Updated Researches implied-Bandage Lens increase Evaporation & adverse Wettability

[HOSADOS]

DrG: While I'm not an expert, the researches and scientific literature determine that even the new silicon hydrogel material increase Evaporation and significantly adverse Wettability. So by all means, how could that be helpful to DES patient, severe ones in particular ? am I missing something ?
According to that, people with MGD or lipid problems are the worst candidates for such treatment. On that note, did you see success with patients in this category (MGD/lipid oil problems) or they all just suffered from DES ?

30th Annual Contact Lens Report, Optometric Study Center: April 2006
New Materials Advance Successful Wear-Here’s a look at how silicone hydrogel lenses fit into the elements of successful contact lens wear.
By Brien Holden, Ph.D.
http://www.revoptom.com/index.asp?Ar...239/lesson.htm

Wettability and Deposits
Besides maintaining healthy corneal metabolism and turnover, contact lenses need to uphold normal tear film integrity and stability to prevent bacteria and debris from accumulating on the lens surface. Tear film integrity is important, as it flushes accumulated debris and metabolic by-products from the ocular surface. These may otherwise accumulate and contribute to discomfort, infection and inflammation. Also, components of the tear film that are altered during contact lens wear, particularly during overnight wear, potentially can impair the defense functions of the eye.
The short term impact of silicone hydrogel lens wear on the tear film is no different from that of conventional hydrogel lens wear. Generally, lenses cause increased tear evaporation and increased rates of tear thinning and break-up time

[HOSADOS]

Effect of Contact Lens Materials on Tear Physiology.
ARTICLES
Optometry & Vision Science. 81(3):194-204, March 2004.
THAI, LEE CHOON BSc, MCOptom; TOMLINSON, ALAN DSc, FCOptom, FAAO; DOANE, MARSHALL G. PhD
Abstract:
Purpose. This study measured evaporation rate, thinning characteristics, and lipid layer changes in the prelens tear film (PLTF) associated with wearing of different soft contact lens materials, in an attempt to determine the biocompatibility of the material with the PLTF. Methods. Twenty habituated contact lens wearers wore five different soft materials in a random order on the left eye at visits separated by at least 24 h. The soft contact lens materials were polymacon (Optima 38), omafilcon A (Proclear Compatibles), phemfilcon A (DuraSoft 2), balafilcon A (PureVision), and etafilcon A (Acuvue). Tear film evaporation rate was measured by a modified Servo Med Evaporimeter and tear thinning time by HirCal grid. Tear film structure, elimination rate, and lens wetting ability were recorded dynamically with a Doane tear film video interferometer and graded according to a new system developed for the study. Baseline measurements were taken of the precorneal tear film before lens insertion, and PLTF was determined 30 min after commencing lens wear.
Results. No statistically significant differences were found for any of the baseline (precorneal tear film) data. There was also no significant difference in evaporation rate change (analysis of variance) and in tear thinning time (Friedmann) between the five contact lenses. In the PLTF structure grading, omafilcon A had significantly more stable grades than phemfilcon A (Friedmann, p = 0.0033) and polymacon (p = 0.004). In PLTF observation of tear thinning and elimination rate, there was a significantly slower rate of elimination observed for omafilcon A than phemfilcon A (Friedmann, p = 0.0023) and polymacon (p = 0.0023). There was no significant difference in the overall PLTF wetting ability grading between any of the lenses worn.

Conclusion. Generally, all soft contact lens materials significantly and adversely affected tear physiology by increasing the evaporation rate and decreasing tear thinning time. The surface wetting ability of all contact lens materials exhibited no significantly difference irrespective of the special surface treatments. Only in PLTF structure and in PLTF elimination rate were differences found from the conventional low water content materials; omafilcon A was better in PLTF structure and in PLTF elimination.
(C) 2004 American Academy of Optometry

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If anyone is interested I could email the full research in PDF

[HOSADOS]

relevant paragraphs to Silicone hydrogel lenses

Since, some argued this research doesn't refer to Silicone hydrogel lenses here are some relevant quotes from this articles to Silicone hydrogel lenses .

*This is a very interesting and comprehensive research. Those who're interested could email for the full research.

Effect of Contact Lens Materials on
Tear Physiology
LEE CHOON THAI, BSc, MCOptom, ALAN TOMLINSON, DSc, FCOptom, FAAO, and MARSHALL G. DOANE, PhD
Department of Vision Sciences, Glasgow Caledonian University, Scotland (LCT, AT), Schepens Institute, Boston, Massachusetts (MGD)
Optometry & Vision Science. 81(3):194-204, March 2004.

In recent years, new additions to this list are some hydrogel materials that are biomimetic and others that are composed of silicone hydrogel polymers.

The biomimetic hydrogel material omafilcon A is a synthetic analogue of the natural phospholipid phosphatidycholine incorporated into the copolymer backbone. The essential element is phosphorylcholine, a substance present in the outer surface of red blood cell membranes, which is the primary, natural material responsible for cell membrane biocompatibility.24 The structure of the biomimetic material is able to create or mimic the biological surface and convince the host to accept it.25 Initial findings suggest that the material’s resistance to hydration and lens deposits help to reduce CLDE and increase contact lens wearers comfort and decrease spoilation.15, 17–19, 21, 26, 27

Silicone hydrogel lenses were first patented by Tanaka et al.28 in the United States. Silicone chains are used as the backbone of the hydrogel polymer, and by increasing the silicone content, the oxygen permeability is improved. However, potential problems with these new hydrogels are decreased wetting ability and increased lipid interaction due to the migration of hydrophobic moieties to the lens surface. To enhance the compatibility of these silicone hydrogel lenses with the anterior ocular structures, the surfaces are treated with gas plasma techniques. The end products are hydrogel lenses that have low water content and high Dk value, which overcomes the major barrier of corneal hypoxia in contact lens wear.They are the first generation of new range of extended-wear products with a real potential for continuous wear.

The purpose of this study was to compare the effect of different soft contact lens materials on aspects of tear physiology that feature in the etiology of CLDE. In this comparative study, five different soft contact lens materials were chosen from each of the four U.S. Food and Drug Ainistration classification groups. They included two commonly prescribed daily wear lenses, one disposable lens, one silicone hydrogel lens, and one “biocompatible” lens. The physiological effects on the preocular tear film of these lenses was evaluated by measuring the preocular tear film evaporation rate, tear thinning time (TTT), and the interferometric observation of tear film structure, wetting capability, and rate of elimination. The physiological effect was assessed by comparing the measurements before and after lens insertion. The aim of this study was to determine whether any of these five lens materials could, through improve biocompatibility, preserve the integrity of the tear film.

DISCUSSION
This study showed that there was little difference in the effect of the contact lens materials on the prelens tear film; all materials had significant negative effects on normal tear physiology, with increases in evaporation rate and decreases in TTT. Only the thin film interferometry showed any statistically significant effects between materials for the pre-contact lens tear film. The evaporation rate of the prelens tear film was about 35% higher than that of the precorneal tear film. All contact lens materials had similar effects in increasing the rate. This is consistent with the various studies of the effect of contact lenses on tear physiology,4, 34–36, 48, 49 and this effect is independent of the initial water content or material of the lenses. The differences in evaporation rate between the five hydrogel lens types were small compared with the large increase in rate produced by the presence of any contact lens. This may mask any significant level of difference between the lenses.

Others have reported that hydrogel lenses create localized tear thinning at the lens edge,49 which interferes with the continuity of the lipid layer of the prelens tear film8, 56 and the spread of mucin over the cornea.46 This disrupts the lipid layer36, 44, 46 and causes a decrease in the tear stability4, 46 and break-up time.6, 51, 57

The categorization of the prelens tear film structure in this study was performed using the same criteria as that for precorneal tear film. From our data, we observed that the lipid layer spreads evenly, but is thinner across the anterior surface of the prelens tear film. Similar results were found in other studies.62, 65 This thinner prelens tear film is prone to contamination by microparticles on the contact lens surface,66, 67 which, in turn, affects the integrity ofthe lipid layer.

The wetting of all the contact lens materials in this study was essentially the same irrespective of any surface treatment. In previous studies, the material and the water content of the hydrogel lens was not found to influence the surface wetting ability of the contact lens.68–72 The wetting ability of hydrogel contact lenses increases after 15 min in vivo and reaches a maximum level after 30 min,73 probably as a result of interaction with the tears. However, with longer wear, greater mucous coating of the lens occurs,71 and the hydrophilic properties of the contact lens are reduced.73 Therefore, the advancing and receding contact angles, provided by manufacturers, may not be the best way to define in vivo wettability.68 Also, the manufacturing techniques for the hydrogel contact lenses are important because the chemical structure of the surface is directly influenced by the method of preparation, causing possible differences in wettability.68, 74

CONCLUSIONS
All soft contact lens materials significantly and adversely affect tear physiology by increasing the evaporation rate and decreasing TTT. These changes are large, and the differences between various soft lens materials in this study were statistically insignificant. Thin film interferometric observations of the prelens tear films indicate that the surface wetting ability of all contact lens materials are not significantly different irrespective of the new biomimetic materials or the special plasma-treated surface of silicone hydrogel lens. The structure of the prelens tear film and the rate of elimination show some differences between lenses. The biomimetic hydrogel contact lens has a better prelens tear film structure than the conventional low water contact hydrogel lenses. The new biomimetic lens also has a lower prelens tear film elimination rate than conventional low water content lenses. The new biomimetic lens material shows some promising results that warrant further investigation. The prelens tear film characteristics on the silicone hydrogel lenses are similar to those of the tear films over conventional low water content hydrogel materials.

[Rebecca Petris]

Hosados, DrG is not a participant in this forum so I have moved this thread in to the forum where other optometrists incl. Dr. Bazan can reply if they wish.

[Justin]

DrG: While I'm not an expert, the researches and scientific literature determine that even the new silicon hydrogel material increase Evaporation and significantly adverse Wettability. So by all means, how could that be helpful to DES patient, severe ones in particular ? am I missing something ?
According to that, people with MGD or lipid problems are the worst candidates for such treatment. On that note, did you see success with patients in this category (MGD/lipid oil problems) or they all just suffered from DES ?

Hi HOSADOS. Great post. I just finished replying to your other post, and knowing you have such a good grasp on what is going on with CLs and tear film interaction is impressive. I like the fact that you are able to analyze literature and form logical questions. Keep up the posting.

Some SiHy lenses are known to be heavy lipid depositors. So in an eye with a lid margin disease that is producing abnormal lipids you can have issues. In these cases, I think that some of the relief a person may feel may be short lived and can lead to the progression of the condition. Among other reasons, contact lens use can also cause corneal desensitization (or prevent nerve activation), which would allow one to find some relief, but this is only ******g the issue. Perplexing, as we all know, CLs can be a major source of discomfort. Because of the multi factorial nature of cl use and a patients condition, there is a lot of variability. A lot....if one is utilizing a CL as a DES therapy, it is important to optimize the normal protocol of fitting and evaluating a lens, and to monitor closely.