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OphInBiometry - Biometry

The OphInBiometry module captures biometry measurements essential for intraocular lens (IOL) power calculation in cataract surgery. It integrates with biometry devices and provides comprehensive IOL calculation using multiple formulas.

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Overview

Biometry is a critical pre-operative investigation for cataract surgery, measuring:
  • Axial length (AL): Eye’s front-to-back measurement
  • Keratometry (K): Corneal curvature in multiple meridians
  • Anterior chamber depth (ACD): Distance from cornea to lens
  • Lens thickness (LT): Crystalline lens measurement
  • White-to-white (WTW): Horizontal corneal diameter
These measurements enable accurate IOL power calculation for optimal post-operative refractive outcomes.
Accurate biometry is essential for achieving target refraction after cataract surgery. Modern optical biometry provides precision to within 0.01mm.

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Core Element - Measurement

Element_OphInBiometry_Measurement extends SplitEventTypeElement
The biometry module uses a split event type element, providing separate data capture for left and right eyes.

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Measurement Properties

Axial Length:
  • axial_length_left: Left eye AL (mm)
  • axial_length_right: Right eye AL (mm)
  • Typical range: 22-26mm
  • High precision: ±0.01mm
Keratometry:
  • Flat K (K1)
  • Steep K (K2)
  • Axis of steep meridian
  • Average K (calculated)
  • Delta K (K2 - K1, measures astigmatism)
Additional Measurements:
  • Anterior chamber depth (ACD)
  • Lens thickness (LT)
  • Central corneal thickness (CCT)
  • White-to-white diameter (WTW)
  • Pupil diameter
Lens Selection:
  • lens_id_left: Selected IOL for left eye
  • lens_id_right: Selected IOL for right eye

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Data Sources

Biometry data typically comes from:

Optical Biometry

Modern gold standard: IOLMaster, Lenstar, Argos - uses partial coherence interferometry or swept-source OCT

Ultrasound Biometry

Traditional A-scan ultrasound when optical methods fail (dense cataracts)

Keratometry

Manual keratometry or automated keratometry/topography for corneal measurements

Manual Entry

Direct entry of measurements from device printouts

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Device Integration

The biometry module supports integration with common biometry devices:

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Supported Devices

IOLMaster 500/700:
  • Optical biometry using partial coherence interferometry
  • Non-contact measurement
  • Highly accurate axial length measurement
  • Integrated keratometry
  • WTW measurement
  • Data export via network or file
Integration Features:
  • Direct device communication
  • Automatic data import
  • Quality control metrics
  • Multiple measurement averaging
Lenstar LS 900:
  • Optical low coherence reflectometry
  • Comprehensive biometry package
  • Measures AL, ACD, LT, CCT, WTW, K
  • High repeatability
  • Integrated calculation platform
Import Capabilities:
  • CSV file import
  • Network integration
  • Bilateral measurement support
Argos (Alcon Movu):
  • Swept-source OCT biometry
  • Fast acquisition
  • Dense cataract capability
Aladdin (Topcon):
  • Combination topographer and biometer
  • Placido disc topography
  • Optical biometry
AL-Scan (Nidek):
  • Combination device
  • Optical and ultrasound modes
Traditional Ultrasound:
  • Contact or immersion technique
  • Used when optical methods fail
  • Requires skilled technique
  • Velocity assumptions critical
  • Manual data entry typically required
When Used:
  • Dense brunescent cataracts
  • White cataracts
  • Posterior subcapsular plaques
  • Vitreous opacities
  • Corneal opacities

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Data Import Workflow

1

Perform Measurement

Acquire biometry measurements on device (typically 3-5 readings per eye)
2

Device Quality Check

Review device quality metrics and measurement consistency
3

Export from Device

Export data via network, file, or prepare for manual entry
4

Create Biometry Event

In OpenEyes, create new biometry event for patient
5

Import or Enter Data

Import file or manually enter measurement values
6

Verify Import

Confirm imported values are correct and complete
7

Save Event

Save biometry event to patient record

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IOL Power Calculation

The module supports multiple IOL calculation formulas to determine optimal lens power.

