Can postlens tear thickness be measured using three-dimensional in vivo confocal microscopy?

Confocal microscopy through focusing (CMTF) of the cornea produces a three-dimensional display of corneal structure and intensity profiles that allow objective measurements of corneal sublayer thickness. This study investigated the feasibility of using in vivo CMTF to measure postlens tear thickness (PLTT).

Two rabbits and one human were evaluated in this study. Both rigid gas-permeable (RGP) and silicone hydrogel contact lenses were used. After contact lens insertion, CMTF scans were performed by rapidly focusing from the tip of the objective, through the contact lens, PLTT, and full thickness of the cornea, and into the anterior chamber of the eye. PLTT was calculated by measuring the distance between the posterior surface of the lens and the anterior surface of the epithelium.

With a steep RGP lenses, the position of the posterior lens surface and the superficial epithelial surface was easily visualized using three-dimensional CMTF, and the PLTT could be measured. However, when a properly fit RGP or silicone hydrogel contact lens was used, the strong signal from the posterior surface of the lens washed out the signal from the front surface of the cornea in most cases, and the PLTT could not be directly assessed. We also found that the PLTT was sensitive to applanation of the confocal objective lens.

The data suggest that the tear film under a properly fit contact lens is less than the 9-microm axial resolution of our confocal microscope. Furthermore, the potential for undetected eye movement and accidental touching of the contact lens raises questions regarding the overall reliability of the measurements.