Effective Ocular Biometry and Intraocular Lens Power Calculation

European Ophthalmic Review, 2016;10(2):94–100 DOI: https://doi.org/10.17925/EOR.2016.10.02.94

Abstract:

Since the introduction of phacoemulsification, cataract surgery has evolved remarkably. The use of premium intraocular lenses (IOLs) (aspheric, toric, multifocal), refractive lens exchange and patients after refractive surgery procedures require extremely precise clinical measurements and IOL calculation formulas to achieve desired postoperative refraction. For many years, ultrasound biometry has been the standard for measurement of ocular parameters. The introduction of optical biometry (fast and non-invasive) has replaced ultrasound methods and is now considered as the clinical standard for ocular biometry. Recently, several modern optical instruments have been commercially launched and there are new methods available, including the empirical, analytical, numerical or combined methods to determine IOL power. The aim of this review is to present current techniques of ocular biometry and IOL power calculation formulas, which will contribute to achieve highly accurate refractive outcomes.
Keywords: Biometry, ocular biometry, optical biometry, optical biometry devices, intraocular lenses, IOLs, IOL power calculation, IOL power calculation formulas
Disclosure: Magdalena Turczynowska, Katarzyna Koźlik-Nowakowska, Magdalena Gaca-Wysocka and Andrzej Grzybowski have nothing to disclose in relation to this article. No funding was received in the publication of this article. This study involves a review of the literature and did not involve any studies with human or animal subjects performed by any of the authors.
Authorship:-All named authors meet the International Committee of Medical Journal Editors (ICMJE) criteria for authorship of this manuscript, take responsibility for the integrity of the work as a whole, and have given final approval to the version to be published.
Received: August 16, 2016 Accepted September 18, 2016
Correspondence: Magdalena Turczynowska, Department of Ophthalmology, Stefan Żeromski Specialist Municipal Hospital in Kraków, os. Na Skarpie 66, 31-913 Kraków, Poland. E: m.turczynowska@gmail.com
Open Access: This article is published under the Creative Commons Attribution Noncommercial License, which permits any non-commercial use, distribution, adaptation and reproduction provided the original author(s) and source are given appropriate credit.

An erratum to this article can be accessed by the link below.

Cataract surgery is currently the most frequently performed surgical technique worldwide. Since the introduction of phacoemulsification by Kelman in 1967, surgical technology and construction of implanted intraocular lenses (IOLs) have undergone considerable improvement. Small, sutureless incisions and the use of foldable intraocular lenses reduced the incidence of complications and surgically induced astigmatism.1,2 Furthermore, the use of premium intraocular lenses (aspheric, toric, multifocal or a combination) allows the patient to become fully spectacle-independent.3 The improvement of surgical treatment results in rising expectations of patients. The key issue is to achieve the desired refractive outcome. Essential for this purpose are precise measurements of the eye and selection of the optimal IOL calculation formula. The aim of this article is to present current techniques of ocular biometry and IOL power calculation formulas, which will contribute to achieve highly accurate refractive outcomes.

Ocular biometry
The first step to achieve satisfactory postoperative refractive outcome is accurate ocular biometry. Biometry enables the measurement of the various dimensions of the eye, including axial length (AL), anterior chamber depth (ACD), lens thickness (LT) or central corneal thickness (CCT). These values, together with the keratometry are essential for the IOL power calculation. Precision of measurements is crucial, as a 0.1 mm error in AL results in a refractive error of about 0.27 diopter (D).4

