|Year : 2017 | Volume
| Issue : 2 | Page : 496-501
Post-LASIK biometry methods in myopes (comparative study)
Saber H Elsayed1, Abd El-Rahman E Sarhan1, Moataz F Elsawy1, Ahmed M Nadi Kamal Selim2
1 Department of Ophthalmology, Faculty of Medicine, Menoufia University, Menoufia, Egypt
2 Department of Ophthalmology, Mansoura University, Menoufia, Egypt
|Date of Submission||28-Mar-2016|
|Date of Acceptance||26-Jun-2016|
|Date of Web Publication||25-Sep-2017|
Ahmed M Nadi Kamal Selim
El-Mansoura, Dakahlia, El-Mansoura, 35517
Source of Support: None, Conflict of Interest: None
This study aimed to find a practical accurate method to overcome the errors of intraocular lenses (IOLs) power calculation in patients undergoing cataract extraction and IOL implantation following previous laser-assisted in-situ keratomileusis (LASIK).
Biometry for patients who had undergone LASIK.
Patients and methods
This study was carried out on 100 eyes to find a practical accurate method to overcome the errors of IOL power calculation in patients undergoing cataract extraction and IOL implantation following previous LASIK.
Shammas no history method showed an average error (+0.46 ± 0.69 D), with 84% of cases within ± 0.5 D and 92% within ± 1 D. Rosa no history method showed an average error (0.7 ± 1.31 D), with 16% of cases within ± 0.5 D and 64% within ± 1 D. Ferrara no history method showed an average error (1.78 ± 1.15 D), with 20% of cases within ± 0.5 D and 32% within ± 1 D. Haigis-L no history method showed an average error (−0.48 ± 1.25 D), with 56% of cases within ± 0.5 D and 68% within ± 1 D.
Shammas method was the most accurate method, followed by the Haigis-L method, the Rosa method, with more dioptric errors, and finally the Ferrara method, with the largest dioptric error.
Keywords: cataract extraction, errors of intraocular lenses, IOL implantation, laser assisted in situ keratomileusis, power calculation
|How to cite this article:|
Elsayed SH, Sarhan ARE, Elsawy MF, Nadi Kamal Selim AM. Post-LASIK biometry methods in myopes (comparative study). Menoufia Med J 2017;30:496-501
|How to cite this URL:|
Elsayed SH, Sarhan ARE, Elsawy MF, Nadi Kamal Selim AM. Post-LASIK biometry methods in myopes (comparative study). Menoufia Med J [serial online] 2017 [cited 2019 Apr 19];30:496-501. Available from: http://www.mmj.eg.net/text.asp?2017/30/2/496/215436
| Introduction|| |
An increasing number of cataract surgeries in eyes after myopic keratorefractive surgery are expected within the next few decades. Although cataract extraction may be possible without major technical obstacles, intraocular lens (IOL) power calculation is problematic .
Accurate IOL power calculation is highly dependent on accurate K reading, especially in eyes that have been subjected to postrefractive surgery. Thus, keratometric power measurement is an essential step for the requirements of these patients. Several methods are recommended to determine the effective corneal power as accurately as possible .
| Patients and Methods|| |
Analysis was carried out of the preoperative and 1-month postoperative data of 100 eyes that were subjected to laser-assisted in-situ keratomileusis (LASIK) for correction of myopia and myopic astigmatism. The patients of this study were recruited randomly (60%) from among the patients from the Menoufia University outpatient clinic and the remaining (40%) were recruited from other centers.
Patients of any age, both sexes, and those with myopic and myopic astigmatism were included.
Patients with hyperopic and hyperopic astigmatism, posterior segment disorders, those who had undergone a complicated LASIK procedure, those with collagen diseases, diabetes mellitus, or hormonal changes (such as pregnancy), and patients with a history of any previous refractive or ocular surgery and ocular injuries were excluded.
A complete preoperative assessment was performed including age, a thorough assessment of medical history to exclude patients who had collagen diseases, diabetes mellitus, or hormonal changes (such as pregnancy), and those with an ocular history to exclude any previous refractive, or ocular surgery and ocular injuries.
A thorough ophthalmic examination included the following:
- Visual acuity measurement using Landolt's broken ring chart.
- Manifest refraction using an autorefractometer.
- Evaluation of the best-corrected visual acuity.
