Technical note

Precision and reliability study of amodified keratometric technique formeasuring the radius of curvature ofsoft contact lenses

Nadia-Marie Quesnel and Pierre SimonetV

Fcole d'Optometrie. Universite de Montreal, Montreal (Quebec) H3C 3J7. Canada )

(Received 20 September 1993, in revised form 17 Jatiuary 1994)

V T IThe back optic zone radius of soft contact lenses cannot be meisured with conventional methods used for rigidlenses. Although a variety of techniques has been proposed, none has received wide clinical acceptance, A studywas conducted to determine the precision and limitations of a method using a Zeiss keratometer. A total of 192lenses from categories 1 (Optima 38) and 4 (Hydrocurve n 55%) of the FDA classification were used. Theselenses were divided into six groups according to their water content and refractive power. The difference versusthe meanof two different measurements (repeatability) has been plotted to determine the bias (meanof the differencesrelative to zero), the 95% limits of agreement and the repeatability coefficient of the technique. Variation betweenmeasurements was also assessed by calculating the reproducibility coefficient R for each sofl lens. The precisionexpressed by the SD of a series of ten measurements on one lens in each group, was computed for comparisonwith previous studies. The repeatability of the technique expressed in term of 95% limits of agreement(range ± 0.058 to ± 0.198 mm) meets the American Standards Tolerances for each group of soft lenses studied.The coefficieni of reproducibility R for each soft lens studied seems to be related to lens flexibility and dioptricpower. The technique used gives the same degree of precision as other keratometric methods described in previousstudies. The ciinical use of this method is discussed with regard to practical factors. ,

Ophthal. Physiol. Opt., 1994, Vol. 14, 320-325, July

An accurate and reliable method of measuring soft contactlens back optic zone radius (BOZR) is an essential part ofquality control for the manufacturer and would consider-ably assist the practitioner in evaluating the situation whereletises ordered do not perform as expected. Most manu-facturers do not check the hydrated soft lens parameters,but check the dry lens and rely on a consistent expansionof the lens to the hydrated state'.

A wide variety of soft lens base curve measurementtechniques using a keratometer has been proposed'"^, butnone has enjoyed widespread acceptance. The purpose ofthis study was to determine the precision, reliability andlimitations of a method using a keratometer which did notneed any modification to the illumination system or to themeasuring scale. Also, instead of using a wet cell not easilyavailable to practitioners, a contact lens case cap was used.The repeatability and reproducibility of the technique wasassessed by the use of statistical analysis methods whichgive more information about the variability of themeasurements than the SD of a series of measurements asused in previous studies.

Correspondence to Dr P, Simonei,

Materials and method

Instrument

A Zeiss keratometer, model D-7082, mounted on the Zeiss30 SLM slit lamp (Humphrey Instruments (Carl ZeissCanada), Don Mills, Ontario, Canada) was used (Figure/). This keratometer is designed on the Helmholtz principleand its measuring scale ranges from 4.0 to 11,20 mm (indivisions of 0.01 mm). Maximum illumination and x 12magnification of the slit lamp were chosen because, even ina dark room, mire images were too faint when highermagnifications were used.

An inclined mirror attached to a special support, asshown in Figure J, was used for the observation of thekeratometer's mires images given by the lens completelyimmersed in saline solution. These mires consisted of twooutline and two solid crosses.

The optical systems of this method of measurement canbe treated as a single equivalent mirror so that the vertexof the mirror and the centre of curvature of the equivalentmirror are displaced towards each other as illustrated inFigure 2. Point A is the vertex and point C is the centre ofcurvature of the back surface of the lens. A ray emergingfrom the vertex A on the actual mirror surface will pass

320 Ophthal. Physiol. Opt., 1994, Vol. 14, July©1994 Butterwonh-Heinemann lor British College of Optometrists0275-5408/94/03320-06

Measurement of soft contact lense curvature: N-M. Quesnel and P. Simonet

Figure 1 The Zeiss keratometer model D-7082 and the special supportholding the inclined mirror and ihe contact lens case cap

