Background: The Thorington method is a refractile technique utilizing the Stenopaeic slit, a test frame accoutrement comprised of a phonograph record with a narrow, slit-shaped aperture in the centre. It is now seldom used in the subjective refraction to happen the chief acmes of astigmia due to the general perceptual experience that the Jackson ‘s cross cylinder method or the Fan & A ; Block method supply a higher grade of truth. However, the Thorington method may still be really utile peculiarly in instances of high astigmia and hapless ocular sharp-sightedness, or in state of affairss where the retinoscopic physiological reaction is hard to see This survey compares the truth and efficiency of finding the chief acmes of astigmia in low vision patients between the Thorington method and and the Jackson ‘s cross cylinder method.
Method: 31 pupils set abouting the Bachelor of Optometry programme at the University of Auckland were recruited in the survey. Following rectification of any refractile mistake of the participant, a dense cataract was simulated utilizing an opaque plastic sheet, and a variable grade of astigmatic mistakes at different axes were induced with four picks of subtraction cylinder lens. The chief acme of the induced astigmia and the clip taken to make the end-point were recorded for the Thorington method and the Jackson ‘s cross cylinder method, and this was repeated three times with a different inducement cylinder lens each clip. The participants were besides required to finish a questionnaire on the comparative easiness of each refractive technique.
Consequences: There was no clinically important difference in the disagreements between the cylindrical constituents induced and the cylindrical constituents measured when the two refractile techniques were compared. The Thorington method took, on norm, less than half the clip to make the end-point in comparing to the Jackson ‘s cross cylinder method. No clinically important difference was found in the patient easiness between the two refractile techniques.
Decision: The Thorington method was found to be merely every bit accurate as the Jackson ‘s cross cylinder method in happening the chief acme of astigmia in persons with decreased ocular sharp-sightedness, while being quicker to obtain the measuring than the Jackson ‘s cross cylinder method. Hence, the Thorington method could be a good option to the Jackson ‘s cross cylinder method particularly in instances where the patient finds it hard to spot which of the two JCC positions is better.
New Zealand has a quickly ageing population. It is estimated that more than a one-fourth of the population will be over 65 old ages of age by the late 2030s, and by 2051, the 65+ age group will make 1.33 million ( 1 ) . As the population ages, more and more people are enduring from low vision and sightlessness. Harmonizing to the 2000 US Census, the prevalence of ‘blindness ‘ ( defined as the best corrected ocular sharp-sightedness of worse than 6/60 in the better seeing oculus ) and ‘low vision ‘ ( defined as the best corrected ocular sharp-sightedness of worse than 6/12 in the better seeing oculus ) in the 40+ age group was 0.78 % and 1.98 % , severally ( 2 ) . When the figures are put in the context for New Zealand, it may be deduced that there are about 30 000 grownups with ‘blindness ‘ and 80 000 grownups with ‘low vision ‘ . In peculiar, surveies have identified that 73 % of people aged between 65 and 74 old ages, 91 % of people aged between 75 and 84 old ages, and 95 % of people aged 85 or over have cataracts ( 3 ) . These high Numberss pose a great concern particularly in New Zealand due to drawn-out public infirmary waiting lists of patients necessitating cataract surgery ( 4 ) . This means that there is a fast turning demand for the optometric diagnosing and direction of ocular damage such as due to cataract as the bulk of patients with ‘low vision ‘ or even ‘blindness ‘ still have a utile staying vision. Hence, any betterment in the current refraction techniques will take to a more accurate and efficient rectification of refractile mistake in low vsion patients, thereby cut downing the socioeconomic costs associated with vision loss.
Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the oculus to project a point object into a aggressively focussed image on the retina. It is a really common refractile mistake, and about 85 % of the population have some grade of mensurable astigmia.
The Thorington method is a refraction technique utilizing the Stenopaeic slit. The Stenopaeic slit is a test instance accoutrement which is comprised of a phonograph record with a narrow, slit-shaped aperture in the centre. It is fundamentally an elongated pinhole which removes the peripheral beams in the meridian perpendicular to the slit, insulating the optic acme on which to execute the refraction ( 5 ) . As the orientation of the Stenopaeic slit alterations, the retinal fuzz size in an astigmatic oculus alterations due to an axial displacement of the circle of least confusion ( COLC ) . When the Stenopaeic slit is aligned with one of the chief acmes of astigmia, it “ alterations ” the spherocylindrical refractile mistake into a strictly spherical one, giving the sharpest image in comparing to other orientations as the refractile mistake is the smallest in this orientation ( 6 ) .
