Ocular Biometry: Measurements, IOL Calculations and What to Expect

Author: Dr Val Phua
Estimated reading time: 23 minutes

Ocular biometry is the measurement of the eye’s dimensions and optical properties.

It is performed most commonly before:

Cataract surgery
Refractive lens exchange
Intraocular-lens implantation
Selected phakic intraocular-lens procedures
Certain glaucoma or retinal operations

The measurements are used to select the power of the intraocular lens, or IOL, that will be implanted during cataract or lens-replacement surgery.

Ocular biometry may measure:

Axial length
Anterior corneal curvature
Total corneal power
Corneal astigmatism
Anterior-chamber depth
Lens thickness
Central corneal thickness
White-to-white corneal diameter
Pupil size
Other anterior-segment dimensions

Modern optical biometers can acquire several of these measurements during one non-contact scan.

The information is then entered into an IOL power-calculation formula.

The formula estimates:

Which IOL power should be implanted
Where the lens is likely to sit after surgery
The expected postoperative prescription
Whether the eye is likely to be focused for distance, intermediate or near vision

Ocular biometry is quick and painless, but the interpretation can be complex.

An inaccurate result may arise from:

Dry eye
Corneal irregularity
Contact-lens wear
Poor fixation
Dense cataract
High myopia
Posterior staphyloma
Previous LASIK, PRK or radial keratotomy
Keratoconus
Previous retinal surgery
Silicone oil inside the eye
Device or operator error
Use of an unsuitable calculation formula

Modern swept-source optical biometers and newer IOL formulas have improved refractive accuracy. Nevertheless, refractive prediction remains less reliable in eyes with unusual anatomy or previous corneal surgery, and no method can guarantee a perfect postoperative prescription. Large registry data continue to show that a meaningful minority of otherwise routine cataract operations finish more than 0.50 dioptres from the predicted target. (pubmed.ncbi.nlm.nih.gov)

The Quick Answer

What Is Ocular Biometry?

Ocular biometry is a series of measurements used to calculate the power of an artificial lens before cataract or refractive lens surgery.

The most important measurements are generally:

Axial length
Corneal power
Corneal astigmatism

Modern formulas may also use:

Anterior-chamber depth
Lens thickness
Central corneal thickness
White-to-white diameter
Patient age
IOL-specific constants

Is Ocular Biometry Painful?

No.

Routine optical biometry is:

Non-contact
Painless
Quick
Performed without anaesthetic drops
Safe to repeat

Ultrasound biometry may touch the anaesthetised cornea or use a fluid-filled immersion shell, but it should not be painful.

How Long Does It Take?

A straightforward optical-biometry examination commonly takes approximately 5 to 15 minutes.

More time may be required when:

The cataract is dense
Fixation is poor
The cornea is irregular
Several devices are used
Measurements need to be repeated
Contact-lens warpage is suspected
Previous refractive surgery complicates the calculation

Why Is Biometry Needed Before Cataract Surgery?

During cataract surgery, the cloudy natural lens is removed and replaced with an artificial IOL.

The surgeon must select the IOL power before surgery.

A lens that is too strong may leave the eye more short-sighted than intended.

A lens that is too weak may leave the eye more long-sighted.

Does the Machine Choose the Final IOL?

Not by itself.

The machine provides measurements and may display formula results.

The surgeon must decide:

Whether the measurements are reliable
Which calculation formula is appropriate
Which IOL model will be used
Which refractive target is desired
Whether astigmatism should be corrected
Whether unusual anatomy requires additional tests
Whether the measurements should be repeated

Can Biometry Guarantee Spectacle Independence?

No.

The calculation is a prediction.

Postoperative vision may still be affected by:

Residual refractive error
Astigmatism
Corneal irregularity
Dry eye
Macular disease
Glaucoma
Optic-nerve disease
IOL position
Healing
Neuroadaptation
The optical design of the IOL

What Does the Biometer Measure?

Axial Length

What Is Axial Length?

Axial length is the distance from the front of the cornea to the back of the eye.

It is generally measured in millimetres.

In a typical adult eye, axial length is often approximately 22 to 25 millimetres, although healthy and diseased eyes may fall outside this range.

Why Is Axial Length Important?

Axial length is one of the most important variables in IOL power calculation.

In general:

A shorter eye requires a stronger positive-powered IOL.
A longer eye requires a lower-powered IOL.
An extremely long eye may require a very low-powered or negative-powered IOL.

A small axial-length error can produce a clinically meaningful refractive error.

The effect varies with eye length, IOL power and formula, but accurate axial-length acquisition is central to avoiding refractive surprise.

Short Eyes

Short eyes commonly have:

Shallow anterior chambers
Thick natural lenses
Crowded anterior segments
Higher-powered IOL requirements

IOL calculations are more difficult because small errors in estimated postoperative lens position can produce larger refractive consequences.

A recent comparison of formulas in eyes shorter than 22 millimetres found meaningful differences between formulas and showed that performance was also influenced by anterior-chamber depth. Kane performed well overall, while PEARL-DGS performed particularly well in one subgroup of very short eyes with deeper chambers. (pubmed.ncbi.nlm.nih.gov)

Long Eyes

Long eyes are commonly associated with axial myopia.

