|Year : 2019 | Volume
| Issue : 2 | Page : 68-73
Predictability of intraocular lens power calculation in eyes with average axial lengths: optical versus ultrasonic biometry
Amin F Ellakwa1, Mohammed S Abd Elaziz1, Marwa A Zaky1, Waleed M Nagy2
1 Ophthalmology Department, Faculty of Medicine, Menoufia University, Shebeen El-Kom, Egypt
2 Ophthalmology Department, Faculty of Medicine, Giza Memorial Institute, Cairo, Egypt
|Date of Submission||02-Feb-2019|
|Date of Acceptance||01-Apr-2019|
|Date of Web Publication||24-Jul-2019|
Waleed M Nagy
Al Nubaria 22773
Source of Support: None, Conflict of Interest: None
Objective The aim of this study was to compare the partial coherence interferometry to ultrasound (US)-based biometry in intraocular lens (IOL) power calculation in eyes with average axial length (AL).
Patients and methods One hundred eyes with AL of 21–24 mm having cataract as the only ocular pathology were included in the study from November 2016 to October 2018. Fifty eyes were subjected to US biometry and 50 eyes were subjected to Zeiss IOL-Master optical biometry followed by IOL power calculation. All patients underwent phacoemulsification by experienced surgeons with intra-bagal implantation of one-piece soft hydrophilic intraocular lens. AL, keratometric reading, anterior chamber depth, and intraocular lens power were compared. Actual postoperative spherical equivalent (SE), mean absolute error, and predicted error were calculated.
Results No statistically significant difference was found between the two groups regarding the AL, keratometric reading, anterior chamber depth, IOLs power, predicted postoperative SE, and actual postoperative SE (P=0.36, 0.20, 0.57, 0.39, 0.31, and 0.09, respectively). The US group had significantly higher predicted error and mean absolute error than IOL-Master group (P=0.03 and 0.01, respectively).
Conclusion IOL-Master optical biometry is slightly more accurate than US biometry for intraocular lens power calculation in eyes with average AL, whereas A-scan biometry is still a cost-effective method.
Keywords: axial length, partial coherence interferometry, ultrasound biometry
|How to cite this article:|
Ellakwa AF, Abd Elaziz MS, Zaky MA, Nagy WM. Predictability of intraocular lens power calculation in eyes with average axial lengths: optical versus ultrasonic biometry. Delta J Ophthalmol 2019;20:68-73
|How to cite this URL:|
Ellakwa AF, Abd Elaziz MS, Zaky MA, Nagy WM. Predictability of intraocular lens power calculation in eyes with average axial lengths: optical versus ultrasonic biometry. Delta J Ophthalmol [serial online] 2019 [cited 2019 Sep 23];20:68-73. Available from: http://www.djo.eg.net/text.asp?2019/20/2/68/263418
| Introduction|| |
Cataract surgery requires achievement of good postoperative refractive results. In addition to good operative techniques, intraocular lens (IOL) quality, and retina identification, a precise calculation of the intraocular lens power is of crucial importance to achieve good results after refractive cataract surgery. Accurate calculations primarily depend on the accuracy of preoperative biometric data, including the axial length (AL) of the eye, the anterior chamber depth (ACD), and the keratometry values. Incorrect calculation of the lens power is the main reason for patient dissatisfaction and lens replacement in modern cataract surgery .
A previous study of ultrasound (US) biometry reported that 54% of the errors in predicted refraction after IOL implantation can be attributed to errors in AL measurements, 8% to keratometric error, and 38% to incorrect estimation of the postoperative effective lens position .
A-scan US biometry is the contact method that requires the use of a topical anesthetic and the previously done keratometry on a manual or automated keratometer . It has a longitudinal resolution of 200 µm and an accuracy of AL measurement of 100–120 µm. An error of 100 µm in AL measurement leads to a 0.28 D of postoperative error .
IOL-Master is a noncontact method which measures the lens power by laser beam and is performed without using a local anesthetic . It has emerged as a new modality for biometry with the advantages of being fast, noninvasive and less dependent on technician expertise . This technique of optical biometry is reported to have a resolution 12 µm and precision of 0.3–10 µm in AL measurement . The current IOL-Master model; the IOL-Master 500, uses partial coherence interferometry with a 780-nm laser diode infrared light to measure the AL. The advantages of optical biometry over applanation US include the reduced risk of trauma and infection, increased patient comfort, and improved accuracy and repeatability of measurements. At present, optical biometry with the IOL-Master 500 is considered the gold standard for AL measurement. The IOL-Master device uses dual-beam partial coherence interferometry to measure the reflection of the infrared laser from internal tissue interfaces, that is, the optical path length from the anterior surface of the cornea to the retinal pigment epithelium. The keratometric readings are calculated by analyzing the anterior corneal curvature at six reference points in a 2.4-mm diameter optical zone. Measurement of ACD is performed through a lateral slit illumination .
