Isaac Ramos

Dynamic ultra high speed Scheimpflug imaging for assessing corneal biomechanical properties

OBJECTIVE: To describe a novel technique for clinical characterization of corneal biomechanics using non-invasive dynamic imaging. METHODS: Corneal deformation response during non contact tonometry (NCT) is monitored by ultra-high-speed (UHS) photography. The Oculus Corvis ST (Scheimpflug Technology; Wetzlar, Germany) has a UHS Scheimpflug camera, taking over 4,300 frames per second and of a single 8mm horizontal slit, for monitoring corneal deformation response to NCT. The metered collimated air pulse or puff has a symmetrical configuration and fixed maximal internal pump pressure of 25 kPa. The bidirectional movement of the cornea in response to the air puff is monitored. RESULTS: Measurement time is 30ms, with 140 frames acquired. Advanced algorithms for edge detection of the front and back corneal contours are applied for every frame. IOP is calculated based on the first applanation moment. Deformation amplitude (DA) is determined as the highest displacement of the apex in the highest concavity (HC) moment. Applanation length (AL) and corneal velocity (CVel) are recorded during ingoing and outgoing phases. CONCLUSION: Corneal deformation can be monitored during non contact tonometry. The parameters generated provide clinical in vivo characterization of corneal biomechanical properties in two dimensions, which is relevant for different applications in Ophthalmology.

Corneal Densitometry in Keratoconus

Purpose: The aim of this study was to compare corneal densitometry measured by Scheimpflug tomography in normal and keratoconic eyes and to assess the differences in densitometry values among the stages of keratoconus. Methods: Keratoconic and normal corneas were examined using the Pentacam. Corneal densitometry was measured over a 12-mm diameter area, divided by annular concentric zones and depths. Keratoconus was classified according to the topographic keratoconus classification. Results: We enrolled 1 eye randomly selected from each of 172 patients with normal corneas (N) and 98 patients with bilateral keratoconus (KC). There were significant differences between the groups for densitometry measurements in 2 annuli: central 2.0 mm in diameter (N = 16.85 ± 2.42, KC = 18.93 ± 2.78, P = 0.0001) and annulus 2.0 to 6.0 mm in diameter (N = 15.18 ± 2.18, KC = 16.16 ± 1.71, P = 0.005), and total diameter (N = 24.89 ± 6.18, KC = 16.71 ± 2.3, P = 0.033). Divided by layers, the inner parts of anterior (120 &mgr;m), central (from 120 &mgr;m to the last 60 &mgr;m), and posterior (last 60 &mgr;m) layers were also higher in the KC group (P < 0.001). There were differences according to the stages of KC for corneal densitometry of the central annuli at total thickness, anterior and central layers. More advanced cases presented a higher backscatter (P < 0.05). The anterior layer presented the smallest overlap between groups and KC stages. Conclusions: The densitometry map reveals that light backscatter was higher in the central portion of the anterior keratoconic cornea than in the normal cornea. The densitometry level is higher in more advanced stages.

Scheimpflug imaging for laser refractive surgery.

Purpose of review To review the principles and clinical applications of Scheimpflug corneal and anterior segment imaging with special relevance for laser refractive surgery. Recent findings Computerized Scheimpflug imaging has been used for corneal and anterior segment tomography (CASTm) in different commercially available instruments. Such approach computes the three-dimensional image of the cornea and anterior segment, enabling the characterization of elevation and curvature of the front and back surfaces of the cornea, pachymetric mapping, calculation of the total corneal refractive power and anterior segment biometry. CASTm represents a major evolution for corneal and anterior segment analysis, beyond front surface corneal topography and single point central corneal thickness measurements. This approach enhances the diagnostic abilities for screening ectasia risk as well as for planning, evaluating the results, managing complications of refractive procedures, and selecting intraocular lens power, type, and design. In addition, dynamic Scheimpflug imaging has been recently introduced for in-vivo corneal biomechanical measurements and has also been used for anterior segment imaging of femtocataract surgery. Summary Scheimpflug imaging has an important role for laser refractive surgery with different applications, which continuously improve due to advances in technology.

