Biomechanical customization: The next generation of laser refractive surgery

Analysis of corneal biomechanical properties 25 years after myopic photorefractive keratectomy

PURPOSE

To evaluate corneal biomechanical changes using Corvis ST in patients treated with photorefractive keratectomy (PRK) 25 years ago.

METHODS

In this study, 32 post-PRK and 38 normal eyes underwent Corvis ST (CST) assessments. The measured CST factors were: time of highest concavity (HC), time of applanation 1 (AT1), time of applanation 2 (AT2), length of applanation 1 (AL1), length of applanation 2 (AL2), velocity of applanation 1 (AV1), velocity of applanation 2 (AV2), deformation amplitude (DA), peak distance (PD), integrated radius (IR), Ambrosio relational thickness horizontal (ARTh), stiffness parameter at first applanation (SP-A1), DA ratio (2 mm), Belin/Ambrosio enhanced ectasia display (BAD) and corneal biomechanical index (CBI).

RESULTS

The mean [± standard deviation (SD)] age was 51.4 ± 7.36 years in PRK, 51.4 ± 3.62 in control group. PRK was performed 24.69 ± 1.78 years ago. ARTh, SP-A1, AT1, AL1, and AL2 were lower in PRK. PD, AT2, DA ratio (2 mm), and IR were statistically higher in PRK (P < 0.01). In PRK and control group the mean value of CBI was 0.91 ± 0.11 and 0.50 ± 0.27 (P < 0.001), and mean value of BAD was 3.34 ± 1.53 and 1.1 ± 0.70 (P < 0.001). In PRK 71.9% of eyes were classed "high risk CBI plus diseased BAD" and 25% remained in the "high risk CBI and normal BAD" group.

CONCLUSIONS

In this study, most of the post-PRK eyes which were clinically and topographically normal were classified as "high risk CBI plus diseased BAD" and had significantly worse CBI and BAD values than the control group. This leads to the conclusion that CBI and BAD alone are not appropriate to evaluate post-PRK ectasia.

Clinical Outcomes of Laser Asymmetric Keratectomy to Manage Postoperative Adverse Effects–A Retrospective Clinical Trial

Background:

Laser asymmetric keratectomy reduces the regional asymmetry of corneal thickness.

Objective:

We aimed to describe the clinical outcomes of laser asymmetric keratectomy keratectomy with laser refractive surgery performed to resolve the adverse effects following ophthalmic surgeries.

Methods:

We compared the preoperative and postoperative outcomes and complaints of blurring after performing laser asymmetric keratectomy with laser refractive surgery in 24 eyes of 16 patients with a deviation sum in corneal thickness in four directions >80 µm. Laser asymmetric keratectomy with laser refractive surgery, with full integration of the Vision Up software, was used to analyze the corneal thickness deviation, employed selective laser ablation to create central symmetry on the thicker cornea to reduce regional asymmetry of corneal thickness, simultaneously correcting the refractive power and myopic shift. The pre-and postoperative clinical and topographic findings were analyzed.

Results:

The patients’ age was 37.57±22.30 (range, 23–65) years. The follow-up period was 16.56±3.23 months. The spherical equivalent (p=0.026), sphere (p=0.022), uncorrected distance visual acuity (LogMAR, p=0.045), blurring score (p=0.000), central corneal thickness (p=0.024), sum of deviations in corneal thickness in four directions (p=0.02), distance between the maximum posterior elevation and visual axis (p=0.04), and kappa angle (p=0.031), significantly decreased postoperatively. The efficiency and safety indices were 0.96±0.11 and 1.00, respectively. There was no myopic regression or blurred vision postoperatively.

Conclusion:

Performing laser asymmetric keratectomy with laser refractive surgery improved corneal symmetry and visual acuity and reduced blurring.

A Novel Approach to Enhancement Linked Laser Asymmetric Keratectomy Using Semi-Cylindrical Ablation Pattern in Patients with Myopic Regression After Laser Refractive Surgery

Purpose

We aimed to introduce a new technique to reduce regional asymmetry of corneal thickness by assessing its effectiveness in four patients with myopic regression after laser refractive surgery (LRS).

Patients and Methods

Four patients (four eyes) with myopic regression after LRS were included in this study. A new technique of enhancement with laser epithelial keratomileusis-linked laser asymmetric keratectomy using semi-cylindrical ablation pattern (E-LAK-SCAP) with full integration of the Vision-Up software for analyzing the corneal thickness deviation can be used to create central symmetry by blocking laser ablation on the thin cornea. It reduces the regional asymmetry of the corneal thickness, thus improving corneal symmetry and correcting the refractive power and myopic shift due to E-LAK-SCAP. We measured refraction, visual acuity, intraocular pressure (IOP), central corneal thickness (CCT), corneal irregularities in the 3.0mm, and 5.0 zones on Orbscan maps, the sum of corneal thickness deviations in four directions (SUM), distance between the maximum posterior elevation (best-fit-sphere [BFS]) and the visual axis (DISTANCE), and angle kappa before and after LRS and E-LAK-SCAP. Blurring scores were measured before and after E-LAK-SCAP.

