Refractive complications of cataract surgery after radial keratotomy

An intraocular lens power calculation formula based on optical coherence tomography: a pilot study

PURPOSE

To develop an intraocular lens (IOL) power calculation formula based on optical coherence tomography (OCT) that would not be biased by previous laser vision correction.

METHODS

Twenty-seven eyes of 27 cataract patients without prior laser vision correction who underwent phacoemulsification were included in the study. An optical coherence biometer (IOLMaster, Carl Zeiss Meditec) measured anterior corneal curvature and axial eye length. A high-speed (2000 Hz) anterior segment OCT prototype mapped corneal thickness and measured anterior chamber depth and crystalline lens thickness. Posterior corneal curvature was computed by combining IOLMaster keratometry with OCT corneal thickness mapping. A new IOL formula was developed based on these parameters. One month after phacoemulsification, the manifest refraction spherical equivalent (MRSE) was measured. The prediction error in postoperative MRSE of the OCT-based IOL formula was compared with that of three theoretic formulae: SRK/T, Hoffer Q, and Holladay II.

RESULTS

The mean prediction error in postoperative MRSE of the OCT-based formula was 0.04+/-0.44 diopters (D). The SRK/T was the best of the theoretic formulae, and its prediction error was -0.35+/-0.42 D. Twenty-one (78%) eyes were within 0.50 D using the OCT formula compared to 18 (67%) eyes using the SRK/T. No statistically significant differences were noted among the formulae.

CONCLUSIONS

For cataract patients without prior laser vision correction, the OCT-based IOL formula was as accurate as the current theoretic formulae. This new formula is based on direct OCT assessment of the posterior curvature and avoids the calculation errors inherent in conventional IOL formulae.

Determining corneal power using Pentacam after myopic photorefractive keratectomy

PURPOSE

To assess the accuracy of Pentacam Scheimpflug camera for corneal power measurement in eyes with previous photorefractive keratectomy for myopia.

METHODS

In this comparative interventional case series, 35 eyes of 35 patients who had myopic photorefractive keratectomy were studied. Corneal power was measured by conventional topography and Pentacam Scheimpflug camera, and equivalent keratometry readings (EKR) in different central corneal rings (0.5 to 4.5 mm), true net power and simulated keratometry (K) measurements as well as those obtained using Shammas no-history, Koch-Maloney and Haigis methods were compared with clinical history method.

RESULTS

All corneal power measurements except for the topography simulated K and true net power values were statistically similar to the clinical history values. Simulated keratometry and 4.5-mm EKR values were more closely correlated with clinical history method. Shammas formula, Pentacam simulated K and 3-, 4- and 4.5-mm EKR provided a 95% confidence interval within +/-0.50 D of the mean clinical history method value, among these, the width of the 95% limits of agreement (LoA) was narrower for Shammas and Pentacam simulated K and 4.5-mm EKR values; however, considerably large 95% LoA were found between each of these values and those obtained with the clinical history method. Estimated preoperative keratometry was statistically similar to the preoperative measurement; however, estimated refractive change was different from actual value.

CONCLUSIONS

The Pentacam 4.5-mm EKR and simulated keratometry may be used as an alternative to clinical history method to predict corneal power when pre-keratorefractive surgery data are unavailable; however, wide LoA should be considered in the calculations.

Cataract surgery after refractive surgery

PURPOSE OF REVIEW

To review recent contributions addressing the challenge of intraocular lens (IOL) calculation in patients undergoing cataract extraction following corneal refractive surgery.

RECENT FINDINGS

Although several articles have provided excellent summaries of IOL selection in patients wherein prerefractive surgery data are available, numerous authors have recently described approaches to attempt more accurate IOL power calculations for patients who present with no reliable clinical information regarding their refractive history. Additionally, results have been reported using the Scheimpflug camera system to measure corneal power in an attempt to resolve the most important potential source of error for IOL determination in these patients.

SUMMARY

IOL selection in patients undergoing cataract surgery after corneal refractive surgery continues to be a challenging and complex issue despite numerous strategies and formulas described in the literature. Current focus seems to be directed toward approaches that do not require preoperative refractive surgery information. Due to the relative dearth of comparative clinical outcomes data, the optimal solution to this ongoing clinical problem has yet to be determined. Until such data are available, many cataract surgeons compare the results of multiple formulas to assist them in IOL selection for these patients.

Intraocular lens power calculation after corneal refractive surgery

Cataract surgery after corneal refractive surgery can be challenging for the ocular surgeon due to the difficulty with accurate intraocular lens (IOL) power determination and unexpected refractive surprises. As clinicians have done more work, a number of error sources have been determined. Furthermore, an increasing number of methods to avoid these refractive surprises have been proposed. The combination of this work has resulted in recommendations for the modification of standard IOL power calculations to improve outcomes. The following article includes a brief on, and by no means, inclusive, error sources and ways to compensate for them.

Intraocular lens power calculation after previous laser refractive surgery

Methods to attempt more accurate prediction of intraocular lens power in refractive surgery eyes are many, and none has proved to be the most accurate. Until one is identified, a spreadsheet tool is available and can be used. It automatically calculates all the methods for which data are available on a single sheet for the patient's chart. The various methods and how they work are described.

Use of the pentacam true net corneal power for intraocular lens calculation in eyes after refractive corneal surgery

PURPOSE

To assess the accuracy and validity of true net corneal power of the Pentacam system to provide a keratometry reading for calculating intraocular lens (IOL) power in postoperative refractive surgery eyes.

METHODS

Refraction, an automated keratometry reading, and true net corneal power were measured for 30 eyes that required cataract surgery and had previously undergone refractive surgery. Target refraction values calculated with the SRK/T formula using true net corneal power were compared with postoperative manifest refraction values.

RESULTS

Using true net corneal power, the mean deviation from the desired postoperative cataract refractive outcome was 0.47 +/- 0.56 diopters (D); the actual refraction was within +/- 0.50 D of the intended refraction for 70% of eyes (21/30) and within +/- 1.00 D for 93% of eyes (28/30).

CONCLUSIONS

The true net corneal power can be used as a keratometry reading for eyes with previous refractive surgery requiring cataract surgery.

The comparison of central and mean true-net power (Pentacam) in calculating IOL-power after refractive surgery

PURPOSE

To compare the accuracy of central true net corneal power (cTNP) and mean true net corneal power (mTNP) of the Pentacam system to give a keratometry (K) reading for calculating IOL (intraocular lens) power in eyes following refractive surgery.

METHODS

Refraction, an automated K-reading (Km), cTNP and mTNP were measured for 15 eyes that required cataract surgery and had previously undergone refractive surgery. The difference between postoperative manifest refraction values and target refraction values calculated with the SRK/T formula using cTNP were compared with the one using mTNP.

