Calculation of Intraocular Lens Power Using Orbscan II Quantitative Area Topography After Corneal Refractive Surgery (original) (raw)

Intraocular Lens Power Calculation after Corneal Refractive Surgery

Journal of Ophthalmic & Vision Research

Purpose: To report refractive outcomes following phacoemulsification (PE) and posterior chamber intraocular lens (PCIOL) implantation in eyes with previous corneal refractive surgery. Methods: In this retrospective comparative study, 18 consecutive eyes of 14 patients with previous keratorefractive surgery for myopia including photorefractive keratectomy (PRK, 6 eyes; 33.3%) and laser in situ keratomileusis (LASIK, 12 eyes; 66.7%) underwent PE+PCIOL. Computerized corneal topography was employed to determine the flattest keratometric reading within the 3-mm central zone. This value was inserted into the Sanders-Retzlaff-Kraff/T (SRK/T) formula to calculate IOL power. IOL power selected for implantation was 1 D greater than the calculated value described above. Results: Mean age and follow-up period were 54.1±11.5 years and 29.9±26.3 months, respectively. Mean implanted lens power was 18.56±3.86 D which was not significantly different from mean back-calculated IOL power for target refraction (19.04±4.16 D) (P=0.28). There was no significant difference between mean target refraction (-0.94±0.52 D) and achieved postoperative spherical equivalent refractive error (-0.62±1.06) at final follow-up (P=0.28). The achieved spherical equivalent refractive error was within ±0.50 D of intended refraction in 8 (44.4%) eyes, within ±1.0 D in 11 (61.1%) eyes, and within ±2.0 D in 16 (88.9%) eyes. In a subgroup of patients (5 eyes) with complete pre-refractive surgery data, the difference between post-refractive surgery keratometry method and all other methods (P=0.02) and between the current method and the Feiz-Mannis method (P=0.01) was statistically significant. Conclusion: The method suggested herein is simple and independent of pre-refractive surgery data with results comparable to other commonly used methods.

Comparison of Newer IOL Power Calculation Methods for Eyes With Previous Radial Keratotomy

Investigative ophthalmology & visual science, 2016

To evaluate the accuracy of the optical coherence tomography-based (OCT formula) and Barrett True K (True K) intraocular lens (IOL) calculation formulas in eyes with previous radial keratotomy (RK). In 95 eyes of 65 patients, using the actual refraction following cataract surgery as target refraction, the predicted IOL power for each method was calculated. The IOL prediction error (PE) was obtained by subtracting the predicted IOL power from the implanted IOL power. The arithmetic IOL PE and median refractive PE were calculated and compared. All formulas except the True K produced hyperopic IOL PEs at 1 month, which decreased at ≥4 months (all P < 0.05). For the double-K Holladay 1, OCT formula, True K, and average of these three formulas (Average), the median absolute refractive PEs were, respectively, 0.78 diopters (D), 0.74 D, 0.60 D, and 0.59 D at 1 month; 0.69 D, 0.77 D, 0.77 D, and 0.61 D at 2 to 3 months; and 0.34 D, 0.65 D, 0.69 D, and 0.46 D at ≥4 months. The Average pro...

Consideration of the posterior corneal curvature for assessment of corneal power after myopic LASIK

Acta Ophthalmologica Scandinavica, 2004

Purpose: To evaluate the effect of a separate measurement of the anterior and posterior corneal surface to calculate the total refractive power of the cornea after myopic laser in situ keratomileusis (LASIK). Methods: A total of 39 eyes of 21 patients (aged 33 AE 9 years) were included in this prospective, non-randomized, comparative study. These involved 19 myopic corrections (À 3.5 AE 1.6 dioptres) and 23 refractive corrections of myopic astigmatism (sphere: À3.7 AE 1.6 D, cylinder: À1.2 AE 0.4 D). All procedures were accomplished with the Keratom II 1 . Coherent-Schwind excimer laser and the Moria Model One 1 microkeratome (150 mm head) at the Medical Education Centre, La Trinidad, Caracas, Venezuela. Subjective refractometry, Bausch & Lomb 1 keratometry and Orbscan 1 slit-scanning corneal topography analysis were performed before and 3 months after LASIK. Corneal power was assessed directly using keratometry (K1) and Orbscan videokeratography (T1). Corneal power was calculated using the preoperative keratometric (K2, 'gold standard', clinical history method) or topographic power (T2, clinical history method) and spherical equivalent change. A composite value was derived from the Orbscan anterior and posterior surface power and central pachymetry (T3). Results: Three months postoperatively, corneal power ranged in a descending order from T1 (42.33 AE 1.78 D), K1 (40.82 AE 2.20 D), K2 (40.42 AE 2.36 D), T2 (40.03 AE 2.51 D) to T3 (38.78 AE 2.23 D). On average, T1 exceeded the gold standard by 1.9 D and the gold standard exceeded T3 by 1.6 D. K2, T1, T2 and T3 correlated significantly with K1 (r ¼ 0.975, p < 0.001; r ¼ 0.909, p < 0.001; r ¼ 0.963, p < 0.001; r ¼ 0.853, p < 0.001, respectively). The differences T1ÀK2 (r ¼ À 0.699, p < 0.001) and T3ÀK2 (r ¼ À 0.499, p ¼ 0.001) correlated highly inversely and K1ÀK2 correlated borderline inversely (r ¼ À 0.325, p ¼ 0.043) with the intended refractive correction. Conclusion: After myopic LASIK, refractive corneal power is overestimated by direct keratometric and especially videokeratoscopic measurements. The higher the intended refractive correction, the greater is this error. A separate measurement of both refractive surfaces of the cornea tends to underestimate but may enhance accuracy of the total refractive corneal power if the history of the patient is unknown.

A new formula for intraocular lens power calculation after refractive corneal surgery

Journal of refractive surgery (Thorofare, N.J. : 1995), 2006

When calculating the power of an intraocular lens (IOL) with conventional methods in eyes that have previously undergone refractive surgery, in most cases the power is inaccurate. To minimize these errors, a new IOL power calculation formula was developed. A theoretical formula empirically adjusted two variables: 1) the corneal power and 2) the anterior chamber depth (ACD). From the average curvature of the entrance pupil area, weighted according to the Stiles-Crawford effect, the corneal power is calculated by using a relative keratometric index that is a function of the actual corneal curvature, type of keratorefractive surgery, and induced refractive change. Anterior chamber depth is a function of the preoperative ACD, lens thickness, axial length, and the ACD constant. We used our formula in 20 eyes that previously underwent refractive surgery (photorefractive keratectomy [n = 6], laser subepithelial keratomileusis [n = 3], laser in situ keratomileusis [n = 6], and radial kerato...

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

Graefe's Archive for Clinical and Experimental Ophthalmology, 2012

Background Precise 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. Methods A 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 pre-K values from Orbscan total-mean power. Anterior chamber depth measures obtained with the IOL Master and Orbscan II were compared. Results Mean 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. Conclusions Orbscan measurements used as the post-K values into the double-K method provide a precise IOL calculation, especially in post myopic refractive surgery patients.