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Calculation Formulas

Sanders-Retzlaff-Kraff TheoreticalCharacteristics:
  • Theoretical vergence formula
  • Good for average eyes (AL 22-26mm)
  • Widely used and validated
  • A-constant optimized for each IOL
Best For:
  • Standard axial lengths
  • Routine cataract surgery
  • General purpose calculation
Holladay 1 & 2Holladay 1:
  • Surgeon factor (SF) personalization
  • Good accuracy for most eyes
  • Well-established formula
Holladay 2:
  • Incorporates additional variables (ACD, LT, WTW, age, refraction)
  • Better for extreme eyes
  • Improved accuracy in long/short eyes
Three-Constant FormulaFeatures:
  • Uses measured ACD (not predicted)
  • Three constants: a0, a1, a2
  • Particularly good for short eyes
  • Optimizable per surgeon
Advantages:
  • Better for unusual axial lengths
  • Accounts for anterior chamber variation
  • Good for post-refractive surgery eyes
Modern Advanced FormulaCharacteristics:
  • Incorporates AL, K, ACD, LT, WTW
  • Excellent for extreme axial lengths
  • Very accurate across all eyes
  • Continuously refined with big data
Best For:
  • Long eyes (greater than 26mm)
  • Short eyes (less than 22mm)
  • High accuracy requirements
  • Modern practice standard
Pattern Recognition FormulaMethod:
  • Radial basis function algorithm
  • Pattern matching to large database
  • Self-validating with bounds checking
  • “Out of bounds” warning system
Use Cases:
  • Unusual eye configurations
  • Cross-checking other formulas
  • When traditional formulas uncertain
Additional Options:
  • Hoffer Q: Good for short eyes (less than 22mm)
  • T2: Modification of SRK/T
  • Olsen: Uses ray tracing
  • Kane: New generation AI formula
  • Pearl-DGS: Deep learning formula
  • EVO: Emmetropia Verifying Optical

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Formula Selection Guide

Short Eyes (under 22mm)

  • Barrett Universal II
  • Haigis
  • Hoffer Q

Average Eyes (22-26mm)

  • SRK/T
  • Holladay 1
  • Barrett Universal II

Long Eyes (over 26mm)

  • Barrett Universal II
  • Haigis
  • SRK/T

Post-Refractive

  • Haigis-L
  • Barrett True K
  • Specialized calculators

Toric IOLs

  • Barrett Toric
  • Holladay 2 Toric
  • Online calculators

Premium IOLs

  • Barrett Universal II
  • Formulas with high accuracy
  • Consider multiple formulas

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Target Refraction

Determining the desired post-operative refraction: Common Targets:
  • Emmetropia (0.00): Distance vision priority, both eyes
  • Mini-monovision: Dominant eye emmetropic, non-dominant eye -0.75 to -1.50
  • Full monovision: Dominant eye distance, non-dominant eye -2.00 to -2.50
  • Myopia (-1.00 to -2.00): Patient preference, reduces presbyopic symptoms
  • Matching fellow eye: Consistency with previous cataract surgery
Considerations:
  • Patient occupation and hobbies
  • Previous refractive history
  • Fellow eye refraction (if already operated)
  • Presence of macular disease (may prefer slight myopia)
  • Patient preference and lifestyle

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IOL Selection

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Lens Types

OphInBiometry_LensType_Lens
The module maintains a database of available IOL models:
Monofocal IOLs:
  • Single focal point
  • Optimized for distance vision
  • Reading glasses needed for near
  • Most predictable outcomes
  • NHS standard
Toric IOLs:
  • Corrects astigmatism
  • Requires precise alignment
  • Stable rotation critical
  • Reduces spectacle dependence
Extended Depth of Focus (EDOF):
  • Enhanced intermediate vision
  • Some near capability
  • Reduced halos vs multifocal
  • Good for computer users
Multifocal IOLs:
  • Distance and near focal points
  • Reduced spectacle dependence
  • Potential for halos/glare
  • Patient selection critical
  • Private/premium option
Accommodating IOLs:
  • Designed to change power
  • Variable effectiveness
  • Less common currently

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Lens Properties

Each IOL model has specific characteristics:
  • A-constant (or SF, ACD): Formula-specific constant for IOL power calculation
  • Power range: Available powers (typically +5.0 to +30.0D)
  • Power steps: Increments (0.50D or 1.00D)
  • Material: Acrylic, silicone, PMMA
  • Haptic design: Single-piece, three-piece
  • Optic diameter: Typically 6.0mm
  • Overall diameter: Typically 13.0mm
  • Incision size: Required for insertion
  • Manufacturer: Alcon, J&J, Bausch & Lomb, Zeiss, Rayner, etc.

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Formulary Management

Institutions configure their available IOL inventory:
  • Standard IOLs for NHS/routine cases
  • Premium IOLs for private patients
  • IOL constants optimized for local outcomes
  • Preferred models by surgeon
  • Stock management integration

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Clinical Workflow

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Pre-Operative Assessment

1

Indication for Surgery

Patient scheduled for cataract surgery during examination
2

Biometry Appointment

Patient attends for pre-operative biometry measurement
3

Measurement Acquisition

Technician/optometrist performs biometry on optical biometer
4

Data Import

Measurements imported into OpenEyes biometry event
5

IOL Calculation

System calculates IOL powers using configured formulas
6

Surgeon Review

Surgeon reviews calculations and selects target refraction and IOL
7

IOL Selection

Final IOL power and model recorded in biometry event
8

Integration with Operation Note

Biometry data available during operation note creation

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Integration Points

Examination

Cataract surgical management element references biometry status

Operation Booking

Biometry completion tracked as pre-operative requirement

Operation Note

IOL details auto-populate from biometry event

Surgical Whiteboard

Biometry completion status displayed on theatre board

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Special Situations

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Post-Refractive Surgery Eyes