Calculation challenges may include:

Very low-powered IOLs
Posterior staphyloma
Poor fixation
Macular disease
Formula-specific long-eye bias
Greater sensitivity to measurement endpoint and lens design

Modern formulas such as Barrett Universal II, EVO, Kane, Cooke K6 and Hill-RBF generally outperform or equal many older formulas in long eyes, but no formula is uniformly best across every axial-length subgroup. Studies of eyes longer than 30 millimetres found that performance differed even among modern formulas. (pubmed.ncbi.nlm.nih.gov)

Keratometry

What Is Keratometry?

Keratometry measures corneal curvature and estimates corneal focusing power.

The report commonly includes:

K1: the flatter principal meridian
K2: the steeper principal meridian
Mean keratometry
Astigmatism magnitude
Astigmatism axis

Why Is Corneal Power Important?

The IOL calculation must account for the focusing power already provided by the cornea.

In general:

A steeper cornea has greater optical power.
A flatter cornea has less optical power.

An error in keratometry can lead to:

Incorrect spherical IOL power
Incorrect toric IOL power
Incorrect toric axis planning
Residual postoperative astigmatism

How Is Keratometry Measured?

Depending on the device, keratometry may be obtained using:

Reflected light spots
Placido rings
Telecentric keratometry
Scheimpflug imaging
Swept-source OCT
Corneal topography or tomography

Different technologies measure different areas of the cornea and use different mathematical assumptions.

Even when two devices correlate strongly, their keratometry and astigmatism results may not be fully interchangeable. (pubmed.ncbi.nlm.nih.gov)

Anterior Keratometry Versus Total Keratometry

Standard Keratometry

Traditional keratometry measures the anterior corneal surface.

It estimates total corneal power using an assumed relationship between:

Anterior corneal curvature
Posterior corneal curvature
Corneal thickness
Refractive indices

This assumption works reasonably well in many untreated corneas.

Total Keratometry

Total keratometry attempts to incorporate both:

Anterior corneal curvature
Posterior corneal curvature

Modern systems may use:

Swept-source OCT
Scheimpflug tomography
Ray tracing
Combined imaging methods

Total keratometry may be particularly helpful in:

Previous LASIK or PRK
Keratoconus
Unusual posterior corneal shape
Toric IOL calculation
Selected premium-IOL planning

Total keratometry should not automatically be substituted into every traditional formula. Formula and device compatibility matter, and some studies have found no advantage—or reduced accuracy—when total keratometry was used with formulas not optimised for it. (pubmed.ncbi.nlm.nih.gov)

Posterior Corneal Astigmatism

The back surface of the cornea contributes to total corneal astigmatism.

Ignoring posterior corneal astigmatism tends to:

Overestimate total with-the-rule astigmatism
Underestimate total against-the-rule astigmatism

Modern toric calculators may:

Predict posterior corneal astigmatism statistically
Measure it directly
Incorporate total keratometry
Combine measured and predicted information

Algorithms that account for the posterior cornea reduce systematic toric-IOL prediction error compared with calculations based only on anterior corneal astigmatism. (pubmed.ncbi.nlm.nih.gov)

Measured posterior corneal values are not automatically superior to validated predicted values. Studies comparing measured and predicted methods have produced mixed results, partly because posterior-surface measurements remain sensitive to device, scan quality and calculation method. (pubmed.ncbi.nlm.nih.gov)

Anterior-Chamber Depth

What Is Anterior-Chamber Depth?

Anterior-chamber depth is the distance between the cornea and the natural lens.

The exact anatomical endpoints differ between devices.

Some report:

Distance from the corneal epithelium to the anterior lens surface
Internal anterior-chamber depth from the corneal endothelium to the lens

The report should be interpreted according to the device definition.

Why Does It Matter?

Anterior-chamber depth helps formulas estimate the effective lens position.

It is also clinically relevant in:

Short eyes
Angle-closure risk
Phakic IOL assessment
Anterior-segment surgery
Eyes with unusual lens anatomy

Lens Thickness

Lens thickness measures the front-to-back thickness of the natural crystalline lens.

It may influence prediction of where the IOL will sit after surgery.

Newer formulas may use lens thickness together with:

Axial length
Anterior-chamber depth
Corneal power
White-to-white diameter
Age

Lens thickness usually increases with age and may contribute to anterior-chamber crowding. Biometric studies show that axial length, anterior-chamber depth and lens thickness are anatomically interrelated rather than independent measurements. (pubmed.ncbi.nlm.nih.gov)

Central Corneal Thickness

Central corneal thickness is not required by every IOL formula.

It may nevertheless be measured by swept-source or anterior-segment biometers.

It can assist with:

Corneal assessment
Glaucoma evaluation
Total corneal-power calculation
Refractive-surgery history
Identification of unusual anatomy

White-to-White Diameter

White-to-white is the visible horizontal distance across the cornea from one limbal border to the other.

It may be used in:

Some IOL formulas
Phakic IOL sizing
Anterior-segment analysis
Selected implant planning

White-to-white measurements can vary meaningfully between devices because the anatomical boundary is identified differently. Studies have found weaker agreement for white-to-white than for axial length, so measurements should not be transferred uncritically between machines. (pubmed.ncbi.nlm.nih.gov)

Pupil Size

Pupil size may be measured for:

Multifocal or EDOF lens counselling
Refractive surgery
Night-vision assessment
ICL planning
Centration analysis

It is not a principal input for most standard monofocal IOL formulas.

How Optical Biometry Works

Partial-Coherence Interferometry

Partial-coherence interferometry uses reflected light to determine the optical length of the eye.

