Soft Contact Lens Design, Material, the Tear Film and the Co

Soft Contact Lens Design, Material, the Tear Film and the Consequences for Corneal Drying

Contact lens materials must have desirable properties in order to be commercially successful.

Hydrogels that have a low water content containing hydrophobic monomers have the highest density of the common hydrogels.

• Dessication
• Tear film hyperosmolarity
• Loss of supportive factors
• Potential for symptoms of discomfort,

reduced vision and ocular surface damage

31. Bron A J, Tiffany J M, Gouveia S M, Yokoi N, and Voon L W (2004) Functional aspects of the tear film lipid layer. Experimental Eye Research 78: 347-360.

32. Bräuer L, and Paulsen F P (2008) Tear film and ocular surface surfactants. Journal of Epithelial Biology and Pharmacology 1: 62-67.

33. Johnson M E, and Murphy P J (2004) Changes in the tear film and ocular surface from dry eye syndrome. Progress in Retinal and Eye Research 23: 449-474.

34. Foulks G N (2007) The correlation between the tear film lipid layer and dry eye disease. Survey of Ophthalmology 52: 369-374.

35. Iskeleli G, Karakoc Y, Ozkok A, Arici C, Ozcan O, and Ipcioglu O (2013) Comparison of the effects of first and second generation silicone hydrogel contact lens wear on tear film osmolarity. International Journal of Ophthalmology 6: 666-670.

36. Kanski J J, and Bowling B (2011) Clinical Ophthalmology: A Systematic Approach. 7 ed. Edinburgh: Elsevier.

37. Hashemi H, Khabazkhoob M, Kheirkhah A, Emamian M H, Mehravaran S, Shariati M, and Fotouhi A (2013) Prevalence of dry eye syndrome in an adult population. Clinical and Experimental Ophthalmology Epub ahead of print, doi: 10.1111/ceo.12183.

38.Moss S E, Klein R, and Klein B E (2000) Prevalence of and risk factors for dry eye syndrome. Archives of Ophthalmology 118: 1264-1268.

39. Lee A J, Lee J, Saw S M, Gazzard G, Koh D, Widjaja D, and Tan D T H (2002) Prevalence and risk factors associated with dry eye symptoms: a population based study in Indonesia. British Journal of Ophthalmology 86: 1347-1351.

40. Doughty M J, Fonn D, Richter D, Simpson T, Caffery B, and Gordon K (1997) A patient questionnaire approach to estimating the prevalence of dry eye symptoms in patients presenting to optometric practices across Canada. Optometry and Vision Science 74: 624-631.

41. Gayton J L (2009) Etiology, prevalence, and treatment of dry eye disease. Clinical Ophthalmology 3: 405-412.

42. Giannoni A, and Nichols J (2012) 2012 annual report on dry eye diseases. Contact Lens Spectrum 27: 26-30.

43. Holly F J, and Lemp M A (1977) Tear physiology and dry eyes. Survey of Ophthalmology 22: 69-87.

44. Purslow C (2010) The interaction between contact lenses and the tear film. Optometry Today February 26th; 36-42.

45. Hosakaa E, Kawamoritab T, Ogasawaraa Y, Nakayamab N, Uozatob H, Shimizub K, Dogrud M et al. (2011) Interferometry in the evaluation of precorneal tear film thickness in dry eye. American Journal of Ophthalmology 151: 18-23.

46. Yokoi N, and Komuro A (2004) Non-invasive methods of assessing the tear film. Experimental Eye Research 78: 399-407.

47. Wong H, Fatt I, and Radke C J (1996) Deposition and thinning of the human tear film. Journal of Colloid and Interface Science 184: 44-51.

48. Mengher L S, Bron A J, Tonge S R, and Gilbert D J (1985) Effect of fluorescein instillation on the pre-corneal tear film stability. Current Eye Research 4: 9-12.

49. Mengher L S, Pandher K S, and Bron A J (1986) Non-invasive tear film break-uptime: sensitivity and specificity. Acta Ophthalmologica 64: 441-444.

50. Holly F J (1980) Tear film physiology. American Journal of Optometry and Physiological Optics 57: 252-257.

51. Arita R, Itoh K, Inoue K, Kuchiba A, Yamaguchi T, and Amano S (2009) Contact lens wear is associated with decrease of meibomian glands. Ophthalmology 116: 379-384.

52. Nichols J J, and Sinnot L (2011) Tear film, contact lens, and patient factors associated with corneal staining. Investigative Ophthalmology and Visual Science 52: 1127-1137.

53. Efron N, and Brennan N A (1987) The clinical relevance of hydrogel lens water content. Journal of the British Contact Lens Association 10: 9-10, 12-14.

54. Fatt I (1989) A predictive model for dehydration of a hydrogel contact lens in the eye. Journal of the British Contact Lens Association 12: 15-31.

