|Year : 2020 | Volume
| Issue : 1 | Page : 103-104
Reply to queries on ocular biometry
Prashob Mohan1, Arup Chakrabarti2
1 Department of Cornea and Refractive Surgery, Giridhar Eye Institute, Kochi, Kerala, India
2 Chakrabarti Eye Care Centre, Thiruvananthapuram, Kerala, India
|Date of Submission||19-Feb-2020|
|Date of Acceptance||19-Feb-2020|
|Date of Web Publication||17-Apr-2020|
Dr. Prashob Mohan
Department of Cornea and Refractive Surgery, Giridhar Eye Institute, Kochi, Kerala
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Mohan P, Chakrabarti A. Reply to queries on ocular biometry. Kerala J Ophthalmol 2020;32:103-4
We would like to thank Dr Rose Mary Tomy for her valuable comments on our article. We hope that we can answer her queries to her satisfaction.
In case of a scarred cornea, as long as a reasonable keratometry can be obtained (manual/automated), we would prefer to use the average keratometry of the same eye in calculating intraocular lens (IOL) power. As far as highly irregular scarred corneas are concerned, if no keratometric values can be obtained, we would prefer to use K readings from the other eye (such eyes will most probably need a keratoplasty as well for visual rehabilitation).
In eyes that have undergone pars plana vitrectomy, certain adjustments may have to be made in the IOL power calculated depending on the indication for vitrectomy.
In silicone oil-filled eyes, the axial length determination may be challenging if ultrasound biometry is used. The vitreous cavity depth will have to be recalculated, as sound travels slower in silico ne oil. If media permits, optical biometry has a definite advantage in this regard.
In cases where the retina is detached, ultrasound biometry may give an erroneous axial length as it measures axial length only up to the internal limiting membrane. Optical biometers measure axial length up to the retinal pigment epithelium and are more accurate.
In macular diseases, the increased/decreased macular thickness as measured by OCT should be accounted for while estimating axial length using ultrasound. Optical biometers do not need such an adjustment. The phenomenon of parafoveal fixation may also have to be taken into account in such cases.
We would recommend using modern IOL power calculation formulae such as Barrett Universal 2 or Hill radial basis function which are freely available on the Internet in all cases.
Estimated lens position (ELP) is the Holy Grail as far as ocular biometry is concerned. Modern theoretical IOL power calculation formulae do a good job at calculating ELP and no adjustments are necessary for mature, brown, or intumescent cataracts.
Optic capture of an IOL placed in the sulcus leads to an effective power closer to in the bag placement of the IOL. Although there are no nomograms developed for sulcus-placed IOLs with optic capture, studies seem to suggest that the myopic shift associated with sulcus placement of an IOL intended for the bag, without optic capture, is mitigated to a certain extent by optic capture. The adjustments in power necessary for sulcus placement of IOLs are given on the website www.doctor-hill.com/iol-main/bag-sulcus.htm. We would recommend going for a power closer to in the bag fixation when optic capture is performed.
Surgically induced astigmatism (SIA) is calculated in the same way regardless of the tool used for axial length measurement. For calculating one's own SIA, Dr. Warren Hill has designed a website sia-calculator.com which uses a vector method to arrive at a centroid value of astigmatism. The average SIA for various surgeons who performed temporal clear corneal phacoemulsification (incision size between 2.2 and 2.8 mm) according to Dr. Hill ranged from 0.08 to 0.14. In case one has not calculated her/his own SIA, it would be prudent to put in a value of 0.12.
A constants/surgeon factor needed to be personalized for use in the older generation formulae. In modern formulae such as the Barrett Universal 2, no personalization of a constant seems necessary. The A constant values (lens factor) of the most commonly used IOLs are provided in the calculator itself. For the rest, the SRK-T A constant from the ULIB website (http://ocusoft.de/ulib/c1.htm) may be used. The A constants provided, however, are for optical biometry and if using ultrasound biometry, the A constants will have to be adjusted. (They are generally less than those for optical biometry.) For this, we would recommend starting with the nominal A constant for ultrasound biometry mentioned on the IOL packaging box and revising the A constant after doing sufficient number of cases based on the postoperative refraction.
We hope we have been able to answer your questions with sufficient clarity.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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Rahman R, Bong CX, Stephenson J. Accuracy of intraocular lens power estimation in eyes having phacovitrectomy for rhegmatogenous retinal detachment. Retina (Philadelphia, Pa). 2014;34:1415–20.
Sun HJ, Choi KS. Improving intraocular lens power prediction in combined phacoemulsification and vitrectomy in eyes with macular oedema. Acta Ophthalmol. 2011;89:575–8.
Millar ER, Merchant K, Steel D. The Effect Of Anterior Capsulorhexis Optic Capture Of A Sulcus Fixated Iol Implant On Refractive Outcome. Invest Ophthalmol Vis Sci. 2012;53:6646–6.