Scheimpflug-Based Tomography and Biomechanical Assessment in Pressure-Induced Stromal Keratopathy

PURPOSE To report the tomographic and biomechanical findings before and after treatment of a case of pressure-induced stromal keratopathy (PISK), which was misdiagnosed as diffuse lamellar keratitis (DLK). METHODS A case report of a referred patient with supposed diagnosis of DLK after LASIK in the right eye. Scheimpflug-based corneal tomography and biomechanical assessment were provided by the Pentacam HR and CorVis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany). RESULTS A layer of corneal opacity beneath the flap with a presumably fluid-filled interface area was observed on slit-lamp biomicroscopy. Scheimpflug image from Pentacam revealed a hyperreflective area underneath the flap interface. Goldmann applanation tonometry was 12 mm Hg, whereas CorVis intraocular pressure was 53.5 mm Hg with deformation amplitude of 0.42 mm. Two days after starting oral and topical ocular hypotensive therapy, CorVis intraocular pressure was 14 mm Hg and deformation amplitude was 1.02 mm. CONCLUSIONS Ocular hypertension in PISK was associated with lower deformation response, along with steepening and thickening of the cornea.

Comprehensive anterior segment normal values generated by rotating Scheimpflug tomography.

Purpose To identify normal values for tomographic parameters that are considered useful in screening patients for refractive surgery. Setting Private center, Albany, New York, USA. Design Database study. Methods A Pentacam HR Scheimpflug system was used to examine 1 randomly selected eye of patients to determine normal values of 21 parameters considered the most clinically applicable for surgical screening. Normality of data was evaluated using the Kolmogorov‐Smirnov test. Statistical analyses were performed using the Student t test to compare means and the 2‐paired sample Wilcoxon signed‐rank test. Results are displayed in 95.0% and 97.5% confidence intervals (CIs). Results The study evaluated 341 adults. High‐end outliers at the 97.5% CI were 46.1 diopters (D) for flat keratometry (K), 47.4 D for steep K, 3.4 D for astigmatism, 3.8 &mgr;m for anterior chamber depth, 4 &mgr;m for front apical elevation, 5 &mgr;m for front elevation at the thinnest point, and 12 &mgr;m for front elevation in the central 4.0 mm. Respective posterior elevation values were 7 &mgr;m, 13 &mgr;m, and 25 &mgr;m, with a progression index maximum of 1.53 and mean of 1.19, difference between apical and thinnest pachymetric reading of 7 &mgr;m, a maximum K of 48.2 D, and an inferior–superior ratio of 1.44 D. Low‐end outliers were a maximum Ambrósio relational thickness of 335 and a mean of 425, minimum pachymetry of 479 &mgr;m, thickness at the apex of 481 &mgr;m, and central 4.0 mm corneal volume of 6.31 mm3. Conclusion Scheimpflug‐derived corneal tomography identified key refractive surgery parameters that may be useful in screening refractive surgical patients. Financial Disclosure Drs. Ambrósio and Belin are consultants to Oculus Optikgeräte GmbH. No other author has a financial or proprietary interest in any material or method mentioned.

Variability of subjective classifications of corneal topography maps from LASIK candidates