55. Mackie I A (1985) Contact lenses in dry eyes. Transactions of the Ophthalmological Societies of the United Kingdom 104: 477-483.

56. Zantos S G, Orsborn G N, Walter H C, and Knoll H A (1986) Studies on corneal staining with thin hydrogel contact lenses. Journal of the British Contact Lens Association 9: 61-64.

57. Gispets J, Solá R, and Varón C (2000) The influence of water content of hydrogel contact lenses when fitting patients with ‘tear film deficiency’. Contact Lens and Anterior Eye 23: 16-21.

58. Ramamoorthy P, Sinnot L, and Nichols J J (2008) Treatment, material, care, and patient-factors in contact lens-related dry eye. Optometry and Vision Science 85: 764-772.

59. Efron N (2012) Contact Lens Complications. 3 ed. Edinburgh: Elsevier Saunders.

60. Fonn D (2007) Targeting contact lens induced dryness and discomfort: what properties will make lenses more comfortable. Optometry and Vision Science 84: 279-285.

61. Schafer J, Mitchell G L, Chalmers R L, Long B, Dillehay S, Barr J, Bergenske P et al. (2007) The stability of dryness symptoms after refitting with silicone hydrogel contact lenses over 3 years. Eye and Contact Lens 33: 247-252.

62. Chalmers R, Long B, Dillehay S, and Begley C (2008) Improving contact-lens related dryness symptoms with silicone hydrogel lenses. Optometry and Vision Science 85: 778-784.

63. Aakre B M, Ystenaes A E, Doughty M J, Austrheim Ø, Westerfjell B, and Lie M T (2004) A 6-month follow-up of successful refits from daily disposable soft contact lenses to continuous wear of high-Dk silicone-hydrogel lenses. Ophthalmic and Physiological Optics 24: 130-141.

64. Dillehay S, and Miller M B (2007) Performance of lotrafilcon B silicone hydrogel contact lenses in experienced low-Dk/t daily lens wearers. Eye and Contact Lens 33: 272-277.

65. Jones L, May C, Nazar L, and Simpson T (2002) In vitro evaluation of the dehydration characteristics of silicone hydrogel and conventional hydrogel contact lens materials. Contact Lens and Anterior Eye 25: 147-156.

66. Peterson R C, Wolffsohn J S, Nick J, Winterton L, and Lally J (2006) Clinical performance of daily disposable soft contact lenses using sustained release technology. Contact Lens and Anterior Eye 29: 127-134.

67. Szczesna-Iskander D H (2014) Comparison of tear film surface quality measured in vivo on water gradient silicone hydrogel and hydrogel contact lenses. Eye and Contact Lens 40: 23-27.

68. McNamara N A, Polse K A, Brand R J, Graham A D, Chan J S, and McKenney C D (1999) Tear mixing under a soft contact lens: effects of lens diameter. American Journal of Ophthalmology 127: 659-665.

69. Snyder C (2000) Preocular tear film anomalies and lens-related dryness. In: Silbert J A [ed.] Anterior Segment Complications of Contact Lens Wear. (2 edn.) Oxford: Butterworth Heinemann; 3-21.

70. Bennet E S, and Weissman B A (2004) Clinical Contact Lens Practice. London: Lippincott Williams and Wilkins.

71. Kymionis G D, Plaka A, Kontadakis G A, and Astyrakakis N (2011) Treatment of corneal dellen with a large diameter soft contact lens. Contact Lens and Anterior Eye 34: 290-292.

72. Holly F J (1985) Physical chemistry of the normal and disordered tear film. Transactions of the Ophthalmological Societies of the United Kingdom 104: 374-380.

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9. http://www.varpa.es/optics.htmlhttp://inhabitat.com/next-gen-contact-lenses-could-send-emails-directly-to-your-eyes/ [Accessed: 6 February 2014]

10. http://northwestoptical.com/lens-options/contact-lenses/ [Accessed: 7 February 2014]

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13. http://thinkingmomsrevolution.com/where-do-you-see-your-child/ [Accessed: 9 February 2014]

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15. http://www.healio.com/ophthalmology/journals/osli/%7B6a6a5eda-bdb4-4962-b6c1-437e336a869a%7D/ultra-high-resolution-optical-coherence-tomography-for-imaging-the-anterior-segment-of-the-eye [Accessed: 9 February 2014]

16. http://visionwellnesscenter.com/dry-eye-syndrome/ [Accessed: 12 February 2014]

17. http://www.mydryeyes.com/What_Is_A_Healthy_Tear_Film [Accessed: 12 February 2014]

18. http://medical-dictionary.thefreedictionary.com/lacrimal+drainage+system [Accessed: 12 February 2014]

19. http://medical-dictionary.thefreedictionary.com/tear+film [Accessed: 13 February 2014]

20. http://www.warrenphotographic.co.uk/02303-rain-drops-on-water [Accessed: 13 February 2014]

21. http://www.everystockphoto.com/photo.php?imageId=1044456 [Accessed: 15 February 2014]

