Accuracy of optical biometry combined with Placido disc corneal topography for intraocular lens power calculation

The role of posterior corneal power in 21st century biometry: A review

PURPOSE

Intraocular lens (IOL) calculation and biometry have evolved significantly in recent decades. However, present outcomes are still suboptimal. Our objective is to summarize the results reported in the literature with regard to a new variable, the value of the relationship between anterior and posterior corneal curvature in the biometric calculation of IOL power.

METHODS

We have created a narrative revision of the existing evidence regarding the posterior to anterior corneal curvature ratio in IOL calculation.

RESULTS

The corneal posterior/anterior ratio (P/A ratio), also called Gullstrand ratio, has a standard deviation of 2.4% in normal people, hence causing a possible IOL power miscalculation error of up to 0.75 diopters (D). This error is magnified in pathological corneas or in those with previous refractive surgery. Including the P/A ratio in the IOL formula reduces errors in the calculation of IOL power.

CONCLUSIONS

Measurement of the posterior corneal surface should be recommended prior to IOL calculation, given the demonstrated results regarding the P/A ratio for IOL power calculation. Regarding toric IOL calculation, we suggest incorporation of all internal astigmatic vectors, for instance, posterior corneal surface, IOL tilt induced toricity, and retinal astigmatism. All of these factors may improve surgical outcomes.

Recent developments in intraocular lens power calculation methods-update 2020

For many decades only a few formulas have been available to calculate the intraocular lens (IOL) power for patients undergoing cataract surgery: the Haigis, Hoffer Q, Holladay 1 and 2 and SRK/T. In recent years, several new formulas for IOL power calculation have been introduced with the aim of improving the accuracy of refraction prediction in eyes undergoing cataract surgery. These include the Barrett Universal II, the Emmetropia Verifying Optical (EVO), the Kane, the Næser 2, the Olsen, the Panacea, the Pearl DGS, the Radial Basis Function (RBF), the T2 and the VRF formulas. Although most of them are unpublished so that their structure is unknown, we give an overview of each formula and report the results of the studies that have compared them. Their performance in short and long eyes is provided and a special focus is given on the issue of segmented axial length, which is a promising method to obtain more accurate outcomes in short and long eyes. Here, the group refractive index originally developed for the IOLMaster may not represent the best method to convert the optical path length into a physical distance. The issue of posterior and total corneal astigmatism (TCA) is discussed in relation to toric IOLs; the latest formulas for toric IOLs and their results are also reported.

An Advanced Lens Measurement Approach (ALMA) in post refractive surgery IOL power calculation with unknown preoperative parameters

PURPOSE

To test a new method to calculate the Intraocular Lens (IOL) power, that combines R Factor and ALxK methods, that we called Advance Lens Measurement Approach (ALMA).

DESIGN

Retrospective, Comparative, Observational study.

SETTING

Department of Medicine and Surgery, University of Salerno, Italy.

METHODS

Ninety one eyes of 91 patients previously treated with Photorefractive Keratectomy (PRK) or Laser-Assisted in Situ Keratomileusis (LASIK) that underwent phacoemulsification and IOL implantation in the capsular bag were analyzed. For 68 eyes it was possible to zero out the Mean Errors (ME) for each formula and for selected IOL models, in order to eliminate the bias of the lens factor (A-Costant). Main outcome, measured in this study, was the median absolute error (MedAE) of the refraction prediction.

RESULTS

In the sample with ME zeroed (68 eyes) both R Factor and ALxK methods resulted in MedAE of 0.67 D. For R Factor 33 eyes (48.53%) reported a refractive error <0.5D, and 53 eyes (77.94%) reported a refractive error <1D, For ALxK method, 32 eyes (47.06%) reported a refractive error <0.5 D, and 53 eyes (77.94%) reported a refractive error <1 D. ALMA method, reported a MedAE of 0.55 D, and an higher number of patients with a refractive error <0.5 D (35 eyes, 51.47%), and with a refractive error <1 D (54 eyes, 79.41%).

CONCLUSIONS

Based on the results obtained from this study, ALMA method can improve R Factor and ALxK methods. This improvement is confirmed both by zeroing the mean error and without zeroing it.

Regional changes in corneal shape over a 6-month follow-up after femtosecond-assisted LASIK

PURPOSE

To assess the regional changes in corneal shape after femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK) in patients with different myopia extents.

SETTING

Eye Hospital, Wenzhou Medical University, Wenzhou, China.

DESIGN

Retrospective case series.

METHODS

A retrospective study of myopic eyes treated with FS-LASIK was conducted to assess the shape changes within different corneal regions after surgery. Corneal curvature was measured in the central region (0 mm to 3.0 mm diameter), pericentral region (3.0 mm to 6.0 mm diameter) and peripheral region (6.0 mm to 9.0 mm diameter) preoperatively and from 1 week to 6 months postoperatively.

RESULTS

The study comprised 608 myopic eyes. During the 6-month follow-up, the anterior cornea became steeper in the central and pericentral regions, but flatter in the peripheral region (P < .01), representing a partial, gradual, yet significant reversal of the immediate change in corneal shape after laser ablation. In contrast, the posterior surface experienced significantly less change than the anterior surface, with the cornea becoming slightly flatter (P < .01) in the central region at 1 week postoperatively, and steeper elsewhere (P < .05), and then remaining stable during the rest of the follow-up. On the other hand, the anterior astigmatism had significant decreases in the central region (P < .01) and slight increases in the peripheral region (P < .01) 1 week postoperatively, and that remained stable over the follow-up period. In contrast, there were little or non-significant changes in the posterior astigmatism throughout the follow-up (P > .05).