The aim of this study was to compare the partial coherence interferometry to US-based biometry in intraocular lens power calculation in eyes with average AL.
| Patients and methods|| |
This is a prospective comparative randomized clinical study that included 100 eyes of 100 (61 females and 39 males) patients scheduled for phacoemulsification cataract surgery. Patients were randomized to undergo biometry by either the IOL-Master 500 (Carl Zeiss Meditec AG, Jena, Germany) or Compact Touch Ultrasound (Quantel Medical, France) for IOL power calculation, in the Outpatient Clinic of the Memorial Institute of Ophthalmic Research at Giza, in a time period from November 2016 to October 2018. The study was approved by the Local Ethical Committee of Menoufia University. All patients have been informed about the goal of the study and signed a written informed consent before the study. They underwent examination by slit lamp, refraction, and biometry before surgery with the following inclusion and exclusion criteria.
The study included patients with significant cataract (nuclear sclerosis grade 1–3) which was suitable for phacoemulsification with implantation of intra-bagal intraocular lens with no ocular pathology except age-related cataract, no history of intraocular surgery, and AL of 21–24 mm as measured by IOL-Master or US biometry.
Eyes with any ophthalmic pathology other than cataract that may cause visual impairment such as amblyopia, glaucoma, optic neuropathy, macular degeneration, macular edema, retinal detachment, proliferative diabetic retinopathy, or ocular inflammation were excluded. Eyes whose AL could not be measured by IOL-Master, because of dense ocular media opacities such as corneal scars or high density posterior subcapsular cataract, were also excluded. In addition, eyes with corneal refractive surgery, corneal opacities, or irregularities such as scarring, dystrophy, and ectasia were also excluded. Inability to make secure ‘in-the-bag’ implantation of the IOL owing to posterior capsule rupture, weak zonules, vitreous loss, or zonular rupture as well as the use of corneal sutures were among the exclusion criteria.
All patients underwent uncomplicated phacoemulsification with secure intra-bagal IOL implantation. The IOL power was calculated by the SRK/T formula. The IOL used in the study was one-piece hydrophilic soft acrylic IOL with A-constant 118.
Two months after cataract surgery, patients underwent non-cycloplegic refraction. They had been divided into two groups: group 1 (A-scan US biometry) and group 2 (IOL-Master biometry). The actual postoperative spherical equivalent (SE) was recorded by auto-refractometer (Topcon Auto-Refractometer, Tokyo, Japan). The mean absolute error (MAE) was calculated, which is the difference between actual postoperative SE and the predicted postoperative SE; this is the absolute value of numeric error. Predicted error (PE) had been calculated by subtracting the predicted SE from the actual postoperative SE; this is helpful for indication of tendency for myopic shift (negative PE value) or hyperopic shift (positive PE value). Proportions that have absolute errors less than 0.25 D, from 0.25 to 0.5 D, from 0.5 to 1 D, and more than 1 D were estimated.
Results had been collected, statistically analyzed, and tabulated by SPSS statistical package, version 20 (SPSS Inc. released 2011, Armonk, New York, USA: IBM Corporation, New York, New York, USA). Two types of statistical analysis had been done: (a) descriptive statistics, which were expressed in number, mean percentage, and SD and (b) analytic statistics.
Student’s t-test was used to compare quantitative variables between two groups that have normally distributed data, whereas Mann–Whitney’s test was used to compare quantitative variables between two groups that have not normally distributed data.
χ2 was used to study association between qualitative variables. Whenever any of the expected cells were less than five, Fischer’s exact test with Yates correction was used. Z test was used to compare two proportions in two groups. P value of less than 0.05 was considered statistically significant.
| Results|| |
The demographic details of both groups are summarized in [Table 1].
