Clinical Instrumentation in Contact Lens Practice

Keratometry

The keratometer has been partially superseded by the corneal topographer, but it is still useful in providing a measurement of the radius of curvature and continues to be used to select the first contact lens. Keratometers are also available combined with an autorefractor or as a hand-held version that is useful for measuring central corneal curvature in small children who require contact lenses (see Chapter 24 ).

The keratometer is a poor guide to overall corneal shape as it assesses only the central 3.0–3.5 mm of the cornea. At a fixed viewing distance, an object of known size will be imaged and the image size will depend on the radius of curvature of the reflecting surface ( Fig. 8.1 ). The working distance of the keratometer is usually monitored through a Scheiner disc or similar system. This produces a doubled image of the object in the eyepiece unless the instrument is used at the exact working distance required by the instrument design ( Fig. 8.2 ).

The eye is constantly moving, even during apparently steady fixation. It is therefore difficult to measure directly the size of an image reflected by the cornea. However, if the image is doubled by passing it through a prism or a doubly-refractive crystal, then when the base of one resultant image is aligned with the top of the other, the displacement will equal the exact height of the object ( Fig. 8.3 ). This principle can be used in the form of either:

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  fixed doubling, where a predetermined amount of doubling is incorporated and the mire moved until the image produced is of the predetermined height (e.g. Javal-Schiotz- or Zeiss-type mires)

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  variable doubling, where the object is set to a predetermined size while the doubling system is varied until the image is displaced through its exact height (e.g. Bausch & Lomb type mires, Fig. 8.2 ).

The mires reflected from the corneal surface vary in appearance among manufacturers ( Figs 8.2, 8.4 & 8.5 ).

As the distance of the eye and change in image size resulting from its reflection from the cornea is now known, the radius of curvature can be calculated. It is read on an internal or external scale in millimetres or in dioptres – the latter making the assumption that the refractive index of the cornea is on average 1.3375, including a compensation for the back surface of the cornea having a power of –10% of the power of the front surface (see Chapter 7 ).

The instrument must be focused before use. As the cornea usually has two principal meridians at 90° to each other, the instrument is first rotated until the horizontal limbs of the mires are coincident. If the keratometer is a one-position instrument (e.g. Bausch & Lomb–type mires), the image is doubled in two directions at right angles to each other, which provides a means of measuring the two meridians simultaneously. The instrument is then adjusted until the two parts of the mire are superimposed. The reflected mires show any corneal distortion, lens flexure and tear film stability. Two-position instruments (e.g. those incorporating Zeiss or Javal-Schiotz mires) only assess the corneal curvature in one meridian and therefore need to be rotated by 90° to measure the second principal meridian. The two meridians may not be at 90° to each other if the astigmatism is irregular. Javal-Schiotz mires are usually of two different colours so that any overlapping of the mires produces a change in mire colour, aiding precise alignment. With the mires aligned in the steeper meridian, rotation of the instrument head through 90° will result in one mire-step overlap for each dioptre of astigmatism ( Fig. 8.5 ).

Errors in the use of a keratometer involve:

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  features of the instrument design

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    inaccuracies in paraxial ray theory

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    the assumption that the peripheral areas from which the mires are reflected have the same curvature as the corneal pole

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  operator-induced errors

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    inaccurate alignment

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    focusing errors

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    proximal accommodation

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    orientation of the instrument

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  patient induced

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    poor fixation

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    corneal distortion

Topographers using a Placido disc

A corneal topographer (or photo/video keratoscope) is an automated version of the Placido disc. It uses a bowl or cone to act as the illumination source to reflect off the tear film coated cornea. Instruments using this technology include the Oculus Keratograph (bowl technology) and the Medmont E300 (cone) ( Fig. 8.6a and b ). A camera attached to an internal or external computer images the rings as they are reflected off the central to mid-peripheral 10 mm of the tear film coated cornea. Image capture can be manually triggered when the image is centred and in focus (often highlighted by indicator scales on the screen) or activated automatically, to plot the contours of the corneal surface.

Image processing detects the separation of the rings in multiple meridians, which can be interpreted as curvature at that position of the anterior corneal surface. The data are displayed in the form of contour maps and simulated keratometry readings in the principal axes (see Fig. 8.9 ). The latter is generated from the innermost rings, the diameters of which most nearly equate to a conventional keratometer. The average asphericity of the cornea can also be calculated. Measures of asphericity include:

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  eccentricity = e

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  shape factor = p

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  asphericity parameter = Q

where e ² = 1 – p = –Q

The average asphericity of the human eye (Q) is about –0.2 to –0.3 (e = 0.45–0.55) and varies with meridian. It is similar across ethnicity ( , ).

