Estimation of intraocular lens power I. List the indications/contraindications

A. Indications 1. Calculation of intraocular lens (IOL) power required for surgical

correction of refractive error 2. Evaluation for anisometropia

B. Contraindication 1. Open globe injury

II. Describe the pre-procedure/therapy evaluation A. Measurement of corneal refractive power (keratometry)

1. Values obtained with manual or automated keratometry or using topography

2. None of these methods reliably determine central corneal power following keratorefractive procedures, resulting in unanticipated refractive outcomes without compensatory adjustments

B. Laser interferometry (e.g. IOL master or Lenstar technology) 1. No anesthesia 2. No contact with the eye 3. Increased reproducibility, ease, and speed of measurements 4. Increased accuracy secondary to measurement of refractive rather

than anatomic axial length C. Immersion A-scan

1. Topical anesthesia 2. Water bath 3. Shell

D. Contact A-scan 1. Topical anesthesia 2. Care taken not to indent the eye

III. List the possible sources of errors in measurement, their prevention and

management A. Poor fixation or failure to find the visual axis accurately as in high myopes +/-

posterior staphyloma 1. Optical methods (laser interferometry) better for measuring axial length

in cases of staphyloma 2. Consider B-scan in these cases

B. Repeat scans for long and short axial lengths 1. In general, there should be repeat axial lengths for short eyes less than

22 millimeters and for long ones greater than 25 millimeters 2. Decreased accuracy with laser interferometer when measuring dense

cataract or posterior subcapsular cataract C. Contact A-Scan can cause inaccurate measurement (underestimate) of axial

length because of corneal compression D. Steps to minimize inaccuracies include:

Cataract/Anterior Segment 3 © 2013, AAO

1. Check both eyes 2. Repeated axial length measurements, particularly if >0.3 mm

difference between eyes 3. Independent technician if >0.3 mm difference or use different

technique (is immersion) to confirm 4. Repeat keratometry measurements, particularly if >0.5 D difference

between eyes in average K’s 5. Clinical correlation is required to explain all significant differences in

axial length or average keratometry values

IV. Describe the considerations in interpretation for this diagnostic procedure A. Examine quality of scans B. Examine reproducibility C. Compare both eyes D. Look for correlation with refraction E. Describe appropriate character of good spikes F. Special consideration in eyes with silicone oil

V. IOL Formulas

A. First generation 1. SRK 2. P = A-0.9 K – 2.5 L

B. Second generation 1. SRK II 2. A constant modified based on the axial length

C. Third generation 1. Holladay

a. Third generation formula and based on linear regression methods

b. Best for axial lengths 24-26 mm 2. Hoffer Q

a. Consider for short axial lengths 3. SRK-T

a. Consider for long axial lengths D. Recent theoretical formulas

1. Haigis, Holliday II, Olsen 2. Incorporate additional measurements (anterior chamber depth, lens

thickness, and horizontal corneal diameter) 3. Improve accuracy in predicting the effective lens position of the IOL to

be implanted 4. Improved accuracy of IOL calculations in cases of previous

keratorefractive surgery E. Optimization of lens constants for a specific IOL based on an individual

surgeon’s actual refractive outcome is recommended

Additional Resources

Cataract/Anterior Segment 4 © 2013, AAO

1. AAO, Basic and Clinical Science Course. Section 11: Lens and Cataract, 2013-2014.

2. AAO, Cataract Surgery and Intraocular Lenses, 2nd edition, 2001. 3. AAO, Focal Points: Modern IOL Power Calculations: Avoiding Errors

and Planning for Special Circumstances, Module #12, 1999. 4. AAO, Focal Points: A-Scan Biometry and IOL Implant Power

Calculations, Module #10, 1995. 5. Hennessy MP, Franzco, Chan DG. Contact versus immersion biometry

of axial length before cataract surgery. J Cataract Refract Surg 2003; 29:2195-8.

6. AAO, Focal Points: Refractive Lens Exchange, Module #6, 2007.

Cataract/Anterior Segment 5 © 2013, AAO