Over the past few years, there has been an explosion of devices combining different forms of anterior segment imaging and ocular biometry. These devices allow the user to carry out many or most of the measurements required to screen cataract surgery candidates, establish the parameters for surgery, and select the most appropriate IOL to optimize refractive outcomes.
The technologies behind these combined instruments are quite varied, and, although to date there is not a great deal of scientific literature comparing their performances, they have all been reported individually to perform well. Although these devices provide a lot of information simultaneously, there is still more potentially useful information that could be extracted with little or no change in hardware, and it is therefore expected that the range of applicability and the overall performance of these devices will expand in the near future.
In this article, we first describe the characteristics of some of these devices with which we have clinical experience. In a final section, we discuss some of the ways these devices can best be used in practice and their potential for the future.
CHARACTERISTICS OF SELECTED DEVICES
Lenstar LS 900 (Haag-Streit). Reported to be a precise noncontact biometer,1-3 the Lenstar LS 900 is capable of making repeatable2,4 and reproducible5 measurements based on optical low-coherence reflectometry (OLCR). The device provides high-resolution measurements of axial length (AL); anterior chamber depth (ACD); and corneal, lens, and retinal thicknesses. It also performs pupillometry, white-to-white (WTW) distance, and keratometry (K) measurement. Repeatability of in vivo measurement is reported to be ±35 μm for AL, ±2.3 μm for central corneal thickness (CCT), ±40 μm for ACD and WTW, and ±80 μm for lens thickness (LT).
Pupil diameter reportedly has no effect on these measurements,6 and built-in software allows calculation of IOL power even for patients with previous refractive surgery.7
According to the manufacturer, the Lenstar measures through clear or cataractous crystalline lenses, but it is also capable of measuring aphakic, pseudophakic, and silicone–oil-filled eyes. The Lenstar and IOLMaster (Carl Zeiss Meditec) have been reported to make comparable ocular biometry measurements in all these situations.3 The Lenstar has also been reported to make repeatable off-axis AL measurements8 and accurately measure choroidal thickness.9
For calculating toric IOL implants, the Lenstar includes two tools. First, the T-Cone Toric Platform provides true Placido-disc–based topography of the central 6 mm of the anterior cornea, according to the manufacturer. Second, the EyeSuite IOL Toric planner enables the surgeon to predict the exact location to perform the incision in order to achieve the minimum residual astigmatism.
The Lenstar has other applications in addition to measuring ocular biometry. Its usefulness to detect zonular weakness has been reported;10 this information can be used in preoperative planning to decide the best surgical procedure. The device can also assess changes in LT during accommodation—information that might be useful to predict variations in lens vault with accommodation11 and, therefore, to choose an IOL design with the most appropriate vault. It is also reported to be useful in judging an eye's predisposition for developing retinal vein occlusion.12
Aladdin (Topcon). Combining optical biometry and a Placido-disc–based topography system, the Aladdin allows the determination of intraocular distances, AL, pachymetry, corneal curvature, WTW distance, and pupillometry. Additional features include keratoconus screening and measurement of corneal wavefront aberrations with up to 7 mm diameter. As with the Lenstar, the Aladdin includes a software tool to aid in the selection of IOLs, even for patients with previous refractive surgery. The biometry values obtained with the Aladdin have been reported to be valid, reproducible, and comparable with those generated by the standard IOLMaster platform.13
IOLMaster 700. As the first system to integrate swept-source OCT technology into biometry, the main advantage of the IOLMaster 700 is that all measurement calipers are shown on the full-length OCT image from the cornea to the retina. As a result, the measurements can be visually verified to reduce potential sources of error. Refractive surprises are also reduced because the OCT images allow the user to identify irregular geometries, such as lens tilt, and to check for correct fixation during measurement.
This device also performs distance-independent K measurements, which is useful for capturing measurements in restless patients and is associated with robust and repeatable measurements. The device acquires 2,000 scans per second, and this high speed is related to its highly repeatable measurement capability. The standard deviation of repeatability is less than 15 μm for AL, ACD, CCT, and LT measurements; 0.09 D for corneal radii; and 90 μm for WTW distances.
The IOLMaster 700 incorporates a reference image feature for markerless toric IOL alignment. Both the keratometric measurements and the reference image can be transferred to the Callisto eye surgical information management system (Carl Zeiss Meditec), which can project information into surgical field assistance functions. With this system integrated with the OPMI Lumera 700 surgical microscope (Carl Zeiss Meditec), data such as incision size and position can be projected directly into the oculars of the microscope.
The IOLMaster 500 has been shown to be an excellent noncontact biometer in eyes with age-related cataracts.14 Based on that experience, the IOLMaster 700 may be expected to be a useful biometer in eyes with age-related cataracts because it uses the same measurement principle. Nevertheless, further studies are needed to confirm this.
