Biomechanical Analysis of Tomographically Regular Keratoconus Fellow Eyes Using Corvis ST

Abstract

Background Keratoconus is a bilateral, yet asymmetric disease. In rare cases, the second eye may show no signs of tomographic changes. The purpose of this study was to analyze the biomechanical characteristics in tomographically regular keratoconus fellow eyes.

Materials and Methods This retrospective, consecutive case series analyzed 916 eyes of 458 patients who presented to our keratoconus clinic between November 2020 and October 2022. Primary outcome measures included best-corrected visual acuity (BCVA), tomographic Scheimpflug analysis using Pentacam AXL (Oculus, Wetzlar, Germany), and biomechanical assessment using Corvis ST (Oculus, Wetzlar, Germany). Tomographic changes were assessed via analysis of the anterior and posterior curvature, K-max, thinnest corneal thickness (TCT), the Belin/Ambrosio Deviation Display (BAD-D), and the ABCD-Grading. Biomechanical changes were analyzed using Corvis Biomechanical Index (CBI) and Tomographic Biomechanical Index (TBI).

Results Of 916 eyes, 34 tomographically regular fellow eyes (7.4%) were identified and included in the analysis. Overall, the mean BCVA was − 0.02 ± 0.13 logMAR. Tomographic analysis showed mean K-max of 43.87 ± 1.21 D, mean TCT of 532 ± 23 µm, and mean BAD-D of 1.02 ± 0.43. Biomechanical analysis demonstrated mean CBI of 0.28 ± 0.26 and mean TBI of 0.34 ± 0.30. While normal CBI-values were observed in 16 (47%) of 34 eyes, only 13 eyes (38%) showed a regular TBI and only 7 eyes (21%) showed regular TBI and CBI. The sensitivity of CBI and TBI to detect a tomographically normal keratoconus fellow eye was 53% and 62%, respectively.

Conclusion A highly asymmetric corneal ectasia with regular tomographic finding in a fellow eye is rare among keratoconus patients. In such cases, a biomechanical analysis may be useful in detecting early signs of corneal ectasia. In our analysis, the TBI showed high sensitivity for detecting a biomechanical abnormality in tomographically regular fellow eyes.

Fazitbox

Bereits bekannt:

• Die Erkennung einer frühen Ektasie ist als Screening vor refraktivchirurgischen Eingriffen essenziell.

• Die Verwendung von tomografischen und biomechanischen Indizes wie z. B. des BAD-D, CBI und TBI erhöht die Wahrscheinlichkeit ein sehr frühes Keratokonus-Stadium von einem gesunden Auge zu unterscheiden.

Neu beschrieben:

• Der TBI zeigte eine etwas höhere Sensitivität als der CBI (62% vs. 53%), den Keratokonus in einem tomografisch unauffälligen Keratokonus-Partnerauge zu erkennen.

• 21% der Keratokonus-Partneraugen konnten weder durch den CBI noch durch den TBI als auffällig erkannt werden.

Conclusion Box

Already known:

• Detection of early ectasia is an essential screening step which must be performed prior to refractive surgical procedures.

• The use of tomographic and biomechanical indices such as BAD-D, CBI, and TBI increases the likelihood of distinguishing an eye with very early stage keratoconus from a healthy eye.

Newly described:

• The TBI showed slightly higher sensitivity than the CBI (62% vs. 53%) for detecting keratoconus in a tomographically unremarkable keratoconus partner eye.

• 21% of the keratoconus partner eyes could not be recognized as conspicuous, either by CBI or TBI.

Key words

keratoconus - subclinical keratoconus - biomechanics - Corvis ST - CBI - TBI

Publication History

Received: 16 December 2022

Accepted: 05 July 2023

Article published online:
11 August 2023

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