Corneal Anterior Power Calculation for an IOL in Post-PRK Patients.

PURPOSE: After corneal refractive surgery, there is an overestimation of the corneal power with the devices routinely used to measure it. Therefore, the objective of this study was to determine whether, in patients who underwent photorefractive keratectomy (PRK), it is possible to predict the earlier preoperative anterior corneal power from the postoperative (PO) posterior corneal power. A comparison is made using a formula published by Saiki for laser in situ keratomileusis patients and a new one calculated specifically from PRK patients. METHODS: The Saiki formula was tested in 98 eyes of 98 patients (47 women) who underwent PRK for myopia or myopic astigmatism. Moreover, anterior and posterior mean keratometry (Km) values from a Scheimpflug camera were measured to obtain a specific regression formula. RESULTS: The mean (±SD) preoperative Km was 43.50 (±1.39) diopters (D) (range, 39.25 to 47.05 D). The mean (±SD) Km value calculated with the Saiki formula using the 6 months PO posterior Km was 42.94 (±1.19) D (range, 40.34 to 45.98 D) with a statistically significant difference (p < 0.001). Six months after PRK in our patients, the posterior Km was correlated with the anterior preoperative one by the following regression formula: y = -4.9707x + 12.457 (R² = 0.7656), where x is PO posterior Km and y is preoperative anterior Km, similar to the one calculated by Saiki. CONCLUSIONS: Care should be taken in using the Saiki formula to calculate the preoperative Km in patients who underwent PRK.

Can We Use the Fellow Eye Biometric Data to Predict IOL Power?

PURPOSE: To study the correlation between right (RE) and left eye (LE) keratometry readings (K) and axial lengths (AL) in a population-based sample of normal subjects. METHODS: In a cross-sectional retrospective study conducted at S. Giuseppe Moscati Hospital, Avellino, Italy, 4516 eyes of 2258 patients with a mean age of 67 ± 16.36 years (range 18-96 years) were included. Partial coherence interferometry data obtained in right (RE) and left (LE) eyes were analyzed and correlated. RESULTS: The average K was 44.01 ± 1.50 diopters (D) (range 39.09-49.89 D) in the RE and 44.04 ± 1.53 D (range 39.63-51.89 D) in the LE (p = 0.0075). 4.5% of the patients (101) presented with differences in the corneal power ≥ 1 D, corresponding to a difference of roughly 1 D in the IOL power. The average AL was 23.89 ± 1.77 mm (range 19.09-35.15 mm) in the RE and 23.84 ± 1.68 mm (range 19.23-35.04 mm) in the LE (p = 0.0018). 19.2% of the patients (433) presented with differences in the AL ≥ 0.4 mm, corresponding to a difference of roughly 1 D in the IOL power. CONCLUSIONS: In calculating the IOL power, we must be aware of these results when we measure the fellow eye to validate the measurements in the first eye. In the case of postcataract refractive error, the outcome could be used for the second eye only when symmetric biometric findings are present.

Intraocular Pressure Evaluation after Myopic Refractive Surgery: A Comparison of Methods in 121 Eyes.

PURPOSE: To compare nine different formulas, developed to overcome the underestimation of intraocular pressure (IOP) readings after myopic photorefractive keratectomy (PRK). METHODS: The correlations between the modified IOP and the preoperative ones in 121 eyes of 72 patients who underwent myopic PRK were evaluated. RESULTS: The correlation and percentage of eyes in the range of ±2 mmHg were: Emara (R(2) = 0.1686, 74%), Munger (R(2) = 0.1665, 71%), Rosa (R(2) = 0.2489, 82%), Rashad (R(2) = 0.8593, 47%), Duch (R(2) = 0.1631, 82%), Arimoto (R(2) = 0.1638, 77%), Chihara (R(2) = 0.5389, 80%), Svedberg (R(2) = 0.1269, 54%), Kohlhaas (R(2) = 0.1074, 77%). In the case of known preoperative IOP, combining Rashad and Chihara's formulas (R(2) = 0.7730, 97%) were obtained. In the case of unknown preoperative IOP, combining Rosa and Duch's formulas (R(2) = 0.2484, 84%) were obtained. CONCLUSION: Our results suggest calculating IOP with the average of Rashad and Chihara's formulas if the preoperative IOP is known and the average of Rosa and Duch's formulas if it is unknown.

IOL power calculation after corneal refractive surgery.

PURPOSE: To describe the different formulas that try to overcome the problem of calculating the intraocular lens (IOL) power in patients that underwent corneal refractive surgery (CRS). METHODS: A Pubmed literature search review of all published articles, on keyword associated with IOL power calculation and corneal refractive surgery, as well as the reference lists of retrieved articles, was performed. RESULTS: A total of 33 peer reviewed articles dealing with methods that try to overcome the problem of calculating the IOL power in patients that underwent CRS were found. According to the information needed to try to overcome this problem, the methods were divided in two main categories: 18 methods were based on the knowledge of the patient clinical history and 15 methods that do not require such knowledge. The first group was further divided into five subgroups based on the parameters needed to make such calculation. CONCLUSION: In the light of our findings, to avoid postoperative nasty surprises, we suggest using only those methods that have shown good results in a large number of patients, possibly by averaging the results obtained with these methods.

Trans epithelial corneal collagen crosslinking for progressive keratoconus: 6 months follow up.

PURPOSE: To evaluate keratoconus biomechanical changes after transepithelial corneal collagen cross linking (TE CXL) using riboflavin and ultraviolet A (UVA). SETTING: Second University of Naples, Naples, Italy. DESIGN: Prospective non comparative case series study. METHODS: Patients with progressive keratoconus were examined, before and during a 6 months follow up after TE CXL, with a Pentacam, an Ocular Response Analyzer and an IOLMaster. Best corrected visual acuity (BCVA), refraction, corneal thinnest point (CTP), keratometry readings at the keratoconus apex (Kmax), axial eye length (AL), corneal volume (CV) anterior chamber volume (ACV), anterior chamber depth (ACD), corneal hysteresis (CH) and corneal resistance factor (CRF) were evaluated. RESULTS: Thirty-six eyes of 36 patients with progressive keratoconus were analyzed. Six months after treatment there was a significant improvement in BCVA (p<0.01), no significant changes in refraction (p=0.57), CTP (p=0.07), Kmax (p=0.88), AL (p=0.07), CV (p=0.38), ACV (p=0.07), ACD (p=0.7), CH (p=0.1) and CRF (p=0.3). CONCLUSIONS: According to our results TE CXL stabilizes most of the patients with progressive keratoconus, without affecting in negative way the corneal elasticity.