Results

The uncorrected far visual acuity (LogMAR) increased after LRS and E-LAK-SCAP. SUM (µm) increased after LRS in three cases, but decreased in all four cases after E-LAK-SCAP. DISTANCE increased after LRS, but decreased after E-LAK-SCAP. The spherical equivalent, CCT, decreased after LRS and E-LAK-SCAP. Blurring scores decreased after E-LAK-SCAP, and angle kappa was similar before and after LRS, but decreased after E-LAK-SCAP. IOP was similar before and after both LRS and E-LAK-SCAP.

Conclusion

E-LAK-SCAP improved corneal symmetry by reducing the SUM and DISTANCE, showing good postoperative visual acuity, and blurring was reduced postoperatively. There was no myopic regression in the one-year postoperative period.

Comparison of outcomes of laser refractive surgery (LRS) alone and LRS with laser asymmetric keratectomy in patients with myopia: A retrospective study

To compare and analyze the postoperative 1-year outcomes of laser refractive surgery (LRS) alone vs LRS with laser asymmetric keratectomy (LAK), in patients with myopia, for preventing and resolving LRS complications.This retrospective study compared the preoperative and 1-year postoperative outcomes between the control and comparison groups using a sum of deviations in corneal thickness in 4 directions >80 μm. The control group included 41 patients with myopia (41 eyes) who underwent LRS. The comparison group included 33 patients (33 eyes) who received LAK-linked LRS. Age, spherical equivalent (SE), sphere, cylinder, uncorrected distance visual acuity (UDVA), pupil size, kappa angle, central corneal thickness, corneal irregularity in the 3.0 mm zone on Orbscan maps (SUM), distance between the maximum posterior elevation (best-fit-sphere) and the visual axis (DISTANCE), postoperative blurring scores, frequency of postoperative myopic regression, and efficiency index were compared.Preoperative age (P = .198), SE (P = .686), sphere (P = .562), cylinder (P = .883), UDVA (P = .139), pupil size (P = .162), kappa angle (P = .807), central corneal thickness (P = .738), corneal irregularity (P = .826), SUM (P = .774), and DISTANCE (P = .716) were similar between the 2 groups. The 1-year postoperative SE (P = .024), sphere (P = .022), corneal irregularity (P = .033), SUM (P = .000), DISTANCE (P = .04), blurring scores (P = .000), and frequency of postoperative myopic regression (P = .004) were significantly decreased in the comparison group compared to the control group. UDVA (P = .014) and the efficiency index (P = .035) were higher in the comparison group.LAK with LRS improved corneal symmetry by reducing the SUM and DISTANCE. UDVA and efficiency index were also improved and blurring and myopic regression were reduced postoperatively.

In vivo human corneal natural frequency quantification using dynamic optical coherence elastography: Repeatability and reproducibility

Reliable and quantitative assessment of corneal biomechanics is important for the detection and treatment of corneal disease. The present study evaluates the repeatability and reproducibility of a novel optical coherence tomography (OCT)-based elastography (OCE) method for in vivo quantification of corneal natural frequency in 20 normal human eyes. Sub-micron corneal oscillations were induced by repeated low-force (13 Pa) microliter air pulses at the corneal apex and were observed by common-path phase-sensitive OCT imaging adjacent to a measurement region of 1-6.25 mm². Corneal natural frequencies were quantified using a single degree of freedom model based on the corneal oscillations. Corneal natural frequencies ranged from 234 to 277 Hz (coefficient of variation: 3.2%; n = 286 for a 2.5 × 2.5 mm² area; time: 28.6 s). The same natural frequencies can be acquired using a smaller sampling size (n = 9 for 1 mm²) and a shorter time (0.9 s). Spatial distribution and local changes in natural frequencies can be distinguished using denser sampling (e.g., 26 × 41 points for 2.5 × 5 mm²). This novel optical method demonstrates highly repeatable and reliable in vivo measurements of human corneal natural frequencies. While further studies are required to fully characterize anatomical and structural dependencies, this method may be complementary to the current OCE methods used to estimate Young's modulus from strain- or shear-wave-based measurements for the quantitative determination of corneal biomechanics.