Ultrasound biometry For many years, the only way to measure the AL of the eye was with ultrasound biometry. This technique measures the distance from the surface of the corneal apex to the internal limiting membrane (ILM). Good alignment along the ocular axis is important and that requires patient cooperation (which can be difficult in children or patients with mental disorders). In cases where a probe has direct contact with the cornea, there is a risk of a corneal damage or infection. Therefore, a topical anaesthetic and proper disinfection of the probe are required. Occurring inter-individual differences are highly dependent on the pressure exerted on the eye by the ultrasound probe. High pressure results in corneal indentation and shortening of the AL. Immersion ultrasound minimises the indentation of the cornea as it uses a saline-filled shell between the probe and the eye. Clinical studies have shown that immersion biometry is more accurate and more reliable than ultrasound biometry performed in contact mode.5–8 A limitation of ultrasound biometry is low image resolution, as a consequence of using a long, low-resolution wavelength (10 MHz) to measure small dimensions. In addition, differences in retinal thickness near the fovea or the presence of other macular pathologies contribute to inconsistent measurements.9,10

by immersion ultrasound biometry,11 but this new method is fast, easy to reproduce by different examiners, non-invasive and non-contact. Repeatability and reproducibility of measurements obtained using this technique are high and the results are less dependent on operators’ skills. However, it is difficult to obtain a measurement in the presence of a dense cataract or other opacities such as corneal scar and vitreous haemorrhage. Optical biometry measures the distance from the corneal surface to the retinal pigment epithelium (RPE). It may be associated with overestimation of measurements of about 0.15–0.5 mm.12 Optical biometry can also be successfully performed in pseudophakic or silicone oil-filled eyes. Furthermore, in high myopic eyes, due to the presence of posterior staphyloma, it may give better results than conventional ultrasound techniques for measuring the AL.

Optical biometry devices
New optical biometry devices provide measurements not only of AL but also other important variables, such as: keratometry, ACD, LT, CCT, pupil size (PS) or white-to-white distance (WTW). To measure the AL of the eye, currently available devices use different technologies. IOLMaster 500 (Carl Zeiss Meditec, Jena, Germany), AL-Scan (Nidek, Aichi, Japan) and Pentacam AXL (Oculus, Menlo Park, California, US) use partial coherence interferometry (PCI) technology. Lenstar LS 900 (Haag-Streit, Koeniz, Switzerland), Aladdin (Topcon, Tokyo, Japan), Galilei G6 (Ziemer, Port, Switzerland) and OA-2000 (Tomey GmbH, Nürnberg, Germany) use optical low-coherence interferometry (OLCR). Swept source OCT (ss-OCT), used by the IOLMaster 700 (Carl Zeiss Meditec, Jena, Germany) and ARGOS (Movu, Santa Clara, California, US) devices, is the newest technology to be implemented in biometry.

The IOLMaster 500 was the first optical biometer and was introduced in autumn 1999. The device is based on the PCI principle and measures AL using infrared light (λ=780 nm) of short coherence emitted by semiconductor laser diode. Furthermore, it measures keratometry, analysing the anterior corneal curvature at six reference points at approximately 2.3 mm optical zone. The ACD is measured using slit-lamp illumination and is defined as a distance from the corneal epithelium and to the anterior lens surface. WTW is obtained by analysing the image of the iris using an infrared light source (wavelength 880 nm). All measurements are performed simultaneously. IOLMaster 500 is currently considered as a gold-standard biometer.13–15 Its repeatability and reproducibility have been assessed in several studies.16–19

AL-Scan uses an 830 nm infrared laser diode for AL measurement with PCI. It also measures keratometry (K) at 36 measurement points in two circles with diameters of 2.4 mm and 3.3 mm, reflected from the corneal surface. WTW and PS are obtained by analysing the image of the iris and fitting the best circle with the lowest error square to the detected edge. ACD and CCT are measured with an incorporated Scheimpflug camera with a 470 nm monochromatic light. The device was introduced for clinical practice in Europe in 2012. Srivannaboon et al.20 compared the repeatability and reproducibility of ocular biometry and IOL power obtained with AL-Scan and IOLMaster 500. AL-Scan provided excellent repeatability and reproducibility for all measured parameters (AL, K, ACD and WTW). Agreement with the IOLMaster 500 was good except for the WTW. This can be caused by different algorithms used by these devices for edge detection around iris image. Furthermore, the light source used for WTW measurements is different: AL-Scan uses a green light source (wavelength 525 nm) and IOLMaster uses an infrared light source (wavelength 880 nm). Kaswin et al.21 evaluated the agreement in AL, K, ACD measurements and IOL power calculations with AL-Scan and IOLMaster 500. They reported excellent correlation in AL measurements and K readings as well as good agreement in ACD measurements between these two biometers. The IOL power calculations were also highly comparable between these devices.