It was a major advance over routine contact ultrasound because it offered:

Non-contact acquisition
High repeatability
Reduced corneal-compression error
Measurements along the visual axis

Earlier partial-coherence devices may fail in dense posterior subcapsular, mature or white cataracts because insufficient light reaches and returns from the retina.

Optical Low-Coherence Reflectometry

Optical low-coherence reflectometry also uses interference of reflected light.

Depending on the instrument, it may measure:

Axial length
Corneal curvature
Anterior-chamber depth
Lens thickness
Central corneal thickness
White-to-white diameter

Swept-Source OCT Biometry

Swept-source optical biometry uses a rapidly varying longer-wavelength light source.

Potential advantages include:

Greater penetration through dense cataracts
Cross-sectional visualisation of ocular structures
Confirmation of fixation
Measurement of the whole eye
Total keratometry in selected devices
Improved detection of abnormal retinal measurement endpoints

Studies consistently show that swept-source biometers obtain axial length successfully in more dense cataracts than older partial-coherence systems. In one prospective study, the failure rate was approximately 21% with swept-source OCT compared with 69% using partial-coherence interferometry in a dense-cataract cohort. (pubmed.ncbi.nlm.nih.gov)

Newer swept-source systems may obtain measurements in most eyes that failed older optical biometers, although mature white, very dense mixed and selected posterior subcapsular cataracts can still prevent reliable acquisition. (pubmed.ncbi.nlm.nih.gov)

Optical Biometry Versus Ultrasound Biometry

Optical Biometry

Possible advantages include:

Non-contact measurement
High repeatability
No corneal compression
Rapid acquisition
Measurement along the fixation axis
Multiple biometric parameters in one sitting
Better refractive predictability in many routine eyes

Optical biometry generally produces more predictable postoperative refraction than older ultrasound-based workflows in routine cataract populations. (pubmed.ncbi.nlm.nih.gov)

Ultrasound Biometry

Ultrasound uses sound waves reflected from anatomical interfaces inside the eye.

It remains useful when optical biometry is unsuccessful or unreliable because of:

Dense white cataract
Severe posterior subcapsular cataract
Corneal opacity
Vitreous haemorrhage
Poor fixation
Certain retinal conditions
Unusual intraocular media

Contact or Applanation Ultrasound

The ultrasound probe touches the anaesthetised cornea.

Possible limitations include:

Corneal compression
Operator dependence
Off-axis alignment
Variable measurement endpoint

Compression can shorten the measured axial length and produce an incorrect IOL calculation.

Immersion Ultrasound

An immersion shell filled with fluid is placed between the probe and the eye.

The probe does not press directly on the cornea.

Potential advantages include:

Less corneal compression
Better alignment of the ocular echoes
Improved reproducibility

Immersion ultrasound remains an important backup when reliable optical measurement cannot be obtained. Optical and immersion methods often agree closely, but systematic differences may remain because light and sound use different physical pathways and retinal endpoints. (pubmed.ncbi.nlm.nih.gov)

Is Ultrasound Less Accurate?

Not necessarily in every eye.

A carefully performed immersion ultrasound examination can be highly accurate.

However, ultrasound is generally more operator-dependent and does not provide the same fixation confirmation or multimodal cross-sectional information as modern swept-source optical biometry.

If optical and ultrasound values differ significantly, the surgeon should investigate rather than automatically averaging them.

What Happens During Ocular Biometry?

Step 1: Reviewing the Eye History

The clinical team asks about:

Previous LASIK, PRK, SMILE or radial keratotomy
Previous cataract surgery in the other eye
Contact-lens wear
Keratoconus
Corneal cross-linking
Corneal transplant
Retinal surgery
Silicone oil
Eye trauma
Glaucoma
Dry eye
Desired postoperative vision

Step 2: Positioning

The patient rests:

The chin on a chin rest
The forehead against a support

The operator aligns the machine with the eye.

Step 3: Fixation

The patient looks at an internal target.

Good fixation is important because the measurement should correspond to the visual axis and the relevant foveal location.

Step 4: Blinking

The patient is asked to blink before keratometry is captured.

This creates a smoother tear film.

Step 5: Image Acquisition

The machine obtains multiple measurements.

Depending on the device, these may include:

Axial length
Keratometry
Total keratometry
Anterior-chamber depth
Lens thickness
Corneal thickness
White-to-white diameter

Step 6: Quality Check

The operator evaluates:

Signal quality
Fixation
Consistency between repeated readings
Corneal mire or ring quality
Anatomical segmentation
Whether the retinal peak is plausible
Whether the two eyes are reasonably symmetrical
Whether the values match the patient’s refraction and clinical history

Step 7: Repeating Inconsistent Measurements

Measurements may be repeated when:

Axial-length readings vary
Keratometry is inconsistent
Astigmatism axis changes substantially
One eye differs unexpectedly from the other
The result does not match the spectacle prescription
The scan shows poor fixation
The ocular surface is unstable

Large inter-eye or inter-visit biometric discrepancies are associated with worse refractive outcomes, supporting repeat measurement when the result appears anatomically or clinically implausible. (pubmed.ncbi.nlm.nih.gov)

Step 8: Additional Tests

The surgeon may request:

Corneal topography
Corneal tomography
Anterior-segment OCT
Ultrasound biometry
Macular OCT
Endothelial cell count
Repeat biometry after ocular-surface treatment
Measurement using a second biometer
Intraoperative aberrometry

Contact Lenses Before Biometry

Contact lenses can alter corneal shape and therefore keratometry.