CONCLUSIONS

Postoperative corneal shape changes were different in different regions. There were shape changes in individual corneal regions during the 6-month follow-up period that represented reverse trends; however, the changes were much smaller than the short-term changes observed 1 week after surgery.

Accuracy of swept-source optical coherence tomography based biometry for intraocular lens power calculation: a retrospective cross-sectional study

Background

To evaluate the accuracy of biometric measurements by a swept-source optical coherence tomography (SS–OCT) based biometry for intraocular lens (IOL) power calculation.

Methods

This retrospective observational study enrolled 431 patients undergoing cataract surgery. The charts were reviewed to investigate the failure rate of axial length (AL) measurement of the SS–OCT biometer, partial coherence interferometry (PCI), and A–scan ultrasonography (US) according to cataract type and severity. AL and keratometry in 164 eyes with the same IOL inserted were measured using the SS–OCT biometer, PCI, and A–scan US. The SRK/T formula was used to calculate IOL power. The mean absolute error (MAE) and percentage of eyes with a prediction error (PE) of ±0.50 D were compared.

Results

The AL measurement failure rate was 0.00% for A–scan US, 2.32% for the SS–OCT biometer, and 15.31% for PCI. The number of eyes measured using three devices (SS–OCT biometer, PCI, and A–scan US) was 128 (Group A) and the number of eyes measured using two devices (SS–OCT biometer and A–scan US) was 36 (Group B). The score of posterior subcapsular opacity was significantly different between two groups (p < .001). The SS–OCT biometer and PCI showed significantly lower MAE compared to A–scan US in Group A (p = 0.027). Using SS-OCT biometer, MAE showed no significant difference between Group A (0.36 ± 0.27) and Group B (0.36 ± 0.31) (p = 0.785). Whereas, MAE of A-scan US was significantly higher than Group A (0.47 ± 0.39) in Group B (0.64 ± 0.36) (p = 0.023).

Conclusions

Using biometry with advanced OCT is useful in clinical practice as it is more effective in obtaining biometric measurements in the eyes with PSC and provides accurate measurements for IOL power calculation regardless of cataract type and severity.

Trial registration

Retrospectively registered. Registration number: KC16RISI1020. Registered 03 January 2018.

Biometry in cataract surgery: a review of the current literature

PURPOSE OF REVIEW

To review the literature in 2017 and 2018 pertaining to biometry for cataract surgery and report pertinent findings.

RECENT FINDINGS

New devices using swept-source ocular coherence tomography can measure axial length in dense cataracts more frequently than common biometers. Computer-assisted registration may be superior to intraoperative aberrometry for toric intraocular lens (IOL) placement. Soft contact lenses may not require removal as long before biometry as previously thought. The Barrett Universal II IOL formula has been found to perform well at all axial lengths.

SUMMARY

New swept-source ocular coherence tomography biometers are more frequently successful at measuring axial length in dense cataracts which promises to improve refractive outcomes. Accuracy in toric IOL placement is likely to increase with improved devices. It may not be necessary to have patients remove soft contact lens any more than 2 days prior to biometry. The Barrett Universal II IOL formula may be used confidently for most eyes. Advancements acknowledged, purchasing new equipment will not be necessary for all surgeons.

A pilot study of intraocular lens explantation in 69 eyes in Chinese patients

AIM

To study the effects of intraocular lens (IOL) explantation and demographic characteristics.

METHODS

Retrospective non-comparative case series. Clinical data recorded from patient charts included the following: demographic, preoperative and postoperative characteristics; complications; surgical methods, and changes in visual acuity.

RESULTS

A total of 69 eyes in 67 Chinese patients who received IOL explants were studied. The patients' mean age at the time of explantation was 46.1 years old [SD 22.5 (6-85)], and 37 patients were female (55.2%). Regarding employment, 47.8% were farmers, 23.9% were retired, 16.4% were students, 4.5% were unemployed, 3% were workers, and 4.5% were other (including staff members, teachers and officers). The main reasons for explantation were dislocation/decentration in 41 cases (59.4%) and retinal detachment in 10 cases (14.5%). The third most prevalent cause was incorrect lens power in 7 eyes (10.1%). The remaining reasons were endophthalmitis in 6 cases (8.7%), posterior capsular opacity in 3 eyes (4.3%), and impacting retinal surgery operation in 2 cases (2.9%). The main comorbidities were high myopia in 18 eyes (26.1%), trauma in 8 eyes (11.6%), retinal detachment in 6 eyes (8.7%), congenital cataracts in 8 eyes (11.6%), and Marfan's syndrome in 2 eyes (2.9%). The mean time from implantation to explantation was 4.0y [SD 4.2 (0.005-15)]. Treatment after explantation included posterior chamber IOL implantation in 44 eyes (63.8%) and aphakia in 25 eyes (36.2%). After surgery, the best corrected visual ability (BCVA) was improved in 50 cases (72.5%), including 28 patients (40.6%) in whom visual ability was improved by more than two lines.

CONCLUSION

Dislocation/decentration is the main cause for explantation, and high myopia is a main risk factor. Posterior chamber IOL implantation remains the most elected treatment after explantation.