|Year : 2019 | Volume
| Issue : 3 | Page : 202-205
Department of Ophthalmology, Chaithanya Eye Institute, Palarivattom, Kochi; Vettam Eye Clinic, Mulanthuruthy, Ernakulam, Kerala, India
|Date of Web Publication||31-Dec-2019|
Dr. Sanitha Sathyan
Vettam Eye Clinic, Perumpilly, Mulanthuruthy, Ernakulam - 682 314, Kerala
Source of Support: None, Conflict of Interest: None
The evolution of cataract surgery to a refractive procedure owes a lot to the advances in the field of ocular biometry. Advanced biometry machines and choice of intraocular power formulae have hugely improved the accuracy of measurements and visual outcomes of the patient. This section explores the recent trends and practice patterns in ocular biometry.
Keywords: Ocular biometry, IOL power, recent advances
|How to cite this article:|
Sathyan S. Ocular biometry. Kerala J Ophthalmol 2019;31:202-5
| Introduction|| |
Ocular biometry has evolved to a specialized field with the advent of newer biometry machines and advanced formulae, with better predictable visual outcomes. Improvement in biometry techniques has helped in refining the armamentarium of the cataract surgeon and has supported the evolution of advanced technology intraocular (IOL) lenses. This panel discussion touches on the current practice patterns in ocular biometry.
| Panelists|| |
- Dr. Arulmozhivarman (Senior Consultant in Cataract Services, Uma Eye Clinic, Chennai, Tamil Nadu, India)
- Dr. Saikumar S J (Senior Consultant in Cataract and Glaucoma Services, Giridhar Eye Institute, Kochi, Kerala, India)
- Dr. Minu Mathen (Senior Consultant in Cataract Services, Chaithanya Eye Hospital and Research Center, Thiruvananthapuram, Kerala, India)
- Dr. Mathew Kurian (Senior Consultant in Cataract Services, Chaithanya Eye Institute, Palarivattom, Kochi, Kerala, India).
1. What is your preferred method of intraocular biometry?
Dr. Arulmozhivarman: Now, my preferred method of biometry is swept source optical coherence tomography (OCT)-based optical biometer-IOL Master 700.
Dr. Saikumar S J: When I started my IOL career 25 years ago, the only device for biometry was the contact ultrasound method. Since the introduction of optical coherence biometry, I have shifted over to that method in almost all cases. Exceptions are very dense cataracts, thick posterior subcapsular cataracts, and very ill patients in whom optical coherence biometry cannot be done. With the newer swept-source OCT machines, optical coherence biometry can be done in most of the dense cataracts.
Dr. Minu Mathen: My choice is optical biometry (Lenstar/IOL Master) in all cases except in very dense and mature cataracts. In dense and mature cataracts, my choice will be immersion biometry.
Dr. Mathew Kurian: Optical biometry when possible is my preferred method. One of the newer devices that also measure the white to white and anterior chamber depth is preferred as this would allow the use of newer formulae like the Barrett Universal II.
In instances like mature cataract, vitreous hemorrhage or corneal opacities that preclude axial length measurement by optical biometry, axial length measurement by immersion biometry is preferred to contact Ascan. Other parameters are measured on the optical biometer and the measured axial length is then entered manually to derive the IOL power.
2. What is your choice of IOL formulae in “normal,” “short,” and “long” axial lengths?
Dr. Arulmozhivarman: Barrett Universal II Toric formula is suited for all axial lengths and gives consistently accurate results.
Dr. Saikumar S J: SRK-T was the standard formula for all eyes for a long period of time. It is used to give good results and is still a very reliable and robust formula. I used Hoffer Q for very short eyes, especially those with an axial length of 20 mm or less. Now, since the introduction of the Barretts Universal formula, it has become the standard formula for all axial lengths. The IOL Master 700 has all the Barretts formulas integrated into it. The Barretts Universal formula for monofocal and multifocal lenses, the Barretts Toric for Toric IOL calculation, and the Barretts True K for postrefractive surgery eyes. The Barretts Toric formula is very accurate since it takes into account posterior corneal astigmatism as well.
Dr. Minu Mathen: My choice for all axial lengths is the Barretts Universal II formula.