PURPOSE To evaluate the variability of subjective corneal topography map classification between different experienced examiners and the impact of changing from an absolute to a normative scale on the classifications. METHODS Preoperative axial curvature maps using Scheimpflug imaging obtained with the Pentacam HR (Oculus Optikgeräte, Wetzlar, Germany) and clinical parameters were sent to 11 corneal topography specialists for subjective classification according to the Ectasia Risk Scoring System. The study population included two groups: 11 eyes that developed ectasia after LASIK and 14 eyes that had successful and stable LASIK outcomes. Each case was first reviewed using the absolute scale masked to the patient group. After 3 months, the same cases were represented using a normative scale and reviewed again by the same examiners for new classifications masked to the patient group. RESULTS Using the absolute scale, 17 of 25 (68%) cases had variations on the classifications from 0 to 4 for the same eye across examiners, and the overall agreement with the mode was 60%. Using the normative scale, the classifications from 11 of 25 (44%) cases varied from 0 to 4 for the same eye across examiners, and the overall agreement with the mode was 61%. Eight examiners (73%) reported statistically higher scores (P < .05) when using the normative scale. Considering all 550 topographic analyses (25 cases, 11 examiners, and two scales), the same classification from the two scales was reported for 121 case pairs (44%). CONCLUSION There was significant inter-observer variability in the subjective classifications using the same scale, and significant intra-observer variability between scales. Changing from an absolute to a normative scale increased the scores on the classifications by the same examiner, but significant inter-observer variability in the subjective interpretation of the maps still persisted.

Assessing ectasia susceptibility prior to LASIK: the role of age and residual stromal bed (RSB) in conjunction to Belin-Ambrósio deviation index (BAD-D)

Purpose: To compare the ability to detect preoperative ectasia risk among LASIK candidates using classic ERSS (Ectasia Risk Score System) and Pentacam Belin-Ambrosio deviation index (BAD-D), and to test the benefit of a combined approach including BAD-D and clinical data. Methods: A retrospective nonrandomized study involved preoperative LASIK data from 23 post-LASIK ectasia cases and 266 stable-LASIK (follow up > 12 months). Preoperative clinical and Pentacam (Oculus; Wetzlar, Germany) data were obtained from all cases. Mann-Whitneys test was performed to assess differences between groups. Stepwise logistic regression was used for combining parameters.The areas under the Receiver Operating Characteristic (ROC) curves (AUC) were calculated for all parameters and combinations, with pairwise comparisons of AUC (DeLongs method). Results: Statistically significant differences were found for age, residual stromal bed (RSB), central corneal thickness and BAD-D (p 0.05). ERSS was 3 or more on 12/23 eyes from the ectasia group (sensitivity = 52.17%) and 48/266 eyes from the stable LASIK group (18% false positive). BAD-D had AUC of 0.931 (95% CI: 0.895 to 0.957), with cut-off of 1.29 (sensitivity = 87%; specificity = 92.1%). Formula combining BAD-D, age and RSB provided 100% sensitivity and 94% specificity, with better AUC (0.989; 95% CI: 0.969 to 0.998) than all individual parameters (p>0.001). Conclusion: BAD-D is more accurate than ERSS. Combining clinical data and BAD-D improved ectasia susceptibility screening. Further validation is necessary. Novel combined functions using other topometric and tomographic parameters should be tested to further enhance accuracy.

Enhanced ectasia screening: the need for advanced and objective data.

