Analysis of tear film breakup on Etafilcon A hydrogel lenses

The relationship of pre-corneal to pre-contact lens non-invasive tear breakup time

Purpose

To examine the relationship between pre-corneal and pre-contact lens tear film stability (TFS), and to determine whether pre-corneal TFS is a reliable predictor of subsequent pre-lens TFS after a contact lens is placed on the eye.

Methods

667 records met inclusion criteria and were extracted from a soft contact lens multi-study database. Multivariable linear mixed effects models were fit to examine the association between pre-corneal and pre-lens TFS, adjusting for potential confounders and accounting for repeated measures. Receiver Operating Characteristic (ROC) analysis was employed to assess the predictive performance of pre-corneal TFS for subsequent pre-lens TFS. TFS was quantified for this analysis as the non-invasive tear breakup time (NITBUT).

Results

Pre-corneal NITBUT was significantly related to the pre-lens NITBUT at both 10 min (p<0.001) and 2–6 hrs (p<0.001) post-lens insertion. However, the sensitivities of pre-corneal NITBUT for predicting symptom-associated thresholds of pre-lens NITBUT ranged from 50–65%, and specificities ranged from 57–72%, suggesting poor-to-moderate diagnostic performance.

Conclusions

Despite the association of pre-corneal and pre-lens TFS, the inherent lability and sensitivity to environmental exposures of the tear film introduce significant variability into NITBUT measurements. Using pre-corneal NITBUT to identify likely successful contact lens candidates prior to fitting is thus not sufficiently accurate to be relied upon in the clinical setting.

Evaluation of commercial soft contact lenses for ocular drug delivery: A review

Soft contact lenses have generated growing interest in ocular drug delivery due to their potential to enhance drug bioavailability in ocular tissues. Commercially available soft contact lenses offer several advantages for ocular drug delivery as they are manufactured on a large scale, which guarantees the availability of a consistent and reproducible product, and their favorable safety profile is well-established through broad clinical use. Here we review the rationale for using commercially available soft contact lenses for ocular drug delivery; summarize the evolution of the materials used in contact lens fabrication; and explore various methods used to improve the drug release characteristics and its tissue penetration. While significant progress has been made, several issues still require further attention for the commercial launch of a viable drug-eluting contact lens product, including control of initial burst release, shelf-life stability, and drug loss during processing or storage.

Zeta potential of tear samples: A tool to explore the effects of wear of contact lenses

PURPOSE

The aim was to develop a method to assess the electrostatic properties of human tear samples, and to evaluate their modifications induced by the wear of contact lenses (CLs).

METHOD

The barrier method was developed for the measurement of the isoelectric point (IEP) on relatively small quantities. The method was applied to compare three groups: tears (T_NW) of non-wearers, tears (T_{W_etaf}) of regular wearers of etafilcon A CLs, and tears (T_{W_omaf}) of regular wearers of omafilcon A CLs. Zeta potential (ζ) as a function of pH was measured by a Zetasizer Nano ZS90 (Malvern Instruments) on 40%-diluted samples, obtained by mixing 57 μL of tears of different subjects of the same group with 85 μL of HCl aqueous solution. IEP was deduced as the pH at which ζ is zero, i.e. the net electric charge on tear constituents being neutralized.

RESULTS

Within an error of about 0.05, IEPs were found to be 2.90 (T_NW), 2.80 (T_{W_omaf}), and 3.16 (T_{W_etaf}). On average, a lower H⁺ concentration is needed to neutralize the surface charge of the tear components of etafilcon A wearers, compared to both T_NW and T_{W_omaf}.

CONCLUSION

IEP measurements on tear samples of wearers of different types of CLs are proposed in order to enhance the knowledge on the modifications of the profile of charged species in tears. The T_{W_etaf} results, compared to those of the other groups, are compatible with an increase, due to the wear of etafilcon A CLs, of the relative concentration of high-IEP proteins.

Tear film dynamics on soft contact lenses

PURPOSE

To analyze and model the dynamics of tear film surface quality (TFSQ) in a group of subjects with healthy eyes, before and during contact lens (CL) wear, and in a group of subjects with dry eyes (DEs).

METHODS

Four sets of TFSQ measurements with lateral shearing interferometry were retrospectively analyzed on two groups of subjects. The first group included 13 CL wearers for which TFSQ measurements corresponding to baseline, Best CL, and Worst CL were selected. The second group included 13 DE subjects. The acquired TFSQ time series were fit with a power-exponential model. Tear film surface quality stability time, best TFSQ, and integrated poststability time characterizing the tear film deterioration process were derived.

