The Business of Contact Lenses

Understanding Splines Will Help Your Contact Lens Practice

By Clarke D. Newman, OD, FAAO

To set your contact lens practice apart, you need to become, in perception and in fact, a contact lens expert. There is no way around it. Part of being an expert means you must have more than a facile understanding of the technologies used to create the lenses and care solutions you prescribe.

New design and, more importantly, fabrication techniques enable manufacturers to create lenses that are more comfortable and biocompatible for a wider range of surface topographies. One such technology is the incorporation of spline curves into lens designs.

In both hydrogel and GP lens designs, the aim is to have the posterior surface of the lens closely approximate the anterior surface of the eye, so that the load of the lens is more widely distributed on the ocular surface. Splines enable the posterior lens surface to follow the contour of the ocular surface, regardless of the irregularity, and this close alignment helps improve comfort. Splines are used in multifocal, scleral and quadrant-specific lenses, as well as in toric lens stabilization methods.

Splines Simplified

What is a spline? I could discuss polynomial expressions, smoothing, quadratics, interpolation and cubics, but that might cause some readers to turn the page quickly. Instead, I will try to simplify this subject by describing an example.

Picture a grid on a piece of paper placed over a horizontally oriented cork board. At each point where the grid lines cross, imagine a push pin stuck through the grid into the board. Imagine that the pins are all different heights and that we cover this array of pin heads with a piece of very thin cloth. The shape of the cloth is a surface spline. The math just gives us the polar coordinates needed to define where the pin heads are oriented in space relative to a reference point on the grid surface. Now, let's relate this example to contact lenses.

Splines in Contact Lens Design

Instead of a grid, picture a round clock face with 10 pins radiating from the center of the circle out to the edge, evenly spaced along each clock hour meridian. If the pins we place from the center to the edge along each clock hour are progressively shorter (but the same length at each point along each meridian), when we drape the cloth over them, we have a spherical surface.

If we change the pins along the 4 o'clock, 5 o'clock, 6 o'clock, 7 o'clock and 8 o'clock meridians and use shorter pins than we did in the other meridians, the cloth collapses in the bottom quadrant. This shape represents quadrant-specific lenses.

Starting with the spherical shape again, if we replace the pins near the edge along the 4 o'clock and 8 o'clock meridians with longer pins and place the cloth over them, we will have bumps in the cloth near the edge at 4 o'clock and 8 o'clock. This is the shape that used to stabilize rotation in the newer hydrogel toric lenses.

As these examples illustrate, splines let us create toroidal surfaces using aspheric curves. Splines also allow us to create multifocal lenses that have intermediate transition zones between distance and near zones. Sound familiar?

Positive Results

Understanding spline curves will help you understand the optics of the newer hydrogel lenses. Further, that understanding will help you visualize the specialty GP lenses that will help set your practice apart from those around you—quadrant-specific, scleral and multifocal GP lenses. CLS

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Dr. Newman has been in private practice in Dallas, Texas, since 1986, specializing in vision rehabilitation through contact lenses as well as corneal disease management, optometric medicine and refractive surgery. He is also a consultant or advisor to B+L. You can reach him at cdnewman@mindspring.com.