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1. O The orbital spikes O are low reflective behind the scleral spike The eye is divided ultrasonically into three compartments See Figures 10 5 and 10 6 1 The anterior chamber depth ACD s measured between the anterior corneal surface C1 and the anterior lens surface L1 using a velocity of 1532 m s If needed the corneal thitkness is measured between the anterior C1 and posterior C2 surfaces of the cornea using a velocity of 1620 m s The thickness of a normal cornea is approximately 0 5 mm 7 12 2 The lens thickness is measured between the anterior lens surface L1 and the posterior lens surface L2 using a velocity of 1641 m s Actually 1640 5 m s is the calculated sound velocity in the normal crystalline lens The sound velocity varies in cataractous eyes with a slower velocity average 1590 m s in the intumescent cataracts due to their high water content and a higher velocity in the posterior capsular cataracts In most cases of nuclear sclerosis with or without subcapsular changes the sound velocity averages 1641 m s 3 The vitreous cavity s depth is measured between the posterior lens surface L2 and the anterior surface of the retina R using a velocity of 1532 m s A manual measurement of the axial length is used with older ultrasound units not equipped with an electronic read out and is rarely used nowadays Using calipers a measurement is taken from the scale in microseconds However this rea
2. a 10 MHz sound bean has a resolution of approximately 0 03 mm By comparison the 780 nm partially coherent light source used in optical coherence biometry has a wavelength of 0 0000000975 mm And since the smaller the wavelength the higher the resolution there is simply no comparison between the two Fundamental Principle 3 Variability for an on axis A scan measurement is an artifact of position Here is an OCT 3 macular thickness plot from a normal eye that illustrates this So for the exercise of A scan biometry we have to take into account the inherent resolution limitations of a 10 MHz sound wave and its inability to discriminate between the foveal center and the foveal shoulder This may not sound like a large error but it s helpful to keep in mind that these types of errors are cumulative Thickness Chart Microns 6007 S0017 400 400 200 Loo g 100 200 300 400 500 Limited resolution As mentioned above a 10 MHz sound bean has a resolution of approximately 0 03 mm By comparison the 780 nm partially coherent light source used in optical coherence biometry has a wavelength of 0 0000000975 mm And since the smaller the wavelength the higher the resolution there is simply no comparison between the two The Haigis Formula Intraocular Lens Power Calculations IOL power calculations have become the main focal point of refractive surgery in a relatively short period of time As recently as 1977 the
3. 21 69 2207 3 61 21 69 AVG 3 61 21 69 STD D 01 0 00 m ha io iE rt re fa 10 L CALACULATION Formula SRK II HAGIS Const 118 50 A 118 50 A IOL REF IOL REF 26 50 0 60 30 50 0 72 27 00 0 20 31 00 0 36 27 50 0720 31 50 0 03 28 00 0 60 32 00 0 32 28 50 1 00 32 50 0 68 Clinical Accuracy New enhanced algorithms for rapid measurement 5 measurements presented on a single screen Multiple velocities and lens type for greater accuracy Statistical analysis of axial length vitreous anterior Chamber depth and lens thickness Continuous and manual readings Data analysis Edit scan data to IOL power calculations ALL current generation IOL power calculation formulae View multiple power calculations using different formulae in a single screen Transducer Frequency 10MHZ 10 Clinical accuracy 1 0 1 mm Theoretical accuracy 0 05 mm Axial length 15 40 mm Gain range 90db Fixation light internal fixation RED Measurement modes Contact Immersion IOL Power calculation formulas SRK T Binkhort SRK II Hoffer Q Holladay Eye Modes Phakic Aphakic Pseudo Phakic Auto and Manual Readings Clinical Accuracy 0 4mm Internal Fixation Light Red Freeze Foot Pedal or Console World wide Customer support We have well knit service network across the globe at Strategic locations Our Service engineers undergo AKAS in plant training course to enable qui
