Magnification and accommodation with phakic intraocular lenses

Monte-Carlo simulation for calculating phakic supplementary lenses based on a thick and thin lens model using anterior segment OCT data

BACKGROUND

Phakic lenses (PIOLs, the most common and only disclosed type being the implantable collamer lens, ICL) are used in patients with large or excessive ametropia in cases where laser refractive surgery is contraindicated. The purpose of this study was to present a strategy based on anterior segment OCT data for calculating the refraction correction (REF) and the change in lateral magnification (ΔM) with ICL implantation.

METHODS

Based on a dataset (N = 3659) containing Casia 2 measurements, we developed a vergence-based calculation scheme to derive the REF and gain or loss in ΔM on implantation of a PIOL having power PIOLP. The calculation concept is based on either a thick or thin lens model for the cornea and the PIOL. In a Monte-Carlo simulation considering, all PIOL steps listed in the US patent 5,913,898, nonlinear regression models for REF and ΔM were defined for each PIOL datapoint.

RESULTS

The calculation shows that simplifying the PIOL to a thin lens could cause some inaccuracies in REF (up to ½ dpt) and ΔM for PIOLs with high positive power. The full range of listed ICL powers (- 17 to 17 dpt) could correct REF in a range from - 17 to 12 dpt with a change in ΔM from 17 to - 25%. The linear regression considering anterior segment biometric data and the PIOLP was not capable of properly characterizing REF and ΔM, whereas the nonlinear model with a quadratic term for the PIOLP showed a good performance for both REF and ΔM prediction.

CONCLUSION

Where PIOL design data are available, the calculation concept should consider the PIOL as thick lens model. For daily use, a nonlinear regression model can properly predict REF and ΔM for the entire range of PIOL steps if a vergence calculation is unavailable.

Changes in Accommodative and Binocular Function following Phakic Intraocular Lens for High and Low-to-Moderate Myopia

The aim was to evaluate accommodative and binocular function of phakic intraocular lens implantable collamer lens (ICL) in high and low-to-moderate myopia. Prospective comparative cohort study with 38 myopic patients who underwent ICL implantation were divided into two groups of 19 patients, each one based on the spherical equivalent (SE): high-power (SE ≤ −6 D) and low-to-moderate (SE > −6 D). The push-up amplitude of accommodation (AA), monocular accommodative facility (MAF), distance and near ocular deviation, near convergence amplitude, near point convergence (NPC), stereopsis, and accommodative convergence/accommodation (AC/A) ratio were assessed before surgery and 1 week and 1 month postoperatively. The mean residual refractive error at 1 month after surgery improved in both groups, 0.18 ± 0.34 D and 0.09 ± 0.26 D, respectively (p < 0.001). There was a significant decrease in AA in both groups between preoperatively and at 1-week (p = 0.001; p = 0.008, respectively) and 1-month follow-up (p = 0.001; p = 0.008). For the rest of the binocular measurements, no statistically significant postoperative changes were found in any group. This finding suggests follow-up studies on amplitude of accommodation in phakic intraocular lens ICL implantation.

Binocular visual function after unilateral versus bilateral implantation of segmented refractive multifocal intraocular lenses: a pilot study

PURPOSE

To evaluate binocular visual function after unilateral and bilateral implantation of segmented refractive multifocal intraocular lenses (MIOLs).

METHODS

This prospective comparative pilot study included patients who underwent SBL-3 (Lenstec; + 3.00 D) implantation at Peking University Third Hospital. Patients were divided into two groups (monocular or binocular surgery). Thirty-two patients with emmetropic presbyopic contralateral eyes and 49 patients with bilateral SBL-3 implantation within a week between eyes were included in the unilateral SBL-3 and bilateral groups, respectively. At 3-month follow-up, the main outcomes were binocular uncorrected distant, intermediate, and near visual acuity (UDVA, UIVA, and UNVA). Secondary outcomes included binocular best-corrected visual acuity at all distances, defocus curve, contrast sensitivity, photic phenomena, spectacle independence, patient satisfaction, and National Eye Institute Visual Function Questionnaire (NEI-VFQ-25) score. The essential perceptual phenomena constituting binocular vision, simultaneous perception, fusion, and stereopsis were also evaluated.

RESULTS

Both groups showed similar binocular UDVA and UIVA, but UNVA was significantly better in the bilateral group (0.07 ± 0.07 versus 0.12 ± 0.07, P = 0.008). Better binocular defocus curve at intermediate to near focal points, higher percentage of spectacle independence at near distance, and NEI-VFQ-25 near activity scores were observed in the bilateral group. No significant differences in contrast sensitivity, photic phenomena, overall satisfaction, other NEI-VFQ-25 subscales, fusional amplitude, and stereoacuity were found between groups.

