decentration can lead to undesired complications such as decreased visual acuity , astigmatism , glare , and monocular diplopia , which would overcome the purposes of the surgery ( 1 , 2 ) . to minimize the risk of decentration
measurement of the angle kappa between the visual and papillary axes plays an important role in this process .
positive and negative angle kappa values correspond to the reflection of corneal light nasally and temporally , respectively ( 3 ) .
a positive angle kappa  5 is physiologic , whereas values > 5 can lead to pseudostrabismus ( 4 ) .
hyperopic eyes show large angle kappa in comparison to myopic eyes ; therefore , a small decentration in these eyes can lead to severe complications .
the angle kappa is commonly measured in cases of strabismus and plays an important role in the preoperative assessment .
one method that can be used to measure the angle kappa is the orbscan ii device , which is widely used preoperatively in photoablation surgeries .
the purpose of this study was to determine the mean angle kappa and its intercepts among normal young adults , who comprise the target population for keratorefractive surgery .
in a non - random simple sampling , a total of 977 cases among the patients , who were referred to the khatam eye center ( a specialized eye hospital in mashhad , northeast of iran ) from march 2011 to february 2012 and claimed to be healthy , were enrolled in this observational cross - sectional study .
exclusion criteria were as follows : history of any deviation or strabismus , with or without orthoptic or surgical treatment ; any intraocular , corneal , or keratorefractive surgery ; using contact lens ; any corneal anomaly ; any ophthalmic or systemic drug consumption ; and hyperopic spherical refraction > + 3.00 diopters ( d ) , myopic spherical refraction < -5.00 d , or cylindrical refraction > 2.00 d. these criteria were defined to exclude cases with refractive errors that tend to induce pathologies .
this study was registered with the ethics committee of mashhad university of medical sciences , and clearance was obtained .
considered ethical codes for this study were 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 10 , 11 , 14 , 17 , and 20 .
complete ophthalmologic and orthoptic examinations were performed , including slit lamp biomicroscopy and dilated pupil fundoscopy . the orbscan ii device ( bausch and lomb , technolas , ny )
one acquisition was done per eye , unless an unacceptable result was obtained that would mandate repetition . for measurements with the orbscan ii device
the examiner used the joystick of the device to make adjustments such that the final result would be a sharply focused image of the eye .
descriptive analysis was used to determine the mean values and the student 's t - test was used to compare differences .
in a non - random simple sampling , a total of 977 cases among the patients , who were referred to the khatam eye center ( a specialized eye hospital in mashhad , northeast of iran ) from march 2011 to february 2012 and claimed to be healthy , were enrolled in this observational cross - sectional study .
exclusion criteria were as follows : history of any deviation or strabismus , with or without orthoptic or surgical treatment ; any intraocular , corneal , or keratorefractive surgery ; using contact lens ; any corneal anomaly ; any ophthalmic or systemic drug consumption ; and hyperopic spherical refraction > + 3.00 diopters ( d ) , myopic spherical refraction < -5.00 d , or cylindrical refraction > 2.00 d. these criteria were defined to exclude cases with refractive errors that tend to induce pathologies .
this study was registered with the ethics committee of mashhad university of medical sciences , and clearance was obtained .
considered ethical codes for this study were 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 10 , 11 , 14 , 17 , and 20 .
complete ophthalmologic and orthoptic examinations were performed , including slit lamp biomicroscopy and dilated pupil fundoscopy . the orbscan ii device ( bausch and lomb , technolas , ny )
one acquisition was done per eye , unless an unacceptable result was obtained that would mandate repetition . for measurements with the orbscan ii device
the examiner used the joystick of the device to make adjustments such that the final result would be a sharply focused image of the eye .
descriptive analysis was used to determine the mean values and the student 's t - test was used to compare differences .
a total of 977 healthy young adults , aged 18 to 45 years , participated in this observational cross - sectional study .
the study population consisted of 614 females and 363 males ( p < 0.001 ) .
figure 1 shows the distribution of angle kappa in degrees in different ages according to sex .
the mean angle kappa was 4.97  1.38 , with a mean of 5.00  1.36 in males and 4.96  1.30 in females ( p = 0.63 ) .
the means of angle kappa in the right and left eyes were 5.16  1.44 and 4.78  1.31 ,
the means of horizontal ( x - axis ) angle kappa were -0.44  0.28 and 0.37  0.25 in the right and left eyes , respectively ( p = 0.012 ) . the means of vertical ( y - axis ) angle kappa were -0.11  0.31 and -0.07  0.34 in the right and left eyes , respectively ( p = 0.01 ) .
the mean of horizontal angle kappa was -0.02  0.49 , with a mean of -0.02  0.50 in males and -0.02  0.49 in females ( p = 0.93 ) .
the mean vertical angle kappa was -0.09  0.32 , with a mean of -0.09  0.33 in males and -0.09  0.32 in females ( p = 0.74 ) ( table 2 ) .
we measured the variables in the right and left eyes separately ( tables 3 and 4 ) .  
the angle kappa plays an important role in refractive surgery , and inaccurate preoperative measurements of this angle can lead to serious surgical complications associated with decentration ( 1 , 5 ) . in patients with strabismus , precise measurement of the angle kappa prevents the under - calculation or over - calculation of the degree of deviation ( 3 ) . in patients with a large angle kappa , the lens of prescription glasses can result in a large prismatic effect , which can disturb the best - corrected visual acuity ( 6 ) .
angle kappa is increased with age ( 7 ) . to achieve more accurate results in the preoperative assessment , we measured the angle kappa in a large sample of a normal population .
