Corneal topography is a powerful way to evaluate properties of the corneal surface. It’s especially useful for fitting speciality contact lenses.
Taking high-quality topography maps depends on many factors—not least your topography instrument.
When it comes to corneal topographers, Placido disc systems such as Medmont’s E300 and meridia™ have long been one of the most popular because of their usability, data quality and affordability.
They work by projecting a series of concentric rings onto the cornea and photographing the rings to determine slope and curvature and pick up any irregularities.
Placido disc topographers offer a choice between a large cone or small cone, and both have their pros and cons.
Which placido cone size should you choose?
Generally speaking, small cone Placido topographers are extremely accurate while large cones are simpler to manipulate, so data collection can be easier.
The shorter working distance of a small cone Placido disc topographer projects more rings onto the cornea than a large cone, offering a much larger capture field and a higher concentration of measured points.
While the longer focal distance of large cones means they don’t encroach far into a patient’s space, shadowing from nose bridges, eyebrows, lids and lashes can impact data capture. It can also be more difficult for rings to reach the peripheral cornea. This makes it unlikely to achieve 10mm of data; 6-9mm is more realistic. As such, many large cone topographers use extensive extrapolation to fill in the data gaps.
When comparing ease of maintenance, large cones can be easier to clean as its rings are on the outside of the cone rather than inside like a small cone. That said, if space is a consideration, small cone topographers win out with their smaller footprint.
One thing to consider about small cones is that they require a high degree of user accuracy, as they’re more susceptible to alignment error. To counter this, the instrument must be well-designed, and the user must be steady with their movements.
Medmont Placido topographers use a small cone.
Our Australia-based research and development team has eliminated common limitations of a small cone to make our topographers among the most accurate, detailed and user friendly. We’re also proud to say, thanks to composite eye capture, our small cone Placido ring corneal topographers cover the largest surface area on the market.
How do we do it? Medmont’s intelligent small cone design packs powerful features required for accurate anterior surface mapping, including:
- precise distance measurement,
- dense ring data close to the corneal apex
- extreme corneal coverage, and
- highly concentrated point density.
These features are enabled and optimised by sophisticated software algorithms that extract out the accuracy to the highest degree.
For example, distance determination is crucial in order to determine curvature correctly with Placido topography. With Medmont, precise distance alignment isn’t necessary: our topographers can measure distance dynamically and adjust accordingly.
Further, only Medmont offers a direct optical profile measurement of the eye across the cone—impossible with large cones. This allows precise measurement that also perfectly syncs with the topography image to avoid movement errors.
What do the experts say?
Randy Kojima, FAAO uses a medmont meridia™ extensively for research purposes at Pacific University, and previously used our E300 USB corneal topographer.
He says, “Medmont does a great deal with its determination of the anterior surface and the way it addresses the shorter focal distance. The potential for inaccuracy is neutralized by determining the front surface and distance from the camera to the apex of the eye.”
What about the argument that larger cones are more accurate because focal distance errors aren’t a concern when the camera is so far from the cornea? “Medmont knows exactly where the front of the eye is, and focal error is essentially neutralised.”, says Kojima.
“The algorithm that determines eye shape from the data points is incredibly impressive. In my opinion, this is one of the things that make the medmont meridia™ the most accurate instrument on the market.”
Practitioner makes perfect.
Regardless of cone size, to capture optimal topography, it’s your job to understand your instrument’s capabilities and work around its limitations.
This involves communicating with your patient to ensure proper alignment—more so with a larger cone which is prone to data loss from varying nasal bridge anatomy and eyelash shadows.
With a small cone, if a patient has deep-set eyes or prominent brows ask the patient to move their chin forward and pivot their head so the nose points away from the cone.
Watch Randy Kojima show you to position your patient with a medmont meridia™.
Which cone size is right for your practice?
Ultimately, only you can answer that.
If data quality is your main priority, small cone placido topographers are highly accurate and offer a larger capture field. Yes, they require more precise patient alignment and operator steadiness—but we believe the result is well worth it.
Want to learn more?
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