Optometric Management Special Edition

2015

Issue link: https://optometricmanagementsupplements.epubxp.com/i/587112

Contents of this Issue

Navigation

Page 19 of 59

18 S P E C I A L E D I T I O N 2 0 1 5 • O P T O M E T R I C M A N A G E M E N T . C O M FE ATURE HOW HARD ARE YOUR PATIENTS' E YE WORKING? WHAT SCIENCE TELLS US Tribology is the science of the interaction between sur- faces in motion. It deals in a rigorous way with things we experience every day: friction, lubrication and wear. In recent years, tribology has been applied to the study of factors that infuence comfort in contact lens wear. 2 It makes intuitive sense that, all other things being equal, the less friction between the eyelid and a contact lens, the more comfortable and the less symptomatic contact lens wear will be. And, indeed, there is evidence to support this. 3 Because more people drop out of lens wear as a result of discomfort than for any other reason, it is little wonder that factors related to discomfort — like friction and lubri- cation — are of great interest to the contact lens industry and practitioners. 1,4 THE PHYSICS OF COMFORT Friction is the force that acts at the interface of two solid surfaces to resist their sliding past one another. 5 Te level of resistance is a property of the surfaces called coefcient of friction (CoF). CoF is derived from two separate forces: (1) the force needed to keep the objects sliding and (2) the force holding the surfaces together (the "normal" force). Both forces may be directly measured and a CoF calculated as shown in the following equation: COEFFICIENT OF FRICTION (CoF) = FRICTIONAL FORCE ÷ NORMAL FORCE Te larger the CoF, the greater the resistance as a result of friction and, thus, the greater the frictional energy re- quired to slide the surfaces past one another over a given distance. When both the frictional force and the distance through which it works are known, the frictional energy can be determined using the equation below: FRICTIONAL ENERGY = SLIDING WORK = FRICTIONAL FORCE × DISTANCE MEASURING FRICTIONAL ENERGY We have said that the work done by the eyelid in blink- ing is equal to the frictional energy required to move the lid over the cornea (or a contact lens). Additionally, the fric- tional energy that is generated is a function of the CoF, as both equations contain the Frictional Force term. Because a relationship between CoF and contact lens comfort has been established, the measurement of frictional energy loss in a simulated blink should also give us insight into how diferent contact lenses will perform on the eye. Although it can't be measured directly, a number of devices for estimating the frictional forces between the eyelid and a contact lens have been described, including instruments that aim to replicate the critical features of the on-eye situation. 1 Tese in-vitro and ex-vivo measurements made under physiologically relevant conditions represent a signifcant step forward in predicting contact lens comfort. Of course, for comparisons of lubricity to be meaningful, they must replicate not just the conditions of the eye but the physiology of contact lens wear. For example, it is one thing to compare the frictional energy loss with two fresh- from-the-package contact lenses; but can this tell us what the lenses will be like afer they have been worn for 10, 12 or 14 hours? To be truly applicable to contact lens wear, it is necessary to look at friction through time and under the conditions that the lens is likely to be worn. FRICTIONAL ENERGY THROUGHOUT THE DAY In a simulated eye-blink experiment using a mi- crotribometer (Figure 1), a device that directly mea- sures the normal force and frictional force, enabling the determination of frictional energy, ACUVUE OASYS Brand Contact Lenses 1-Day with HydraLuxe Technology were cycled in a tear-like fuid for up to 18 hours. Te results were impressive. In Figure 2 (page 19), the blue line represents the frictional energy of the lenses mea- sured through time that approximates a long day of wear, in this case, 18+ hours. It is important to note that the fric- tional energy starts low and stays low for the duration of lens wear. If the line instead curved upwards, that increase in frictional energy could contribute to a degradation in performance. Lens-to-lens variability, as indicated by the lighter blue "spread" in the confdence intervals, is also very tight, meaning that the frictional energy of the lenses are con- sistent from one lens to the next. Together, these factors suggest a favorable and robust tribological profle for the lenses under physiological conditions. CREATING LESS WORK What is it about ACUVUE OASYS 1-Day Lenses with Figure 1: Microtribometer used to measure Friction and Distance for computation of Frictional Energy. A mucin covered counter-surface is moved over the contact lens exposed to TLF (tear-like fuid) at forces and speed to simu- late the eyelid. Photo courtesy of SuSoS, AG.

Articles in this issue

Links on this page

Archives of this issue

view archives of Optometric Management Special Edition - 2015