THE PARAOPTOMETRIC: MEASURING THE CURVATURE OF THE CORNEA

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Diagnostic machines such as the Pentacam from Oculus provide valuable information about the anterior and posterior curvature of the cornea.

TIPS FOR OPTOMETRIC ASSISTANTS AND TECHNICIANS ON HOW TO IMPROVE THEIR INSTRUMENTATION SKILLS.

The measurement of the cornea has become increasingly important in the ophthalmic practice. The corneal surface accounts for approximately 70% of the eye’s total refractive power.

KERATOMETER CHALLENGES
For many years, the manual keratometer was the only means of providing information about the curvature of the cornea. However, this instrument was limited in its measurement of the corneal shape as it measures only two points””the flattest and the steepest meridians. It does this by measuring the margin at which the corneal curvature begins to flatten in order to determine the corneal curvature. On the downside, it ignores the central and peripheral areas.

The keratometer only measures the anterior surface of the cornea and assumes that the cornea is symmetrical. So, if the cornea is irregular it will find the steepest meridian, but that may not be the steepest area of the cornea.

TOPOGRAPHY AS THE “˜RING’ LEADER
In recent years, the corneal topographer has become the “go to” method of measurement as it provides both a qualitative and quantitative evaluation of corneal curvature. Concentric rings (known as Placido rings) are projected onto the cornea and create a virtual image. The anterior surface of the cornea acts like a convex mirror, so the size of the image is determined by its curvature.

If the patient has a steep cornea, the image produced will be smaller; conversely a flat cornea will produce a larger image. The picture of the reflected rings is captured by an acquisition camera. The distance from one ring to the next is used to determine the curvature of the ring. The images of all the rings are then analyzed and compared to the target size and the deviation of reflected rings.

The rings are displayed as an image by computer software as the curvature of the corneal surface. Most topographers measure 8,000-10,000 different points on the cornea. The reflections of the corneal rings are affected by the quality and quantity of the tear film, and the irregularity of the corneal surface. This irregularity can cause distortion to the reflected images and lead to inaccurate measurement of the corneal curvature.

Because this technology is limited to the measurement of the anterior corneal surface, it doesn’t provide information about the posterior corneal surface. Measurement of the posterior corneal surface has become increasingly more important in such procedures as refractive surgery. Corneal topography measurements are used in assessment, diagnosis, and management of corneal diseases, abnormalities, and deformities. Conditions such as keratoconus can be accurately measured and followed by providing a color map of the corneal cone.

SURGERY SUGGESTIONS
Pre-operative corneal topography can provide useful information for those patients about to have corneal transplant, and cataract or refractive surgery. It can map corneal irregularities, scarring, or irregular corneal astigmatism which may be a deciding factor to whether the patient will have surgery, and what type of surgery will be performed.

Post-operatively, the corneal topography can impart data about the healing of the cornea, and information about irregular astigmatism, which often occurs secondary to penetrating keratoplasty.

IMPROVING LENS FITTINGS
The topographer is also used often in contact lens fitting. A contact lens that does not fit the cornea properly may cause damage to the corneal epithelial cells. The topographer can also evaluate changes to the cornea due to contact lens wear and improve the management and success rate of complex contact lens fits. Some corneal topography machines also provide information related to wavefront analysis which can reveal myopia, hyperopia, and astigmatism.

Stereo Optical’s AKRcornea 500 uses Placido ring technology to give an accurate representation of the corneal topographic surface.

MAPPING IT OUT
The Axial Map on the topographer displays the variations in corneal curvature by use of a pseudo-color map. The warm colors represent areas of corneal steepening, while the cooler colors represent the flatter areas. The Tangential Map uses the pseudo-color scale to represent the changes in the dioptric values of the cornea. It offers better imaging of corneal defects, as well as their location.

Diagnostic machines such as the Pentacam (from Oculus) provide valuable information about the anterior and posterior curvature of the cornea by taking an optical cross-section of the cornea using a rotating Scheimpflug camera. The camera obtains 50 Scheimpflug images in less than two seconds. The Pentacam delivers a detailed, high-resolution image of the entire cornea, has the ability to measure the thickness of the cornea from limbus to limbus, and also provides lens densitometry information. The patient is asked to fixate straight ahead on a target while the technician manually focuses and aligns the image of the patient’s eye that is reflected on the computer screen.

Just as valuable: a combination autokerato-refractometer-topographer, such as the AKRcornea 500 made by Stereo Optical, which uses Placido ring technology to give an accurate representation of the corneal topographic surface. This instrument also provides information for contact lens fitting and can analyze corneal deformities.

The Orbscan II (from TECHNOLAS Perfect Vision, Inc.) uses a scanning slit system to measure the corneal surface. The slits are projected onto the cornea and then are captured and analyzed, thereby providing the curvature of the anterior, posterior corneal surfaces, and corneal thickness.

Ocular Coherence Tomography (OCT) generates a three-dimensional cross-section of the cornea by utilizing optical interferometry. This machine displays a high-resolution image of the cornea to within 3 to 20 µm and can reveal pathology as well as corneal thickness.

EYES WIDE OPEN
High Frequency Ultrasound Biomicroscopy (UBM) imparts a high resolution scan of the superficial structures of the anterior segment. The UBM is done with the patient in the supine position and the eye open. A special cup is placed between the open lids and filled with saline while the transducer is placed in the shell approximately 2mm away from the corneal surface.

The layers of the corneas can be seen and differentiated with a resolution of 40 microns. This is the only technology that allows detailed measurement of the thickness of the different corneal layers and can give a topographic map of those layers. The UBM also allows visualization of the depth of the anterior chamber, and ciliary body.

Assessing the cornea has become a part of the standard of care in the ophthalmology examination. The corneal measurements can be obtained and determined in a number of different ways. These newer technologies discussed measure both curvature and shape, enabling quantitative assessment of corneal depth, elevation, and power.

Janet Hunter is president of Eye Source LLC and specializes in ophthalmic technician training.

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