Unintended changes in ocular biometric parameters during a 6-month follow-up period after FS-LASIK and SMILE

Background

Corneal refractive surgery has become reliable for correcting refractive errors, but it can induce unintended ocular changes that alter refractive outcomes. This study is to evaluate the unintended changes in ocular biometric parameters over a 6-month follow-up period after femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK) and small incision lenticule extraction (SMILE).

Methods

156 consecutive myopic patients scheduled for FS-LASIK and SMILE were included in this study. Central corneal thickness (CCT), mean curvature of the corneal posterior surface (K_pm), internal anterior chamber depth (IACD) and the length from corneal endothelium to retina (ER) were evaluated before and after surgery over a 6-month period.

Results

Both the FS-LASIK and SMILE groups (closely matched at the pre-surgery stage) experienced flatter K_pm, shallower IACD and decreased ER 1 week post-surgery (P < 0.01), and these changes were larger in FS-LASIK than in SMILE group. During the 1 week to 6 months follow up period, K_pm, IACD and ER remained stable unlike CCT which increased significantly (P < 0.05), more in the FS-LASIK group.

Conclusions

During the follow up, the posterior corneal surface became flatter and shifted posteriorly, the anterior chamber depth and the length from the corneal endothelium to retina decreased significantly compared with the pre-surgery stage. These unintended changes in ocular biometric parameters were greater in patients undergoing FS-LASIK than SMILE. The changes present clear challenges for IOL power calculations and should be considered to avoid affecting the outcome of cataract surgery.

Analysis and Causation of All Inaccurate Outcomes After WaveLight Contoura LASIK with LYRA Protocol

Purpose

This study analyzes every eye that had an outcome greater than 0.25D of sphere or astigmatism from planned goal after treatment with WaveLight Contoura with LYRA Protocol.

Methods

The study included 266 consecutive eyes treated with LASIK Contoura using the LYRA Protocol. All LASIK procedures were performed on the WaveLight EX500 excimer laser. Flaps were created with either the Alcon WaveLight FS200 femtosecond laser or the Moria M2 microkeratome. Eyes that were off by >0.25 diopters (D) sphere or cylinder from the targeted goal within 3 months after surgery were identified and analyzed for cause. Topographical, higher-order aberration, and epithelial maps were created.

Results

Causes for inaccurate outcomes were biomechanical corneal change from LASK flap creation (9.78% of total eyes), pre-operative epithelial compensation of corneal higher-order aberration (4.1% of total eyes), changes to lamellar corneal tension from laser ablation causing a hyperopic shift (1.9% of total eyes), epithelial thickening over the ablation area post-operatively causing a refractive change (1.5% of total eyes), and posterior astigmatism (0.75%).

Conclusion

The causes of the majority of inaccurate outcomes have not been properly defined and must be incorporated into further improving outcomes. Current and planned advances in technology do not address the majority of these causes.

Biomechanical Changes to the Cornea from LASIK Flap Creation Resulting in Inaccurate Ablations and Suboptimal Refractive Outcomes with Topographic-Guided Ablation

Purpose

This study documents a biomechanical corneal change related to corneal flap creation in certain patients leading to an irregular ablation pattern and an inaccurate refractive outcome.

Methods

This retrospective study included consecutive eyes treated with primary LASIK Contoura using the LYRA Protocol. All LASIK procedures were performed on the WaveLight EX500 excimer laser. Flaps were created with either the Alcon WaveLight FS200 femtosecond laser or the Moria M2 microkeratome. Eyes that were off by greater than or equal to 0.50 diopters (D) sphere or cylinder from the targeted goal within 3 months after surgery were identified. Topographical, higher order aberration, and epithelial maps were created. Of these eyes, approximately 10% of eyes were found to have undergone a biomechanical change upon flap creation that led to an inaccurate outcome.

Results

Six representative cases are presented that demonstrate the biomechanical change, outcomes, and treatment. All patients demonstrated an elliptical, irregular ablation pattern on post-operative topography, lateralized the thinnest point of the cornea relative to the corneal apex on Pentacam pachymetry maps, and irregular corneal epithelial thickening at the periphery of the elliptical ablation.

Conclusion

A biomechanical change during flap creation can occur in certain types of corneas during LASIK flap creation and subsequent treatment with topographic-guided ablation leading to an irregular ablation and suboptimal refractive outcomes.

