A PROJECT ON“ACCURATE BIOMETRY &POST-OPERATIVE OUTCOMES”The primary intent of this project is to present in a simple and illustrative manner the actual role ofbiometry in IOL power calculation for cataract surgery to provide the patient best vision. Its contenthave been so organized to provide useful information regarding the post-operative error due toerroneous biometric measurements before cataract surgery.CONTANTS: 1.INTRODUCTION 2. TECHNIQUES OF BIOMETRY:- ITS FEATURES 3.DIFERENT FORMULAS USED IN BIOMETRIC CALCULATIONS. 4.STEPS IN SELECTING CORRECT IOL 5.DIFFICULT EYES 6.COMMON A- SCAN ERRORS 7.A STUDY 8.RESULTS 9.CONCLUSION1.INTRODUCTION • Heightened patient expectations for precise post-operative refractive results have spurred the continued improvements in biometry and intraocular lens calculations. In order to meet these expectations, attention to proper patient selection, accurate keratometry and biometry, and appropriate intraocular lens power formula selection with optimized lens constants are required. Intra ocular lens implantation in cataract surgery is a rapidly evolving art towhich ultrasonic biometry adds the finishing touch. The pre-operative calculation of IOL powertoday, is an established ophthalmological technique which is being adopted by an increasing numberof ophthalmic surgeons the world over. The precision in 100 power calculations pre-operatively givemore predictable and accurate post-operative results, which is the goal one strives to achieve.
• Biometry is the process of measuring the power of the cornea (keratometry) and the length of the eye, and using this data to determine the ideal intraocular lens power .It is a one dimensional time amplitude display. • The A-scan uses reflected sound waves to measure intraocular lengths. A sound beam passes through the eye, strikes various interfaces (media with different densities), and part is reflected back (echo). The “A” stands for amplitude, which is a measure of the reflectivity or strength of the echo (spike height) and depends upon the density of the media, incidence of the sound beam (perpendicularity), and gain. The gain refers to amplification of echoes, which is directly proportional to sensitivity, indirectly proportional to resolution, and adjustable on the machine. 2. Technique:- There are two technique 1.contact technique and 2. Immersion techniqueContact technique:• Probe in contact with cornea• Hand-held or slit-lamp• Most popular but least accurate(0.10 mm at best)• Takes longer to do than immersion!• Corneal compression always a factor• Tech-dependent• IOP dependent• Greater risk of corneal abrasion• Greater variation of readings so must delete then do more• If must do contact, gentle on/off technique• Recline patient for less compression,Sit down on an adjustable stool• Place machine where screen easily seen• Monitor ACD carefully Hansen Shells2) Immersion Technique• Probe immersed inshell of saline • Most accurate/no corneal compression (0.0126 - 0.05 mm depending on manufacturer)
FEATURES OF IMMERSION TECHNIQUENo tech-dependency• Less risk for corneal abrasion• Faster technique• More consistency• Less repeating• “Gold Standard of CarePROCEDURE:-Hansen Shells•Anesthetize eye•Place shell on limbus •Fill with contact lens saline by squirting stream against inner rim•Place probe in saline and align3.DIFERENT FORMULAS USED IN BIOMETRIC CALCULATIONS • Introduced by SANDERS,RETZALLF and KRAFF
P = A – 2.5L- 0.9K where, P=IOL POWER A=CONSTANT SPECIFIC FOR EACH LENS L=AXIAL LENGTH K=AVERAGE KERATOMETRY IN ‘D’ • Used for axial length between 22-24.5 mm • Problem:-predict too small value in short eye and too large value in long eye. SRK2 P = A - (2.5 L) - (0.9 K) + CValues in Modified SRK - II where, P : IOL power • A : A - constant • L : Axial length • K : Average K- reading • C : Correction factor • AL < 20mm C=3, 20 - 21mm C=2 • 21 - 22mm C=1, 22 - 24.5 C=0, >24.5mm C= - 0.5 SRK2 MODIFIED: • AL < 20 mm C = 1.5 ,AL 20 – 20.99 mm C=1 • AL 21- 21.99 mm C = 0.5,AL 22- 24.5 mm C=0 • AL 24.5 – 26 mm C = -1 , AL > 26 mm C = -1.5 SRK-T Non linear theoretical optical formula • For post operative -RETINAL THICKNES,CORNEAL R.I. • More accurate for long eye (if axial length>28.0mm)HOFFER Q • Used for ACD.
