Biomechanical responses of healthy and keratoconic corneas measured using a noncontact scheimpflug-based tonometer.
OBJECTIVE
To examine the repeatability of measurements obtained using a noncontact Scheimpflug-based tonometer (Corvis ST) and investigate potential differences in these parameters between healthy and keratoconic (KC) corneas.


METHODS
Forty-five keratoconic eyes and 103 healthy eyes were examined using biomicroscopy, corneal tomography and the Corvis ST (CST).


RESULTS
Intraocular pressure and central corneal thickness (CCT) were highly repeatable (intraclass correlation [ICC] > 0.70, coefficient of variation [CV] < 0.20). Deformation amplitude (DA) and applanation-1 time (A1T) were fairly repeatable (ICC > 0.60, CV < 0.20). There was no association between DA and age, sex, or ethnicity in healthy eyes. There was a greater mean DA in the KC group compared with 46 age-matched healthy eyes (KC 1.37 ± 0.21 mm, healthy 1.05 ± 0.11 mm, P < 0.001). Multivariate analysis showed DA in KC was predicted by IOP, CCT, and the steepest simulated keratometry value (P = 0.03, P = 0.03, P = 0.001 respectively, R(2) = 0.75). A subgroup analysis of healthy and KC eyes with comparable mean CCT and IOP highlighted a statistically significant difference in mean DA (KC 1.25 ± 0.08 mm, thin healthy 1.13 ± 0.09 mm, P = 0.006). Receiver-operating characteristic analysis showed an area under the curve of 0.77 (95% CI 0.61-0.93, P = 0.006) but no ideal cutoff value for DA.


CONCLUSIONS
Key parameters assessed by the CST are repeatable. Keratoconus is associated with greater DA than in healthy eyes, even when controlled for CCT and IOP. Deformation amplitude may be a useful adjunct in keratoconus assessment and monitoring, but cannot solely discriminate between healthy and keratoconic corneas.
Department of Ophthalmology, New Zealand National Eye Centre, University of Auckland, New Zealand.
Noor Q
NQ
Ali
Department of Ophthalmology, New Zealand National Eye Centre, University of Auckland, New Zealand.
Dipika V
DV
Patel
Department of Ophthalmology, New Zealand National Eye Centre, University of Auckland, New Zealand.
Charles N J
CN
McGhee
D000293:Adolescent; D000328:Adult; D000368:Aged; D001696:Biomechanical Phenomena; D003315:Cornea / Q000000981:diagnostic imaging / Q000473:pathology / Q000503:physiopathology*; D019781:Corneal Topography; D004867:Equipment Design; D005260:Female; D006801:Humans; D007429:Intraocular Pressure; D007640:Keratoconus / Q000175:diagnosis / Q000503:physiopathology*; D008297:Male; D033401:Microscopy, Acoustic; D008875:Middle Aged; D012016:Reference Values; D015203:Reproducibility of Results; D014065:Tonometry, Ocular / Q000295:instrumentation*; D055815:Young Adult
Corvis ST; corneal deformation amplitude; diagnostic instruments; non-contact tonometry; repeatability