Biometry for IOL Power Calculation

Objectives

• Calculate appropriate IOL power before cataract surgery.
• Plan target refraction, usually emmetropia, mild myopia, or monovision.
• Identify eyes likely to have refractive surprise, such as very short, very long, post-refractive surgery, or silicone-oil-filled eyes.

Parameters measured

1. Axial length (AL): Distance along the visual axis from cornea to retina. It is the most influential biometric variable.
   • Average adult AL: approximately 23.5-24 mm.
   • Short eye: <22 mm.
   • Long eye: >26 mm.
2. Keratometry (K): Curvature of the anterior corneal surface in the flat and steep meridia, expressed in dioptres or corneal radius (mm).
   • Required for spherical IOL power.
   • Keratometry/topography also guides astigmatism management and toric IOL selection.
3. Additional modern parameters: Anterior chamber depth (ACD), lens thickness (LT), white-to-white corneal diameter (WTW), central corneal thickness, and pupil size depending on the device/formula.

Methods of axial length measurement

IOL power calculation formulae

• Older formula: SRK
  [ P = A - 2.5(AL) - 0.9(K) ] where P = IOL power and A = A-constant.
• Common modern formulas: SRK/T, Hoffer Q, Holladay 1/2, Haigis, Barrett Universal II, Hill-RBF, Kane, Olsen.
• Modern formulas estimate effective lens position (ELP) and may incorporate ACD, LT, and WTW.

• Short eyes (AL <22 mm): Hoffer Q, Haigis, Hill-RBF, or Kane may perform well.
• Long eyes (AL >26 mm): Barrett Universal II, Haigis with optimized constant, Holladay, and Kane are commonly used.
• Use an optimized surgeon-specific lens constant where available. Kanski's Clinical Ophthalmology, pp. 331-332.

Sources of error and refractive surprise

• Poor fixation or misalignment of the measuring beam.
• Corneal surface disease, dry eye, irregular astigmatism, or inaccurate K readings.
• Corneal contact-lens-induced warpage.
• Irregular globe contour, posterior staphyloma, retinal pathology.
• Dense media opacity causing optical biometry failure.
• Silicone oil in vitreous.
• Previous LASIK, PRK, RK, or other corneal refractive surgery.
• Incorrect IOL constant, incorrect IOL labeling, or an inaccurate prediction of ELP.

Special situations

• Post-corneal refractive surgery: Standard K readings and conventional IOL calculations are unreliable. Use dedicated methods/formulas such as Barrett True-K or Haigis-L, preferably compare more than one method, and counsel regarding residual refractive error.
• Dense cataract: Use immersion ultrasound if optical biometry is unsuccessful.
• Silicone oil-filled eye: Use appropriate device settings or alternative ultrasound velocity correction.
• Extreme AL, keratoconus, previous surgery: Repeat measurements, assess topography/tomography, compare formulae, and individualize target refraction.

Key exam points

• Biometry = AL + K at minimum.
• Optical biometry is first-line; immersion A-scan is the preferred backup.
• AL measurement error produces a substantial IOL power error, especially in short eyes.
• Accurate corneal measurements, optimized lens constants, and selection of an appropriate formula are at least as important as the formula itself, as emphasized by the AAO discussion of IOL formulas.
• Counsel patients with previous refractive surgery and abnormal eyes about possible postoperative refractive surprise.