Possible effects include:

Corneal flattening
Local steepening
Astigmatism-axis changes
Irregularity
Tear-film disruption

The required discontinuation period depends on:

Soft or rigid lens type
Orthokeratology
Duration of wear
Lens fit
Degree of corneal warpage
Stability of repeat measurements

Rigid and orthokeratology lenses often require a substantially longer discontinuation period than ordinary soft lenses.

The goal is not simply to complete a fixed waiting period. The goal is to demonstrate stable:

Refraction
Keratometry
Topography
Tomography

Dry Eye and Ocular-Surface Disease

The tear film forms the first optical surface of the eye.

Dry eye may cause:

Variable keratometry
Variable astigmatism magnitude
Variable astigmatism axis
Irregular topography
Inconsistent IOL calculations

Ocular-surface disorders that may affect biometry include:

Meibomian gland dysfunction
Blepharitis
Epithelial basement membrane dystrophy
Salzmann nodular degeneration
Pterygium
Corneal scars
Recurrent corneal erosion

Epithelial basement membrane dystrophy can be subtle but has caused clinically significant spherical and cylindrical refractive surprises when unrecognised before premium-IOL surgery. (pubmed.ncbi.nlm.nih.gov)

When measurements are inconsistent, it may be safer to:

Treat the ocular surface
Repeat the examination
Delay final IOL selection
Reassess corneal topography

Understanding IOL Power Calculation

What Is an IOL Formula?

An IOL formula is a mathematical or data-driven model that predicts the postoperative refractive result for a particular IOL power.

The formula combines several variables, which may include:

Axial length
Corneal power
Anterior-chamber depth
Lens thickness
White-to-white diameter
Central corneal thickness
Age
IOL design
Surgeon or lens constants

Effective Lens Position

The formula must estimate where the IOL will sit after surgery.

This predicted position is called the effective lens position.

It does not refer only to the physical postoperative distance from the cornea.

It is an optical calculation representing how the final IOL position affects refractive power.

Errors in effective-lens-position prediction are particularly important in:

Short eyes
High-powered IOLs
Unusual anterior-chamber anatomy
Previous glaucoma or lens surgery

IOL Constants

Each IOL model has constants used by the formula.

Examples include:

A-constant
Surgeon factor
ACD constant
Lens factor

The constant accounts partly for:

IOL design
Lens position
Surgical technique
Measurement method
Formula behaviour

Optimising constants using postoperative outcomes can improve the accuracy of a surgeon’s or clinic’s results.

The Refractive Target

The target refraction is the intended postoperative prescription.

Possible targets include:

Plano or near zero for distance
Mild myopia
Monovision
Mini-monovision
A specific near target
A balanced target between the eyes

The best target depends on:

Patient preference
Previous spectacle habits
Vision in the other eye
IOL type
Occupation
Reading requirements
Tolerance of anisometropia
Macular and optic-nerve health

Formulas may be slightly less accurate when targeting intentional myopia than when targeting emmetropia. A 2024 comparison found greater formula prediction error when the intended target was approximately −2.00 D rather than plano. (pubmed.ncbi.nlm.nih.gov)

Older IOL Formulas

Traditional formulas include:

SRK/T
Hoffer Q
Holladay 1
Haigis
Holladay 2

These formulas remain useful and have extensive clinical experience.

Their performance may be very good when:

The eye falls within a suitable anatomical range
The formula constant is optimised
The IOL model is well represented
Measurements are reliable

Modern IOL Formulas

Newer formulas include:

Barrett Universal II
Kane
Emmetropia Verifying Optical
PEARL-DGS
Hill-RBF
Hoffer QST
Cooke K6
Olsen
Castrop
VRF and VRF-G
Other evolving formula systems

Some are primarily based on vergence optics.

Others incorporate:

Artificial intelligence
Machine learning
Large outcome databases
Thick-lens optics
Additional biometric variables

Systematic reviews generally find that Barrett Universal II, Kane and PEARL-DGS are among the most consistently accurate contemporary formulas, while performance varies by eye length and study population. (pubmed.ncbi.nlm.nih.gov)

Is One Formula Always Best?

No.

Formula performance can vary according to:

Axial length
Anterior-chamber depth
Corneal power
Lens thickness
IOL model
IOL power
Refractive target
Previous corneal surgery
Population
Device
Constant optimisation

Modern-formula comparisons often find only small average differences in routine eyes, while larger differences emerge in short, long or surgically altered eyes. (pubmed.ncbi.nlm.nih.gov)

Why Surgeons May Compare Several Formulas

Comparing several formulas may help identify:

An outlier calculation
Formula disagreement in unusual anatomy
A possible measurement problem
Greater uncertainty in the predicted result

Agreement between formulas does not prove that the result is correct because all formulas rely on the same input measurements.

Cataract Density and Optical Measurement Failure

Dense cataract can block or scatter the biometer’s light.

Measurement may be difficult in:

Mature white cataract
Dense posterior subcapsular cataract
Dense nuclear cataract
Mixed advanced cataract

Swept-source OCT has improved acquisition through dense lenses, but failure still occurs.

When optical measurement fails, options include:

Repeat measurement with another swept-source device
Enhanced retinal visualisation mode
Immersion ultrasound
B-scan-guided ultrasound
Comparison with the other eye
Review of previous measurements

Different swept-source biometers may have different acquisition rates and may not produce interchangeable values for every parameter. (pubmed.ncbi.nlm.nih.gov)

Poor Fixation and Retinal Disease

Optical biometry depends partly on fixation.

