IOL Power Calculation in Short Axial Length Eyes

Definition of "Short Eye"

Why Short Eyes Are Particularly Challenging

1. Effective lens position (ELP) prediction error - In short eyes, even a small miscalculation in ELP produces a disproportionately large refractive error, because the focal length changes are magnified in a shorter optical system.
2. Higher IOL powers required (typically > 25 D), and the relationship between power and refraction is steeper at higher dioptric values.
3. Steep corneas (common in short eyes) interact with corneal refractive index assumptions in unexpected ways.
4. Shallow anterior chamber makes it harder to predict where the IOL will ultimately sit.
5. Mean spherical error (MSE) after surgery is consistently higher in short eyes compared to eyes of normal length.

IOL Power Calculation Formulas

Generation Classification

Formula Recommendations by Axial Length

Key Formulas in Detail

• Historically the most recommended formula for AL < 22 mm
• Uses a personalized ACD (pACD) to predict ELP
• Incorporates a tangent function relating corneal curvature to ELP
• Still widely used and performs well, but newer AI formulas now match or surpass it

• Uses three constants (a0, a1, a2) - requires individual optimization
• The a0 constant shifts ELP; a1 relates to measured ACD; a2 relates to AL
• With properly optimized constants it performs well across a wide range, including short eyes
• Also has the Haigis-L variant for post-refractive surgery eyes

• A newer AI/theoretical hybrid incorporating AL, keratometry, ACD, lens thickness, central corneal thickness, and sex
• Multiple independent studies now rank it as one of the most accurate for short eyes
• A 2025 study (Luo et al., Zhongshan) found Kane had the highest overall accuracy in eyes with AL < 22 mm

• An AI-based formula
• A 2025 network meta-analysis (Zheng et al., BMC Ophthalmology) of 21 formulas across 756 short eyes (AL < 22 mm) found Pearl-DGS to be the top-performing AI-based formula, with the highest percentage of eyes achieving prediction error within ±0.50 D
• Particularly strong in eyes with relatively shorter AL combined with deeper ACD

• Incorporates a theoretical model of the lens and its relationship to biometric parameters
• Performs well across the full AL spectrum
• Less consistently ranked at the top for very short eyes compared to Kane and Pearl-DGS

• Pattern-recognition AI trained on large datasets
• Performs well when the biometric profile of the eye falls within the bounds of its training data
• Recommended in Kanski's alongside Hoffer Q, Haigis, and Kane for short eyes

Measurement Considerations in Short Eyes

Biometry Method

• Optical coherence biometry (partial coherence interferometry, e.g., IOLMaster, Lenstar) is preferred - non-contact, highly reproducible
• Immersion A-scan is more reliable than contact A-scan if optical biometry fails (dense cataracts, posterior subcapsular opacities)
• Contact A-scan is least reliable due to corneal indentation artefact, which falsely shortens AL

Parameters to Measure

• Keratometry (K1, K2)
• Axial length
• Anterior chamber depth (ACD)
• Lens thickness (LT) - especially important for 4th/5th generation formulas
• Corneal white-to-white diameter

Confounders of Axial Length Measurement

• Poor patient fixation (most common)
• Irregular globe contour
• Staphyloma (in myopic long eyes)
• Silicone oil in vitreous
• Dense cataract blocking optical biometry signal

Practical Strategies to Reduce Refractive Surprise

1. Use multiple formulas and compare - If results from Hoffer Q, Kane, and Barrett UII agree within 0.5 D, you can proceed with confidence. Wide divergence warrants extra caution.
2. Target slight myopia - Many surgeons aim for -0.25 D rather than emmetropia, since postoperative hyperopia is less well tolerated than mild myopia in short eyes.
3. Personalized IOL constants - If a consistent systematic error is seen across cases, optimize the A-constant or equivalent. This significantly reduces mean prediction error.
4. Beware of nanophthalmos (AL < 20 mm) - these eyes are also prone to uveal effusion and angle-closure glaucoma after surgery; the refractive challenge is compounded by surgical risk.
5. Use the ESCRS online calculator - allows simultaneous comparison of multiple formulas and has been validated on large datasets.
6. AI-based formulas (Kane, Pearl-DGS, Hill-RBF) are increasingly preferred as first-line in short eyes due to superior ELP prediction.

Recent Evidence Summary

• Pearl-DGS was the top AI-based formula for PE within ±0.50 D
• AI-based formulas as a class were superior to conventional vergence formulas

• Kane formula showed highest overall accuracy in short eyes
• Pearl-DGS was superior specifically in eyes with shorter AL combined with deeper ACD - suggesting that the "best" formula may need to be individualized based on ACD as well as AL

Summary

Clinical note: No single formula is universally superior for all short eyes. Combining Kane or Pearl-DGS with Hoffer Q as a cross-check, using optical biometry with full parameter capture (AL, K, ACD, LT), and targeting mild myopia represents current best practice.