Figure 2 The equivalent mirror effect which results in a reduced backoptic zone radius when a liquid-immersed lens is measured bykeratomeiry', A Is the vertex and C is the centre of curvature of the backsurface of the lens, A ray emerging from the vertex A on the actual mirrorsurface will pass through the saline and be refracted at the surface toappear to come from A' such that A'O = AO/n, where n, is the refractiveindex of saline-, A ray leaving the mirror surface normally and directedtoward C will be refracted by the saline and imaged al C such thatOC' =OC/«,, The distance A 'C is the radius of curvaiure of theequivalent mirror. Therefore. Ihe radius of ctirvature in saline [r^^^„^) isshoner ihan ihe radius of curvature in air ir^J

through the saline and be refracted at the surface to appearto come from A' such that A'O ^ A0/«, where n^ is therefractive index of saline'. A ray leaving the mirrorsurface normally and directed toward C will be refracted by

the saline and imaged at C such that OC = OC/n,. Thedistance A 'C is the radius of curvature of the equivalentmirror. Therefore, the radius of curvature in air (r̂ ,r) isequal to the radius of curvature in saline (r,̂ i,n̂ } multipliedby the refractive index of the saline solution", plus acompensation factor since a keratometer calibrated forconvex surfaces will slightly underestimate the radii ofconcave surfaces'̂ by ^ 0.03 mm.

Calibration

The keratometer scale is calibrated for readings of convexsurfaces in air. Therefore, the instrument must be recali-brated for measurement of concave surfaces in saline""\This was done by repeated measurements (n = 10) of theBOZR of nine afocal pwlymethyl methacrylate (PMMA)lenses (range 7.39-8.55 mm) with a AO Radiuscope(model 11800) (Innova, Laval, Quebec, Canada) and withthe Zeiss keratometer.

To simulate the experimental procedure used for softlenses, PMMA lenses were immersed in saline solution forthe measurement with the keratometer. A calibration curve(Figure 3) was obtained by plotting the mean radius readingobtained in saline with tbe keratometer (variable .v) againstthe mean radius value obtained in air with the Radiuscope(variable v) for each PMMA lens. The linear regressionequation (v = -0.03811 + 1.3569.ic) was used to calculatethe true BOZR which will be identified as tbe correctedBOZR. Tbe correlation coefficient of the linear regressionwas r = 0.996.

Tbe calibration system used (Figure 3) eliminates theneed to apply compensatory factors to keratometricreadings. It was not necessary to recalibrate from a convexto a concave surface or to allow for tbe effect of tberefractive index of the saline on tbe measurement of BOZR.This is the major advantage of the technique described.

Contact Lenses

A total of 192 unused soft contact lenses from categories 1(Optima 38, Bausb & Lomb, Richmond Hill, Ontario.Canada) and 4 (Hydrocurve II 55%. PBH, Missauga,

E 7,40 -

7,20

5.40 5,50 5,60 5,70 5,80 5,90 6.00 6,10 6.20 6,30 6,40

BOZR measurements in saline with keratometer (mm)

Figure 3 Calibration curve for convening keratometer readings to trueor corrected back oplic zone radius (BOZR), For each PMMA lens, ihemean radius reading obtained in saline with ihe keratomeler is plottedalong the abscissa and the mean radius value obtained in air with theRadiuscope is plotted along the ordinate. The true or corrected BOZRvalue is obtained by the equation: y = -0,03811 + 1.3569A:, Correlationcoefficient r is 0.996,

Ophthal. Physiol. Opt., 1994, Vol. 14, July 321

Measurement of sofi contact lenses curvature: N-M. Quesnel and P. Simonet

Table 1 Parameters and grouping of soft contact lenses used in the study

Sofi lenses Material Groupnumber

Number oftenses

Nominal back vertexpower (D)

Nominal back optic zone Diameterradius (mm) fmm)

Bausch & Lomb Optima 38Batjsch & Lomb Optima 38Bausch & Lomb Opiima 38Pilkington Barnes-Hind Hydrocurve [I 55%Pilkington Barnes-Hind Hydrocurve I! 55%PilWinglon Barnes-Hind Hydrocurve U 55%

PolymaconPolymaconPolymaconBufilcon ABull Icon ABufilcon A

I23456

323232323232

Data

0,00+ 5,00- 5.00

0.00+ 5.00-5,00

anahsis

777555

14.014.014.014.514.514.5

Ontario. Canada) of the FDA classification were used.These lenses were divided into six groups (32 lenses in eachone) according to their refractive power {piano, + 5.00 D,-5.00 D) and vi'ater content (lower or higher than 50%).Nominal BOZR was 8.7 mm for all Optima lenses and 8.5mm for all Hydrocurve II lenses. Other parameters andgrouping of these soft lenses are listed in Table / .

