Ultrastructural and Immunohistochemical Findings after Linear Excimer Laser Keratectomy

Can we produce a human corneal equivalent by tissue engineering?

Tissue engineering is progressing rapidly. Bioengineered substitutes are already available for experimental applications and some clinical purposes such as skin replacement. This review focuses on the development of reconstructed human cornea in vitro by tissue engineering. Key elements to consider in the corneal reconstruction, such as the source for epithelial cells and keratocytes, are discussed and the various steps of production are presented. Since one application of this human model is to obtain a better understanding of corneal wound healing, the mechanisms of this phenomenon as well as the function played both by membrane-bound integrins and components from the extracellular matrix have also been addressed. The analysis of integrins by immunohistofluorescence labelling of our reconstructed human cornea revealed that beta(1), alpha(3), alpha(5), and alpha(6) integrin subunits were expressed but alpha(4) was not. Laminin, type VII collagen and fibronectin were also detected. Finally, the future challenges of corneal reconstruction by tissue engineering are discussed and the tremendous applications of such tissue produced in vitro for experimental as well as clinical purposes are considered.

Astigmatism Outcomes Following Spherical Photorefractive Keratectomy for Myopia

BACKGROUND

Our aim was to examine the change in astigmatism after spherical photorefractive keratectomy (PRK). These effects are essential to optimizing photoastigmatic refractive keratectomy (PARK) to correct astigmatism to within fractions of a diopter.

METHODS

We retrospectively reviewed 98 eyes of 178 patients with mild to moderate myopia and cylinder < or = 1.00 diopter (D) treated with spherical PRK (VISX 20/20 STAR excimer laser system); 31 eyes had epithelium removed mechanically with a blade and 67 eyes by a laser-scrape technique.

RESULTS

Refractive astigmatism was reduced by greater than 0.25 D in 27 eyes (28%); refractive astigmatism was induced by greater than 0.25 D in 31 eyes (32%); the average vector-corrected difference between an eye's astigmatism before and after surgery was 0.01 +/- 0.52 D (P = .85). Eyes with high topographic astigmatism but low refractive cylinder before PRK showed an average of 0.07 +/- -0.60 D change in refractive cylinder after PRK. Refractive astigmatism of more than 0.25 D was induced in 16 eyes (44%) that received manual removal of epithelium versus 15 eyes (24%) that received laser removal of epithelium (odds ratio 2.51, P < .01).

CONCLUSIONS

When using PRK for astigmatism correction, refractive cylinder before surgery rather than topographic astigmatism may be the most appropriate method for targeting the astigmatism correction, especially when the two values are discordant. An unpredictable mild increase or decrease in astigmatism may be expected.

Extracellular matrix changes in human corneas after radial keratotomy

Extracellular matrix and basement membrane alterations were identified in human corneas after radial keratotomy. Ten normal and five radial keratotomy autopsy corneas (two at 6 months post surgery, and three at 3 years post surgery) were studied by immunofluorescence with antibodies to 28 extracellular matrix and basement membrane components. Outside of radial keratotomy scars, all studied components had a normal distribution. Of stromal extracellular matrix, only type III collagen accumulated around the scars. The basement membrane around epithelial plugs had a normal composition except for type IV collagen. Its alpha1-alpha2 chains, normally present only in the limbal basement membrane, appeared around all plugs. alpha3 and alpha4 chains were very weak or absent in these areas, contrary to nonscarred areas. This basement membrane pattern was similar to the normal limbal but not to the central corneal pattern. Keratin 3 also had a limbal-like, suprabasal expression in the plug epithelium. The stroma around the scars accumulated tenascin-C, fibrillin-1, types VIII and XIV collagen, all of which were absent from normal corneal basement membrane and extracellular matrix. Only tenascin-C showed less staining in anterior scars 3 years post surgery than 6 months post surgery, but still persisted in posterior scars. Incomplete scar healing was evident even 3 years post radial keratotomy. It was manifested by the accumulation of abnormal extracellular matrix in the anterior and posterior scars and by the limbal-like pattern of type IV collagen isoforms in the basement membrane around epithelial plugs.