the anterior and total cornea. As the measurement area moved to the periphery, the proportion of WTR increased. Conclusions Mean corneal powers and astigmatisms on total cornea with keratoconus change depending on calculation methods and measurement areas.

Cataract surgery considerations in patients with prior history of keratoconus and ectasia

“…TCRP can be measured over the apex (TCRP apex ) or the pupil (TCRP pupil ). Significantly higher mean K values have been found with TCRP apex measured at multiple optical zones [24]. Although this may be of limited relevance in patients with central cones, using TCRP pupil in peripheral cones would be less likely to overestimate corneal power, decreasing the likelihood of hyperopia postoperatively.…”

“…Pentacam provides multiple measurements of corneal power, including simulated K (SimK), true net power (TNP), equivalent keratometry reading (EKR), and total corneal refractive power (TCRP). Studies comparing these parameters note that TCRP [5,23 ] and TNP [24] measure flatter values compared with SimK, whereas SimK values were significantly greater than EKR at increased corneal zone diameters [25].…”

Title: Cataract Surgery in Patients with Underlying Keratoconus: A Focused Review

“…Moreover, the group also found that the TCRP may be greater than or less than simK depending on the measurement area. 39 This highlights the need for further research to help inform clinician interpretation of various topography-derived data and ultimately help guide appropriate integration of this information into IOL power calculation.…”

Measuring corneal astigmatism using OCT in keratoconus

“…Anterior segment optical coherence tomography (OCT) and Scheimpflug imaging are imaging modalities that can directly measure the anterior and posterior corneal surfaces and assess net corneal astigmatism. 8–12 OCT has several advantages over Scheimpflug imaging; modern OCT systems have higher axial resolution, scan at faster speeds, and can use custom topography software to reduce motion error. 11,13,14…”

“…Anterior segment optical coherence tomography (OCT) and Scheimpflug imaging are imaging modalities that can directly measure the anterior and posterior corneal surfaces and assess net corneal astigmatism. [8][9][10][11][12] OCT has several advantages over Scheimpflug imaging; modern OCT systems have higher axial resolution, scan at faster speeds, and can use custom topography software to reduce motion error. 11,13,14 The goal of this study was to determine the accuracy and repeatability of net corneal astigmatism measured with a spectral-domain OCT system in keratoconic patients and compare them with a Scheimpflug imaging topographer using different analytic zone diameter (3 mm vs 4 mm) and map centration (corneal vertex vs pupil center).…”