Keratometry PDF

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Keratometry: What is it mainly used for?:  measure the curvature of any reflecting surface but is mainly used to measure the radius of curvature of the anterior corneal surface  The measurement had been used to acquire an objective measurement of parts of the refractive state of the eye  The anterior corneal surface is responsible for about 70% of the total optical power of the eye because:  anterior corneal surface is the only optical component of the eye which is surrounded by air and has a steep radius of curvature (7.7mm)  The refractive status of the eye and overall astigmatism depends strongly on the anterior corneal surface  The main applications for keratometers include:  Contact lens fitting : helps with the selection of the first lens before finding the ideal one. Used to assess changes in the shape of the cornea as a result of contact lens wear  Detection of corneal distortions: eg keratoconus, keratoglobus, contact induced corneal distortions  Healing: following corneal surgery  Calculating the power of the IOL: when combined with refraction and ultrasonic axial length measurements , the power of an intraocular lens implant can be determined, which will replace the eyes own lens  Determining astigmatism (and/or spectacle Rx): more for cataract surgery and contact lens fitting than for standard refraction  Checking and measuring contact lenses. Keratometer design Principle:  The cornea acts a convex mirror as it reflects about 3-4% of the incident light  The assumption that the anterior surface of the cornea is of spherical shape, its curvature can be determined on the basis of the image it forms of an object  Convex mirror: the cornea creates a virtual, erect image of any object in front of it

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If size of the object (h) and its distance from the cornea (l) are known and the size of the image is measured (h') then the radius of curvature of the reflecting surface (r) can be calculated All keratometers consist of two small mires (test targets) and it is the distance between their images (h') that is measured Due to the optical properties of the cornea, the image is rather small and a compound microscope is required to observe it First image of the mires is created by the cornea are imaged by the microscope objective in the focal plane of the eyepiece which in turn projects it into infinity The second image of the mires by the objective can be compared with a scale and is measured

Doubling system: 

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One problem is that the eyes move constantly (physiological nystagmus) without intention this makes it impossible to accurately compare the moving images of the mires with a fixed scale at the eyepieces focal plane The solution is a doubling system. - such a system creates two images of each of the two mires and the separation between them can be adjusted

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Whenever the two central images are coincident their separation equals the size of the image created by the cornea ( distance between the mire images) Once this has been achieved any small eye movement will not disturb the images as they all move together by the same amount

There are several ways adjustable doubling can be achieved and keratometers can be divided into two groups, depending upon the technique they use:  Variable mire position with fixed doubling  Variable doubling with fixed mire position Wollaston Prism:  Easiest and cheapest way to achieve doubling is using a doubling prism

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Prism has the shape of a cube and splits an incidental pencil of rays into two pencils These two pencils are of opposite polarisation and have a constant angle α of 1˚ between them This effect of the prism is fixed and the variability of the doubling is achieved via altering the position of the mires which are mounted on an arc This method utilises constant doubling and requires a change of the position of the mires hence a change in image size or distance between mires h

m Parallel Glass Plates:  Based on the fact that a parallel glass plate causes a parallel shift of incident rays  The technique employs two plates that each cover half of the aperture and can be rotated with respect to each other  The separation between the images is given by the amount of rotation of the plates

Biprism: employs a biprism which could be in front or behind the objective  The two prisms covering each half of the aperture are of the same magnitude but of  opposite sign and therefore achieves doubling  The mire image can be adjusted by moving the prism along the horizontal axis

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Several problems arise that are all connected with the accuracy of corneal curvature measurements arising in various forms of distance dependencies Mire positions (first distance dependence)  The radius measurement depends not only on object and image size but also on the distance (l) between the instrument and the patients cornea.- referred to as distance dependence  Using the instrument at two different distances results in different measurements of (h') - different estimates of the radius of curvature for the same surface  This problem can be eliminated if the mires are placed a long distance away from the patient  Littmann introduced a method with two collimator lenses  With this method the mires are positioned behind the collimators so that they are optically imaged at infinity. - this makes the measurement independent of the distance between the mires and the patient so is termed distance independent

 The mire distance independence is an important criterion for the accuracy of keratometers but is currently only employed in one instrument 2. Instrument position (second distance dependence):  Problem is a consequence of an incorrect position of the whole keratometer either too close to the patients cornea or too far away. ( whole keratometer: mires, objective, doubling system and eyepiece)  This is independent from the first dependence as those instruments that are mire position corrected suffer from it  The instrument position is coupled to the position of its mires in the instruments which are not corrected for mire position  Originates in the doubling system and not the position of the mires  Incorrect position causes the amount of doubling to change and coincident mire images to be seen as not aligned - originates in the nature of doubling prism

