according to china 's ministry of health , china has approximately 4 million cataract victims , with 500,000 new cases being diagnosed each year . as a developing country , especially in rural china , poverty and limited access to health care , due to the uneven distribution of health care sources , can make it very difficult for these people to obtain proper treatment . cataract surgical rate ( csr ) is still very low in rural china . lifeline express hospital eye - train ( lehet ) , the first charge - free cataract surgery project founded in 1997 , is a quite important way to restore vision for the low - income rural people in china . independent of cost or other factors , the first expectation from surgeons and patients is good postoperative visual outcomes . to meet these expectations , attention to accurate biometry measurements the biometry is indispensable to the surgeons and patients as it might indicate the prognosis and safety of the coming operation . in the biometric parameters , axial length ( al ) and corneal curvature are the most important however , the distribution and determinants of al have been assessed in only a few population - based studies of older persons [ 410 ] , of which there is still no study of rural chinese population , especially in middle china , having cities with extremely long history . in 2011 and 2012 , our hospital ( peking university people 's hospital , puph ) had three missions of lehet in middle china . in this study , we explored the biometric parameters of adult cataract patients who had cataract surgeries on lehet in these missions and all were rural people . our hospital , puph , had four missions of lehet , zhoukou in henan province and songyuan in jilin province in 2011 , yuncheng in shanxi province and sanmenxia in henan province in 2012 . the sites were selected by the office of lehet , and they were blind to our hospital before the mission start . yuncheng ( n 35.03 ; e 111.01 ; altitude : 369.53  m ) in shanxi province and zhoukou ( n 33.62 ; e 114.66 ; altitude : 50.50  m ) in henan province have thousands of years of history . sanmenxia ( n 34.77 ; e 111.20 ; altitude : 376.08  m ) in henan province was built in the 1950s and also is a rural city . residents in this new city partly immigrated from the whole of china , such as northeast china and west china . in 2011 , the pure annual income of rural people was 5601.40  cny ( about 889.11  usd ) in shanxi province and 6604.03  cny ( 1048.26  usd ) in henan province , much lower than beijing 14735.68  cny ( about 2339.00  usd ) cited from china statistical yearbook 2012 . based on the sixth national census of china 2010 ( http://www.stats.gov.cn/ )   and the 2010 annual survey data of china disabled persons ' federation ( http://www.cdpf.org.cn ) , cataract surgical rate ( csr ) was calculated as in table 1 . any patients who wanted to have the charge - free cataract operations on lehet registered at the base hospital ( a local hospital selected by the office of lehet ) . after the systemic and ocular examinations and signing the informed consent at the base hospital , the patients were sent to lehet . preoperatively on lehet , all patients underwent a complete ophthalmological examination , that is , measurement of presenting visual acuity ( va ) by means of snellen charts ( performed by the nurses from the base hospital ) , intraocular pressure evaluation ( iop ) by noncontact tonometer ( canon tx-10/tx - f , tokyo , japan ) by the trained nurses from puph , slit lamp examination ( topcon sl-1e , tokyo , japan ) , and fundus examination ( 90 dioptre , volk optical , mentor , oh ) with dilated pupil by the ophthalmologists from puph . corneal curvature by auto - keratometer ( nikon speedy - k , tokyo , japan ) , axial length ( al ) and b - scan by ultrasonic system ( odm-2100 , meda , tianjing , china ) , and corneal endothelial counting ( cec ) by specular microscope ( topcon sp-3000p , tokyo , japan ) were performed by the trained technicians from puph on the patients suitable for operation . the flatter ( k1 ) and steeper corneal curvature ( k2 ) were read directly from the auto - keratometer , and k was calculated as the average of k1 and k2 . corneal radius ( cr ) was calculated from the formula cr  ( millimeter , mm ) = 1000  0.3375/k  ( diopter , d ) . the srk / t formula for normal or long axial length ( al more than 25.00  mm ) and hoffer q formula for short axial length ( al less than 22.00  mm ) were used to calculate the power of intraocular lens ( iol ) and the estimated postoperative refractive errors were less than 0.25  d except patients with high myopia . lehet was equipped with specular microscope , sp-3000p , in the first half of year 2012 ; the patients of zhoukou and part of yuncheng had no cec measurement . exclusion criteria for this study are as follows : age less than 20 years , al equal to or more than 27.00  mm , and history of intraocular surgery . the study was in accordance with the tenets of the declaration of helsinki and has been approved by the institutional review board of puph . the student t - test was used to compare age and chi - square test was used to compare the female ratio between the groups . a p value less than 0.05 statistical analysis was performed using statistical product and service solutions software ( spss version 20.0 , armonk , new york , usa ) . 