No statistically significant difference was found between the two groups regarding the AL, average keratometric readings, ACD, IOL power, and the predicted postoperative refraction ([Table 2]).
|Table 2 Comparison between the two groups regarding the preoperative parameters|
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No statistically significant difference was found between the two groups regarding actual postoperative SE. However, the US biometry group had significantly higher PE and MAE than the IOL-Master group ([Table 3] and [Figure 1]).
|Table 3 Comparison between the two groups regarding the postoperative parameters|
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The percentage of patients with MAE from 0 to 0.25 D were significantly higher in the IOL-Master group, whereas the percentage of patients with MAE more than 1 D were significantly higher in US biometry group. No significant difference was found between the two groups in the MAE from 0.25 to 0.5 D and from 0.5 to 1.0 D ([Table 4] and [Figure 2]).
| Discussion|| |
Sahin and Hamrah , Drexler et al. , Zaldivar et al. , and Aristodemou et al.  have reported that measurement of the AL with IOL-Master produces significantly more precise IOL power calculation and refractive outcome in cataract operation after excluding poor-quality readings, with signal-to-noise ratio less than 2.1. This is owing to avoiding possible compression on the eye with A-scan US. In addition, the IOL-Master measures the AL along the visual axis, whereas in US biometry a misalignment between the measured axis and the visual axis occurs. IOL-Master, also, measures the distance between the anterior corneal surface and the retinal pigment epithelium, whereas A-scan biometry measures the distance between the internal limiting membrane and the corneal vertex; the resulting differences of the measured AL were between 150 and 350 µm. However, in this study, the mean AL difference between the two methods was statistically insignificant. Similarly, Fontes and Castro  and Solanki et al.  also reported that the mean AL difference between the two methods was statistically insignificant. In contrast, Cvetkovic et al.  and Cech et al.  have reported that the AL measured using the IOL-Master was significantly longer than the AL measured using A-scan.
The mean ACD difference between the two methods, in this study, was statistically insignificant. The design of our study offered a limitation as ACD measurements were obtained with only one of the two methods used. Gopi and Sathyan  have reported that ACD measured with the applanation US was significantly shorter by −0.06±0.25 mm than that measured by the IOL-Master. This can be explained by corneal indentation by the US probe which is responsible for shorter values. In addition, the IOL-Master does not measure the axial ACD because the slit source comes from the temporal side resulting in deeper ACD value.
In this study, the mean IOL power difference between the two methods using SRK/T formula was statistically insignificant. Solanki et al.  and Cech et al.  had used SRK/T formula to calculate the IOL power in all patients, whereas Cvetkovic et al.  and Fontes and Castro  had used Holladay formula to calculate the IOL power in all patients. This study just includes ALs of 21–24 mm. Karabela et al.  have reported that SRK/T formula is an accurate option to predict refractive error after surgery in eyes with medium AL.
Jin et al.  have reported that the PE is the actual postoperative SE minus the target postoperative SE. The sign of the PE denotes the direction of the departure from the target SE. In other words, a negative error of prediction value means that the patient ended up with a more myopic refraction than intended, whereas a positive error of prediction value means that the patient ended up with a more hypermetropic refraction than intended. In this study, the mean PE difference between the two methods was statistically significant. The US biometry group had significantly higher PE than the IOL-Master group. Findl et al.  have reported that the MAE is the absolute value of the PE, which denotes the interval of the refraction from zero, without care whether the departure from zero was in the myopic or the hypermetropic direction. The PE has the disadvantage of averaging both positive and negative errors, whereas the MAE is a more useful measure of the true size of the error. In this study, the MAE difference between the two methods was statistically significant. The US biometry group had significantly higher MAE than the IOL-Master group. Rose and Moshegov  reported similar results, where the MAE difference between US and IOL-Master was statistically significant, improving a 0.65 D error to a 0.42 D error. This reflects a 34% improvement with the IOL-Master compared with US in IOL power calculation using SRK/T lens formula. The PE difference was not statistically significant. A similar study by Findl et al.  using the SRK II formula reported a 27% improvement with the IOL-Master.
In this study, the percentage of patients with MAE from 0 to 0.25 D was significantly higher in IOL-Master group, whereas the percentage of patients with MAE more than 1 D was significantly higher in the US biometry group.
Olsen  reported that the IOL-Master offers superior reproducibility of AL measurement in comparison with applanation US biometry. If measurement using US biometry was done correctly, results of both methods correspond significantly and the methods are mutually replaceable.
The strengths of this study are the large sample size, uniformity of the biometric data, same surgical technique, experienced surgeons, and the use of one formula, which gives the advantage that the postoperative results can be compared with the preoperative prediction easily. However, the weakness of the study is the use of only one biometry technique per eye.
| Conclusion|| |
The optical biometry in IOL power calculation is slightly more accurate, but the US biometry is adequate and a cost-effective method if the optical biometry cannot be used.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4]