4. Bio Meter An Experience Affordable Quality About AKAS Medical AKAS Medical was started in 1996 as a small firm with a team of professionals headed by Mr Arjunsooraj V CEO The techonology and product innovation is headed by Mr Arunkrishna V The promoters are qualified engineers and Management graduates with shared Vision AKAS started with manufacture of drug delivery systems and now has grown many folds in the field of Ophthalmology Critical Care Disposables Nutrition Online Medical stores www akasdoctor com Today AKAS Medical has its head office at Chennai in India and manufacturing plants and marketing offices at various locations globally It has grown through sheer expertise and knowledge sharing in the field of medicine and technology AKAS Medical other than following the ISO 9001 quality management system has itself developed a quality management system apt for the medical devices quality and has named it as AKAS Quality System AQS in short AKAS pays lot of importance on Safety on the therapeutic and diagnostic products that it manufactures Today AKAS Products and solutions are accepted world wide by the medical fraternity AKAS believes in being socially responsible and stand by its mission to make health care affordable and available for everyone AKAS believes in not just selling products but in offering solutions on the whole and also in education of its customers to enhance their experience
5. ater A i o The axial length AL is conventionally measured with ultrasonography using a biometry unit Measurement of the AL is achieved using an immersion or a contact technique It is always advisable to have both eyes measured for comparison purposes Immersion A Scan Biometry TECHNIQUE The technique described herein can be used with any ultrasound unit equipped with a solid A scan probe and mobile electronic gates 1 The patient is placed in a supine position on a flat examination table or in a reclining examination chair and a drop of local anesthetic is instilled in both eyes 2 A scleral shell is applied to the eye The most commonly used scleral shells are the Hansen shells Hansen Ophthalmic Development Laboratory Coralville IA Figure 10 1A and the Prager shells ESI Inc Plymouth MN Figure 10 1B The Hansen shells are available in 16 18 20 22 and 24 mm diameter While the 20 mm shell fits most eyes the larger cup provides a better fit in bigger eyes with large palpebral fissures and the smaller cups fit better in the presence of a narrow palpebral fissure The newest Prager shell features single handed immersion biometry a Luer fitting to facilitate tubing changes an autostop for exact manufacturer specified probe depth and six centering guides to ensure perpendicularity Each shell is polished allowing direct visualization of fluid levels Other types of scleral shells are also available from differe
6. ck and right support on field Our customer care team is always available to clarify your queries on products and its operations or training as and when required We constantly take up feed back from customers to evaluate our product performance and service support Contact Cel 91 98840 79116 Tel 91 44 3253 3333 Fax 91 44 2635 0030 support akasmedical com Global Support Enhanced Product Life Service support Contact number is stuck on the rear side of the equipment FOR SUPPORT n cats scosso 7016 1D for immediate reference Support hehas medical com Genuine Spares at economical Prices from manufacturer s desk Service Directly from the Manufacturers desk Genuine Spares available on demand Factory trained Service cj CRM software for complaints engineers registration and follow up Ordering informati Please email for quote to enquiry akasmedical com with the following information First name Surname Title Company Position Department Address Telephone Fax E mail AKAS Medical 240 1 Periya colony Athipet Ambattur Chennai 600 058 www akasmedical com Cel 91 98403 79116 Tel 91 44 3253 3333 Fax 91 44 2635 0030 enquiry akasmedical com Ascan PDF Mar 09