RESULTS

The mean deviation from the desired post-cataract refractive outcome was 0.60 diopter (D)+/-0.47 (standard deviation) using cTNP; 0.75+/-0.54 using mTNP (p=0.386). The actual refraction was within +/-0.50D of the intended refraction for 60% (cTNP) and 33.3% (mTNP) of eyes, and within +/-1.00D for 93% (cTNP) and 66.7% (mTNP) of eyes.

CONCLUSIONS

Although not statistically significant, the cTNP showed better accuracy than mTNP to give a keratometry (K) reading for post-refractive surgery eyes requiring cataract surgery.

Posterior corneal curvature measurements with peripheral fitting zones before and after myopic LASIK using Orbscan II

PURPOSE

To compare pre- and postoperative posterior corneal curvature measurements in peripheral fitting zones using the Orbscan II topographer in patients undergoing myopic LASIK.

METHODS

Retrospective analysis of preoperative and 3-month postoperative Orbscan II data of 194 eyes that underwent myopic LASIK at a university eye center. Posterior corneal power was estimated using the peripheral 7- to 10-mm fitting zones. The pre- and postoperative values were analyzed and compared.

RESULTS

The mean difference in estimated pre- and postoperative power of the posterior cornea was -0.04 +/- 0.16 diopters (P < .01).

CONCLUSIONS

The difference in posterior corneal curvature measurement following myopic LASIK using the peripheral fitting zone with the Orbscan II, as compared to the preoperative values, is clinically insignificant.

Biometry and intraocular lens power calculation

PURPOSE OF REVIEW

Heightened patient expectations for precise postoperative refractive results have spurred the continued improvements in biometry and intraocular lens calculations. In order to meet these expectations, attention to proper patient selection, accurate keratometry and biometry, and appropriate intraocular lens power formula selection with optimized lens constants are required. The article reviews recent studies and advances in the field of biometry and intraocular lens power calculations.

RECENT FINDINGS

Several noncontact optical-based devices compare favorably, if not superiorly, to older ultrasonic biometric and keratometric techniques. With additional improvements in the internal acquisition algorithm, the new IOL Master software version 5 upgrade should lessen operator variability and further enhance signal acquisition. The modern Haigis-L and Holladay 2 formulas more accurately determine the position and the shape of the intraocular lens power prediction curve.

SUMMARY

Postoperative refractive results depend on the precision of multiple factors and measurements. The element with the highest variability and inaccuracy is, ultimately, going to determine the outcome. By understanding the advantages and limitations of the current technology, it is possible to consistently achieve highly accurate results.

Accuracy of Orbscan II in the Assessment of Posterior Curvature in Patients With Myopic LASIK

PURPOSE

To evaluate the accuracy of Orbscan II measurements in assessing posterior corneal curvature in patients undergoing myopic LASIK.

METHODS

Using the Orbscan II, posterior corneal curvature was assessed pre- and postoperatively in 304 eyes that underwent myopic LASIK. The radius of curvature and corneal refractive power in diopters (D) were compared using the paired sample t test.

RESULTS

The mean pre- and postoperative radius of posterior corneal curvature were 6.49 +/- 0.26 mm and 6.35 +/- 0.30 mm, respectively. Mean pre- and postoperative posterior corneal power were -6.17 +/- 0.25 D and -6.32 +/- 0.30 D, respectively, and the difference (0.14 +/- 0.14 D) was statistically significant (P < .01).

CONCLUSIONS

Although the derived value for the power of the postoperative LASIK posterior corneal surface is overestimated using the Orbscan II, this small difference may not be clinically important. Orbscan II measurements can therefore be used (with caution) to measure posterior corneal curvature in patients with myopic LASIK for the assessment of intraocular lens power based on the Gaussian optics formula.

Artisan iris-fixated toric phakic and aphakic intraocular lens implantation for the correction of astigmatic refractive error after radial keratotomy

We report 2 patients who had radial keratotomy (RK) to correct myopia. The first patient developed a postoperative hyperopic shift and cataract. Nine years post RK, she had intracapsular cataract extraction and implantation of an Artisan aphakic intraocular lens (IOL). Twenty years post RK, hyperopia and astigmatism progressed to +7.0 -5.75 x 100 with a best corrected visual acuity (BCVA) of 20/20. Due to contact lens intolerance, the Artisan aphakic IOL was exchanged for an Artisan toric aphakic IOL. Three months later, the BCVA was 20/20 with +1.0 -0.50 x 130. The second patient demonstrated residual myopic astigmatism 6 years after bilateral RK and had become contact-lens intolerant. An Artisan toric phakic IOL was implanted in both eyes. Four months later, the BCVA was 20/25 with a refraction of +0.25 -1.0 x 135 and 20/20 with a refraction of -1.0 x 40. Both patients were satisfied with the visual outcomes.

No-history method of intraocular lens power calculation for cataract surgery after myopic laser in situ keratomileusis

PURPOSE

To prospectively evaluate the no-history method for intraocular lens (IOL) power calculation in 15 cataractous eyes that had previous myopic laser in situ keratomileusis (LASIK) and for which the pre-LASIK K-readings were not available.

SETTING

Private practice, Lynwood, California, USA.

METHODS

The predicted IOL power was calculated in each case. Also calculated were the mean arithmetic and absolute IOL predictor errors, range of the prediction errors, and number of eyes in which the error was within +/-1.00 diopter (D).

RESULTS

The mean arithmetic IOL prediction error was -0.003 D +/- 0.63 (SD), and the mean absolute IOL prediction error was 0.55 +/- 0.31 D (range -0.89 to +1.05 D). Fourteen eyes (93.3%) were within +/-1.00 D. The results of the Shammas post-LASIK formula compared favorably to the results obtained with the optimized Holladay 1 (P = .42), Hoffer Q (P = .25), Haigis (P = .30), and Holladay 2 (P = .19) formulas and were better than the results obtained with the optimized SRK/T formula (P = .0005).

CONCLUSION

The no-history method is a viable alternative for IOL power calculation after myopic LASIK when the refractive surgery data are not available.

Estimation of true corneal power after keratorefractive surgery in eyes requiring cataract surgery: BESSt formula

PURPOSE

To describe a new formula, BESSt, to estimate true corneal power after keratorefractive surgery in eyes requiring cataract surgery.

SETTING

Moorfields Eye Hospital, London, United Kingdom.

METHODS

The BESSt formula, based on the Gaussian optics formula, was developed using data from 143 eyes that had keratorefractive surgery. The formula takes into account anterior and posterior corneal radii and pachymetry (Pentacam, Oculus) and does not require pre-keratorefractive surgery information. A software program was developed (BESSt Corneal Power Calculator), and corneal power was calculated in 13 eyes that had keratorefractive surgery and required cataract surgery.

RESULTS

In the eyes having phacoemulsification, target refractions calculated with the BESSt formula were statistically significantly closer to the postoperative manifest refraction (mean deviation 0.08 diopters [D] +/- 0.62 [SD]) than those calculated with other methods as follows: history technique (-0.07 +/- 1.92 D; P = .05); history technique with double-K adjustment (0.13 +/- 2.39 D; P = .05); Holladay 2 with K-values estimated with the contact lens method (-0.76 +/- 1.36 D; P = .03); Holladay 2 with K-values from Atlas topographer (Humphrey) (-0.55 +/- 0.61 D; P<.01). Using the BESSt formula, 46% of eyes were within +/-0.50 D of the intended refraction and 100% were within +/-1.00 D.

