The Topcon CA-200F

However, I am becoming increasingly dependent upon one particular piece of equipment, the topographer. As an optometrist in an independent practice, I am fortunate enough to be involved in specialist contact lens fittings. However, contact lens patients or not, the topographer is an important diagnostic tool that I could not live without.

How it works

The Topcon CA-200F corneal analyser essentially uses the historic principles of Placido Disc to assess the curvature of the anterior cornea by interpreting reflections from the tear film. These reflected mires are then analysed to create a detailed curvature, typically like a contour map of a mountain. 

One of my passions in practice is myopia management and I actively fit patients with contact lenses to help slow their progression of myopia. A topographer plays an essential role in this management, especially in orthokeratology. 

A good topographic map is required initially for baseline data and the ordering of lenses, and then future fitting and management is based off differential maps. The corneal analyser can also be linked with software systems from external companies for empirical lens ordering.

A keratometer is still widely used by practitioners in routine contact lens fittings. However, by using a topographer to assess the curvature beyond the central corneal 3mm, I have a better overall indication of the shape to which I am fitting a contact lens; especially considering it is safe to say that most contact lenses fit beyond that central 3mm.

A topography map also guides the fitting of lenses to irregular corneas, including astigmatism. The corneal analyser has a contact lens fitting simulation for rigid gas permeable lenses, which presents the expected fluorescein pattern for a wide range of different manufacturers’ lenses and the ability to alter the BOZR and diameter and assess the change in fit.

"A topographer is an essential piece of equipment in practice. With a quick capture, it can provide such a large amount of data"

Other uses

The topographer is not solely for use in contact lenses. It can be a diagnostic tool for corneal ectasias, such as keratoconus and pellucid marginal degeneration. 

Patients can be more accurately monitored for progression and interventions such as onward referral for collagen cross-linking or surgical intervention, which can be provided in a more timely and efficient manner for both primary and secondary care. 

The corneal analyser has the added benefit of a keratoconus screening analysis, which looks at asymmetry of the corneal and apical gradient to display a probability of whether the cornea is reflective of keratoconus. 

It can also be used both at pre- and post-refractive surgery, for example, photorefractive keratectomy and laser-assisted-in-situ keratomileusis, post-keratoplasty and disorders such as pterygia and corneal scars, which may induce astigmatism. 

The corneal analyser can display the data in various ways, including 3D, axial or instantaneous maps, plus multiple and differential maps. Higher order aberrations can influence a patient’s quality of vision and the corneal analyser has the added benefit of also performing wavefront analysis and presenting the zernike analysis of the cornea. 

A topographer is an essential piece of equipment in practice. With just a quick capture, it can provide such a large amount of data. It is important in optometry to be able to offer patients the best level of care, whether that is the opportunity to fit orthokeratology to manage myopia or fit contact lenses to irregular corneas. 

Due to the growing demands in both optometry and ophthalmology, I believe that it is important to be able to manage more patients in primary care, but also provide good referrals at the correct time into secondary care when necessary. Without a topographer, I do not believe that this would always be achieved.