The Pentacam AXL device consists of a Scheimpflug camera which rotates around the eye and a PCI-based optical biometer. It was introduced in autumn 2015. In addition to anterior segment tomography, ACD, CCT and WTW measurements, corneal topography, anterior and posterior corneal surface and spherical aberrations, it also has integrated AL measurement. Calculation of toric IOLs is based on the total corneal refractive power and it takes into account the influence of the posterior corneal surface. To our knowledge, no study has yet evaluated the repeatability, reproducibility and accuracy of biometry measurements obtained using this device.

The Lenstar LS 900 biometer is based on OLCR. Using a 820 μm superluminescent diode as light source, it allows the measurement of the AL, CCT, LT and ACD. The retinal thickness can also be determined from the scans, but this requires subjective alignment of a cursor. It also uses 950 μm light to assess by image analysis central corneal curvature using two rings of diameter 1.65 mm and 2.30 mm of 16 light spot each. WTW and PS are obtained by fitting the best circle with the lowest error square to the detected edge. Optional T-cone module complements this device with a Placido topography of the central 6 mm corneal zone. Several studies confirmed Lenstar’s repeatability, reproducibility and agreement with other biometry devices. Generally, Lenstar provided results that correlated very well with those of the IOLMaster. Excellent agreement has been shown between the AL measurements taken by Lenstar and IOLMaster,22–25 but only good22 or moderate24 agreement Optical biometry The introduction of optical biometry has steadily replaced ultrasound methods and is now considered the clinical standard for ocular biometry. The results are comparable to those achieved between these two devices in ACD measurements. In some cases, small but statistically significant differences in K and ACD measurements were reported.24 However, in a few studies, the AL measurements taken by Lenstar were slightly higher than the IOLMaster measurements, but the differences were not clinically significant.22,23 The Lenstar was unable to take measurements due to lens opacities in a similar number of patients to the IOLMaster.23