The Thorington method is now seldom adopted in the subjective refraction to happen the axes of refractile mistake of an astigmatic oculus because of the general perceptual experience that the Jackson ‘s cross cylinder ( JCC ) method and the Fan & A ; Block method supply a greater truth. However, these latter techniques can be rather boring to execute on some patients with decreased vision due to difficulty patients can hold with observing any alterations between the positions produced by two lenses of differing axis or power. In instances of high astigmia and hapless ocular sharp-sightedness ( 7 ) , or in state of affairss where the retinoscopic physiological reaction is hard to see ( 8 ) , the Thorington method is known to be really utile. Harmonizing to the early literature, the Stenopaeic slit is besides utile for the rectification of refractile mistake in patients with optical anomalousnesss such as cataract or keratoconus ( 9 ) .
It is non possible, with planar optics, to decently account for the optical behavior of a 3-dimensional oculus in a 3-dimensional universe. Harmonizing to Munoz-Escriva and Furlan ( 2001 ) , the consequence of the Stenopaeic slit in forepart of an astigmatic oculus can be best described within the dioptric power infinite which is a vector in a 3-dimensional infinite ( 10 ) . Thibos et al derived mathematical expression to break up the power of a spherocylindrical lens into three independent co-ordinates ( ten, Y, omega ) in the signifier of M ( spherical power ) , J0 ( JCC power ) and J45 ( JCC power ) where ( 11 ) :
M = S + C/2 J0 = -C/2 cos 2I± J45 = -C/2 sin 2I±
The vector representation of a spherocylindrical lens in a 3-dimensional infinite allows the simple summing up of different spherocylindrical lens powers through vector add-on and the rating of discrepancy in astigmia ( 12 ) . Through another survey in 2001, Munoz-Escriva and Furlan analysed the optical rules of the Stenopaeic slit in the dioptric power infinite in state of affairss where the slit is non positioned along on of the chief acmes, and deduced an equation to obtain the residuary refractile mistake in forepart of the astigmatic oculus at any slit orientation ( 13 )
There have been a few other surveies on the Thorington method in the 1960 ‘s and 1970 ‘s sing its methodological analysis and optics, but a survey on the clinical efficiency of the Thorington method in comparing with other subjective refraction techniques has non been attempted yet.
The end of this undertaking is to look into the efficiency of finding the chief orientation of astigmia utilizing the Stenopaeic slit for subjective refraction in an oculus with hapless ocular sharp-sightedness. Due to a broad assortment optic pathology that leads to reduced vision, the current research will merely affect low vision secondary to cataract, a common cause of ocular damage in the old age group. This undertaking aims to compare the truth and efficiency of finding the chief acmes of astigmia between the Stenopaeic slit and JCC methods under the simulation of a dense cataract utilizing opaque sheets of plastic, and topics with normal vision will be recruited to supply informations.
The specific purposes of this survey are: 1 ) To find if there is a clinically important difference in the chief acmes of astigmia obtained by the two methods. 2 ) To find if there is a statistically/clinically important difference in the clip taken for happening the chief acmes of astigmia between the Thorington method and the JCC method under the simulation of heavy cataract. 3 ) To find if there is a statistically important difference in the comparative easiness between the two methods in the position of participants.
A sum of 31 topics between the ages of 16 and 50 were recruited in this survey. The minimal age demand of 16 was chosen so that all topics can do a voluntary consent to take part in the research. The maximal age bound of 50 was chosen because older people are more likely to hold media opacities and other optic conditions that may bring forth a confusing consequence on our measurings. All participants were required to hold a best corrected ocular sharp-sightedness of 6/6 or better in at least one oculus.
A short preliminary scrutiny was conducted on each topic to look into for any preexistent optic conditions which may adversely impact the ocular public presentation, and to mensurate the refractile mistake utilizing Zeiss Humphrey automatic refractor ( model 599 ) . The exclusion standards included important media opacities such as cataract or corneal cicatrix, and other optic conditions that may potentially impact the ocular public presentation. The oculus with less astigmia was chosen to obtain informations. An over-refraction was performed on the prescription found by the auto-refractor to rectify for any residuary refractile mistake.
