Carlos G. Arce

Calculation of intraocular lens power using Orbscan II quantitative area topography after corneal refractive surgery.

PURPOSE To present the prospective application of the Orbscan II central 2-mm total-mean corneal power obtained by quantitative area topography in intraocular lens (IOL) calculation after refractive surgery. METHODS Calculated and achieved refraction and the difference between them were studied in 77 eyes of 61 patients with previous radial keratotomy (RK), RK and additional surgeries, myopic LASIK, myopic photorefractive keratectomy (PRK), or hyperopic LASIK who underwent phacoemulsification without complications in 3 eye centers. All IOL calculations used the average from the central 2-mm Orbscan II total-mean power of maps centered on the pupil without the use of previous refractive data. Six IOL styles implanted within the bag were used. RESULTS Using the SRK-T formula, the overall calculated refraction was -0.64+/-0.93 diopters (D). The overall achieved spherical equivalent refraction (-0.52+/-0.79 D; range: -3.12 to 1.25 D; 95% confidence interval [CI]: -0.70/-0.34 D) was +/-0.50 D in 53% of eyes, +/-1.00 D in 78% of eyes, and +/-2.00 D in 99% of eyes. The overall difference between the calculated and achieved refraction (0.12+/-0.93 D, P=.27; range: -2.18 to 2.62 D; 95% CI: 0.09/0.33 D) was +/-0.50 D in 39% of eyes, +/-1.00 D in 77% of eyes, and +/-2.00 D in 96% of eyes. This difference was +/-1.00 D in 77% of eyes with RK (P=.70), 82% of eyes with myopic LASIK (P=.34), and 90% of eyes with myopic PRK (P=.96). In eyes with RK followed by LASIK, a trend toward undercorrection was noted (P=.03). In eyes with hyperopic LASIK, a trend toward overcorrection was noted (P=.005). CONCLUSIONS In eyes with previous corneal refractive surgery, IOL power calculation can be performed with reasonable accuracy using the Orbscan II central 2-mm total-mean power. This method had better outcomes in eyes with previous RK, myopic LASIK, and myopic PRK than in eyes with hyperopic LASIK or RK with LASIK.

Orbscan II and double-K method for IOL calculation after refractive surgery

BackgroundPrecise IOL calculation in post-refractive surgery patients is still a challenge for the cataract surgeon. The purpose of this study is to test whether adding Orbscan II values into the double-K method improves IOL calculation in this group of patients.MethodsA prospective study with 43 eyes previously submitted to refractive surgery that underwent cataract extraction. IOL calculation was performed with double-K method. Post-K value was derived from Orbscan total-mean power map. The average corneal curvature of the general population (43.8D) was used as the pre-K value. Refraction results 30 days after surgery were compared with refraction that would be obtained if we used: (1) post-K values from keratometry, (2) post-K values from topography, and (3) pre-K values from Orbscan total-mean power. Anterior chamber depth measures obtained with the IOL Master and Orbscan II were compared.ResultsMean postoperative spherical equivalent (SE) was −0.25 ± 1.10 D in eyes submitted to radial keratotomy , –1.04 ± 1.42 D in eyes previously submitted to myopic Lasik, and +0.05 ± 1.76 D in those submitted to hyperopic surgeries. Had we inputted post-K values derived from keratometer and from topography, we would have obtained significantly higher postoperative refractive errors in eyes previously submitted to myopic refractive surgery (p < 0.05). Refractions using pre-K derived from the central 8 mm Orbscan instead of 43.8 D were similar in all studied groups (p > 0.05). Anterior chamber depth measured with IOL Master or Orbscan were similar.ConclusionsOrbscan measurements used as the post-K values into the double-K method provide a precise IOL calculation, especially in post myopic refractive surgery patients.

O cálculo do poder das lentes intra-oculares e o Orbscan-II. Parte 1: O poder óptico da córnea normal

PURPOSE: To evaluate the dioptric power of the normal central cornea. METHODS: Retrospective analysis of combined slit-scanning and Placido-ring topography in 30 normal eyes. RESULTS: Keratometric-mean, anterior-mean, thickness-mean, and total-mean powers; keratometric-optical and total-optical, keratometric-axial and total-axial, and total-tangential powers; equivalent power and the addition of anterior and posterior mean powers, were similar (p>0.8361) in the four analyzed central areas with 0.04, 1.0, 3.0, and 5.0 mm diameter. The posterior-mean power decreased (p=0.0030) from -6.62 D to -6.35 D. The average thickness increased (p=0.0231) from 524 µm to 554 µm, representing 0.13 D of total-mean corneal power. Keratometric-optical (p 0.4907), and to the addition of anterior-mean and posterior-mean powers (p>0.3868). Anterior-mean power was about 7.5 greater than posterior-mean power. CONCLUSIONS: The real power of the cornea calculated using both surfaces, its thickness, and the physiologic refraction indices is smaller (less positive) than the power usually accepted and determined by the anterior corneal surface radius and the keratometric index of refraction (F<K). Anterior-mean/posterior-mean power ratio is not ten-to-one as conventionally assumed. Power of posterior surface and thickness vary according to the size of the analyzed area. However, power of anterior surface and total power of the cornea remain the same. Components of total corneal power and pachymetry seem to be independent of each other.

Sectorial and Annular Quantitative Area Pachymetry With the Orbscan II

PURPOSE To present a new application of quantitative area pachymetry of the cornea. METHODS Sectorial quantitative area pachymetry of small corneal zones located at the center and surrounding areas is possible by means of the statistical analysis device and tools menu of Orbscan II (Bausch and Lomb). RESULTS Average pachymetry of 1 central zone and 12 small zones around the center may be shown in the pachymetry map by simply formatting the system. Average pachymetry, standard deviation, rate of thickness variation (slope), and volume of analyzed annular corneal zone can also be automatically shown in a data window. The examination results can be printed and kept on record. CONCLUSIONS The method is simple and descriptive. This type of preoperative evaluation may guide the manual or laser preparation of channels for intracorneal segments and the corneal trephination for lamellar or penetrating keratoplasty.

Orientaçöes para o polimento e retoque das lentes de contato rígidas gás permeáveis

Objetivo: Relatar, passo a passo, orientacoes basicas de polimento e retoque de lentes de contato rigidas gas permeaveis (RGP). Metodos: Com uma unidade de retoque pode polir-se adequadamente as superficies de lentes RGP e modificar-se bordas, espessura e curvatura perifericas, diâmetro e poder optico. Conclusoes: Os procedimentos recomendados servem para restabelecer a qualidade de lentes deterioradas, prolongar seu tempo de vida ou melhorar o conforto e as condicoes de adaptacao nos pacientes que usam este tipo de correcao