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The magnitude of the error can be reduced if the mires are at infinity The error can be eliminated by one of two optical methods : replacing the prism as the doubling system with two parallel plates OR positioning the prism at the focal point of the objective (telecentric arrangement)  For example:  With the aid of two parallel glass plates on top of each other the mire images, once aligned stay aligned regardless of the instruments distance from the patients cornea  Both their size and their final position changes but this does not affect coincidence  The error is eliminated in two instruments : Rodenstock with parallel plates and Zeiss with a telecentric system)  For any other instrument: only way to minimise the error is to:  correctly focus the eyepiece which guarantees that the second image (h'') are imaged at the correct position which guarantees a correct distance of the instrument from the cornea 3. Eyepiece Focussing Errors (third distance dependence)  If the mires are at infinity or the instrument is positioned at the correct distance from the cornea the source of error is : the measurement depends on the setting of the eyepiece  The eyepiece creates a magnified image of the intermediate image (h'')  When the eyepiece is not set for the correct distance a physically correct separation setting (s=h'') will be seen as an incorrect alignment and therefore result in inaccurate measurement)  The eyepiece would not be set for the correct distance due to:  inaccurate setting of the eyepieces focus  accommodation of the observer  If the eyepiece is focused anywhere else other than the correct position the observer will see two images that are not coincidence even if the images would be at the correct focus  The observer will re-adjust the separation of the images and read of the wrong radius of curvature - this is a problem because of the large depth of focus of instrument and observer  The error can be minimised or avoided via accurately focusing the eyepiece  To facilitate this is a graticule/crosshair is invariably placed within the instrument upon which the eyepiece has to be focused prior to making a measurement  The focus is done the same way as for a focimeter  It is important that accommodation is avoided but is not always straightforward due to instrument myopia  The fancy optical method to avoid eyepiece focussing errors is based on the same principle as the scheiner disc:  Only at the position of the image (given by the power of the optical system) sees an observer one single image  Everywhere else ( if the observer was myopic or hyperopic) two images of the one object would be observed  The scheiner disc principle has been applied to the keratometer by splitting up the aperture into three parallel glass plates.  The upper and lower plates are identical and together with the central plate constitute the doubling system

 The upper and lower plates act as two Scheiner discs and guarantee that the observer only sees two images (instead of three) when focussed in the correct plane  Base down for the upper and lower and base up for the central plate : each parallel plate requires a small prismatic effect  Symmetrical Aperture Splitting: is a method that can avoid the eyepiece focussing error (with or without accommodation)  The following factors make some instruments more accurate than others:  the distance dependence of the mire positions  the distance between the instrument and cornea  the eyepiece focussing (in combination with accommodation)  A general strategy should therefore involve correct eyepiece focussing to minimise errors

Corneal Curvature Measurements: Astigmatism:  

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Theory behind keratometry is reflection on a spherical surface - not always valid especially in the case of gross deformations of the cornea as in keratoconus The corneal surface is not spherical so what does one measure?  The part of the cornea that have an impact on the measurements with keratometers is the parts which reflect the light coming from mires  Because of limited aperture stops ( there are two effective stops because of the doubling) within the instrument, only small parts of the cornea contribute to the measurement :  These parts fall within an annulus on the cornea - because of this the central part of the cornea (along the optical axis) does not contribute to the measurement.  The diameter of this annulus is typically around 3mm The radius measured is that along any one meridian By rotating the instrument any other meridian can be measured and this is done in the case of astigmatism A toric surface has two extreme radii of curvature at right angles to each other, two measurements are carried out in meridians that are approximately orthogonal ( right angle) to each other - used to decide whether a cornea is spheric or toric To discriminate between toric/spheric is to rotate the instrument and watch the mire images :  If the cornea is spheric, neither the relative position of the images to each other nor their general appearance will change  With toric surfaces a change of relative position (because of different curvatures) as well as misalignment (and to a lesser extend distortion) of the mire images (because of the different amounts of magnification in the two principle meridians) is observed  A toric cornea is measured with the two main meridians along 15˚ and 105˚ If the instrument is initially set with the mires positioned horizontally, the initial image will show misaligned mire images Rotating the keratometer aligns the images The axis of the keratometer and one of the two principal meridians of the toric cornea are parallel

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The doubling is modified until coincidence is seen The keratometer is rotated by 90˚to find the second meridian A second alignment is found in a meridian which is not exactly perpendicular to the first Gross deviations indicate corneal distortions Once the second meridian is found the mires have to be re-adjusted

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Two position instruments have another set of mire images on either side of those shown - due to the doubling but are not relevant for measurements Two position instruments: keratometers which require the instrument to be rotated for the measurement of the second meridian One position: instruments have to be rotated to find the first meridian but not to measure the second - second rotation also required with these instruments whenever the principle meridians of the cornea are not exactly orthogonal to each other

Specific Types of Keratometers: Bausch & Lomb type:  One position instrument with variable doubling and fixed mires  Employs two apertures ( A and B) which act as Scheiner discs, so that the central image of the mire is seen double when the keratometer is not at the correct distance from the cornea  The central image is therefore used to correctly position the instrument - the eyepieces have to be focused first  The other two apertures (C and D) are part of the doubling which is achieved by two prisms one horizontal and one vertical  Employing two prisms in this fashion gives three mires which fall on perpendicular planes and can therefore be used to measure both principle meridians without rotating the instrument  Both prisms can be adjusted independently  This instrument suffers from first and second dependencies - due to the prisms but the eye-piece focussing error ( and therefore to some extent the mire position) is minimised by the use of Scheiner discs 