Galilei G6 (Ziemer). This device combines optical biometry, dual-Scheimpflug imaging, and Placido-disk topography. The optical biometry component of the Galilei G6 measures axial and intraocular distances; the double-Scheimpflug camera supplies high-precision pachymetry and elevation data and a 3-D reconstruction of the anterior segment; Placido-disk topography measures the anterior corneal surface and allows the detection of surface irregularities.
According to the company, the repeatability standard deviation is 0.015 mm for AL, 1.13 µm for CCT, 0.015 mm for ACD, 0.035 mm for LT, 0.05 D for simulated K, and 0.024 mm for WTW distance.
The Galilei G6 can be used for numerous functions, including toric IOL planning based on biometric measurements and K readings; planning and positioning of arcuate incisions for astigmatism correction during cataract surgery; and calculation of IOL power after LASIK. Using its corneal topography acquisitions, the device can be used to perform a complete screening for keratoconus and corneal inlay implants and to plan and follow up keratoplasty surgery.
Previous versions of the Galilei have been used to measure changes in the anterior segment with accommodation,15,16 assess morphologic changes in healthy corneas due to contact lens use,17 and measure angle kappa and how it changes with accommodation.18 Additionally, moderate repeatability in the determination of corneal wavefront aberrations has been reported.19 It is expected that the Galilei G6 performs at least as well as previous versions, although these data are yet to be published.
AL-Scan (Nidek). The AL-Scan optical biometer measures AL, K, pachymetry, ACD, WTW, and pupil diameter. Partial coherence interferometry is used to measure AL; Scheimpflug imaging is used to measure ACD and corneal thickness; and LEDs of differing wavelengths are used to measure pachymetry, WTW, and pupil diameter. According to company information, these measurements can be taken even in eyes with dense cataracts. Nevertheless, the device incorporates an ultrasound biometer to measure eyes with extremely dense opacities.
Additionally, the AL-Scan calculates IOL implant power based on its own parameter measurements, and these values have been reported to be comparable with those of the IOLMaster.20 Excellent repeatability and reproducibility values in healthy21,22 and keratoconic23 corneas have been reported.
FOR YOUR CONSIDERATION
Despite the well-documented performance of state-of-the-art technology for ocular assessment and measurement described above, it is good practice not to rely completely on these measurements without the following considerations:
• Align the device and the patient precisely prior to measurement;
• Always take more than one measurement;
• Take automatic scans when possible (and check them); and
• Always ask the patient to blink and try to acquire images after allowing the tear film to spread (about 4 seconds after blink).24
Some devices, such as the AL-Scan, incorporate an autotracking system to ensure centration. Use functions such as these whenever possible, but also be sure to verify the results.
Manufacturers will no doubt continue to combine multiple successful technologies into single devices, and this will give ophthalmologists a lot to talk about and research for years to come. Aside from the manufacturers' recommended applicability of each device, there is still much more potentially useful information that can be extracted from each of these with little or no change in hardware. The technologies combined in these units have shown great potential for further applications individually, and it is therefore to be expected that these combinations will only expand the possibilities in preoperative evaluation.
For example, OCT images may be used to objectively grade lens and corneal opacities, measure retinal thickness,25 and document changes in lens thickness with accommodation.26 Scheimpflug technology has the potential to objectively grade opacities in the ocular media,27,28 and it could potentially be used to assess changes in the anterior chamber during accommodation. All of this added information could potentially be used to improve considerably the assessment of surgery candidates, optimize the selection of the most appropriate IOL design, and allow other applications yet to be dreamed of. n
1. Holzer MP, Mamusa M, Auffarth GU. Accuracy of a new partial coherence interferometry analyser for biometric measurements. Br J Ophthalmol. 2009;93:807-810.
2. Shammas HJ, Hoffer KJ. Repeatability and reproducibility of biometry and keratometry measurements using a noncontact optical low-coherence reflectometer and keratometer. Am J Ophthalmol. 2012;153:55-61.e52.
3. Rohrer K, Frueh BE, Walti R, Clemetson IA, Tappeiner C, Goldblum D. Comparison and evaluation of ocular biometry using a new noncontact optical low-coherence reflectometer. Ophthalmology. 2009;116:2087-2092.
4. Buckhurst PJ, Wolffsohn JS, Shah S, Naroo SA, Davies LN, Berrow EJ. A new optical low coherence reflectometry device for ocular biometry in cataract patients. Br J Ophthalmol. 2009;93:949-953.
5. Cruysberg LP, Doors M, Verbakel F, Berendschot TT, De Brabander J, Nuijts RM. Evaluation of the Lenstar LS 900 non-contact biometer. Br J Ophthalmol. 2010;94:106-110.
6. Bakbak B, Koktekir B, Gedik S, Guzel H. The effect of pupil dilation on biometric parameters of the Lenstar 900. Cornea. 2013;32:e21-e24.
7. Shammas H, 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:(1)31-36.
8. Schulle K, Berntsen D. Repeatability of on- and off-axis eye length measurements using the Lenstar. Optom Vis Sci. 2013;90:16-22.
