Contact lens ppt should be learned by all

Editor's Notes

• #7: Maintain stable dimensions. Be durable when handled by wearers. Be transparent with minimal light loss. Be optically regular so its optics are predictable. Have physical properties which allow the creation and retention of high quality surfaces. Require minimal maintenance by wearer. Be easy to fabricate lenses from.
• #8: Characterizing a Material Manufacturers rely on in vitro data because it can be obtained readily. However: Tests are often over-simplifications of the real situation. Testing procedures often vary between manufacturers, and there are few standards. Tests frequently do not reflect the actual clinical situations relevant to the specification being determined.
• #9: Important Material Properties Oxygen permeability is a material property and not a lens property. Good wettability is necessary for long-term lens tolerance. Scratch resistance is essential to the maintenance of good optical surface properties. Rigidity (rigid lenses) is a key determinant of the minimum lens thickness necessary to resist lens warpage on the eye, particularly if the cornea is astigmatic. Material must be stable if lens parameters are to remain as manufactured. For comfort, good vision and minimal adverse responses, the lens must resist deposits. The lens should withstand normal handling and wearing, i.e. not break easily. Flexibility (soft lenses) is also a key factor and should allow the lens to conform to the ocular surface.                                                    
• #10: Permeability to Oxygen One of the most important properties of a contact lens material is its permeability to oxygen (Dk). This property is an inherent material property (like specific gravity or refractive index). It is not a function of lens thickness, shape or back vertex power (BVP).
• #11: Oxygen Transmissibility Oxygen transmissibility, Dk/t, is the Dk of the material (its permeability) divided by the lens thickness. The thickness t may be tc (geometric centre thickness) or tLocal depending on the transmissibility being calculated. D = diffusion coefficient of the material. k = solubility of the gas in the material.
• #12: Measuring Oxygen Transmissibility The JDF - Dk1000™ Coulometric Oxygen Permeation Instrument is illustrated (left). It is one of several types of instruments used to measure the oxygen transmissibility (Dk/t) of contact lenses. This device uses the coulometric technique. The lens to be tested is mounted in an environment-controlled cell. Data is fed to either a chart recorder (as in this illustration) or a computer (data logger). Dk is determined indirectly from Dk/t and thickness measurements. For further information on the coulometric and polarographic techniques, see Fat, I (1971) and Winterton et al. (1987).
• #13: A Comparison of Dk Measuring Techniques The table compares polarographic (with and without correction (cor) factors for edge effects, boundary layers, etc.) and coulometric techniques of Dk determination for several rigid gas permeable (RGP) materials. Note that for most materials, the corrected polarographic and coulometric results are in fair agreement. At higher Dks the differences between the measuring techniques become more apparent, and for high Dk (100) materials, coulometric results are more accurate. RGP results are presented here because RGP permeabilities (Dks) cover a much wider range than hydrogels. Theoretically, a volumetric system can also be used. However it is not in current use.
• #14: Oxygen Transmissibility In vitro measurements of oxygen transmissibility are used to determine the Dk in a laboratory. The Dk/t is calculated from measured thickness. In vivo, there are three main indirect methods which infer the oxygen transmissibility: Corneal swelling after overnight wear. EOP or the Equivalent Oxygen Percentage. EOP states the oxygen concentration of a gas mixture (the balance is nitrogen and water vapour) which produces a corneal response equivalent to that resulting from wearing the contact lens. Corneal oxygen demand following lens removal. The oxygen demand is measured immediately after lens removal. The demand is directly related to the oxygen tension that existed under the lens and any oxygen debt lens wear may have produced.
• #15: EOP The EOP may be either static (non-blinking) or dynamic (normal blinking). Dynamic EOPs for both rigid and soft lenses are approximately 2 to 3% higher than static values (Efron, 1991). EOP may be determined using either rabbit or human eyes. It is dependent on the lens thickness used and the altitude of the laboratory. EOP determination is a two-stage procedure. Using gas mixtures and air, calibrate the cornea’s demand over five minutes, without a lens. This procedure establishes a series of curves of corneal oxygen demand versus the oxygen concentration of the atmosphere provided (usually via close-fitting goggles). The gases are humidified to prevent corneal dessication. Measure the cornea’s oxygen demand after five minutes of lens wear and compare it with the calibration data. The lens is worn for five minutes in air and the oxygen demand then ascertained. By comparing this data with that determined during calibration, an estimate of the equivalent oxygen level available under the lens, in vivo, can be made.
• #16: The slide illustrates the EOPs under various types of lenses and materials. The lenses used represented typical configurations, particularly with regard to tc and BVP.
• #17: Low Oxygen Transmissibility Can Result in Corneal Changes The following corneal conditions are indicative of inadequate oxygen transmissibility of a contact lens: Epithelial microcysts. Thought to be spherules of disorganized cellular growth, necrotic tissue and cellular debris which accumulate between epithelial cells and which probably contain metabolic by-products. The number of microcysts seen increases during extended wear, particularly if the lenses have a low oxygen transmissibility (Dk/t). As the Dk/t of lenses increases, the incidence of microcysts decreases in both daily and extended wear (see Brown, 1971, Zantos, 1981, Holden and Sweeney, 1991). Polymegethism. An increase in the range of endothelial cell sizes believed to be a result of hypoxia. Corneal pH. An acidic shift results from carbon dioxide retention. Oedema. Oedema-induced reduction in the efficacy of the endothelial pump results in fluid retention and swelling of the cornea. Endothelial blebs. Transient changes in the appearance of the endothelial mosaic thought to be due to hypoxia-induced pH changes in the cornea.
• #18: Preventing Oedema We know that oxygen is required for normal corneal metabolism. However, it has not been established with certainty just how much is actually required. Opinions vary widely. The following figures from Holden and Mertz, 1984 are widely accepted as being a useful guide: Dk/t = 24 for Daily Wear (DW). Dk/t = 87 for Extended Wear (EW). These values were determined using contact lenses as the test stimuli.
• #19: Correlation of Oedema with Measured Dk/t This slide shows data from studies of overnight corneal oedema with RGP and soft lenses of various oxygen transmissibilities (Dk/ts). It can be seen that the lens types had little influence on the results. However, the oxygen transmissibilities had significant effects on the oedema levels measured.