Simulation of Cataract
An opaque plastic sheet was placed in forepart of the selected oculus to imitate a dense cataract which reduces the best corrected ocular sharp-sightedness to around 6/24.
A subtraction cylinder lens was placed in the test frame to imitate astigmia. A random figure generator was used to delegate the power and the axis of astigmia out of the four options: -1.00DC x 180, -1.00DC x 045, -2.00DC x 180, and -2.00DC x 045. An appropriate spherical lens was included with the cylindrical lens so that the induced refractile mistake ( assorted astigmia ) has a spherical equivalent of 0.00D.
The refraction technique to be evaluated foremost was determined indiscriminately, and the chief acme of astigmia and the clip taken to make the end-point utilizing the Stenopaeic slit method and the Jackson ‘s cross-cylinder method were measured. The participants were non cognizant that they were being timed. The refraction was performed monocularly, with the other oculus being occluded throughout the experiment. The low vision simulator remained changeless for both techniques.
1 ) The Thorington method:
A +0.50DS fogging lens was placed in the test frame to travel the posterior line of focal point closer to the retina. A logMAR chart with multiple lines of letters at 6m distance was used as the screening mark. It was confirmed that the participant can read at least two lines above the bound of ocular sharp-sightedness with the low vision simulator and the fogging lens in topographic point. The Stenopaeic slit was ab initio positioned at 90° , and the participant was instructed to maintain revolving the slit utilizing the boss on the test frame until the place that gives the clearest position of the letters was found. The axis of the rectifying subtraction cylinder ( i.e. the place perpendicular to the slit orientation ) and the clip taken to make the end-point were recorded. This was repeated three times, each with a different inducement cylinder lens assigned indiscriminately. Care was taken to aline the slit precisely in forepart of the student centre at all times.
2 ) The Jackson ‘s cross-cylinder method:
As for the Thorington method, a +0.50DS fogging lens was used. A chart with rotationally symmetric points of the size tantamount to two lines above the bound of ocular sharp-sightedness with the low vision simulator and the fogging lens in topographic point was used as the screening mark. Initially, a A±1.00D handheld JCC lens was twirled with the grip foremost at 45° and so at 180° . The consequences of these two kinks were used to find the axis place at which a -1.00DC rectifying lens was placed ( the axis place was at the mid-point of the two places that produced the clearer position of the points ) . The spherical tantamount power was maintained by adding a +0.50DS lens. To look into the orientation of the probationary correcting cylinder, the bracketing technique ( i.e. altering the axis of the rectifying subtraction cylinder lens in 20° measure followed by 10° measure and so 5° measure on each reversal ) was used with a dual reversal standard ( i.e. the participant coverage that the mark is clearer in the same way when the axis of the subtraction cylinder was changed in the opposite way by an excess 5° on reversal ) being used to turn up the two borders of the axis scope across which the two JCC positions were identical. The concluding axis was found from the norm of the two reversal points found by this dual reversal technique. The clip taken to obtain the two reversal points was recorded. This was repeated three times, each with a different randomly assigned bring oning cylinder.
At the terminal of the experiment, the participants were asked to finish a questionnaire to measure the two refraction techniques subjectively. The questionnaire was comprised of three inquiries that were to be answered on a ocular parallel graduated table runing from 1 to 5. For each refraction technique, the participants were required to measure the easiness of understanding the instructions, the easiness of doing judgements, and the grade of assurance in their responses. Besides, the patient was asked which of the two methods was easier overall.
The power and axis constituents of astigmia were converted into power vectors ( J0, J45 ) in a 3-dimensional infinite as described by Thibos ( 11 ) . The mated t-test was used to find whether there is a statistically important difference in the J0 and J45 between the two refraction techniques. The Levene trial was done to look into the premise of equal discrepancies. The Welch ‘s trial was used to compare the J0 and J45 found and the J0 and J45 induced for each type of bring oning cylinder lens, and find whether there is a statistically important difference in disagreements between the two methods. The correlativity coefficient is normally used to measure the grade of understanding between a new method and a preexistent method. Nevertheless, a high correlativity merely means that there is a strong association between two methods, but it does non needfully bespeak a high degree of understanding ( 14 ) . Furthermore, the correlativity coefficient can supply a utile estimation of understanding between two methods merely if the pre-existing method has a really low grade of mistake. Because all refractile techniques have a variable grade of built-in mistake, and due to a possibility of residuary astigmia that may be present even after the rectification of refractile mistake, the Bland-Altman ( prejudice ) analysis was used to measure the grade of understanding in the vector constituents between the Stenopaeic slit method and the Jackson ‘s cross-cylinder method ( 14 ) .
