orthokeratology today
Checking Ortho-K Accuracy
BY JOHN MOUNTFORD, DIP. APP. SC, FAAO, FCLSA

The crucial factor in the accuracy and efficacy of a reverse geometry lens is the lens sagittal height. If this measurement is too high, then the lens is effectively too deep and you won't achieve the refractive change. If it's too low, it'll cause lens decentration, a loss of the effect as well as induced astigmatism. What we need is a quick, accurate and inexpensive method of measuring lens sag.

Figure 1. The "sag check" program calculates the sag of the lens if the various curves are known.

Calculating Sag Height

The simple ortho-k lens rule is: lens sag = corneal sag + tear layer thickness where the lens touches the cornea at the junction of the alignment curve and edge lift. Manufacturers have computer programs that can give you the sag height of the lens at this point without revealing the proprietary curves that constitute the lens back surface.

Alternatively, EyeDeal Software and Design's OrthoTools program can supply the sag height of the lens at any given point along the surface. Don Noack has a program (Sag Check) that, when supplied with the various curves and band widths, calculates the lens sag at the chord required (Figure 1).

Get Out Your Tools

Figure 2 shows a simple and inexpensive device that can allow you to check the actual sag of the lens you received from the lab against the sag values they gave you. Any instrument maker can create it for about $850. It consists of a Mitutoyo dial thickness gauge mounted on a solid steel upright, which is embedded into a solid block of granite approximately 2.5 inches thick. The gauge is attached to a mini processor that calculates the mean and standard deviation of a number of repeated readings. Place the lens in a special block that prevents lateral movement. E-mail me for the plans and specifications of the gauge and measuring blocks (at no charge) at besty@bigpond.net.au.

	Figure 2. The lens sag gauge consisting of a Mitutoyo Absolute dial thickness gauge, solid base and measuring blocks.

Let's Look at an Example

Let's look at a five-zone lens that has a total diameter of 10.6mm. The lens rests on the corneal surface at the junction of the first and second reverse curves or at a chord of 8.6mm. The block has an inner diameter of 10.7mm, which will allow 0.05mm of lens displacement laterally. This will, by calculation, result in a lens sag error of ±0.7µm.

Place the lens upside down in the block so it rests on the central platform. Now measure the lens thickness five times with the gauge. The miniprocessor will then print out the main and standard deviation of the readings. Now invert the lens and measure the total height at a chord of 8.6mm. Now measure the lens with the gauge (five repeat readings) and record the mean value. The effective sag of the lens equals the total height value minus the center thickness.

If the value you measure varies more than 5µm from that calculated by software or the lab, then the lens almost certainly won't work properly. The instrument has a high degree of accuracy (±1µm) and a repeatability of ±2µm.

Dr. Mountford is an optometrist in private practice specializing in advanced contact lenses for keratoconus, post refractive surgery and pediatric aphakia. He is a visiting contact lens lecturer to QUT and UNSW, Australia.