The desperate last gasps of rigid contact lenses.
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Child↗
Biomedical subjects
Nathan Efron
Publications and source records attributed to Nathan Efron.
Impact of manufacturing technology and material composition on the surface characteristics of hydrogel contact lenses.
BACKGROUND: Our aim was to investigate the impact of manufacturing method and material composition on the surface characteristics of hydrogel contact lenses. METHODS: Five lens types were examined; three polyhydroxyethyl methacrylate (pHEMA) lenses, each manufactured by a different technique, namely, lathing, spin-casting and cast-moulding, a HEMA/methacrylic acid cast-moulded lens and a HEMA/glycerol methacrylate cast-moulded lens. Six lenses of each type were examined (front and back) using scanning electron microscopy (SEM). Additionally, both surfaces of three lenses from each of the pHEMA lens groups were examined, partially hydrated, using an atomic force microscope (AFM). Qualitative data were gathered for both SEM and AFM studies in addition to root-mean-square (RMS) roughness values for the lenses investigated with AFM. RESULTS: The surfaces of the lathed lenses were covered in lathing/polishing marks. RMS roughness values for the anterior surface (10.9 +/- 4.3 nm) were significantly greater (p = 0.02) than those of the posterior surface (9.3 +/- 0.8 nm). The two surfaces of the spun-cast lens appeared similar by SEM but AFM RMS roughness values were greater (p = 0.02) for the anterior (12.3 +/- 1.8 nm) than the posterior (5.8 +/- 1.9 nm) surface. Both SEM and AFM showed similar topographic appearances for the surfaces of the cast-moulded pHEMA lens, although RMS roughness values were greater (p = 0.02) for the anterior (5.8 +/- 0.9 nm) than the posterior (3.9 +/- 0.3 nm) surface. All three cast-moulded lenses had more processing debris than the lathed and spun-cast pHEMA lenses. Overall, the surfaces of the lathed lens were 'rougher' than those of the cast-moulded lens (p = 0.01). CONCLUSION: The surface topographies of the hydrogel contact lenses are dependent on the method of manufacture. Cast-moulded lenses are associated with apparently 'stickier' surfaces, which may be indicative of surface degradation during the manufacturing process.
Biomedical Technology↗
Incidence and morbidity of hospital-presenting corneal infiltrative events associated with contact lens wear.
AIM: To determine the incidence and morbidity (visual loss) of hospital-presenting corneal infiltrative events (CIEs) associated with the wearing of current generation contact lenses. METHODS: All contact lens wearers presenting with any form of corneal infiltrate/ulcer to a hospital centre in Manchester, UK, were surveyed in this 12-month, prospective, hospital-based epidemiological study. A clinical severity matrix was used to quantify the overall severity of presenting signs and symptoms. The size of the hospital catchment population and the wearing modalities (daily wear [DW] or extended wear [EW]) and lens types used in that population were estimated from relevant demographic and market data to facilitate the calculation of incidence. We also attempted to ascertain, from their eye care practitioners, the visual acuity (VA) of patients suffering from CIEs prior to and at about six months following attendance at the hospital. RESULTS: During the survey period, 118 patients presented with CIEs of varying severity. The annual incidence (cases per 10,000 wearers) for all wearing modalities and lens types is 21.3 (95 per cent confidence interval 17.8 to 25.5). The incidence of CIEs for each wearing modality and lens type is: DW rigid, 8.6 (3.9 to 18.7); DW hydrogel daily disposable, 14.0 (9.3 to 21.0); DW hydrogel (excluding daily disposable), 20.4 (15.9 to 26.2); DW silicone hydrogel, 55.9 (9.9 to 309.6); EW rigid, zero (0.0 to 1758.8); EW hydrogel, 144.6 (66.4 to 311.8) and EW silicone hydrogel, 118.6 (75.2 to 186.7). The risk of developing a CIE with EW lenses was 8.1 (5.3 to 12.5) times greater than that with DW lenses (p < 0.0001). Although there was no difference between EW hydrogel and EW silicone hydrogel lenses with respect to the risk of developing CIEs, the clinical severity of CIEs was greater with EW hydrogel lenses (p = 0.04). Results of VA for pre- and post-hospital attendance were obtained from 38 patients, none of whom lost more than one line of VA. For the study population, zero patients (95 per cent CI: 0 to 9.2 per cent) suffered a significant loss of VA as a result of developing a CIE. CONCLUSIONS: Overall, there is an eight times higher incidence of CIEs in wearers who sleep in contact lenses compared with wearers who use lenses only during the waking hours. For those who choose to routinely or intermittently sleep in soft contact lenses, silicone hydrogels are the lens of first choice because CIEs are less clinically severe with this lens type compared with hydrogel lenses. The rate of significant visual loss as a result of developing a CIE is low.
