PURPOSE. Measure lens thickness profile and calculate oxygen transmissibility of a range of commercially available soft toric lenses of different materials and lens designs. METHOD. Axial lens thickness was measured at precisely controlled intervals across each of 4 principal semi-meridians of 7 toric hydrogels using the ISO64979 approved low force gauge. 3 independent sets of readings were taken and averaged for each lens. Independent compression factors were calculated using manufacturers quoted thickness values for each lens. Measured values, along with calculated edge and boundary corrected Dk based on manufacturers quoted water contents, were used to generate transmissibility profiles across vertical and horizontal meridians of each lens.
RESULTS. Significant variations in oxygen transmission occur across different commercially available hydrogel toric lenses (average values ranged from 5 to 23). As expected, low water content, thicker toric hydrogels give lowest levels of oxygen to the central and peripheral cornea whilst more recently available soft toric designs allow significantly greater levels of oxygen to the cornea along both the horizontal and vertical lens merdians.
CONCLUSIONS. Although there is currently much discussion on improved Dk/t with new silicone hydrogel materials to support extended wear, limitations of oxygen delivery through soft toric lenses is often neglected. Soft toric lens designs can achieve lens stability with prism stabilisation, dynamic stabilisation or a combination. More sophisticated designs that employ modified prism or thin dynamic stabilisation zones can result in thinner lens profiles. This combined with higher water content materials results in improved oxygen transmissibility. Interestingly, the lens material with the highest water content and hence highest Dk, did not have the highest transmissibility across the lens surface. Only one lens design in this study approached the transmissibility levels of 24x10-9 to avoid corneal oedema during daily wear as established by Holden and Mertz (1984). This work shows the practitioner and patient benefits from more modern soft toric designs.