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Refractive Lens Surgery Mark Packer, MDT, I Ophthalmol Clin N Am 19 (2006) 77 – 88 Refractive Lens Surgery Mark Packer, MDT, I. Howard Fine, MD, Richard S. Hoffman, MD Drs. Fine, Hoffman & Packer, 1550 Oak Street, Suite 5, Eugene OR 97401, USA Axial length measurement remains an indispens- IOLMaster (Zeiss Humphrey Systems, Jena, Ger- able technique for intraocular lens (IOL) power many) with measurements obtained by immersion calculation. Recently, partial coherence interferome- ultrasound using the Axis II (Quantel Medical, try has emerged as a new modality for biometry [1]. Clermont-Ferrand, France). The postoperative refrac- The postoperative results achieved with this mo- tions of patients undergoing cataract extraction with dality have been considered analogous to those posterior chamber IOL implantation were also exam- achieved with the ultrasound immersion technique [2]. ined to determine the accuracy of the immersion Although reportedly ‘‘user-friendly’’ and less depen- ultrasound technique. dent on technician expertise than are ultrasound Fifty cataractous eyes underwent preoperative methods, noncontact optical biometry is limited by axial length measurement with both the Axis II and dense media such as posterior subcapsular cataract. A IOLMaster. For the Axis II immersion technique, the second limitation of the optical method is the lack of Praeger shell was employed. Patients were placed in a a lens thickness measurement, which is a required sitting position in an examination room chair with the variable in the Holladay II IOL power calculation head reclined gently against the headrest. The average software, version 2.30.9705. According to Holladay, ‘‘Total Length’’ reported by the unit was entered into the lens thickness can be estimated by the formula, the Holladay II IOL power calculation formula. For 4.0 + (age/100). Also, optical biometry can provide the IOLMaster, the selected axial length with the keratometry measurements, obviating the need for a highest signal-to-noise ratio was used as the basis for second instrument. comparison. The measured axial lengths were plotted Immersion ultrasound has long been recognized and a linear regression trendline fit to the data. The as an accurate method of axial length measurement Pearson correlation coefficient was determined to and is generally considered superior to applanation assess the relationship between the immersion and ultrasound techniques [3,4]. The absence of corneal the optical measurementsP according to the formula, depression as a confounding factor in measurement r = 1/(1 À n) ((x À m)/s)((y À m)/s). reduces the risk of intertechnician variability in Keratometry was performed with the IOLMaster. technique. In addition to having a short learning The three reported sets of values were compared for curve, immersion ultrasound has no limitations in consistency and were correlated with the axis and terms of media density and measurement capability. magnitude of the eye’s preoperative astigmatism. An On the other hand, optical biometry may be superior averaged value of three measurements or of the two in eyes with posterior staphyloma because of more closest measurements (in case one measurement precise localization of the fovea. appeared to be an outlier) was entered into the formula. The authors have compared axial length mea- In selected cases, autokeratometry (HARK 599, Zeiss surements obtained by optical biometry using the Humphrey Systems) or computerized corneal topog- raphy (EyeSys Technologies, Houston, Texas) was used to better delineate the preoperative keratometry. T Corresponding author. The corneal white-to-white diameter was determined E-mail address: [email protected] (M. Packer). with the Holladay-Godwin Corneal Gauge. 0896-1549/06/$ – see front matter D 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.ohc.2005.09.005 ophthalmology.theclinics.com 78 packer et al One surgeon (IHF) performed all of the surgery. the axial lengths measured by immersion was The Holladay II IOL power calculation formula was 23.40 mm (range, 21.03 to 25.42 mm), whereas the used to select the IOL for implantation in each case. mean of the optically measured axial lengths was This program automatically personalized the sur- 23.41 mm (range, 21.13 to 25.26 mm). Technicians geon’s A-constant during the course of the study. noted that immersion measurements required 5 min- To provide uniform results, the Collamer IOL utes, whereas optical measurements required about (CC4204BF, Staar Surgical, Monrovia, California) 1 minute. was implanted in all 50 eyes. The surgical technique has been described previously [5]. Briefly, a temporal clear corneal incision is followed by continuous curvilinear capsulorrhexis, cortical cleaving hydro- Surgical outcomes assessment dissection