• #21: Carbon Dioxide Permeability of Lens Materials The ratios of carbon dioxide permeability to oxygen permeability for the main material categories are: 21:1 for hydrogels. 7:1 for rigid gas permeable lens materials. 8:1 for silicone elastomers materials. (Ang, Efron, 1989)
• #22: RGPs - Better Physiologically than Soft Contact Lenses (SCLs)? RGP materials have a higher Dk than hydrogels, in fact, some have a higher Dk than 100% water - the hypothetical limit of conventional hydrogel technology. Significantly less than the total corneal area is covered by an RGP lens, and the uncovered part of the cornea is able to respire approximately normally. The tear pump under an RGP lens is a minor contributor to corneal oxygenation. It has been shown that little tear mixing or exchange occurs under an SCL. Our knowledge of how contact lenses affect corneal metabolism is incomplete. It has already been established that the carbon dioxide transmissibility (Dk CO2) is greater for hydrogels than RGPs, but the clinical significance of this is not yet established fully.                                                                                
• #23: Wettability In vitro tests: Sessile drop: (Water-in-Air) A drop of pure water is placed on the test surface. The angle between the tangent to the drop’s surface at the point of contact and the horizontal test surface ( {theta}) is measured (see slide 24). a zero angle = completely wettable a low angle = somewhat wettable a large angle (especially > 90°) = poorly wettable. When the bubble is expanded by adding more water, the advancing angle is determined. By withdrawing some water, the drop decreases in size and the receding angle can be measured. Receding angles are usually smaller (indicating better wetting), because the angle involves surfaces previously wetted (slide 25).                                                     Wilhelmy plate: A flat sample of the test material is lowered into water. This allows assessment of the advancing angle to be made, using a method of measurement similar to the sessile drop technique. Similarly, after withdrawing the sample a little, the receding angle can be measured. Again the receding angle is smaller.           Captive bubble: An air bubble is introduced under a lens, convex side down in a wet-cell. The bubble floating against the underside of an immersed lens is viewed in profile, and contact angles measured. By increasing or decreasing bubble size the advancing or receding angles respectively can be measured. Note that in this method the receding angle is greater (i.e. the reverse of the other methods) because previously unwetted surfaces are involved.       In vivo tests: Tear coverage: Assessment of the ability of tears to form a complete film over the lens surface. An incomplete tear film is illustrated in slide 23. Break Up Time (BUT): An assessment of the ability of a lens to retain a complete tear film. Even if a complete tear film forms and is retained, the aqueous component evaporates and the lipid layer diffuses into the aqueous layer. Eventually, lipids contaminate the mucous layer rendering it hydrophobic. This results in a local break in the tear film. The time from the cessation of blinking to the first appearance of a break in the tear film is measured. Very short times indicate poor wettability.        
• #26: Wilhelmy plate: A flat sample of the test material is lowered into water. This allows assessment of the advancing angle to be made, using a method of measurement similar to the sessile drop technique. Similarly, after withdrawing the sample a little, the receding angle can be measured. Again the receding angle is smaller.
• #27: Captive bubble: An air bubble is introduced under a lens, convex side down in a wet-cell. The bubble floating against the underside of an immersed lens is viewed in profile, and contact angles measured. By increasing or decreasing bubble size the advancing or receding angles respectively can be measured. Note that in this method the receding angle is greater (i.e. the reverse of the other methods) because previously unwetted surfaces are involved.
• #28: In vivo tests: Tear coverage: Assessment of the ability of tears to form a complete film over the lens surface. An incomplete tear film is illustrated in slide 28. Break Up Time (BUT): An assessment of the ability of a lens to retain a complete tear film. Even if a complete tear film forms and is retained, the aqueous component evaporates and the lipid layer diffuses into the aqueous layer. Eventually, lipids contaminate the mucous layer rendering it hydrophobic. This results in a local break in the tear film. The time from the cessation of blinking to the first appearance of a break in the tear film is measured. Very short times indicate poor wettability.
• #29: Flexibility In vitro tests: Rigidity: The force required to produce a pre-determined deformation (bend) of a standard sample mounted in a prescribed manner. Cornea and Contact Lens Research Unit (CCLRU) method: A lens sample of a standardized design and parameters is loaded across its diameter. The ‘force versus % change in diameter’ curve is determined. This is done using a Vitrodyne™ apparatus (see illustration). Within its glass, controlled-environment chamber, precise loads can be applied pneumatically, and the change in diameter measured accurately by a linear displacement transducer. The detailed image (slide 31) shows the metal bellows which apply the load, the displacement transducer immediately behind them and the jaws with a lens mounted between. The apparatus can be used for both soft and RGP lenses. In vivo test: Residual astigmatism: The more rigid a lens material is, the less it will conform to the shape of the cornea. In the case of corneal astigmatism, a more rigid lens will flex less or not at all, and therefore no significant residual astigmatism will be induced.
• #32: What Other Properties Do We Require From a Contact Lens Material? Optical quality: material must be transparent with little light transmission loss material must be optically homogeneous, i.e. its refractive index should not be subject to regional variation unless such variation is intentional and well controlled. All lens materials must be biocompatible since they are in intimate contact with a physically and physiologically sensitive organ for extended periods of time. In particular, the material should contain virtually no leachable unreacted chemical components which may affect the cornea and/or conjunctiva. From a manufacturer’s point of view, ease of manufacture is essential. Use of difficult materials will be adversely reflected in the cost, the reliability of the finished product and the responsiveness of the manufacturer to requests to use it.
• #33: Optical Properties Refractive index (n): This inherent property of a material ultimately governs the practical thickness of the fabricated lens. Generally, a higher refractive index is better, provided it does not mean other aspects, e.g. specific gravity, offset any gains. Spectral transmission should be uniform across the visible spectrum so as not to result in any changes in colour perception due to selective absorption of particular wavelengths or bands of wavelengths. Tints, including light handling tints, may alter the wearer’s colour perception. Dispersion refers to the differences in refractive index for each wavelength of light. The principle refractive index is that measured for the green mercury line (546.07 nm). The principle dispersion is the refractive index difference demonstrated by a material for blue (479.99 nm) and for red (643.85 nm) light. Chromatic aberrations are affected by dispersion. The greater the dispersion the greater the chromatic aberrations (longitudinal and transverse). Any optical inhomogeneity, translucency or opacity in a lens material will result in the scattering of light. This in turn may result in haze, veiling glare and light loss when a lens made from such a material is worn.  
