Ординатура / Офтальмология / Английские материалы / Wavefront Analysis Aberrometers and Corneal Topography_Boyd_2003

Alcon® EH-290 Eye Map corneal Topography System is a large 23 narrow modified Placido disk system. The modified patented Placido cone design is supposed to be very accurate and sensitive. Easy and intuitive to use (software, developed by SynApps Software, runs under Windows™), it

offers advanced contact lens software, keratoconus detection, corneal statistics information and advanced communication software. (Editor’s Note: For a more complete description of the use of Alcon’s Eye Map EH-290 please see Chapter 4 - Part 2, page 108- BFB).

Tomey® TMS-3 auto-topographer is a cheap, small and portable fully automatic self-topog- rapher that requires no operator alignment. The patient places his or her face on an ergonomically designed face rest and the automated topographer is activated by proximity sensors, automatically taking the measurements. The software, that can be installed in a pre-owned PC, runs under Windows™

operating system. The software is very complete and comprehensive, and includes a contact lens wizard with interactive fluorescein displays. Optional software packages include: Height and Height Change Maps, Klyce Corneal Statistics, Keratoconus Screening and the Contact Lens Wizard. The low level lights cone is intended to produce minimal glare and disturbance for the patient.

OCULUS ® Keratograph (Figure 32) and Haag-Streit ® Keratograph CTK 922 (Figure 33) are very similar instruments sold under different brand names and different packaging. They are compact systems that can fit any refractive unit and include built-in keratometer in connection with the topography system. The software runs under Windows™ operating system and is easy to use, with automatic measurement. The Oculus® can be an integrated computerised system (Keratograph CTK) or an independent system linked to a preowned computer. A non-contact measurement large Placido system with 22 rings in a hemisphere and 22.000 measuring points try to guarantee a high resolution.

The working distance of 80 mm is enough to make the patient feel comfortable. The light system (warm coloured) is intended to produce minimal glare and disturbance for the patient.

They have an interesting software that allows contact lens-fitting in three simple steps: automated contact lens recommendation with a database that includes 20.000 lens geometries from all major contact lens manufacturers, and can be easily enlarged, and realistic fluo-image simulation of contact lens adaptation (Figure 34). There is a possibility of measuring the back surface of rigid gas permeable contact lens through optional Lens Check software.

There is also an optional statistics software package called Datagraph, intended for refractive surgeons.

This systems allows wonderful comprehensive kinetic three-dimensional analysis of corneal

topography for simple explanation to the patient (Figure 35, 36). Fourier surface analysis (Figure 34-e) is available and new software is under development for refractive surgery and contact lens fitters.

Also optional is the Topolink software, that integrates the corneal topography data and some but not all excimer laser software .

The Oculus Keratograph now comes with the software version 1.56, which includes the automatic Keratoconus-detection module "Indices" (Figure 37) and the additional module "ZernikePolynomials" for fine analyses of cornea-induced aberrations.

For those instruments working with software 1.55 or later there is an additional software module available as option, called "Pupillometry". It allows user to measure not only the size of the pupil under different light conditions but also the reaction time and the speed of the pupillary changes in size Figures 38 and 39 ).

This is a truly revolutionary instrument for the study of the cornea. It combines a slit scanning system and a Placido disk (with 40 rings) to measure the anterior elevation and curvature of the cornea and the posterior elevation and curvature of the cornea. It offers a full corneal pachimetry map with white to white measurements.

ORBSCAN IIz™ takes a series of slit-beam images of two scanning slit-lamps projected beams at 45 degrees, to the right or left of the instrument axis. During the exam, the patient fixates on a blinking red light coaxial with the imaging system. Forty images are taken by the system, 20 with slit beams

projected from the right and 20 from the left. The 20 images are acquired in 0.7 seconds each. Simultaneously, a tracking system measures the non voluntary movements of the eye during the exam.

Orbscan IIz™ is able to measure anterior chamber depth, angle kappa, pupil diameter, simulated keratometry readings (3 and 5 central mm of the cornea), and the thinnest corneal pachimetry reading. It offers every traditional map apart form those of posterior corneal surface. Elevation topography of the anterior cornea enables clinicians to more accurately visualise the shape of abnormal corneas, which should lead to more accurate diagnoses and better

Chapter 3: Fundamentals on Corneal Topography

surgical results. It has proven to be and extraordinary tool for research and for the refractive surgeon.

The system is able to acquire over 9000 data points in 1.5 seconds, which is fast, but not enough for the patient to feel comfortable. Not every patient can avoid blinking, and in some cases measurements have to be repeated. A faster processing speed would be desirable, although we feel very comfortable with the system.

Easy to use and running under Microsoft® Windows™ NT 4.0 operating system, the major disadvantage is the high price, that makes it not affordable for most ophthalmologists. Any colour printer running under NT 4.0 can be used. Three dimensional views of the different maps are available (see figure 41 in this chapter).