9. Zengin M, Karahan E, Yilmaz S, Cinar E, Tuncer I, Kucukerdonmez C. Association of choroidal thickness with eye growth: a cross-sectional study of individuals between 4 and 23 years. Eye (Lond). 2014;28:1482-1487.
10. Bosnar D, Kuzmanovic Elabjer B, Busic M, Bjelos Roncevic M, Miletic D, Barac J. Optical low-coherence reflectometry enables preoperative detection of zonular weakness in pseudoexfoliation syndrome. Graefes Arch Clin Exp Ophthalmol. 2012;250:87-93.
11. Read SA, Collins M, Woodman E, Cheong S. Axial length changes during accommodation in myopes and emmetropes. Optom Vis Sci. 2010;87:656-662.
12. Szigeti A, Schneider M, Ecsedy M, Nagy ZZ, Recsan Z. Association between retinal vein occlusion, axial length and vitreous chamber depth measured by optical low coherence reflectometry. BMC Ophthalmol. 2015;15:45.
13. Mandal P, Berrow EJ, Naroo SA, et al. Validity and repeatability of the Aladdin ocular biometer. Br J Ophthalmol. 2014;98:256-258.
14. Mylonas G, Sacu S, Buehl W, Ritter M, Georgopoulos M, Schmidt-Erfurth U. Performance of three biometry devices in patients with different grades of age-related cataract. Acta Ophthalmol. 2011;89:e237-241.
15. Sisó-Fuertes I, Domínguez-Vicent A, Del Águila-Carrasco A, Ferrer-Blasco T, Montés-Micó R. Corneal changes with accommodation using dual Scheimpflug photography. J Cataract Refract Surg. 2015;41:981-989.
16. Domínguez-Vicent A, Monsálvez-Romín D, Albarrán-Diego C, Sanchis-Jurado V, Montés-Micó R. Changes in anterior chamber eye during accommodation as assessed using a Dual Scheimpflug system. Arq Bras Oftalmol. 2014;77:243-249.
17. Del Águila-Carrasco A, Domínguez-Vicent A, Pérez-Vives C, Ferrer-Blasco T, Montés-Micó R. Assessment of corneal morphological changes induced by the use of daily disposable contact lenses. Cont Lens Anterior Eye. 2015;38:28-33.
18. Dominguez-Vicent A, Monsalvez-Romin D, Perez-Vives C, Ferrer-Blasco T, Montes-Mico R. Measurement of angle Kappa with Orbscan II and Galilei G4: effect of accommodation. Graefes Arch Clin Exp Ophthalmol. 2014;252:249-255.
19. Cerviño A, Dominguez-Vicent A, Ferrer-Blasco T, García-Lázaro S, Albarrán-Diego C. Intrasubject repeatability of corneal power, thickness, and wavefront aberrations with a new version of a dual Scheimpflug-Placido system. J Cataract Refract Surg. 2015;41:186-192.
20. Kaswin G, Rousseau A, Mgarrech M, Barreau E, Labetoulle M. 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.
21. Huang J, Savini G, Li J, et al. Evaluation of a new optical biometry device for measurements of ocular components and its comparison with IOLMaster. Br J Ophthalmol. 2014;98:1277-1281.
22. 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;40:709-715.
23. Yagci R, Guler E, Kulak AE, Erdogan BD, Balci M, Hepsen IF. Repeatability and reproducibility of a new optical biometer in normal and keratoconic eyes. J Cataract Refract Surg. 2015;41:171-177.
24. Montés-Micó R, Alió J, Muñoz G, Charman W. Temporal changes in optical quality of air-tear film interface at anterior cornea after blink. Invest Ophthalmol Vis Sci. 2004;45:1752-1757.
25. Sato A, Fukui E, Ohta K. Retinal thickness of myopic eyes determined by Spectralis optical coherence tomography. Br J Ophthalmol. 2010;94:1624-1628.
26. Neri A, Ruggeri M, Protti A, Leaci R, Gandolfi S, Macaluso C. Dynamic imaging of accommodation by swept-source anterior segment optical coherence tomography. J Cataract Refract Surg. 2015;41:501-510.
27. Grewal D, Jain R, Brar GS, Grewal SP. Pentacam tomograms: a novel method for quantification of posterior capsule opacification. Invest Ophthalmol Vis Sci. 2008;49:2004-2008.
28. Grewal DS, Brar GS, Grewal SP. Correlation of nuclear cataract lens density using Scheimpflug images with Lens Opacities Classification System III and visual function. Ophthalmology. 2009;116:1436-1443.
Alejandro Cerviño, PhD
• Associate Professor, Department of Optics, Optometry, and Vision Sciences, University of Valencia, Burjassot, Spain
• Financial disclosure: None
Alberto Domínguez-Vicent, MSc
• Predoctoral Research Fellow, Department of Optics, Optometry, and Vision Sciences, University of Valencia, Burjassot, Spain
• Financial disclosure: None