Comparison between Surgical Outcomes of LASIK with and without Laser Asymmetric Keratectomy to Avoid Conventional Laser Refractive Surgery Adverse Effects

This study compared one-year postoperative outcomes of laser refractive surgery combined with laser asymmetric keratectomy (LAK) and laser in situ keratomileusis (LASIK)for myopia correction in middle-aged patients (aged 40–49 years) with a total corneal thickness deviation (summed across four directions) ≥ 80 microns. The control group (n = 26; 52 eyes) underwent LASIK; the comparison group (n = 26; 52 eyes) underwent combined laser refractive surgery and LAK. Age, spherical equivalence, uncorrected visual acuity (near and far), corneal irregularity on the Orbscan map, sum of corneal thickness deviations in four directions, corneal thickness distribution, distance between the maximum posterior elevation (best-fit sphere; BFS) and visual axis, and postoperative blurring scores were analysed retrospectively between the groups. Both groups had similar preoperative findings. Postoperatively, the sum of corneal thickness deviations in four directions (p = 0.000), distance between maximum posterior elevation (BFS) and visual axis (p = 0.003),blurring score (p = 0.001), and corneal irregularity in the 3.0 and 5.0 mm zones on the Orbscan map (p = 0.033 and p < 0.0001, respectively) were significantly lower in the comparison group (p = 0.000). LAK reduced total corneal thickness deviation, improved corneal symmetry, and reduced blurring scores significantly, one-year postoperatively. LAK could resolve shortcomings of LASIK, producing better surgical outcomes.

Regional changes in corneal shape over a 6-month follow-up after femtosecond-assisted LASIK

PURPOSE

To assess the regional changes in corneal shape after femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK) in patients with different myopia extents.

SETTING

Eye Hospital, Wenzhou Medical University, Wenzhou, China.

DESIGN

Retrospective case series.

METHODS

A retrospective study of myopic eyes treated with FS-LASIK was conducted to assess the shape changes within different corneal regions after surgery. Corneal curvature was measured in the central region (0 mm to 3.0 mm diameter), pericentral region (3.0 mm to 6.0 mm diameter) and peripheral region (6.0 mm to 9.0 mm diameter) preoperatively and from 1 week to 6 months postoperatively.

RESULTS

The study comprised 608 myopic eyes. During the 6-month follow-up, the anterior cornea became steeper in the central and pericentral regions, but flatter in the peripheral region (P < .01), representing a partial, gradual, yet significant reversal of the immediate change in corneal shape after laser ablation. In contrast, the posterior surface experienced significantly less change than the anterior surface, with the cornea becoming slightly flatter (P < .01) in the central region at 1 week postoperatively, and steeper elsewhere (P < .05), and then remaining stable during the rest of the follow-up. On the other hand, the anterior astigmatism had significant decreases in the central region (P < .01) and slight increases in the peripheral region (P < .01) 1 week postoperatively, and that remained stable over the follow-up period. In contrast, there were little or non-significant changes in the posterior astigmatism throughout the follow-up (P > .05).

CONCLUSIONS

Postoperative corneal shape changes were different in different regions. There were shape changes in individual corneal regions during the 6-month follow-up period that represented reverse trends; however, the changes were much smaller than the short-term changes observed 1 week after surgery.

Development and clinical verification of numerical simulation for laser in situ keratomileusis

To develop and validate numerical models of the laser in situ keratomileusis (LASIK) procedure through considering its effect on corneal biomechanical behavior. 3D finite element models of the human eye were developed to simulate LASIK. The models' predictions of post-operative corneal elevation, corneal refractive power with vector decomposition (M_-c-pos, J_0-c-pos, J_45-c-pos) and refractive error correction (M_-rec, J_0-rec, J_45-rec) were compared against clinical data obtained for 28 eyes of 28 patients. A parallel exercise was conducted to estimate the post-operative corneal shape using a shape subtraction method (SSM) - which does not consider the effects of LASIK on corneal mechanical behavior - and the results are compared with the finite element method (FEM). A significant decrease in elevation differences between FEM predictions and clinical data was found compared with the differences between SSM results and clinical data (p = 0.000). In addition, there were no significant differences in post-operative equivalent sperical corneal refractive power between FEM results and corresponding clinical data (M_-c-pos: p = 0.501), while SSM showed significant differences with clinical data (M_-c-pos: p = 0.000). Further, FEM achieved a predicted value of M_-c-pos within ± 1.00D accuracy in 100% of cases, compared with 57% achieved by the SSM. M_-rec predicted by FEM was not significantly different from clinical results (p = 0.085), while SSM overestimated it (p = 0.000). The match between LASIK numerical model predictions with clinical measurements improved significantly when the procedure's effect on corneal biomechanical behavior was considered. This outcome has important implications on efforts to develop planning tools for refractive surgery.

Corneal Biomechanical Properties after FS-LASIK with Residual Bed Thickness Less Than 50% of the Original Corneal Thickness

Background

The changes in corneal biomechanical properties after LASIK remain an unknown but important topic for surgical design and prognostic evaluation. This study aims to observe the postoperative corneal biomechanical properties one month after LASIK with amount of corneal cutting (ACC) greater than 50% of the central corneal thickness (CCT).