CONCLUSION

Unilateral implantation of segmented refractive MIOL provided desirable distant visual acuity and high patient satisfaction, but inferior intermediate and near visual outcomes compared with bilateral implantation.

[Simple method to estimate object to image magnification and aniseikonia following cataract surgery]

BACKGROUND AND PURPOSE

Aniseikonia as one of the major risk factors for asthenopic problems is mostly overlooked in modern cataract surgery. The purpose of this study was to develop a simple calculation scheme for clinicians to predict the object to image magnification in a pseudophakic eye with biometric data.

METHODS

The calculation scheme for object to image magnification in the pseudophakic eye is based on a vergence calculation of the lens power with theoretical optical formulae. From the biometric data, which are typically derived from both eyes during lens power calculation, the vergences in front of and behind the 3 or 4 refractive surfaces of the pseudophakic eye model are used to predict the magnification for objects at infinity or objects located at a finite measurement distance (e.g. 5 m).

RESULTS

With a formula-based lens power calculation a pseudophakic eye model is set up with 3 or 4 refractive surfaces (postoperative spectacle refraction; thick cornea described by anterior surface or thick cornea characterized by anterior and posterior surfaces; intraocular lens). The vergence in front of and behind each refractive surface is derived by means of linear Gaussian optics. The quotient of the product of all vergences in front of the surfaces and the product of all vergences behind the respective surfaces describes the object to image magnification of the eye. A comparison of the object to image magnification of both eyes yields the retinal image size disparity or aniseikonia. This calculation strategy is shown in a step-by-step approach exemplarily for the Haigis and Hoffer‑Q formulae (3 surfaces) and the Castrop formula (4 surfaces).

CONCLUSION

If during planning and lens power calculation biometry is performed for both eyes, ocular magnification of both eyes can be easily derived with this calculation scheme and aniseikonia can be extracted from a comparison of magnification of both eyes. Such a simple prediction should be established as a standard for precataract biometry and lens power calculation for early detection and avoidance of asthenopic complaints after cataract surgery.

Calculation of ocular magnification in phakic and pseudophakic eyes based on anterior segment OCT data

PURPOSE

The purpose of this study is to develop a straightforward mathematical concept for determination of object to image magnification in both phakic and pseudophakic eyes, based on biometric measures, refractometry and data from an anterior segment optical coherence tomography (OCT).

METHODS

We have developed a strategy for calculating ocular magnification based on axial length measurement, phakic anterior chamber and lens thickness, keratometry and crystalline lens front and back surface curvatures for the phakic eye, and axial length measurement, anterior chamber and lens thickness, keratometry and intraocular lens power, refractive index and shape factor for the pseudophakic eye. Comparing the magnification of both eyes of one individual yields aniseikonia, while comparing the preoperative and postoperative situation of one eye provides the gain or loss in ocular magnification. The applicability of this strategy is shown using a clinical example and a small case series in 78 eyes of 39 patients before and after cataract surgery.

RESULTS

For the phakic eye, the refractive index of the crystalline lens was adjusted to balance the optical system. The pseudophakic eye is fully determined and we proposed three strategies for considering a potential mismatch of the data: (A) with spherical equivalent refraction, (B) with intraocular lens power and (C) with the shape factor of the lens. Magnification in the phakic eye was -0.00319 ± 0.00014 and with (A) was -0.00327 ± 0.00013, with (B) was -0.00323 ± 0.00014 and with (C) was -0.00326 ± 0.00013. With A/B/C, the magnification of the pseudophakic eye was on average 2.52 ± 2.83%/1.31 ± 2.84%/2.14 ± 2.80% larger compared with the phakic eye. Magnification changes were within a range of ±10%.

CONCLUSIONS

On average, ocular magnification does not change greatly after cataract surgery with implantation of an artificial lens, but in some cases, the change could be up to ±10%. If the changes are not consistent between the left and right eyes, then this could lead to post-cataract aniseikonia.

A systematic review about the impact of phakic intraocular lenses on accommodation

Phakic intraocular lenses (pIOL) are the main treatment for patients who have either high ametropia or contraindications for laser refractive surgery. The main feature that makes this kind of lenses suitable for its implantation in young adults searching for independence of optical prescription is the conservation of accommodation, since lens extraction is not required. A systematic review has been performed to evaluate the scientific literature on the effect of pIOL implantation on accommodation. Critical assessment of the articles included in the review was achieved using the tool Critical Appraisal Skills Programme in its Spanish form (CASPe). After revising the complete text of 10 articles pre-selected, two quasi-experimental pre-post studies evaluating the outcomes of a specific model of posterior chamber pIOL were included in the systematic review. The CASPe scoring of both studies were 5/11. According to this outcome, the evidence describing the impact of the pIOL implantation on the accommodative function can be defined poor. Some trends are reported as the decrease in the amplitude of accommodation, a decrease positive relative accommodation and improvement of accommodation. However, these results should be confirmed in future controlled studies.

