Persistent Haze and Disorganization of Anterior Stromal Collagen Appear Unrelated Following Phototherapeutic Keratectomy

Structural control of corneal transparency, refractive power and dynamics

The cornea needs to be transparent to visible light and precisely curved to provide the correct refractive power. Both properties are governed by its structure. Corneal transparency arises from constructive interference of visible light due to the relatively ordered arrangement of collagen fibrils in the corneal stroma. The arrangement is controlled by the negatively charged proteoglycans surrounding the fibrils. Small changes in fibril organisation can be tolerated but larger changes cause light scattering. Corneal keratocytes do not scatter light because their refractive index matches that of the surrounding matrix. When activated, however, they become fibroblasts that have a lower refractive index. Modelling shows that this change in refractive index significantly increases light scatter. At the microscopic level, the corneal stroma has a lamellar structure, the parallel collagen fibrils within each lamella making a large angle with those of adjacent lamellae. X-ray scattering has shown that the lamellae have preferred orientations in the human cornea: inferior-superior and nasal-temporal in the central cornea and circumferential at the limbus. The directions at the centre of the cornea may help withstand the pull of the extraocular muscles whereas the pseudo-circular arrangement at the limbus supports the change in curvature between the cornea and sclera. Elastic fibres are also present; in the limbus they contain fibrillin microfibrils surrounding an elastin core, whereas at the centre of the cornea, they exist as thin bundles of fibrillin-rich microfibrils. We present a model based on the structure described above that may explain how the cornea withstands repeated pressure changes due to the ocular pulse.

Birefringence analysis of collagen supraorganization in cat corneas with tropical keratopathy

OBJECTIVE

To evaluate the birefringent properties of the cornea and examine the supraorganizational aspects of collagen fibers in cats with tropical keratopathy.

PROCEDURE

In this study, 10-micrometer-thick sections of corneal tissue from cats with tropical keratopathy were examined, both in the opaque and transparent areas of the anterior stroma. Control samples were obtained from healthy cat corneas. Polarized light microscopy was employed to evaluate the birefringent properties using two distinct methods. The first method involved measuring the optical retardation associated with corneal birefringence, while the second method assessed the alignment/waviness of the birefringent collagen fibers. Differences were significant when p < .05.

RESULTS

Tropical keratopathy resulted in a significant rise (p < .05) in optical retardation in both opaque and transparent regions of the cat cornea. In the anterior stroma, both the opaque zones and transparent tissue exhibited a higher degree of collagen fiber packing than the control corneas. However, no significant differences (p > .05) in alignment were observed between the transparent tissue of the diseased cornea and the healthy corneas.

CONCLUSION

Supraorganizational changes in collagen fiber packing are not restricted to lesion zones in cat corneas affected by tropical keratopathy. Such alterations also occur in the corneal tissue of the anterior stroma adjoining the lesions. Therefore, it is plausible that the transparent tissue of the anterior stroma in corneas affected by the disease may have functional abnormalities, despite its macroscopic healthy appearance. Additional investigations are required to clarify the implications of these potential defects and their conceivable contribution to tropical keratopathy.

Birefringence Analyses Reveal Differences in Supramolecular Characteristics of Corneal Stromal Collagen Fibrils Between Falconiformes and Strigiformes