Methods

FS-LASIK was performed in 10 left rabbit eyes with ACC being 60% (L60) and 65% (L65) of the CCT, while the right eyes (R) were the control. After 4 weeks, rabbits were executed and corneal strip samples were prepared for uniaxial tensile tests.

Results

At the same strain, the stresses of L65 and L60 were larger than those of R. The elastic moduli of L60 and L65 were larger than those of R when the stress was 0.02 MPa, while they began to be less than those of R when stress exceeds the low-stress region. After 10 s relaxation, the stress of specimens L65, L60, and R increased in turn.

Conclusion

The elastic moduli of the cornea after FS-LASIK with ACC greater than 50% of the CCT do not become less under normal rabbit IOP. The limit stress grows with the rise of ACC when relaxation becomes stable.

Influence of Pachymetry and Intraocular Pressure on Dynamic Corneal Response Parameters in Healthy Patients

PURPOSE

To evaluate the influence of pachymetry, age, and intraocular pressure in normal patients and to provide normative values for all dynamic corneal response parameters (DCRs) provided by dynamic Scheimpflug analysis.

METHODS

Seven hundred five healthy patients were included in this multicenter retrospective study. The biomechanical response data were analyzed to obtain normative values with their dependence on corrected and clinically validated intraocular pressure estimates developed using the finite element method (bIOP), central corneal thickness (CCT), and age, and to evaluate the influence of bIOP, CCT, and age.

RESULTS

The results showed that all DCRs were correlated with bIOP except deflection amplitude (DefA) ratio, highest concavity (HC) radius, and inverse concave radius. The analysis of the relationship of DCRs with CCT indicated that HC radius, inverse concave radius, deformation amplitude (DA) ratio, and DefA ratio were correlated with CCT (rho values of 0.343, -0.407, -0.444, and -0.406, respectively). The age group subanalysis revealed that primarily whole eye movement followed by DA ratio and inverse concave radius were the parameters that were most influenced by age. Finally, custom software was created to compare normative values to imported examinations.

CONCLUSIONS

HC radius, inverse concave radius, DA ratio, and DefA ratio were shown to be suitable parameters to evaluate in vivo corneal biomechanics due to their independence from IOP and their correlation with pachymetry and age. The creation of normative values allows the interpretation of an abnormal examination without the need to match every case with another normal patient matched for CCT and IOP. [J Refract Surg. 2016;32(8):550-561.].

High intercorneal symmetry in corneal biomechanical metrics

Backgroud

To evaluate the symmetry of corneal biomechanical metrics, measured using an ocular response analyzer (ORA) and self-built corneal inflation test platform, in bilateral rabbit corneas and to investigate their relationship with physical intraocular pressure (IOPp).

Methods

Twenty fresh enucleated eyes from ten rabbits were used for ex vivo whole ocular globe inflation. IOP was increased from 7.5 to 37.5 mmHg with 7.5 mmHg steps and biomechanical metrics were acquired using the ORA. At least 3 examinations were performed at each pressure stage. Two biomechanical metrics, corneal hysteresis (CH) and corneal resistance factor (CRF) were recorded and analyzed as a function of IOPp. Corneal specimens were then excised from the intact ocular globe and tested under inflation conditions up to 45.7 mmHg posterior pressure. The experimental pressure-deformation data was analyzed using an inverse modeling procedure to derive the stress-strain behavior of the cornea.

Results

A comparison of corneal shape parameters showed no statistically significant difference (P > 0.05) between bilateral eyes. Similarly, there were no statistically significant differences in values of CH, CRF and corneal stiffness (as measured by the tangent modulus, Et) between bilateral eyes (CH: F = 0.94, P = 0.54; CRF: F = 4.42, P = 0.35; Et: F = 3.15, P = 0.12) at different pressure levels. IOPp was highly correlated with CRF while the relationship with CH was less pronounced.

Conclusions

An obvious interocular symmetry in biomechanical metrics is found in this research. IOP has been shown to have important influences on the value of CRF provided by ORA.

Dynamic curvature topography for evaluating the anterior corneal surface change with Corvis ST

Background

The measurement of dynamic parameters, such as the length of applanation and the amplitude of deformation, is significant for evaluating corneal properties. Most of the corneal properties (related to shape) including the anterior corneal curvature and the thickness of cornea can be easily measured using some existing techniques. However, they only provide the static or pseudo-dynamic analysis. Based on Corvis ST images, the dynamic features after corneal boundaries detection and parameter estimation will be helpful for corneal analysis.