7. d position with the contact technique and the probe is brought forward to touch the cornea The patient is conventionally examined in the supine position with the immersion technique and the solid probe is kept 5 mm to 10 mm away from the cornea These differences in the methods of examination mainly the corneal indentation and the subsequent shallowing of the anterior chamber are responsible for the shorter measurement obtained with the contact technique z JAAR RSL FHREIU 1 bagr 169411153 Figure 10 10A A ANL 25 10m LENS 4 Plan ACD 3 69 scan display of a phakic eye using immersion biometry YH ii Apto ca i bbdd CONTACTI PHAKIC 1 1532 1641 1532 0 Figure 10 10B A scan RL 24 98n0 LENS 4 219 ACD 2 49Hn display of the same phakic eye using contact biometry Note the shallower anterior chamber depth ACD and shorter axial length AXL with this technique Wide incon baila ALL cal E Rat i o DST YA Ba aa parley ALII Lu Clinical Application Figure 10 10 shows measurements obtained from the same eye with an immersion technique Figure 10 10A and a contact technique Figure 10 10B Note the shallower ACD 3 49 instead of 3 69 mm and shorter AL 24 98 instead of 25 18 mm obtained with the contact technique Biometry Methods Explained Routine A scan biometry is an indispensable tool for ophthalmology but has limitations in resolution and an inability to consis
8. ding represents the travel time it takes the ultrasound beam to reach the tissue under examination and return to the probe thus representing twice the actual measurement It is then divided by 2 and converted to millimeters using an average velocity Some biometers give the readings directly in millimeters using an average sound velocity This velocity is reported in meters per second m s Most biometers use an average velocity of 1550 to 1555 m s A velocity of 1553 m s is recommended Most modern biometers use separate sound velocities for the different eye components The biometer provides an ACD the lens thickness and the total AL INAEKSHI FHARIC bdo A REI 25 Hran PWG oh fend REU Han IS R S TATA al Figure 10 5 Ultrasound display of the different echospikes during immersion biomet ry identifying the initial spike IS the anterior C1 and posterior C2 corneal surfaces the anterior L1 and posterior L2 lens surfaces the retina R sclera S and orbital tissues O Figure 10 6 A scan display of a phakic eye where the initial spike has been removed from the screen display and identifying the corneal C1 amp C2 lens L1 amp L2 and retinal R spikes Contact A Scan Biometry TECHNIQUE The contact technique for AL measurement is an alternative to immersion A scan biometry It does not use a scleral shell Instead the probe comes in contact with the cornea to generate the
9. first echospike 1 Adrop of local anesthetic is instilled in both eyes The patient is examined in the seated position with the chin correctly positioned on a free standing chin rest 2 The ultrasound probe is attached to a zero weight balance glide to prevent any pressure on the eye during examination The probe s movements are controlled through a joystick handle Figure 10 7 The probe is positioned in front of the eye and the patient is asked to fixate the red light within the probe The probe is then brought forward to gently touch the cornea without indenting it 4 Attention is then focused on the screen The probe is moved slightly up and down or to the side to optimize the echospikes displayed on the oscilloscope A Polaroid picture or a print out is obtained Other probes are mounted on a Goldmann tonometer holder and are designed to be used with a slit lamp Figure 10 8 The examination is performed in the same manner as previously described Hand held compact biometric rulers such as the Bio Pen Mentor O amp O Norwell MA have lost some of their popularity and are now rarely used Probes used with the contact technique should be cleaned after each examination to prevent contamination CONTACTI 184 HAETC 1 J i 15 _ RAL 29 Glu LENS E den ADD 2 97ww Figure 10 9 A scan display of a phakic eye measured with contactA scan biometry Since the probe is in contact with the e
10. gently moved until it is properly aligned with the optical axis of the eye and an acceptable A scan echogram is displayed on the screen A SCAN PATTERN OF THE PHAKIC EYE The A scan pattern of a normal phakic eye examined with an immersion technique displays the following echospikes from left to right Figure 10 5 The cup is placed between the lids and The ultrasound probe is immersed The Kohn shell methylcellulose 1 is poured into the cup in the solution keeping it 5 to 10 mm away from the cornea IS The initial spike IS is produced at the tip of the probe It has no clinical significance Many units will allow the technician to move the whole A scan pattern to the left and remove the IS from the picture Figure 10 6 C The corneal spike C is double peaked representing the anterior C1 and posterior C2 surfaces of the cornea L1 The anterior lens spike L1 is generated from the anterior surface of the lens L2 The posterior lens spike L2 is generated from the posterior surface of the lens and is usually smaller than L1 R The retinal spike R is generated from the anterior surface of the retina It is straight high reflective and tall whenever the ultrasound beam is perpendicular to the retina as it should be during AL measurement S The scleral spike S is another high reflective spike generated from the scleral surface right behind the retinal spike and should not be confused with it