References:
1. George R, Rupauliha P, Sripriya AV, et al., Comparison of endothelial cell loss and surgically induced astigmatism following conventional extracapsular cataract surgery, manual small-incision surgery and phacoemulsification, Ophthalmic Epidemiol, 2005;12:293–7.
2. Zheng L, Merriam JC, Zaider M, Astigmatism and visual recovery after ‘large incision’ extracapsular cataract surgery and ‘small’ incisions for phacoemulsification, Trans Am Ophthalmol Soc, 1997;95:387–410.
3. Javitt J, Steinert R, Cataract extraction with multifocal intraocular lens implantation: a multinational clinical trial evaluating clinical, functional and quality-of-life outcomes, Ophthalmology, 2000;107:2040–8.
4. Basic and Clinical Science Course, Section 3: Clinical Optics. (2011-2012 ed.) American Academy of Ophthalmology. pp. 211–223.
5. Ossoinig KC, Standardized echography: basic principles, clinical applications and results. Int Ophthalmol Clin, 1979;19:127–210.
6. Schelenz J, Kammann J, Comparison of contact and immersion technique for axial length measurement and implant power calculation, J Cataract Refract Surg, 1989;15:425–8.
7. Shamma’s HJF, A comparison of immersion and contact techniques for axial length measurements,J Am Intraocul Implant Soc, 1984;10:444–7.
8. Olsen T, Nielsen PJ, Immersion versus contact technique in the measurement of axial length by ultrasound, Acta Ophthalmol, 1989;67:101–2.
9. Lee AC, Qazi MA, Pepose JS, Biometry and intraocular lens power calculation, Curr Opin Ophthalmol, 2008;19:13–7.
10. Frings A, Dulz S, Skevas C, et al.,Postoperative refractive error after phacovitrectomy for epiretinal membrane with and without macular oedema, Graefes Arch Clin Exp Ophthalmol, 2015;253:1097–104.
11. Haigis W, Lege B, Miller N, Schneider B, Comparison of immersion ultrasound biometry and partial coherence interferometry for intraocular lens calculation according to Haigis, Graefes Arch Clin Exp Ophthalmol, 2000;238:765–73.
12. Grzybowski A, Gaca-Wysocka M, Current knowledge of the lens, Przegląd Okulistyczny, 2014:4:1–4.
13. Drexler W, Findl O, Menapace R, et al., Partial coherence interferometry: a novel approach to biometry in cataract surgery, Am J Ophthalmol, 1998;126:524–34.
14. Packer M, Fine IH, Hoffman RS, Immersion A-scan compared with partial coherence interferometry: outcomes analysis, J Cataract Refract Surg, 2002 Feb;28:239–42.
15. Bhatt AB, Schefler AC, Feuer WJ, et al., Comparison of predictions made by the intraocular lens master and ultrasound biometry, Arch Ophthalmol, 2008;126:929–33.
16. Kielhorn I, Rajan MS, Tesha PM, et al., Clinical assessment of the Zeiss IOLMaster, J Cataract Refract Surg, 2003;29:518–22.
17. Lam AK, Chan R, Pang PC, The repeatability and accuracy of axial length and anterior chamber depth measurements from the IOLMaster™, Ophthalmic Physiol Opt, 2001;21 477–83.
18. Sheng H, Bottjer CA, Bullimore MA, Ocular component measurement using the Zeiss IOLMaster, Optom Vis Sci, 2004;81:27–34.
19. Santodomingo-Rubido J, Mallen EA, Gilmartin B, Wolffsohn JS, A new non-contact optical device for ocular biometry, Br J Ophthalmol, 2002;86:458–62.
20. Srivannaboon S, Chirapapaisan C, Chonpimai P, Koodkaew S, Comparison of ocular biometry and intraocular lens power using a new biometer and a standard biometer, J Cataract Refract Surg, 2014;40:709–15.
21. Kaswin G, Rousseau A, Mgarrech M, et al., Biometry and intraocular lens power calculation results with a new optical biometry device: comparison with the gold standard, J Cataract Refract Surg, 2014;40:593–600.
22. Hoffer KJ, Shammas HJ, Savini G, Comparison of 2 laser instruments for measuring axial length, J Cataract Refract Surg, 2010;36:644–8
23. Buckhurst PJ, Wolffsohn JS, Shah S, et al., A new optical low coherence reflectometry device for ocular biometry in cataract patients, Br J Ophthalmol, 2009;93:949–53
24. Holzer MP, Mamusa M, Auffarth GU, Accuracy of a new partial coherence interferometry analyser for biometric measurements, Br J Ophthalmol, 2009;93:807–10