- Slit-lamp biomicroscopy to examine the cornea, excluding the presence of any corneal disease, determination of anterior chamber depth (ACD), and noting any abnormalities in the iris or in the lens.
- Fundus examination: dilated fundus examination to exclude any posterior segment disorder.
- Pentacam corneal topography: to determine the simulated keratometry readings(Sim K1 and Sim K2) from the corneal topography printout.
Postoperative data were collected after 1 month including the following:
- Visual acuity.
- Manifest refraction using an autorefractometer.
- Evaluation of the best-corrected visual acuity.
- Slit-lamp biomicroscopy.
- Fundus examination.
- Biometry: [IOL master/A-scan ultrasound (US)]
Axial length (AL) measurement.
Corrected corneal refractive power was determined.
The samples (100 eyes) were divided into four groups to determine the corrected corneal power; each group included 25 eyes:
- Shammas no history method.
- Haigis method.
- Ferrara method (adjusted refractive index method).
- Rosa method.
IOL power calculation was carried out by the adequate formula using the different corneal readings from the clinical history method and no history methods equations.
The clinical history method was considered as the standard. Then, It was compared with the no history methods for shammas.
The data were grouped into four tables, a table for each method, and the average error of each method was calculated.
| Results|| |
The study was carried out on 100 eyes from 51 patients ranging in age from 19 to 39 years who had undergone the LASIK procedure. They were chosen according to the inclusion criteria mentioned before.
The ages of the patients ranged from 19 to 39 years, with a mean of 28.65 ± 6.31 years [Table 1]. Among the patients, there were 33 women (64.7%) and 18 men (35.3%). In total, 51 eyes were right sided and 49 were left sided [Table 2].
Statistics of the four groups
Group 1: clinical history method versus Shammas no history method
Shammas no history method showed an average error (+0.46 ± 0.69 D) compared with the gold-standard clinical history method, with 84% of cases (21 eyes) within ± 0.5 D and 92% (23 eyes) within ±1 D.
The most common error was + 0.5 D, which occurred in 48.0% of cases (12 eyes) [Table 3].
Group 2: clinical history method versus Rosa no history method
Rosa no history method showed an average error (0.7 ± 1.31 D) compared with the gold-standard clinical history method, with 16% of cases (four eyes) within ±0.5 D and 64% (16 eyes) within ±1 D.
The most common was + 1.0 D, which occurred in 36.0% of cases (nine eyes) [Table 4].
Group 3: clinical history method versus Ferrara no history method
Ferrara no history method showed an average error (1.78 ± 1.15 D) compared with the gold-standard clinical history method, with 20% of cases (five eyes) within ±0.5 D and 32% (eight eyes) within ±1 D.
The most common error was +1.5 D and +2.0 D, which occurred in 24.0% of cases (six eyes) [Table 5].
Group 4: clinical history method versus Haigis-L no history method
Haigis-L no history method showed an average error (−0.48 ± 1.25 D) compared with the gold-standard clinical history method, with 56% of cases (14 eyes) within ±0.5 D and 68% (17 eyes) within ±1 D.
The most common error was 0.0 D, which occurred in 28.0% of cases (seven eyes) [Table 6].
| Discussion|| |
Cataract is one of the most common diseases affecting the transparency of the crystalline lens of the eye. The treatment of cataract includes removal of the cataractous lens and implantation of a new IOLs instead of natural crystalline lens. To achieve maximum visual acuity after surgery, the preoperative biometry must be accurate and an accurate IOL power must be used .
Various formulae have been developed to calculate the IOL power on the basis of various measurements (biometry). To calculate the IOL power, we need measurements such as the keratometric value (K), AL of the eye, and ACD of the eye ball. Biometry consists of these all measurements and this information is fed into a variety of formulae to calculate IOL power .
The measurement of axial eye length is one of the most important steps for IOL power calculation and it can be measured using a laser interferometer-based system (IOL master) or an US-based system .
The A-scan US biometry is the most commonly used method for this essential part of the preoperative evaluation of cataract surgery patients .
There are mainly two methods in A-scan biometry: the contact method or applanation method and the immersion method. The contact method is performed by gently placing the probe on the corneal vertex and directing the sound beam through the visual axis. The immersion technique of biometry is accomplished by placing a small scleral shell between the patient's lids, filling it with saline, and immersing the probe into the fluid, being careful to avoid contact with the cornea .
The keratometric reading needed to calculate the IOL power can be measured using a keratometer. Manual and automatic keratometers are available nowadays to measure the K-value .