Poor fixation may occur because of:

Macular degeneration
Macular scar
Amblyopia
Advanced glaucoma
Optic-nerve disease
Dense cataract
Nystagmus
Neurological disease

If the patient fixates eccentrically, the machine may measure towards a retinal location that does not represent the true foveal axis.

High Myopia and Posterior Staphyloma

A posterior staphyloma is an outpouching of the back of a highly myopic eye.

The anatomically longest point may not coincide with the fovea.

The clinically relevant measurement should correspond to the visual axis rather than simply the deepest part of the staphyloma.

Swept-source biometers that display cross-sectional retinal images can help confirm:

Fixation
Foveal measurement location
Posterior staphyloma anatomy

Studies in myopic eyes have shown that fixation status and posterior staphyloma materially affect agreement between biometers. (pubmed.ncbi.nlm.nih.gov)

Ocular Biometry After LASIK or PRK

Previous corneal laser surgery changes the relationship between:

Anterior corneal curvature
Posterior corneal curvature
True corneal power
Effective lens-position estimation

After myopic LASIK or PRK, standard keratometry may overestimate corneal power.

This can lead to selection of an IOL that is too weak and a postoperative hyperopic surprise.

After hyperopic laser treatment, the direction and magnitude of error differ.

Historical Data

Useful historical information may include:

Pre-laser keratometry
Pre-laser refraction
Amount of laser correction
Operative report
Previous topography
Stable post-laser refraction

However, many patients no longer have these records.

No-History Formulas

Modern no-history approaches include:

Barrett True-K No History
Haigis-L
Shammas
EVO post-refractive methods
PEARL-DGS post-refractive methods
Hoffer QST post-refractive methods
ASCRS calculator averages

Barrett True-K generally performs better than several older no-history methods after myopic LASIK or PRK, although prediction remains less accurate than in untreated corneas. (pubmed.ncbi.nlm.nih.gov)

A 2025 multicentre study found similar average accuracy among several newer no-history methods, with the ASCRS average and Barrett True-K No History among the best-performing approaches in that cohort. (pubmed.ncbi.nlm.nih.gov)

Total Keratometry After Laser Surgery

Measured total keratometry may improve understanding of the altered cornea.

Formulas specifically designed to incorporate total keratometry may be used.

Results remain device- and formula-dependent, and studies do not support treating every measured total-cornea value as automatically superior to validated formula estimates. (pubmed.ncbi.nlm.nih.gov)

Intraoperative Aberrometry

Intraoperative aberrometry measures the eye’s optical power during cataract surgery after the natural lens has been removed.

It may provide an additional estimate in:

Previous LASIK or PRK
Previous radial keratotomy
Toric IOL surgery
Unusual eyes

It does not replace accurate preoperative biometry.

Comparative studies generally find intraoperative aberrometry broadly comparable with modern post-refractive formulas, with the relative advantage depending on the type of previous surgery. (pubmed.ncbi.nlm.nih.gov)

Ocular Biometry After Radial Keratotomy

Radial keratotomy, or RK, creates radial corneal incisions that flatten the central cornea.

Biometry is difficult because RK may cause:

Irregular central curvature
Small effective optical zones
Diurnal fluctuation
Progressive hyperopic shift
Unusual anterior-to-posterior corneal relationships
Unstable keratometry
Postoperative refractive fluctuation

Specialised formulas may include:

Barrett True-K for RK
Double-K methods
Haigis
Kane
PEARL-DGS
VRF
ASCRS post-RK calculator methods

Even with modern formulas, outcomes are less predictable than in untreated eyes.

A network meta-analysis found that Barrett True-K History and Partial History performed well when prior information was available, while several other specialised methods were reasonable alternatives when it was not. (pubmed.ncbi.nlm.nih.gov)

Patients should be counselled about:

Greater refractive uncertainty
Postoperative fluctuation
Possible need for spectacles
Possible enhancement or lens-based correction
The need to delay final refraction until stability

Ocular Biometry in Keratoconus

Keratoconus alters:

Anterior corneal curvature
Posterior corneal curvature
Corneal thickness
Corneal regularity
The relationship between measured keratometry and true optical power

Biometry becomes less repeatable as keratoconus becomes more advanced, particularly when corneal power exceeds approximately 55 D or the ocular surface is poor. (pubmed.ncbi.nlm.nih.gov)

Why Standard Formulas May Fail

Standard formulas may produce a hyperopic surprise because:

The steep anterior cornea may cause total corneal power to be overestimated.
The posterior corneal relationship is abnormal.
Keratometry may be irregular.
Effective lens position may be predicted incorrectly.

Keratoconus-Specific Formulas

Options include:

Barrett True-K Keratoconus
Kane Keratoconus

Recent systematic reviews and network meta-analyses generally rank Barrett True-K Keratoconus and Kane Keratoconus among the most accurate approaches, while acknowledging that outcomes remain less predictable in advanced disease. (pubmed.ncbi.nlm.nih.gov)

IOL Choice in Keratoconus

A toric IOL may be considered when:

Astigmatism is regular
Measurements are stable and repeatable
The patient obtains useful spectacle-corrected vision
Contact-lens dependence is not essential for visual quality

A toric IOL may be unsuitable when:

Astigmatism is highly irregular
The patient relies on a rigid or scleral lens
The cone is unstable
Measurements vary substantially

Toric IOL Biometry

A toric IOL corrects corneal astigmatism as well as spherical refractive error.