This study aimed to assess the birefringent properties of corneal stromal collagen fibrils in birds of the orders Falconiformes (diurnal) and Strigiformes (predominantly nocturnal) to compare their supramolecular organizations. Twenty-two corneas of Falconiformes (Caracara plancus, n = 8; Rupornis magnirostris, n = 10; and Falco sparverius, n = 4) and 28 of Strigiformes (Tyto furcata, n = 16; Pseudoscops clamator, n = 6; and Athene cunicularia, n = 6) were processed histotechnically into 8 μm thick sections. Corneal optical retardation values related to the form and intrinsic fractions of the total birefringence of collagen fibrils were measured using a polarized light microscope equipped with phase compensators. In addition, the coherence coefficients that inform the local orientation of the fibrils were calculated through video image analysis. All assessments were conducted both in the anterior and posterior stroma of the cornea. Differences were significant when p < 0.05. The results showed supraorganizational differences between fibrils in the anterior stroma of Falconiformes and Strigiformes. The optical retardation values were greater (p < 0.0001) for Falconiformes, indicating that the corneas of these birds contain more collagen fibrils or more aggregated collagen fibrils. In contrast, the coherence coefficients were higher (p = 0.016) for Strigiformes, indicating that the collagen fibers in these birds are highly aligned and have few undulations. A multivariate data matrix constructed for Euclidean distance calculations showed that the dissimilarity between Falconiformes and Strigiformes corneas, in terms of the supraorganization of stromal collagen fibrils, was 4.56%. In conclusion, it is possible that the supraorganizational differences reported in this study may be sources of variation in the visual quality of Falconiformes and Strigiformes. This study provides the necessary evidence to encourage further research associating corneal optical performance to supramolecular characteristics of corneal stromal collagen.

Natural history of corneal haze after corneal collagen crosslinking in keratoconus using Scheimpflug analysis

PURPOSE

To analyze corneal haze after corneal collagen crosslinking (CXL) for progressive keratoconus using Scheimpflug densitometry.

SETTING

Auckland District Health Board, Auckland, New Zealand.

DESIGN

Prospective randomized controlled study.

METHODS

Both eyes of all patients were examined preoperatively and 1, 3, 6, and 12 months postoperatively. One eye of each patient was treated with corneal CXL, with the contralateral eye serving as the control. Examinations included uncorrected distance visual acuity and corrected distance visual acuity (CDVA), slitlamp biomicroscopy, and Scheimpflug tomography (Pentacam).

RESULTS

Thirty-six eyes of 36 patients were enrolled. The mean preoperative corneal densitometry was 19.0 ± 3.2 (SD). Postoperatively, the mean densitometry peaked at 1 month, decreasing to baseline level after 6 months without application of topical corticosteroids. The development of haze was significant compared with the contralateral untreated eyes (P < .01). The anterior (120 μm) and inner (central 0.0 to 2.0 mm) zones of the cornea had the highest densitometry after treatment. Lower preoperative central corneal thickness (CCT) was significantly correlated with higher corneal densitometry (P = .03). However, the preoperative CCT, keratometry, and astigmatism did not influence the difference in densitometry between the treated eyes and untreated eyes. There was no evidence of a relationship between densitometry and CDVA (P = .30).

CONCLUSIONS

After corneal CXL, patients with keratoconus developed transient corneal haze in the anterior central cornea that might not require specific treatment. Thin corneas were associated with higher densitometry; thus, there might be a greater expectancy of corneal haze in patients with advanced keratoconus.

FINANCIAL DISCLOSURE

None of the authors has a financial or proprietary interest in any material or method mentioned.

Clinical Study of Mitomycin C in Reducing Haze Formation After Ultraviolet A/Riboflavin Crosslinking for Keratoconus

OBJECTIVES

To evaluate the safety and efficacy of mitomycin C (MMC) in haze formation after ultraviolet A/riboflavin corneal crosslinking (CXL) for progressive keratoconus.

METHODS

A total of 60 keratoconic eyes of 48 patients were enrolled in this prospective comparative study after obtaining informed consent. In the CXL group, standard corneal CXL was performed, whereas in the CXL+MMC group, 0.02% MMC was used for 30 s soon after CXL. Comprehensive ophthalmologic examinations were performed on all patients before surgery and at 1, 3, 6, and 12 months after surgery.

RESULTS

The epithelium recovered within 3 to 4 days after CXL, and the healing time was comparable in the two groups. There was no significant endothelial cell density loss after CXL in both groups. Eyes in both groups showed improvement of uncorrected distance visual acuity (Snellen) and best-corrected visual acuity (Snellen; P<0.05), and there was a decrease in K-max, cylinder degree, and central corneal thickness (CCT) (P<0.05). There was no significant statistical difference between the groups regarding postoperative K-max reduction, refraction, and CCT (P>0.05). Corneal haze scores were significantly higher in the CXL group at 1 and 3 months after CXL (P=0.012 and P=0.028, respectively), but were similar to the MMC group at 6 and 12 months after surgery (P=0.329 and P=0.543, respectively).

CONCLUSIONS

Prophylactic intraoperative use of 0.02% MMC can significantly reduce CXL-associated haze formation, especially in the early postoperative period, and no signs of weakening CXL efficacy were observed.

One-Year Follow-Up of Changes in Corneal Densitometry After Accelerated (45 mW/cm2) Transepithelial Corneal Collagen Cross-Linking for Keratoconus: A Retrospective Study

PURPOSE

The objective of this study is to investigate changes in corneal densitometry after accelerated transepithelial corneal collagen cross-linking (ATE-CXL) for patients with progressive keratoconus (KC).

METHODS

Seventeen progressive KC patients who underwent ATE-CXL (KC group) were examined and compared against 17 non-KC myopes (control group). For the KC group, corneal topography and densitometry were evaluated preoperatively and at 1, 6, and 12 months postoperatively. Manifest refraction spherical equivalent and best spectacle-corrected distant visual acuity were assessed preoperatively and at 12 months postoperatively. These parameters were also evaluated in the control group.

RESULTS

Preoperatively, in the KC group, the densitometry values of the total layer over the annular diameters (Φ) 0 to 2 and Φ 2 to 6 mm were 18.47 ± 1.81 and 16.62 ± 1.60, respectively. In the control group, the values were 14.98 ± 1.18 and 13.39 ± 1.33, respectively, significantly lower than those of the KC group (both post hoc P values < 0.001). At postoperative month 12, the densitometry values of Φ 0 to 2 and Φ 2 to 6 mm of the total layer in the KC group were 16.88 ± 1.57 and 15.28 ± 1.40, which were significantly lower than the preoperative values (post hoc P = 0.012 and 0.030, respectively). However, they were still higher than those of the myopes (post hoc P = 0.002 and 0.001, respectively).

CONCLUSIONS

KC patients have much higher corneal densitometry values than myopes without KC. The KC patients' corneal densitometry values decreased significantly when measured at 12 months after ATE-CXL. However, they remain higher than those of the myopes.

Corneal structure and transparency

The corneal stroma plays several pivotal roles within the eye. Optically, it is the main refracting lens and thus has to combine almost perfect transmission of visible light with precise shape, in order to focus incoming light. Furthermore, mechanically it has to be extremely tough to protect the inner contents of the eye. These functions are governed by its structure at all hierarchical levels. The basic principles of corneal structure and transparency have been known for some time, but in recent years X-ray scattering and other methods have revealed that the details of this structure are far more complex than previously thought and that the intricacy of the arrangement of the collagenous lamellae provides the shape and the mechanical properties of the tissue. At the molecular level, modern technologies and theoretical modelling have started to explain exactly how the collagen fibrils are arranged within the stromal lamellae and how proteoglycans maintain this ultrastructure. In this review we describe the current state of knowledge about the three-dimensional stromal architecture at the microscopic level, and about the control mechanisms at the nanoscopic level that lead to optical transparency.

In Vivo Confocal Microscopy after Corneal Collagen Crosslinking

In vivo confocal microscopy (IVCM) findings of 84 patients who had undergone conventional epithelium-off corneal collagen crosslinking (CXL) and accelerated CXL (ACXL) were retrospectively reviewed. Analysis confirmed that despite a significant decrease in the mean density of anterior keratocytes in the first 6 postoperative months, cell density after CXL and ACXL returned to baseline values at 12 months. The demarcation lines observed after treatments represent an expression of light-scattering (reflectivity changes) through different tissue densities. Temporary haze of the anterior-mid stroma after conventional CXL represents an indirect sign of CXL-induced stromal collagen compaction and remodeling. IVCM showed that treatment penetration varies to some extent, but that the endothelium is not damaged and is correlated with CXL biomechanical effects. IVCM of limbal structures shows no evidence of pathological changes. Regeneration of subepithelial and stromal nerves was complete 12 months after the operation with fully restored corneal sensitivity and no neurodystrophic occurrences. IVCM allowed detailed high magnification in vivo micromorphological analysis of corneal layers, enabling the assessment of early and late corneal modifications induced by conventional and accelerated CXL. IVCM confirms that CXL is a safe procedure, which is still undergoing development and protocol adjustments.

From nano to macro: studying the hierarchical structure of the corneal extracellular matrix

In this review, we discuss current methods for studying ocular extracellular matrix (ECM) assembly from the 'nano' to the 'macro' levels of hierarchical organization. Since collagen is the major structural protein in the eye, providing mechanical strength and controlling ocular shape, the methods presented focus on understanding the molecular assembly of collagen at the nanometre level using X-ray scattering through to the millimetre to centimetre level using non-linear optical (NLO) imaging of second harmonic generated (SHG) signals. Three-dimensional analysis of ECM structure is also discussed, including electron tomography, serial block face scanning electron microscopy (SBF-SEM) and digital image reconstruction. Techniques to detect non-collagenous structural components of the ECM are also presented, and these include immunoelectron microscopy and staining with cationic dyes. Together, these various approaches are providing new insights into the structural blueprint of the ocular ECM, and in particular that of the cornea, which impacts upon our current understanding of the control of corneal shape, pathogenic mechanisms underlying ectatic disorders of the cornea and the potential for corneal tissue engineering.

Complications of corneal collagen cross-linking

Cross-linking of corneal collagen (CXL) is a promising approach for the treatment of keratoconus and secondary ectasia. Several long-term and short-term complications of CXL have been studied and documented. The possibility of a secondary infection after the procedure exists because the patient is subjected to epithelial debridement and the application of a soft contact lens. Formation of temporary corneal haze, permanent scars, endothelial damage, treatment failure, sterile infiltrates, and herpes reactivation are the other reported complications of this procedure. Cross-linking is a low-invasive procedure with low complication and failure rate but it may have direct or primary complications due to incorrect technique application or incorrect patient's inclusion and indirect or secondary complications related to therapeutic soft contact lens, patient's poor hygiene, and undiagnosed concomitant ocular surface diseases.

Natural history of corneal haze after collagen crosslinking for keratoconus and corneal ectasia: Scheimpflug and biomicroscopic analysis

PURPOSE

To determine the natural history of collagen crosslinking (CXL)-associated corneal haze measured by Scheimpflug imagery and slitlamp biomicroscopy in patients with keratoconus or ectasia after laser in situ keratomileusis.

SETTING

Cornea and refractive surgery subpecialty practice, United States.

DESIGN

Prospective randomized controlled clinical trial.

METHODS

The treatment group received ultraviolet-A/riboflavin CXL therapy. The control group received riboflavin alone without epithelial debridement. To objectively measure CXL-associated corneal haze, corneal densitometry using Scheimpflug imagery was measured and the changes in haze were analyzed over time. A similar analysis was performed using clinician-determined slitlamp haze. Correlation of CXL-associated corneal haze with postoperative outcomes was analyzed.

RESULTS

The mean preoperative corneal densitometry was 14.9 ± 1.93 (SD) (Pentacam Scheimpflug densitometry units). Densitometry peaked at 1 month (mean 23.4 ± 4.40; P<.001), with little change at 3 months (mean 22.4 ± 4.79; P = .06) and decreased between 3 months and 6 months (19.4 ± 4.48; P<.001) and between 6 months and 12 months. By 12 months, densitometry had not completely returned to baseline in the entire cohort (mean 17.0 ± 3.82; P<.001) and the keratoconus subgroup; however, it returned to baseline in the ectasia group (16.1 ± 2.41; P = .15). The postoperative course of slitlamp haze was similar to objective densitometry measurements. Increased haze, measured by densitometry, did not correlate with postoperative clinical outcomes.

CONCLUSIONS

The time course of corneal haze after CXL was objectively quantified; it was greatest at 1 month, plateaued at 3 months, and was significantly decreased between 3 months and 12 months. Changes in haze did not correlate with postoperative clinical outcomes.

Light transmission in the human cornea as a function of position across the ocular surface: theoretical and experimental aspects

This article investigates the theoretical basis for differences in visible light transmission through the human cornea as a function of distance from the center. Experimentally, transmission decreases approximately linearly up to 3 mm from the central axis, then quadratically beyond this. It is known that collagen fibril number density and collagen fibril radii change from the central region to the corneal periphery. We modeled, using the direct-summation-of-scattered-fields method, the effects these ultrastructural changes would be expected to have on light transmission, accounting for the increase in corneal thickness from center to edge. Fibril positions for the modeling were obtained from electron micrographs of human cornea. Theoretically, transmission remains fairly constant across the central cornea; then, as the fibril diameter increases, the predicted scattering increases. Interfibrillar spacing changes alter the refractive index ratio between matrix and fibril; this was modeled in our theoretical deductions. Fibril number density had a minimal effect on light propagation. Our theoretical deductions were in broad agreement with our experimental data. It is concluded that the reduced transparency in the peripheral stroma is primarily caused by changes in fibril radius and an increase in refractive index ratio between the fibril and the interfibrillar substance.

Correlation Between TGF-β1 in Tears and Corneal Haze Following LASEK and Epi-LASIK

PURPOSE

To compare the incidence and degree of corneal haze formation following laser subepithelial keratomileusis (LASEK) and epithelial laser in situ keratomileusis (epi-LASIK), and examine its correlation with tear film transforming growth factor-beta1 (TGF-beta1) levels.

METHODS

This prospective, interventional, clinical trial included 20 eyes (20 patients) randomly assigned to undergo LASEK or epi-LASIK. The level of TGF-beta1 in tear fluid was measured preoperatively and 1, 3, and 5 days postoperatively. Corneal haze was graded at 1 and 3 months after surgery, and the relationship with TGF-beta1 levels was determined.

RESULTS

Mean preoperative spherical equivalent refraction was -4.50 +/- 1.44 diopters (D) (range: -1.50 to -6.00 D) for LASEK eyes and -4.90 +/- 1.26 D (range: -1.75 to -6.00 D) for epi-LASIK eyes. Although mean corneal haze scores at 1 month were significantly higher in LASEK-treated eyes than in epi-LASIK treated eyes (P=.031), these scores were similar at 3 months (P=.608). Tear fluid TGF-beta1 levels were similar in LASEK and epi-LASIK eyes before surgery (P=.458) and significantly higher in the LASEK group at 1, 3, and 5 days postoperatively (P=.015, P=.023, and P=.039, respectively). A positive correlation was noted between tear TGF-beta1 levels on the first postoperative day and the degree of corneal haze at 1 month (r=0.501, P=.016).

CONCLUSIONS

Less corneal haze was noted after epi-LASIK than LASEK. A positive correlation between corneal haze and tear fluid TGF-beta1 levels on the first postoperative day suggest a possible mechanism for the observed difference.

Wound healing in the cornea: a review of refractive surgery complications and new prospects for therapy

PURPOSE

The corneal wound healing response is of particular relevance for refractive surgical procedures since it is a major determinant of efficacy and safety. The purpose of this review is to provide an overview of the healing response in refractive surgery procedures.

METHODS

Literature review.

RESULTS

LASIK and PRK are the most common refractive procedures; however, alternative techniques, including LASEK, PRK with mitomycin C, and Epi-LASIK, have been developed in an attempt to overcome common complications. Clinical outcomes and a number of common complications are directly related to the healing process and the unpredictable nature of the associated corneal cellular response. These complications include overcorrection, undercorrection, regression, corneal stroma opacification, and many other side effects that have their roots in the biologic response to surgery. The corneal epithelium, stroma, nerves, inflammatory cells, and lacrimal glands are the main tissues and organs involved in the wound healing response to corneal surgical procedures. Complex cellular interactions mediated by cytokines and growth factors occur among the cells of the cornea, resulting in a highly variable biologic response. Among the best characterized processes are keratocyte apoptosis, keratocyte necrosis, keratocyte proliferation, migration of inflammatory cells, and myofibroblast generation. These cellular interactions are involved in extracellular matrix reorganization, stromal remodeling, wound contraction, and several other responses to surgical injury.

CONCLUSIONS

A better understanding of the complete cascade of events involved in the corneal wound healing process and anomalies that lead to complications is critical to improve the efficacy and safety of refractive surgical procedures. Recent advances in understanding the biologic and molecular processes that contribute to the healing response bring hope that safe and effective pharmacologic modulators of the corneal wound healing response may soon be developed.

Grading of corneal transparency

AIM

To examine the academic literature on the grading of corneal transparency and to assess the potential use of objective image analysis.

METHOD

Reference databases of academic literature were searched and relevant manuscripts reviewed. Annunziato, Efron (Millennium Edition) and Vistakon-Synoptik corneal oedema grading scale images were analysed objectively for relative intensity, edges detected, variation in intensity and maximum intensity. In addition, corneal oedema was induced in one subject using a low oxygen transmissibility (Dk/t) hydrogel contact lens worn for 3h under a light eye patch. Recovery from oedema was monitored over time using ultrasound pachymetry, high and low contrast visual acuity measures, bulbar hyperaemia grading and transparency image analysis of the test and control eyes.

RESULTS

Several methods for assessing corneal transparency are described in the academic literature, but none have gained widespread use in clinical practice. The change in objective image analysis with printed scale grade was best described by quadratic parametric or sigmoid 3-parameter functions. 'Pupil image scales' (Annunziato and Vistakon-Synoptik) were best correlated to average intensity; however, the corneal section scale (Efron) was strongly correlated to variations in intensity. As expected, patching an eye wearing a low Dk/t hydrogel contact lens caused a significant (F = 119.2, p < 0.001) 14.3% increase in corneal thickness, which gradually recovered under open eye conditions. Corneal section image analysis was the most affected parameter and intensity variation across the slit width, in isolation, was the strongest correlate, accounting for 85.8% of the variance with time following patching, and 88.7% of the variance with corneal thickness.

CONCLUSION

Corneal oedema is best determined objectively by the intensity variation across the width of a corneal section. This can be easily measured using a slit-lamp camera connected to a computer. Oedema due to soft contact lens wear is not easily determined over the pupil area by sclerotic scatter illumination techniques.

Transparency, swelling and scarring in the corneal stroma

PURPOSE

This paper briefly reviews current explanations for corneal transparency and uses a well-developed model to try to explain the increased light scattering either accompanying corneal swelling or following phototherapeutic keratectomy (PTK).

METHODS

The direct summation of fields (DSF) method was used to compute light transmission as a function of wavelength. The method requires input of a number of structural parameters. Some of these were obtained from electron micrographs and others were calculated from X-ray diffraction data.

RESULTS

By swelling sections of stroma cut from different depths in the tissue, we have shown that fluid entering the cornea causes more swelling in the posterior lamellae than in the anterior lamellae. Furthermore, posterior lamellae can reach a higher final hydration than anterior lamellae. Collagen-free regions ('lakes') exist in corneas swollen in vitro and in Fuch's dystrophy corneas, many of which may be caused by the death of cells. The DSF method shows that local fibril disordering, increased refractive index mismatch, and increased corneal thickness together can account for a 20% increase in light scattering in a Fuch's dystrophy cornea at H=5.8 compared to the normal cornea. Additional scattering is probably caused by 'lakes'. The DSF method applied to PTK rabbit stroma with high levels of haze suggests that the newly deposited collagen is not the cause of the increased light scattering.

CONCLUSIONS

Fluid is not uniformly distributed within the corneal stroma when the cornea swells. Increased hydration of posterior lamellae may be because of known differences in the glycosaminoglycans between the anterior and posterior stroma. Lamellar interweave in the anterior stroma probably limits the extent to which the constituent lamellae can swell. The DSF method can be used to account for increased light scattering in oedematous corneas but cannot account for haze following PTK.