• #34: Biocompatibility Contact lenses materials should: Be inert, i.e. they should not react with, or cause other materials to react with, the eye tissues, the tears or lens care products with which they come in contact. Not take part in any enzymatic, activity or catalyse reactions between themselves and/or other chemical species. Contain no leachables, especially hydrogels, since the movement of water through a polymer is potentially a vehicle for the transfer of undesirable materials from inside the lens to the external eye. Common leachables include unreacted monomers, cross-linking agents, unbound or unprecipitated tinting chemicals, hydration accelerators and other chemicals used in lens manufacture. Not be selectively absorbing of metabolites, toxins, micro-organisms and other substances present in the environment. Not exhibit excessive electrophoresis which may result in the selective absorption, deposition or separation of chemical or biochemical entities from the environment. Exhibit low friction in situ. The material should be capable of accepting and retaining a good surface finish which, when wetted, will exhibit low friction. This will allow smooth lens movement on the eye and safe digital rubbing as part of the care regimen. Be electrically compatible. A lens material should not disturb the cornea’s surface electrical properties (surface is negative with respect to the posterior cornea, with the potential difference believed to be in the range 20-40 mV). Generally, RGP lenses have a greater effect on the transcorneal potential than soft lenses. Not induce inflammatory or immunological responses in the anterior eye, even after prolonged exposure as in extended wear. These requirements also apply to other materials used in lens manufacture which could find their way into a wearer’s eye.                
• #35: Ease of Manufacture For a contact lens material to be considered easy to work with during the manufacturing process it should: Be homogeneous. Contact lens materials must be homogenous if the final product, especially its surface quality, is to be predictable and the resultant product reproducible. Have consistent mechanical properties. A material’s mechanical properties are an important determinant of its behaviour during manufacture. For the lenses produced to be consistent, the variation in mechanical properties between individual batches of material must be small. Be stress-free and dimensionally stable. If lens materials, especially in the button form, are delivered with internal stresses, it is probable that these will be relieved at some stage of the manufacturing process. This can lead to distortions or other shape irregularities in the finished product. Should a material exhibit dimensional instability, then alterations in the lens shape or dimensions can occur at any stage of manufacture or after. Such alteration may occur rapidly or slowly, the latter making it very difficult to pinpoint whether the cause lies with a material property, manufacturing procedure or storage system. Be durable and resist local heating. A material must be able to withstand the rigours of manufacturing, particularly the curve cutting and polishing steps. The latter generates significant localized heating which may affect the surface quality and/or the surface properties of the end product. Be easy to polish/retain surface finish. Regardless of the material or the manufacturing method, the material must readily accept and retain a quality surface finish. This is especially true when the lens expands significantly during hydration. In this case, a quality surface generally requires a surface that expands uniformly. Materials used for molded lenses must also be capable of accepting and retaining the surface quality imparted to them by molds which have a high quality finish. Have predictable hydration characteristics. Regardless of whether the lens is rigid or soft, a material’s behaviour at the hydration step is of paramount importance. This is especially so for soft lenses formed from xerogel buttons (material is anhydrous or in the ‘dry’ state). Batch-to-batch consistency, especially in terms of expansion on hydration, is perhaps the greatest determinant of reproducibility of a lens series using the same material. To achieve the required levels of reproducibility, manufacturers often purchase large quantities of a single batch of lens material, then study and characterize the batch carefully before passing it to manufacturing.                                                                            
• #37: Poly(Methyl MethAcrylate) (PMMA) Patented: 1934-Nov-16 by ICI (UK). Used in contact lenses late ‘30s (Feinbloom 1936, Mullen and Obrig, 1938). Readily machined and polished. Fairly wettable when clean. Easy to care for. Rigid. 0.2 - 0.5% water when hydrated fully. Almost zero oxygen permeability. Produces ‘spectacle blur’ and in the long-term, polymegethism and/or ‘corneal exhaustion syndrome’.
• #38: Rigid Gas Permeable Materials Early attempts to replace PMMA: Once the shortcomings of PMMA as a contact lens material were understood, attempts were made to find better materials. Early attempts included: Cellulose Acetate Butyrate (CAB): An engineering plastic which scratched readily, had a low Dk and was difficult to lathe. However it was relatively wettable. Siloxane Acrylates (SAs): Patented in early 1970s. Eventually led to successful materials from many manufacturers. Variations are still in use. t-Butyl Styrene: A novel material which combined high refractive index and low specific gravity - both desirable properties. However, the surface was prone to scratching and some solutions were reported to reduce lens wettability.      
• #39: Rigid Gas Permeable Materials CAB Introduced by Eastman, mid 1930s and used in haptic shells by Teissler, 1937. Not used in RGP corneal contact lenses until 1972/73 (RX-56 lenses by Rynco Scientific Corp, see Stahl et al, 1974). Slightly flexible, water resistant. Can be molded or lathed. Well tolerated, possibly because hydroxyl groups result in 2% water content and reasonable wettability. Material stability lower than PMMA. Dk in range 4 - 8 (i.e. from half to approximately the same as polyhydroxyethyl methacrylate (HEMA or PHEMA)). Incompatible with the preservative benzalkonium chloride.
• #41: Siloxane Acrylates Properties: The era of successful RGP materials was heralded by the introduction of Polycontm, the original siloxane acrylate material, in the late 1970s (patented 1974). A PMMA backbone gives the material its dominant physical properties, especially rigidity. Si-O-Si bond is flexible and extensible. This results in significant increases in oxygen permeability but a reduction in material rigidity. Dks in low to medium range are achievable. A wetting agent may be incorporated to enhance lens wettability, usually methacrylic acid (MA). Material chemistry results in a net negative charge on the lens surface, especially if methacrylic acid is used as the wetting agent.
• #42: Siloxane Acrylates (SAS) Advantages: SAs have higher oxygen permeabilities than all the lens materials which preceded their development. At the time of their introduction this resulted in improved corneal physiology. The lower rigidity of SAs allows lenses to conform more closely to the shapes of the corneas on which they are placed. This reduces the likelihood of the lenses being displaced from the cornea during normal use. By offering improved physiology, and to a lesser extent lower rigidity, larger diameter lenses could be fitted. This allowed the optic zone diameter to be increased, thereby overcoming some of the problems of smaller optic zones, especially in low light levels.