CONCLUSIONS

The BESSt formula was statistically significantly more accurate than the other techniques tested. Thus, it could significantly improve intraocular lens power calculation accuracy after keratorefractive surgery, especially when pre-refractive surgery data are unavailable.

Anterior Chamber Depth Measurement before and after Photorefractive Keratectomy

PURPOSE

To measure the anterior chamber depth (ACD) with 2 different devices before and after photorefractive keratectomy (PRK).

DESIGN

Noncomparative case series.

PARTICIPANTS

One hundred forty-three eyes of 143 patients who had undergone PRK with refractive errors ranging from -13.13 diopters (D) to +7 D (mean, -3.67+/-3.58) were analyzed.

METHODS

The ACD values preoperatively and at 1, 3, and 6 months postoperatively were measured with the Orbscan II and IOL Master. The results were analyzed using the Pearson correlation.

MAIN OUTCOME MEASURE

Anterior chamber depth.

RESULTS

The instruments showed good agreement between the measurements before and after surgery. A significant decrease between the preoperative and 1-month postoperative measurements was found in the ACD measured from the epithelium with Orbscan II (P<0.01) and IOL Master (P<0.01). A nonsignificant decrease with both IOL Master (P>0.01) and Orbscan II (P>0.01) was found between 3 and 6 months after surgery. The ACD measured from the endothelium using the Orbscan II showed a significant difference only between the 3- and 6-month follow-up data (P<0.01).

CONCLUSIONS

The 2 devices showed good agreement, and the changes detected postoperatively seem to be related not only to corneal thinning but also to anterior segment remodeling.

Intraocular lens power calculation after laser in situ keratomileusis: Aphakic refraction technique

PURPOSE

To evaluate the accuracy of a new method of intraocular lens (IOL) power calculation for eyes having cataract extraction after previous laser in situ keratomileusis (LASIK).

SETTING

Clinical private practice and ambulatory surgical center, Astoria, New York, USA.

METHODS

This retrospective study was of 12 eyes of 9 patients who presented for cataract extraction after previous LASIK. Cataract removal was performed under topical anesthesia without IOL implantation. Approximately 30 minutes later, a manifest aphakic refraction was performed. The calculation of the IOL power was obtained by using an algorithm derived from previous experience with secondary IOL implantation (Mackool algorithm). The patient then returned to the operating room for lens implantation (aphakic refraction technique).

RESULTS

The refractive error 2 weeks postoperatively, defined as the difference between the intended and actual refractive outcome, ranged from 0.50 diopter (D) of unintended hyperopia to 0.75 (D) of unintended myopia.

CONCLUSIONS

The aphakic refraction technique provided an extremely accurate postoperative refraction in eyes having cataract with IOL implantation surgery after previous LASIK. Although the pool sample was small (12 eyes) and the range of the aphakic refraction was limited (+8.50 to 12.375 D), the technique was found to be remarkably accurate.

Intraocular lens power calculation using ray tracing following excimer laser surgery

PURPOSE

To evaluate intraocular lens (IOL) power calculation using ray tracing in patients presenting with cataract after excimer laser surgery.

METHODS

Ten eyes of seven consecutive patients who presented for cataract surgery following excimer laser treatment without any pre-refractive biometry data were enrolled in this prospective clinical study. Preoperatively, IOL power calculation was performed using a ray tracing software called OKULIX. Keratometry data (C-Scan) were imported and axial length (IOLMaster) was entered manually. Accuracy of IOL power calculation was investigated by subtracting attempted and achieved spherical equivalent.

RESULTS

Mean spherical equivalent was -3.51+/-2.77 D (range -10.38 to -0.5 D) preoperatively and -1.01+/-1.08 D (range -2.5 to +0.75 D) postoperatively. Mean error was 0.31+/-0.84 D, mean absolute error was 0.74+/-0.46 D, and IOL calculation errors ranged from -1.39 to +1.47 D. A total of 40% of eyes were within +/-0.5 D, 70% within +/-1.0 D, and 100% within +/-1.5 D. Three eyes with corneal radii over 10 mm showed calculation errors exceeding +/-1.0 D. Mean best-corrected visual acuity increased from 20/60 to 20/30 postoperatively.

CONCLUSIONS

IOL power calculation after excimer laser surgery can be difficult, especially when pre-refractive keratometry values are not available. In these cases, ray tracing combined with corneal topography measurements provides reliable and satisfactory postoperative results. However, it is advisable to be careful when calculating IOL power for eyes with corneal radii exceeding 10 mm because of slightly higher prediction errors.

A New Formula for Intraocular Lens Power Calculation After Refractive Corneal Surgery

PURPOSE

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.

METHODS

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 keratotomy [n = 5]) and compared our results to other formulas.

RESULTS

Mean postoperative spherical equivalent refraction was +0.26 diopters (D) (standard deviation [SD] 0.73, range: -1.25 to +/- 1.58 D) using our formula, +2.76 D (SD 1.03, range: +0.94 to +4.47 D) using the SRK II, +1.44 D (SD 0.97, range: +0.05 to +4.01 D) with Binkhorst, 1.83 D (SD 1.00, range: -0.26 to +4.21 D) with Holladay I, and -2.04 D (SD 2.19, range: -7.29 to +1.62 D) with Rosa's method. With our formula, 60% of absolute refractive prediction errors were within 0.50 D, 80% within 1.00 D, and 93% within 1.50 D.

CONCLUSIONS

In this first series of patients, we obtained encouraging results. With a greater number of cases, all statistical adjustments related to the different types of surgery should be improved.

Intraocular Lens Power Calculation Following LASIK: Determination of the New Effective Index of Refraction

PURPOSE

To determine the new corneal effective index of refraction (rN) following LASIK to be used for accurate keratometry reading (K-reading).

METHODS

A total of 332 eyes that underwent myopic LASIK were divided into two groups (group A [n = 137] and group B [n = 1951). In each group, patients were divided into four subgroups according to the amount of spherical equivalent refraction of myopic LASIK ablation (subgroup 1 [< -4.0 D], subgroup 2 [-4.0 to < -8.0 D], subgroup 3 [-8.0 to -12.0 D], and subgroup 4 [> -12.0 D]). In each subgroup of group A, K-reading was measured by the clinical history method and the new corneal effective index (rN) was determined using paraxial formula: (K-reading = (rN-1)/Ra), where Ra is the radius of curvature of the anterior corneal surface. In group B, the anterior radius of curvature of the cornea was determined by automated K-reading, and K-reading was measured in each subgroup using the new effective index in paraxial formula, clinical history method, and automated K-reading.

