Comparative study of some physiologically important properties of six brands of disposable hydrogel contact lenses.

PURPOSE

This study examines the water content, refractive index, oxygen permeability (Dk), oxygen transmissibility (Dk/L), and lens thickness profile for six brands of disposable hydrogel contact lenses.

METHODS

Refractive index was measured with a Bausch & Lomb Abbe Model 3L laboratory refractometer. Water content was read from the scale in the refractometer eyepiece. Oxygen permeability and transmissibility were measured by a Rehder polarographic unit using the procedure described in International Standards Organization (ISO) Standard 9913-1.2. Lens thickness profiles were measured on a Rehder Electronic Thickness Gauge equipped with a rotating ball anvil.

RESULTS

Water content ranged from 38% to 62%. Lower water content lens materials had an oxygen permeability of about 7 x 10(-11) (cm2/sec)(mL O2/mL x mm Hg), whereas the permeability for the higher water content materials ranged from 20 x 10(-11) to 30 x 10(-11).

CONCLUSIONS

In theory, it is possible to design contact lenses of approximately the same transmissibility-when starting with materials of different permeabilities-by adjusting lens thickness. This was not the case with the six brands studied here. Some brands of contact lenses had significantly higher transmissibilities than others, but all had FDA approval. The clinical significance of different contact lens transmissibilities based on different definitions of thickness is discussed.