hashemi et al . previously measured the angle kappa distribution in a population of tehran residents with a wide age range ( 6 ) .
their findings encouraged us to restrict our sampling to the population that was most likely to undergo refractive surgery ( age range , 18 - 35 years ) .
therefore , our results might be more reliable for estimating the angle kappa in this population ( 8) .
we used the orbscan ii device because of its very precise measurements and excellent reproducibility with only one acquisition per eye . to reduce the risk of systematic bias , a single operator used the orbscan ii .
the orbscan ii finds the center of the pupil and finds where the perpendicular axis to the pupil center intercepts the cornea . in the reports ,
the plus or cursor sign ( + ) shows the corneal apex and the small " k " shows the intercept of the pupillary axis .
the cornea periphery is assumed 360 , initiating at the 3-oclock position and rotate counterclockwise .
for example , the locus is at 346.47. therefore , the kappa angle is 4.59 at ( @ ) 346.47. the third and fourth are the intercepts in x - axis and y - axis , respectively .
the dimension of the intercept is divided into x - axis ( horizontal ) and the y - axis ( vertical ) . in our example , it is + 0.19 mm in the left of the corneal apex and -0.11 mm below the corneal apex . among the 977 individuals ( 1954 eyes ) who were examined ,
there were more women than men ( p < 0.001 ) , and the values obtained from women would have more effect on the overall results .
the angle kappa for both men and women decreased with age , according to the linear regression test .
we observed a significant difference between the mean angle kappa in the right and left eyes .
no previous study had obtained this result , which did not seem to be accidental due to the large number of cases .
further investigations are needed to resolve this difference . in comparison to the reported value of 4.96  1.38 in this study , hashemi et al .
reported a mean angle kappa of 5.46 previously ( 6 ) ; however , we used a different age range with a smaller refractive error than they did .
because a higher refractive error can result in larger angle kappa , lower angle kappa results were expected in our study .
( 4 ) used an age range similar to ours ( 20 - 40 years ) and observed an average angle kappa of 5.22. in wachler et al .
the angle kappa was reportedly greater in patients with hyperopia ( 8) while hashemi et al .
unfortunately , we did not divide patients into groups on the basis of their refraction and hence , we could not comment on the angle kappa distribution among different refractive groups . in another recently published study , performed in our center , the angle kappa changes after photorefractive surgery were evaluated in a population different from this study s and it was found that in 96 eyes , the angle kappa had not changed significantly after the surgery ( 4.97  1.24 preoperatively and 4.99  1.10 at six months postoperation ) .
findings of the current study were the same as our previous population ( 9 ) .
moreover , we found just another document about the measurement of kappa angle in normal population .
basmak et al . in 100 normal individuals , compared synaptophore with the orbscan , and the effect of refraction on the kappa angle .
they found that a correlation exists between positive refractive errors and large positive angle kappa values .
moreover , data gathered by orbscan showed a little bit higher values in comparison to synaptophore ; however , the orbscan system provides angle kappa values with quantitatively established precise measurements ( 10 ) .
park et al . have reviewed the concept of angle kappa , its measurement and distribution in normal populations , and its implications in refractive surgery and concluded that evidences about the kappa angle effects on refractive surgery were growing .
decentered treatment might be induced by ignoring it and visual symptoms might aggravate the vision .
in treating refractive errors in modern era , compensating kappa angle is important in achieving optimal correction ( 11 ) .
although a large sample size in this study could assume a good evaluation of the amount of angle kappa in refractive surgery cases , one of our limitations was ignoring the spherical equivalent of patients for more precise classification of angle kappa , which is known to be affected by this variable .
 background : the angle kappa is important in proper centration of corneal ablation in keratorefractive surgery . 
 orbscan ii device is widely used preoperatively in photoablation surgeries and can be used to measure the angle kappa.objectives:this study aimed to determine the mean angle kappa and its intercepts in healthy young iranian adults.patients and methods : in this cross - sectional study , orthotropic patients ( age range , 18 - 35 years ) who were referred to the khatam eye hospital ( mashhad , iran ) were included . 
 exclusion criteria were as follows : history of any eye deviation or strabismus with or without orthoptic or surgical treatment ; any intraocular , corneal , or keratorefractive surgery ; contact lens use ; any corneal anomaly ; any ophthalmic or systemic drug consumption ; and hyperopic spherical refraction > + 3.00 diopters ( d ) , spherical refraction > -5.00 d , or cylindrical refraction > 2.00 d. all of the parameters were measured by the same operator through an orbscan ii device.results:a total of 977 healthy participants who aged 18 to 45 years were included consecutively . 
 the study population consisted of 614 females and 363 males . 
 the average angle kappa was 5.00  1.36 at 240.21  97.17 in males and 4.97  1.30 at 244.22  94.39 in females ( p = 0.63 ) . 
 the average horizontal ( x - axis ) angle kappa was -0.02  0.49 , with a mean of -0.02  0.50 in males and -0.02  0.49 in females ( p = 0.93 ) . 
 the average vertical ( y - axis ) angle kappa was -0.09  0.32 , with a mean of -0.09  0.33 in males and -0.09  0.32 in females ( p = 0.74).conclusions : by using the normal angle kappa determined in this study , pseudodeviations can be identified more precisely in those who might undergo keratorefractive surgery . 
1. Background
2. Objectives
3. Patients and Methods
3.1. Patients
3.2. Examinations
3.3. Statistical Analysis
4. Results
5. Discussion