One-year results of a prospective multicenter study of the Casebeer system of refractive keratotomy. Casebeer Chiron Study Group

Corneal multiphoton microscopy and intratissue optical nanosurgery by nanojoule femtosecond near-infrared pulsed lasers

Multiphoton microscopy including multiphoton autofluorescence imaging (MAI) and second-harmonic generation (SHG) is being used as a novel diagnostic tool to perform tissue nonlinear optical tomography with submicron resolution. The three-dimensional corneal ultrastructure of whole depth has been viewed without any staining or mechanical slicing. Compared with photodisruptive surgical effects occurring at TW/cm2 light intensity, multiphoton imaging can be induced at MW-GW/ cm2 photon intensity. The intratissue surgical effect including nanojoule (nJ) femtosecond laser ablation and flap generation was induced through multiphoton nonlinear absorption at a wavelength of 800 nm and ascertained by the histological outcomes. More interesting, the multiphoton microscopy based on nonlinear absorption of femtosecond laser pulses at the wavelength of 715-930 nm emitted from solid-state Ti:sapphire system is acting as a precise non-invasive monitoring tool to determine the interest of region, to visualize and verify the outcomes in in vivo intrastromal laser nanosurgery. Overall, these data suggest that multiphoton microscopy is a highly sensitive and promising technique for studying the morphometric and biomechanical properties of biological tissues and that the nJ ultrashort Lasers can be used as a ultra-precise nanoscalpel for performing intratissue surgery.

Human corneal thickness and its impact on intraocular pressure measures: a review and meta-analysis approach

We determined the "normal" central corneal thickness (CCT) value in human corneas based on reported literature values for within-study average CCT values, and used this as a reference to assess the reported impact of physiological variables (especially age and diurnal effects), contact lens wear, pharmaceuticals, ocular disease, and ophthalmic surgery on CCT. With the expected CCT and its variance defined, it should be possible to determine the potential impact of differences in CCT in intraocular pressure (IOP) assessments, especially by applanation tonometry, using a meta-analysis approach. Some 600 sets of CCT data were identified from the worldwide literature over the period of 1968 through mid-1999, of which 134 included IOP measures as well. The within-study average CCT values and reported variance (SD) was noted along with the number of eyes and any special characteristics, including probable ethnic origin of the study subjects. Various sets of data were subjected to statistical analyses. From 300 data sets from eyes designated as normal, the group-averaged CCT was 0.534 mm. From 230 data sets where interindividual variance was reported, the group-averaged CCT was 0.536 mm (median 0.536 mm; average SD of 0. 031 mm, average coefficient of variation = 5.8%). Overall, studies using slit-lamp-based pachometry have reported marginally lower CCT values (average 0.530 mm, average SD 0.029 mm) compared to ultrasound-based studies (average 0.544, average SD 0.034 mm), which perhaps reflects the type of individual studied (non-surgical vs. pre-surgical patients) rather than the technique itself. A slight chronological increase in reported average CCT values (approximately 0.006 mm/decade) was evident, but a substantial chronological increase was evident for ultrasound pachometry studies (approximately 0.015 mm/decade). Within the meta-analysis-generated average and variance, age had no obvious impact on CCT measures for *whites, although an age-related decline in CCT is evident for non-whites. Any diurnal effects are likely concealed within the expected variance in CCT. Contact lens wear and pharmaceuticals generally produced changes in CCT that were well within the expected variance in CCT. Of the ocular diseases, only those associated with collagen disorders (including keratoconus) or endothelial-based corneal dystrophies (e.g., Fuchs) were likely to result in decreases or increases, respectively, of CCT beyond the normal variance. Routine contact lens wear and diseases such as diabetes seem unlikely to produce changes in CCT of a magnitude that would justify pachometry as a monitoring method beyond routine slit-lamp evaluation. Increases in CCT beyond the expected variance were reported after a range of intraocular surgeries (cataract operations, penetrating keratoplasty), whereas photorefractive surgery produces a measurable decrease in CCT. A meta-analysis of possible association between CCT and IOP measures of 133 data sets, regardless of the type of eyes assessed, revealed a statistically significant correlation; a 10% difference in CCT would result in a 3. 4 +/- 0.9 mm Hg difference in IOP (P </= 0.001, r = 0.419). The observed phenomenon was much smaller for eyes designated as healthy (1.1 +/- 0.6 mm Hg for a 10% difference in CCT, P = 0.023, r = 0. 331). For eyes with chronic diseases, the change was 2.5 +/- 1.1 mm Hg for a 10% difference in CCT (P = 0.005, r = 0.450), whereas a substantial but highly variable association was seen for eyes with acute onset disease (approximately 10.0 +/- 3.1 mm Hg for a 10% difference in CCT, P = 0.004, r = 0.623). Based on the meta-analysis, normal CCT in white adults would be expected to be within +/-11.6% (+/-2 SD) of 0.535 mm, i.e., 0.473-0.597 mm (95% CI, 0.474-0.596). The impact of CCT on applanation tonometry of healthy eyes is unlikely to achieve clinical significance, but for corneas of eyes with chronic disease, pachometry should be performed if the tonometry reveals IOP readi

Advances in refractive surgery. New options expand the scope of corrective procedures

The Food and Drug Administration's approval of the excimer laser in 1995 heralded the age of laser refractive surgery in the United States. Since then, other advances in refractive surgery have spurred great public interest in and enthusiasm for these procedures. In this article, the authors review the major types of refractive surgery and discuss their indications, results, and complications.

Radial Keratotomy Clear Zone Diameter Errors

PURPOSE

To examine the variability of achieved clear zone diameters following radial keratotomy and to determine if refractive outcomes are correlated to clear zone diameter errors.

METHODS

Twenty-six patients with radial keratotomy (51 eyes) were studied retrospectively. The central clear zone diameter for each radial incision meridian was measured using a slit-lamp and a photograph. The achieved clear zone diameters were statistically compared to the intended clear zone sizes and to the refractive outcomes.

RESULTS

Twenty-nine of the 51 eyes met various exclusion criteria. Ninety individual slit-lamp clear zone measurements were obtained for the remaining 22 eyes. Sixty-two of 90 (69%) achieved clear zone diameters were more than 0.25 mm different than the intended clear zone size. A maximum difference of +/- 0.80 mm from the intended clear zone marker size was observed. The average clear zone diameter error did not statistically differ between the three refractive outcome groups (undercorrection, emmetropia, and overcorrection).

CONCLUSIONS

In selected cases of centripetal radial keratotomy, the achieved clear zone diameter can vary widely from the intended clear zone size despite extensive surgeon experience.