Dr. Mathew Kurian: In normal eyes (22–26 mm axial length), the formula of choice would be the SRK-T or the Holladay 1, while the Hoffer Q in short eyes (<22 mm) and the SRK-T or the Holladay 1 with the Wang-Koch axial length modification in long eyes (>26 mm) is preferred.
However, as described by Holladay, the anterior segment may not be proportional to the axial length [Table 1]., The advent of the Barrett Universal II vergence formula has given a single formula that is applicable across most eyes. The Hill-RBF formula and the Super Ladas formula also hold the same promise.
3. How do you decide on the determination of intraocular power in patients postrefractive surgery?
Dr. Arulmozhivarman: In postrefractive eyes, in addition to keratometry, it would be best to look at topography too. We should record posterior corneal curvature and incorporate in the IOL calculation formula. Haggis L gives the best accuracy in postrefractive eyes.
Dr. Saikumar S J: Most postlaser-assisted in-situ keratomileusis (LASIK) patients will not have pre-LASIK K values. I regularly use the online calculator available on the American Society of Cataract and Refractive Surgery (ASCRS) website and choose the option of entering only the present post LASIK biometry details. By using either the Holladay or Shammas formula, I get very good IOL power values. I always overcorrect by 0.75–1 D so that the patient ends up with slight myopia. Now I have started using the True K available on the IOL Master 700 but always cross-check with the online calculator.
Dr. Minu Mathen: I refer to the ASCRS Website for postrefractive surgery IOL power calculation.
Dr. Mathew Kurian: In postrefractive surgery eyes, only 70%–75% of eyes are within ± 0.5 D of the postoperative refractive target. The challenges are as follows:
- The axial length must be adjusted as these are usually long eyes
- If there has been prior keratorefractive surgery, the corneal power needs to be estimated by the corresponding methodology, appropriate for the type of keratorefractive surgery performed
- The Double K method using the K that was measured before the cornea was altered must be used to estimate the effective lens position.
Validation of the measurements and power calculation is necessary if the patient has had prior exicmer laser-based keratorefractive surgery and the calculated IOL power for standard phacoemulsification is <+17.0 D or >+23.0 D.
The online ASCRS calculator by Koch, Wang, and Hill gives one-stop solution where all the available measurements are entered and the mean, minimum, and maximum IOL powers are given in summary.
The Barrett True K formula is suitable for myopic, hyperopic, and radial keratotomy eyes and can be used with or without the history of the change in refraction that was produced by the refractive procedure.
4. What is your take on biometry in silico n oil-filled eyes?
Dr. Arulmozhivarman: If one is using an optical biometer, there is no need to change anything. If one is using ultrasound biometer, appropriate changes in the velocity of sound in silico n oil should be incorporated. Most modern ultrasound biometers have this option.
Dr. Saikumar S J: Biometry in silico ne oil-filled eyes always used to be a challenge during the days of ultrasound biometry. This is because the refractive index of silicone oil is grossly different from that of vitreous, and hence, the axial length is overestimated by approximately 30%. This overestimation leads to the implantation of lower power lenses, and finally, after the removal of silicone oil, there is significant residual hypermetropia. Various studies have proposed various correction formulas to overcome this error. As a rough estimate, measured axial length multiplied by 0.70 gives the actual axial length. This can still be used in very mature cataracts where optical coherence biometry cannot be done.
But nowadays in most eyes, optical coherence biometry is used in the silicone oil mode for the calculation of IOL power. In my experience, this gives good postoperative refractive outcomes without the need for a correction factor.
Dr. Minu Mathen: In optical biometry, it can be performed in silico ne oil mode, and I take the biometry values as such. If optical biometry is not possible, I use immersion ultrasound mode and multiply the axial length by a correction factor of 0.71.
Dr. Mathew Kurian: Most optical biometers measure eyes with silicone oil with minimal difficulty.
Immersion ultrasound is preferred to contact A-scan. The refractive index for that particular silicone oil should be used for calculating the axial length. Till the silicone oil is removed, the patient is going to have hyperopic refraction.
A conversion factor is available for ultrasound biometers that have no option for silicone oil setting. In machines that have the silicone oil option, one needs to select the right velocity of sound.
A confounding factor could be whether the eye partially filled with silicone oil and if so how much of the silicone oil bubble is in the path of the ultrasound beam.
5. Can you mention the situations in which you would like to validate axial length/keratometry measurements with different machines or by multiple readings on the same machine by different observers?
Dr. Arulmozhivarman: Accurate keratometry is very important for accurate refractive outcomes. If K values are doubtful, they have to be repeated by more than one observer with different instruments, particularly in post-LASIK patients. Very long eyes with posterior staphyloma will need more than one measurement preferably with different instruments.
Dr. Saikumar S J: Regarding axial length, our protocol is to first try the optical coherence biometry in all cases. In very dense cataracts, ultrasound biometry with the contact method is used. Very rarely in eyes with the axial length of more than 30 mm, B-scan ultrasonography can be used, but my experience with this technique has been quite disappointing since the accuracy is not very good.
With regard to keratometry, we have access to four devices, namely the Lenstar, IOL Master 700, Verion, and corneal topography. For routine monofocal IOLs, either Lenstar or IOL Master 700 is used. Both have the Barretts formulae, and there is hardly any difference between the two as far as refractive outcomes are concerned. But being a swept-source OCT technology, the IOL Master 700 can be used in most mature cataracts.
For toric and multifocal IOLs, we use either Lenstar or IOL Master 700 along with Verion, as it gives intraoperative guidance in IOL placement. In most cases, these two machines are usually in agreement. If there is a significant variation between the IOL Master 700 and Verion, we have the option of using the corneal topography and we take the values which matches closely with the topography values. If all three are in disagreement, it is usually due to an ocular surface issue. The values can be taken on a different day with proper lubrication. If the disagreement persists, it is better to avoid multifocal IOLs.
Dr. Minu Mathen: For all toric and multifocal IOLs, I do keratometry and axial length with two machines, and final keratometry readings are obtained from corneal topography. I always depend on biometry reading taken before any eye drop is applied on the eye and also treat the dry eye/OSD before finalizing the calculation.
Once it is treated, one will stop getting different values in different instances. I would reconfirm and repeat if the difference in IOL powers of both eyes is more than one diopter. Furthermore, if there is a residual postoperative refractive error of more than 0.5 diopter in the other eye.
Dr. Mathew Kurian: When remeasurement is needed, it is better done by the second technician without prior knowledge of the first reading.
Remeasure the keratometry in both eyes if
- Corneal curvature is <41 D or more than 47 D
- The amount of corneal astigmatism is >2.50 D
- The difference in the corneal cylinder is more than 1 D between eyes
- The average corneal power difference between the two eyes is >0.09 D. Average corneal power = (K1 + K2)/2.
- The corneal cylinder correlates poorly with the refraction cylinder
- The corneal diameter is <10.75 mm or >13.0 mm
- The patient cannot adequately fixate (e.g. mature cataract, macular hole, etc.,).
Remeasure the axial length in both eyes if
- Axial length measurement is <22 mm or more than 25 mm in either eye
- The difference between the two eyes is more than 0.3 mm that cannot be correlated with the patient's oldest refraction
- The axial length measurement does not correlate with the refraction
- An uncooperative patient.
A second person should repeat the axial length measurements, keratometry readings, and rerun the IOL power calculations for both eyes if:
- The IOL power for emmetropia is >3.00 diopters different than anticipated
- There is a difference in IOL power ≥1.00 diopter between the two eyes.
| Conclusion|| |
All the panelists preferred partial coherence interferometry- based biometry as their technique of choice and Barretts Universal II as the formula of choice. In postrefractive surgery eyes, calculations based on ASCRS website were accepted. In case of silicon oil-filled eyes, optical biometer with silicone oil mode was preferred by the panelists. Immersion A-scan was used in cases where optical biometers could not be used. The panelists also have more or less similar indications for repeat checking of IOL powers.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
| References|| |
Holladay JT, Gills JP, Leidlein J, Cherchio M. Achieving emmetropia in extremely short eyes with two piggyback posterior chamber intraocular lenses. Ophthalmology 1996;103:1118-23.
Hoffman RS, Vasavada AR, Allen QB, Snyder ME, Devgan U, Braga-Mele R, et al
. Cataract surgery in the small eye. J Cataract Refract Surg 2015;41:2565-75.