Optical Biometry Measurements for Future

IOL’s

Aaron Graham, BA, CCOA

Financial Disclosure

Ground Rules

The Good

Exclusive Эксклюзивные Exclusif Exclusiva

Superior верхний Supérieur Superior

Elite Элитные élite elisión

Specialist Специалист Spécialiste Especialista

The Good

The Good

The Good

The Good

The Good

The Good

The Good

The Good

The Good

Good καλός Buono Versus έναντι Contro Bad κακός Cattivo

The Bad

IndifferentБезразлично Indifférent Indiferente

Inferior Нижняя Inférieur Inferior

Generic родовой Générique Genérico

Apathetic апатичный Apathique Apático

The Bad

The Bad

The Bad

The Bad

The Bad

The Bad

The Bad

The Bad

The Bad

Finer Points Estimates of the resolution of the human eye are

some where around 576 mega pixels (24000 x 24000 pixels) for a 120 degree field of view.

However, it must be noted that the human eye itself has only a small spot of sharp vision in the middle of the retina, the fovea centralis, the rest of the field of view being blurry.

The angle of the sharp vision being just few degrees in the middle of the view, the sharp area thus barely achieves even a single mega pixel resolution.

The experience of wide sharp human vision is in fact based on turning the eyes towards the current point of interest in the field of view, the brain thus perceiving an observation of a wide sharp field of view.

First surgical procedure 6th century B.C. "couching", in which a curved needle was used to

push the lens into the rear of the eye and out of the field of vision. The eye would later be soaked with warm clarified butter and then bandaged.

Couching continued to be used throughout the Middle Ages and is still used in some parts of Africa and in Yemen.

1748 First lens extraction (Modern Era)

1884 First local anesthetics (Cocaine Drops) 1940 First IOL (Harold Ridley)

1960 Phacoemulsification

Cataract Surgery Saga

Stats

According to surveys of members of the American Society of Cataract and Refractive Surgery, approximately 2.85 million cataract procedures were performed in the United States during 2004 and 2.79 million in 2005.

Eye Measurements

Average Range

• Axial Eye Length 23.5mm 22.00-24.5

• AC Depth 3.24mm Varies with AL

• LT 4.25mm up to 6.9 w/cat

• Pachymetry 0.55mm

• Keratometry 43.0-44.0 Usually within 1D of

each other

Normal Measurements

23.5mm

K:43 - 44 D

3.24 mm

550μm

4.25 mm*12.5mm

0.5 – 8mm

*LT is Age dependant andcontinuously growing

245 μm

Contact Method – Contact Biometry is the most common technique used for measuring eye length, but is by far the most inaccurate. This method requires placing the A-Scan probe directly on the Cornea. This will result on Corneal compression and the amount of compression will vary depending on the experience and technique of the user.Studies have shown that this method can result in erroneous eye length measurements between 0.14mm and 0.36mm too short. (.25D - .75D)

A-Scan Methods

• Immersion Method – The Immersion method of Ultrasound Biometry is by far the most accurate way to measure the AL of the eye using Sound. This method requires that the probe is placed in a “bath” of saline solution directly over the eye. With the immersion of the probe the cornea is not touched thus eliminating the compression errors.

Optical Biometry – Uses Optical Low-Coherence Reflectometry, a similar technology that is used in OCT devices. This technology results in highly accurate measurements of the eye using light in comparison to sound. The added benefit is that this technology is also non contact and can be performed with the patient sitting comfortably in a chair without the need for any topical anaesthesia, and without the risk of damage to the cornea.

The Down SideSince optical Biometry uses light there is a higher probability of the “scatter” effect. Meaning that if the light beam is reflected prior to the RPE then the signal returning to the device sensor will be very weak if detected at all. This will result in low SNR. Patients with Dense PSC, Extreme Corneal Abnormalities, or White Cataracts are very tough to measure.

Long Eyes

• Optical up to 40mm• Ultrasound 45mm (Up to 60)

< 5% of patients have AL > 26mm

Lenstar LS 900 (Haag-Streit)IOL Master (Carl Zeiss)OA 1000 (Tomey) to be launched 2000?