RESULTS

In group A, the new effective index of refraction was 1.3355, 1.3286, 1.3237, and 1.3172 in the four subgroups, respectively. In group B, the mean K-reading measurements using rN in paraxial formula, clinical history method, and automated K-reading were: 40.33 +/- 1.68 D, 40.33 +/- 1.67 D, and 40.54 +/- 1.69 D, respectively, in subgroup 1; 37.96 +/- 1.26 D, 38.03 +/- 1.38 D, and 38.98 +/- 1.28 D, respectively, in subgroup 2; 35.77 +/- 1.75 D, 35.84 +/- 1.85 D, and 37.29 +/- 1.83 D, respectively, in subgroup 3; and 34.03 +/- 1.49 D, 34.15 +/- 1.84 D, and 36.21 +/- 1.59 D, respectively, in subgroup 4. In all subgroups of group B, the results of K-reading obtained using the new effective index of refraction were statistically similar to the results obtained by clinical history method (P > .05). Automated K-reading statistically overestimated the K-reading values in subgroups 2, 3, and 4 of group B (P < .001).

CONCLUSIONS

The use of the new corneal effective index of refraction allows for an accurate derivation of K-reading from the anterior radius of curvature.

Effect of Corneal Curvature and Corneal Thickness on the Assessment of Intraocular Pressure Using Noncontact Tonometry in Patients After Myopic LASIK Surgery

PURPOSE

To evaluate the effect of corneal curvature and corneal thickness on the assessment of intraocular pressure (IOP) using noncontact tonometry (NCT) in patients after myopic LASIK surgery.

METHODS

All patients who had myopic LASIK in a university-based eye clinic between February 2002 and May 2002 were retrospectively analyzed. Preoperative NCT was compared with postoperative NCT, postoperative corneal thickness, and postoperative corneal curvature.

RESULTS

The difference between the mean preoperative NCT (15.46 +/- 2.50 mm Hg) and postoperative NCT (6.30 +/- 1.57 mm Hg) was significant (9.16 +/- 1.96 mm Hg, P < 0.010). Preoperative NCT significantly correlated with postoperative NCT (P < 0.001), postoperative corneal thickness (P = 0.006), and postoperative anterior corneal curvature (P < 0.010).

CONCLUSIONS

Both corneal thickness and anterior corneal curvature affect IOP assessment in patients with myopic LASIK. Although correction formulas can be used to estimate the actual IOP, alternative methods should be investigated to assess IOP independent of corneal thickness and curvature.

Correcting the corneal power measurements after myopic laser in situ keratomileusis

To describe and evaluate a refraction-derived method and a clinically derived method to calculate the correct corneal power for intraocular lens (IOL) power calculations after laser in situ keratomileusis (LASIK) and to compare the results to the commonly used history-derived method. Retrospective analysis were conducted in consecutive case from clinical practice. For each patient, we established the pre-LASIK and post-LASIK spectacle refraction, the pre-LASIK (K(pre)) and post-LASIK K readings (K(post)). We then calculated the pre- and post-LASIK refraction at the cornmeal plane and the amount of correction obtained by the refraction surgery (deltaSEQco). The cases were divided into two groups. Group I was used to derive two formulas. The values obtained with the two methods were compared with the K by history-derived method (K(c.hd) in group II to validate the results. The K values calculated by using the refraction-derived method (K(c.rd)) and the K values calculated using the clinically derived method (K(c.cd)) correlated highly with K(c.hd). The correct corneal power for intraocular lens (IOL) power calculations after LASIK can use refraction-derived method and clinically derived method instead of history-derived method when some refractive parameters are not available.

Contact lens overrefraction variability in corneal power estimation after refractive surgery

PURPOSE

To evaluate the accuracy and precision of the contact lens overrefraction (CLO) method in determining corneal refractive power in post-refractive-surgery eyes.

SETTING

Refractive Surgery Service and Contact Lens Service, University of Illinois, Chicago, Illinois, USA.

METHODS

Fourteen eyes of 7 subjects who had a single myopic laser in situ keratomileusis procedure within 12 months with refractive stability were included in this prospective case series. The CLO method was compared with the historical method of predicting the corneal power using 4 different lens fitting strategies and 3 refractive pupil scan sizes (3 mm, 5 mm, and total pupil). Rigid lenses included 3 9.0 mm overall diameter lenses fit flat, steep, and an average of the 2, and a 15.0 mm diameter lens steep fit. Cycloplegic CLO was performed using the autorefractor function of the Nidek OPD-Scan ARK-10000. Results with each strategy were compared with the corneal power estimated with the historical method. The bias (mean of the difference), 95% limits of agreement, and difference versus mean plots for each strategy are presented.

RESULTS

In each subject, the CLO-estimated corneal power varied based on lens fit. On average, the bias between CLO and historical methods ranged from -0.38 to +2.42 diopters (D) and was significantly different from 0 in all but 3 strategies. Substantial variability in precision existed between fitting strategies, with the range of the 95% limits of agreement approximating 0.50 D in 2 strategies and 2.59 D in the worst-case scenario. The least precise fitting strategy was use of flat-fitting 9.0 mm diameter lenses.

CONCLUSIONS

The accuracy and precision of the CLO method of estimating corneal power in post-refractive-surgery eyes was highly variable on the basis of how rigid lense were fit. One of the most commonly used fitting strategies in clinical practice--flat-fitting a 9.0 diameter lens-resulted in the poorest accuracy and precision. Results also suggest use of large-diameter lenses may improve outcomes.

Intraoperative optical refractive biometry for intraocular lens power estimation without axial length and keratometry measurements

PURPOSE

To correlate intraoperative aphakic autorefraction to conventional emmetropic intraocular lens (IOL) calculations and derive an empiric predictive model for IOL estimation based on optical refractive biometry without axial length and keratometry measurements.

SETTING

Institutional Review Board of the University of Southern California, Los Angeles County General Hospital, Los Angeles, California, USA.

METHODS

A pilot group of 22 eyes of 22 patients scheduled for cataract surgery were enrolled in a prospective trial. All patients had a standard preoperative workup with subsequent cataract extraction and IOL implantation according to conventional biometric measurements and IOL calculations. Intraoperative autorefractive retinoscopy was used to obtain aphakic autorefraction and to measure the aphakic spherical equivalent before lens implantation. A linear regression analysis was used to correlate the aphakic spherical equivalent to the final adjusted emmetropic IOL power to empirically derive a refractive formula for IOL calculation (optical refractive biometry method). A second validation series of 16 eyes was used in a head-to-head comparison between the optical refractive biometry and the conventional IOL formulas. A subset of 6 eyes from the validation series were post-refractive cases having subsequent cataract surgery.