Experimental procedure

The lens under measurement was placed concave side up ina contact lens case cap filled to the top with unpreservednormal saline solution. Care had to be taken when mea-uring that the edge of the lens did not come into contactwith that of the container at any point, and that the lens didnot adhere to the container. When the reflected mires wereof bad quality or doubtful accuracy, the lens was replacedin the container with tweezers. It was easier to correctlyplace PMMA lenses and low water content soft lenses andto obtain a clear image of the reflected mires. Each lens wasmeasured twice (test and retest). The lens was then removedfrom the container and the saline solution was changedbetween each measurement so that independent measure-ments were obtained.

For measuring, the two centre crosses of the mire, onesolid and one outline, had to be aligned one in the other.For positive or negative contact lenses, two sets of mirereflections were seen by the observer, one reflected fromthe front surface and the other from the back surface of thelens. For low power lenses (± 0.75 to ± 3.75 D), measure-ment of the BOZR by alignment of mire reflections seen inthe observation system was not possible because the imagesfrom the front and back surfaces of the lens were so closetogether that it was diflicult for the observer to decidewhich image belonged to which surface. Depending onwhether the lens had a positive or negative refractivepower, the mires reflected from the back surface becameeither smaller or larger respectively, than the ones reflectedfrom the front surface. The measurements were easier forvery low power ( ± 0.25 and ± 0.50 D) and piano lensessince no doubling of the mire reflections was observed.

All measurements were done by the same investigator tokeep the same criterion of mires alignment. The ambientvariables such as temperature, pH and osmolarity ortonicity of the saline solution used were not measured.However, all bottles of ALLERGAN Lens Plus bufferedsaline (Allergan Optical, Markham, Ontario, Canada) usedthroughout the experiment had the same serial number,suggesting that the pH and the osmolarity (0.9% NaCl)were invariable, and all measurements were carried out onthe same day at room temperature (22°C).

For one soft lens in each group, repeated measurements(n = 10) of the BOZR were used to assess the repeatabilityof the technique in terms of SD of a series of measures. Themean corrected value of these measures was computed forcomparison with the nominal base curve of these lenses.

In describing the accuracy of any measurement technique,validity and repeatability should be assessed when possible.To establish the validity, a reference standard technique isnecessary. Since this standard system of measurement forthe BOZR of soft contact lenses did not exist, the validityof the technique described was not determined.

In accordance with the suggestions of Bland and Aitman^repeatability of the technique was assessed by calculatingthe numeric difl'erence between the values obtained for eachlens on two different measurements (i.e. test—retest) and bysubsequent analysis of the distribution of those differencesfor each group of soft lenses. In this analysis, for eachgroup of soft lenses, the bias was assessed statistically asthe mean of the differences compared to zero (paired t-test)^^ The degree of repeatability (the limits of agree-ment) was the interval over which 95% of the differencesbetween the two measurements of each soft lens weresituated^**. Thus, 95% limits of agreement was the meandifference ± 1.96 x SD of the difl'erences observed in agiven group of soft lenses^ ̂ Differences in BOZR outsidethese 95% limits of agreement have only a 5% probabilityof being due to measurement error alone and a 95% chanceof being a true difference in BOZR.

The 95% limits of agreement were computed in order tobe compared with the Soft Lens Tolerances limits publishedby the American National Standards Institute for low andhigh water content lenses (±0.15 and ±0.20 mmrespectively )^

The repeatability coefficient, defined for this study as1.96 X SD of the test—retest differences^ were determinedfor each group of 32 soft lenses; the lower the coefficient,the better the repeatability. The repeatability coefficientwas also computed for two difFerent samples, one groupingall Optima 38 lenses except the afocal ones (n = 64); theother grouping all Hydrocurve II 55% lenses except theafocal ones (n =64).

To assess repeatability for each lens, the coefficient ofreproducibility R was computed. The value of thiscoefficient was derived from the absolute value ofdifferences between two measurements (test—retest) overthe mean of the two same measurements'*. This coefficientis lowest when reproducibility is best.

ResultsFigures 4a and b present the difference versus mean plotsfor BOZR measurements of the 32 piano lenses (Figure 4a)and the 64 ± 5.00 D {Figure 4b) Optima lenses. Figures 5aand b present the difference versus means plots for BOZRmeasurements of the 32 piano lenses (Figure 5a) and the 64± 5.00 D (Figure 5b) Hydrocurve 11 55% lenses. For eachgroup of soft lenses, the differences do not vary in anysystematic way over the range of BOZR measurements andthere does not appear to be any relation between thedifference and the value of the BOZR.

322 Ophthal. Physiol. Opt., 1994, Vol. 14, July

Measurement of sofi contact lense curvature: N-M. Quesnel and P. Simonet

Optima piano lenses Optima 1 5,00 D lanM«

o.^^ 10,20 -0,18 -0,16 -0 14 -0.1? -

f 0.10 -E 0.08 -r 0.06 -•N 0.04 -g 0 02 -c CM -•; -0 02 -i -0,04 -S -0,06 -g -0,08 -5 -0,10 -

•012 -•0,14 --0,16 --0,1B --0.20 -•0 22

Limil Di iMtance

Uppsriimil Dfagteamafll |35%j

Uiw« limn ol agtetnent (95%)

Limn ot lole^ance

0.22 -|0 20 •0.18 •0.16 •0.14 •o.ie •

c" 0.10 -I o.oe •^ 0,06 •N 0.04 •S 0 02 -c 0,00 -Z -0 02 •I -0 04 -£ -0.06 -g -0.08 -Q -0.10 -

•0.12 -•0,14 ••0,1 G •-0,1 a ••0,20 --0,22 -

Limir ol loterance

Uppef bm) ol agremem (95%)

Limn ol MleraFice

6.05 6,10 6,15 6,20 6,25 6.30 6 35 6 40 645 6 50 6 55 6.60 6,65 6,70 6.75

Mean BOZR (mm)

6,05 6.10 6,15 6.20 6.25 6,30 6.35 640 645 650 6,55 6,60 6,65 6,70 6,75

Mean BOZR (mm)

Figure 4 (a) Difference versus mean plot for back optic zone radius (BOZR) of each Optima 38 piano lens (n = 32), The differences do not vary in anysystematic way over the range of BOZR measurements. There does not appear to be any relation between the difference and the value of the BOZR,(h) Difference versus mean plot for BOZR of each Optima 38 ± 5.00 D lens (« = 64), The differences do not vary in any systematic way over ihe rangeof BOZR measurements. There does not appear to be any relation between the difference and the value of the BOZR

Hydrocurva II S5% piano IMISSB0 22 -10.200,18 -0 1 6 •0.14 •0.12 •0.10 •0,08 •0.06 • , ,0.04 -0.02 -0.00 •

-0.02 ••0.04 ••0 06 •-0.08 •-0,10 ••0,12 •

-0,14 •-0,16 --0.18 --0,20 • a —-0,22 -I——r

Limn of totsf anco

Upper liirM oj aerajnent (95%)

Limn ol tolerance

Hydrocurv* II55% ±5.00 D lenses

?•?? 1 Um« o( iDlwineB0 20 • -.•.-.-.-.-.--;-:;::;:r--—.-.---."-;/-::-::-.---v.-.v-.-.--:;-:;--.---.---"-•Q13 . upper Irrntotagrsmsni |95%|

0.16 ' • • •0.14 • •

0.12 • • « " • * * *"E" 0.10 • • • , •

| S : S 2 : •.. •N 0,04 • , , •

2 ; - : «• -0.02 •g -0,04 -• 0.06 - . • . ^a •o.oa - , • • • • • •5 • o i o - • • , • ,

-0.14 • «•0.16 --0.18 -020 • h -//-.v^"-"-::-;;-^^-v--;^-^^^ri-

•0.22 -I - 1 < 1 1 r~—. 1 1—

Lowet limn oi agrenwrn (95%)

Limil ol Iderancs

6.05 6 10 6.15 6.20 6.25 6.30 6.35 6.40 6.45 6.50 6.55 6.60 6 65 6,70 6.75

Mean BOZR (mm)

605 6.10 6.15 6,20 6,25 6 30 6.35 640 6,45 6.50 6 55 6 60 6.65 6 70 6 75

Mean BOZR (mm)

Figure 5 (a) Difference versus mean plot for back optic zone radius (BOZR) of each Hydrocurve II piano lens (n = 32), The differences do not vary inany systematic way over the range of BOZR measurements. There does not appear to be any relation between the difference and the value of the BOZR.(b) Difference versus mean plot for BOZR of each Hydrocurve II ± 5,00 D lens [n = 64), The differences do not vary in any systematic way over therange of BOZR measurements. There does not appear to be any relation between the difference and the value of the BOZR

Figure 6 shows the distribution of the values of thereproducibility coefficient R among the six groups of 32soft lenses. The results indicate tbat higher coefficients(lower reproducibility) are more frequent for HydrocurveII 55% than for Optima 38 lenses.

The bias, 95% limits of agreement, repeatability coeffi-cients and reproducibility coefficients R for BOZR measure-

ments of eacb one of tbe six groups of 32 soft lenses are listedin Table 2. Two other groups are added in Table 2, one of allOptima 38 lenses except the afocal ones; the other of allHydrocurve II 55% lenses except the afocal ones. All biaswere not statistically different from zero (paired /-test,P > 0.05).

The repeatability coefficient for eacb group of soft lenses

Table 2 Bias. 95% limits of agreement, repeatability coefficient and reproducibility coefficient R for each group of soft contact lenses

Lenses Bias' (mm) SDt(mm) 95% limits of agreement* Repeatability Reproducibility coefficient,(mm) coefficient** R***

Optima 38 (0,00 D)Opiima 38 (+ 5,00 D)Optima 38 (-5,00 D)Optima 38 (-h 5.00 and -5.00 D)Hydrocurve II 55% (0,00 D)Hydr<Kurve II 55% (+5.00 D)Hydrocurve U 55% (-5,00 D)Hydrocurve [I 55% (4 5,00 and -5.00 D)

0,00!0.0060.0020,0020,0010,0000.0010,000

0,0300.0540.0550,0540,0710,1010.0990,099

-0.057-0.111-0,106-0,108-0.138-0.198-0,193-0,194

to 0.060!o 0,10010 0,10910 0,104to 0,141to 0,198to 0.195to 0-195

0.0580.1050.1070,1060.1400.1980-1940,194

0,08% to 0-79%0.15% lo 1.40%0,08% to 1.69%0.08% to 1.69%0,40% to 1.67%0,47% to 2.56%0,76% to 2,14%0,47% to 2,56%

tMean of all the differences between iwo measurements (lest-retest) for each group of sofil lenses. None of these amounts of bias is statisticallydifferent from zero (paired f-tesi).JSD of all the differences between two measurements (tesl-retest) for each group of sofi lenses.*95% limits of agreement = bias ± 1,96 x SD of all the differences between two measurements (tesl-relest),••Repeatability coefficient = 1,96 x SD ot" all the differences between iwo measurements (lest-reiest),'••Derived from absolute value of difference between Iwo measurements (tesl-retest) over mean of the two same measurements.

Ophthal. Pbysiol. Opt.. 1994, Vol. 14, July 323

Measurement of sofi contact lenses curvature: N-M. Quesnel and P. Simonet

DO|»ima38 piano (n=32)

D optima 38 *S.OO D (n=32)

S optima 38 -5.00 D (n=32)

• Hydrocurve II 55% piano (n=32)

0 Hydrocurve II55% .f 5.00 D (n=32)

S Hydrocurve II 55% -5.00 D (n=32}

0,00 to 0,50 0,51 to 1,00 1.01 to 1.50 1,51 to 2.00 2.01 to 2.50 2.51 tO 3.00

Ranges of reproducibllity coefficient R (%)

Figure 6 Disiribution of reproducibility coefficient R values among soft lenses groups. The reproducibility is better when the coefficieni R is lower

varies from ±0.058 to ±0.198 mm depending on lensmaterial and refractive power. The repeatability coefficientsare higher for the ± 5.00 D Hydrocurve n 55% lenses (0.198and 0.194 mm respectively), and lower for the Optima pianolenses (0.058 mm). For the same material, + 5.00 D and-5.00 D lenses have similar repeatability coefficients, whilepiano lenses have lower repeatability coefficients thanthe ± 5.00 D lenses. From Table 2. as well as from theobservation of Figures 4 and 5, it appears that the variabilityof the keratometric technique is lower for soft contact lenseswith low water content (Optima 38). The variability ofmeasurements is also increased when the soft lenses werenot afocal, i.e, when two sets of reflected mires were seenin the observation system of the keratometer.

The mean and SD of repeated measures (n = 10) of theBOZR of one soft lens chosen in each group are listed inTable 3. The mean corrected BOZR and nominal BOZR ofthese lenses are also listed in Table 3. The results indicatethat the corrected BOZR of the soft lens chosen in eachgroup was steeper than the BOZR specified by themanufacturer. For all six lenses studied, these differences(range 0.06 to 0.15 mm) were within the limits of toleranceof the American National Standards\

Discussion

The variability of radius of curvature measurementsdepends on tbe quality of the optical and mechanical

systems of the keratometer'. The variation for the Zeissmodel D-7082 keratometer, as stated by the manufacturer,is ± 0.02 mm. According to Chaston'. for measurementstaken in saline this variability could be increased by 25%.Therefore, the measurement variation with this keratometercould be as high as ± 0.025 mm; this value is consistentwith our results for PMMA lenses used for calibration,since the variability of BOZR in terms of SD observedduring this operation ranged from O.(X)7 to 0.017 mm.

The repeatability coefficients for each group of 32 softlenses (range ± 0.058 to ±0.107 mm for the Optima38; ±0.140 to ±0.198 mm for the Hydrocurve II 55%)are within the limits of tolerance stated by the AmericanNational Standards' for low and high water content lenses(±0.15 and ±0.20 mm respectively). This suggests thatthe keratometer and the technique used are valid for anaccurate measurement of the BOZR of soft contact lenseswith a nominal power of piano or ± 5.00 D.

Variation in the measurements of the BOZR of softcontact lens with the Zeiss keratometer seems to dependpartially on the lens material, refractive power, watercontent and flexibility. As a matter of fact, the variability,assessed by the SD, is lower for PMMA lenses used forcalibration compared to soft lenses (either Optima 38 orHydrocurve II 55%). Furthermore, among soft lenses, theOptima piano show a better repeatability compared toHydrocurve II 55%, whether the repeatability is expressedin terms of SD for the measurement of one lens, or in terms

Table 3 Nominal back optic zone radius (BOZR) and mean measured BOZR of one soft lens in each group

Soft lensesMean BOZR* and SD

(mm)Mean corrected BOZR**

and SD (mm)Nominal BOZR

(mm)Dijference in BOZR'

(mm)

Optima 38 {0,00 D)Opimia 38 (+5-00 D)Optima 38 (-5,00 D)Hydrocurve 11 55% (0,00)Hydrocurve n 53% (+5.00 D)Hydrocurve II 55% (-5,00 D)

6,347 ±0.0306.369 ±0.0826.354 ± 0,0876.202 ±0.1076.247 ±0.1296,182 ±0.127

8.57 ±0.048.60 ±0.118,58 ±0,128.38 ±0,148,44±O,I88,35 + 0,17

8.708,708.708.508.508.50

0.13OMQM0.120.060,15

*Mean BOZR: mean measured BOZR of one lens chosen in each group,"Corrected BOZR = -0,03811 + 1,3569 x mean BOZR (mm).'Difference between corrected BOZR and nominal base curve of ihe lens measured.

324 Ophthal. Physioi, Opt., 1994, Vol. 14. July

Measurement of sofi contact lense curvature: N-M. Quesnel and P. Simonet

of coefficient of repeatability and coefficient ofreproducibility R for each group of soft lenses.

The decreased repeatability for ± 5.00 D lenses comparedwith piano for both Optima 38 and Hydrocurve II 55%presumably reflects tbe greater difficulty for the observer toalign the reflected mires when two sets of them were seenin tbe observation system of the keratometer.

It seems tbat tbere is a trend for decreased variability inBOZR measurements with decreased water content since allgroups of Optima 38 lenses sbow a better repeatability thanall groups of Hydrocurve II 55% lenses. More lenses withdifl̂ erent water contents, and particularly lenses witb higherthan 55% water content, sbould be used in further studiesto verify tbat hypothesis.

It is difficult to compare the repeatability of the techniquedescribed with the results of previous studies since they havereported only the SD of their measurements. Although, theprecision of tbis technique, expressed in term of SD of aseries of ten measures (range ± 0.030 to ± 0.129 mm), issimilar with the ones reported by Chaston"'" ". Holden'"'and Johansen et al.'^ who also used a modified keratometrictechnique to measure the BOZR of soft contact lenses.

The difference between tbe nominal and correctedmeasured BOZR could not be attributed to the position ofthe lens in the cell (concave down or up)'. Tbis differencecould be attributed to characteristics (pH. tonicity.temperature) of the saline solution used" "•, Thediscrepancy between nominal and measured radius ofcurvature may be due to tbe fact that tbe tecbnique used bymost manufacturers measures tbe sagittal height instead ofthe BOZR as does the keratometer.

The clinical use of this technique is limited by somepractical factors. Even though it is not necessary to use awet cell, the special support has to be set in place on thechin rest and adjusted properly to support the mirror andthe contact lens case cap (Figure 7). It can take as long as10 min to do the whole procedure including the correctpositioning of the lens in the case cap in order to get goodquality reflected mires. It was not possible to measure theBOZR of Spectrum lenses (CIBA Vision, Missauga.Ontario. Canada) because the reflected mire images werenot visible. Also it was not possible to measure the BOZRof low power lenses (between ± 0^75 D and ± 4.00 D)which are the most frequently prescribed in clinic. The costof the Zeiss keratometer and slit lamp also limits theclinical use of this technique.

The results suggest that the test-retest repeatability of thetechnique is sufficient to provide an appropriate verificationsystem for research purposes. For example, the techniquecan be used to determine the effect of any treatment ordisinfection procedure on tbe base curve of piano or highpower soft lenses. In addition, like all keratometrictechnique, it allows the detection and measurement of backsurface toric lenses which is not possible with sagittalheight measuring devices.

Conclusion

The back optic zone radii are probably the most clinicallysignificant variables in fitting medium to low standardthickness soft contact lenses, but paradoxically they are tbemost difficult to check.

The repeatability of measurements, expres.sed in term ofSD (range 0.030-0,129 mm) with the Zeiss model D-7082keratometer is similar to that obtained in previousstudies'"'"'"" witb different keratometers.

The repeatability of the technique described, expressed interm of SD or in term of 95 % limits of agreement meets theAmerican Standards Tolerances'* for all soft contact lensesstudied.

The results suggest that the technique described iscapable of a clinically acceptable level of accuracy.Measurements of .soft contact lens BOZR with thistechnique were not difficult to carry out but did. however,require more care and time than would be usual forRadiuscope measurements of rigid lenses. The level ofrepeatability is sufficient to provide an appropriateverification technique for research purposes.

The use of the repeatability coefficient and reproduci-bility coefficient R should be considered for the assessmentof tbe precision or repeatability of a technique, since itgives more information about the variability of themeasurements than tbe SD of a series of measures.

Acknowledgements

Tbe autbors want to thank the Canadian OptometricEducation Trust Fund and the NSERC Canada, for theirfinancial support to Nadia-Marie Quesnel and PierreSimonet respectively. We also thank Bausch & Lomb andPilkington Barnes-Hind for supplying the lenses, andHumphrey Instruments, a division of Carl Zeiss Canadawho supplied the keratometer used in this study.

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