Eyes with previous corneal refractive surgery (LASIK, PRK, RK) require special handling:
Problems:
  • Standard keratometry inaccurate (measures front surface only)
  • IOL formulas assume normal cornea
  • Hyperopic surprise common if not adjusted
Solutions:
  • Historical method: Use pre-refractive K and refractive change
  • Contact lens method: Over-refraction with contact lens
  • Corneal topography: Central K values or effective refractive power
  • OCT-based: Anterior segment OCT total corneal power
  • Specialized calculators: Barrett True-K, Haigis-L, Shammas
  • Intraoperative aberrometry: Real-time measurements

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Dense Cataracts

Optical biometry may fail with very dense cataracts: Alternatives:
  • Attempt multiple measurements
  • Immersion ultrasound A-scan
  • Use fellow eye data (if applicable)
  • Empiric IOL selection (if bilateral dense cataracts)

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Extreme Axial Lengths

Short Eyes (under 21mm):
  • Risk of hyperopic surprise
  • Use Hoffer Q, Haigis, or Barrett formulas
  • Consider piggyback IOL if AL less than 17mm
Long Eyes (over 26mm):
  • Risk of myopic surprise
  • Barrett Universal II excellent choice
  • Haigis also good option
  • Consider IOL exchange strategy

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High Astigmatism

For corneal astigmatism >1.50D:
  • Consider toric IOL
  • Calculate cylinder power needed
  • Plan IOL axis alignment
  • Consider limbal relaxing incisions as alternative/adjunct
  • Topography helpful for irregular astigmatism

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Quality Assurance

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Measurement Quality Checks

Axial Length

  • SNR (signal-to-noise ratio) adequate
  • Multiple measurements agree (SD less than 0.05mm)
  • Value physiologically plausible (18-32mm)
  • Consistent with patient age and refraction

Keratometry

  • Mires properly aligned
  • Measurements repeatable
  • Values physiologically normal (40-48D typically)
  • Consistent with topography if available

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Calculation Verification

Cross-Check Methods:
  • Use multiple formulas
  • Compare results across formulas
  • Verify lens constants
  • Check extreme values
  • Review against clinical expectation
Red Flags:
  • Large discrepancy between formulas (greater than 0.50D)
  • Calculation outside expected range
  • Unusual lens power for axial length
  • Inconsistent with fellow eye

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Outcome Monitoring

Post-Operative:
  • Record actual refraction achieved
  • Calculate prediction error
  • Optimize IOL constants over time
  • Audit outcomes by surgeon
  • Identify systematic biases

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Configuration

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Module Setup

'modules' => [
    'OphInBiometry' => [
        'class' => 'application.modules.OphInBiometry.OphInBiometryModule',
    ],
],

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IOL Formulary

Administrators configure:
  • Available IOL models
  • A-constants (or surgeon factors)
  • Power ranges and steps
  • Default selections
  • Formula preferences

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Device Interfaces

Setup device integration:
  • Network endpoints for biometers
  • File import locations
  • Data mapping configuration
  • Quality control rules

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Best Practices

Multiple Measurements

Always obtain at least 3 measurements per eye and review consistency

Check Both Eyes

Measure both eyes even for unilateral cataract - helps verify plausibility

Quality Metrics

Review device quality indicators - SNR, SD, alignment scores

Multiple Formulas

Use at least 2-3 formulas and understand why they might differ

Document Target

Clearly record target refraction and rationale

Verify Before Surgery

Final check of IOL selection on day of surgery

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Troubleshooting

Causes:
  • Dense cataract
  • Posterior subcapsular opacity
  • Corneal opacity
  • Poor patient fixation
  • Vitreous hemorrhage
Solutions:
  • Multiple acquisition attempts
  • Optimize patient positioning
  • Different fixation target
  • Pupil dilation if PSC plaque
  • Resort to ultrasound A-scan
Understanding Differences:
  • Normal to have 0.25-0.50D variation
  • Formulas optimize for different situations
  • Check if eye is unusual (very short/long)
  • Review formula suitability for case
Resolution:
  • Use average of most appropriate formulas
  • Weight toward formula best for this eye
  • Document rationale for selection
  • Consider IOL exchange strategy if uncertain
Check:
  • Correct patient selected
  • Left/right eye mapping correct
  • Units correct (mm vs µm)
  • All fields populated
  • Device export settings
Fix:
  • Re-export from device
  • Manual entry if import fails
  • Verify against device printout

Examination

Cataract assessment and surgical decision

Operations

Surgical documentation with IOL details

User Guide

Biometry workflows and procedures

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Further Information

IOL Constant Optimization

Improving calculation accuracy over time

Device Integration

Configuring biometry device connections

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