Other Types of Corneal Topographers

Other systems are not simply based on a Placido disc ( ); they use different approaches to produce corneal contour maps:

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  Orbscan uses slit scanning.

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  Pentacam (Oculus) uses Scheimpflug imaging ( Fig. 8.7 ).

  

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  Sirius (CSO Italia) combines a Scheimpflug camera with Placido technology.

ORBSCAN II (BAUSCH & LOMB) scans a series of slit sections of the cornea in addition to the traditional Placido disc rings to produce a topographical map. The camera plane is at 45° to the light slit to improve the depth of field of the optic section (like swinging the illumination system on a slit-lamp to better view the corneal layers).

PENTACAM (OCULUS) (see Fig. 8.7 ) uses a Scheimpflug camera set in a rotating wheel to directly measure the corneal topography and analyse the cornea while a second static camera within the fixation target (a monochromatic slit) monitors fixation ( ). In addition, the iris camera lens measures the horizontal visible iris diameter.

CASSINI TOTAL CORNEAL ASTIGMATISM (CASSINI) uses multicoloured (red, green and yellow) light emitting diodes (LED) spaced throughout the bowl and uses ray tracing to measure the relative position of each point in order to produce a map. The manufacturers suggest that this has fewer errors than from ring overlap in irregular corneas or tear film distortion. The machine also utilises the second Purkinje reflections of each LED to quantify posterior corneal curvature.

These topographers are able to give corneal curvature data for the anterior and posterior cornea, and the Orbscan II and the Pentacam can provide further information about the anterior segment ( Fig. 8.8 ).

THE EYE SURFACE PROFILER (EAGLET EYE) is a corneal and scleral topographer that can measure the curvature and sagittal height of up to 20 mm diameter of the anterior surface of the eye. The instrument captures the image by projecting two Moiré fringe patterns onto the eye after instilling fluorescein (see Fig. 14.28 ).

Contour maps, plots or scans

These are generated from the point contour values, with similar values connected to form zones of equal curvature. The zones are coloured in spectral order, with the red end (warm colours) corresponding to steeper (shorter) corneal radii and the blue end (cooler colours) corresponding to flatter (longer) corneal curvatures.

Relative scales grade the image presentation to cover the entire difference in curvature across the image, highlighting any differences occurring regardless of their magnitude. Absolute scales are set by the user and attribute each scale increment to a set radius or power change. Careful note must be made of the type of scale used and the magnitude of the increments in order to correctly interpret contour maps (see Chapter 19 ).

There are four main ways in which contour maps are presented.

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  SAGITTAL (OR AXIAL) MAPS ( Fig. 8.9a ) determine the radius of curvature of the cornea at each measured point. This is based on a single refracting surface formula (paraxial ray theory). It assumes rotational symmetry of the surface and predicts that all rays will be focused on the axis of symmetry ( Fig. 8.9a ).

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    Advantages – easy to verify, has the highest repeatability and is the most widely used; possible to correlate the anterior surface shape with the refractive status, e.g. determines the type and shape of astigmatism.

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    Disadvantages – distorts the position of the apex and features such as ablation areas.

  

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  TANGENTIAL MAPS (or instantaneous representation) ( Fig. 8.9b ) calculate the actual radius of curvature measured at a tangent (90°) to its surface. This is based on a mathematical derivation of the radius of curvature with radii centres not restricted to a single axis.

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    Advantages – gives a more accurate representation of the position of the apex and other corneal structures and a better corneal shape for comparing the plot to an observed contact lens fluorescein pattern.

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    Disadvantages – difficult to verify and has a lower repeatability than the sagittal plot.

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  CORNEAL HEIGHT OR ELEVATION MAPS ( Fig. 8.9c ) (or X, Y, Z coordinates or Z values) – based on the difference in height from a reference sphere (the reference sphere can vary among instruments).

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    Advantages – they are the most direct measure of corneal shape and can predict areas of corneal touch of a contact lens on an irregular cornea.

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    Disadvantages – they have the lowest repeatability.

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  REFRACTIVE POWER MAPS ( Fig. 8.9d ) convert the detected curvature at any point into presumed refractive power based on assumptions of the refractive index of the cornea. Clinicians tend to think in terms of power rather than radii, but the true power of the cornea is based on more than front surface curvature.

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    Advantage – can infer the quality of vision from the corneal surface especially after corneal surgery.

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    Disadvantage – makes presumption that corneal has standard refractive index in all individuals.

Most instruments have software to simulate the expected fluorescein patterns of specific lenses (custom-made by the manufacturer or the practitioner's own design), allowing improved empirical fitting accuracy ( Fig. 8.10 ; and further material available at: https://expertconsult.inkling.com/ ). Comparative data are also available that show, for example, how keratoconus changes over time ( Fig. 8.11 ).