Zeiss:      

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Completely distance independent It is optically more complex and more expensive than any other keratometer Only instrument to achieve complete independence Two position instrument with variable doubling and fixed mires The mires are optically projected into infinity which eliminates first distance dependence The two prism systems serve the purpose of dividing the beam into equal energy pencils which acts similar to the Scheiner discs in that it avoids any mis-positioning and eyepiece focussing error of the instrument The prisms are not used for doubling Two negative lenses that can be shifted laterally and so create different amounts of prismatic effects take the part of the actual doubling system

Javal-Schiotz-type:  Two position fixed doubling instrument with variable mire positions  The mires mounted on an arc, can be rotated around the cornea and hence the distance between them (object size h) altered.  It is mechanically, as well as optically comparatively simple and inexpensive  Has none of the refinements required for distance independence ( neither first, second nor third)  It suffers a lot from focussing and positioning errors  Correctly focussing the eyepiece is of utmost importance General Set up:

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Focus the eyepiece a) Screw the eyepiece out to its maximum b) slowly push it in until the crosswire just becomes clear - avoids accommodation Align the patient and the instrument a) Ensure patients head is at correct height in headrest - align with canther mark b) Shine a pen torch towards the patient through the eyepiece. - if correctly aligned, this should be reflected from the centre of the cornea. / if not readjust the height/position of the instrument and/or patient c) Some instruments have a spike projecting from the side which can be used to line up the instrument d) use the instruments microscope and focus on the (very dimly illuminated) lid margins. From there move up/down and right/left until the mire images can be seen e) always cover the eye which is not being measured Focus the instrument on the mires a) Should just require moving the instrument towards the cornea. b) Make sure that both the cross wires of the eyepiece and the mire images are in focus at the same time c) For instruments employing the Scheiner disc principle make sure that the central mire image is seen single Take a measurement:  The majority of instruments give two values: a curvature in mm (usually between 78) and a corneal power in dioptres (usually between 42D and 44D)  The only sensible unit is distance as the keratometers measure the radius of curvature of a reflecting surface  The dioptical power of the corneal front surface cannot be determined without knowing its refractive index - values for refractive index have to be assumed a) The image of the mires should be complete and sharp. If they are not complete:  re-align the instrument  check that the lashes/lids are not in the way; relevant when measuring vertical meridian b) Rotate the instrument and observe the mires. If they change their relative position to each other, the cornea in front of you is toric c) Find one of the two principle meridians (the mires should be aligned) d) Adjust the doubling until mires are coincident e) Read and record the curvature and the axis eg 7.8 @ 170 f) Rotate the instrument by 90˚. Fine adjust this second axis until the mires are aligned again- not required to do for one position instruments g) Adjust the doubling until mires are in coincidence for the second meridian h) Read and record the curvature and the second axis

Corneal Shape Measurement (Keratoscopy) 

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Keratometry is usually restricted to a central annular region on the cornea- cannot provide a full description of the corneas shape, such as the amount of flattening towards the periphery or the precise location of the corneal apex( point of shortest radius) Keratometry can be used to measure peripheral radii of curvature - requires patient to fixate at peripheral targets not at the central fixation target within the keratometer Main application of this technique includes:

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Contact lens fitting ( improve the alignment between CL and cornea) Diagnosis and treatment of corneal dystrophies ( keratoconus, keratoglobus) Aid treatment following corneal surgery (keratoplasty) Monitor effects of radial keratotomy (radial incisions used to alter corneal shape) and photorefractive keratectomy (laser modification of corneal shape)

Placido Disc Based Techniques: Common form of keratoscopy Consists of series of concentric black and white rings which may be internally illuminated The image of the rings reflected from the cornea is observed through a viewing hole at the centre  The bright bands are often narrower than the dark ones, making the reflected pattern easier to see  The device is very useful for looking qualitatively at corneal topography  useful for irregular corneas

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Placido rings are placed on the inside of a hemisphere (dome) The design of the dome ensures that the image formed by corneal reflection approximates to a flat surface The underlying assumption is that of an average corneal shape The image is then recorded photographically or on video and analysed in detail with a microprocessor The analysis is complicated and involves comparison with images from surfaces of known contour Examples are the CMS( Corneal Modelling System) - uses 32 rings. TMS ( Topographic Modelling System - 25 to 30 rings) EyeSye - 8 rings Although they measure up to several thousand points, there are limitations with their accuracy, mainly due to errors from paraxial approximations

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Flourescein in combination with contact lens  A rigid contact lens of known curvature and shape is used in combination with a dye  The amount of tears between CL and cornea can be evaluated and the shape of the cornea inferred  This is a qualita...