Tear Film Exchange: Just One More Reason RGP Contact Lenses May Be A Better Choice

The first hard (rigid gas permeable) contact lens was introduced mainstream in the 1970s.  If you or your parent recounts their lens wearing experience from this decade, it usually is not with fond memories.  The lenses weren't comfortable, you had to clean them with much greater care, and they were a lot more expensive.  So when disposable hydrogel (low oxygen permeable) soft lenses hit the market in 1987, and then later high oxygen permeable (silicone hydrogel) soft lenses became available in 2002, many thought RGP contact lenses would go the way of the dinosaur.  Who would want hard contact lenses they kept for a year or two at a time when they could have a new, fresh, inexpensive, comfortable lens exchanged out monthly (or even more often!)?

Debris (in this case microscopic balls of mucin) trapped underneath your contact lens can result in increased risk for inflammation and infection of your eye with contact lens wear.  It's much more common to get debris like mucin balls under soft contact lenses as compared to under RGP lenses. via

The truth however, defies that expectation.  RGPs continue to make up around 7-10% of the contact lens wearing market, and the number of wearers has not been steadily declining, but instead remaining stable over the last decade.  The two biggest reasons for the resilience of RGP lenses in the marketplace are visual quality and excellent ocular health benefits.  Soft contact lens wearers suffer from microbial infections at an average rate of 4/10.000 wears per year; RGP wearers only have infections at a rate of 1.2/10,000 wears per year.  So if we are throwing away soft lenses more regularly, why do we have more infections with these lenses than the older generation RGP materials?  The answer lies to a large extent with how your tear film moves underneath the contact lens.  Tear film movement is responsible for washing away debris, bacteria, and cycling nutrients in a regular flow out from underneath the contact lens with every blink of your eye.

Understanding Tear Film Exchange

Imagine the front surface of your eye.  Your cornea is continuously coated in tear film.  That tear film expresses with every blink, moving across your eye and then drained down your nasolacrimal duct through the puncta openings in your eyelid.  The tear film helps keep your eye hydrated; it's essential for comfort, oxygen transmission, and clear vision, and also is responsible for flushing away debris and contaminants that your eye gets exposed to regularly in the atmosphere.

There are multiple layers to your tear film, and when you place a contact lens on the eye, the tear film must split to both coat the lens surface and the space between your cornea and your contact lens. via

Now imagine putting your contact lens on your eye.  When the lens sits on the cornea, it splits your tear film in half; some of the tear film now goes over top of your contact lens, but some of it remains underneath the lens, sandwiched between your lens and the cornea.  In order to properly flush "dirty" tear film off the eye with your blink, you must move not just the tear film from over top of the contact lens, but also the tear film from the behind the lens. This is the limiting step with soft contact lenses.  Tear exchange is measured in units of tear turnover time; T95 represents the time in minutes for 95% of the tear volume to be eliminated from under the lens.  The lower the T95, the faster (healthier) the movement of tear film from under the lens. Studies show the average T95 for soft contact lenses is around 30 minutes; the  average T95 for RGP contact lenses is around 5 minutes.

Why do RGPs exchange tear fluid so efficiently?  

1. Smaller Diameter:  The average RGP is about 5 mm smaller than the average soft contact lens, and because it is smaller, it is easier for tear film to move freely underneath.

   RGP lenses are typically between 9-10 mm in size via

   A well-fit soft contact lens is around 14 mm in size and covers the entire cornea via

2. More Movement on the Eye:  RGPs are properly fit to have more up and down movement on the eye than soft lenses are traditionally fit.  The properly fit RGP lens will move 1-2 mm vertically with blink.  A soft contact lens typically moves around 0.5 mm.  The lesser movement of soft lenses is one of the reasons that they are considered to be significantly more comfortable on the eye than RGPs.  Mobile RGP lenses bump against your eyelids as you blink, creating lens awareness (let's face it-- that's a euphemism for discomfort) that's a hallmark of RGP wear. But the con of soft contact lenses eliminating this comfort problem caused by greater movement is a tighter fit that traps tear film and the debris caught inside it longer underneath the lens.  This is one of the reasons that doctors must carefully assess the fit of soft contact lenses every year for proper on-eye movement.  A decrease in lens movement with blink of just 0.2 mm can increase the wearer's risk for an inflammatory or infectious event 4.5 times higher.  Just a little too tight can mean the difference between you getting an infection this year!