Manufacturers

Optical Biometer FeaturesFeature Device

IOL Master Lenstar Tomey OA 1000

Axial Length X X X

White to White

X X X

Keratometry X X X

ACD X X X

Pachymetry X X

Lens Thickness

X ???

Retinal Thickness

X ???

Pupillometry X ???

Visual Axis X ???

Patients demand at least 20/20 vision after surgery

Multifocal / Accommodating IOL’s

Toric lenses

Patient specific anatomical anomaliesVery Long / Short Eyes / Astigmatism

Modern Technology / Knowledge

Measurement along the Visual Axis

Visual axis(Patient fixates on the measurement beam)

Ultrasound BiometerTheoretical Optical Axis

Optical BiometerVisual axis

≈ 90°

What about the “Dust Cover”?

aka CorneaTo ensure proper K value along central visual axis Keratometry should be measured at the same time as Axial Length.

Poor Alignment

* Image courtesy of Karen Bachman and Cara Fletcher from the Cincinnati Eye Institute

39.25 / 43.25

37.50 / 42.00 44.75 / 45.25

40.75 / 43.50 35.25 / 40.50

* Measurements obtained using Reichert Manual Keratometer Model # 12990

41.25 D

39.75 D45.00 D

42.12 D 37.88 D

IOL Data

Alcon SN60WFSRK/T using 119.05

www.doctor-hill.comAL = 23.50CCT + AD = 3.79

24.0 D

25.5 D19.5 D

23.0 D 27.5 D

+ 4.50 D

+6.00 D0.00 D

+3.50 D +7.00 D

US vs. optical Biometry Axial length

US Biometry accuracy ±0.04 mm (theoretical)±0.1 to 0.12 mm (real)

Optical Biometry accuracy ±0.03 mm (theoretical)±0.03 mm (real)

US depends on user (0.1mm ≡ 0.28 D)Compression of the eyeMisalignment of the probe

Achievable accuracy in post-op. refractive errorUS Biometry based St. Dev.: 0.8DOptical Biometry based St. Dev.: 0.25 to 0.5D

US vs. Optical BiometryCorneal Curvature

US Biometry requires a 2nd deviceAnterior Chamber Depth

Compression of the anterior chamber (US)Misalignment of the probe (US)

Contact (US) vs. Non contact (Optical)Transmission of bad germs (US) Risk of corneal injuries (US)Compression of the eye (US, unless

immersion)No limitation with the cataract penetration

(US)

US vs. optical BiometryWhy is Optical- Superior US Biometry:

Higher AccuracyNon contactAll measurements in one deviceOperator independentSpeedPatient comfortSpecial cases like very long (high myopic)

eyes

US vs. Optical BiometryHigh myopic cases (very long eyes)

US Biometer on theoretical optic axisOptical Biometer on the visual optical line

US Biometer

Optical Biometer Known Cases of up to more than 1.5mmor 3.0 to 3.5 D of post operative Myopia

IOL Formulas

When intraocular lenses were first becoming a viable alternative to aphakia after cataract removal, back in the 1970's,  the IOL power was estimated by consulting a chart of expected outcomes given a patient's refractive history.  It soon became obvious that this method was inadequate and formulas were developed based on keratometry readings, axial length, and past results.  Formulas continue to improve and are now based on theoretical optical models.

Biometry Formulas• 1st Generation – The first theoretical formula (based on

Geometric Optics as applied to schematic eye models) was developed in 1967. These formulas were very primitive and usually resulted in large amounts of post-cataract surgery refractive errors. (Regression)

• 2nd Generation – With an extreme need for increased IOL Calculation, the second generation formulas (Hoffer, SRK II) listed manual correction factors for long or short eyes. (These formulas are now considered obsolete.) (Regression)

• 3rd Generation – In 1988 Dr. Holladay published a formula (Holladay I)that predicted the AC Depth on the basis of K Height and the distance from the iris plane to the IOL optical plane called the “Surgeon Factor”. This change in the physics greatly increased the visual outcomes for Cataract Surgery. This generation also includes the SRK/T and Hoffer Q.

• 4th Generation – Consist of Holladay II as well as the modern post-refractive formulas

Lens Constants

• A-Constants are used with all IOL formulas, and are determined by the anticipated position within the eye.