RESULTS

The proposed power-exponential model was better suited (higher correlation values) for the TFSQ dynamics on CL rather than for those of baseline and DE measurements. The average baseline TFSQ Stability Time was significantly longer (p < 0.001) than those in the DE group and with both the best and worse CL. The average Best TFSQ achieved at baseline was statistically significantly better (p = 0.03) than that for the DE group. The average best TFSQ was significantly better (p < 0.01) for the Best CL than for that of the Worst CL. Deterioration of TFSQ on both best and Worst CL was substantially faster than that achieved for the DE group.

CONCLUSIONS

The observed distinct change in the sign of the TFSQ velocity on contact lenses suggests a two-phase dynamics in which the postblink stability phase is followed by a phase of dewetting. Lens material properties influence the first phase but play little role after the dewetting process occurs.

Mechanical testing of hydrogels in cartilage tissue engineering: beyond the compressive modulus

Injuries to articular cartilage result in significant pain to patients and high medical costs. Unfortunately, cartilage repair strategies have been notoriously unreliable and/or complex. Biomaterial-based tissue-engineering strategies offer great promise, including the use of hydrogels to regenerate articular cartilage. Mechanical integrity is arguably the most important functional outcome of engineered cartilage, although mechanical testing of hydrogel-based constructs to date has focused primarily on deformation rather than failure properties. In addition to deformation testing, as the field of cartilage tissue engineering matures, this community will benefit from the addition of mechanical failure testing to outcome analyses, given the crucial clinical importance of the success of engineered constructs. However, there is a tremendous disparity in the methods used to evaluate mechanical failure of hydrogels and articular cartilage. In an effort to bridge the gap in mechanical testing methods of articular cartilage and hydrogels in cartilage regeneration, this review classifies the different toughness measurements for each. The urgency for identifying the common ground between these two disparate fields is high, as mechanical failure is ready to stand alongside stiffness as a functional design requirement. In comparing toughness measurement methods between hydrogels and cartilage, we recommend that the best option for evaluating mechanical failure of hydrogel-based constructs for cartilage tissue engineering may be tensile testing based on the single edge notch test, in part because specimen preparation is more straightforward and a related American Society for Testing and Materials (ASTM) standard can be adopted in a fracture mechanics context.

Future directions in non-invasive measurements of tear film surface kinetics

PURPOSE

To review the methods for dynamic, non-invasive, and objective assessment of tear film surface quality and to outline their current state-of-the-art and their future potential.

METHODS

Among the methods available, high-speed videokeratoscopy, lateral shearing interferometry, and dynamic wavefront sensing are being considered.

RESULTS

The principles of operations, their advantages and disadvantages, and limitations of each method are being outlined. The possible future directions of each method are also discussed.

CONCLUSIONS

To gain a better understanding of tear film, its structure and function, it is essential to combine macroimaging technologies with those focusing on tear film microstructure. In this way, one can envisage a clinical device that could help, in future, early diagnosis of dry eye syndrome and development of better materials for contact lenses and eye lubricants.

Tear Film Surface Quality with Soft Contact Lenses Using Dynamic Videokeratoscopy

Purpose

To investigate changes in tear film surface quality after commencing soft contact lens wear.

Methods

Tear film surface quality (TSQ) was assessed during the interblink period using dynamic videokeratoscopy at 25 Hz. A quantitative value of TSQ is derived for each raw Placido ring image. Eleven young subjects with normal tear characteristics participated in the study. Dynamic videokeratoscopy was taken three times per day; in the morning, at lunchtime, and in the afternoon. This was done on two baseline days (bare eye) and on the first and seventh days of lens wear for a conventional hydrogel lens and following a week of no lens wear, for a further week of silicone hydrogel lens wear. Additionally clinical tests to assess TSQ were conducted and subjects were also asked to rate the subjective dryness of their eyes.

Results

All lens wear measurements showed a significant worsening of TSQ compared to bare eye measurements (repeated measures ANOVA, P<0.01). A significant diurnal change was found on the first day of silicone hydrogel contact lens wear, where TSQ improved during the day (P=0.045). However, no diurnal changes were found in TSQ for the other lens wearing days or for the bare eye condition (P>0.05). The subjective rating of dryness correlated with TSQ values (Pearson's r=0.62, P<0.05) for the bare eye condition, but not during contact lens wear. TSQ derived from the right and left bare eyes of the same individuals showed a significant correlation (Pearson's r=0.61, P<0.05).

Conclusions

The measurement of TSQ using dynamic videokeratoscopy differentiates between bare eye and lens wearing conditions. It also shows a small systematic improvement in tear surface quality during the first day of silicone hydrogel lens wear and a significant association with subjective dryness for the bare eye condition.