22. http://www.buzzle.com/articles/clogged-drain.html [Accessed: 15 February 2014]

23. http://www.softchalk.com/lessonchallenge10/lesson/k21%20final/livingthings/softchalk-challenge-2010_print.html [Accessed: 15 February 2014]

24. http://www.eyehealthweb.com/bulging-eyes/ [Accessed: 16 February 2014]

25. http://www.revophth.com/content/d/therapeutic_topics/i/1302/c/25069/ [Accessed: 16 February 2014]

26. http://getdryeyetreatment.com/blog/chronic-blepharitis-treatment/chronic-blepharitis-treatment-2/ [Accessed: 17 February 2014]

27. http://revision4gcses.wordpress.com/science/chemistry-2/polymers/ [Accessed: 18 February 2014]

28. http://pslc.ws/macrog/kidsmac/pmma.htm [Accessed: 18 February 2014]

29. http://theorganicsolution.wordpress.com/2012/05/23/the-chemistry-of-the-contact-lens/ [Accessed: 18 February 2014]

30. http://www.oculist.net/downaton502/prof/ebook/duanes/pages/v4/v4c016a.html [Accessed: 18 February 2014]

31. http://optometrist.com.au/corneal_neovascularization/ [Accessed: 18 February 2014]

32. http://www.clspectrum.com/articleviewer.aspx?articleid=12955 [Accessed: 18 February 2014]

33. http://www.matheson-optometrists.com/atlas_of_eye_conditions/atlas_of_eye_conditions_pg_1.html [Accessed: 18 February 2014]

34. http://www.contactlensheadlines.com/category/contact-complications/ [Accessed: 18 February 2014]

references

1. Morgan P B, and Efron N (2006) A decade of contact lens prescribing trends in the United Kingdom (1996–2005). Contact Lens and Anterior Eye 29: 59-68.

2. Papas E, Wolffsohn J S, and Jones L (2010) Innovation in contact lenses: basic research and clinical science. Journal of Optometry 3: 123-124.

3. Morgan P B (2009) Taking stock of the UK contact lens market. [online] Contact Lens Monthly. Available at: http://www.opticianonline.net/assets/getAsset.aspx?ItemID=3710 [Accessed: 2 January 2014].

4. Morgan P B, Woods C A, Tranoudis I G, Helland M, Efron N, Orihuela G C, Grupcheva C N et al. (2013) International contact lens prescribing in 2012. Contact Lens Spectrum 28: 31-38.

5.Young G (2010) Soft lens design and fitting. In: Efron N [ed.] Contact Lens Practice. Edinburgh: Butterworth Heinemann Elsevier; 109-118.

6. Young G, Holden B, and Cooke G (1993) Influence of soft contact lens design on clinical performance. Optometry and Vision Science 70: 394-403.

7. Bleshoy H, Guillon M, and Shah D (1994) Influence of contact lens material surface characteristics on replacement frequency. International Contact Lens Clinic 21: 82-95.

8. Tighe B J (2007) Contact lens materials. In: Phillips A J, and Speedwell L [eds.] Contact Lenses. (5 edn.) Edinburgh: Butterworth Heinemann Elsevier; 59-78.

9. Kopecek J (2009) Hydrogels: from soft contact lenses and implants to self-assembled nanomaterials. Journal of Polymer Science Part A: Polymer Chemistry 47: 5929-5946.

10. Nicolson P C, and Vogt J (2001) Soft contact lens polymers: an evolution. Biomaterials 22: 3273-3283.

11. Tranoudis I G, and Efron N (2004a) Tensile properties of soft contact lens materials. Contact Lens and Anterior Eye 27: 177-191.

12. Grey C P (1986) Changes in contrast sensitivity when wearing low, medium and high water content soft lenses. Journal of the British Contact Lens Association 9: 21-25.

13. Compan V, Lopez L M, Andrio A, Lopez-Alemany A, and Refojo M F (1999) Determination of the oxygen transmissibility and permeability of hydrogel contact lenses. Journal of Applied Polymer Science 72: 321-327.

14. French K (2005c) Contact Lens Material Properties. Part 3 - Oxygen performance. Optician 230 (6030); 16-21.

15. Holly F J, and Refojo M F (1975) Wettability of hydrogels. I. Poly (2-hydroxyethyl methacrylate). Journal of Biomedical Materials Research 9: 315-326.

16. French K (2005a) Contact Lens Material Properties. Part 1 - Wettability. Optician 230 (6022); 20-28.

17. Maldonado-Codina C (2010) Soft lens materials. In: Efron N [ed.] Contact Lens Practice. (2 edn.) Edinburgh: Butterworth Heinemann Elsevier; 67-85.

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19. Kita M, Ogura Y, Honda Y, Hyon S, Cha W, and Ikada Y (1990) Evaluation of polyvinyl alcohol hydrogel as a soft contact lens material. Graefe's Archive for Clinical and Experimental Ophthalmology 228: 533-537.

