Biometry & IOL power calculation
Calculation
of IOL power
1.
Refractive:
IOL power = 18+ 1.25 X Preoperative
refraction (SE)
2.
Theoretical formulae.
3.
Second generation formulae.
4.
Third generation formulae:
Holladay II, SRK-T & Hoffer Q.
5.
AI hybrid.
Holladay’s Data Screening Criteria to
identify unusual measurement and require further validation. Repeat measurement
if:
1.
Axial length < 22.0 mm or
> 25.0 mm.
2.
Average corneal power < 40.0
Diopters or > 47.0 Diopters.
3.
Calculated emmetropic IOL power
> 3.0 Diopters of average power for the specific lens type.
4.
Between eyes, the difference
in:
a. Average corneal power > 1.0 Diopter.
b. Axial length > 0.3 mm.
c. Emmetropic IOL power > 1.0 Diopter.
Optiwave refractive analysis:
Independent of AL & K readings.
The Impact of Optical Biometry
In ultrasound biometry, axial length
measurement error alone accounted for 54% to 68% of the total prediction error.
In optical biometry, the source of error from axial length measurement
decreased substantially from 0.65 D to 0.43 D or 30 to 40% of the total
prediction error.
Despite the improvement in AL
measurement, this precision is not reflected in reducing prediction error. This
less than encouraging improvement was probably overshadowed and supplanted by
the ACD prediction error, a function of IOL power calculation formulas.
RCOphth
guidelines:
In axial length < 22.00 mm, use
Haigis or Hoffer Q.
In axial length between 22.00 and 26.00
mm, use SRK/T, or Barrett Universal II if it is installed on the biometry
device and does not need the results to be transcribed by hand.
In axial length > 26.00 mm, use
Haigis or SRK/T
Modern Evidence: Haigis vs Hoffer Q
Multiple studies show Haigis often matches or exceeds
Hoffer Q in short eyes:
Moschos et al (2014): Haigis had lowest median
absolute error (MedAE) in 55 eyes (AL <22 mm), outperforming Hoffer Q,
Holladay 1, SRK/T.
Eom et al (2014): Equivalent MedAE (0.40 D) overall,
but Haigis superior in shallow ACD (<2.5 mm), where differences widen
as ACD decreases (R²=0.644). Shallow ACD exacerbates Hoffer Q's
tendency to predict hyperopic shifts.
2025 meta-analysis
(1178 eyes): Haigis ranked highly (with Kane, Olsen), though Kane edged out
overall.
Recent study (2025): Kane best overall for AL <22
mm, but Haigis competitive.
Pathophysiology note: Short eyes risk postoperative
hyperopia due to posterior IOL displacement from crowded bags; Haigis uses
three IOL constants (a0,a1,a2) optimising effective lens position (ELP) via
preoperative ACD.
When to
repeat biometry?
· Previous measurements are > 4 years old.
· IOL exchange is required.
· Patient has had previous corneal surgery or a progressive corneal disease.
· Had a major eye operation since last biometry
· Axial length may have changed due to eye disease
· Considerable change in measurement system/devices.
· Recalculate as necessary if there is a change in type or supplier of the
IOL used routinely.
Patient
safety terminology
National Reporting & Learning
System/ Learn From Patient Safety Events
1.
Adverse incident:
2.
Near Misses:
3.
Never event:
4.
Unexpected outcome:
