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Science behind friction factor


Research into what influences comfort in contact lens wear is breaking new ground, and the implications for current prescribing practice could be significant.

Kathrine Osborn Lorenz, optometrist and head of global strategic claims for Johnson & Johnson Vision Care, is exploring how this research can be interpreted.

In her paper ‘Friction a factor in contact lens comfort’ published in the January-February 2014 issue of Advanced Ocular Care, Ms Osborn Lorenz looks at ongoing research that is showing that the degree of friction between a lens and eyelid—known as the dynamic coefficient of friction (dCoF)—is the strongest factor in determining comfort.

Ms Osborn Lorenz notes that independent researchers first reported in 2009 that dCoF was more closely correlated with end-of-day contact lens comfort than water content, oxygen permeability, modulus and other factors long thought to be associated with comfort.

The professional affairs director at Johnson & Johnson Vision Care ANZ, Luke Cahill, says this discovery that friction is the overwhelming factor in contact lens comfort signals a major shift in thinking.

‘As a profession, we need to be evidence-based to make judgments. We need to look at what the science tells us, and not be swayed by marketing,’ Mr Cahill said.

Since the 2009 study, additional research has shown that dCoF accounts for about 80 per cent of the differences in comfort among contact lens brands, Ms Osborn Lorenz says.

‘We know comfort is a driving force keeping patients in lenses as drop-out is often linked to lack of comfort. We know six or seven factors make up what is subjectively considered “comfort”, depending on the specifications of the lens,’ Mr Cahill said.

‘The scientific reason underlying comfort is often overlooked. As practitioners, we need to look at the science, as Kathrine Osborn Lorenz has presented, and demonstrate strong clinical judgement.’

Ms Osborn Lorenz’s paper in Advanced Ocular Care on what to look for in studies that attempt to measure and compare the contact lens coefficient of friction, acknowledges that measuring friction is too invasive to be performed in vivo.

She notes that laboratory tests typically measure friction by rubbing the test surface (contact lens) after it has been bathed in liquid or solution (to replicate human tear film), against a counter surface that acts as the eyelid.

Citing differences in solutions, counter surface materials, and the speed and pressure used to represent the force of the eyelid, Ms Osborn Lorenz says results can vary significantly. She says there are no internationally recognised standards for measuring dCoF in vitro, making it difficult to compare results for contact lens friction across studies conducted with different methods.

Mr Cahill says that despite the fact that the science behind measuring friction is not yet standardised, this ground-breaking area of research is exciting and promising.

‘This research casts some doubt on the previously-accepted notion of the dominance of moisture and wettability influencing comfort, and instead investigates the smoothness of the lens surface as leading the way to comfort.’


Osborn Lorenz K. Friction a factor in contact lens comfort. Advanced Ocular Care 2014; January/February.

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