20. Hyon S, Cha W, Ikada Y, Kita M, Ogura Y, and Honda Y (1994) Poly(vinyl alcohol) hydrogels as soft contact lens material. Journal of Biomaterials Science, Polymer Edition 5: 397-406.

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Technological advances have had a huge impact on the contact lens market in the UK...

1

• The first contact lenses were made from glass and filled with jelly.

Contact Lens Markets...

• Increased dominance of soft lens materials
• Aspherics to facilitate optimal distance vision
• Multifocals having a greater range of clear focus
• Easier astigmatic correction through more dependable toric designs

7.2%

2

14.7%

14.6%

• In the 1930s, suitable polymers were discovered and their continued development has meant that contact lenses can now be made from hard, soft and gas permeable materials.

phema

2.

Total diameter

Polymerising the HEMA monomer forms polyHEMA (PHEMA). Upon hydration, PHEMA transforms from the hard glassy material into the familiar soft hydrogel lens material. It wasn’t introduced as a commercial material until 1970, called polymacon. PHEMA is typical of other hydrogels and remains the most important single material of its type over a wide range of biomedical applications.

10

The UK market is half that of Japan & America but greater than most European markets.

3

false

8

Due to their affinity for water, the structure of PMMA

can be made more hydrophilic by the incorporation of

hydroxyl groups (OH).

In the late 1950s, Otto Wichterle and Drahoslav Lim

discovered this and invented the first soft lens monomer,

2-hydroxyethyl methacrylate (HEMA).

EWC

true

9

pmma

Not always, by adapting monomer combinations and cross-linking agents it is possible to produce high water content lenses with good stability.

11

12

BOZR

Equilibrium water content is most important in hydrogels

TD is also often of little use as...

• Soft lenses shrink when raised from room to eye temperature.
• Lenses of similar diameter can vary greatly in sagittal depth and because lenses flatten peripherally, sagittal depth can have a considerable effect on diameter.

The first polymer used to make contact lenses

was polymethyl methacrylate (PMMA),

a glassy thermoplastic material providing good optical clarity,

surface properties, ease of processing and sterilisation

compared to original glass lenses.

The primary function of hydrogel chemical groups is to attract and bind water within the structure.

29

8

At low levels of water content, all water within the hydrogel is tightly bound. A medium-level water content hydrogel will have tightly and loosely bound water, free water occurring in high water content lenses.

4

Most common...

Traditionally, BOZR is the main parameter modified to optimise lens fit, but it is actually of little use since...

lubricate the eyelashes

11

5

EWC is determined by the attraction of water and defined as:

Lens type: Soft (90% of those fitted internationally)

Material: Silicone hydrogel

Modality: Daily disposable

8

EWC (%) = weight of water/weight of hydrated gel x 100%

The tear film lubricates the cornea, conjunctiva and eyelids.

28

• Even thick soft lenses require a substantial change in BOZR to significantly affect lens movement.

However, it had the disadvantage of being

virtually impermeable to oxygen.

To achieve a specific water content, a range of monomers is chosen to balance those which are more hydrophilic with those which are less so.

8

material

• Lenses of similar BOZR but from different manufacturers can fit very differently due to differences in back surface design.

A cross-linking agent is also required to form a network with elastic stability. Increasing the density of cross-linking,

decreases water content.

Contact lenses are prescription medical devices which rest on the cornea to refract light rays and ensure their focus on the retina.

back vertex power

11

5

High WC lenses have poor stability.

True or False?

a) 8.7-28.7%

By changing the chemical nature of the polymer chain and their arrangement together, it is possible to manipulate a polymer’s physical properties.

• Low - high minus powers = little difference in fit

design

14

8

10

dry eye

Oxygen performance can be improved with:

• A looser arrangement of polymer chains = increased diffusion rate.
• Increased water content = increased diffusion rates and solubility.
• Decreased lens thickness = increased transmissibility.

As water content increases, refractive index decreases.

If there is microphase separation of water within the gel, regions of differing refractive index occur, affecting optical transmittance.

tear film

Plus lenses show more post-blink movement than minus.

True or False?

6

• Plus power = more post-blink movement

8

Worldwide prevalence

is between...?

a) 8.7-28.7%

b) 10.3-25.4%

c) 6.9-29.8%

Which of these is NOT

a function of the tear film?

• Provide a smooth refractive surface
• Lubricate the eyelashes
• Defence against pathogens
• Supply the cornea with nutrients

EWC affects:

density

optical transmittance

refractive index

oxygen permeability

mechanical strength

dimensional stability

comfort

contrast sensitivity

wettability

Practitioners have little control over soft contact lens design; although custom-made lenses are available they are relatively expensive.

Therefore, soft lens parameters rely on the manufacturer’s judgement and are specified in terms of....

