Andrew J. Quantock

Wound healing in response to keratorefractive surgery

Over one million Americans have undergone refractive keratoplasty since the introduction of radial keratotomy into the United States in 1978. There are now a number of alternative techniques available for reshaping the corneal surface to alter ocular refractive errors. Numerous technologic advances in the past decade now enable us to perform these procedures in a safer and more reliable fashion. The ability to control precisely the refractive outcome, however, continues to elude us and appears to be limited, in part, by interindividual variability in the wound healing response. Presently, we review the corneal wound healing response to various keratorefractive approaches and suggest some interventional strategies which might enable us to modulate more precisely our refractive results.

Corneal Iron Lines Associated With the Intrastromal Corneal Ring

Corneal epithelial iron lines commonly occur, and their shape is characteristically influenced by the underlying corneal surface topography. We studied a pattern of iron deposition, observed after implantation of the Intrastromal Corneal Ring (KeraVision, Inc., Santa Clara, California). In five of ten patients undergoing placement of the Intrastromal Corneal Ring, an arcuate pattern of epithelial iron deposition was observed eight to 12 months postoperatively. There was no significant clinical or topographic difference over time among the corneas of patients who developed an iron line vs those who did not. Of the five patients with an arcuate epithelial iron line, three underwent explantation of their Intrastromal Corneal Ring at 12 months according to the study protocol. Iron deposition either reverted to a Hudson-Stähli pattern or disappeared in each of these three patients.

CHARACTERIZATION OF A CENTRAL CORNEAL CLOUDINESS SHARING FEATURES OF POSTERIOR CROCODILE SHAGREEN AND CENTRAL CLOUDY DYSTROPHY OF FRANCOIS

A 56-year-old black woman with full-thickness mosaic pattern central corneal cloudiness, similar in appearance to central cloudy dystrophy and posterior crocodile shagreen, underwent corneal transplantation. Atypical features included decreased vision, photophobia, and epithelial involvement, with occasional foreign body sensation. Numerous 0.5-2.0-µm-diameter lacunae were present in the corneal stroma and Bowmans layer, and a saw-toothed lamellar pattern was often evident in the corneal stroma. Soybean agglutinin (SBA), a lectin that binds N-acetyl-galactosamine residues, bound diffusely to stromal foci exhibiting similar size and distribution to the lacunae observed by electron microscopy. An absence of histochemically detectable lipid associated with these lacunae suggests that SBA reacted with glycoconjugates other than glycolipids. Biochemical analyses revealed similar contents of keratan sulfate, chondroitin/dermatan sulfate, and collagen as in normal controls, suggesting that the SBA binding moieties are associated with a glycoprotein or proteoglycan that is structurally or compositionally different from those found in normal cornea. This patient may represent an extreme variant of Vogts or Francosis central corneal clouding or a previously undescribed corneal dystrophy.

Synchrotron X-ray diffraction in atypical macular dystrophy

This report documents the first case of X-ray diffraction techniques aiding the diagnosis of a corneal dystrophy with a clinically ambiguous presentation. Post-operatively, a high-angle synchrotron X-ray diffraction pattern was obtained from an in vitro portion of a pathological cornea. This pattern displayed two X-ray reflections which we recently demonstrated to be unique to the high-angle X-ray diffraction patterns of both type I and type II macular dystrophy corneas; on the basis of this evidence we were able to offer a post-operative diagnosis of macular corneal dystrophy. An electron microscopical evaluation of the cornea revealed stromal lacunae at all levels and an extensive layer of vacuoles, predominantly between Bowmans layer and the anterior stroma. These vacuoles were often associated with large proteoglycan filaments, as identified by Cuprolinic blue staining. Abnormally large collagen fibrils were documented, for the first time, in a macular dystrophy cornea; they existed in localised regions, frequently adjacent to the vacuoles and abnormal proteoglycans, and could well have implications for corneal transparency. We propose that the dystrophy is an atypical variant of macular corneal dystrophy which is encompassed by the heterogeneous nature of the condition.

Analysis of high-angle synchrotron x-ray diffraction patterns obtained from macular dystrophy corneas.

&NA; Six macular dystrophy corneas from six individuals were investigated by synchrotron x‐ray diffraction. Immunochemical evidence demonstrated that five of the patients had type I macular corneal dystrophy and that one had type II. Analysis of the high‐angle x‐ray diffraction patterns showed that the intermolecular spacing of macular dystrophy corneal collagen increases with hydration from the dry state in a fashion similar to the intermolecular spacing of normal human corneal collagen. The highangle x‐ray diffraction pattern from all six corneas contained two “extra reflections” not obtained from other human corneas, normal or pathologic. The reflections, which arise from 4.61 and 9.62 Å periodic structures, are independent of the type of macular dystrophy. Furthermore, their position is independent of corneal hydration. We propose that a glycosaminoglycan structure is the origin of the unique extra reflections.

A Combined Incision Technique of Radial Keratotomy: A Comparison to Centripetal and Centrifugal Incision Techniques in Human Donor Eyes

PURPOSEnTo evaluate and compare the extent of corneal flattening and the anatomy of the incision grooves produced by centrifugal, centripetal, and combined incisions in a human donor eye model.nnnMETHODSnTwenty-seven eyes, divided into three groups of nine, received eight-incision radial keratotomy using the centrifugal, centripetal, or combined incision technique. Corneal curvature was evaluated using a topography system, and the anatomy of the incision was assessed by light microscopy and scanning electron microscopy.nnnRESULTSnThe average central corneal flattening at the 3-mm clear zone was 4.16 +/- 1.47 diopters (D) for centrifugal incisions; 7.71 +/- 2.77 D for centripetal incisions; and 9.26 +/- 1.75 D for combined incisions. The difference in corneal flattening between eyes that received centrifugal versus either centripetal or combined incisions was significant (P < 0.01), whereas the difference between combined and centripetal incisions was not significant (P = 0.174). Anatomic differences were observed within the incision grooves produced by the three techniques.nnnCONCLUSIONnThe authors describe a novel combined incision technique of radial keratotomy that provides the potential safety of a centrifugal incision with the effect of a centripetal incision.

