Abstract
Purpose:: A function attributed to the Meibomian lipid layer is that it prevents evaporation from the tear film. This is based on the concept that the Meibomian lipid layer forms an oily blanket over the aqueous layer. It is very difficult to test this idea under controlled conditions, and in many studies where this has been attempted, the results have been equivocal. Given that there is now evidence for proteins being part of this outer layer, they are also likely to be involved in the evaporative protection of the eye. Therefore, the purpose of this study was to test the effects of human Meibomian lipids and tear proteins on evaporation under controlled conditions.
Methods:: One millilitre of water, a buffer with salt concentrations similar to that found in tears, or these containing proteins was placed in a ceramic vessel giving a set surface area, and known amounts of Meibomian lipids were spread on the surface. The vessel was then placed in a sealed chamber on the balance of a synchronised high resolution thermogravimetry infrared spectrometer. The experimental chamber was heated to 37°C and purged with dry nitrogen gas. The rate of mass change (evaporation) was measured at a resolution of 10-7g and the evaporated species was characterised by vibrational spectroscopy.
Results:: Surprisingly, even with high amounts of Meibomian lipids on the different subphases (with or without proteins), evaporation (30mg/hr) was slightly higher than without Meibomian lipids (27mg/hr; ~0.1µL/blink). Water was identified as the evaporated species. Mineral oil on the surface stopped evaporation.
Conclusions:: Unlike current dogma, Meibomian lipids are not preventing evaporation from the surface of the eye, and in any case if the Meibomian lipids were absent, the estimated evaporation rate between blinks (12 sec) is not enough to give a significant difference in tear volume. Therefore preventing evaporation is not a major role of the lipid layer of the tear film and there was no evidence that protein interaction with Meibomian lipids affected the evaporation rate over lipids alone.
Methods:: One millilitre of water, a buffer with salt concentrations similar to that found in tears, or these containing proteins was placed in a ceramic vessel giving a set surface area, and known amounts of Meibomian lipids were spread on the surface. The vessel was then placed in a sealed chamber on the balance of a synchronised high resolution thermogravimetry infrared spectrometer. The experimental chamber was heated to 37°C and purged with dry nitrogen gas. The rate of mass change (evaporation) was measured at a resolution of 10-7g and the evaporated species was characterised by vibrational spectroscopy.
Results:: Surprisingly, even with high amounts of Meibomian lipids on the different subphases (with or without proteins), evaporation (30mg/hr) was slightly higher than without Meibomian lipids (27mg/hr; ~0.1µL/blink). Water was identified as the evaporated species. Mineral oil on the surface stopped evaporation.
Conclusions:: Unlike current dogma, Meibomian lipids are not preventing evaporation from the surface of the eye, and in any case if the Meibomian lipids were absent, the estimated evaporation rate between blinks (12 sec) is not enough to give a significant difference in tear volume. Therefore preventing evaporation is not a major role of the lipid layer of the tear film and there was no evidence that protein interaction with Meibomian lipids affected the evaporation rate over lipids alone.
| Original language | English |
|---|---|
| Article number | 2382636 |
| Pages (from-to) | 431 |
| Journal | Investigative opthalmology & visual science |
| Volume | 48 |
| Publication status | Published - 2007 |