Oxygen performance can be expressed in terms of:

oxygen permeability (Dk)

oxygen transmissibility (Dk/t)

oxygen flux

equivalent oxygen potential (EOP).

• Most contact lens polymers are made from monomers characterised by the existence of a carbon-carbon double bond, which opens to form a linked chain.

• The way in which the structural and functional groups interact with each other and their surrounding environment determines how polymer chains interact, entangle, behave and the polymer’s resultant properties.

13

When a hydrogel is placed on the eye, the anterior lens surface will progressively lose water, affecting surface properties of the hydrogel, particularly wettability.

False

Mechanical stability includes:

• tensile strength
• dimensional stability
• tear strength
• modulus of rigidity

8

12

centre thickness

Polymer chains rotate in response to a change in environment-hydrophilic chains rotating to the surface when in contact with aqueous fluids or burying within the gel when exposed to more hydrophobic interfaces e.g. air/lipids.

clinical

assessment

8

The following questions are based on the 7 subject areas covered by this CAL

A grey colour characterises a normal, uniform distribution.

Water-content dependent and relates to ease of handling and dehydration

5

All hydrogels have adequate wettability when fully hydrated, irrespective of initial water content. However, problems occur when the lens dehydrates, responds dynamically to air and lipids, leading to deposition of tear components and the onset of dryness symptoms.

• back optic zone radius

(BOZR or base curve)

• total diameter
• centre thickness
• back vertex power
• material

8

Interference pattern classification states that a grey pattern is more characteristic of lipid layer instability.

True or False?

11

classification

edge thickness & design

15

Wettability can be assessed by using the sessile drop technique or by using a captive air bubble to assess contact angle. The lower the contact angle, the more completely a liquid wets the lens surface.

16

Thicker edge = looser fit

Determined by chemical structure and water content.

In general, increasing water content reduces durability, particularly tear strength.

However, by adapting monomer combinations and cross-linking agents it is possible to produce high water content lenses with good stability and elasticity.

Polymers

5

However, most lenses are offered in a limited range of parameters e.g. two BOZRs in one diameter.

Edge design is dependent on manufacturing method...

5

BS EN ISO 18369-1: 2006 standard classifies each lens/material using a six-part code.

17

8

Lathing = thicker edge

(but greater range of parameters)

management

clide

Stem, filcon=WC > 10%

Moulding = extremely thin knife edges

Prefix=particular chemical formulation

• A polymer is a substance with a molecular structure built mainly or completely from a large number of single repeating units (monomers) bonded together.

• These monomers may be the same or if more than one type of repeating monomer unit is used, the structure is called a co-polymer.

Series suffix=revision of chemical formula

For marginal dry eye patients, are prism ballast or dynamic stabilisation the best choice of toric design?

nelfilcon A

Approximately 40% of contact lens wearers experience dryness.

True or False?

The Food and Drug Administration (FDA) classification system then categorises the material into one of four subsections...

17

Polyvinyl alcohol (PVA) confers greater tensile strength and higher WC than PHEMA, as well as less protein absorption

Soft lenses come in a wide range of modalities, however the rate at which contact lens performance is adversely affected is proven to be highly patient dependent.

true

19, 20

Group suffix (roman numeral)=

WC and ionic character

Dk range=oxygen permeability

7

Vinyl pyrrolidone (VP) and Methacrylic acid (MA) are commonly used to increase WC

18

Modification code 'm'=lens has a surface modification

Methyl methacrylate (MMA) is used to lower water content or to improve strength and rigidity

18

Introduction

27

Design

References

prism ballast

Oxygen is more soluble in silicone than water, meaning that as water content decreases, Dk increases. Consequently, all silicone hydrogel lenses provide significantly more corneal oxygen than hydrogels.

silicone hydrogels

Material

23

QUIZ

Introduced in the late 1990s, next major milestone in contact lens science since HEMA.

This was overcome either with the incorporation of more polar groups into the TRIS molecule or using specifically-developed macromers to confer particular properties on the final polymer.

Silicone hydrogels have been proven to reduce the chance of hypoxic signs, including corneal swelling and reduce/resolve chronic limbal hyperaemia and corneal neovascularisation.

The Tear Film

Management

8

Increased inferior thickness protects the PoLTF at the inferior cornea from depletion

23

Clinical Assessment

The first two silicone hydrogel lenses were:

Purevision (Bausch and Lomb) and Air Optix Night and Day (Cibavision)

balafilcon A lotrafilcon A

When a contact lens is applied, the pre-corneal tear film is divided into two components.

Dry Eye

Contact Lens Induced

Dry Eye

Silicon had successfully been incorporated into rigid lens materials (with a monomer commonly called TRIS) before but its hydrophobic nature posed more of a challenge for hydrogels. It is poorly miscible with hydrophilic monomers, leading to opaque, phase-separated materials.

32

31

The pre-lens tear film (PLTF) acts like the pre-corneal tear film, protecting the lens surface from drying and deposition, it is therefore most important in wearing comfort and stable vision.