IOL master is a combined biometric instrument that measures quickly and precisely parameters of the human eye needed for IOL power calculation using a noncontact technique. It also incorporates the software to calculate the IOL power from various formulae .
On the basis of their deviation, IOL power formulae are grouped into theoretical formulae and regression formulae. The theoretical formulae were derived from the geometric optics as applied to the schematic eyes using theoretical constants. It is based on three variables: the AL of the eye ball, K reading, and the estimated postoperative ACD. The regression formulae were developed to overcome the drawbacks of the theoretical formulae. The regression formulae are based on a regression analysis of the actual postoperative results of implant power as a function of the variables of corneal power and AL .
Calculation of IOL power in patients who have undergone previous refractive surgery has been the subject of a significant amount of research over the past decade, including many reviews and editorials .
It is becoming an increasingly important issue for two particular reasons. First, millions of patients over the past decade have undergone laser refractive surgery (i.e. LASIK, laser-assisted epithelial keratomileusis, photorefractive keratectomy) and as they age, they will eventually undergo cataract surgery. According to surveys of members of the American Society of Cataract and Refractive Surgery, approximately one million refractive surgery procedures were performed in the USA per year in 2004 and 2005 .
Second, with the increasing popularity of multifocal and accommodating lenses, there is a greater necessity to obtain accurate results in this group of patients as small errors in lens power choice can result in patient dissatisfaction or the need for further surgery .
Patients who have undergone previous LASIK, photorefractive keratectomy, or laser-assisted epithelial keratomileusis are exacting and challenging individuals who would like to achieve a specific refractive result to minimize their dependence on glasses after surgery. They have already elected to undergo previous refractive surgery and will be expecting the same level of success with their cataract surgery .
As myopic laser refractive surgery has been the most commonly used corrective procedure over the last decade, the majority of research and interest has focused on this topic. There are, however, significant differences in the calculation methodology after other refractive procedures such as hyperopic LASIK or radial keratotomy .
It is important for the cataract surgeon to be able to perform IOL calculations in patients with previous myopic laser refractive surgery as well as to understand the differences in the calculation methodology for other procedures .
The two important measurements that aid accurate IOL power calculation are the AL and the corneal curvature. Their importance is highlighted by the fact that an error of 1.0 D in keratometry would result in an equivalent error in IOL power in an eye of average AL. In eyes shorter than 22 mm, the magnitude of the error in IOL power estimation would increase .
After corneal refractive surgery, the most important change is in the corneal shape. Thus, any inaccuracies in IOL power calculation in this situation arise from these changes .
Keratorefractive surgery for myopia flattens the anterior corneal radius, but leaves the posterior corneal radius mostly unchanged. Because the central cornea has been flattened, keratometry may read a falsely large area. Rather than measuring several points within a central 2.5–3.5 mm area, the measurement may instead be at 6.0 mm. With the nonincisional forms of keratorefractive surgery, such as LASIK, the index of refraction of the cornea is probably changed. Automated keratometry and corneal topography analysis therefore use incorrect assumptions to measure corneal power. The cornea can no longer be compared with a sphere centrally. The back surface is no longer 1.2 mm steeper than the front surface as in a normal cornea. For an anterior corneal radius of 7.5 mm, using the standardized keratometric index of refraction of 1.3375, the corneal power would be 45 D, thereby overestimating the total power by 0.56 D. Most IOL calculations today used a net index of refraction of 1.3333 (4/3) and the anterior radius of the cornea to calculate the net power of the cornea. Using this, the total power of a cornea = 44.44 D. The major source of error in such patients, therefore, lies in the accurate determination of the keratometric power .
After keratorefractive surgery for myopic correction, the optical zone of the cornea becomes flatter. Usually, inside the pupillary area, the central cornea becomes flatter than the portion of the cornea that lies over the marginal zone of pupil; therefore, its prolate shape becomes oblate (with reverse asphericity) .
In postmyopic refractive laser in-situ keratomileusis, the practitioner will measure the keratometry values from the steeper part of the cornea rather than the centrally flattened portion. The K is estimated to be high, thereby estimating a falsely lower IOL power giving rise to a hyperopic surprise .
