DR RAGHU NAGARAJU , keratometry. Dr Agarwal's eye hospital, Banglore

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Presentation Transcript

Keratometry and Placido disc : 

Keratometry and Placido disc Dr Raghu

Slide 2: 

Keratometry is the optical technique used to measure the anterior curvature of central cornea i.e.central 3-4 mm of cornea . The keratometry is expressed in Dioptric power or diaptric curvature. Keratometer also called ophthalmometer

Historical background : 

Historical background In 1691 description of corneal curvature was done by Christoph Scheiner . First model of keratometer was invented by Jesse Ramsden Herman Von Helmholtz invented the revised model of keratometer The first practical keratometer that was suited for clinical use was developed by Javal and Schiotz

Slide 4: 

Principle of keratometry; Optical principle involved is the relationship between the size of an object and size of the image of that object reflected from surface. Radius of curvature is determined by the apperant size of the image of bright object (mires) viewed by the reflection from anterior corneal surface which acts as a convex mirror r= 2 X h1/h r= radius of curvature, h=height of object, h1=height of the image D=( n1-n) /r X 1000 n1= refractive index of cornea (1.337),n=refractive index of medium from which light originates (air=1)

Slide 5: 

Keratometer works based on 2 concepts Fixed object size with variable image size (variable doubling) ex; B&L Fixed image size with variable object size (fixed doubling) ex; J&S

Slide 6: 

Doubling principle; Because of involuntary eye movement image formed on cornea would be constantly moving. To overcome this Ramsden devoloped Doubling technique. A prism is introduced into the optical system so that 2 images are formed .The prism is moved until the images touch each other.

Slide 7: 

Basically there two types keratometer Manual keratometer Auto keratometer

Bausch and Lomb keratometer : 

Bausch and Lomb keratometer Works on the principle of constant object size and variable image size. The size of image on cornea depends on radius of curvature

Slide 12: 

Typical keratometer mire pattern

Slide 13: 

Mire pattern with proper focus when first K reading is determined

Slide 14: 

Mire pattern with proper focus when second K reading is determined

Javal and schiotz keratometer : 

Javal and schiotz keratometer Works on the principle of variable object size and constant image size.

Slide 17: 

Object consist of 2 mires ,one stepped (green) and other square mire (red) Radius of curvature of the cornea is measured in one meridian and then the entire optical system is rotated 900 about its central axis for the measurement of radius of curvature in second meridian.

Slide 18: 

If the black line that bisect the test images are not continuous axis adjustment must be made by rotating the arc until the black line is continuous. Then the meridian is the principal meridian nearest horizontal. Arc rotated 900 and if the inner test target overlaps then astigmatism with the rule is present and inner targets are apart there is against the rule astigmatism is present .then again the knobs are adjusted till the inner target just touches. Then the meridian is the principal meridian nearest vertical

Clinical procedure : 

Clinical procedure The room lighting should be adjusted to avoid stray reflections on the cornea. The keratometer is cleaned Focus the eye piece; Plane white paper is placed in the plane of head rest Looking through the eye piece fix a distance target with other eye until the cross hairs appear in sharp focus.

Slide 20: 

Patient is seated confortably Adjustment of the instrument height Adjust chin rest Eye is focused in center of condenser A central fixation target within the instrument is provided and must be viewed by the patient. The instrument position is accomplished by the outer canthus of the patient’s eye is aligned with the marker on the condenser Cross hairs are focused in the center of lower right circle of the corneal image Then the instrument is locked

Slide 21: 

Proper focussing Locate principal meridian First the right lower circle is focused until it appears single and clear. The two crosses are aligned till they are continous. The crosses and minus are aligned and then the K reading measurment is taken . Contact lenses should be removed at least 48 hours before keratometry because their longterm use can induce a reversible corneal flattening (~0·05 mm).

Automatic keratometers : 

Automatic keratometers Automatic keratometers have the advantage of virtually eliminating operator subjectivity. The mires of automated keratometers are generally light emitting diodes and the corneal image positions of the mires are detected using solid state detectors. The fast response of such detectors overcomes problems associated with eye movement, thus there is no need for doubling devices.