17

44

30

It can be assessed by assessing tear film quality or NIBUT.

Both these lenses must be treated to ensure their hydrophobic surface is suitable for wear. However, despite treatment it is generally accepted that the hydrophobic interaction of silicone with the tear film gives these lenses comparatively inferior wettabililty to conventional hydrogels.

17

14

The tear film is the

most anterior structure of the eye.

During a blink, lid closure proceeds from the temporal to nasal side of the eye, distributing tears evenly over the ocular surface and allowing drainage through the lacrimal puncta.

24

Newer materials (galyfilcon A and senofilcon A) don’t rely on surface treatments to improve wettability but have moisture-rich wetting agents e.g. PVP contained within the material.

21

15

What makes contact lens wear successful?

21

Lotrafilcon A has a contact angle > 40°degrees, but newer materials e.g. delefilcon A have an angle < 10°degrees.

most common:

The post-lens tear film (PoLTF) lubricates the rear lens surface, provided adequate tear exchange occurs. This can be assessed using fluorescein.

A thinner PoLTF is associated with poor tear exchange which can lead to a build-up of debris and metabolic by-products between the lens and cornea, potentially compromising corneal integrity.

1.

Early silicone hydrogel lenses had high moduli, making them easier to handle and more durable, but also causing mechanical irritation when worn on an extended wear basis. Newer lenses have a lower modulus.

44

22

21

The tear film has 7 major functions

A harmonious relationship between the lens material, tear film and cornea

• Daily disposable material: conventional hydrogel
• Material overall: silicone hydrogel

Research shows that contact lenses can cause a decrease in the number of functional meibomian glands proportional to lens wear duration.

Lens dehydration decreases wettability and comfort, altering lens parameters and compromising the corneal epithelial surface

25

44

53

51

42

13

So how do lenses cause dry eye?

• When a lens is placed on the eye evaporation occurs from the front surface, lowering water content.

• This evaporation occurs during the time in the blink cycle when the tear film is broken, but upon its reformation, the hydrogel takes up water.

A contact lens sits within the aqueous layer, making the PLTF thinner than the pre-corneal, resulting in a very thin if not absent lipid layer.

18

• When the amount of water lost through evaporation is equal to that absorbed by the lens no further time-averaged water loss occurs.

54

and...

1) Maintain an optically smooth refractive surface

44

This reduces tear film stability, causing dry spot formation and rupture. Increased PLTF evaporation occurs, followed by lens dehydration and thinning of the PoLTF due to lens absorption.

water content

35

6) Remove corneal and conjunctival foreign bodies

A high water content lens has to lose more water than one of a lower water content before equilibrium is reached.

Contact lens wear has also been shown to cause tear film hyperosmolarity (due to higher evaporation rates of the PLTF), changes in tear film mucins, electrolytes and tear proteins.

54

The most common method of CLIDE management is to refit with a different lens; either one with a more frequent replacement schedule or a different lens material of the same modality

how to Predict clide

using fda?

20

This is thought to be the mechanism of contact lens induced dry eye (CLIDE)

The tri-laminar structure of the tear film was first identified by Wolff in 1954; consisting of the innermost mucous, aqueous and outermost lipid layers.

Normal tear film dynamics is essential for the health and function of the eye; specifically...

the adequate production of tears,

retention on the ocular surface,

and subsequent elimination.

A thicker pre-corneal tear film before lens fitting is more likely to form a continuous lipid layer during lens wear, than one which is already thinned.

Consequently, symptom-free patients with marginal deficiencies are likely to be symptomatic if contact lenses are fitted.

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55, 56, 57

44

Consequently, it has been found that lenses of lower WC will remain hydrated to a reasonable extent for a longer period than those of higher WC.

Dehydration at the anterior surface is less likely to reach the cornea-lens interface in the case of a low WC lens, more corneal punctate staining is seen with high WC lenses.

The chance of dryness for group II and IV materials is 2 to 3 times more likely than those in group I

Tests focus on

assessing...

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50

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• tear film stability
• tear secretion rate
• ocular surface disease

2) Lubricate the cornea and conjunctiva

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7) Defence against pathogens via specific

and nonspecific antibacterial substances.

In addition...

is dehydration the

answer?

5) Supply the cornea with nutrients and remove by-products of metabolism from the corneal surface

It is proven that corneal staining is more generally associated with contact lens factors (i.e. material and water content) as opposed to care solution, medical, demographic or other ocular surface factors.

21

All lenses have the potential to cause dryness symptoms so management is essential.

After all, approximately 40% of lens wearers have dry eye and it is the principle reason for ceasing contact lens wear.

Although some studies have found a correlation between water content, dehydration and discomfort, others have found that increasing water content increases the free water content and ratio of free-to-bound water, but it is not possible to use this as a predictor of hydrogel dehydration in vivo.

Dry eye assessment is essential in clinic practice

16

18

42

Decreased corneal sensitivity is associated with contact lens wear

Decreased tear secretion and reduced blink rate.

DRYING

Consequently, other factors must be involved in the relationship between water content and CLIDE...

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52

...but what

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Compromised lipid quality/quantity predisposes to KCS, particularly true in meibomian gland dysfunction. Decreased meibum secretion causes thinning of the lipid layer and evaporation of underlying aqueous.

Infrequent/incomplete blinking has many causes, including proptosis, facial palsies and prolonged near tasks (particularly VDU work) and leads to KCS since reduced tear elimination and clearance results in increased residency time of tear film components (e.g. toxic cell waste products) which may activate or potentiate ocular surface inflammation.

33

Ciba Vision’s new Dailies Total 1 silicone hydrogel lenses employ a unique water gradient technology that provides excellent wettability and improved tear film surface quality compared to its predecessor.

lubrication

• 0.25-0.3mm is considered normal
• Dry eye patients have a thinner tear film, reflected in a reduced tear meniscus height.

Frothing in the tear film or at the eyelid margin can indicate meibomian gland dysfunction.

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33

45

But what do patients complain of?

25

24

25

4) Allow leukocytes access to the cornea and conjunctiva

17

67

3) Lubricate the eyelids

8

7

4

Aqueous Layer Deficiency

22

Non-invasive, direct methods...

Blepharitis often co-exists with KCS since blepharitis provides antigenic and proinflammatory substances, adversely influencing lipid production.

i.e. an under-production or excessive loss of tears

• The meibomian glands in the tarsal plate of the eyelid produce meibum which forms the lipid layer, composed of polar and nonpolar lipids, nonpolar esters and wax.

• The polar section, composed mainly of phospholipids aligns over the aqueous layer, leaving the nonpolar section more superficial.

• The meibomian lipids are well adapted to their function; forming a thin, smooth film, with a thickness and composition that prevents evaporation of underlying aqueous.

25

33

High water content materials have been

associated with considerable tear film deposition;

• group II lenses more prone to lipid deposits
• and group IV lenses more likely to attract protein species.

Resultant changes to the PLTF lipid layer and the altered contact lens surface lead to increased evaporation and reduced wettability.

The effect of deposition also explains why frequent lens replacement, thorough hygiene and appropriate disinfecting solution are desirable for dry eye patients.

• Tear meniscus height
• Tear film quality
• Lipid layer
• Tear break up time (TBUT)

To enhance lens wettability,

PVA and PVP (widely used in artificial tears) have been

incorporated into lens materials.

Ciba Vision Focus Dailies lenses are composed of a cross-linked functionalised PVA macromer with the addition of non-functionalised PVA that can elute from the lens into the tear film with each blink. PVA is a successful tear film stabiliser, improving lens comfort by decreasing the surface tension of tears or mimicking tear film mucin.

Johnson and Johnson 1 Day Acuvue lenses have PVP locked into the lens matrix which acts as a hydrophilic humectant, attracting and retaining moisture to ensure all-day hydration and to decrease the coefficient of friction of the lens surface.

36

• It may be classified by the presence or absence of Sjögren’s syndrome, an autoimmune exocrinopathy affecting the lacrimal and salivary glands. Non-sjögren’s may be caused by congenital alacrima and different types of acquired lacrimal deficiency.

58

33

17, 66

Disruption of this balanced equilibrium can lead to Dry Eye Syndrome.

24

27

• Tear interferometry can be used to assess lipid layer thickness and fluidity.

• Interference pattern classification means that a grey colour characterises a normal, uniform distribution.

• An increase in colours observed implies a more non-uniform distribution and no lipid layer pattern at all means the cornea is partially exposed.

• Since most aqueous production is a reflex response, decreased surface sensitivity (caused by ocular surface inflammation, contact lenses, corneal surgery, diabetes and herpatic keratitis) must be related to a decrease in lacrimal secretion.

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33

• Tear film thickness is estimated to be between 34-45μmicrometers by laser interferometry. This value is larger than that obtained by earlier non-optical methods and can be explained by their probable exclusion of the mucous layer with these earlier techniques.
• Reflection spectra suggests a thickness of approximately 3μmicrometers, and this value is similar to that of Fluctuation Analysis by Spatial Image Correlation (FASIC), namely 3.05μmicrometers.
• Consequently, the exact value is debatable.
• The tear film has a pH of 7.45 and osmolarity of 300-305mOsm/kg.

6

59

28

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• Topical and systemic medications are also a known cause of decreased tear production e.g. anti-histamines, beta-blockers and anti-depressants.

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34

33

60

29

68

Dryness

Grittiness

Burning

Particles and debris consisting of lipid-contaminated mucin in the tear film that move with blinking are characteristic of the affected tear film quality in dry eye.