Manual and automated keratometers evaluate the radius of curvature designated by four points in orthogonal meridians separated about 3.2 mm apart with manual keratometers or about 2.6 mm apart in automated keratometers. Corneal optics in keratometers is assumed to be spherocylindrical. Normal corneas are nearly spherical or prolate. A patient who, before the refractive surgery, is myopic will develop a relatively flatter cornea in the center and it will become steeper in the periphery. Postrefractive surgery corneas, however, are oblate .
In 1989, Holladay was the first to publish and popularize two methods in an attempt to predict the actual corneal power in refractive surgery eyes. Hoffer referred to them as the clinical history method and the contact lens method .
This approach has been confirmed and has been used as the gold standard in studies comparing the accuracy of different methods for IOL power calculation in eyes with previous corneal refractive surgery .
A limitation is that the clinical history method requires pre-LASIK keratometry, pre-LASIK refraction, and post-LASIK stable refraction, which renders this method unpractical as it cannot be used if these pre-LASIK data are not available.
To date, there is no available method that can estimate the true K reading with absolute certainty when there no pre-LASIK data are available, but there are many methods that have been under trial recently, with very variable results.
A number of regression formulae have been developed to calculate the IOL power for patients when no historical information is available. These formulae use the postoperative K measurement and an adjustment on the basis of collected data from refractive surgery patients. These formulae are still a less accurate way of calculating the postrefractive surgery IOL power than the formulae that use the historical data.
Therefore, it is essential to use a standard nonhistory-dependent method for IOL calculation in patients who have undergone a previous LASIK procedure.
In this study, we have tested some of the methods that do not need pre-LASIK data; we chose keratometry and IOL master as they are the standard investigations used for ocular biometry and IOL power calculation.
The methods that we investigated are Shammas no history method, Rosa no history method, and Ferrara no history method, which are keratometry based, and Haigis-L no history method, which is IOL master based.
We considered the clinical history as a standard and compared it with each of the above four methods to find the most accurate method.
The first method is Shammas no history method, which yielded fairly accurate results in all study groups examined to date, and Shammas and Shammas reported an average error in IOL power prediction of 0.55 ± 0.31 D in a sample of eyes without known preoperative corneal power in his study .
Th is method was first published by Dr. John Shammas in 2007 in the Journal of Cataract and Refractive Surgery. Since then, the efficacy of shammas no histrory method had been compared with other types of calculation methods; no history as well as history-based methods and documented in several peer-reviewed papers in a successful way.
In our study, Shammas no history method showed an average error in IOL power prediction of 0.46 ± 0.69 D in a sample of 25 eyes.
The next methods examined were Rosa no history method and Ferrara no history method, which seemed to be inaccurate, especially the Ferrara method.
In 2010, Kenneth J. Hoffer carried out a large-scale study in which the Rosa and Ferrara methods were investigated and they reported significant errors, with an average error in IOL power prediction of 1.90 ± 1.10 D for the Rosa method and for 3.64 ± 1.45 D for the Ferrara method .
Rosa's method is based on a correlation between the AL and the corneal radius correction factor. Ferrara's method is based on a correlation between the length and the theoretical variable refractive index.
In our study, we obtained more accurate results; the Rosa no history method showed an average error in IOL power prediction of 0.7 ± 1.31 D in a sample of 25 eyes. The Ferrara method showed an average error in IOL power prediction of 1.78 ± 1.15 D in a sample of 25 eyes.
Finally, the Haigis-L no history method was studied. In 2003, Haigis reported an average error in IOL power prediction of 0.62 ± 0.55 D in his study .
In our study, the Haigis-L no history method showed an average error in IOL power prediction of − 0.48 ± 1.25 D in a sample of 25 eyes.
This study has some limitations and further investigation is warranted. The main limitation is that the sample size was relatively small and a larger sample is required before safe standardization of these methods, especially Shammas no history method for keratometry-based post-LASIK biometry.
| Conclusion|| |
Shammas no history method was the most accurate method with the smallest average dioptric error.
In terms of the Haigis-L method, although more cases showed zero error than the Shammas method, but came in the second place with a larger average dioptric error.
This was followed by the Rosa method, which led to more dioptric errors than the Shammas and Haigis-L methods.
Finally, the Ferrara method came showed the largest dioptric error, with overestimation of the IOL power.
Our recommendation is that Shammas no history method should be used for keratometry-based biometry and the Haigis-L method should be used for IOL master-based biometry.
Rosa and Ferrara methods have lower accuracy than Shammas and Haigis-L methods; thus, they are not recommended.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]