Corneal Pachymetry in Prediction of Refraction After Cataract Surgery

Muneera A. Mahmood, MD, FRCOphth Martin F. Wilkes, MD

None of the authors have financial interests to disclose.

Background

Corneal pachymetry is a ready available and easily obtainable measurement To our knowledge, the potential association between corneal pachymetry and post-operative refraction following cataract surgery has not been looked at Given that many assumptions are made regarding prevailing Intraocular lens implant (IOL) calculation formulas (pachymetry, ratio of Ant/Post corneal curvature, refractive index of the cornea), it seems plausible that corneal thickness may have an effect on refraction

Purpose

To determine if pre-operative corneal pachymetry is a predictor of refraction after cataract surgery, independent of axial length and keratometry measurements, used in conventional biometry predictive formulas.

Methods & Study Design

IRB approved retrospective review of 91 eyes of 91 patients operated on between January 1, 2004 and March 30, 2009 Pachymetry measurements obtained via Anterior Segment OCT (AS-OCT; Zeiss Visante ) or Ultrasound (US; DGH Technology Inc, Pachette 2) Each patient had complete pre-operative evaluation including: Visual acuity (VA) and Manifest Refraction Slit lamp and dilated fundoscopic examination Pre-operative IOL measurements obtained via immersion A-scan biometry (Innovative Imaging Inc.) or IOL master (Zeiss) IOL power was calculated using the SRK/T formula Post-operative refraction and best-corrected visual acuity were recorded 8-10 weeks following surgery.

Inclusion Criteria

Placement of an SA60AT IOL in the bag Post-op VA of at ≥ 20/60 Pre-op corneal pachymetry measurement (AS OCT or US) Exclusion Criteria History of prior refractive surgery or scleral buckling Complicated cataract surgery with history of posterior capsule rupture Post-operative cystoid macular edema

Demographics

Patients: Mean age: Average Pachymetry (µ): 91 (all male) 71.9

547.7

Results

Strength of prediction of post-operative refraction based on pachymetry was determined by multivariable linear regression analysis Calculation yielded the following equation: Post-op Refraction = -1.59 + (1.18 x SRK/T prediction) +( 0.0030 x Pachymetry) Slope of 0.0030 D / µ : each 100 µ change in thickness changes the post-operative refraction by 0.3 D.

The p-value of the above calculation is 0.057.

Results

1.5

1 y = 0.0032x - 1.7239

R 2 = 0.0445

0.5

Error of predicted refraction 0 400 -0.5

-1 -1.5

-2 450 500 550 600 650 700 -2.5

-3 Pre-operative Pachymetry The positive correlation between pachymetry and the error of predicted refraction is displayed

Conclusions

Pachymetry appears to have a considerable and possibly clinically significant effect on post-operative refraction, although not statistically significant in our study

Limitations/Considerations

Small sample size Variability in method of obtaining pachymetry readings Variability in Manifest Refraction, obtained by different technicians

References

1.

Haigis W. Occurrence of erroneous anterior chamber depth in the SRK/T formula. J Cataract Refract Surg. 1993 May; 19(3):442-6.

2.

Jabbour J, Irwig L, Macaskill P, Hennessy MP. Intraocular lens power in bilateral cataract surgery: whether adjusting for error of predicted refraction in the first eye improves prediction in the second eye. J Cataract Refract Surg. 2006;32:2091-7.

3.

Keriechbaum K, Findl O, Preussner P, Koppl C, Wahl J, Drexler W. Determining postoperative anterior chamber depth. J Cataract Refract Surg 2003; 29:2122-2126.

4.

Olsen T, Corydon L, Gimbel H. Intraocular lens power calculation with an improved anterior chamber depth prediction algorithm. J cataract Refract Surg. 1995 May ; 21(3):313-9.

5.

Olsen T. Sources of error in intraocular lens power calculation. J cataract Ref Surg. 1992 Mar; 18(2):125-9.