RESULTS

Intraoperative retinoscopic autorefraction was successfully obtained in all 22 patients in the pilot group and all 16 patients in the validation group. The spherical equivalent of the aphakic autorefraction correlated linearly with the final adjusted emmetropic IOL power (P<.0001, with adjusted r(2)=.9985). The relationship was sustained over an axial length range of 21.43 to 25.25 mm and an IOL power range of 12.0 to 25.5 diopters (D). In a subsequent validation series of 10 standard and 6 post-laser in situ keratomileusis (LASIK) cataract cases, the optical refractive biometry method proved to be a better predictive model for IOL estimation than conventional formulas; 83% of the LASIK eyes and 100% of the normal eyes were within +/-1.0 D of the final IOL power when aphakic autorefraction was used, compared with 67% of LASIK eyes and 100% of the normal eyes, using the conventional methodology.

CONCLUSIONS

A new model for IOL power calculation was derived based on an optical refractive methodology that breaks away from the conventional art introduced by Fyodorov in the 1960s. A purely refractive algorithm is used to predict the power of the IOL at the time of surgery without the need for axial length and keratometry measurements. This method bypasses some limitations of conventional biometry and shows promise in the post-refractive cataract cases.

Keratometry for Intraocular Lens Power Calculation Using Orbscan II in Eyes With Laser in situ Keratomileusis

PURPOSE

To compare the results of comeal keratometry after laser in situ keratomileusis (LASIK) obtained by the Gaussian optics formula and the clinical history method.

METHODS

Sixty-one consecutive patients (121 eyes) who had undergone LASIK were recruited in this retrospective case-controlled study. The K-value obtained from the Gaussian optics formula (CalK) based on postoperative corneal topography by Orbscan II (Bausch & Lomb, Rochester, NY) and ultrasound pachymetry was compared with that obtained from the clinical history method (estK). Keratometry measured by these two methods was compared using the paired sample t test and Pearson correlation coefficient.

RESULTS

A high correlation was noted between K-value obtained by the clinical history method and the Gaussian optics formula (R = 0.97, P < .001). The mean difference between the two methods is 0.13 diopters (P = .06).

CONCLUSIONS

K reading derived from the Gaussian optics formula correlated closely to that obtained from the clinical history method and would be especially useful in patients with no preoperative LASIK treatment data.

Topography-based intraocular lens power selection

PURPOSE

To provide mathematical tools for selecting intraocular lens (IOL) power for normal eyes and for "odd" eyes, particularly after corneal refractive surgery.

SETTING

Universitats-Augenklinik, Mainz, Germany.

METHODS

First, IOL power is selected based on the radii and numerical eccentricity of the cornea, extracted from corneal topography in a consistent numerical model of the cornea. To fine-tune the result, the visual impression is simulated by blurred Landolt rings superimposed on the retinal receptor grid. The calculation uses numerical ray tracing of the whole pseudophakic eye comprising all monochromatic errors. The error contributions of the influencing parameters, such as anterior and posterior corneal shape and corneal thickness, are quantified in detail. The method is verified in IOL power selection for normal eyes and for eyes after corneal refractive surgery.

RESULTS

The main difference between normal corneas and corneas after refractive surgery results from different asphericities. Normal corneas are prolate, with typical numerical eccentricities of 0.5, whereas corneas after laser surgery for myopia are oblate. This causes the main difference (hyperopic shift up to 2.0 diopters) in IOL power selection. Shifts in the posterior corneal radius and corneal thickness are of minor importance.

CONCLUSION

Intraocular power selection after corneal refractive surgery should be based on all the information corneal topography provides.

Comparison of intraocular lens power calculation methods in eyes that have undergone LASIK

OBJECTIVE

To compare methods of calculating intraocular lens (IOL) power for cataract surgery in eyes that have undergone myopic LASIK.

DESIGN

Noncomparative case series.

PARTICIPANTS

Eleven eyes of 8 patients who had previously undergone myopic LASIK (amount of LASIK correction [+/-standard deviation], -5.50+/-2.61 diopters [D]; range, -8.78 to -2.38 D) and subsequently phacoemulsification with implantation of the SA60AT IOLs (Alcon Surgical, Inc., Fort Worth, TX) were included (refractive error after cataract surgery, -0.61 +/- 0.79 D; range, -2.0 to 1.0 D).

METHODS

We evaluated the accuracy of various combinations of: (1) single-K versus double-K (in which pre-LASIK keratometry is used to estimate effective lens position) versions of the IOL formulas; the Feiz-Mannis method was also evaluated; (2) 4 methods for calculating corneal refractive power (clinical history, contact lens overrefraction, adjusted effective refractive power [EffRP(adj)], and Maloney methods); and (3) 4 IOL formulas (SRK/T, Hoffer Q, Holladay 1, and Holladay 2). The IOL prediction error was obtained by subtracting the IOL power calculated using various methods from the power of the implanted IOL, and the F test for variances was performed to assess the consistency of the prediction performance by different methods.

MAIN OUTCOME MEASURES

Mean arithmetic IOL prediction error, mean absolute IOL prediction error, and variance of the IOL prediction error.

RESULTS

Compared with double-K formulas, single-K formulas predicted lower IOL powers than the power implanted and would have left patients hyperopic in most cases; the Feiz-Mannis method had the largest variance. For the Hoffer Q and Holladay 1 formulas, the variances for EffRP(adj) were significantly smaller than those for the clinical history method (0.43 D2 vs. 1.74 D2, P = 0.018 for Hoffer Q; 0.75 D2 vs. 2.35 D2, P = 0.043 for Holladay 1). The Maloney method consistently underestimated the IOL power but had significantly smaller variances (0.19-0.55 D2) than those for the clinical history method (1.09-2.35 D2; P<0.015). There were no significant differences among the variances for the 4 formulas when using each corneal power calculation method.

CONCLUSIONS

The most accurate method was the combination of a double-K formula and corneal values derived from EffRP(adj). The variances in IOL prediction error were smaller with the Maloney and EffRP(adj) methods, and we propose a modified Maloney method and second method using Humphrey data for further evaluation.

New formula for calculating intraocular lens power after laser in situ keratomileusis

PURPOSE

To assess the validity and accuracy of a proposed formula for keratometry (K) readings after laser in situ keratomileusis (LASIK).

SETTING

The Eye Center and the Eye Foundation for Research, Riyadh, Saudi Arabia.

METHOD

This studied comprised 34 eyes that had LASIK surgery. Refraction and an automated K-reading (auto-K) were performed preoperatively. Refraction, auto-K, and K-reading assessment by the clinical history method and the proposed formula were performed 4 to 12 weeks postoperatively. The proposed formula is K(postop) = K(preop) - [(N(c) - 1) x (R(a-postop) - R(a-preop))/(R(a-postop) x R(a-preop))], where K(postop) is the K-reading after LASIK, K(preop) is the K-reading before LASIK, N(c) is the index of refraction of the cornea (1.376), R(a-postop) is the radius of curvature of the anterior corneal surface after LASIK, and R(a-preop) is the radius of curvature of the anterior corneal surface before LASIK.