Planning should consider:

Magnitude of total corneal astigmatism
Astigmatism axis
Posterior corneal astigmatism
Incision location
Surgically induced astigmatism
Estimated IOL position
Toric IOL model
Lens rotation

Measurement Repeatability

Before selecting a toric IOL, the surgeon may compare:

Optical-biometer keratometry
Corneal topography
Corneal tomography
Manifest refraction

Large disagreement may indicate:

Dry eye
Epithelial disease
Contact-lens warpage
Irregular astigmatism
Poor fixation
Incorrect scan quality

Surgically Induced Astigmatism

The cataract incision can alter corneal astigmatism.

The toric calculation may include the surgeon’s estimated surgically induced astigmatism.

This estimate may be derived from previous surgical outcomes.

Posterior Corneal Measurement

Modern toric calculators that incorporate predicted or measured posterior corneal astigmatism generally reduce systematic over- and undercorrection compared with anterior-cornea-only calculations. (pubmed.ncbi.nlm.nih.gov)

Biometry for Multifocal and EDOF Lenses

Multifocal, trifocal and extended-depth-of-focus lenses are less tolerant of certain optical imperfections.

Assessment may include:

Repeat biometry
Corneal topography
Corneal tomography
Macular OCT
Dry-eye assessment
Pupil evaluation
Higher-order aberration assessment
Optic-nerve evaluation

Small residual errors may be more noticeable with premium IOLs because image quality is affected not only by focus but also by contrast and light distribution.

Patients should understand that accurate biometry reduces but does not eliminate the possibility of:

Residual prescription
Night glare
Halos
Spectacle use
Enhancement surgery

Biometry After Retinal Surgery

Previous retinal surgery may affect measurement because of:

Silicone oil
Scleral buckle
Macular displacement
Poor fixation
Retinal detachment
Vitrectomy-related anatomical change

Silicone Oil

Silicone oil changes the optical and ultrasound properties of the eye.

The biometer must use the correct measurement mode or adjustment.

Uncorrected axial-length errors can produce a substantial refractive surprise.

A 2024 study found that correcting the axial-length measurement for silicone oil reduced systematic hyperopic prediction error across several modern formulas. (pubmed.ncbi.nlm.nih.gov)

Scleral Buckle

A scleral buckle may:

Increase axial length
Induce astigmatism
Alter eye shape

The eye should be measured after its anatomy and refraction have stabilised.

Macula-Off Retinal Detachment

A detached macula may create an incorrect retinal measurement endpoint.

The surgeon may need:

Ultrasound
Fellow-eye comparison
Previous axial-length records
Repeat biometry after retinal repair
Careful retinal-image review

High Eye Pressure

Extremely elevated intraocular pressure may temporarily lengthen the eye.

A 2026 study of eyes undergoing cataract surgery after marked pressure reduction found measurable axial shortening and improved prediction after adjustment for the pressure-related change. This is a specialised situation rather than a routine correction for ordinary glaucoma. (pubmed.ncbi.nlm.nih.gov)

Why Measurements From Different Devices May Differ

Different biometers may use different:

Wavelengths
Refractive indices
Segmentation boundaries
Keratometry zones
Corneal-power assumptions
White-to-white detection methods
Fixation targets
Formula implementations

Studies comparing swept-source devices commonly find excellent agreement for axial length but less agreement for:

Total keratometry
Astigmatism
Lens thickness
White-to-white diameter
Some anterior-chamber measurements

Serial comparison and IOL calculation are generally most reliable when the same validated device and formula workflow are used. (pubmed.ncbi.nlm.nih.gov)

Signs That Biometry Should Be Repeated

Repeat or confirm the measurements when:

Axial length differs unexpectedly between eyes
IOL powers differ more than expected between similar eyes
The result conflicts with the spectacle prescription
Keratometry varies between scans
Astigmatism axis is unstable
Scan quality is poor
The patient recently wore contact lenses
The cornea is dry or irregular
Dense cataract obscures the retinal signal
The device shows poor fixation
Previous refractive surgery is not recognised by the calculation
The planned IOL power appears anatomically implausible

A case report of a contaminated optical surface producing axial lengths above 35 millimetres and a postoperative refractive error of +14.00 D illustrates why implausible measurements should be questioned and confirmed with another method. (pubmed.ncbi.nlm.nih.gov)

Comparing the Two Eyes

The two eyes are commonly similar, but they are not always identical.

Comparison may include:

Axial length
Corneal power
Astigmatism
Anterior-chamber depth
IOL power
Spectacle prescription

A large difference may be genuine in:

Anisometropia
Unilateral keratoconus
Previous surgery
Trauma
Retinal disease
Amblyopia

It should nevertheless prompt review.

Using the First Eye to Refine the Second Eye

When cataract surgery is performed on separate days, the first eye’s result may provide useful information.

The surgeon may compare:

Predicted refraction
Actual postoperative refraction
IOL model
Biometry values
Healing and lens position

A first-eye error may help refine the second-eye plan, but the full error should not necessarily be transferred because:

The eyes may differ
Measurement errors may be eye-specific
Corneal astigmatism may differ
IOL position may differ

What Is a Refractive Surprise?

A refractive surprise occurs when the postoperative prescription differs meaningfully from the intended target.

Possible causes include:

Incorrect axial length
Incorrect keratometry
Unstable tear film
Previous corneal surgery
Incorrect formula or constant
Unexpected IOL position
Wrong IOL power
IOL labelling or implantation error
Toric IOL rotation
Capsular changes
Surgical complication
Postoperative corneal change

What Happens if the Result Is Off Target?