External Factors Affecting Data Acquisition During Corneal Topography Examination

PURPOSE

To analyze the factors affecting data acquisition during corneal topography examination with the Medmont E-300 videokeratoscope and to provide strategies to minimize their effects.

METHODS

Sixty eyes from thirty young adults were examined. A second observer registered incidences with the potential to affect data acquisition. Those factors were correlated with the difficulty of measurements as judged subjectively by the practitioner who performed the examination. Measurements of axial curvature were analyzed to evaluate the variability expressed as intrasession and intersession coefficient of variation and the standard error of the mean (SEM).

RESULTS

The level of difficulty rated by the practitioner was in general low, with 70% of the eyes being easy or very easy to measure. For the remaining 30% of the eyes, corneal topography measurements were considered to be difficult (27%) or very difficult (3%). Of the external parameters investigated, only fixation instability (P<0.001, chi2) and the need for head repositioning (P=0.024, chi2) were associated significantly with a higher level of difficulty, as rated subjectively by the practitioner. Further analysis showed that some external factors, including those previously mentioned and others related to tear instability, affect the variability of measurements at certain corneal locations, particularly in the vertical meridian when related to tear instability and in the horizontal meridian when related to the need for head repositioning on the chin rest owing to physiognomy interferences with the keratoscope cone. Intersession SEM improved when three readings from each session were considered.

CONCLUSIONS

The level of subjective difficulty found during videokeratoscopy examination is correlated strongly with fixation instability and the need for head reorientation in the chin rest, whereas tear-related events seem to be less relevant in the practitioner perception of test ease or difficulty. Those factors have relevance in measurement variability.

Dynamic wettability properties of a soft contact lens hydrogel

The wettability of poly[2-hydroxyethyl methacrylate-co-methacrylic acid] (pHEMA-MAA) soft contact lenses was investigated in the absence and presence of block copolymer surfactants and lysozyme using the sessile drop method. The advancing dynamic contact angles (Thetaw/a) values are reported for water as a function of sequential wetting and drying cycles. The Thetaw/a values for the pHEMA-MAA in the absence of surfactant and lysozyme increased from approximately 20 degrees to 100 degrees as the number of cycles increased from two to ten, and they were independent of the pHEMA-MAA bulk water content. The change from the highly hydrophilic to hydrophobic pHEMA-MAA surface could not be reversed using the sequential wetting and drying cycles even under repeated exposures to saline solution. The effect of block copolymer surfactants with different molecular weights (MW) and hydrophilic-lipophilic balance (HLB) values on the pHEMA-MAA wettability were also studied. Low Theta(w/a) values were observed for pHEMA-MAA hydrogels that were treated with T1304 (MW 10500, HLB 14) and T904 (MW 6700, HLB 15). The surface tension data indicated that these surfactants were incompletely desorbed from the pHEMA-MAA and that the rate of desorption was slow in the timescale of the cycling experiments. Comparatively, poor wettability was observed for pHEMA-MAA surfaces presoaked in T304 (MW 1650, HLB 16) and T1107 (MW 15000, HLB 24) as Thetaw/a values greater than 90 degrees were measured for these surfactants. The surface tension data indicated that the rate of desorption of T304 and T1107 from the pHEMA-MAA was rapid and that they had a low affinity to the pHEMA-MAA. High contact angles were observed for the pHEMA-MAA hydrogels treated with lysozyme and also for the T1107 presoaked pHEMA-MAA that was also treated with lysozyme. Zero wetting angles throughout the sequential cycling were observed for the T1304 pre-treated pHEMA-MAA that had been treated with lysozyme. These results suggested that the adsorbed lysozyme on the pHEMA-MAA hydrogel had no significant influence on its wetting properties when the hydrogel was pre-treated with T1304.

Eye complaints in the office environment: precorneal tear film integrity influenced by eye blinking efficiency

To achieve a common base for understanding work related eye complaints in the office environment, it is necessary to merge approaches from indoor air science, occupational health, and ophthalmology. Based on database searches, it is concluded that precorneal tear film (PTF) alteration leads to eye complaints that may be caused by: (1) thermal factors (low relative humidity; high room temperature); (2) demanding task content (attention decreases blinking and widens the exposed ocular surface area); and (3) individual characteristics (for example, tear film alterations, blinking anomalies, gland dysfunctions, and use of contact lenses). These factors and conditions are able to progressively increase water evaporation and faster thinning of the PTF, which causes dryness and dry spot formation on the cornea, possibly followed by corneal and conjunctiva epithelial alterations and eye complaints. Another possible cause of eye complaints is certain irritating chemical compounds, in addition to oxidation mixtures that are formed in reactions between ozone and unsaturated organic compounds (alkenes). The effect may be exacerbated by low relative humidity.