RESULTS

Twenty patients (10 men, 10 women) were included in the study. The mean age of the patients was 30.58 years +/- 17.68 (SD) (range 18 to 44 years). Preoperatively, the mean spherical equivalent (SE) was -4.99 +/- 2.82 diopters (D) (range -1.12 to -15.00 D), the mean R(a) was 7.76 +/- 0.32 mm (range 7.33 to 8.50 mm), and the mean auto-K reading was 43.45 +/- 1.73 D (range 39.62 to 46.00 D). Postoperatively, the mean SE was +0.02 +/- 0.63 D (range -2.75 to +1.00 D), the mean R(a) was 8.63 +/- 0.53 mm (range 7.80 to 9.92 mm), and the mean K-reading assessed by auto-K, clinical history method, and the proposed formula was 39.17 +/- 2.35 D (range 34.00 to 43.25 D), 38.79 +/- 2.52 D (range 33.1 to 42.78 D), and 38.69 +/- 2.51 D (range 33.1 to 43.0 D), respectively. The results obtained by the proposed formula were similar to those obtained by the clinical history method (P =.098). Auto-K readings significantly overestimated the K-values (P<.0001) when compared to the proposed formula and clinical history method.

CONCLUSION

The proposed formula was simple, objective, not dependent on refraction, and as accurate as the clinical history method in determining K-readings after LASIK.

Intraocular lens power calculation after corneal refractive surgery

PURPOSE OF REVIEW

Keratorefractive procedures designed to decrease refractive errors have gained enormous popularity among ophthalmologists and patients. As the post-refractive surgery patient population ages, visually significant cataracts will develop. With advances in techniques for cataract extraction and intraocular lens implantation, cataract surgery has evolved into a refractive surgical procedure as well as an operation to improve best corrected visual acuity. This raises expectations in terms of desired postoperative refractive status and uncorrected visual acuity. Although performing modern cataract surgery in post-refractive surgery eyes is technically no more complicated than operating on virgin eyes, the calculation of intraocular lens power for a desired refractive target can be challenging and complicated. This has become increasingly apparent as case reports of "refractive surprises" after cataract surgery appear in the literature more frequently.

RECENT FINDINGS

This paper reviews the current clinical experience with intraocular lens power determination after cataract surgery in post-keratorefractive patients, provides an overview of possible sources of error in intraocular lens power calculation in these patients, and analyzes methods to minimize intraocular lens power errors.

SUMMARY

The clinical and routine methods of intraocular lens power determination after keratorefractive surgery need to be modified to improve accuracy. Our knowledge of this subject is still evolving. Given the enormous impact of this problem on clinical practice, awareness of the shortcomings and suggested methods to improve accuracy can be valuable to clinicians.

Difficult lens power calculations

PURPOSE OF REVIEW

Although cataract extraction seems to be feasible without major technical obstacles, the surgical technique has changed completely, and patients are no longer satisfied with good spectacle-corrected vision but anticipate complete visual rehabilitation after cataract surgery, without correction. To fulfill this desire, toric or accommodative intraocular lenses are of increasing popularity, and the intraocular lens power calculation after keratorefractive surgery has been improved.

RECENT FINDINGS

In this review article, we provide an overview of different mathematical strategies of calculating the intraocular lens power with standard formulas and with new algorithms, such as paraxial or numeric ray-tracing. These enhanced techniques may improve the validity of lens power calculation due to reduction of the prediction error, especially in cases with high or excessive corneal astigmatism and after refractive laser surgery. Furthermore, a new calculation scheme for the determination of bitoric eikonic intraocular lenses allows a distortion-free imaging in astigmatic eyes. The biometric determinants for the different formulas and calculation schemes are discussed in detail.

SUMMARY

In difficult cases, standard calculation schemes are overemployed and new mathematical algorithms are necessary to adequately address these problems. Ray-tracing algorithms and other complex mathematical computation schemes are of increasing interest and will more and more replace conventional calculation formulas for determination of intraocular lens power.

Correcting the corneal power measurements for intraocular lens power calculations after myopic laser in situ keratomileusis

PURPOSE

To describe and evaluate a refraction-derived method and a clinically derived method to calculate the correct corneal power for intraocular lens (IOL) power calculations after laser in situ keratomileusis (LASIK) and to compare the results to the commonly used history-derived method.

DESIGN

Interventional case series.

METHODS

Retrospective analysis of consecutive cases from clinical practice. Two hundred randomly selected eyes from 200 patients were evaluated before and after LASIK surgery. For each patient, we established the pre-LASIK and post-LASIK spectacle refraction, the pre-LASIK (Kpre) and post-LASIK K readings (Kpost). We then calculated for each case the pre- and post-LASIK refraction at the corneal plane and the amount of correction obtained by the refractive surgery (CRc). The cases were divided into two groups. Group I was used to derive the two formulas. The K values were calculated using the history-derived method (Kc.hd) in which Kc.hd = Kpre - CRc. Kc.hd was compared with Kpost. The average difference was 0.23 diopters for every diopter of myopia corrected. This value was used to calculate the corneal power using the refraction-derived method (Kc.rd) where Kc.rd = Kpost -0.23CRc. A regression equation was used to develop a clinically derived method (Kc.cd) where Kc.cd = 1.14Kpost -6.8. The values obtained with the two methods were compared with the Kc.hd values in group II to validate the results.

RESULTS

Both Kc.rd and Kc.cd values correlated highly with Kc.hd when plotted on a scattergram (P <.001), and there was no statistically significant difference between the mean keratometric values (P >.5).

CONCLUSIONS

The corneal power measurements for intraocular lens power calculations after LASIK need to be corrected to avoid hypermetropia after cataract surgery by either the history-derived method, the refraction-derived method, or the clinically derived method.

Intraocular lens power calculation after refractive surgery

PURPOSE

To analyze the results of phacoemulsification cataract surgery in eyes that had had refractive surgery and to compare the predictability of various methods of intraocular lens (IOL) power calculation.

SETTING

Instituto de la Visión, Buenos Aires, Argentina.

METHODS

The study involved 7 cases that had phacoemulsification after radial keratotomy or laser in situ keratomileusis. The spherical equivalent (SE) and visual acuity were evaluated preoperatively and postoperatively to assess the changes before cataract development. The IOL power calculated with conventional keratometry (CK), adjusted keratometry, the clinical history method (CHM), corneal topography (CT), and the contact lens method (CLM) was compared with the final refractive and keratometric results measured with the BackCalcs (Holladay(R) IOL Consultant Program, Holladay Consulting, Inc.) to assess the accuracy and predictability of each method.

RESULTS

The mean SE was -4.82 diopters (D) +/- 5.13 (SD) before phacoemulsification and +0.19 +/- 1.01 D after phacoemulsification, and the mean best corrected visual acuity was 0.39 +/- 0.07 (20/50) and 0.80 +/- 0.06 (20/25), respectively.

CONCLUSIONS

Post-phacoemulsification refraction in cases with previous refractive surgery appeared to be predictable when the appropriate calculation method was applied. When all the data were available, the CHM provided the best results. Adjusted keratometry and CT seemed to be more accurate than CK and the CLM.

Analysis of intraocular lens power calculation in post-radial keratotomy eyes

PURPOSE

To determine whether refractive complications can be prevented by applying the currently most accurate method of intraocular lens (IOL) power calculation in the post-radial keratotomy (RK) patient.

SETTING

Department of Ophthalmology, University of California, Davis, Sacramento, and American Eye Institute, Cedars Sinai Medical Center, Los Angeles, California, USA.

METHODS

Twenty-four eyes having cataract surgery after RK were studied retrospectively for the final postcataract refraction and for the target refraction used in selecting the IOL. Nine of the eyes were further studied for the keratometry (K) values obtained with different methods and for the theoretical postoperative refraction with an IOL aiming for plano or -1.50 diopters (D) based on the known flatter calculated K, axial length, power of the implanted IOL, and refraction after cataract surgery.

RESULTS

Implantation of an IOL aiming for plano in the 24 post-RK eyes would have resulted in a hyperopic refraction in 83.4% cases. The choice of an IOL targeted at myopia reduced the frequency of hyperopia to 42.0% (24 cases). Selection of an IOL calculated with a flatter calculated K and aiming for plano decreased the frequency of hyperopia to 44.4%; however, aiming for -1.50 D still resulted in hyperopia in 44.4% of eyes (9 cases).

CONCLUSIONS

Unintentional hyperopia can be significantly decreased but not eliminated as a complication of post-RK cataract surgery. The accuracy of the IOL power determination can be improved if myopia is targeted as the postcataract surgery refractive error and the flatter calculated K is used in the IOL determination.

[Pitfalls of IOL power prediction after photorefractive keratectomy for high myopia -- case report, practical recommendations and literature review]

BACKGROUND AND PURPOSE

Published experience with eyes after keratorefractive correction of myopia indicates that insertion of the average keratometric readings into standard IOL power predictive formulas will frequently result in substantial undercorrection and postoperative hyperopic refraction or anisometropia after cataract surgery depending on the amount of myopia corrected previously. The purpose of this paper is to discuss the accentuated differences of various approaches to minimize IOL power miscalculations by describing a case report of a patient with excessive myopia as well as a review of the literature.

PATIENT AND METHODS

A 50-year old lady presented for cataract surgery on her left eye after having PRK seven years ago elsewhere (refraction - 25.5 - 3.0/20 degrees, central keratometric power 43.0 diopters [D]). Central power before cataract extraction was measured to be 35.5 D (Zeiss Keratometer) and 36.5 D (TMS-1 topography analysis) and refraction was - 3.0 D (before onset of index myopia). Orbscan slit scanning topography analysis displayed an anterior surface power of 36.8 D and a posterior surface power of - 9.3 D. Total axial length was 31.93 mm (optical biometry using Zeiss IOL-Master). The contralateral eye after PRK suffering from a comparable excessive myopia had required an exchange of the IOL implant because of intolerable anisohyperopia of + 6.0 D after primary cataract extraction elsewhere.

RESULTS

Corrected corneal power values for the left eye were calculated as follows: (1) spherical equivalent (SEQ) change at spectacle plane 19.0 D, (2) SEQ change at corneal plane 26.2 D, (3) separate consideration of anterior and posterior curvature 27.5 D, (4) consideration of the IOL power misprediction on the fellow eye 29.5 D, (5) subtraction of 24 % of the SEQ change at the spectacle plane from the actually measured keratometry value 29.7 D, (6) clinical estimate from regression analysis performed earlier 30.5 D, (7) change of anterior surface power 34.5 D. Deciding for a presumably "real" corneal power of 28.0 D the Haigis formula was used to aim for - 2.0 D since the patient preferred to read uncorrected. Thus, a 21.0 D IOL was implanted uneventfully in the capsular bag. The stable refraction postoperatively was - 3.5 - 1.0/20 degrees and visual acuity increased to 20/30. Therefore, the "real" power of that cornea must have been around 30 D.

CONCLUSIONS

After corneal refractive surgery, various techniques to determine the current corneal power should be compared and the value around which results tend to cluster should be relied on to avoid hyperopia after cataract surgery with lens implantation. In those cases where keratometry and refraction before PRK/LASIK are available, the gold standard is still to subtract the change of the SEQ at the corneal plane from the preoperative central keratometric power, although in the present case report the subtraction of 24 % of the SEQ change at the spectacle plane from the measured corneal power value seemed to produce the best result. Pure subtraction of the SEQ change at the spectacle plane from the corneal power value before refractive surgery has to be avoided in eyes with excessive myopia. The most reliable corrected power value should be inserted in more than one modern third-generation formula (such as Haigis, Hoffer Q, Holladay 2, SRK/T) and the highest power IOL should be implanted. In all instances, the cataract surgeon has to make sure that the corrected K-reading is not wrongly re-converted within the IOL power calculation formula used.

Measuring corneal power for intraocular lens power calculation after refractive surgery. Comparison of methods

PURPOSE

To find a more accurate and predictable method for intraocular lens (IOL) power calculation in eyes after refractive surgery.

SETTING

Department of Ophthalmology, Kangnam St. Mary's Hospital, Seoul, Korea.

METHODS

The accuracy of the following methods for calculating IOL power in 132 eyes after PRK or LASIK was compared: manual keratometry, hard contact lens, refraction-derived keratometry at the corneal plane, and the refraction-derived keratometry at the spectacle plane. Based on this comparison, the IOL power was calculated in the 2 eyes of a patient using refraction-derived keratometry at the spectacle plane with the SRK II formula. Cataract surgery with IOL implantation was then performed.

RESULTS

The largest corneal power values were obtained using a manual keratometer and the smallest using refraction-derived keratometry at the spectacle plane (P <.001). In the patient having cataract surgery with IOL implantation, near target refraction was achieved with minimal error in IOL power.

CONCLUSIONS

If the corneal power is known before refractive surgery, the use of the smallest value of those obtained using refraction-derived keratometry and the hard contact lens method is recommended. However, if the corneal power before refractive surgery is unknown, the use of the hard contact lens method is recommended.

A New Method of Calculating Intraocular Lens Power After Photorefractive Keratectomy

PURPOSE

To find a method of calculating intraocular lens (IOL) power that may be independent of preoperative data, in eyes that have developed a cataract after refractive surgery.

METHODS

Prior to and 1 month after PRK, the SRK/T formula was used to calculate IOL power in 88 eyes of 65 patients with a preoperative spherical equivalent refraction between -16.25 to +0.25 D (mean -5.39 +/- 3.19 D). IOL power was calculated by utilizing the spherical equivalent refraction as target both before and after PRK. Utilizing the postoperative corneal radius measurement (R2), an underestimation of the IOL power was found. For this reason, the mean postoperative corneal radius (R3) that gave the same IOL power found before surgery was calculated for each patient. The R3/R2 ratios were plotted against the axial eye length and a linear regression formula was used to calculate R2 correcting factors that gave the new corneal radius (R4). Patients were divided into classes according to axial eye length, and the mean R3/R2 ratios for each class were calculated and used to recalculate the new mean radius (R5). IOL power for emmetropia was calculated in all patients by utilization of R3, R4, R5, the historical method, and the "true corneal power" method.

