Dr. Binesh Tyagi, T16176, Dr. Vandana John Serrao, Dr. Sheetal Brar, Dr. Sri Ganesh
PURPOSE: To compare Toric IOL calculators with and without posterior corneal curvature consideration.
METHODS: Barrett’s calculator, which considers the posterior corneal curvature, was compared with Eyecryl Toric and Tecnis Toric calculators. Each of the three was used for a group of 25 eyes, a total of 75 eyes. IOLs used were Eyecryl toric or AMO Tecnis Toric IOLs. Post op residual astigmatism at intended axis and ideal axis was determined by iTrace Toric back calculator (Zaldiver Caliper).
RESULTS: On 15 days, 3 and 6 months post-operative follow ups, Barrett’s calculator was found to be within 0.5 diopter of residual astigmatism 71% of the time compared with only 31% for the Eyecryl toric or the Tecnis Toric calculator. Ideal axis was within +/- 5 degree of intended axis in 65% in Barrett’s as compared to 34% in others.
CONCLUSION: Toric IOL calculators, which consider posterior corneal astigmatism, give more accurate and predictable results.
Key words: Posterior corneal curvature, Toric IOL, Biometry
Introduction
Pre operative astigmatism in cataract patients is relatively common. According to different studies, 15% to 29% of cataract patients have more than 1.5 diopters of refractive astigmatism(1). It was recognized by Javal et al in late nineteenth century, that there is a difference between the refractive and anterior corneal astigmatism of the humaneye: and thereafter postulated an approximately linear relationship between cornealastigmatism and refractive astigmatism, which became known as Javal’s rule(2).
Until recently, the source of this difference was unknown. The role of the posterior cornea was initially dismissed because of the small difference between the indices of refraction of the cornea and aqueous(3).The retina was dismissed on the basis of theory and confirmed by a clever retinoscopic study by Flüeler and Guyton.(4)
However, several studies using different methodologies(5) recently reported that the posterior cornea has astigmatism that ranges from 0.26 to 0.78 D. In a study(6) it was found that in most eyes, the posterior corneal steep meridian is aligned vertically and led to theory that for toric IOL implantation, ignoring the posterior corneal astigmatism would result in overcorrection in eyes having with-the-rule (WTR) anterior corneal astigmatism and under correction in eyes with against-the-rule (ATR) astigmatism(7).
Ueno et al(8) noted that superior vertical thickness is greater than the inferior, indicating that the posterior cornea produces asymmetric astigmatism and some higher-order aberrations as well. Therefore, the only way to confirm the accuracy of these values is by refracting eyes and eliminating other sources of refractive astigmatism, which is best done in pseudophakic eyes. (7)
With more understanding, focus was again on correcting corneal astigmatism. Several techniques exist to correct corneal astigmatism. These include limbal relaxing incisions, opposite clear corneal incisions, excimer laser refractive procedures, femtosecond laser–assisted astigmatic keratotomy and toric intraocular lens (IOL) implantation in cataract cases.
Toric IOLs were first made and used to correct corneal astigmatism by Shimizu et al in1994.(9) Its implantation during cataract surgery results in less spectacle dependence postoperatively in patients with pre-existing corneal astigmatism.
Till recently, available toric calculators were not taking into consideration the posterior curvature, but now there are newer systems which take into account posterior curvature of the cornea as well for IOL power and axis calculation. Clinicians can also use regression approaches such as the Baylor nomograms (10) and theoretical formulas such as the Barrett toric intraocular lens formula (www.ascrs.org) to address posterior corneal astigmatism.
In our study, we compared refractive outcomes of tecnis toric and eyecryl toric online calculators, which did not take into account posterior corneal curvature (updated versions of both consider it now) to Barrett’s toric calculator.
Material and methods
It is a prospective, single Hospital based, randomized, interventional study done at a tertiary eye care centre in south India.
Sample size: 75 (25 in each of 3 groups)
Period of study: March 2016 onwards, study duration 1year.
Study was approved by local institutional review board. 75 consecutive suitable patients were recruited and randomized into three groups of 25 each.