Management depends on:

Magnitude of the error
Symptoms
IOL type
Corneal suitability
Time since surgery
Vision in the other eye
Patient goals

Options may include:

Spectacles
Contact lenses
Corneal laser enhancement
Supplementary intraocular lens
IOL exchange
Toric IOL rotation
Observation

A recent systematic review confirmed that several surgical approaches can correct residual ametropia, but each carries different benefits and risks. (pubmed.ncbi.nlm.nih.gov)

Can Biometry Be Performed After Pupil Dilation?

Yes, but routine measurements are commonly obtained before dilation.

Dilation may alter:

Pupil-centre measurements
Selected anterior-chamber dimensions
Fixation
Tear-film quality

The effect depends on the device and parameter.

Can Biometry Be Performed During Pregnancy?

The test uses light or ultrasound rather than ionising radiation.

It can be performed when clinically required.

Elective lens surgery is generally postponed during pregnancy because:

Refraction may fluctuate
Corneal measurements may change
Medication exposure should be minimised
The procedure is usually non-urgent

Can Children Have Ocular Biometry?

Yes.

Biometry may be required for:

Congenital cataract
Traumatic cataract
Lens abnormalities
Research or myopia monitoring
Paediatric IOL planning

Young children may require:

Handheld devices
Ultrasound
Sedation or anaesthesia
Age-adjusted IOL planning

The eye continues growing, so paediatric IOL targeting differs from adult cataract surgery.

Common Myths

“Biometry Is Just Measuring Eye Length”

False.

Axial length is critical, but modern biometry also assesses corneal power, astigmatism and anterior-segment anatomy.

“The Machine Automatically Chooses the Correct Lens”

False.

The surgeon must validate the measurements, formula and target.

“The Highest-Technology Biometer Cannot Be Wrong”

False.

No device is immune to poor fixation, ocular-surface problems, dense cataract, segmentation error or equipment problems.

“A Difference of Less Than One Millimetre Is Too Small to Matter”

False.

Axial-length errors much smaller than one millimetre can produce clinically meaningful refractive errors.

“The Same Formula Is Best for Every Eye”

False.

Formula performance varies with anatomy and surgical history.

“Total Keratometry Is Always More Accurate”

False.

It must be used with an appropriate formula and reliable measurement.

“Ultrasound Biometry Is Obsolete”

False.

It remains essential when optical measurement fails or appears unreliable.

“Both Eyes Should Always Have the Same IOL Power”

False.

The eyes may have different axial lengths, corneal powers or refractive goals.

“Previous LASIK Does Not Matter Once Cataract Develops”

False.

Previous corneal laser surgery materially affects IOL calculation.

“A Normal Corneal Topography Guarantees Accurate Biometry”

False.

Dry eye, axial-length error, formula limitations and internal eye anatomy may still affect the result.

“Premium Lenses Can Correct Any Biometry Error”

False.

Premium lenses usually require greater—not less—measurement precision.

“An Exact Calculation Means Spectacles Will Never Be Needed”

False.

The eye can still heal differently from the prediction, and other optical or retinal limitations may remain.

“A Refractive Surprise Means the Surgery Was Technically Poor”

Not necessarily.

It can occur after an otherwise uncomplicated operation because biometry and lens-position prediction remain probabilistic.

Frequently Asked Questions

Why Must I Repeat My Biometry?

Common reasons include:

Dry eye
Contact-lens wear
Inconsistent keratometry
Poor fixation
Dense cataract
Unexpected difference between the eyes
Previous laser surgery
Planning a toric or premium IOL

Why Must I Stop Contact Lenses?

Contact lenses may temporarily reshape the cornea and alter the measured IOL power or astigmatism.

How Long Must I Stop Contact Lenses?

The duration depends on the lens type and corneal stability.

The surgeon may repeat measurements until they are consistent.

Should I Use My Dry-Eye Drops Before Biometry?

Use prescribed ocular-surface treatment as directed.

A stable tear film generally improves measurement repeatability.

Does Biometry Test My Retina?

Some swept-source biometers display a cross-sectional retinal image to confirm the measurement endpoint.

This does not replace a full retinal examination or macular OCT.

Why Do I Need a Macular OCT Before Cataract Surgery?

Macular OCT may detect:

Epiretinal membrane
Macular degeneration
Macular hole
Diabetic macular oedema
Other causes of limited visual potential

These conditions may affect IOL choice and postoperative expectations.

Why Do My Two Biometry Reports Give Different IOL Powers?

Possible reasons include:

Different formulas
Different lens constants
Different keratometry
Different devices
Different refractive targets
Measurement variability
Use of total versus standard keratometry

What Does a 0.00 D Target Mean?

It means the calculation is aiming approximately for no spherical refractive error at distance.

It does not guarantee perfect unaided distance vision.

Why Would My Surgeon Target Mild Myopia?

Possible reasons include:

Near-vision preference
Monovision
Previous adaptation to myopia
Desire to reduce reading-glasses dependence
Balancing the two eyes

Can I Choose My Target?

The target should be discussed with the surgeon.

The final recommendation depends on:

Eye health
IOL type
Prescription in the other eye
Visual priorities
Tolerance of imbalance

Why Is My IOL Power a Positive Number?

Most cataract IOLs are positive-powered lenses because the eye requires substantial focusing power after the natural lens is removed.