• Surgeon Factor – is used with the first Holladay formula, and is determined by the distance from the Iris plane to the Optical plane of the implant.

• Effective Lens Position (ELP) is used for the Holladay II formula, and is based on the depth of the AC following Cataract surgery with the new IOL in place.

Types of Formulas Regression formulas are based upon

mathematical analysis of a large sampling of post-operative results.  The most familiar regression formula is the SRK formula. The basic SRK formula  works well for eyes in the "average" measurement range; 22.5 to 25.0 mm in axial length, with certain combinations of K readings.  The formula does not work well for "long" (>25 mm) or "short" (<22.5 mm) eyes. 

The advantage of a simple regression formula is that it is relatively simple to calculate.  A factor can be added to a simple regression formula to compensate for a long or a short eye. (These formulas have been discredited by their authors.)

Theoretical formulas are optical formulas based on the optical properties of the eye.  They do a better job of predicting post-op outcomes for long and short eyes.

Theoretical Formulas Holladay -- Produced by Jack Holladay MD.  It has been

considered to be most accurate for eyes with an axial length between 22 and 26mm.  Available in Holladay IOL Consultant (HIC) V2.2 software.  The software will also do calculations for piggyback IOLs, post-refractive surgery patients, and personal constants for the surgeon. 

The latest version of the software, called Holladay II, requires the pre-op refraction, the anterior chamber depth, the lens thickness, and the "horizontal white-to-white"  (corneal diameter), as well as the K's and axial length.  The software claims that the additional information improves accuracy. The anterior chamber depth and the lens thickness can be obtained from most A-scan instruments in the process of obtaining the axial length measurement.

Haigis – This formula has been shown to be as accurate as the Holladay II for all eye lengths once optimized. Most other formulas only use one constant such as ACD, A-Constant, or ELP for a given IOL, Dr. Haigis’ algorithm uses three (A0, A1, A2).

A1 – Measured ACD A2 – Measured AL A0 – Regression based on Surgical Outcomes

The Haigis formula is “Optimized” by Dr. Haigis in Europe and Dr. Warren Hill in the U.S. after a sample of 200-300 post-op results are sent directly to Dr. Haigis or Hill.

Hoffer Q -- Produced by Kenneth Hoffer MD.  It has been considered to be the most accurate formula for short eyes and does well for short eyes <22mm.  It recommends the best calculation based on axial length. 

SRK II – Is a formula used in short eyes <22mm as well as in long eyes >24.5mm it uses calculations based on AL and Keratometry (Not in use by most physicians)

Olsen – The Olsen formula addresses four areas of concern: the calculation of corneal power, the measurement of axial length, the anterior chamber depth prediction, and the IOL optics. (European)

SRK-T – Uses empirical regression methodology for optimization of the post-operative ACD prediction, of the retinal thickness correction factor, and of the corneal refractive index.

Formula Requirements

Haigis Hoffer Q SRK/2 SRK/T HOLLADAY 1 HOLLADAY 2 Olsen

Axial Length YES YES YES YES YES YES YES

ACD YES NO NO NO NO YES* YES

Keratometry YES YES YES YES YES YES YES

Lens Thickness NO NO NO NO NO YES YES

Corneal Thickness NO NO NO NO NO NO NO

White to White NO NO NO NO NO YES YES

Pupil Size NO NO NO NO NO NO NO

Visual Axis NO NO NO NO NO NO NO

 

Formula Preferences Short Eyes (<22.0mm) Hoffer Q Average Eyes (22.1-24.4mm) Hoffer Q /

Holladay I / SRK/T Medium-Long Eyes (24.5-25.9mm) Holladay I / Hoffer Q Long Eyes (25.0mm +) SRK/T / Holladay

I(Holladay II All eye lengths.)(Haigis All eye lengths

w/optimization)

Average Eye

Short Eye

Long Eye

Importance of your Measurements!

Remember!!

PCIOLFor every .1mm error in

Calculations, there is a .19D Post-Op SURPRISE!

So 1.0mm equals about 2.0D

ACIOLFor every .1mm error in

Calculations, there is a .12D Post-Op SURPRISE!