RESULTS

Within +/-0.50 D from the IOL power calculated with R3, R4 gave 35 (39.3%) IOLs, while R5 gave 40 (45.5%) IOLs; the clinical history method gave 24 (27.3%) IOLs and "true corneal power" gave 23 (26.1%) IOLs, with a statistically significant difference P<.001).

CONCLUSIONS

Our theoretical method, based on correlation between axial eye length and corneal radius correcting factors, may represent an effective method of calculating IOL power after PRK, especially if the history of the patient is unknown.

Intraocular lens power calculations after laser in situ keratomileusis

PURPOSE

To compare the accuracy of several techniques for calculating intraocular lens (IOL) power after laser in situ keratomileusis (LASIK).

METHODS

Retrospective review of 10 eyes from nine patients undergoing phacoemulsification after LASIK. Corneal power (K) was measured by manual keratometry (MK), refractive history (RH), contact lens overrefraction (CTL), videokeratography (VK), and an average of the refractive history and contact lens methods (AVG 2). Results were compared with the back-calculated K value generated by the Holladay IOL Consultant program. Age-matched patients undergoing phacoemulsification without previous refractive surgery served as controls.

RESULTS

Mean spherical equivalent postoperative refraction was +0.21 diopter (D) (SD, 1.54; range, -2.25 to +2.25 D) for patients undergoing cataract extraction after LASIK versus -0.56 D (SD, 0.66; range, -2.375 to +0.5 D; p= 0.16) for controls. Thirty percent of cases versus 90% of controls were within 1 D ( p= 0.002) of emmetropia. Forty percent of cases versus no controls were more than 1 D hyperopic ( p= 0.08). The mean differences for each method compared with the back-calculated K values were MK, +0.82 D; VK, +1.24 D; RH, -0.76 D; CTL, +0.91 D; AVG 2, +0.08 D. The mean absolute deviations from the back-calculated K values were MK, 1.91 D; VK, 2.01 D; RH, 1.68 D; CTL, 1.62 D; AVG 2, 1.42 D.

CONCLUSION

Significant refractive errors occurred with each of the methods investigated for determining IOL power after LASIK. RH, CL, or AVG 2 provided the most accurate results.

Intraocular lens calculation for cataract after previous radial keratotomy

PURPOSE

The purpose of this study is to discuss the major reasons for the hyperopic shift after cataract surgery following radial keratotomy (RK) and to find potential methods to improve the prediction of intraocular lens power (IOLP).

METHODS

The results for 18 cataract surgery eyes that had undergone RK were analysed retrospectively. The IOLP was calculated using Haigis, Hoffer Q, and SRK/II formulae with different keratometric measurements. Theoretical IOLP miscalculation ((delta)IOLP(K)) was assessed in order to define the optimal combination of different keratometric parameters and intraocular lens prediction formula.

RESULTS

The research revealed a significant temporary hyperopic shift after cataract surgery, the maximum value of which was noted in the first week post-operatively. Standard keratometers and placido-topography systems overestimate comeal power after RK. Corneal power overestimation correlates significantly with the spherical equivalent change after RK. Theoretical formulae (Haigis, Hoffer Q) produced the lowest (delta)IOLP(K) using calculated corneal powers derived from spherical equivalent change.

CONCLUSIONS

Standard autokeratometry significantly overestimates corneal power after RK performed for moderate and high myopia. The use of calculated corneal power and third-generation theoretical formulae seems to be the most appropriate for IOLP prediction.

Methods of estimating corneal refractive power after hyperopic laser in situ keratomileusis

PURPOSE

To evaluate and compare methods of estimating corneal refractive power after hyperopic laser in situ keratomileusis (LASIK).

SETTING

Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA.

METHODS

Using the clinical history method (HisRP) as the standard, the accuracy of values of corneal refractive power derived from the EyeSys Corneal Analysis System or Humphrey Atlas computerized videokeratography, the thin lens formula, and corneal topographic measurements modified according to the amount of LASIK-induced refractive change were evaluated.

RESULTS

Thirty-four eyes of 19 patients were evaluated using EyeSys and 27 eyes of 16 patients were examined using Humphrey Atlas. Although the values derived from corneal topography and the thin lens formula correlated well with HisRP, the differences between HisRP and topographic values increased significantly with increasing hyperopic correction. When the values of corneal topographic measurements were modified according to the amount of LASIK-induced refractive change, the accuracy was significantly improved (all P <.05) and the percentages of eyes within 0.5 diopter (D) and 1.0 D of the HisRP values exceeded 71% and 94%, respectively.

CONCLUSION

Using the clinical history method as the standard, the most accurate method for determining corneal refractive power in hyperopic LASIK eyes was to adjust the postoperative corneal topographic measurement according to the amount of LASIK-induced refractive change.

A comparative analysis of five methods of determining corneal refractive power in eyes that have undergone myopic laser in situ keratomileusis

OBJECTIVE

To evaluate the accuracy of computerized videokeratography, keratometry, and the Gaussian optics formula for measuring corneal refractive power in patients after myopic laser in situ keratomileusis (LASIK).

DESIGN

Noncomparative case series.

PARTICIPANTS

One hundred eyes of 63 patients (mean age, 45.0 +/- 10.9 [standard deviation] years) who underwent LASIK were included in the study.

METHODS

Using the clinical history method as the standard, we evaluated the accuracy of values of corneal refractive power derived from computerized videokeratography (the EffRP value of the EyeSys Corneal Analysis System, which averages corneal refractive power over the central 3 mm), keratometry (K), the Gaussian optics formula (GauRP), and values of EffRP and keratometry as modified according to the amount of LASIK-induced refractive change.

MAIN OUTCOME MEASURES

Correlation of measured corneal power values to those obtained using clinical history method (HisRP).

RESULTS

Although the values for HisRP were significantly correlated with postoperative EffRP and K values and with GauRP, postoperative EffRP and K values were higher than HisRP (0.87 +/- 0.68 diopters [D] and 1.16 +/- 1.10 D, respectively), and GauRP were lower than HisRP (0.44 +/- 0.66 D) (P < 0.001 for all three comparisons). The differences between HisRP and both postoperative EffRP and K values increased significantly with the amount of myopic correction. The most accurate results were obtained by modifying the postoperative values of EffRP according to the amount of LASIK-induced refractive change; 70% of these values were within +/- 0.5 D and 94% within +/- 1 D of HisRP values.

CONCLUSIONS

Using the clinical history method as the standard, we found that the most accurate method for determining corneal refractive power in post-LASIK eyes was to adjust the postoperative corneal measurement according to the amount of LASIK-induced refractive change.