Material

The study included 40 eyes in normal group (ranging from 19 to 45 years old) and 30 eyes in keratoconus group (ranging from 16 to 40 years old). These eyes were examined by Corvis ST and for each one a sequence of 140 images was obtained. Besides, 11 subjects of each group were also tested by Pentacam.

Methods

By analyzing the video from the Corvis ST imaging, the fully dynamic curvature topography is proposed to evaluate the response of the anterior corneal surface to the air puff. The new method not only quantitatively measures the intact variation of anterior corneal surface but also provides an intuitive way to observe the dynamic change of the anterior corneal surface in the whole air stream process. The proposed method consists of three main steps: cornea segmentation, curvature estimation and integrated visualization. An automatic segmentation method based on the combination of prior knowledge with phase symmetry and asymmetry theory is firstly presented to detect the corneal boundaries. The Landau-new method is then used to estimate the anterior corneal surface. The corneal dynamic topography is finally obtained by combining the dynamic parameters with the original Corvis ST video, which is an improvement of the fusion technique proposed by Li et al.

Results and conclusion

By comparing the segmentation results with manual method and built-in method of Corvis ST, the accuracy and robustness of our proposed segmentation method is demonstrated. The correctness of the estimated corneal anterior curvatures is also evaluated by comparing it with that of Pentacam which is considered to be able to provide the first-class measurement currently. The dynamic topography may be used to distinguish the dynamic behavior of normal corneas from that of keratoconus.

Electronic supplementary material

The online version of this article (doi:10.1186/s12938-015-0036-2) contains supplementary material, which is available to authorized users.

Evaluation of the relationship of corneal biomechanical metrics with physical intraocular pressure and central corneal thickness in ex vivo rabbit eye globes

The relationship of corneal biomechanical metrics provided by the Ocular Response Analyzer (ORA) and Corvis ST (CVS) with physical intraocular pressure (IOPp) and central corneal thickness (CCT) was evaluated. Thirty fresh enucleated eyes of 30 rabbits were used in ex vivo whole globe inflation experiments. IOPp was measured with a pressure transducer and increased from 7.5 to 37.5 mmHg in steps of 7.5 mmHg while biomechanical data was acquired using the ORA and CVS. At least 3 examinations were performed at each pressure level, where CCT and twelve biomechanical metrics were recorded and analyzed as a function of IOPp. The biomechanical metrics included corneal hysteresis (CH) and corneal resistance factor (CRF), obtained by the ORA. They also included the applanation times (A1T, A2T), lengths (A1L, A2L) and velocities (A1V, A2V), in addition to the highest concavity time (HCT), peak distance (PD), radius (HR) and deformation amplitude (DA), obtained by the CVS. The variation of CCT and the twelve biomechanical metrics for the 30 rabbit eyes tested across the 5 pressure stages considered (inter-pressure differences) were statistically significant (P = 0.00). IOPp was highly to moderately correlated with most biomechanical metrics, especially CRF, A1T, A1V, A2V, PD and DA, while the relationships with CH, A2T, A1L and HCT were poor. IOP has important influences on most corneal biomechanical metrics provided by CVS and ORA. Two biomechanical metrics A1V and HR were influenced by CCT after correcting for the effect of IOP in most pressure stages, while the correlation with others were weak. Comparisons of research groups based on ORA and CVS with different IOPs and CCTs may lead to possible misinterpretations if both or one of which are not considered in the analysis.

Reliability of corneal dynamic scheimpflug analyser measurements in virgin and post-PRK eyes

Purpose

To determine the measurement reliability of CorVis ST, a dynamic Scheimpflug analyser, in virgin and post-photorefractive keratectomy (PRK) eyes and compare the results between these two groups.

Methods

Forty virgin eyes and 42 post-PRK eyes underwent CorVis ST measurements performed by two technicians. Repeatability was evaluated by comparing three consecutive measurements by technician A. Reproducibility was determined by comparing the first measurement by technician A with one performed by technician B. Intraobserver and interobserver intraclass correlation coefficients (ICCs) were calculated. Univariate analysis of covariance (ANCOVA) was used to compare measured parameters between virgin and post-PRK eyes.

Results

The intraocular pressure (IOP), central corneal thickness (CCT) and 1_st applanation time demonstrated good intraobserver repeatability and interobserver reproducibility (ICC≧0.90) in virgin and post-PRK eyes. The deformation amplitude showed a good or close to good repeatability and reproducibility in both groups (ICC≧0.88). The CCT correlated positively with 1_st applanation time (r = 0.437 and 0.483, respectively, p<0.05) and negatively with deformation amplitude (r = −0.384 and −0.375, respectively, p<0.05) in both groups. Compared to post-PRK eyes, virgin eyes showed longer 1_st applanation time (7.29±0.21 vs. 6.96±0.17 ms, p<0.05) and lower deformation amplitude (1.06±0.07 vs. 1.17±0.08 mm, p<0.05).

Conclusions

CorVis ST demonstrated reliable measurements for CCT, IOP, and 1_st applanation time, as well as relatively reliable measurement for deformation amplitude in both virgin and post-PRK eyes. There were differences in 1_st applanation time and deformation amplitude between virgin and post-PRK eyes, which may reflect corneal biomechanical changes occurring after the surgery in the latter.

Theoretical basis, laboratory evidence, and clinical research of chemical surgery of the cornea: cross-linking

Corneal cross-linking (CXL) is increasingly performed in ophthalmology with high success rates for progressive keratoconus and other types of ectasia. Despite being an established procedure, some molecular and clinical aspects still require additional studies. This review presents a critical analysis of some established topics and others that are still controversial. In addition, this review examines new technologies and techniques (transepithelial and ultrafast CXL), uses of corneal CXL including natural products and biomolecules as CXL promoters, and evidence for in vitro and in vivo indirect effectiveness.

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.

Analytical optimization of the ablation efficiency at normal and non-normal incidence for generic super Gaussian beam profiles

We suggest a general method to determine the optimum laser parameters for maximizing the ablation efficiency for different materials (in particular human cornea) at different incidence angles. The model is comprehensive and incorporates laser beam characteristics and ablative spot properties. The model further provides a method to convert energy fluctuations during ablation to equivalent ablation deviations in the cornea. The proposed model can be used for calibration, verification and validation purposes of laser systems used for ablation processes at relatively low cost and would directly improve the quality of results.

Impact of ablation efficiency reduction on post-surgery corneal asphericity: simulation of the laser refractive surgery with a flying spot laser beam

We developed a rigorous simulation model to evaluate ablation algorithms and surgery outcomes in laser refractive surgery. The model (CASIM: Corneal Ablation SIMulator) simulates an entire surgical process, which includes calculating an ablation profile from measured wavefront errors, generating a shot pattern for a flying spot laser beam, simulation of the shot-by-shot ablation process based on a measured or modeled beam profile, and healing of the cornea after surgery. Using simulated post-surgery corneal shapes for various ablation parameters and beam fluences, we calculated angular dependence of ablation efficiency and the amount of increase in corneal asphericity. Without considering the effect of corneal healing, our result shows the following; 1) ablation efficiency reduction in the periphery depends on the peak fluence of the laser beam, 2) corneal asphericity increases even in the surgery using an ablation profile based on the exact Munnerlyn formula, contrary to previous reports, and 3) post-surgery corneal asphericity increases by a smaller amount in high fluence small Gaussian beam surgery than in low fluence truncated Gaussian beam. Our model can provide improved ablation profiles that compensate for the change of corneal asphericity and induction of spherical aberration in a flying spot laser system, resulting in better surgery outcomes in laser refractive surgeries.

Airbag induced corneal ectasia

PURPOSE

To report a case of airbag induced corneal ectasia.

METHODS

Case report.

RESULTS

A patient 3 years post-LASIK developed bilateral corneal ectasia worse in the right eye following airbag deployment in a road traffic accident. At last follow up, best corrected vision was 20/40 with -4.00/-4.00 x 25 in the right eye and 20/25 with -1.25/-0.50 x 135 in the left eye.

CONCLUSIONS

This is a rare presentation of trauma induced ectasia in a patient post-LASIK. It is possible that reduction in biomechanical integrity of the cornea from prior refractive surgery contributed to this presentation.

A Longitudinal Study of LASIK Flap and Stromal Thickness with High-speed Optical Coherence Tomography

OBJECTIVE

To assess corneal anatomic changes after LASIK with a high-speed corneal and anterior segment optical coherence tomography (CAS-OCT) system.

DESIGN

Cross-sectional observational study.

PARTICIPANTS

Fifty-one eyes of 26 healthy persons undergoing LASIK.

METHODS

The CAS-OCT prototype operated at a 1.3-mum wavelength and 2000 axial scans/second. The corneas were scanned with a flap profile pattern (horizontal line, 512 axial scans) and a flap map pattern (4 radials, 256 axial scans each). Both patterns are 8 mm long and are centered on the corneal vertex. LASIK flaps were created using either a mechanical microkeratome (Hansatome; Bausch & Lomb, Inc., Rochester, NY) or a femtosecond laser (Pulsion; IntraLase Corp., Irvine, CA). Intraoperative pachymetry was performed using a 50-MHz ultrasound probe. Three OCT scans were obtained on preoperative and post-LASIK visits up to 6 months. An automated algorithm was developed to process the OCT images and to calculate corneal, flap, and stromal bed thickness profiles and maps. The profiles and maps were divided into central (diameter, <2 mm), pericentral (2-5 mm), and transitional (5-7 mm) zones for analysis.

MAIN OUTCOME MEASURES

Corneal, flap, and stromal bed thicknesses as determined by OCT and ultrasound pachymetry.

RESULTS

The flap interface was best detected in the pericentral zone. One week after surgery, the repeatability of OCT flap and stromal bed thickness measurement was 2 to 7 microm by pooled standard deviation for zones inside a 5-mm diameter. The central flap thickness in 24 Hansatome eyes with a 180-microm setting was 143+/-14 microm by OCT and 131+/-17 microm by ultrasound. In the 8 IntraLase cases with a 120-microm setting, it was 156+/-11 microm by OCT and 160+/-19 microm by ultrasound. Eyes with other settings also were analyzed. There were small systematic changes in flap thickness up to 1 week and bed thickness up to 3 months.

CONCLUSIONS

We have developed a method for using high-speed OCT to measure LASIK flap thickness after surgery. The measurement is noncontact, rapid, and repeatable. Profile and map measurements provide more information than point measurements previously demonstrated. This could be valuable for planning LASIK enhancement and characterizing microkeratome performance.

Online optical coherence pachymetry to evaluate intraoperative ablation parameters in LASIK

PURPOSE

The ablation of corneal tissue with the excimer laser can be variable and can lead to miscorrections. The purpose of this study was to evaluate intraoperative ablation parameters during laser-assisted in-situ keratomileusis (LASIK) with online optical coherence pachymetry (OCP).

METHODS

In a prospective, nonrandomized, comparative clinical study, the ablation parameters were continuously assessed intraoperatively with online OCP (Heidelberg Engineering, Lübeck, Germany) in 45 myopic and 10 hyperopic LASIK treatments. The central intraoperative ablation values were compared with the calculated values of the excimer laser (ESIRIS, Schwind, Germany) and the postoperative refraction. The ablation process and the ablation rate in mum per layer, time, and dioptric correction were evaluated in myopic corrections.

RESULTS

In myopic LASIK treatments, a linear ablation process was measured with a mean correlation coefficient of -0.968 +/- 0.04. The intraoperative ablation rate was, on average, 0.59 +/- 0.17 microm per layer, 1.45 +/- 0.48 microm per second, and 24.63 +/- 7.81 microm per corrected diopter. These values were 28.7% to 29.6% higher (P < 0.001) than the calculated values. There was a significant correlation (P < 0.001) for the ablation rate per layer (r = 0.823), per second (r = 0.869), and corrected diopter (r = 0.892), but no correlation (r = 0.21, P = 0.239) between the measured linear ablation process and the postoperative refraction. During hyperopic LASIK treatments, without ablation of the corneal center, there was a significant decrease (P = 0.005) of the stromal thickness by 18.34 +/- 14.13 microm, which corresponded to a mean corneal dehydration rate of 0.27 microm per second.

CONCLUSIONS

Online OCP allowed a clinical evaluation of intraoperative ablation parameters in LASIK. Further studies are needed to assess a possible active control of the excimer laser ablation from these continuous values, which could possibly improve current ablation nomograms.

Binocular Visual Performance After LASIK

PURPOSE

To analyze binocular visual function after LASIK.

METHODS

Eye aberrometry and corneal topography was obtained for both eyes in 68 patients (136 eyes). To evaluate visual performance, monocular and binocular contrast sensitivity function and disturbance index for quantifying halos were measured. Tests were performed under mesopic conditions.

RESULTS

Binocular summation and disturbance index diminished significantly (P<.0001) after LASIK with increasing interocular differences in corneal and eye aberrations. Binocular visual deterioration was greater than monocular deterioration for contrast sensitivity function and disturbance index.

CONCLUSIONS

Binocular function deteriorates more than monocular function after LASIK. This deterioration increases as the interocular differences in aberrations and corneal shape increase. Improvements in ablation algorithms should minimize these interocular differences.

Technological Improvements With the Alcon LADAR 6000 Excimer Laser Platform

PURPOSE

To outline the technological improvements seen in the new Alcon LADAR 6000 laser.

METHODS

New LADAR 6000 features can be categorized into three areas: automation, speed, and control.

RESULTS

Automation features include auto-registration, specialized illumination, scleral vessel registration, and automated prompting. Speed features include a faster ablation rate and less intensive maintenance. Control features include autoregistration, a better graphic user interface, and an improved workspace.

CONCLUSIONS

The technological improvements of the LADAR 6000 help eliminate the variability of measurement, alignment, and delivery needed to optimize custom laser vision correction outcomes.