So 1.0mm equals about 1.2D

Coming soon to your Clinic!!Keep an eye out!!

According to surveys of members of the American Society of Cataract and Refractive Surgery, approximately 948,266 refractive surgery procedures were performed in the United States during 2004 and 928,737 in 2005.

Post-Refractive Surgery Patient

One of the most challenging problems facing modern Cataract Surgery is the Post-Refractive patient. Following refractive surgery (RK, PRK, LASIK, ect) accurate K readings cannot be obtained from topography, automated or manual keratometry because the central cornea has been flattened causing the mires of the measuring device to measure roughly 4.5mm versus 3.0mm for which they were designed. This causes erroneous K readings compromising the effectiveness of all modern IOL formulas.

Post Refractive Formulas

Haigis L - The Haigis-L formula offers predictable outcomes after laser refractive surgery for myopia based only on current measurements without refractive history. Data showed that 98.4% of predictions were within 2 D of targeted refraction, 84% were within 1 D and 61% were within 0.5 D. Among eyes without previous refractive surgery, 99.1 % were within 2 D, 93% were within 1 D and 73.4% were within 0.5 D of targeted refraction. (JCRS 10/2009)

Masket Method - The Masket Method of post-LASIK corneal power estimation is a postoperative regression method developed by Samuel Masket and recently published in the JCRS.

Clinical History Method – Is usable when both the pre-op and post-op Keratometry values are known.

Contact Lens Method - The Contact Lens Method, originally outlined by Dr. Holladay is considered a helpful way to estimate the average central corneal power following radial keratotomy. This technique required a special PMMA contact lens, of a known base curve and power.

Shammas – Used when no pre-op data is available such as refraction and keratometry.

Double K SRK/T – Utilizes pre-op refraction and keratometry

Almost Finished!

I Promise!

Future IOL TechnologyLiquiLens (Vision Solutions): a dual

liquid gravity dependent lens composed of a single optic with 2 immiscible fluids of different refractive index. With down gaze, the liquids change position producing increased power of at least 30 D.

• SmartIOL (Medennium): a thermoplastic flexible acrylic gel polymer that can be manufactured to precise optical specifications and customizable into any size, shape, and power. The polymer undergoes a temperature dependent configuration change: it is converted to a thin rod at room temperature, implanted through a micro incision, and returns to its original shape (approximately 9.5 mm wide and 3.5 mm thick) and power at body temperature. Potentially, wavefront, Toric, and multifocal technology could be imprinted onto the surface of the lens.

Light Adjustable Lens (Calhoun Vision): a photosensitive adjustable foldable 3-piece IOL enabling precise non-invasive post-op adjustment of myopic, hyperopic, and astigmatic errors up to 2 D. The lens is composed of subunits (macromers) embedded in a matrix. Focal UV irradiation from a digital light delivery device (Carl Zeiss Meditec) causes polymerization of macromers, then nonpolymerized macromers diffuse and migrate into the irradiated area causing a power change. Irradiating the lens again locks in the desired configuration. Future possibilities include the potential for multifocal and wavefront adjustments, as well as, readjustments.

Synchrony IOL (Visiogen): another 1-piece foldable lens with 2 optics that are connected by spring-like haptics. The 5.5 mm +32 D anterior optic moves 1.5 mm with respect to the 6 mm minus-power posterior optic to produce 3.3 D of accommodation. It can be injected through a 3.7 mm incision.

Bit of Advice

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Newbury Street 352 Newbury Street Boston MA 02115 617-236-1666 Open Sunday-Thursday 11am - 11pm Friday-Saturday 11am - midnight

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The End!!!!

Contact Info

agraham@haag-streit-usa.com

References/Bibliography NBC Olympics

www.nbcolympics.com Tomey

www.tomey.co.jp/asia/products/products/Info/OA-1000/OA-1000.pdf Dr. Warren Hill, M.D.

http://www.doctor-hill.com JCAHPO

http://www.jcahpo.org Google Images:

http://images.google.com Reference:

http://www.reference.com AAO

http://www.aao.org WebMD:

http://www.webmd.com ASCRS

http://www.ascrs.org