• #43: Siloxane Acrylates Disadvantages: Surface charge and surface chemistry make them more deposit prone. SA materials have relatively ‘soft’ surfaces, hence they scratch more readily. SA materials are relatively brittle and are cracked or broken more easily. Some SA materials, when used in conjunction with particular lens care products, have been known to ‘craze’. This phenomenon is possibly the result of induced internal stresses being relieved, leading to surface and matrix failure. Low rigidity allows the lens to conform to corneal shape, reducing the completeness of correction of corneal astigmatism by a simple spherical lens. Flexure problems with SA materials of relatively low rigidity are possibly related to the siloxane content of the material. SA lens parameters may be influenced by age, their environment, lens care products and the stresses placed on them by storage cases or astigmatic eyes. Lenses may recover slowly, incompletely or not at all. Unknown factors during lens fabrication may also affect lens parameters.
• #46: Fluoro-Siloxane Acrylates (FSAs) The FSA materials were developed as a result of efforts to further increase the Dk of RGP materials and to increase resistance to surface deposition.  
• #47: Fluoro-Siloxane Acrylates Properties: The element fluorine (F) is added to basic SA chemistry to enhance O2 permeability. A lower surface charge results. Some materials may wet a little better. Some materials may resist deposits more. However the allusion to TeflonTM-like (poly(tetrafluoroethylene) or (PTFE)) properties cannot be justified.
• #48: Dks of 40 - 100 or more are achievable. Dks are high enough for extended wear to be a possibility. FSAs are generally more flexible than SAs. Surfaces are relatively easily scratched.
• #50: Perfluoroethers The 3M perfluoroether lens material is novel and in a distinct material category. It should not be confused with FSAs. Production of the Adventtm lens made from this material has been discontinued. A perfluoroether consists of: Fluorine. Oxygen. Carbon. Hydrogen. The fluorofocon A material consists of: Perfluoroether. PVP (poly(vinyl pyrrolidine)). MMA (methyl methacrylate).
• #51: Perfluoroethers Advantages: High Dk, potentially sufficient to support extended wear. No surface charge, thus reducing the likelihood of lens spoilage. High flexibility results in conformity to the corneal shape in situ. This results in stable vision and possibly greater comfort. However, this conformity can also be a disadvantage.
• #52: Perfluoroethers Disadvantages: A low refractive index means a thicker lens for a given prescription. High specific gravity means a heavier lens for a given prescription. These first two points constitute a significant disadvantage. Together they mean a heavier, thicker lens, the reverse of what is desired. The success rate during manufacture is lower than for other materials and the cost of each lens is higher. Manufacture of this lens type is therefore more expensive. The wettability of the fluorocarbon lens surface is only average. The on-eye flexibility and conformity reduces the correction of cornea-induced astigmatism by a spherical contact lens.  
• #53: Perfluoroethers Disadvantages: A low refractive index means a thicker lens for a given prescription. High specific gravity means a heavier lens for a given prescription. These first two points constitute a significant disadvantage. Together they mean a heavier, thicker lens, the reverse of what is desired. The success rate during manufacture is lower than for other materials and the cost of each lens is higher. Manufacture of this lens type is therefore more expensive. The wettability of the fluorocarbon lens surface is only average. The on-eye flexibility and conformity reduces the correction of cornea-induced astigmatism by a spherical contact lens.  
• #56: RGP Manufacturing Care is required during RGP contact lens manufacture, particularly in the choice of compounds which come in contact with the lens blank. This is especially true when blocking and solvent cleaning the button or lens. Care must also be taken with cutting and polishing procedures so as not to produce localized heating of the lens blank, as this may alter the surface properties of the finished lens.
• #57: RGP Manufacturing Poor wettability may be associated with: Over-polishing which may result in localised heating of the lens blank surface with a subsequent alteration of its properties (Walker, 1989). Either alone or in combination with the first point, the use of the incorrect solvent or the incorrect use of a solvent may also adversely affect the wettability of the finished product (Hogg, 1995).
• #58: Fluoro-Siloxane Acrylates and Siloxane Acrylates Manufacturing disadvantages: Have surfaces which are generally ‘softer’ than PMMA, i.e. they scratch more easily. Are more difficult to finish with a highly polished surface. Are more prone to surface ‘burning’ during manufacture. Are more susceptible to solvent damage during manufacture.
• #59: The back optic zone radius (BOZR) of finished lenses has been known to change over time, especially in high minus BVPs. The more exotic materials of high Dk are often difficult to modify, especially in the contact lens practice. The reproducibility of lenses fabricated in these materials is lower than that of less sophisticated materials, especially PMMA and low Dk SAs.
• #60: The manufacturing process is more difficult and requires more care. More sophisticated equipment is required to manufacture lenses from these materials. The combination of manufacturing difficulties and the need for more sophisticated equipment adds to the cost of production using these materials. A lower success rate during manufacture leads to lower production yields. FSAs<SAs<PMMA.
• #62: RGPs Lens Fabrication Techniques Lathing. This is the original method. Lathing is a well understood and longstanding method of fabricating anything that can be made symmetrical about an axis of rotation, e.g. a contact lens. Molding. Molding contact lenses is a more recent adaptation of an old manufacturing technique in which the lens material enters a double-sided mold as a liquid and solidifies in situ as a result of polymerization. Once the mold is broken apart, the lens is in its final form and requires little or no secondary manufacturing or finishing.
• #63: Lathing Advantages: Lathing is a simple technology which requires little adaptation to contact lens manufacturing. Most of the specialised techniques required have already been pioneered in other industries (e.g. the semiconductor and ultra-precision engineering fields). There are few limitations on the parameters that can be lathed (e.g. virtually any radius required can be cut), especially with modern, computer-controlled machines. Most, but not all, materials can be lathed, although some may require special care. Minimal investment is required to start a lathe-based production facility, since each lathe represents a finite investment, is usually an off-the-shelf item and the ancillary equipment is not overly complex or expensive.        