Can an IOL Power Be Negative?

Yes.

Extremely long eyes may require a zero- or negative-powered IOL.

Does a Higher IOL Number Mean a Worse Cataract?

No.

IOL power reflects the eye’s anatomy, not cataract severity.

Why Is My IOL Power Different from My Spectacle Prescription?

The IOL sits inside the eye.

Spectacles sit in front of the eye.

Their powers are not directly equivalent.

Can Biometry Detect Glaucoma?

No.

It may measure parameters relevant to glaucoma, but glaucoma diagnosis requires:

Eye-pressure assessment
Optic-nerve examination
OCT
Visual fields
Gonioscopy

Can Biometry Detect Keratoconus?

Keratometry may raise suspicion, but detailed corneal topography and tomography are required.

Can Biometry Be Done Through a Dense Cataract?

Often yes with swept-source technology.

Ultrasound may be required when optical acquisition fails.

Why Is Ultrasound Needed When Optical Biometry Was Already Done?

Ultrasound may be used to:

Confirm an unexpected result
Measure through an opaque cataract
Assess a posterior staphyloma
Obtain a second independent measurement

Does Cataract Severity Change Axial Length?

The anatomical axial length usually does not change substantially from an ordinary cataract.

However, dense cataract can interfere with how optical systems measure the eye, and cataract-related refractive changes have been associated with small systematic prediction effects. (pubmed.ncbi.nlm.nih.gov)

Why Is Calculation Harder After LASIK?

Laser surgery changes the corneal shape and invalidates some assumptions used by standard keratometry and IOL formulas.

Why Is Calculation Harder in High Myopia?

High-myopic eyes may have:

Extreme axial length
Posterior staphyloma
Poor fixation
Unusual IOL powers
Retinal disease
Greater formula sensitivity

Can the IOL Be Changed if the Power Is Wrong?

Yes, in selected cases.

IOL exchange is another intraocular operation and may become more difficult after the capsule has contracted.

When Is the Final Prescription Checked?

Refraction is commonly checked after the eye has healed and the IOL position has stabilised.

The timing depends on:

Surgical technique
Corneal swelling
Dry eye
IOL type
Whether complications occurred

When to Seek Earlier Eye Assessment

Ocular biometry is normally an elective diagnostic test.

Seek earlier assessment if there is:

Sudden loss of vision
New flashes or a sudden shower of floaters
A curtain or shadow
Severe eye pain
Marked redness
Sudden distortion
Eye trauma
Rapidly worsening vision
Nausea or vomiting with eye pain

These symptoms may indicate a condition requiring treatment before elective lens planning.

An Ocular Biometry Checklist

Information to Report

Previous LASIK, PRK, SMILE or radial keratotomy
Previous cataract surgery
Previous ICL surgery
Corneal cross-linking
Keratoconus
Corneal transplant
Retinal detachment surgery
Scleral buckle
Vitrectomy
Silicone oil
Glaucoma surgery
Eye trauma
Contact-lens use
Dry eye
Current spectacle prescription

Visual Goals to Discuss

Distance vision
Reading vision
Computer vision
Night driving
Monovision
Spectacle independence
Tolerance of halos
Previous experience without glasses
Occupation
Hobbies

Tests That May Be Required

Optical biometry
Repeat optical biometry
Immersion ultrasound
Corneal topography
Corneal tomography
Macular OCT
Anterior-segment OCT
Endothelial cell count
Refraction
Dilated retinal examination

Questions to Ask the Surgeon

Are my measurements reliable and repeatable?
Is my ocular surface stable?
Do I need to stop contact lenses longer?
Which IOL formula is being used?
Have several formulas been compared?
What refractive target is planned?
Why is that target recommended?
Is my axial length unusually short or long?
Is my corneal astigmatism regular?
Has posterior corneal astigmatism been considered?
Would a toric IOL benefit me?
Does previous LASIK or another operation affect the calculation?
Is a premium IOL appropriate?
What is the realistic chance that I will need spectacles?
What options are available if the result is off target?

The Bottom Line

Ocular biometry measures the eye before cataract or lens-replacement surgery.

The most important measurements include:

Axial length
Corneal power
Corneal astigmatism
Anterior-chamber depth
Lens thickness

Modern swept-source optical biometry provides:

Non-contact measurements
High repeatability
Improved penetration through dense cataracts
Fixation and retinal-endpoint assessment
Additional anterior-segment measurements
Total keratometry on selected devices

Ultrasound biometry remains valuable when:

Optical measurement fails
The cataract is extremely dense
Fixation is poor
The result appears implausible
A second independent measurement is required

IOL calculation depends on:

Reliable biometric measurements
An appropriate formula
An optimised IOL constant
The selected IOL model
Prediction of effective lens position
The patient’s desired refractive target

Modern formulas such as Barrett Universal II, Kane, EVO, PEARL-DGS and other contemporary methods generally provide excellent results in routine eyes.

Specialised calculation is often required for:

Very short eyes
Very long eyes
Previous LASIK or PRK
Previous radial keratotomy
Keratoconus
Silicone-oil-filled eyes
Posterior staphyloma
Toric or premium IOLs

The most important message is:

Ocular biometry is not simply a machine-generated IOL number. A reliable surgical plan depends on confirming that the axial length, corneal measurements, ocular-surface condition, eye anatomy, formula and refractive target all make sense together. Measurements that do not fit the clinical picture should be repeated or confirmed before an irreversible lens is implanted.

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