• CD=ACD+0.3(AL-23.5)+(tan k)2+0.1M(23.5 AL2) • Non linear second generation formula • More accurate for eye with axial length between 22-26 mm. • Axial length=AL+0.200mm • SF=A X 0.5663)-65.60 • ACD =0.56+Rag – [SQRT(Rag2-Ag2/4] • Ag=13.5 • Rag=7mm HAIGIS FORMULA • IT USES 3 CONSTANTS a0, a1&a2. to set both position & shape of power prediction . d=effective lens positions d=a0+(a1*ACD)+(a2*AL) ACD=ant chamber depth AL=axial length a0,works in the same way as other const.4.Steps in selecting the correct IOL Identifying the refractive needs of the patient • Emmetropia will be the goal for most patients, but some may benefit from being left intentionally myopic post-operatively (or, rarely, hypermetropic), depending upon their preference and the refraction of the other eye. Anisometropia should be kept below 3 dioptres. The need for reading glasses should be explained and the patient made aware of the options.
Measuring the axial length of the eye • The measurement of axial length measurement has the greatest potential for error in calculating IOL power. Traditionally, contact A-scan ultrasonography is used. This measures the time taken for sound to traverse the eye and converts it to a linear value using a velocity formula. Part of the ultrasound beam reflects back from each surface in the eye – cornea, anterior lens, posterior lens, and retina. The reflected beam is translated into an image showing lines (spikes) for each surface. The distance between the corneal and retinal spikes gives the axial length of the eye.4.1.Tips for accurate measurement of axial length (using applanation): • ensure the machine is calibrated and set for the correct velocity setting (e.g. cataract, aphakia, pseudophakia) • the echoes from cornea, anterior lens, posterior lens, and retina should be present and of good amplitude • misalignment along the optic nerve is recognised by an absent scleral spike • the gain should be set at the lowest level at which a good reading is obtained • take care with axial alignment, especially with a hand-held probe and a moving patient (as described above) • don’t push too hard – corneal compression commonly causes errors • average the 5-10 most consistent results giving the lowest standard deviation (ideally < 0.06 mm) • errors may arise from an insufficient or greasy corneal meniscus due to ointmentor methylcellulose used previously. • Again, an accuracy is essential, as an error of 0.75 D in the keratometry will result in a similar post-operative error. Keratometry may be carried out manually or using an automated or hand-held device. • Again, accuracy is essential, as an error of 0.75 D in the keratometry will result in a similar post-operative error. Keratometry may be carried out manually or using an automated or hand-held device.4.2.Tips for accurate manual keratometry: • calibrate and check the accuracy of the • keratometer • use a dedicated single instrument that is • known to be accurate
• don’t touch the cornea before hand and • ensure a good tear film • adjust the eyepiece to bring the central • cross-hairs into focus• make sure that the patient’s other eye is occluded and that the cornea is centred• take an average of three readings, including the axis• if high or low results are encountered (< 40.00 D or > 48.00 D), it is advisable to have a secondperson check the measurements• repeat if the difference in total keratometric power between the eyes exceeds 1.50 D• in a scarred cornea, use the fellow eye or average the results.4.3.Using the appropriate formulaThe Hoffer Q, Holladay I, and SRK/T formulae are all commonly used, but theSRK I and II regression formulae are now regarded as obsolete.5 More recentformulae, such as the Holladay II or Haigis, are not currently built into biometrysoftware. Where audit software is in use,the personalisation of calculation constantscan increase accuracy. Table indicates which formulae to use.Table 1. Range of axial length and preferred formula • < 20 mm Holladay II • 20-22 mm Hoffer Q • 22-24.5 mm SRK/T / Hoffer Q/Holladay (average) • > 24.5-26 mm Holladay I • > 26 mm SRK /T5.DIFFICULT EYES: • Extremely dense cataracts create difficulties, as they absorb sound as it passes through the lens. A higher gain setting may be necessary to achieve adequate spikes.Posterior staphyloma in myopic eyes not only cause an elongated globe, but often tilt the macula as
well so that the ultrasound beam is deflected. In these cases, it may be necessary to add the A-scan anterior chamber depth measurement to vitreous depth taken from a B-scan.5.1.High Myopia • The eye is misshapen, oval or elongated rather than round.- Macula on a “slope” ;Perpendicularityimpossible.Eyes may be different lengths -- “The shorter theeye the more symmetrical, the longer the eye the more asymmetrical”5.2.Posterior Staphyloma• Can occur in the high myope • Uvea bulging into thin, stretched sclera • Most common in posterior pole • Macula within bulge • Perpendicularity impossible • Measurements vary greatly • OCB helpful • Immersion scanning is to be preferred for axial length measurements, especially for short eyes. • First, with the corneal double peak echo clearly discernible, all 4 ocular landmark echoes (cornea, lens front and rear, back wall) may be adjusted for optimal steepness thus achieving best orientation. • second, no danger of indenting the globe exists thus preventing the axial length from being measured too short which would later result in a postoperative myopization.5.3.IOL related problems in short eyes • The IOL to be used for short eyes must usually have high dioptric powers (30 dpt or more). • Commonly, these powers are not readily available and the lenses themselves must be especially designed and manufactured. • With high-powered lenses a new problem starts to rise: differences between total power and vertex power may not be neglected any more. Due to still different power-labelling policies of IOL manufacturers, the diopter value for a given lens may either denote total or back vertex power.