Contact Lenses↗
In vivo corneal confocal microscopy in keratoconus.
PURPOSE: To evaluate the corneas of keratoconic subjects using in vivo confocal microscopy. METHODS: Slit scanning confocal microscopy was used to evaluate the central cornea of one eye of each of 29 keratoconic subjects (mean age 31 +/- 10 years; range 16-49 years). Quantitative aspects of corneal morphology were compared against data from control subjects. RESULTS: Compared with normal control corneas, epithelial wing cell nuclei were larger (p < 0.0001) and epithelial basal cell diameter was larger (p < 0.05) in the keratoconic cornea. Many of the keratoconic corneas investigated showed increased levels of stromal haze and reflectivity, which appeared to be related to the presence of apical scarring on slit lamp examination. A grading scale was devised to quantify the levels of haze. This scale was shown to provide a measure of the level of scarring present. The anterior keratocyte density (AKD) and posterior keratocyte density were 19% lower (p < 0.0001) and 10% lower (p = 0.004) than in controls, respectively. The reduction in AKD was significantly associated with three factors: a history of atopy, eye rubbing and the presence of corneal staining. The mean endothelial cell density in keratoconus was 6% greater than that of normal controls (p = 0.05). The level of endothelial polymegethism was shown not to be different between keratoconic subjects and matched controls (paired t-test: t = 1.82, p = 0.08). CONCLUSIONS: Confocal microscopy demonstrates significant quantitative alterations of corneal morphology in keratoconus.
Adolescent↗
Risk factors for the development of corneal infiltrative events associated with contact lens wear.
PURPOSE: To identify risk factors for the development of corneal infiltrative events (CIEs) associated with contact lens wear, and to report other relevant clinical characteristics. METHODS: A series of symptomatic contact lens wearers presenting consecutively to a large hospital clinic over a 1-year period were examined. The clinical severity of any CIE was determined with a scoring system, and general and lens-specific information was captured with a questionnaire. Logistic regression analyses were performed to investigate the association between a range of risk factors and the occurrence of any CIE and the subset of cases categorized as severe keratitis. Three quasi-independent control groups were used for this analysis: hospital, lens-matched, and population. The relationship between clinical severity and the delay in attending the hospital was investigated. The prevalence of symptoms and initial actions taken by patients are reported. RESULTS: Factors identified as being associated with an increased risk of development of a CIE include: wearing modality/lens type (greatest risk for extended-wear hydrogel lenses of 7.1 vs. daily wear hydrogel lenses), male gender (relative risk 1.4), smoking (1.4), the absence of relevant ocular (1.8) and general health (2.4) problems, and the late winter months (greatest risk in March of 3.6 vs. July). The overall predictive value of these risk factors for a given individual was low. Shorter time delays in hospital attendance were associated with increasing severity of keratitis. Eye soreness was the most common initial symptom (prevalence 69%), and the most frequent initial course of action taken by the patient was lens removal (prevalence 50%). CONCLUSIONS: Risk factors for the development of contact lens keratitis were identified that, although being of limited predictive value for individual patients, highlight general associations that may assist in the management of contact lens wearers. Such risk factors may also assist in the development of a more complete understanding of the etiology of contact-lens-associated CEIs.
Adolescent↗
Effects of benoxinate hydrochloride 0.4% on the morphological appearance of the cornea using confocal microscopy.