25. Rohrer K, Frueh BE, Wälti R, et al., Comparison and evaluation of ocular biometry using a new noncontact optical low-coherence reflectometer, Ophthalmology, 2009;116:2087–92
26. Mandal P, Berrow EJ, Naroo SA, et al., Validity and repeatability of the Aladdin ocular biometer, Br J Ophthalmol, 2014;98:256–8.
27. Hoffer KJ, Shammas HJ, Savini G, Huang J, Multicenter study of optical low-coherence interferometry and partial-coherence interferometry optical biometers with patients from the United States and China, J Cataract Refract Surg, 2016;42:62–7.
28. Huang J, Savini G, Wu F, et al., Repeatability and reproducibility of ocular biometry using a new noncontact optical low-coherence interferometer, J Cataract Refract Surg, 2015;41:2233–41.
29. Savini G, Hoffer KJ, Barboni P, Influence of corneal asphericity on the refractive outcome of intraocular lens implantation in cataract surgery, J Cataract Refract Surg, 2015;41:785–9.
30. Goebels S, Pattmöller M, Eppig T, et al., Comparison of 3 biometry devices in cataract patients, J Cataract Refract Surg, 2015;41:2387–93.
31. Shin MC, Chung SY, Hwang HS, Han KE, Comparison of Two Optical Biometers, Optom Vis Sci, 2016;93:259–65.
32. Srivannaboon S, Chirapapaisan C, Chonpimai P, Loket S, Clinical comparison of a new swept-source optical coherence tomography-based optical biometer and a time-domain optical coherence tomography-based optical biometer, J Cataract Refract Surg, 2015;41:2224–32.
33. Kunert KS, Peter M, Blum M, et al., Repeatability and agreement in optical biometry of a new swept-source optical coherence tomography-based biometer versus partial coherence interferometry and optical low-coherence reflectometry, J Cataract Refract Surg, 2016;42:76–83.
34. Kurian M, Negalur N, Das S, et al., Biometry with a new sweptsource optical coherence tomography biometer: Repeatability and agreement with an optical low-coherence reflectometry device, J Cataract Refract Surg, 2016;42:577–81.
35. Shammas HJ, Ortiz S, Shammas MC, et al., Biometry measurements using a new large-coherence-length sweptsource optical coherence tomographer, J Cataract Refract Surg, 2016;42:50–61
36. Binkhorst RD, The optical design of intraocular lens implants, Ophthalmic Surg, 1975;6:17–31.
37. Sanders DR, Kraff MC, Improvement of intraocular lens power calculation using empirical data, J Am Intraocul Implant Soc, 1980;6:263–7.
38. Holladay JT, Prager TC, Chandler TY, et al., A three-part system for refining intraocular lens power calculations, J Cataract Refract Surg, 1988;14:17–24.
39. Sanders DR, Retzlaff J, Kraff MC, Comparison of the SRK II formula and other second generation formulas, J Cataract Refract Surg, 1988;14:136–41. 40. Terzi E, Wang L, Kohnen T, Accuracy of modern intraocular lens power calculation formulas in refractive lens exchange for high myopia and high hyperopia, J Cataract Refract Surg, 2009;35:1181–9.
41. Haigis W, Intraocular lens calculation in extreme myopia, J Cataract Refract Surg, 2009;35:906–11.
42. Petermeier K, Gekeler F, Messias A, et al., Intraocular lens power calculation and optimized constants for highly myopic eyes, J Cataract Refract Surg, 2009;35:1575–81.
43. Eom Y, Kang S, Song J, et al., Comparison of Hoffer Q and Haigis Formulae for Intraocular Lens Power Calculation According to the Anterior Chamber Depth in Short Eyes, Am J Ophthalmol, 2014;157:818–24.
44. Ghanem A, El-Sayed H, Accuracy of intraocular lens power calculation in high myopia, Oman J Ophthalmol, 2010;3:126–30.
45. Retzlaff JA, Sanders DR, Kraff MC, Development of the SRK/T intraocular lens implant power calculation formula, J Cataract Refract Surg, 1990;16:333–40.
46. Haigis W, Challenges and approaches in modern biometry and IOL calculation, Saudi J Ophthalmol, 2012;26:7–12.
47. Hirnshall N, Findi O, Intraocular Lens Power Calculation - Still Searching for the Holy Grail, Eur Ophthalmic Rev, 2015;9;13–6