• #64: Lathing Disadvantages: Complex designs are difficult, labour-intensive, or even impossible given the requirement that a lathed product be symmetrical and on a locus of the arcuate sweep of the toolholder. Some computer-controlled machines do not have this latter limitation. The labour-intensive nature of lathing results in a higher unit cost. Since the lathe’s cutting tool leaves a very fine spiral tool track in the lens surface, polishing is usually required. The lathing and/or polishing process may lead to variability in the surface finish due to local heating or variations in the completeness of polishing. Some success has been achieved with research into lathing processes that do not require subsequent polishing. All processes associated with lathing, and the lathing step itself, require significant time. This limits production rates, meaning that volume production is difficult without the installation of many machines working concurrently. Because of the large number of steps involved in lathe-based manufacturing, variations in the overall process are easily introduced. This is reflected in the lower reproducibility usually achieved.
• #65: Molding Advantages: The reduction in the total number of steps and the number of repetitive steps reduces both the time involved in manufacturing and the unit cost. A low unit-cost means that it is feasible to keep manufactured items in stock rather than manufacturing them ‘on demand’ or ‘just in time’. This simplicity and brevity means that volume production is both easier and less expensive to attain. Surface finish is largely dependent on mold surface quality. Once the latter is satisfactory, all subsequent lenses should exhibit similar surface quality. The same applies to lens parameters, which translate to satisfactory levels of reproducibility. Once molds for a complex design are completed satisfactorily, the actual manufacturing process costs no more than for a simpler design. All the major investment is ‘up front’. Economic production then depends on maximum utilization of the initial investment in the complex master mold.
• #66: Molding Disadvantages: Since expensive and specialised machinery is required, the start-up costs are usually high. Some customization may also be required. New molds are required for each new lens series required. This is more expensive than having to change tool settings which is all that a lathe-based system requires. In view of the obvious expense of a new or wider lens series it is usual to limit the parameters and/or the number of lens series manufactured to those which satisfy the maximum demand. The less common needs of the market are ignored. Not all materials lend themselves to molding, either due to their chemical composition or undesired dimensional changes during the polymerization process. Because of the cost of tooling, custom lens making is not usually undertaken. In practical terms, molding is only suited to volume production of common (higher demand) designs and parameters.
• #67: RGP Contact Lenses Bifocals Translating lenses move on the eye when the eye is directed downwards to view a near object. The rest position of the lens amounts to an upward decentration. Concentric (annular), distance centre. The lens mid-periphery has a blended single-power near addition on its front surface. The distance prescription occurs only in the lens centre. The lens edge is conventional (design and thickness) in the interests of comfort and ease of manufacture. Progressive addition. The lens mid-periphery has a blended, variable-power, near addition on its front surface. The near addition increases with increasing displacement from the lens centre. Implanted segment. A higher refractive index segment, usually ‘D’ or crescent-shaped, is incorporated into an RGP button of conventional material. Non-translating Diffractive. A series of concentric zones of alternating distance and near powers incorporated into the back surface. Alternation of powers is intended to make the design less dependent on pupil size. Concentric, distance centre. Similar to the translating type, this lens type incorporates just one distance zone (the centre) and one near zone (the zone of the lens immediately surrounding the centre). The lens periphery follows normal RGP design practice. While minimal movement with non-translating designs is essential, it is quite undesirable physiologically.
• #68: Manufacturing RGP Bifocals Concentric and progressive bifocals are made using conventional lathing or molding techniques. Implanted Segments have a high refractive index segment incorporated in a lens button of conventional material. They are usually ‘D’ shaped or crescent-shaped. Diffractive: concentric zones are molded onto the lens back surface.
• #71: Preliminary Lens Assessment Assessment during manufacture Dry State: BOZR and lens diameter are among the most important measurements. BVP is measured to determine that the final prescription is as ordered. Image quality will give a useful assessment of the quality of the lens optics. Centre thickness, whether it be a series standard or a practitioner request, influences the physiological performance of the final lens and some of its physical properties. Its compliance with the order needs to be confirmed. Edge profile has been shown to be critical to lens comfort. The overall quality of workmanship needs to be assessed to determine the possibility of the finished product delivering less than optimum performance or comfort on the eye. Defects may include edge chips and surface scratches. Wet State: BOZR is critical. It influences the lens fit and the optics of the tear lens. The BOZR will be approximately 0.03 mm flatter after hydration. Image quality is again assessed to determine the optical quality of the product. Vision quality depends on the prescription being accurate and the quality of the optics. Workmanship again needs to be assessed to confirm that hydration has not revealed any previously undetected defects.        
• #72: Changes From Dry To Hydrated State: Expansion resulting from hydration, and its uniformity and predictability, determine the outcome of the hydration step. While hydration is minimal in RGPs, it is still of both manufacturing and clinical significance. Any untoward hydration effect has the potential to produce a toric or even an irregular lens shape. Vision quality may suffer as a result. Standards, whether they be set by the practitioner or a standards authority, normally stipulate a tolerance. It is expected that finished lenses are within these tolerances. Other, less common changes may occur. These may be due to a lack of homogeneity, other variations in the lens material, vagaries of the manufacturing and hydration process or other factors not as yet understood.
• #73: Centre Thickness Affects: Flexure of the lens in situ. Conformity to the shape of the cornea may induce residual astigmatism because of lens flexure. Gas transmissibility, which is inversely proportional to the lens thickness, i.e. Dk/t decreases as thickness increases. If the thickness of the delivered lenses and trial lenses differ markedly, the behaviour of the trial lens may not be truly indicative of the delivered lens. Handling. Apart from inherent material properties such as rigidity, the next most important factor affecting lens handling is thickness. The greater its thickness, the easier the lens is to handle. Acceptable tolerance. Regardless of the tolerance applied, lenses outside a predetermined range should be rejected. Some countries have national standards which can be applied.                      
• #75: Soft Lens Materials Physical Compatibility Mechanical properties of the lens in combination with its fit must allow for lens movement. The lens material must be flexible enough, especially in thicker lenses, to allow the lens to conform somewhat to the anterior eye’s topography. This ensures comfort and satisfactory physiological performance.