PURPOSE: To investigate whether benoxinate hydrochloride 0.4% used to make confocal microscopy more comfortable alters the morphology of the cornea as viewed with the confocal microscope. METHODS: Confocal microscopy was performed on both eyes of 10 subjects prior to instillation of either topical anaesthetic or non-preserved sterile saline, on two randomly ordered occasions. Images of all corneal layers were analysed quantitatively and qualitatively in a masked fashion. RESULTS: The images were similar in appearance in 5/10 subjects, there was greater clarity when anaesthetic was instilled in 4/10 subjects, and in the remaining subject there was greater clarity when saline was used. Anaesthetic had no influence on anterior keratocyte density (AKD), posterior keratocyte density (PKD) or endothelial cell density (ECD). CONCLUSIONS: Local anaesthetic does not affect corneal morphology as imaged using the confocal microscope. However, failure to use anaesthetic may lead to a degradation of image quality due to patient discomfort and excessive eye movements.
Journal Article↗
Confocal microscopy of the corneas of long-term rigid contact lens wearers.
Slit scanning confocal microscopy (Tomey Confoscan P4) was used to evaluate the central cornea of 22 subjects who had been wearing rigid lenses on a long-term, daily wear basis. Anterior and posterior keratocyte densities appeared unaffected by rigid lens wear (P = 0.10 and 0.34 for anterior and posterior keratocyte densities, respectively). Subjects with a previous history of polymethyl methacrylate (PMMA) lens wear showed a reduction in anterior keratocyte density (AKD) (P < 0.0001) and an increased level of haze in the anterior stroma. This may represent previous hypoxic damage. Endothelial cell density (ECD) was unaffected by rigid lens wear (P = 0.36) although an increase in endothelial polymegethism was evident (P < 0.0001). Subjects who had only worn rigid lenses with no history of PMMA lens wear did not show an increase in endothelial polymegethism (P = 0.10). An increased number of microdot opacities compared to the non-lens wearing eye was apparent (P = 0.05). The number of microdot opacities induced by rigid lenses, as reported in this study, appears to be less than that reported by others in respect of soft lenses.
Journal Article↗
In-eye performance of soft contact lenses made from different materials.
The in-eye performance of soft contact lenses may be affected by the material from which they are fabricated and may alter during wear. This study describes clinical and laboratory experiments that were conducted in order to examine the in-eye performance of eight soft contact lenses manufactured from different materials; these materials (and nominal water contents) were: HEMA/VP 40%, HEMA/VP 55%, HEMA/VP 70%, VP/MMA 55%, VP/MMA 70%, HEMA 40%, HEMA/MAA 55% and HEMA/MAA 70% (HEMA: 2-hydroxy-ethyl methacrylate, VP: vinyl pyrrolidone, MMA: methyl methacrylate, MAA: methacrylic acid). Two lenses from each of the eight soft contact lens groups were used in experiments concerning their parameter stability. Six subjects were fitted with lenses for 1 day. Verification of back optic zone radius, total diameter, back vertex power, centre thickness and water content was undertaken at 20 degrees C. In vitro measurements of total diameter were taken at 35 degrees C before lens fitting and after 6 h of lens wear. In vivo measurements of lens centration, up-gaze lag, post-blink movement, total diameter and subjective assessment of comfort were taken (a) immediately after lens insertion (1 min), (b) 20 min later and (c) 6 h later (same day). The majority of differences of the in vivo parameters between lens types (with respect to lens centration, lag, movement on blink, lens total diameter and comfort) were not found to be statistically significant throughout the 6 h wearing period. It was also found that lenses increase in diameter when first placed on the eye and decrease in diameter when they were removed from the eye. The information generated in study concerning the in-eye performance of soft contact lens materials may assist the contact lens industry and contact lens practitioners in developing and prescribing soft lenses with optimal performance characteristics.
Journal Article↗
Parameter stability of soft contact lenses made from different materials.
The parameters of soft contact lenses may alter during wear. A series of clinical and laboratory experiments were conducted in order to examine the parameter stability of eight soft contact lenses manufactured from different materials. The following materials (and nominal water contents) were used: HEMA/VP 40%, HEMA/VP 55%, HEMA/VP 70%, VP/MMA 55%, VP/MMA 70%, HEMA 40%, HEMA/MAA 55% and HEMA/MAA 70% (HEMA: 2-hydroxy-ethyl methacrylate, VP: vinyl pyrrolidone, MMA: methyl methacrylate, MAA: methacrylic acid). Two lenses from each of the eight soft contact lens groups were used in experiments concerning the parameter stability. Six subjects were fitted with lenses for 1 day. Verification of back optic zone radius, total diameter, back vertex power, centre thickness and water content was undertaken at 20 degrees C. In vitro measurements of water content, oxygen transmissibility, total diameter and back optic zone radius were taken at 35 degrees C before lens fitting and after 6h of lens wear. Distortion, discolouration and lens quality were assessed before and after lens wear. When the temperature was raised from 20 to 35 degrees C, a significant reduction in lens water content for all the lens types was observed, as well as a significant reduction in total lens diameter for the majority of the lens types. Water content, oxygen transmissibility, total diameter and back optic zone radius of all the lens types reduced, following a 6h open eye wearing period. For the majority of the lens types, these changes were found to be statistically significant. Distortion, discolouration and quality of the lenses remained unchanged throughout the study with the exception of the HEMA/MAA 70% lens. Correlating a number of parameters generated in this study, gave the following conclusions. High water content materials exhibit a low relative change in oxygen transmissibility following a 6h wear period. Soft contact lens dehydration leads to a decrease in oxygen transmissibility and total diameter, following a 6h wear period. These results will assist practitioners in predicting the alterations that occur in soft contact lens parameters and oxygen performance as a result of lens wear.
Journal Article↗
Tensile properties of soft contact lens materials.
The strength of contact lens materials is an important consideration with respect to resistance to damage during lens handling and long term durability, and may govern some aspects of in-eye lens performance. The tensile properties of hydrogel contact lenses manufactured from eight different materials were examined in a series of clinical and laboratory experiments using the Instron 1122 Universal Testing Instrument. Lenses from the following eight materials (and nominal water contents) were used: HEMA/VP 40%, HEMA/VP 55%, HEMA/VP 70%, VP/MMA 55%, VP/MMA 70%, HEMA 40%, HEMA/MAA 55% and HEMA/MAA 70% (HEMA: 2-hydroxy-ethyl methacrylate, VP: vinyl pyrrolidone, MMA: methyl methacrylate, MAA: methacrylic acid). Tensile strength, elongation-at-break and Young's modulus were measured. A technique was devised that enables three parallel-sided specimens of identical width to be cut from a single contact lens with good accuracy. It was found that materials made from HEMA/MAA--although having a very low tensile strength and elongation-at-break--exhibit only a moderate Young's modulus. Materials made from HEMA/VP exhibit high-to-moderate tensile strength, high elongation-at-break and moderate-to-low Young's modulus. Materials made from VP/MMA exhibit high tensile strength and high-to-moderate elongation-at-break, but the Young's modulus is high for the 55% water content and low for the 70% water content materials. The HEMA 40% material exhibits a moderate tensile strength, a low elongation-at-break and a high Young's modulus. This experiment highlights the necessity of developing an accepted standard test methodology for contact lens material stiffness evaluation, in order to derive useful comparative information. Six subjects were fitted with the same lenses for one day. In vitro measurements of total diameter and back optic zone radius were taken at 35 degrees C before lens fitting and after 6h of lens wear. Lens water content, the relative change in lens total diameter (%deltaTD) and relative change in lens back optic zone radius (%deltaBOZR) were calculated and correlated with the tensile properties of the eight hydrogel lens materials. It is concluded that hydrogel materials with high stiffness and strength display less tendency to change their geometric parameters and materials with a high water content do not necessarily have the weakest mechanical properties.
Journal Article↗
Water properties of soft contact lens materials.
The properties of water in soft contact lenses such as the water content, free-to-bound water ratio, and the extent to which soft lenses dehydrate during wear, are key determinants of their in eye performance and oxygen transmissibility characteristics. This study describes clinical and laboratory experiments that were conducted in order to examine the state of water in eight soft contact lenses manufactured from different materials. Specifically, lenses made from the following eight materials (and nominal water contents) were used: HEMA/VP 40%, HEMA/VP 55%, HEMA/VP 70%, VP/MMA 55%, VP/MMA 70%, HEMA 40%, HEMA/MAA 55% and HEMA/MAA 70% [HEMA = 2-hydroxy-ethyl methacrylate, VP = vinyl pyrrolidone, MMA = methyl methacrylate, MAA = methacrylic acid]. Differential scanning calorimetry (DSC) was used for measuring the free water content in the materials listed above. Some noticeable differences in water properties among soft contact lens materials having approximately the same water contents were revealed. Low water content materials exhibited a simple endotherm and all water melted around 0 degrees C. On the other hand, medium and high water content materials exhibited multiple melting endotherms, representing a broad range of interactions between water and the polymer. Low water content soft contact lenses have approximately the same amount of bound water as those with much higher water contents. Six subjects were then fitted with the same lenses for one day. In vitro measurements of water content and oxygen transmissibility were taken at 35 degrees C, both before lens fitting and after 6 h of lens wear. Water content and oxygen transmissibility were correlated with the water properties of the soft contact lens materials. The relative change in lens water content (%deltaWC) and relative change in lens oxygen transmissibility (%deltaDk/t) were calculated and correlated with the water properties of the eight soft contact lens materials. It was concluded that (a) oxygen transmissibility, free water content and free-to-bound water ratio are increased when the water content of a contact lens is increased and (b) water content, free water content and free-to-bound water ratio cannot be used for the prediction of soft contact lens dehydration in vivo.
Journal Article↗
Particles at the laser in situ keratomileusis flap interface.
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Corneal Stroma↗
Hydrogel contact lens dehydration in controlled environmental conditions.
PURPOSE: To ascertain the impact of environmental conditions on hydrogel contact lens dehydration. METHODS: Six young adult subjects wore an ACUVUE lens in one eye and a Proclear lens in the other eye for 200 minute sessions in arid, temperate, and arctic conditions, maintained in a purpose-built environmental chamber in an aerospace medical facility. Dehydration was determined by measuring lens water content before and after each session using a soft contact lens refractometer. Comfort of the lenses was assessed at the conclusion of each session. RESULTS: Lens dehydration was similar for the three environmental conditions (F2 = 0.3, P = 0.75). There was less dehydration with the Proclear lens compared with ACUVUE lens (F1 = 43.0, P = 0.001). No differences were detected with respect to lens comfort between the three environmental conditions (F2 = 2.0, P = 0.18) or the two lens types (F1 = 1.4, P = 0.29). CONCLUSION: Soft contact lens dehydration is unaffected by environmental extremes.
Adult↗
Characterization of the surface of conventional hydrogel and silicone hydrogel contact lenses by time-of-flight secondary ion mass spectrometry.
PURPOSE: To characterize the surfaces of unworn conventional hydrogel and silicone hydrogel contact lenses. METHODS: Near-identical formulations of poly(hydroxyethyl methacrylate) (pHEMA) were used to manufacture lathe-cut, spun-cast, and cast-molded contact lenses. The surfaces of two of each of these lens types and two of each of two commercially available silicone hydrogel lenses-balafilcon A (PureVision) and lotrafilcon A (Focus Night and Day)-were analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS). RESULTS: The ToF-SIMS spectra revealed the presence of the bulk polymer pHEMA at the surface of all three hydrogel lenses, along with other contaminants, such as poly(dimethyl siloxane), alkyl sulfates, alkyl-aryl sulfonates, dioctyl phthalate, Irgafos 168, sodium, chlorine, aluminum, potassium, calcium, copper, and fluorine, which are primarily derived from the various processing steps undertaken in lens manufacture, handling, and storage. The amount of bulk polymer detected at the surface of the PureVision lens was greater than that detected at the surface of the Night and Day lens. In addition, contaminants similar to those found on the surfaces of the conventional hydrogel lenses were detected. The Focus Night and Day lens appears to be coated with an organo-nitrogen material, which results from the plasma deposition of reactive precursors on the surface. CONCLUSIONS: We confirm that ToF-SIMS has the capacity to characterize the surface chemistry of contact lenses. The ongoing application of this technique can assist researchers and clinicians to understand the clinical performance of contact lenses.
Contact Lenses, Hydrophilic↗
Impact of manufacturing technology and material composition on the clinical performance of hydrogel lenses.
PURPOSE: To establish the clinical impact of three different methods of manufacture used to produce soft contact lenses. METHODS: Clinical performance of five lens types was investigated by undertaking a prospective, double-masked, randomized, crossover study. Three of the lenses were made from poly(hydroxyethyl methacrylate) (pHEMA) by three different manufacturing processes (lathing, spin casting, and cast molding), and the remaining two lenses were cast molded from different materials-hydroxyethyl methacrylate/methacrylic acid and hydroxyethyl methacrylate/glycerol methacrylate (HEMA/GMA). All lenses were specially fabricated for this work at the same manufacturing plant. Thirty-four soft contact lens wearers wore each lens for 1 month on a daily-wear basis. Several clinical variables, such as ocular response, visual acuity, lens fitting, prelens tear film, lens surface dehydration, subjective response, and protein deposition, were measured. RESULTS: In general, the spun-cast pHEMA lens performed inferiorly compared with the other pHEMA lenses. This lens induced significantly more limbal and conjunctival hyperemia than the cast-molded lens and provided poorer low contrast visual acuity (LCVA) than the other two lenses. It dehydrated more and had the least on-eye movement. However, the spun-cast lens deposited the least protein of the pHEMA lenses. In general, the HEMA/GMA lens performed inferiorly compared with the other cast-molded lenses. LCVA was worse with this lens, and subjective responses showed that this lens was thought to give the worst visual performance of the cast-molded lenses. It was also thought to be the most difficult lens to handle. Significantly more breakages occurred with this lens than any other. CONCLUSIONS: Overall, this work has shown that manufacturing method and material composition have a fundamental effect on many clinical properties of a lens. Therefore, method of manufacture is also an important consideration in the overall production of a soft lens.
Adult↗
Corneal endothelial cell morphometry and corneal thickness in diabetic contact lens wearers.
PURPOSE: To assess corneal endothelial cell morphometry and corneal thickness in diabetic patients who wear contact lenses. METHODS: Images of the central corneal endothelium were analyzed quantitatively and qualitatively and corneal thickness was measured in a group of diabetic patients (type 1, N = 26; type 2, N = 4) who wear soft contact lenses and in a group of nondiabetic age-matched control subjects who were also contact lens wearers. RESULTS: Endothelial cell characteristics and corneal thickness values were similar for the two groups (p > 0.05). Four of the diabetic patients (and none of the nondiabetic patients) displayed folds in the endothelial mosaic. CONCLUSIONS: The morphometry of corneal endothelial cells and central corneal thickness values in diabetic patients who wear soft contact lenses were not appreciably different from those found in lens-wearing control subjects.
Adult↗
Non-compliance with lens care and maintenance in diabetic contact lens wearers.
We hypothesize that diabetic contact lens wearers may represent a special group displaying higher levels of compliance with their lens care regimens as a result of learned behaviour relating to maintenance of their diabetic condition. To test this hypothesis, a prospective, single centre, controlled, masked study was performed whereby 29 diabetic contact lens patients and 29 non-diabetic control subjects were issued with disposable hydrogel contact lenses and a multipurpose lens care regimen. All participants were given identical instruction on lens care and maintenance. Compliance levels were assessed at a 12-month aftercare appointment by demonstration and questionnaire. Twenty-four different aspects of compliance were scored, 12 by observation and 12 by questionnaire report, of which only two showed a significant difference between the diabetic and control groups. Although the combined population of contact lens wearers was generally compliant, there were examples of non-compliance in both groups. Neither the duration of diabetes nor the degree of metabolic control appeared to have a significant effect on compliance. The results suggest that eye care practitioners cannot assume that diabetic patients will be more compliant with contact lens care and maintenance than non-diabetic patients.
Adolescent↗
Impact of manufacturing technology and material composition on the mechanical properties of hydrogel contact lenses.
Hydrogel materials are not only different as a result of different co-monomer compositions but also as a result of the manufacturing method used to produce them. Hydrogel contact lenses fabricated by different methods are subjected to markedly different processing steps, which are likely to affect the resultant material network structure of a lens. The mechanical performance of five soft contact lens types was investigated using one conventional technique (tensile test) and three novel techniques (probe, tear and ball milling tests). The five lens groups consisted of three polyhydroxyethyl methacrylate (pHEMA) lenses which were each manufactured by a different technique: lathing, spin-casting and cast-moulding, a HEMA/methacrylic acid (HEMA/MAA) cast-moulded lens and a HEMA/glycerol methacrylate (HEMA/GMA) cast-moulded lens. Overall, the results show that the best mechanical performance is obtained for the pHEMA lenses, with some differences occurring between the lenses. The HEMA/GMA lens showed the worst performance.
Biomechanical Phenomena↗