Measurement of post-lens tear thickness

PURPOSE. A method to measure the tear film beneath a soft contact lens, ref erred to as post-lens tear thickness (PLTT), would have many applications t o contact lens research. In this study a noninvasive technique for measurin g the PLTT is presented, METHODS. The feasibility of measuring the tear layer by optical pachometry was first assessed using a model eye. The baseline corneal thickness (B) of both eyes of 21 subjects was measured, etafilcon-A ionic disposable soft c ontact lenses (58% water) were inserted, and the total thickness (T) of the cornea, contact lens, and PLTT were measured. After the pachometry reading s the lenses were removed and their center thickness (C) determined. The PL TT was calculated using the equation: PLTT = T - (B + C). Two sets of measu rements of T were performed at 15 and 25 minutes after lens insertion. The entire procedure was repeated at a second visit. RESULTS. The pachometry measurements of the small aqueous reservoir between the model eye and the lens closely matched those obtained by direct micros copic measurement. For human PLTT, the mean values (and 95% confidence inte rvals) for right eyes on visits 1 and 2 were 11 (8, 13) and 12 (10, 15) mu m, respectively, and for left eyes were 12 (10, 15) and 11 mu m (8, 14) mu m, respectively. CONCLUSIONS. It is possible to measure the post-lens tear thickness using o ptical pachometry. The variability between repeated measurements suggests t hat with careful sample size planning, the technique is sufficiently precis e to be useful in group assessments of PLTT.