48. Trivedi RH, Wilson ME, Reardon W, Accuracy of the Holladay 2 intraocular lens formula for pediatric eyes in the absence of preoperative refraction, J Cataract Refract Surg, 37:1239–43.
49. Mahdavi S, Holladay J, IOLMaster 500 and integration of the Holladay2 Formula for intraocular lens calculations, Eur Ophthalmic Rev, 2011;5:134–5
50. Barrett G, A formula for all seasons. The Barrett Universal II and True K and the Barrett Toric Calculator provide the best results for toric, post refractive and standard cataract patients, Supplement to: Cataract & Refractive Surgery Today Europe, October 2014.
51. Aramberri J, Intraocular lens power calculation after corneal refractive surgery: double-K method, J Cataract Refract Surg, 2003;29:2063–8.
52. Masket S, Masket SE, Simple regression formula for intraocular lens power adjustment in eyes requiring cataract surgery after excimer laser photoablation, J Cataract Refract Surg, 2006;32:430–4.
53. Shammas HJ, Shammas MC, No-history method of intraocular lens power calculation for cataract surgery after myopic laser in situ keratomileusis, J Cataract Refract Surg, 2007;33:31–6.
54. Haigis W, Intraocular lens calculation after refractive surgery for myopia: Haigis-L formula, J Cataract Refract Surg, 2008;34:1658–63.
55. Suto C, Shimamura E, Watanabe I, Comparison of 2 optical biometers and evaluation of the Camellin-Calossi Intraocular lens formula for normal cataractous eyes, J Cataract Refract Surg, 2015;41:2366–72.
56. Potvin R, Hill W, New algorithm for intraocular lens power calculations after myopic laser in situ keratomileusis based on rotating Scheimpflug camera data, J Cataract Refract Surg, 2015;41:339–47.
57. Preussner PR, Wahl J, Lahdo H, et al., Ray tracing for intraocular lens calculation, J Cataract Refract Surg, 2002;28:1412–9.
58. Jin H, Rabsilber T, Ehmer A, et al., Comparison of ray-tracing method and thin-lens formula in intraocular lens power calculations, J Cataract Refract Surg, 2009;35:650–62.
59. Hill-RBF Calculator, IOL Power Calculations for Cataract Surgery. Available at: www.rbfcalculator.com (accessed 15 September 2016).
60. Section 03: Clinical optics. In: Basic and clinical science (Polish edition), Poland: Elsevier Urban & Partner 2008-2009 pg 236–238, 325,328, 329.
61. Wang J, Chang S, Optical biometry intraocular lens power calculation using different formulas in patients with different axial lengths,Int J Ophthalmol, 2013;6:150–4.
62. Day A, Foster P, Stevens J, Accuracy of intraocular lens power calculation in eyes with axial length <22.00 mm, Clin Experiment Ophthalmol, 2012;40:855–62.
63. Carifi G, Aiello F, Zygoura V, et al., Accuracy of the refractive prediction determined by multiple currently available intraocular lens power calculation formulas in small eyes, Am J Ophthalmol, 2015;159:577–83.
64. Hoffer KJ, The Hofer Q formula: a comparison of theoretic and regression formulas, J Cataract Refract Surg, 1993;19;700–712; errata 1994;20;1677.
65. Olsen T, Calculation of intraocular lens power: a review, Acta Ophthalmol Scand, 2007;85:472–85.
66. Hoffer KJ, Clinical results using the Holladay 2 intraocular lens power formula, J Cataract Refract Surg, 2000;26:1233–7.
67. Maclaren R, Natkunarajah M, Riaz Y et al., Biometry and Formula Accuracy With Intraocular Lenses Used for Cataract Surgery in Extreme Hyperopia, Am J Ophthalmol, 2007;143: 920–31.
68. El-Nafees R, Moawad A, Kishk H, Gaafar W, Intra-ocular lens power calculation in patients with high axial myopia before cataract surgery, Saudi J Ophthalmol, 2010;24:77–80.
69. Aristodemou P, Knox Cartwright NE, Sparrow JM, Johnston RL, Formula choice: Hoffer Q, Holladay 1, or SRK/T and refractive outcomes in 8108 eyes after cataract surgery with biometry by partial coherence interferometry, J Cataract Refract Surg, 2011;37:63–71.
70. Chong EW, Mehta JS, High myopia and cataract surgery, Curr Opin Ophthalmol, 2016;27:45–50.
Keywords: Biometry, ocular biometry, optical biometry, optical biometry devices, intraocular lenses, IOLs, IOL power calculation, IOL power calculation formulas