Hand-held keratometers : 

Hand-held keratometers Portable hand-held keratometers can be used with patients in seated, standing, or supine positions. Ideal for use on infants, individuals with restricted physical mobility, or those supine under general anaesthesia. Care must be taken to hold the instrument parallel to the plane of VISUAL AXIS, To check that the eye is fixating correctly and that the eyelids do not obscure the cornea.

Uses of keratometer : 

Uses of keratometer Estimating the patients corneal astigmatism Monitoring the shape of cornea in keratoconus Keratometer helps in estimating the nature of refractive error Calculating the intraocular lens power Fitting contact lenses Monitoring the corneal curvature after surgery.

Slide 25: 

Accuracy of the measurement is determined by Sharp focusing of reflecting image Precise determination of size

Commercially available keratometers : 

Commercially available keratometers Reichert keratometer (B&L) American optical CLC ophthalmometer Humphrey Autokeratometer Topcon OM-4 Ophthalmometer

Slide 27: 

Measuring range of Corneal radius of curvature6.5 to 9.4mm(minimum reading 0.05mm) Measuring range of Corneal refractive power35.872 to 52D(0.125D steps) Measuring range of Axis of corneal astigmatism0 to 180degree(5degree steps) Light source100v, 120v and 230vAC, 15w

Slide 28: 

Instrument generally have two scales, one giving the corneal radius of curvature in millimeters and the other giving corneal power in dioptres (D). Difficult and complex keratometry Poor fixation; Poor fixation by the patient is the major source of keratometry error. The examiner should ensure that the patient is fixating on the target light by observing the patient’s eye and the reflections of ocular structures viewed both directly and through the keratometer eyepiece. Poor tear film; If the tear film constantly breaks up then it may be necessary to insert a drop of tear substitute measurement.

Slide 29: 

3) Nystagmus; The keratometer should be roughly aligned and then the patient should be asked to close their eyes for 10 seconds. The nystagmus is generally reduced on initial opening of the eyes, which allows fine adjustment of the mire separation. 4) Post refractive surgery; keratometry should be combined with topography.

Slide 30: 

Expand the range of measurment It is used in very low or very high corneal curvatures +1.25D trial lens over eye piece will increase the range by 9 D. -1.0D lens will decrease the reading by 6D

Limitations of keratometry : 

Limitations of keratometry Only central 3mm area is measured Not effective is post refractive eyes Different keratometers gives different readings Poor fixation, poor tear film, nystagmus give fallacious reading Diffcult to perform in childrens

Historical background : 

Historical background Inventor of photokeratoscope was by Placido Clinical implication of keratoscopy was done by Javal

Slide 34: 

Placido disc is also called photokeratoscope They can be self illuminated or externally illuminated. Consists of flat round plate upon which are black and white concentric rings on one side with handle attached. The images of concentric rings are viewed after specular reflection from the cornea through plus lens which increase magnification. Keratoscopy is the method of examination of the anterior curvature of cornea

Principle of placido disc : 

Principle of placido disc It is based on the principle that the size of a reflected image is directly proportional to the radius of curvature of the reflecting surface. Farther the given point on one of the imaged rings is from the center of image pattern greater the radius of curvature at that point on cornea and Closer the rings to each other steeper the cornea.

Slide 36: 

Uses of placido disc Gross abnormality in anterior surface of cornea can be detected. Used in contact lens fitting which aids in contact lens prescription for optimal outcome. Used in monitoring contact lens fitting in keratoconus. In orthokeratology; Good results are expected if keratoscopy shows greater peripheral corneal flattening

Slide 37: 

Various commercially available keratoscope Placido disc klein keratoscope Kera corneoscope Nidek photokeratoscope

Slide 38: 

Nidek photokeratoscope It has a head and chin rest The target is 11 Transilluminated concentric rings

Slide 39: 

Clinical procedure Patient comfortably seated (no glasses) Placido disc is illuminated by lamp or other light source Disc Held at 20 cm from patients eye and perpendicular to the visual axis Clinician views through the hole in center of disc (plus lens if magnification is required )and observes the reflected rings to determine any changes in the rings from circular form

Slide 42: 

Thank You

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