Fig.2: Principal planes of lenses of different shape: 0: convex-plano, 1: biconvex with 1:1 ratio of anterior to posterior lens radius; 3: biconvex with 3:1 ratio. All lenses are situated in the same position (e.g. capsular bag). In order to focus onto the retina R, each lens must have a different focal length f, i.e. different power. • Powers relate to the effective focal length (f, principal plane to focal point) for total power and to vertex distance (back vertex to focal point) for vertex power. • To make things worse, these distances are dependent on the shape of the lens opticWhy things go wrong • No matter how good the system, people will still make mistakes. Some reasons include: • people in a hurry • lack of training or accessible guidelines • reliance on others technical failure (rarely) • human error (often). • Some common mistakes (collected from the UK and overseas departments): • wrong A-constant selected • wrong formula used • wrong K-readings entered by hand (90 degrees out) • biometry print-out stuck in wrong patient’s notes • incorrectly labelled IOL
• wrong patient in theatre • reversed IOL optic wrong IOL implanted (25.5 D implanted Instead of 22.5 D or +30 D instead of +3.0 D). • Some errors of omission include: • no biometry at all • no spectacle prescription or focimetry available • no IOL available on the day • not taking account of the other eye • not discussing the intended outcome with the patient.6.Common A-scan errors include: • Misalignment — if the probe (e.g., sound beam) is not perpendicular to the lens (resulting in a short lens spike) or to the macula (resulting in a poor retina spike). Furthermore, misalignment also occurs if the probe is aligned along the optic nerve (resulting in no scleral spike). • Gain too high — although this increases the sensitivity, it also produces poor resolution of spikes, causes the retina and sclera spikes to merge, combines the anterior and posterior corneal peaks, and cuts off the tops of all spikes (flat-topped rather than pointed spikes). • Falsely short reading — this type of error may be caused by corneal compression (AL & ACD will both be 0.14-0.36 mm shorter than their true values), the beam not being perpendicular, vitreous opacity or membrane, choroidal thickening/effusion, wrong gate position, or incorrect velocity (too slow). • Falsely long reading — such an error occurs from a fluid meniscus (between the probe and cornea), posterior staphyloma, measuring to the sclera instead of the retina spike, wrong gate position, or incorrect velocity (too fast). • Incorrect velocity — if the eye is not phakic, it is important to change the setting so the machine uses the correct velocity for either an aphakic or a pseudophakic eye with the specified IOL material (alternatively for pseudophakes, the aphakic setting can be used with a correction factor). The aphakic scan has 1 less spike (lens spikes absent but spike from capsule or hyaloid face is present), whereas the pseudophakic eye shows multiple spikes in the vitreous due to reverberation artifact from the IOL. Phakic eyes filled with silicone oil also pose a potential for incorrect velocity errors. In this situation, accurate biometry requires using a silicone oil setting (980 or 1040 m/s depending on the type of silicone oil) or a velocity conversion equation for the vitreous portion ((Vc/Vm) x AVL = TVL) (where Vc = velocity (correct), Vm = velocity (measured), AVL = apparent vitreous length, TVL = true vitreous length).
Importance of Accuracy • In short eyes, • 0.1 mm error = 0.25 - 0.3 D post-op surprise! • Therefore, 1.0 mm error = 2.5 - 3.0 D post-op! • • 0.1 mm error = up to 0.75 D post-op! Accurate Biometry • How do I know it’s a good scan? • Understand ultrasound principles • to accurately interpret spik patterns • Use most accurate technique • Use good equipment • Avoid common errorsImproving Surgical Outcomes • Most common causes for post-op surprises: • Axial eye length error • Erroneous K-readings • IOL position • Post-Op Surprises • If measurement too short, • post-op error = myopic direction • If measurement too long, • post-op error = hyperopic direction
7.A STUDY:- The post-operative refraction was evaluated in 100 patients undergoing cataract extraction with posterior chamber IOL implantation. The patients were selected irrespective of their age and sex. Keratometry was carried out on an Appaswamy Keratometer (Baush and Lomb type) and readings were recorded in dioptres. The axial length was measured with the Opthasonic A-scan ultrasonic unit with a built in microprocessor for determining the axial length of the eye, computing the IOL power, to analyse the echo returns, put out audio signals and automatically print the valid measurements while they are being taken. The microprocessor analyses the amplitude of lens and retinal spikes to use as criteria to ensure proper probe alignment. The IOL power was calculated by using the SRK formula. This was done by using the Programme - 4 in the Ophthascan unit in which the SRK formula is