• #78: SCL Materials Oxygen Permeability of SCLs is Influenced by: Water content. In general, the higher the water content, the greater the Dk. This is believed to be the result of oxygen dissolving in the water, especially if it is unbound (free). Chemistry of the polymer. The packing density of a material’s molecules influences the ease with which oxygen may pass through the material. If large, open, but rigid-side molecules are present, the packing density is limited and the permeability is enhanced. If molecular chains are flexible or ‘loosely’ arranged, the packing density is again lower and the Dk higher. Alternatively, densely packed molecules make oxygen passage virtually impossible because of the restricted space between adjacent molecules e.g. PMMA. The presence of crosslinks as well as their length and density also contribute to the ‘packing density’ of the polymer. Method(s) of water retention. The water normally exists as a dipole which can be attached electrostatically to a charged material molecule (bound) or simply locate itself within the inter-molecular spaces (free). The greater mobility of ‘free’ water enhances Dk. Temperature. Higher temperatures increase agitation of molecules, resulting in an increase in potential inter-molecular space and easier passage of O2 through the material. Dks are often quoted at eye temperature (around 34°C) because the figures are higher than at room temperature (usually quoted at around 21°C). pH. As the pH of the lens environment decreases (becomes more acidic), so too does the water content. As it increases (becomes more basic or alkaline), the water content increases (Masnick, Holden, 1972). The magnitude of the change may be dependent on the material’s chemistry as well as its water content. Tonicity. The tonicity of the surrounding medium (tears or lens care products) can affect the water content. Hypertonic solutions decrease the water content, hypotonic solutions increase it.              
• #79: Water Content Influences: (at least for hydrogels) Oxygen permeability. Generally the higher the water content of the lens, the greater the oxygen permeability. Refractive index. Higher water content materials have a lower n. Rigidity (handling). Higher water content materials are generally less rigid and are more difficult to handle when thin. Durability. Higher water content materials are generally less durable. Minimum thickness to prevent pervaporation. Usually this is a problem of higher water content lenses made too thin. Ultra-thin low water content lenses can exhibit the problem in susceptible wearers. Environmental susceptibility including spoilage. High water content lenses, especially if ionic, are more easily spoiled and/or influenced by their environment. Lens care system choice. Not all lens materials are suited to all available lens care regimens.
• #80: Soft Lens Materials: Dk @ 34°C All figures are measured by the coulometric technique at eye temperature (341C). These figures are generally lower than those used in promotional literature but are believed to represent a more rigorous and realistic set of figures.
• #81: Low Water Content: Advantages Lower susceptibility to environmental influences, especially pH, results in lenses which have more stable parameters. Greater rigidity provides easier handling. Higher refractive index allows a thinner lens to be made. Virtually any lens care product can be used. All methods of lens fabrication can be used. Generally more predictable behaviour and lower expansion on hydration results in greater reproducibility. More wettable. Pervaporation staining is less likely because the bulk flow of water through such materials is more difficult.
• #82: Low Water Content: Disadvantages Because of low Dk, only the thinnest lenses provide adequate oxygen for daily wear. The greater rigidity of most of these materials results in less conformity to the topography of the anterior eye, which may result in lower comfort levels. Thin lenses (for adequate Dk/t) are usually more difficult to handle, especially in lower BVPs.
• #84: High Water Content: Disadvantages Greater fragility. More deposit prone. Larger ‘pore’ size, often in combination with an ionic chemistry, increases uptake of foreign material including tear proteins. More susceptible to the environment, especially pH changes. Lower refractive index requires a thicker lens to be made. Less stable parameters, lower reproducibility. Thermal disinfection is not recommended as protein uptake is higher and the risk of protein denaturation greater. This also has rami-fications for trial lens disinfection i.e. high water trial lenses should not be thermally disinfected. Optical quality is more difficult to achieve because the expansion on hydration is very significant. This means a polished xerogel (‘dry’ or anhydrous) lens undergoes major changes on hydration. Surface quality and shape are not necessarily retained. Alternative manufacturing methods, e.g. stabilized soft molding, may overcome these problems. More difficult to manufacture by lathing since a small, steep xerogel lens is required for the hydrated product to have ‘normal’ parameters. Pervaporation staining, due to bulk flow of water through the lens, limits the thinness of the lens.
• #85: Physical Properties: Elasticity Materials: Should have a large elastic limit. For a material exhibiting elastic properties, there is a limit to the extension it can undergo before permanent changes may be induced. This is the material’s elastic limit. A lower limit is the Limit of Proportionality. Within this limit, stress (the load or force per unit area) is proportional to strain (the extension), i.e. Stress (the load)  (Strain (the extension)) Should be strong (high Young’s modulus). Young’s modulus is the constant which relates proportional (i.e. within the material’s proportionality limit) stresses and strains. Stress = Y x Strain Young’s modulus is one of several moduli of elasticity (Barron, 1959). A combination of a large elastic limit and a large Young’s modulus should result in a durable lens, i.e. the lens material can withstand significant extension, but extends little even under significant load. Should recover their shape rapidly following deformation. The recovery should be complete.
• #86: Elasticity: Methods of Determination In vitro Stress versus strain curve determination within the elastic limit. Destructive testing. Exceed elastic limit to point of failure. Standard test methods may not be applicable to soft lens materials: environment must be normal saline standard test piece shapes or sizes cannot always be obtained, especially with spin-cast products clamping/mounting of test pieces is critical to the determination of how and where the samples fail (failure mode) during testing and the results may be influenced by the method(s) used.                
• #87: Elasticity In vivo Because of their influence on post-lens tear film thickness, material rigidity and elasticity influence lens fit: a thin tear film results in a slow or unmoving lens which is interpreted clinically as a ‘tight’ fit similarly, a conforming lens is more difficult to move and remove. an elastic material is desirable as it will survive the repeated deformations of lens removal. Masking of astigmatism. Rigidity and elasticity of the lens material influences ‘on-eye’ behaviour, including lens shape and its recovery following a blink. Vision quality and/or the variability of vision may give an indication of this ‘on-eye’ behaviour. A closely conforming lens will not mask cornea-induced astigmatism significantly. Claims of soft lenses masking 0.75 D or more of astigmatism should be viewed suspiciously.                                                  
• #89: Poly(HydroxyEthyl MethAcrylate) (PHEMA) Original material (1952-1959, patented 1955) by O. Wichterle and D. Lim, Czechoslovakia. A close relative of poly(methyl methacrylate) (PMMA, patented 1934). Its differentiating feature is a polar hydroxyl (OH) group to which the water dipole may bind. Water content is approximately 38% (W/W). PHEMA is still in regular use by many manufacturers.
• #91: While the majority of PHEMA lenses are in the thin and ultra-thin categories, the percentage of lenses (presumably mostly plus lenses) thicker than 0.10 mm is surprising given the poor Dk/t known to result from the combination of low Dk and significant lens thickness.
