Noel Brennan has conducted a retrospective study on over 700 wearers in order to address the questions on comfort in silicone hydrogels.1 Subjects wore a range of lenses for one month and comfort differences were measured between materials. Material properties of silicone hydrogels were correlated with comfort data to identify if any relationships existed.
Professor Noel Brennan speaking about his research on contact lens comfort.
No relationship was found between Dk/t and comfort.2 It has already been shown that Dk/t is not a relevant measure when differentiating between silicone hydrogel lenses in terms of oxygen performance. This research now indicates that Dk/t does not influence comfort either. In fact Noel Brennan concludes that a silicone hydrogel lens should be selected on criteria other than Dk/t.2
A relationship was found between modulus and comfort.2 This is shown in the graph. In general, the lower the modulus, the better the comfort. This tells us that, in general, recommending lenses with a lower modulus should be more comfortable for the wearer.
The strongest correlation was between surface lubricity (coefficient of friction) and comfort.2 A lens with a smoother surface resulted in greater comfort. Professor Brennan concludes with his belief that the defining feature for lens comfort is the surface finish.2
While there are many theories on the causes of discomfort, this finding helps in understanding the potential cause better. It is felt that the relationship between the conjunctival palpebral surface and the contact lens surface leads to discomfort.2 The pathology associated with this is lid wiper epithelopathy and a strong association between staining of this area and comfort has been found.3
Professor Noel Brennan speaking about his research results.
1. Brennan N. Contact lens based correlates of soft lens wearing comfort, Optom Vis Sci 2009; 86: E-abstract 90957. 2. Sulley, A. The best CL for my patient. Optician, March 2011; 30-34. 3. Korb, D et al. Lid-Wiper Epitheliopathy and Dry-Eye Symptoms in Contact Lens Wearers. CLAO. October 2002; 28:4 211-216.
Silicone hydrogel materials have solved the problem of getting enough oxygen to the cornea; lens induced hypoxic changes are a thing of the past.
Dk/t is no longer a relevant measure in differentiating between silicone hydrogel materials both in terms of their oxygen performance and their comfort. All silicone hydrogels have high oxygen flux and all currently available lenses allow 100% corneal oxygen consumption.
Comfort is correlated with both the modulus and surface lubricity of the contact lens, the latter being the strongest relationship with improved comfort seen in lower modulus, lower friction materials.
Achieving a balance of properties in silicone hydrogels is key in order to deliver health and importantly, comfort benefits to the wearer. Careful consideration of the balance of modulus and coefficient of friction (lubricity) should help in recommending more comfortable contact lenses for patients.