incorporated. The basic formula is P = A - 2.5L - 0.9K, where, P = Power of lens for emmetropia A = A Constant of lens to be implanted L = Axial LengthK = Average of Keratometry readings. • For `short eyes , the following relationship will apply to the calculated power automatically. AL 20mm: 2D added to IOL AL 20mm to 21 mm: 1.50D added to estimated IOL power. a AL 21 mm to 22mm: 0.75D added to estimated IOL power. where AL is the axial length. • Depending on the patients occupation and refraction of the other eye, the desired post- operative refraction of the patient was derived. In general, the aim was to make the patient myopic, by 1 to 2 D, so that they would have good near vision without correction. • At the end of one and a half months, retinoscopy was performed and correction was given for distance and near. The difference between the expected post-operative refraction and
the spherical correction for distance was determined. This gave the deviation of the post- operative refraction from the expected value. The results were tabulated and analysed. • Note the 5 high-amplitude spikes and the steeply rising retinal spike separated from the scleral spike• initial spike (probe tip and cornea) anterior lens capsule, posterior lens capsule, retina, sclera and• orbital fat8. Results:- The maximum corrected post-operative visual acuity for distance was 6/6 and the minimum was finger counting at 5 feet. Both patients with visual acuity less than 6/60 were young males. One patient had a traumatic cataract, the other had retinitis pigmentosa. The average keratometry readings show a trend towards the myopic side. The average axial length 23.19 mm falls in the normal range. The average IOL power 20.91 D is close to the standard power of PC IOL ie. 21.0 D. The accuracy of prediction is equally good in all three groups. This is because of the correction factors incorporated in the programme. Sanders, Retzlaff and Kraff in their study using the SRK formula had the following results . The results obtained in this study are comparable to those obtained by other authors. In 99% patients the error of prediction was within ± 3.OD. The maximum error encountered was +3.2 D. These results can be further refined by using immersion instead of applanation for measurement of axial length and by calculating the personal A-constant for each surgeon and lens style. Therefore, the answer to the question - "Is a smaller amount of ametropia worth the time and expense of a precise biometry?" is obvious . Biometry is definitely essential, because the primary reason for IOL implantation is the reduction of high aphakic ametropia. As the calculation of IOL power can be done with no risk to the patient, there are no medical reasons for implanting any "Standard" lens.
CASES:CASE1: • AGE:- 60 • GENDER:- M • PRESENTING COMPLAINTS:- C/O BLURRING OF VISION ( D & N) × 3 MONTH C/O FALLING OF UPPER EYELID LASHES & BURNING SENSATION × 3 MONTH H/O HAVING SPECS BUT NOT USES N/H/O TRAUMA & PRIOR OPHTHALMIC CONSULT OD OS • Vn:- ( WITH GLASS) 6/24 6/24→6/18 • PREVIOUS GLASS Rx:- -3.25 DS -2.75 DS • REFRACTION:- -3.5O DS - 3.50 DS • ACCEPTANCE:- -3.50 DS (6/24+1) -3.50 DS ( 6/18) • AT:- 14 ( mm of HG) 15 SLE:- WNL DIAGNOSIS:- PSC ( NS 2) (OU ) OD>OS ADVICE:- PHACO WITH IOL UNDER LA • K READING:- OD OSHORIZONTAL (180°) 42.50 42.50VERTICAL ( 90°) 42.25 42.25 A-SCAN:- AXL:- 23.89 23.90 ACD:- 2.90 3.51 PCIOL:- +20.50 +20.50
9.CONCLUSION:- Accurate keratometry readings are also important because each diopter of error in the average Kvalue results in an equivalent error in predicted IOL power. The other parameters used in IOLcalculations include A-constant of the IOL, anterior chamber depth, effective lens position, and IOLgeometry. Even with accurate biometry and keratometry measurements, a surprise may occur in thesetting of unusually high or low values for which most IOL formulas are less accurate. The exceptionsto this are the Holladay II formula and the Haigis formula when surgeon optimized, both of which areaccurate for all size eyes.Accurate axial length determination is critical for successful cataract surgery. This is even moreimportant now as patients expect perfect results. Fortunately, we are able to achieve more accurateand predictable outcomes. This is mostly due to improved technology: more accurate biometry,newer IOL calculation formulas, smaller wounds, attention to astigmatism correction, and betterIOLs (aspheric, toric, and presbyopia-correcting). Although we can more frequently obtain a targetrefraction very close to predicted, post-operative refractive surprises still occur. The most commoncause is an axial length error (0.1 mm error = 0.25-0.3 D surprise). A myopic surprise occurs if the ALmeasurement is too short, and a hyperopic surprise occurs if the measurement is too long.Therefore, accurate biometry is essential. • The surgeon should suspect a problem and double check the AL if there is a difference of > 0.3mm between the eyes, or there is a difference of > 0.1mm among readings in the same eye.REFERANCE: