Mathematical Approximation of Orbscan II Central Corneal Thickness to Contact Ultrasound

Central corneal thickness in glaucoma

PURPOSE OF REVIEW

The purpose of this review is to summarize the role of central corneal thickness (CCT) in the clinical management of a glaucoma patient.

RECENT FINDINGS

The prognostic value of CCT is well recognized in patients with ocular hypertension. However, its predictive value in other glaucoma suspects and patients with established glaucoma is less certain. Tonometry artefacts can result from variations in CCT. However, an adequately validated correction algorithm for Goldmann applanation tonometry measurements does not exist. Newer methods of tonometry are potentially less influenced by CCT but are limited in their clinical use. There may also be biological and genetic associations between corneal thickness and glaucoma. Demographics, environmental factors, glaucoma treatment and the measurement device used have a significant influence on CCT, and should be considered when interpreting the effect of cornea thickness in patients with glaucoma. New measurements of the biomechanical properties of the cornea are likely to be better approximations of the globe biomechanics than CCT, but these require further evaluation.

SUMMARY

The clinical significance of CCT is well recognized in the context of glaucoma diagnosis and management, though the extent of its importance remains debatable. Corneal biomechanical properties may be more significantly associated with glaucoma than CCT.

Measurement of Central Corneal Thickness Using Ultrasound Pachymetry and Orbscan II in Normal Eyes

Purpose:

To compare ultrasound pachymetry and Orbscan II for measurement of central corneal thickness (CCT) in normal eyes.

Methods:

The current study was performed at Labbafinejad Medical Center (LMC), Tehran, Iran. Three hundred eyes from 150 healthy individuals referred for keratorefractive surgery were assessed first by Orbscan II and then by ultrasound pachymetry, and CCT values were recorded and compared.

Results:

Overall, Orbscan II overestimated CCT as compared to ultrasound pachymetry by about 2.4% (mean values 547.6 ± 34.7 versus 534.8 ± 34.7, respectively, P < 0.001). The difference was more significant when CCT was less than 500 microns (mean values 493.2 ± 16.9 versus 479.9 ± 15.6, mean overestimation: 2.6%, P < 0.001). There was good linear correlation between the two methods (Pearson's correlation r = 0.968, P < 0.0001).

Conclusion:

Orbscan II has good correlation with ultrasound pachymetry for measurement of CCT in normal eyes; however Orbscan II should not be used to evaluate corneal thickness before keratorefractive surgeries, as it tends to overestimate corneal thickness and may result in undesirable, low residual stromal thickness.

Comparison of central and peripheral corneal thickness measurements with scanning-slit, Scheimpflug and Fourier-domain ocular coherence tomography

PURPOSE

To compare central, regional and relational corneal thickness values obtained with multiple technologies in normal patients and to determine their equivalence and interchangeability.

METHODS

Retrospective analysis of 100 eyes from 50 patients evaluated by ultrasound pachymetry (Pachette II), scanning-slit (Orbscan II), Scheimpflug (Pentacam HR) and spectral-domain ocular coherence tomography (OCT) (RTVue-100) obtained as average values (OCT-A) and point measurements (OCT-P). Measurements included central corneal thickness (CCT) for all technologies and thinnest corneal thickness for scanning-slit, Scheimpflug and OCT. Peripheral thickness measurements were obtained at the 3 mm radius in the superior (S), nasal (N), inferior (I) and temporal (T) regions.

RESULTS

CCT values were: 563.9±36.1μ ultrasound, 570.9±36.1μ scanning-slit, 552.8±33.8μ Scheimpflug, 550.5±32.7μ (OCT-A), 549.4±32.7μ (OCT-P). Ultrasound and scanning-slit were significantly different from each other (p<0.0001), and both were significantly different from all other devices (p<0.0001), while Scheimpflug was similar to OCT-A and OCT-P (p=0.4). Differences between CCT and thinnest corneal thickness were significantly different from all technologies except scanning-slit and OCT-A. For peripheral values, almost all locations' measurements were significantly different from one another (p<0.0001). Superior-inferior values and ratios were also significantly different from one another for almost all devices with no consistent patterns detectible.

CONCLUSIONS

There are significant clinically relevant differences between regional and relational thickness measurements obtained with ultrasound, scanning-slit, Scheimpflug and OCT devices. Screening metrics devised for one system do not appear directly applicable to other measurement systems.