11. in availing AKAS products What are the symptoms of cataracts Early signs of cataracts include blurred or cloudy vision frequent changes in eyeglass or contact lens prescriptions night glare and hazy vision and colors that seem to fade An ophthalmologist must determine if these symptoms are really caused by a cataract or by some other eye problem that may need treatment For an adult a cataract should be removed only when it interferes with lifestyle and makes it difficult to continue normally enjoyable activities Generally there is no such thing as a cataract being ripe or not ripe for removal What matters is whether or not the problem interferes with vision In rare instances a hyper mature cataract may cause elevated eye pressure or inflammation of the eye In this case it must be removed immediately Otherwise removal of a cataract is at the patient s discretion What are the different types of cataracts 1 2 Age related cataract Most cataracts are related to aging Congenital cataract Some babies are born with cataracts or develop them in childhood often in both eyes These cataracts may not affect vision If they do they may need to be removed Secondary cataract Cataracts are more likely to develop in people who have certain other health problems such as diabetes Also cataracts are sometimes linked to steroid use Traumatic cataract Cataracts can develop soon after an eye injury or vears l
12. ique is an important first step in improving the overall accuracy of your A scans Measurement consistency from one measurement to the next is often outstanding due to the lack of corneal compression and the fixed position of the ultrasound probe over the surface of the cornea pag Mphreu A DIVISION OF CARL ZEISS INC AVERAGING MODE PATIENT d6 07 b0 OD RECORD ACD LT AL l 3 06 4 32 23 31 M 2 3 06 4 92 23 351 M 3 3 606 4 92 25 31 M 4 3 06 4 92 27 31 M 3 3 06 4 92 25 51 N OD BIASED AL 23 31 AKAS Bioscale Diagnosis Measurement Modes Contact Immersion Six Formulae SRK I s SRK I Hoffer Q BINK HORST HOLA DAY HAGIS Latest FPGA Technology for fastest readings Aesthetics Slim and Portable Light weight model User friendly Color Touch Screen Display Built in Thermal Printer Packing content Immersion cups Calibration cylinder Probe A scan Stylus A scan Thermal Paper 5 rolls A scan Power Chord 1no User manual 1 no e Product code OPH OPD ATABIO hi Specification MEASUREMENT amp IOL CALCULATION Patient 000000 ID 000000 Doctor 000000 Date 000000 OD RIGHT K1 42 00D LENS CATARACT K2 42 50D GAIN 70 MODE CONTINOUS LT AC AL 4 07 3 61 21 69 4 01 3 61 21 69 4 01 3 61 21 69 4 01 3 61 21 69 3 96 3 61 21 69 3 96 3 61 21 69 2096 3 61 21 69 4 01 3 65 21 69 ISO 3 61
13. nt manufacturers including the Kohn shell Innovative Imaging Inc Sacramento CA Figure 10 2 The Prager shell and the Kohn shell have made immersion biometry easier The use of the Kohn shell is discussed in Chapter 11 In this section we will describe the routine method used with the Hansen shells 4 The Hansen shell is filled with gonioscopic solution Methylcellulose 1 is preferred over the 2 5 concentration too thick and over saline solutions too liquid The solution should be free of air bubbles the presence of bubbles causes variations in the speed of sound and is responsible for noise formation within the ultrasound pattern The easiest way to avoid bubbles is to remove the bottle s nipple and to pour the solution in the cup Figure 10 3 If bubbles do form within the solution they are removed with a syringe and if unsuccessful the cup has to be emptied cleaned repositioned and refilled with gonioscopic solution The Kohn shell is designed to hold the probe tightly and allow a better fit on the eye Because of this tight fit the coupling fluid used in this shell does not have to be methylcellulose instead balanced salt solution or artificial tears could be used 5 The ultrasound probe is immersed in the solution keeping it 5 to 10 mm away from the cornea Figure 10 4 The patient is asked to look with the fellow eye at a fixation point placed at the ceiling Attention is then focused on the screen The probe is