Binocular vision alterations after refractive and cataract surgery: a review

To review binocular and accommodative disorders documented after corneal or intraocular refractive surgery, in normal healthy prepresbyopic patients. A bibliographic revision was performed; it included works published before 1st July 2017 where accommodation and/or binocularity was assessed following any type of refractive surgical procedure. The search in Pubmed yielded 1273 papers, 95 of which fulfilled the inclusion criteria. Few publications reporting binocular vision and/or accommodative changes after refractive surgery in normal subjects were found. The reduction in fusional vergence is the most frequently reported alteration. Anisometropia is an important risk factor for postoperative binocular vision-related complaints. Most diplopia-related visual complaints, irrespective of the surgical procedure, were in fact misdiagnosed preoperative disorders. The preoperative evaluation of patients seeking spectacle/contact lens independence should include a complete binocular and accommodation assessment where parameters such as the phoric posture, accommodative amplitude and facility, near point of convergence, fusional reserves and accommodative convergence/accommodation coefficient are measured. This would allow the identification of risk factors that could compromise the success of the refractive surgery and cause clinical symptoms.

Pre-operative visual evaluation for implantable collamer lenses in highly myopic eyes

PURPOSE

The aim of this study was to evaluate a clinical procedure based on trial rigid gas-permeable (RGP) contact lenses fitting to assess the real pre-operative visual acuity (VA) of highly myopic eyes undergoing surgery with an implantable collamer lens.

METHODS

Highly myopic eyes with a spectacle visual acuity (SVA) of 0.2 logMAR (6/9.6 Snellen) or worse were enrolled. A trial RGP contact lens with high dioptric power was fitted before collamer lens implantation. The trial contact lens residual refractive error was corrected by spectacle over-refraction to estimate contact lens VA (CLVA). The three-month post-operative uncorrected vision and visual acuities were also measured. The safety and efficacy indices of collamer lens implantation were calculated based on pre-operative SVA and CLVA, respectively.

RESULTS

The study enrolled 42 patients with 68 highly myopic eyes with a mean pre-operative spherical equivalent of -16.40 ± 4.43 D. The mean pre-operative SVA was 0.38 ± 0.19 logMAR. Pre-operative trial RGP contact lens fitting resulted in a significant improvement of VA (2.02 ± 1.05 lines), which was statistically insignificantly different from the VA with the implantable collamer lens (2.07 ± 0.8 lines). The safety index of collamer lens implantation was 1.70 ± 0.38 in reference to the pre-operative SVA but significantly decreased to 1.02 ± 0.17 in reference to CLVA. Similarly, the efficacy index of 1.17 ± 0.48 on SVA decreased to 0.71 ± 0.28 on CLVA.

CONCLUSIONS

Trial RGP contact lens fitting was found to indicate the potential implantable collamer lens VA more accurately than spectacle refraction. Consideration should be given to the use a trial RGP contact lens as part of the pre-operative work-up prior to collamer lens implantation in order to provide an assessment of the patient's potential post-operative VA.

[Determination of toric intraocular lenses]

BACKGROUND

In the last decades, toric posterior chamber lenses (TPCLs) for cataract surgery and phakic toric lenses (PTLs) for refractive surgery have become more and more popular for correcting high or excessive corneal astigmatism. The purpose of this article is to present a vergence-based calculation scheme for TPCLs and PTLs.

METHODS

In Gaussian optics (in the paraxial space), spherocylindrical optical surfaces can be described in a mathematically equivalent formulation as vergences. There are dual notations: The standard notation is used for transforming vergences through a homogeneous optical medium, and the component notation is applied to add up the power of a refractive surface to the vergence. Both notations can be used interchangeably. For calculating TPCLs, the vergences in front of and behind the predicted pseudophakic lens position are determined and subtracted. For calculating PTLs, the anterior vergence at the predicted lens position is estimated for the preoperative and postoperative states, and the difference between the two yields the desired lens power. WORKING EXAMPLES: In the 1(st) example, the power of a thin TPCL is determined step by step by applying the presented calculation scheme, which was designed to be transferred directly to a simple computer program (e.g., Microsoft Excel). In the 2(nd) example, the postoperative refraction is estimated for a simulation in which a TPCL similar to that in example 1 is implanted in a slightly misaligned orientation. In a 3(rd) example, the power of a PTL is determined step by step using the above-mentioned calculation scheme.

CONCLUSIONS

The presented calculation scheme allows determination of"thin" TPCLs or PTLs to achieve spherocylindrical target refraction with a cylinder axis at random or to predict the postoperative refraction for any toric lens implanted in any axis. The concept can be easily generalized to"thick" toric intraocular lenses if the geometric data and refraction index of the material are known.