Contact Lens & Anterior Eye
Volume 31, Issue 5 , Pages 228-241, October 2008

Predicted tear layer oxygen tensions under two designs of silicone hydrogel toric lenses☆

Jules Stein Eye Institute, Department of Ophthalmology, David Geffen School of Medicine at UCLA, 100 Stein Plaza, Los Angeles, CA 90095-7003, United States

published online 31 July 2008.

Article Outline

Abstract 

Introduction

The goal of this study was to apply a theoretical analysis of the relationship between contact lens oxygen transmissibility and tear layer oxygen tension to silicone hydrogel toric lenses and thereby model the success of such lenses in decreasing corneal hypoxia.

Method

Lens thickness was measured at different points along the vertical meridian of one prism-ballasted, one dynamic-stabilized silicone hydrogel toric lens design, and one control “traditional” hydrogel toric lens design. Using lens thickness measurements and the nominal oxygen permeability values of these three lens designs, the subsequent oxygen tension in the tear layer trapped between the contact lens and the anterior cornea (P2) were calculated for both open- and closed-eye conditions using a single corneal chamber model with a personal computer program.

Results

We found that cylindrical powers, regardless of lens materials, did not have a statistically significant effect on lens thickness (F=0.30, p=0.5834) to the limitation of our measurements, while contact lens type, spherical power, location on lens, and axis of cylindrical power were all found to do so. When multi-factor ANOVA was applied to the lens thickness and P2 data, contact lens location had a statistically significant effect on P2 values for both prism-ballasted (F=640.16, p<0.0001) and dynamic-stabilized toric lenses (F=352.85, p<0.0001). When the same statistical methodology was used to compare the relative performance of all three lenses on P2 during daily wear, the average P2 values for the three different lens brands were all statistically significantly different, while under closed-eye conditions the average P2 values for both silicone hydrogel toric lenses were no longer statistically significantly different (F=1.00, p=0.3178).

Conclusion

Assuming that the critical P2 was 100mmHg, we predicted that silicone hydrogel toric soft lenses would provide reasonable anterior corneal oxygenation, certainly much enhanced over the environment predicted under traditional hydrogel designs, especially during daily (e.g. open eye) wear. Substantial corneal hypoxia continued to be predicted during extended wear of all lenses, but especially so with the use of traditional hydrogels.

Keywords: Oxygen transmissibility and permeability, Silicone hydrogel, Toric, Contact lens, Prism ballasted, Dynamic stabilized

 

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1. Introduction 

Hypoxia is a classic complication of contact lens wear and is known to interrupt corneal metabolism leading to acute epithelial and stromal edema, and chronic thinning [1], [2]. Other corneal complications associated with hypoxia include endothelial polymegethism and blebs, stromal neovascularization, and limbal hyperemia [2], [3], [4]. Although increased contact lens oxygen transmissibility has also been linked to increased superficial epithelial cell exfoliation and decreased initial microbial binding to corneal cells, protection from microbial infection remains debatable [5], [6], [7], [8].

Silicone hydrogel materials have been specifically developed for contact lenses to dramatically improve corneal oxygen supply (four to six times greater than that available with traditional hydrogel materials) [2], [9] and have been clinically available in spherical designs for about a decade. It has been estimated, however, that about 40% of patients wearing soft lenses would benefit from astigmatic correction [10]. This potential demand for toric soft contact lenses has stimulated the recent development of several silicone hydrogel toric contact lens designs.

Traditional toric hydrogel contact lens designs usually employ a base-down prism to position and stabilize the astigmatic optical axis before the patient's pupil, resulting in an increased lens edge thickness in the inferior portion of the lens’ vertical meridian. Alternate toric lens designs (such as double thin zones or Accelerated Stabilization Design (ASD)) use zones of accelerated slope of thickness in the interpalpebral region of the contact lens to reduce those lid interactions that lead to lens rotation. Toric contact lens thickness profiles therefore vary depending on design, material, and the prescription of the contact lens.

Eghbali et al. [11] calculated that prism-ballasted toric hydrogel contact lens designs have significantly lower oxygen transmissibility in the inferior portions of their vertical meridians where the contact lenses are thickest. This has been known to often lead to hypoxic corneal problems like neovascularization [12].

To extend the investigation of Eghbali et al. [11] to silicone hydrogel toric soft lenses, we herein report lens thicknesses measurements for two designs of silicone hydrogel toric contact lenses (one prism-ballasted and one ASD design) along their vertical meridians, with one traditional toric hydrogel as a control, the calculated theoretical oxygen transmissibility at these locations, and the secondary tear layer oxygen tension values, predicted via a simple single chamber cornea model for both open-and closed-eye conditions.

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2. Methods 

2.1. Lens selection and measurements 

Two silicone hydrogel toric lens designs, one prism-ballasted (Bausch & Lomb PureVision™ Toric) and one ASD (Acuvue Advance for Astigmatism™), were evaluated in this study. A “traditional” hydrogel toric (Bausch & Lomb Soflens 66™ Toric) was also evaluated to serve as a control. All contact lenses were donated by their manufacturers expressly for use in this study. For each contact lens design, 16 different lens prescriptions were measured: combinations of four spherical powers and two cylindrical powers at two different axes (i.e. spherical powers of +4, +2, −2, and −4 with cylinder powers of −1.75 and −1.25 and axis of 90° and 180°).

After being removed from their blister packaging, the contact lenses were oriented to the vertical position via the manufacturer-indicated laser markings, placed on a metal cylinder with a curved spherical top surface upon which are engraved with concentric circles (radii of 3, 6, 7, 7.25, and 7.5mm) [13], and dotted with a permanent marker at five positions along their vertical medians: center of the lens (position 0), 3mm above and below the center (+3 and −3 positions) and 6mm above and below the center (+6 and −6 positions) (Fig. 1). After so dotting each silicone hydrogel toric lens, it was re-soaked in 0.9% saline (or similar contact lens solution) in its blister pack for at least 10min to permit full rehydration before measuring its thicknesses.

Each contact lens was placed on an electric thickness gauge [14] for measurement, starting with the +6 position. A Kimwipe™ (Kimberly-Clark, Irving, TX) was used to lightly remove excess solution on the contact lens surface so that the surplus moisture would not interfere with the measurements. For each position, three consecutive measurements were taken. Each lens was again soaked in the original solution for at least 10min in between measuring each position.

2.2. Calculation of tear oxygen tension under the contact lenses 

Oxygen permeability or Dk (where D is the oxygen diffusion coefficient and k is oxygen solubility) is an intrinsic material property. Dk is directly dependent on a material's oxygen permeable moieties for both rigid and hydrogel plastics, usually silicone and/or water [15], [16]. Oxygen transmissibility is Dk/t in the contact lens literature where t is sample (e.g. lens) thickness [15], [16], [17], [18], [19], [20]. Both Dk and Dk/t are laboratory “in vitro” measurements most commonly determined by the polarographic method of Fatt [15], [21]. Dk is measured in units of 10−11 (cm2/s) (mlO2/mlmmHg). Dk/t, where t is in cm, is measured in 10−9 (cm/s) (mlO2/mlmmHg). Benjamin has proposed “Fatt Dk units” as alternate units for Dk for simplicity [20] and this will be used below. Dk/t is then measured in Fatt Dk/t units.

From the thickness measurements described above, local oxygen transmissibility (or Dk/t) values were calculated by dividing the manufacturer supplied nominal values of oxygen permeability (Dk) (Bausch & Lomb PureVision™ Toric: 99; Acuvue Advance for Astigmatism™: 60; and Bausch and Lomb Soflens 66™ Toric: 25FattDk units, respectively) by the measured mean thickness for each position on each toric contact lens.

Huang et al. [22] extended the Fatt and Ruben [23] single corneal chamber model computer spreadsheet technique to predict tear layer oxygen tension from contact lens Dk/t to spherical soft lenses of 60–150FattDk units. Brennan developed a more complex (eight-chamber) model that plots corneal/contact lens oxygen tension profiles [24]. The Brennan model continues a multilayer revision of Fatt's model cornea proposed by Harvitt and Bonanno [25]. Harvitt and Bonanno [25] corrected for altered oxygen consumption with corneal acidosis and provided newer boundary conditions for both closed eye (60mmHg) [26], [27] and aqueous humor (24mmHg) oxygen tensions [28]. The Brennan model includes several theoretically important improvements: Brennan corrects for spurious oxygen consumption when the oxygen tension falls to zero within the cornea, and he improves several thickness values using more modern estimates. We herein extend the Huang et al. [22] model to silicone hydrogel toric soft lenses. We use several of the Brennan improvements in boundary conditions but not his more complex model (as will be discussed below).

Various constants (corneal thickness or tcor; anterior cornea oxygen tension (P1) for the open or closed eye; whole corneal oxygen consumption or Q; aqueous humor oxygen tension or Paq; and corneal oxygen transmissibility or Dkcor) (see Table 1 for constants used here) were placed into a Microsoft Excel™ spreadsheet developed by Huang et al. [22] after the spreadsheet previously discussed by Fatt and Ruben [23] to allow theoretical calculation of tear layer oxygen tension (P2) under contact lens systems.

Table 1. Corneal parameters and boundary conditions used in the calculations
tcor (μm)Q (μlO2/(cm2h))P1closed (mmHg)P1open (mmHg)Paq (mmHg)Dkcor (FattDkunits)
Fatt and Ruben [23]5009.54551555524.7
Harvitt and Bonnano [25]495Various (by layer)61.415524Various (by layer)
Huang et al. [22]5609.54551555524.7
Brennan [24]532Various (by layer)61.515524Various (by layer)
Current study540/677a9.7601553024.7

Note—tcor: corneal thickness; Q: corneal oxygen consumption; P1closed: eyelid oxygen tension; P1open: open eye oxygen tension; Paq: aqueous humor oxygen tension; Dkcor: corneal oxygen permeability.

aCentral corneal thickness of 540μm and peripheral corneal thickness assumed to be 677μm just inwards from the corneal limbus.

We use the value for closed-eye palpebral oxygen tension accepted by Brennan [24] and Harvitt and Bonanno [25] of 60mmHg rather than Fatt's earlier [29], [30], [31] value of 55mmHg. A value for central human corneal thickness of 540μm (after Doughty and Zaman [32]) is used here rather than the earlier values of 500μm used by Fatt and Ruben [23] or 560μm used by Huang et al. [22]. Alvord et al. [33] suggested that it is not appropriate in such calculations to use central corneal thickness for peripheral corneal predictions so we will use their peripheral corneal value of 677μm for our model (at the ±6 positions). Although the values for Paq at different location of the cornea have been controversial (central vs. peripheral) [34], [35], sample calculations showed that the difference between predicted P2 values obtained using the different proposed Paq for peripheral/central cornea in our computer model yield value small differences (less than 4mmHg [unpublished data]). Therefore, we used only one value (30mmHg) for Paq for both central and peripheral cornea as a compromise between the value of 55mmHg used by Fatt and colleagues [23], [29], [30], [31] and the value of 24mmHg used by Brennan [24] and Harvitt and Bonanno [25].

It is necessary to consider the role of corneal oxygen consumption (Q). Each layer of the cornea has separate values for Q (as well as for thickness, Dk, etc.), based on the individual metabolism and volume of each inherent group of living cells. Consideration of layer-by-layer individual parameters would lead to a much more complex model of corneal oxygenation with several unknowns and equations to be solved simultaneously as has been done by Fatt and colleagues, Harvitt and Bonnano, and Brennan [24], [25], [29], [30], [31].

But for the sake of this simple calculation, where we seek to calculate only P2, it may be reasonable to only use a simpler single chamber model and a whole corneal value for Q. This was first measured at 9.54μlO2/(cm2h) (or 6.58×10−5mlO2/(mls)) by Freeman with rabbit cornea [36] and this value was used by Fatt and colleagues [22], [29], [30], [31]. Bonanno et al. recently estimated human Q at (5–6)×10−5mlO2/(mls), not much different than a previous estimate of about 4×10−5mlO2/(mls) [37], [38]. Considering the variations in this value reported over the years, we will use 5×10−5mlO2/mls (or 9.7μlO2/(cm2h)) as a fair estimate.

Whole corneal Q may not be independent of either oxygen tension or pH. Forty years ago Fatt suggested that Q should be fairly stable at oxygen tensions greater than 20mmHg [39]. As others have found stability in Q at perhaps only a few mmHg [40], changes in corneal Q related to low oxygen tension will not be considered further here. Brennan [24] accepted the Harvitt and Bonanno finding that Q increases with acidosis [41] and this was incorporated into his model but is not used in our simple model.

It is important to further note that we have previously satisfied ourselves that it is reasonable to use our simple model for these kinds of calculations when solely predicting P2; comparison of our model's predictions to those of Brennan's eight-chamber model suggest minimal different P2 values for the open eye [42].

Excel (Microsoft Office 2000 Standard, Microsoft Corporation, Redmond, WA) does the mathematical work. The entries/values were entered into a newly opened Excel spreadsheet on a PC computer running Windows XP (Microsoft Corporation, Redmond, WA) into cells A1–A3, B1–B3, C1–C3, and D1–D3. Cells A1–A3, B1–B3, and C1–C3 form a 3×3 matrix. By use of provided built-in Excel mathematics functions, the matrix A1–A3, B1–B3, and C1–C3 was inverted and placed into cells A6–A8, B6–B8, and C6–C8.

This new matrix is then multiplied by the 3×1 matrix D1–D3. The resulting 3×1 matrix is placed in cells D6–D8. Because Excel cannot directly multiply a 3×3 matrix with a 3×1 matrix, we used sub-steps. The 3×3 matrix is divided into three sub-matrices, each with the dimensions of 1×3. This allows for a simpler matrix multiplication where [1×3]×[3×1]=[1×1]. Multiplying each of the three sub-matrices by the [3×1] matrix gives three number values viewed as a 3×1 matrix in D6–D8.

The mathematics described above placed a value for B in D6, B′ in D7 and C′ in D8 in the spreadsheet (see Fatt and Ruben [23] and Fatt and Weissman [29] for definitions of B, B′ and C′) for each lens at each thickness and boundary conditions. These values were used to calculate oxygen tension values (P) by

where x is tcor, the thickness of the cornea, P will be P2, the predicted oxygen tension at the corneal surface.

P2 under each of the vertical points of each toric soft lens, both the two silicone hydrogels and the control hydrogel, for both open- and closed-eye conditions, were calculated by the method above and placed into Excel tables for presentation as the graphs shown below. We calculated single point (local) oxygen tensions here rather than averages following Fatt and Neumann's [43] concept that the corneal epithelium will be stressed in reflection of the immediately overlying oxygen resistance, particularly as we are evaluating contact lenses of variable thicknesses.

Oxygen flux (j) might be clinically useful as it represents the amount of oxygen actually reaching the cornea per unit time. Flux through a lens can be calculated from j=(Dk/t)(P1−P2). Flux into a cornea can also be calculated by jcor=Qtcor+BDkcor [37]. These values should be identical, with units of μlO2/(cm2h). Morgan and Brennan reported that the relationship of j and Dk/t is approximately linear between Dk/t values of zero to about 50Fatt units. Once the Dk/t value surpasses 50Fatt units, however, the relationship between the two variables approaches an asymptote, which means that the increase in oxygen delivered to the cornea through contact lenses is minimal with Dk/t values ≥50 [44]. Since the silicone hydrogel toric contact lenses used during this study both have Dk/t values close to or over 50 at any given point, it is more advantageous to utilize the P2 values to characterize the performance of these lenses during this study. For comparison, however, we also calculated oxygen flux values presented in Fig. 14, Fig. 15, Fig. 16, Fig. 17, Fig. 18, Fig. 19.

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3. Results 

Our results are shown graphically in Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12, Fig. 13, Fig. 14, Fig. 15, Fig. 16, Fig. 17, Fig. 18, Fig. 19.

3.1. Lens measurement and profile 

As predicted, the toric lens designs that used prism-ballasted technology for lens stabilization (both Bausch & Lomb Soflens 66™ Toric and PureVision™ Toric) were found by measurement to be thicker in their inferior portions regardless of contact lens prescription (Fig. 2, Fig. 3, Fig. 4, Fig. 5). The average thickness at the −6 position was found to be 0.386mm for Soflens 66™ Toric and 0.306mm for PureVision™ Toric lenses in our sample.

The non-prism-ballasted lens (i.e. Acuvue Advance for Astigmatism™) showed a lens profile that roughly corresponds to a concave shape for minus spherical powered lenses but is fairly uniform for the plus powered lenses (Fig. 6, Fig. 7). The average thickness at the −6 position was 0.127mm for the Acuvue Advance for Astigmatism™ lenses in our sample.

Of interest, we note that the magnitude of the cylindrical power does not substantially affect lens thickness for any of our sample lens designs at the limits of this calculation. To test the effect of cylindrical power on thickness we used multi-factor ANOVA with the following factors: contact lens type (e.g. Soflens 66™ Toric), spherical power, location on lens (e.g. +6), and axis of cylindrical power (e.g. 90° or 180°). We found that cylindrical power does not have a statistically significant effect on thickness (F=0.30, p=0.5834). At the same time, however, contact lens type, spherical power, location on lens, and axis of cylindrical power all had statistically significant effects on thickness.

3.2. Theoretical tear oxygen tension under the contact lens 

Calculated tear oxygen tensions for the prism-ballasted contact lens designs (i.e. PureVision™ Toric and Soflens) showed a definite decrease as the lens thickness profile increased for both designs (the correlation between P2 level and lens thickness was −0.58 in the open-eye condition and −0.46 in the closed-eye condition, both statistically significant at the 99% level) (Fig. 8, Fig. 9, Fig. 10, Fig. 11). Both the Soflens 66™ Toric and the PureVision™ Toric P2 trends are similar in shape, with the highest tear P2 values found at the second highest position (+3 position) of the contact lens and the lowest P2 values are generally found at the bottom position (−6 position). Note, however, that the tear P2 values found under the PureVision™ Toric as a group are far greater than those found with the Soflens 66™ Toric design, consistent with the difference in material Dk. As with any simplifying methodology, there are limitations. One such limitation of the computer model employed here is that, under conditions implying very low oxygen transmissibility, it can predict negative P2 values. Because negative values are not possible, we set those values that were predicted to be negative to equal zero. Brennan's eight-chamber model [24] excludes negative stromal oxygen tension values but does not result in much difference in predicted open eye P2 values [42]. In the case of the Soflens 66™ Toric, predictions for P2 are at zero for all lenses in our sample under closed-eye conditions (extended wear) and are at or near zero under open-eye conditions (daily wear) for the bottom half of this lens design as well.

When compared to both prism ballast lenses (Soflens 66™ Toric and PureVision™ Toric), the ASD toric lens (i.e. Acuvue Advance for Astigmatism™) showed relatively uniform thickness throughout the lens regardless of prescription (Fig. 12, Fig. 13). When examined closely, the Acuvue Advance for Astigmatism™ lenses showed higher P2 values in their middles (position 0) for lenses with minus sphere powers and slightly higher P2 values at their mid-peripheries (at the +3 and −3 positions) for lenses with plus sphere powers.

To determine the effect of lens thickness on P2 values (ASD versus prism-ballasted silicone hydrogel toric lens designs) we used multi-factor ANOVA. The factors included in the analysis were contact lens brand (e.g. PureVision™ Toric), spherical power, location on lens (e.g. +6), axis of cylindrical power (e.g. 90° or 180°), and eye condition (e.g. open or closed). Multi-factor ANOVA using only data for PureVision™ Toric indicated that lens location has a statistically significant effect on P2 values (F=640.16, p<0.0001). Multi-factor ANOVA using only data for Acuvue Advance for Astigmatism™ also indicated that location has a statistically significant effect on P2 values (F=352.85, p<0.0001). Location appears to be a relatively more important determinant of P2 for Acuvue Advance for Astigmatism™ than for PureVision™ Toric (for Acuvue Advance for Astigmatism™, 35% of the total sum of squares explained by the model is attributable to location while for PureVision™ the corresponding value is only 21%), probably related to the increase in Dk of the PureVision™ material compared to that of the Acuvue Advanced for Astigmatism™ material.

We also used the same multi-factor ANOVA methodology to compare the relative performance of all three lenses on P2 depending on whether the eye was open or closed, with the following factors: eye condition, lens brand, and interaction between eye condition and lens brand (e.g. open-and-Acuvue Advance for Astigmatism™ or open-and-PureVision™ Toric). While the ranking of the lenses did not change depending on eye condition (PureVision™ Toric had higher average overall P2 values than Acuvue Advance for Astigmatism™, which in turn had higher average overall P2 values than Soflens 66™ Toric, in agreement with their rank order of Dk), the difference in oxygen availability among the three lenses, and especially between Soflens 66™ Toric and the other two lenses, was much greater under open than closed-eye conditions, as would be predicted. Under open-eye conditions, the average P2 values for the three different lens brands are all statistically significantly different, whereas under closed-eye conditions the average P2 values for Acuvue Advance for Astigmatism™ and PureVision™ Toric are no longer statistically significantly different (F=1.00, p=0.3178).

3.3. Oxygen flux 

We used the formula j=(Dk/t)(P1−P2) to obtain oxygen flux values for the measured lens positions of all three contact lens types under both open- and closed-eye conditions. Because P2 has a negative relationship with oxygen flux, the variation in flux values across lens positions is the opposite of that found for P2. This is most easily seen in the Acuvue Advance for Astigmatism™ figures, since the thickness is fairly uniform and symmetric. For example, under open-eye conditions, Acuvue Advance for Astigmatism™ showed oxygen flux values highest at the peripheral lens location, where P2 values were lowest.

For the other two lenses, the asymmetric thickness alters the simple relationship between P2 and flux. While the Soflens 66™ Toric has low P2 values, it also has low flux values due to the greater thickness and lower Dk. The highest flux value occurs at +6mm, where both P2 and thickness are at their minimum. Similarly for the PureVision™ Toric, the highest flux value occurs at +6mm where P2 is close to its minimum and thickness is at its minimum. Due to the higher Dk value of PureVision™ Toric, it obtains a flux of 33.1μlO2/(cm2h) at the +6mm position (with −2.00−1.25×090), the maximum observed for any of the lenses. Under closed-eye conditions, for all of the lenses, the trends in flux values across lens power and location remained similar to that found with open-eye conditions but all of decreased magnitude.

Due to the large number of observations (six positions per lens, eight powers, and three types of lens), the data is presented here in graphical form (Fig. 14, Fig. 15, Fig. 16, Fig. 17, Fig. 18, Fig. 19).

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4. Discussion 

We first acknowledge limitations to our model. One of the limitations of our computer model is that it can result in negative P2 values. As negative values are not possible, we set those values that were predicted to be negative to equal zero. Even if these P2 values are not equal to zero, suffice it to say that they are very low. Modifications to the computer model are beyond the scope of this paper. Also, due to lens design difference, the ASD-stabilized toric lens may seem to have a lens thickness advantage over prism-ballasted toric lens design along the vertical meridian. ASD-stabilized toric lenses, however, may have increased lens thicknesses in their horizontal meridians, not measured here, secondary to the characteristic of the ASD design. Both of these issues may be addressed in future research.

We also acknowledge there may be different aqueous oxygen tensions at various locations in the anterior chamber [34], [35] and it may also differ (potential homogenization with mixing) with ocular movement, but for the sake of this simple calculation, we have used only one value (30mmHg) for Paq under both the central and peripheral cornea.

Both Holden and Mertz [45], Harvitt and Bonanno [25], and others, have proposed critical Dk/t values below which corneal physiology might be compromised (ranging from 25 to 125FattDk/t units), but Fatt stated that Dk/t was an in vitro measurement and a “disappointment” as a measure of in vivo performance [46]. First, oxygen tension under a contact lens on an electric sensor (in vitro) is always zero at the steady-state. It is both undesirable as well as unlikely to reach zero under a lens on a living eye. Secondly, the flux of oxygen across a lens on a polarographic sensor tends to infinity as the lens t decreases to zero—but the in vivo limiting case (e.g. without contact lens on an eye) is an oxygen flux into the cornea sufficient to sustain its metabolism (about 5–10μlO2/(cm2h)) [47], [48].

The debate between using oxygen flux and P2 to best describe the performance of the contact lens material in supplying oxygen to the cornea continues. Although it is more relevant in clinical setting to consider the amount of oxygen reaching the cornea per unit time, in vivo corneal oxygen flux (j) has been reported to be useful only when lens Dk/t values were low [24]. The change in oxygen flux values is minimal with the advance of modern lens materials (values ≥50–100FattDk units) [44].

Polse and Mandell originally used tear layer oxygen tension (P2 as above) as a marker for oxygen supply adequacy and found a “critical value” of 11–19mmHg; others have since raised this value to 70–125mmHg or greater [1], [2], [4], [25], [49], [50].

Brennan proposed using the percent of normal corneal oxygen consumption (Q) as another possible measure, but this has yet to be widely accepted [24].

Fatt and Ruben introduced the concept of “biological oxygen apparent transmissibility” (BOAT) to be a measure of in vivo contact lens oxygen performance: BOAT=(Dk/t)cl(P1−P2/P1) [23]. BOAT, unfortunately, is confusing and has not been used much.

Given the above discussion, as tear P2 seems as good a surrogate as any to us, and is both understandable and usable, we elected to use P2 as our metric here. Inspection of the literature suggests that maintaining a P2 value of about 100mmHg (although some authors argue for higher and some for lower values) should be sufficient for most human corneas under most conditions. Additional support for 100mmHg as a critical P2 value is found in the data of Briggs-Dokubo (unpublished, but reported in Larke JR. The eye in contact lens wear (2/e). Oxford: Butterworths-Heinemann; 1997) wherein very little reduction in corneal lactate production was found with oxygen environments greater than 100mmHg.

The graphs shown in Fig. 8, Fig. 9, Fig. 10, Fig. 11, Fig. 12, Fig. 13 predict that both silicone hydrogels provide corneas with much enhanced P2 values compared to the values found with the “traditional” hydrogel lens. The centers of both positive and negative Acuvue Advance for Astigmatism™ torics provide tear P2 values >100mmHg under daily wear conditions, and the superior three quarters of both positive and negative powered PureVision™ Toric lenses do the same. Extended wear dramatically decreases P2 under all lenses, even more so for traditional toric soft lenses.

Huang et al. [22] previously suggested that contact lens thickness did not contribute much to such calculations with Dk values of 100Fatt units or greater, at least for spherical soft lenses. Our calculations suggest that thickness values do contribute substantially to corneal oxygenation with silicone hydrogel toric lens designs however, and our statistical analysis confirms that the predictions and differences shown in our figures are significant. Only one of the lens designs studied here has a nominal Dk value of 100Fatt units, however, and these lenses all have thicker portions than expected with spherical soft lenses.

In sum, we report toric soft contact lens thicknesses measured along their vertical meridians and the subsequent predicted P2 values with two different types of silicone hydrogel toric lens designs (e.g. prism-ballasted and ASD) compared to a traditional prism-ballasted hydrogel toric lens. Our results show higher P2 values from the superior portions of both PureVision™ Toric and Soflens 66™ Toric while fairly uniform P2 values are found along the vertical meridian of Acuvue Advance for Astigmatism™. Under open-eye condition, results from both PureVision™ Toric and Acuvue Advance for Astigmatism™ predict P2 values of close to or above 100mmHg at all lens locations while Soflens 66™ Toric results in predicted P2 values of about 35mmHg at the superior portion of the lens and 0mmHg at its inferior portion. Under closed-eye condition, results from both PureVision™ Toric and Acuvue Advance for Astigmatism™ predict P2 values of close to or above 15mmHg while Soflens 66™ Toric results in predicted P2 values of 0mmHg at all lens locations. Although most authors now believe that silicone hydrogel lenses will not substantially reduce the incidence of presumed microbial keratitis associated with soft contact lens extended wear (compare 18/10,000/year found with silicone hydrogels [8] to 20/10,000/year with traditional hydrogel extended wear [51], [52]), the ability of silicone hydrogel contact lens materials to enhance oxygen to the anterior cornea is undeniable [6], [7].

Clinicians using traditional toric hydrogel lenses even for daily wear may wish to inform their patients of the increased risks of hypoxia to which such patients are exposed, and encourage them to use silicone hydrogels as they become available in all possible prescriptions.

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Acknowledgement 

We thank both Vistakon and Bausch & Lomb for most kindly supplying us with the sample contact lenses used in this study.

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References 

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☆ Dr. Forister was supported in part by an education grant from Vistakon, a division of Johnson & Johnson Vision Care Inc. Dr. Weissman's participation was supported in part by a research grant from Dr. Marvin Smotrich.

PII: S1367-0484(08)00093-3

doi:10.1016/j.clae.2008.06.003

Contact Lens & Anterior Eye
Volume 31, Issue 5 , Pages 228-241, October 2008

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