14. ove represents our experience with several popular posterior chamber intraocular lenses with axial length measured using the IOLMaster with results displayed in terms of the best possible mean absolute prediction error These data represent outcomes for patients with keratometry below 50 0 D and above 40 0 D and excludes cases of keratoconus or prior keratorefractive surgery For the reasons outlined above your own individual results may vary depending on the geometry of the intraocular lens used and overall accuracy of pre operative measurements such as keratometry ACD and axial length Applanation A scan Biometry pres A scan biometry by applanation requires that the ultrasound probe be A DIVISION OF CARL ZEISS INC placed directly on the corneal surface This can either be done at the slit BIDHETRY MODE lamp or by holding the ultrasound probe by hand 91 42 FH 11 24 99 PATIENT Even in the most experienced hands some compression of the cornea is unavoidable this typically being 0 14 mm 0 28 mm As tit ALO Tae BAIN The popularity of the applanation method is due to how quickly it appears PHAKIC o RECORD Qi to be accomplished 3 Figure A Phakic axial length measurement using the applanation method a Initial spike probe tip and cornea b Anterior lens capsule c Posterior lens capsule d Retina e Sclera f Orbital fat ii Figure B Note the typical applanation measurement variations Measurement
15. s taken by applanation method will frequently show A DIVISION OF CARL ZEISS INC variability from one to the next as a result of inconsistent corneal AVERAGING MODE compression and will be seen even under the most experienced PATIENT guidance 11 24 99 The way to avoid this is to change to the immersion technique as do described below SERRO PEG a sella For further reading we highly recommend the book A scan Axial 2 3 69 4 16 23 89 A Length Measurements by Sandra Frazier Byrne 3 3 60 4 69 22 97 A Also there is an excellent national certification program in s gia il as E h Ophthalmic Biometry available for your technicians OD BIASED AL 23 11 Immersion A scan Biometry y Humphrey Preferred over applanation With the immersion A scan a pp a a A DIVISION OF CARL ZEISS INC technique the probe tip does not come into contact with the cornea BIOMETRY MODE 09 56 AM 06 87 00 PATIENT Vi A 1532 L 1641 P 1332 Figure C Break down of phakic axial length LT 4 92 70 measurements using the immersion technique ACD 3 06 AL 23 31 GAIN CUETON VELOCITY RECORD 61 a Probe tip Echo from tip of probe now moved HUAL away from the cornea and has become visible b Cornea Double peaked echo will show both the anterior and posterior surfaces c Anterior lens capsule Posterior lens capsule e Retina This echo needs to have sharp 90 degree Instead the ultrasound beam is coupled to the eye take off from
16. state of the art IOL power calculation to achieve emmetropia with a posterior chamber IOL was simply adding 19 0 D to the pre cataractous refraction Around 1990 with the formulas available at that time being within 1 00 D of the target refraction was considered perfect Today by paying attention to IOL power calculations and advanced surgical techniques it is possible to be within 0 50 D in 95 percent of surgeries or better and fast becoming the standard 4 IOL Constants Here is a Microsoft Excel spreadsheet that can be used to calculate an average IOL constant based on data from 100 patients for the four popular IOL power formulas Holladay 1 Hoffer Q SRK II and SRK T Constants 4 IOL Calculation Formulas One of the final frontiers in ophthalmology is the consistently accurate calculation of intraocular lens power for all eyes When properly personalized any of the modern IOL power calculation formulas will do a good job for normal axial lengths and normal central corneal powers However for very long or short eyes or for eyes with very flat or very steep corneal powers consistently accurate IOL power calculation has remained elusive Dr Wolfgang Haigis N Haigis Holladay Holladay SRK T Haigis Mala Hoffer Q A gt A SIL nr d a2 x non d SF ACD constant P P optimized optimized optimized optimized 18 00 19 99 20 00 21 99 22 00 25 99 26 00 27 99 28 00 30 00 Minus power IOLs The chart ab