and hydrodelineation, and nuclear dis- assembly using horizontal chopping with high The Holladay IOL Consultant report reflected a vacuum and flow but low levels of ultrasound energy. personalized A-constant of 119.365 (ACD, 5.512) The IOL is inserted into the capsular bag via an compared with the manufacturer’s suggested constant injection device. of 119.0 (ACD, 5.55). The frequency distribution of All patients underwent autorefractometry the postoperative spherical equivalent prediction error (HARK 599) and subjective manifest refraction revealed that 48% of eyes precisely achieved the 2 to 3 weeks postoperatively. Only eyes obtaining targeted refraction. The cumulative distribution graph 20/30 or better best-corrected visual acuity were demonstrated that 92% of eyes measured within included in the study. The postoperative refraction ±0.5 D of the targeted refraction and 100% of eyes was then entered into the Holladay IOL Consultant measured within ±1.00 D of the targeted refraction (Holladay Consulting, Bellaire, Texas). Using the (Fig. 2). The mean absolute error measured 0.215 D, Surgical Outcomes Assessment Program (SOAP), the whereas the mean error of À0.105 reflected the trend spherical equivalent prediction error was measured toward myopia. and analyzed. The near perfect correlation of immersion ultra- sound and optical coherence biometry measurement techniques indicates the high level of accuracy of both of these methodologies. The high rate of Axial length measurements achieving the targeted refraction by using immersion ultrasound measurements and the Holladay II formula The axial length measurements obtained with the compares favorably with previously reported results. Axis II and the IOLMaster correlated highly (Pearson For example, Haigis achieved accurate prediction correlation coefficient, 0.996) (Fig. 1). The mean of within ±1.00 D in 85.7% of eyes by using immer- 25.5 25 24.5 24 23.5 23 IOLMaster 22.5 22 21.5 21 21 21.5 22 22.5 23 23.5 24 24.5 25 25.5 Axis II Fig. 1. Comparison of axial length measurements with immersion ultrasound (abscissa) and optical coherence interferometry (ordinate). The linear regression trendline reflects the high correlation between the two sets of values. refractive lens surgery 79 Fig. 2. Holladay IOL Consultant, Surgical Outcomes Assessment Program. sion ultrasound [2]. Additionally, Sanders and co- suring four sample points to determine the steepest workers [6] have indicated that achievement of about and flattest meridians of the cornea, yielding accurate 90% of eyes within ±1.00 D of the targeted refraction values for the central corneal power. In irregular and a mean absolute error of approximately 0.5 D corneas, such as those having undergone radial kera- represents an acceptable outcome. totomy, laser thermal keratoplasty, hexagonal kera- Technicians report that the immersion ultrasound totomy, penetrating keratoplasty, photorefractive method with the Praeger shell is well tolerated by keratectomy, or laser in site keratomileusis (LASIK), patients and relatively easy to learn. Its applicability the four sample points are not sufficient to provide to all types of cataracts and its ability to generate a an accurate estimate of the center corneal refractive phakic lens thickness represent significant advan- power [11]. Traditionally, three methods have been tages, especially for surgeons who use the Holladay II used to calculate the corneal refractive in these eyes calculation formula. [12]. These approaches include the historical method, the hard contact lens method, and values derived from standard keratometry or corneal topography. The historical method remains limited by its reliance Keratometry after keratorefractive surgery on the availability of refractive data before the keratorefractive surgery. The contact lens method is IOL power calculations for cataract and refractive not applicable in patients with significantly reduced lens exchange surgery have become more precise visual acuity [13]. Use of simulated or actual kera- with the current theoretical generation of formulas tometry values almost invariably leads to a hyperopic and newer biometry devices [7]. Nevertheless, IOL refractive surprise [14]. power calculation remains a challenge in eyes with It has been suggested that using the average prior keratorefractive surgery. The difficulty in these central corneal power rather than topography-derived cases lies in determining accurately the corneal keratometry may offer improved accuracy in IOL refractive power [8–10]. power calculation following corneal refractive sur- In a normal cornea, standard keratometry and gery [15]. The Effective Refractive Power (Eff RP) computed corneal topography are accurate in mea- (Holladay Diagnostic Summary, EyeSys Topogra- 80 packer et
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