Objective:
To compare the residual astigmatic outcome difference with and without considering posterior corneal curvature.
Equipments: IOL master 700, OPMI Lumera 700 microscope with the Callisto markerless eye system, Pentacam (Oculus 7) for keratometry, iTrace (Hoya) to check the residual astigmatism and enhancement by rotation of the toric IOL.
Inclusion criteria:
Age > 18 years
Eyes with Cataract requiring surgery
Corneal astigmatism more than 1.00 dioptres (D) using partial coherence Interferometry (PCI) (IOLMaster, Carl Zeiss Meditec)
Adequate pupil dilatation to visualize the innermost toric axis marks at all follow-ups
At least 1 prominent episcleral vessel around the limbus to act as a reference mark for aligning all follow-up images
Total capsulorrhexis coverage of the IOL optic.
Patient willing and able to comply with scheduled visits and other study procedures
Exclusion criteria:
Irregular corneal astigmatism or presence of Forme-fruste keratoconus on corneal tomography using Scheimpflug scanning- slit imaging device (Pentacam HR)
Difference in astigmatism axis between the PCI keratometry reading (K) and the simulated K reading axes of tomography of more than 10.0 degrees
Any ophthalmic pathology that could have impact on postoperative visual function
History of ocular surgery/ trauma
Patients with intraoperative complications, such as wound-site thermal injury, incisions requiring suturing, posterior capsule rupture, and Descemet membrane detachment.
Uncontrolled/manifest glaucoma
Pathology of the eyelid
External ocular pathologies involving the limbus such as pterygium
Narrow palpebral eyelid fissures and sunken eyes
Axial length was measured using optical coherence interferometry (IOL Master 700,Carl Zeiss Meditec AG).Corneal astigmatism determined by keratometry (IOLMaster 700, Carl Zeiss Meditec AG). The spherical and cylindrical power of the IOL calculated targeting emmetropia using the online calculators by entering IOLMaster keratometry values.
Patients underwent routine phacoemulsification surgery with foldable Toric IOL (tecnis or eyecryl toric IOL) using Callisto system connected to iTrace through FORUM viewer for axis alignment by a single senior surgeon. Patients were evaluated on 1st Post op day, 15th Post op day, 3 months and 6 months.
During follow up visits, subjective refraction was performed, any rotation and alignment was checked using zaldiver caliper of iTrace. Eyes with more than 5 degrees of deviation from our intended axis were excluded from study, and new patients were recruited for the same, so as to compare only planned axis and ideal axis for the same IOL.
All relevant data was entered in online Berdahl & Hardten Astigmatism Fix Calculator at www.astigmatismfix.com to determine the ideal axis for the same IOL.
The results were recorded in tabulated form in Microsoft® Excel® 2016. Charts and graphs were prepared and analyzed by using IBM® SPSS® statistics version 23.0 for Microsoft® windows® and Microsoft® Word® 2016.
Descriptive statistics (mean, standard deviation, median and range) were calculated for relevant Parameters. 2 sample T-tests were performed to determine P values for significance between Barrett’s versus other calculators.
Results:
The 3 study groups included a total of 75 patients (75 eyes), 39 (52%) male and 36 (48%) female, of mean age 64 ± 19 years at the time of toric IOL implantation. There were no significant differences between three groups in terms of gender or BCVA at presentation.
A comparison of the residual cylinder power (Graph 1) for the IOLs implanted with power calculated from different calculator groups among Barrett vs Tecnis vs Eyecryl was (0.44 D ± 0.34 versus 0.68 ± 0.40, versus 0.75 ±0.41). P value for Barrett vs others showed significant difference (0.003). The mean axis difference between planned axis and ideal axis (Graph 2) among Barrett vs Tecnis vs Eyecryl was (5.68 ± 3.9 versus 8.76 ± 4.70, versus 8.16 ±4.78) P value for Barrett vs others showed significant difference (0.009)
There was no significant difference in residual cylinder between 3 and 6 months post operatively in any of the groups.
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