To the Editor: We read with interest the report from Drs. Abdulmassih-Gonçalves and Gonçalves.1 The authors should be commended for publishing a case of ectasia after LASIK, which was detected and addressed by collagen cross-linking. We also appreciate their willingness in sharing the preoperative raw data from rotatingScheimpflug tomography, which generated interesting information that we address in this letter. The tomography examination was taken by the Oculyzer (Wavelight, Inc., Erlangen, Germany), which has the same hardware characteristics as the Oculus Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany). Although the Oculyzer software is designed for topography-guided ablations with the Wavelight excimer lasers, there are significant differences on diagnostic parameters when compared to Pentacam. Objective tomographic parameters, which go beyond curvature and central corneal thickness, are critical for clinical decisions when screening for ectasia risk after laser vision correction.2 Advanced analysis from the raw data generated by the Oculyzer allowed for the calculation of Ambrósio’s Relational Thickness3 and the deviation value from the Belin-Ambrósio Enhanced Ectasia Display (BAD-D), which demonstrates enhanced accuracy for detecting milder forms of ectasia, defined as the fellow eye with relatively normal curvature maps from patients with asymmetric keratoconus (considered as forme fruste keratoconus) (Figure 1).4 BAD-D in the right eye was higher than 1.22, the cut-off value for forme fruste keratoconus (93.62% sensitivity and 94.56% specificity).4 Ambrósio’s Relational Thickness average was lower than 521 μm in both eyes, the cut-off value for forme fruste keratoconus (91.94% sensitivity and 93.05 specificity).4 Ambrósio’s Relational Thickness maximum in the right eye was lower than 386 μm, the cut-off for keratoconus detection (99.17% sensitivity and 97.28% specificity).4 The patient was 21 years old at the time of the Oculyzer examination. Therefore, the Ectasia Risk Score System should be recalculated to 5 for the right eye and 4 for the left eye. Although the consideration of age may be an important factor for the lack of specificity of the Ectasia Risk Score System, there is compelling evidence that supports the younger age being related to lower biomechanical properties of the cornea.2 In a retrospective study involving preoperative clinical and Pentacam data from 23 cases that developed ectasia after LASIK and 266 stable LASIK cases (more than 12 months of follow-up), age was a strong predictor of ectasia risk, being combined in a stepwise logistic regression analysis with BAD-D and residual stromal bed to calculate the Ectasia Susceptibility Score-1. The Ectasia Susceptibility Score-1 represents the percentage of risk for developing ectasia, with a threshold of 6.45%. This allows for 100% sensitivity and 94% specificity, which is better than BAD-D, the best isolated parameter with a cut-off of 1.29 (87% sensitivity and 92.1% specificity).5 Considering the calculated residual stromal bed of 281 and 301 μm,1 the Ectasia Susceptibility Score-1 would be 96.75% and 79% for the right and left eye, respectively. Also, minimal re-

Impact of chamber pressure and material properties on the deformation response of corneal models measured by dynamic ultra-high-speed Scheimpflug imaging

PURPOSE To study the deformation response of three distinct contact lenses with known structures, which served as corneal models, under different chamber pressures using ultra-high-speed (UHS) Scheimpflug imaging. METHODS Three hydrophilic contact lenses were mounted on a sealed water chamber with precisely adjustable pressure: TAN-G5X (41% hydroxyethylmethacrylate/glycolmethacrylate, 550 µm thick), TAN-40 (62% hydroxyethylmethacrylate, 525 µm thick) and TAN-58 (42% methylmethacrylate, 258 µm thick). Each model was tested five times under different pressures (5, 15, 25, 35 and 45 mmHg), using ultra-high-speed Scheimpflug imaging during non-contact tonometry. 140 Scheimpflug images were taken with the UHS camera in each measurement. The deformation amplitude during non-contact tonometry was determined as the highest displacement of the apex at the highest concavity (HC) moment. RESULTS At each pressure level, the deformation amplitude was statistically different for each lens tested (p<0.001, ANOVA). Each lens had different deformation amplitudes under different pressure levels (p<0.001; Bonferroni post-hoc test). The thicker lens with less polymer (TAN-G5X) had a higher deformation (less stiff behavior) than the one that was thinner but with more polymer (TAN-40), when measured at the same internal pressure. The thinnest lens with less polymers (TAN-58) had a lower deformation amplitude (stiffer behavior) at higher pressures than the thicker ones with more polymer (TAN-40 and TAN-G5X) at lower pressures. CONCLUSIONS UHS Scheimpflug imaging allowed for biomechanical assessment through deformation characterization of corneal models. Biomechanical behavior was more influenced by material composition than by thickness. Chamber pressure had a significant impact on deformation response of each lens.

Detection of ectatic corneal diseases based on pentacam.

Pentacam is a rotating Scheimpflug-based corneal and anterior segment tomographer that gives as comprehensive analysis of corneal 3D geometry. With this device the detection of mild keratoconus or ectasia susceptibility is possible. This is fundamental for screening ectasia risk prior to laser vision correction. The identification of susceptible cases at risk for developing progressive iatrogenic ectasia should go beyond (but not over) corneal front surface topography.