Why A Single Percentage Cannot Be Given
Patients often want a single number, a percentage that tells them how much of their outcome is in the surgeon’s hands. Honest answers decline to give one, because the apportionment changes from eye to eye.
What can be said is that three of the four domains actively produce the visual result: planning (biometry, formula choice, lens selection), execution (surgical technique, intraoperative stability), and biology (the patient’s unique ocular and neural characteristics). The fourth domain protects against risk rather than producing the outcome.
How Planning, Execution, And Biology Each Contribute
Planning now carries less variability than it did two decades ago. Modern IOL-calculation formulas have reduced prediction error meaningfully,¹ and their relative accuracy has been well documented in direct comparisons.² In eyes with unusual dimensions, particularly very long or very short axial lengths, formula selection matters more than in average eyes.
Execution contributes a further layer. Effective lens position is influenced by capsulorhexis geometry, incision construction, and the healing response. Norrby’s landmark analysis of sources of error in IOL calculation remains the reference point for understanding where the residual unpredictability sits.³
Biology is the domain that persists regardless of how well the first two are managed. Corneal aberrations, macular health, healing response, and neuroadaptation capacity all belong here. No amount of surgical care abolishes their influence.
The honest answer to “how much of the result is in the surgeon’s hands” depends on the eye. For a healthy eye with average dimensions, rigorous planning and execution account for most of the predictable variation. For eyes at the extremes, biology dominates, and counselling must say so.
References
- Melles RB, Holladay JT, Chang WJ. Accuracy of intraocular lens calculation formulas. Ophthalmology. 2018;125(2):169–178.
- Kane JX, Van Heerden A, Atik A, Petsoglou C. Intraocular lens power formula accuracy: comparison of 7 formulas. J Cataract Refract Surg. 2016;42(10):1490–1500.
- Norrby S. Sources of error in intraocular lens power calculation. J Cataract Refract Surg. 2008;34(3):368–376.
Related Topics
- Why Vision Outcomes Vary After Refractive Surgery – The Four-Domain Model
- Why do patients with the same prescription get different vision after surgery?
- How much of the final result is biology versus planning versus execution?
- Why does lens power calculation matter more than the lens itself?
- How do corneal shape and tear film affect final vision quality?
- Why do some patients take longer than others to neuroadapt to a new IOL?
- How does pre-existing dry eye change vision quality after laser or ICL?
- Why do realistic expectations correlate with better perceived outcomes?
- How much does surgical execution actually affect final vision quality?
- Why can two eyes in the same patient recover differently?
- What the surgeon controls versus what the eye decides
- What increases the risk of a poor outcome?
