Most patients who choose to wear cosmetic colored contact lenses do just fine, but one small design detail may be surprisingly important: the location of the pigments. A study in Eye Contact Lens reports that the location of the pigment in the lenses can be key in preventing infections and lens discomfort.

The investigation looked at 19 commercially available cosmetic contact lenses from six manufacturers and used two methods—bright field microscopy and

OCT—to determine which lenses had a visible clear layer, indicating the majority of the pigment was enclosed within the lens matrix.

Researchers explained if the pigment was located at the top or bottom surface of the lens, pigment particles had direct contact with the conjunctiva or cornea, but designs with the pigment enclosed in the lens matrix eliminate exposure to the ocular surface. 

Earlier studies link colored contact lenses that failed a rub-off test with increased incidence of Pseudomonas aeruginosa adherence, the researchers note. In addition, surface pigments may create a rough lens surface, which can also increase microbial adherence, they wrote in the study.

Investigators found the majority of the cosmetic contact lenses analyzed in the study contained the bulk of the pigment within 0.4μm of the surface or on the surface itself. Investigators noted only one of the six manufactures (five out of 19 lenses) produced designs where the clear layer was identified in either imaging method.

“The extent of pigment enclosure is currently not regulated in most countries by their regulatory agencies; yet, the evidence that the pigment location impacts cosmetic contact lens safety and comfort is growing,” the researchers wrote in their paper.