The mated t-test
The premise of equal discrepancies of each vector constituent between the Stenopaeic slit method and the Jackson ‘s cross-cylinder method was satisfied as the p-value for the difference in average discrepancy of J0 between the two methods and the difference in average discrepancy of J45 between the two methods was 0.4632 and 0.7934, severally. The mated t-test showed that there is no statistically important difference ( p=0.8736 ) in mean J0 found between the two methods ( Table 1 ) . The mated t-test besides showed that there is no statistically important difference ( p=0.5158 ) in mean J45 found between the two methods ( Table 1 ) .
95 % Confidence Interval
Average difference ( D )
S.D. ( D )
Mean-1.96SD ( D )
Mean+1.96SD ( D )
Table 1: The average difference in vector constituents between the Stenopaeic slit method and the Jackson ‘s cross-cylinder method, standard divergence, p-value, and 95 % assurance interval bounds.
The Welch ‘s trial
The difference in vector constituent between the Stenopaeic slit method and the Jackson ‘s cross-cylinder method was grouped into two classs harmonizing to the sum of astigmatic refractile mistake that was induced: ‘Low cyl ‘ ( -1.00DC bring oning cylinder lens ) and ‘High cyl ‘ ( -2.00DC bring oning cylinder lens ) . The Levene trial showed that the premise of equal discrepancies of each vector constituent between the ‘High cyl ‘ group and the ‘Low cyl ‘ group was non satisfied as the p-value for the difference in average discrepancy of J0 between the two groups and the difference in average discrepancy of J45 between the two groups was 0.00041 and 0.00003, severally. The secret plans of the difference in J0 between the two methods against the difference in J45 between the two methods for ‘Low cyl ‘ and ‘High cyl ‘ showed that the informations are more closely packed together with the usage of -1.00DC bring oning cylinder lens in comparing to the usage of -2.00DC bring oning cylinder lens. This suggests that the two refraction techniques are more comparable when the sum of astigmia is comparatively little.
Figure 1: Plots of the difference in J0 versus the difference in J45 for the -1.00DC and the -2.00DC bring oning cylinder lenses.
The differences between the vector constituents found under each refraction technique and the vector constituents induced was calculated and were compared individually for each bring oning cylinder lens. Table 2 shows that the premise of equal discrepancies was satisfied for all types of bring oning cylinder lens. No statistically important difference was found in the difference between the vector constituents found under each refraction technique and the vector constituents induced for all types of bring oning cylinder lens with an exclusion of -2.00DC @ 135 ( p=0.0154 ) .
Mean within group
95 % Confidence Interval
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-1.00DC x 90 J0
-1.00DC x 90 J45
-1.00DC x 135 J0
-1.00DC x 135 J45
-2.00DC x 90 J0
-2.00DC x 90 J45
-2.00DC x 135 J0
-2.00DC x 135 J45
Table 2: The average difference between the induced and the found cylindrical constituents per method, p-value, 95 % assurance interval.
The Bland-Altman analysis
The Bland-Altman analysis demonstrated low degrees of prejudice ( Table 3 ) . No obvious relationship was observed on the secret plans of the difference in J0 or J45 between the two methods against the average J0 or J45 of the two methods for both high and low cylinder powers ( Figure 2 ) . The secret plans showed no consistent prejudice. The scope between the two bounds of understanding for both J0 and J45 was approximately twice as broad with the high cylinder power compared to the low cylinder power, but Bland and Altman claim that if all informations prevarications within the bounds of understanding, there will be no clinically important difference between the two methods ( 14 ) .
Limits of Agreement
S.D. of prejudice
Low cyl J0
Low cyl J45
High cyl J0
High cyl J45
Table 3: Bland-Altman analysis of prejudice, standard divergence, and 95 % bounds of understanding.
Figure 2: Plots of difference versus mean of vector constituent obtained with the Stenopaeic slit method and the Jackson ‘s cross-cylinder method.
Time taken to make the end-point
The premise of equal discrepancies of the clip taken to make the end-point for the two refraction techniques was satisfied ( p=0.1724 ) . The two-sample t-test showed that the true difference in the average clip taken to make the end-point between the two methods lies between 36.55 seconds and 45.26 seconds at the 95 % degree assurance. The difference between the two agencies was statistically important ( p & lt ; 2.2e-16 ) . Figure 3 shows that the average clip taken to make the end-point was 22.36 seconds with the Stenopaeic slit method, 63.26 seconds with the Jackson ‘s cross-cylinder method. The difference in the average clip taken to make the end-point between the two methods would be larger if the possible outlier in the information for the Stenopaeic slit method was removed.
Figure 3: Box-and-whiskers ( Tukey ) representation of the clip taken to make the end-point with the two refraction technique.
The first inquiry was “ how easy was it for you to understand the process? “ , and the response was given on a grade graduated table from 1 to 5, in an increasing degree of easiness. There was a moderate grounds ( p=0.0739 ) that there is a statistically important difference in the easiness of understanding the process between the two refraction techniques. Figure 4 shows that the participants found that the Stenopaeic slit method was comparatively easier to understand than the Jackson ‘s cross-cylinder method.
Figure 4: Tonss of the easiness of understanding the process of the two refraction techniques.
The 2nd inquiry was “ how easy did you it to do judgements? “ , and the response was given on a grade graduated table from 1 to 5, in an increasing degree of easiness. There was no statistically important difference ( p=0.8920 ) between the two methods, bespeaking that the easiness of doing judgements as to make up one’s minding which orientation of the Stenopaeic slit gave the clearest position was similar to the easiness of doing judgements as to make up one’s minding which of the two positions produced by the Jackson ‘s cross-cylinder was better.
Figure 5: Tonss of the easiness of doing judgements in the two refraction techniques.
The 3rd inquiry was “ how confident were you with your responses? “ , and the response was given on a grade graduated table from 1 to 5, in an increasing degree of assurance. No statistically important difference was found between the two methods ( p=0.5470 ) .
Figure 6: Tonss of the assurance with responses in the two refractile techniques.
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Mean ( SD )
Mean ( SD )
4.68 ( 0.54 )
4.10 ( 1.01 )
3.30 ( 0.94 )
3.32 ( 1.01 )
3.58 ( 0.76 )
3.36 ( 0.95 )
Table 4: The average tonss of the inquiries for the two refractive techniques, p-value for the difference between the average tonss.
The participants were besides asked which of the two refractive techniques was easier overall, and 18 participants ( 58.06 % ) chose the Stenopaeic slit method while the staying 13 participants ( 41.94 % ) chose the Jackson ‘s cross-cylinder method.
There was no statistically important difference in the disagreement between the induced cylindrical constituents and the mensural cylindrical constituents when the two refractile techniques were compared except for the J45 constituent of the rectification found for the inducement cylinder lens -2.00 x 180. However, it is hard to find whether this difference is of any clinical importance as the existent axis of the rectifying cylinder is comprised of a combination of the cylindrical constituents J0 and J45. The average difference between the induced cylinder axis and the measured cylinder axis for the -2.00 x 180 bring oning cylinder lens was less than 10i‚° for both the Stenopaeic slit method and the Jackson ‘s cross cylinder method. This indicates that the difference in the disagreement of the measured J45 constituent from the induced J45 constituent between the two refractive techniques for this peculiar inducement cylinder lens may non be clinically important in the context of low vision patients.
The Bland-Altman secret plans showed that the two refractive techniques are every bit valid with regard to the truth and variableness of the consequences.
The clip taken to obtain the measuring was, on mean 41 seconds shorter with the Stenopaeic slit method than with the Jackson ‘s cross cylinder method. This difference may arguably be clinically important sing that the measuring of the axis is required for the other oculus as good.
This survey showed that there was no clinically important difference in the chief acmes of astigmia measured by Stenopaeic slit method when compared with the Jackson ‘s cross cylinder method in patients with decreased ocular sharp-sightedness secondary to cataract. The Stenopaeic slit was found to be more than twice every bit fast as the Jacson ‘s cross cylinder method, but there was no clinically important difference in the grade of patient easiness between the two refractile techniques. Hence, the usage of the Stenopaeic slit may still be advocated these yearss particularly in state of affairss where a low vision patient is holding huge troubles in doing judgements between the two obscured JCC positions.