Lens fit must involve adequate tear exchange and full corneal coverage. Smaller diameter lenses allow substantially better tear mixing. A prism ballast lens design has been found to be preferred for marginal dry eye patients because the increased inferior thickness protects the PoLTF at the inferior cornea from depletion, especially with incomplete blinking.

prevalence

Non-invasive, indirect methods...

30

Instability may be caused by insufficient tear production or excessive tear evaporation, however these often co-exist.

5

36

Research indicates that low water content materials should be chosen over high water content for CLIDE patients, even though it is still debatable whether their desirable properties are due to decreased dehydration or deposition. However, the proportion of low water content fits has decreased significantly since 1996, partly due to their decreased Dk/t, fabrication methods, costs, disposability and the increased popularity of silicone hydrogels.

25

• Symptom questionnaires
• Lid-parallel conjunctival folds (LIPCOF)

69

37, 38, 39, 40

33

70

• The lacrimal and accessory glands of the lid express the watery constituent which composes the aqueous layer and which contains 98% water, allowing lubrication and protection of the ocular surface.

• Antibacterial protection is ensured by the presence of lactoferrin, lysozyme, immunoglobulins, albumin, transferrin, ceruloplasmin, tear specific prealbumin, and glycoproteins.

• The aqueous layer also provides the avascular cornea with nutrients, including glucose, oxygen, protein and inorganic salts.

After initial diagnosis it may be necessary to cease lens wear or reduce wearing time allowing the eye to recover. In extreme cases of corneal drying a large diameter bandage soft contact lens approved for extended wear has been proven as an effective treatment.

Stringy discharge

Transient blurred vision

Lid crusting/redness

1, 60

25

36

so are silcone hydrogels the answer?

3

59, 71

All three layers of the tear film are essential for tear film stability

41

Estimates vary between

8.7-28.7% across the world.

Higher with increasing age

and higher in females.

But there is often a limited correlation between symptoms and signs

25

invasive, indirect methods...

Risk factors: contact lens wear, smoking, diabetes, thyroid disease, caffeine and multivitamin use.

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• Ocular surface staining
• Lid wiper epitheliopathy (LWE)

Dry eye is also known as keratoconjunctivitis sicca (KCS), and refers to a wide spectrum of ocular surface disorders with diverse and often multiple aetiologies. However, tear film instability appears to be characteristic of all dry eye states irrespective of aetiology.

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33

60

34

invasive, direct methods...

• The mucous section is mainly formed by secretory mucins synthesised by conjunctival goblet cells, the lacrimal and accessory glands, but also from membrane bound mucins from conjunctival and corneal epithelial cells.

• The mucoglycoproteinic structure of the mucous layer makes it hydrophilic, ensuring attachment of the aqueous film to the underlying hydrophobic cornea.

• It lubricates and protects the cornea, preventing desiccation and bacterial contamination

19

• Schirmer test
• Phenol red thread test
• TBUT

61, 62, 63, 64

30

• During the first 1-2 weeks of switching from conventional to silicone hydrogels initial discomfort is common due to mechanical irritation associated with modulus, edge thickness and lens design.
• Studies using longer test periods concluded a significant reduction in the frequency and severity of dryness symptoms and a reduction in corneal staining after re-fitting with silicone hydrogels.
• Silicone hydrogels have been found to attract less protein deposits as compared to conventional hydrogels, improving wettability and decreasing friction.
• It has been reported that silicone hydrogels dehydrate to a lesser extent and more slowly than conventional hydrogels.

The most commonly used diagnostic techniques...

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60

TBUT

Symptom questionnaires

Corneal staining

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In conclusion, many different methods of CLIDE management exist and it is important to remember that dry eye is multifactorial, influenced by patient-specific (e.g. blinking) and environmental factors. Therefore, the exact degree of success is unpredictable and patients may require regular monitoring and combined therapy (although it is important to change one parameter at a time) e.g. change of lens material and the use of artificial tears.

35, 65

Hyperosmolarity activates intracellular stress kinases in the epithelial cells of the ocular surface.

This causes ocular surface inflammation, resulting in a self-perpetuating cycle of deterioration; becoming the cause and consequence of cell damage.

Hyperosmolarity also adversely affects normal apoptic control in cell renewal, mucin structure and function and lipid-tear film interactions.

33

Tear break up time (or tear thinning time) refers to the time taken for minimum tear film height to reach the effective range of dewetting intermolecular forces, causing it to rupture.

It is measured non-invasively (NIBUT) using a grid pattern or invasively following fluorescein instillation; however fluorescein has been found to also reduce stability causing a potential underestimation.

When a 10 second cut off point is used as the abnormal/normal border it has a sensitivity of 82% and specificity of 86%.

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48

• Ocular surface damage is detected by surface staining.
• Fluorescein stains damaged epithelial cells (superficial epithelial punctate staining often found in the inferior cornea, called smile staining)
• Rose bengal stains dead epithelial cells and mucous.

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35

Diffuse superficial epithelial punctate corneal staining observed using fluorescein