• #92: After PHEMA Attempts to ‘improve’ on PHEMA were fuelled by patent/legal/ marketing issues. A so-called second generation material was the Griffin ‘Bionite’ Naturalens (1968): a co-polymer of PHEMA and Poly(Vinyl Pyrrolidone) or PVP, 55% water content a direct descendent (vifilcon A), is still in use.
• #93: Other Variants Followed Most were a combination of two (co-polymer) or three (ter-polymer) of the following monomers: PHEMA (poly(hydroxyethyl methacrylate)). PVP (poly(vinyl pyrrolidone)). MA (methacrylic acid). MMA (methyl methacrylate). GMA (glyceryl methacrylate). DAA (diacetone acrylamide). PVA (poly(vinyl alcohol)). In each of these methods, a cross-linking agent is required.
• #95: Classifying a Lens Material To define a material generically, each chemical entity has been given a name (including a version, e.g. A, B, etc.) by the US Adopted Names Council (USANC). Regardless of the marketing name used by a manufacturer, the USAN uniquely identifies the actual material. Materials which differ in water content only (usually by altering the proportions of the ingredients) still carry the same name and version e.g. bufilcon A 45% & 55%. The USAN is written in lower case. Note the inclusion of methacrylic acid renders a hydrogel ionic. Similarly, the use of MA as a wetting agent in RGP materials results in a negative surface charge. The UK also has a lens classification system.
• #96: Ionic Materials Net negative charge on surface due to one or more of the material components being polar and presenting its negative polar ends to the outside world. Non-Ionic Materials Also have charged sites within polymer matrix. However, the charges are internal to the polymer and no polar ends are presented to the outside world. This results in no net surface charge. The degree of charge, rather than its presence or absence, may be clinically relevant.
• #97: Ionic Materials: Advantages More wettable. Polar functional groups at the lens surface increase wettability. The more polar groups present, the greater the wettability. Methacrylic acid can be added for this purpose because at physiological pHs it exists as a negatively charged ion. It has been reported that ionic materials denature tear proteins less than non-ionic materials, even though they contain more protein. Other data contradict this finding. This issue is still under investigation. Ionic Materials: Disadvantages Accumulate deposits more readily. Any charged particles, including positively charged lysozyme, may be attracted to the negatively charged sites in ionic materials. Deposits may be bound, and therefore more difficult to remove. Ionic materials are more susceptible to pH changes, especially their water content.
• #98: Non-Ionic Materials: Advantages Less deposit prone. Do not bind charged particles. Non-Ionic Materials: Disadvantages Denature tear proteins more. Studies on the state of tear proteins have produced conflicting results. Less wettable. It has been claimed that the absence of polar groups at the lens surface may decrease its attractiveness to the water dipole, rendering the surface less wettable.
• #99: Soft Lens Manufacturing Methods Molding - xerogel. Monomers are mixed and then poured into a mold (single or double sided) in the absence of water/water vapour (and usually air/oxygen as well) at tightly controlled temperatures. Spin-casting. An open-backed mold is spun as a small centrifuge. The mold defines the front surface of the lens. Rotational velocity, surface tension and gravity combine to define the back surface. Lathing - xerogel. An anhydrous button of lens material is lathed conventionally in a controlled atmosphere. Manufacturers are researching methods of eliminating the need for surface polishing by the application of high-precision engineering principles and other advances such as air bearings and anti-vibration mounts. Molding/Lathing combination. Usually a combination of molding the back surface and body of the lens and lathing the front surface. Spin-casting/Lathing combination. Usually spin-casting the front surface and body of the lens and lathing the back surface. Molding - Stabilized Soft. In this recent innovation, a space-taking inert diluent is included in the mix of monomers during molding/polymerization. The diluent is replaced by water at a later stage. The final product is quick to hydrate fully, undergoes minimal expansion on final hydration and provides high quality optics and surface finish.
• #101: Cast Molding Monomer in liquid form is introduced into a female mold which defines the lens front surface shape. As with the to RGP process, the mold may be double-sided or open (single-sided). If double-sided, a UV-transparent male mold is mated to the monomer-containing female mold and the two are clipped or clamped together. The process requires strict environmental control, especially of humidity, and in many versions of the process needs to be oxygen-free. The combination is UV irradiated until polymerization is complete. The mold is disassembled and the lens is then further processed and hydrated. Subsequent steps are similar to those for lathed products.
• #102: Soft Lens Manufacturing Lathing The raw material is an anhydrous (xerogel) button. As with the RGP procedure, special contact lens lathes are used, usually numerically controlled by a computer. Numerical control increases both the complexity of design that can be achieved and the level of reproducibility. Requires strict control of environment, especially humidity, since a significant relative humidity will result in partial hydration/expansion of the lens material while the lens is being formed. This results in unpredictable outcomes and adversely affects reproducibility. Cleaning is required after final surfacing to remove polishing compounds and other surface contaminants including the materials used to block-up the lens button. As for RGPs, the use of incorrect solvents or incorrect use of solvents may affect the surface properties of the completed lens and may lead to differential hydration, reduced optical quality and altered wetting properties. Hydration of the lens is required after lens completion. This can be accelerated by initial hydration in a suitable substance other than water, and substitution with water as a final step. The lens is then sealed in normal un-preserved saline prior to sterilization. The packaged product is autoclaved (121° C for 15 minutes or longer) to sterilize the contents. The product can then be stored safely for a long period of time (often of the order of 3 - 5 years if necessary).        
• #104: Soft Lens Manufacturing Spin Casting The raw materials are liquid monomers. Monomers are introduced into a spinning mold in a controlled environment of CO2 at high temperature. Centrifugal force and gravity defines the back surface shape and BOZR. The mold defines the front surface. The process can produce a good surface finish front surface finish depends on mold finish back surface finish depends on surface tension and other surface properties of the resulting polymer. Secondary manufacturing steps may be required, e.g. edge finishing. Subsequent procedures are similar to those for other manufacturing methods.
• #107: Soft Lens Manufacturing Stabilized Soft Molding Developed for volume production requiring quick hydration, good optical quality and good reproducibility. An inert diluent is added to monomers in the mold prior to polymerization. The diluent, which is subsequently replaced by water, takes the space the water will eventually occupy.  