17. tently direct the sound beam to a known location And although we have accepted ultrasound based biometry as our main methodology for the measurement of axial length it s important to keep in mind exactly what we are doing The resolution of a wavelength based measurement is inversely proportional to the wavelength of the measuring device being used The longer the wavelength the lower worse the resolution The shorter the wavelength the higher better the resolution This is why an electron microscope has much higher resolution than a light microscope This is also why we use 50 MHz ultrasound to more precisely image somewhat smaller anterior segment structures rather than 10 MHz ultrasound Things work best when the measuring wavelength is many times shorter than the distances being measured or the resolution desired Fundamental Principle 3 Variability for an on axis A scan measurement is an artifact of position Here is an OCT 3 macular thickness plot from a normal eye that illustrates this So for the exercise of A scan biometry we have to take into account the inherent resolution limitations of a 10 MHz sound wave and its inability to discriminate between the foveal center and the foveal shoulder This may not sound like a large error but it s helpful to keep in mind that these types of errors are cumulative Thickness Chart Microns 6007 500 4007 300 2004 100 g 100 200 300 400 200 Limited resolution As mentioned above
18. the baseline through fluid Because there is no corneal compression the displayed result more closely f Sclera represents the true axial length g Orbital fat Note Be sure to set your ultrasound machine to immersion mode if it doesn t automatically do so or you will get meaningless readings that are several mm too long The immersion technique requires the use of a Prager Scleral Shell pictured at the top of this page or a set of Ossoinig or Hansen Scleral Shells The patient lies supine looking up at the ceiling and the scleral shell is placed between the eyelids and centered over the cornea The scleral shell is then filled with a 40 60 mixture of Goniosol and Dacriose and the probe tip is placed into the solution Align the ultrasound beam with the macula by having the patient look at the probe tip fixation light then simply take your readings as usual In our office we have found that the Prager Scleral Shell is easiest to use and gives very consistent readings The Prager Scleral Shell can be obtained directly from A set of Ossoinig Scleral Shells which are lighter in weight easing probe manipulation can be obtained from Hansen Ophthalmic Development Laboratories at 319 338 1285 When the ultrasound beam is properly aligned with the center of the macula all five spikes cornea anterior and posterior lens capsule retina and sclera will be steeply rising and of maximum height Changing to this techn
19. ye the initial spike and the anterior corneal spike become one I se et a a SD E PP ee ee daj A SCAN PATTERN The A scan pattern of a phakic eye examined with the contact technique Figure 10 9 demonstrates similar echospikes except that the corneal spike is merged with the initial spike since the cornea is in touch with the tip of the probe Variations in Axial Length Measurement Variations in AL measurement are due to the use of different examination techniques and or to the use of different sound velocities by the biometer VARIATIONS DUE TO THE EXAMINATION TECHNIQUE The contact method for AL measurement does not yield the same results as the high precision A scan biometry When measuring the same eye the contact technique yields a shorter measurement than the immersion technique In a prospective study on 180 eyes performed by the author AL measurements were obtained on each eye with both contact and immersion techniques Each eye was measured with the Ocuscan DBR contact Alcon Irvine CA the Ocuscan 400 immersion Alcon and the Kretz 7200 MA immersion units AL measurements obtained with the contact technique were shorter than measurements obtained with the immersion technique by an average of 0 24 mm The two methods of examination differ in the patient s position and the possible corneal applanation by the ultrasound probe The patient is conventionally examined in the seate

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