Proteoglycans contain a 4.6 A repeat in muscular dystrophy corneas: x-ray diffraction evidence

Synchrotron x-ray diffraction patterns from macular corneal dystrophy (MCD) corneas contain an unusual reflection that arises because of an undefined ultrastructure with a periodic repeat in the region of 4.6 A. In this study, we compared with wide-angle x-ray diffraction patterns obtained from four normal human corneas and four MCD corneas. Moreover, portions of two of the MCD corneas were pretreated with a specific glycosidase to shed light on the origin of the 4.6 A reflection. None of the normal corneas produced an x-ray reflection in the region of 4.6 A, whereas all four of the MCD corneas did (MCD type I at 4.65 A and 4.63 A, MCD type II at 4.63 A and 4.67 A). This reflection was diminished after incubation of the MCD tissues with either chondroitinase ABC or N-glycanase. The findings indicate that glycosaminoglycans or proteoglycans contribute to the unusual MCD x-ray reflection and hence most likely contain a periodic 4.6 A ultrastructure. Furthermore, the results imply that periodic 4.6 A MCD ultrastructures reside in either intact, unsulfated lumican molecules and regions of the CS/DS-containing molecules or in a region of a hybrid macromolecular aggregate formed by the interaction of the two molecules.

An erroneous glycosaminoglycan metabolism leads to corneal opacification in macular corneal dystrophy

Macular corneal dystrophy (MCD) is a rare, potentially blinding disease whose fundamental genetic defect and exact pathogenesis are yet to be elucidated. It is, however, an especially interesting pathology, which highlights how an erroneous glycosaminoglycan or proteoglycan metabolism can induce physical symptoms in a specific connective tissue. Based on immunochemical data, MCD is a heterogeneous condition, and at least two types of the disease have been identified. The cornea, cartilage, and serum from MCD type I patients all contain an unsulphated form of keratan sulphate. In contrast, these tissues contain normally sulphated keratan sulphate in MCD type II patients. A normal population of keratan sulphate proteoglycans (and chondroitin/dermatan sulphate proteoglycans) in the cornea seems to be a requirement of corneal transparency. However, a clinical diagnosis of MCD is unable to distinguish between the keratan sulphate positive and negative types of MCD. The histopathology of MCD is fairly well established, and various corneal aberrations—such as fibrillogranular and glycosaminoglycan deposits, abnormal diameter collagen, and collagen-free lacunae—result in a breakdown of the regular corneal architecture that presumably contributes to the subsequent corneal opacification.

Organization of Collagen in the Lyophilized Cornea

PURPOSEnTo investigate the organization of collagen fibrils in the lyophilized cornea.nnnMETHODSnFreshly harvested porcine corneas (n = 10) were lyophilized and examined by synchrotron x-ray diffraction and transmission electron microscopy.nnnRESULTSnCollagen fibrils are highly compacted in lyophilized corneas. They become more widely spaced when the tissue is rehydrated, however, the distribution of imbibed water throughout the stroma is not necessarily homogeneous within an individual cornea, nor is it always similar in specimens that have been rehydrated to similar levels. In lyophilized corneas, the mean center-to-center interfibrilar spacing of the regularly arranged collagen reaches levels found in freshly thawed porcine corneas (between 74% and 78% water by weight) when between 74.3% and 81.6% of the rehydrated lyophilized corneas weight is water.nnnCONCLUSIONnRegularly arranged collagen fibrils are able to reapproximate their original spacings if lyophilized corneal tissue is rehydrated, although the manner in which imbibed water is distributed is somewhat unpredictable.

Histology and ultrastructure of picosecond laser intrastromal photorefractive keratectomy (ISPRK)

Picosecond intrastromal ablation is currently under investigation as a new minimally invasive way of correcting refractive error. When the laser pulses are placed in an expanding spiral pattern along a lamellar plane, the technique is called intrastromal photorefractive keratectomy (ISPRK). We performed ISPRK on six human eye bank eyes. Thirty picosecond pulses at 1000 Hz and 20 - 25 (mu) J per pulse were separated by 15 microns. A total of 3 layers were placed in the anterior stroma separated by 15 microns. The eyes were then preserved and sectioned for light, scanning and transmission electron microscopy. Light and scanning electron microscopy reveals that picosecond intrastromal ablation using an ISPRK pattern demonstrates multiple, coalescing intrastromal cavities oriented parallel to the corneal surface. These cavities possess a smooth appearing inner wall. Using transmission electron microscopy, we noticed tissue loss surrounding some cavities with collagen fibril termination and thinning of collagen lamella. Other cavities we formed by separation of lamella with little evidence of tissue loss. A pseudomembrane lines the edge of some cavities. Although underlying tissue disruption was occasionally seen along the border of a cavity in no case was there any evidence of thermal damage or tissue necrosis. Ablation and loss of tissue in ISPRK results in nonthermal microscopic corneal thinning around some cavities whereas others demonstrate only lamellar separation. Alternative patterns and energy parameters should be investigated to bring this technology to its full potential in refractive surgery.