• #111: Soft Contact Lens Manufacturing Aspheric Template-following lathe. A large scale (e.g. 10X) model is assembled from individual aspects of the lens design (optic, periphery, edge, etc.) and its profile traced by the lathe template follower. The lathe is set to make a lens at a fixed reduction ratio relative to the template (e.g. 1:10). A ‘plunge’ tool is a full or half diameter, full-sized cutter which is shaped to the profile of the lens desired. It remains stationary and is fed slowly into the rotating lens button in a lathe. It machines the inverse of its profile into the button. Large diamonds have been used as the profiled cutter. By changing the x, y co-ordinates of a cutting tool under numerical control, quite complex surface shapes can be lathed on to a lens button. This has largely replaced the more expensive and less flexible ‘plunge’ tool method. Molding - single/double-sided or spin-casting methods are no more difficult to execute than simpler designs once the master molds are created. The master molds are produced by one or more of the methods above.
• #112: Soft Contact Lens Manufacturing Toric Toric machining. A dual-axis cutting tool can be used on a non-rotating lens button to produce the two radii required. By the controlled crimping of a lens blank (not a button) across its diameter, a toric surface can be created. If this toric surface is then worked into a sphere, the release of the crimping pressure will allow the lens to revert to a toric shape. If a lens blank or button is mounted to the side of, and parallel to, the axis of a rotating chuck, the displacement from the axis can define one of the principal radii of a toric surface (see rB in diagram). If the cutting tool moves about an axis tracing out a plane which includes the axis of the rotating chuck, the arc radius can define the second principal radius (see rc in diagram). Since the tool engages the button for a minority of the time (until the button rotates through the remainder of its circular path), the term ‘flying’ cutter is often applied. This method is used for producing double slab-off torics. Molding - single, double-sided or spin-casting can produce torics just as easily as a simpler lens once the master molds are made. Again one or more of the techniques above is used for this purpose. Combinations of the above methods may also be used either as the main method or as a secondary step to produce a particular aspect of the lens design.
• #114: Soft Contact Lens Manufacturing Bifocals Concentric (annular) with distance centre similar to RGP designs. A concentric (annular) with near centre difficult to manufacture because centre curve is steeper than periphery. This lens type is more suited to molding or spin-casting. Concentric (annular) with distance centre, progressive near similar to RGPs.
• #115: Diffractive bifocal similar to RGPs but with fewer zones diffractive optics on back surface. Translating bifocal a design with thin zones to allow for easier deformation of lens during translation. One-piece monocentric design used. in practice too little translation is possible and this has limited the success of this design. Further research is required.
• #119: Changes From The Dry To Hydrated State Expansion on hydration and its regularity and predictability determine the outcome of the hydration step. Any untoward hydration effect has the potential to produce a toric or even irregular lens shape. Vision quality may suffer as a result. Standards, set by the practitioner or a standards authority, normally stipulate a tolerance. It is expected that finished lenses will be within these tolerances. The possibility always exists that other less common changes may occur. These may be due to our incomplete understanding of the process, inhomogeneities or other variations in the lens material and/or vagaries in the manufacturing and hydration processes.              
• #122: Types of Tinted Soft Lenses Transparent tint - full diameter (handling). For cosmetic acceptance, the tint density must be low. If too dark, the lens edge would be highlighted by the lighter sclera. Transparent tint - iris-diameter tint. This is the most common tint for handling or cosmetic purposes. The untinted lens edge remains inconspicuous. Transparent tint - iris-diameter tint with clear pupil. Prosthetic opaque. This tint type is designed for corneal scars, opacities or deformities of the cornea/iris by: blocking light from the anterior eye substituting a realistic image of the iris of the other eye in or on the lens. Cosmetic opaque. A lens incorporating a partial or complete cover of the natural iris and substitute artwork. This lens type is intended to change the appearance and/or the apparent colour of the eye for non-essential reasons (fashion, performing arts, modelling, etc.).
• #123: UV absorber - often full diameter. Most UV tints have little apparent ‘colour’. Consequently the whole lens can be tinted and still be cosmetically acceptable. Tinting the whole lens is also simpler and less labour intensive. UV and a transparent tint. A transparent tint can be used in addition to a UV-absorbing tint. A clear lens with opaque pupil may be used to conceal a hyper-mature cataract, or pupil deformities in a sighted, partially sighted or blind eye. Transparent tint with opaque pupil. A variation on the previous theme. Tints to assist colour defectives. Examples are the X-Chrom and the JLS (by JL Schlanger) lenses. Usually dense tints with quite narrow-band transmission curves. They function by changing the apparent brightness of objects whose colours would otherwise be confused by colour defectives.
• #124: Tinted Soft Contact Lenses Tinting Process Areas not to be tinted need to be protected from dye. The lens is mounted in a mold with flexible gaskets which seal off the ‘clear’ areas. Tint density can be altered by changing dye concentration, time, or temperature or combinations of these. Some colours are the result of a single dye, others are produced by a combination of dyes. In situ, the tint affects both incident and reflected light involving the iris. It is unwise to select tints based on the in vitro appearance of a lens.
• #125: Tinted Soft Contact Lenses Vat Dye Process Water soluble vat dye (reduced form) is prepared. The lens is swollen chemically and then exposed to the dye only in the areas intended. The dye is oxidized in situ, rendering it insoluble in water and locked into the lens polymer matrix. Extensive extraction follows to remove excess dye and restore the lens parameters to normal. Chemically, vat dyes are very stable. However it is more difficult to get a uniform tint with them, and the tint can vary with lens thickness (BVP and design).          
• #126: Tinted Soft Contact Lenses Reactive Dye Process Dye molecules are bound to hydroxyl groups in the lens polymer by forming stable covalent bonds. A tinted polymer is created and the depth of penetration of the tint molecules is small (i.e. surface and just below). Most dyes used are colour-fast textile dyes which have been shown to be non-toxic (many such dyes are not). Extensive extraction removes excess unreacted/unbound dye. While chemically stable, reactive dyes are more susceptible to chlorine compounds and bleaches than vat dyes. It is easier to get a uniform tint density and the density is not affected by lens thickness (BVP and design).
• #132: Research continues into other novel advanced materials, none of which have been released.  
• #133: Manufacturing: Regulatory Aspects Most countries have regulations controlling the manufacturing processes and facilities for therapeutic/medical devices. As well, the ISO 9000 (or similar) quality certification protocol may be applicable.  
• #135:   GMPs - good manufacturing procedures.