3828 cataract patients ( 3828 eyes ) were enrolled in this study , including 1419 males and 2409 females ( male  :  female = 1  :  1.70 ) and 1984 right eyes and 1844 left eyes . there were no statistically significant differences between missions preoperatively in age , gender , and eye operated on . as in table 2 , average age of these cataract patients was 69.50  8.05 , which was 69.10  8.41 for males and 69.74  7.82 for females ( p = 0.019 ) , respectively . in detail , the average age was 68.55  8.12 for males and 69.57  8.07 for females in zhoukou ( p = 0.056 ) , 69.02  8.33 for males and 70.16  7.50 for females in yuncheng ( p = 0.010 ) , and 69.52  8.64 for males and 69.48  7.92 for females in sanmenxia ( p = 0.933 ) . although the average age of females is older than males totally , that of males and females was of no difference for zhoukou and sanmenxia , except that of females which was older than that of males in yuncheng . as shown in tables 3 , 4 , and 5 , not only for males or females , but also for total patients , the preoperative va ( logmar ) of these three groups is as follows : zhoukou > yuncheng > sanmenxia . the patients in sanmenxia had the best preoperative va , even in each gender , significantly . as shown in tables 3 , 4 , and 5 , there was a statistically significant difference in preoperative iop between the patients of yuncheng and zhoukou , yuncheng and sanmenxia . the males , females , and total patients of yuncheng had lower preoperative iop compared with those in zhoukou or sanmenxia . as shown in figure 1 and tables 3 , 4 , and 5 , the patients of zhoukou had lower k1 and k2 , significantly . there was no statistically significant difference in k1 between those of yuncheng and sanmenxia , but k2 of yuncheng was higher than sanmenxia significantly . respectively , both the males and females in zhoukou had lower k1 and k2 . however , for either the males or the females , there was no difference of k1 and k2 between those in yuncheng and sanmenxia . average corneal power ( k ) is an important parameter to calculate the power of iol . in figure 1 and tables 3 , 4 , and 5 , the patients in zhoukou had lower average corneal power ( k ) significantly compared with the other two groups , the same for male and female patients in zhoukou . but there was no significant difference in average corneal power ( k ) between those in yuncheng and sanmenxia , for either the males or the females . the difference between k1 and k2 could be used to indicate the corneal astigmatism , which has the effect on the postoperative visual acuity . in figure 1 and tables 3 , 4 , and 5 , the difference of k1 and k2 for the patients was as follows : zhoukou > yuncheng > sanmenxia . but for the males except that |k1  k2| of zhoukou was higher than sanmenxia significantly , there was no significant difference between zhoukou and yuncheng or between yuncheng and sanmenxia . as seen in figure 2 and tables 3 , 4 , and 5 , al for the patients was as follows : zhoukou < sanmenxia < yuncheng . for the males , al of zhoukou was shorter than the other two cities . for the females , al of yuncheng was longer than the other two sites . there was no significant difference in al between yuncheng and sanmenxia for males or between zhoukou and sanmenxia for females . the al / cr ratio is highly correlated with the spherical equivalent as a previous study . as seen in figure 2 and tables 3 , 4 , and 5 , the patients in zhoukou had the smallest al / cr ratio closer to 3.0 , and yuncheng and sanmenxia had similar ratio . cec is a very important factor to decide the operation scheme and to predict prognosis . as there was no machine in zhoukou at that time , we only could compare cec between those in yuncheng and sanmenxia . as shown in figure 2 and tables 3 , 4 , and 5 , cec of yuncheng was higher than sanmenxia , which was same result for the males . but for the females , there was no significant difference in cec between yuncheng and sanmenxia . this study explored the data of cataract patient , who had the free surgeries on lehet , on ocular biometry of chinese population in rural china . and our study provided the normative data on k1 , k2 , |k1  k2| , average corneal power ( k ) , al , al / cr , and cec of this population ; those were 43.74  1.64  d , 44.75  1.68  d , 1.02  0.86  d , 44.24  1.60  d , 23.04  1.49  mm , 3.03  0.12 , and 2462.36  423.65/mm , respectively . our study showed al in rural chinese population was normally distributed with a positive skew and a big kurtosis ( 1.417 ) . skew and kurtosis have been reported in the distribution of al in the reykjavik eye study , the singapore malay eye study , the singapore indian eye study , and fotedar et al . hence , this is the first report of the appearance of big kurtosis in the distribution of al in rural chinese population . it is worthwhile comparing our findings with those of the tanjong pagar study on adult chinese population in singapore , which also used a - scan . the mean al in that study ( 23.23  1.17  mm ) was a little longer than in our study ( 23.04  1.49  mm ) . moreover , al in our study is shorter than latinos ( 23.38  mm ) in los angeles with a - scan , malay people ( 23.55  mm )   in singapore , indian people ( 23.45  mm )   in singapore , and caucasian people ( 23.44 )   in the blue mountains area in australia with iolmaster , longer than another asian population ( 22.76  mm ) in myanmar with ocuscan . the similarity of al in those studies with a - scan and ours is likely to be explained by the same method of al measurement . the difference in al of these studies might be explained by a greater degree of urbanization in singapore and subsequently a higher rate of axial myopia . those three studies with iolmaster indicated that the race might have significant effect on al compared with region as the similarity of indian and caucasian people . the corneal power k1 , corneal power k2 , and k ( average corneal power ) in our study were not normally distributed with different skews and kurtosis . k2| in our study was normally distributed with a positive skew ( 2.704 ) and a significant kurtosis ( 13.317 ) . moreover , the preoperative visual acuities in the three missions of our study had the same trend as |k1  k2| , both of that of males and females are the same . there is evidence that the al / cr ratio of an emmetropic eye is usually very close to 3.0 , and a higher al / cr ratio was reported to be a risk factor in myopia [ 15 , 16 ] . however , few studies have reported the al / cr ratio . compared with zhoukou , the patients in yuncheng and sanmenxia had similar al / cr ratio , also in males and in females . the singapore indian eye study   showed that the al / cr ratio correlated more highly with the spherical equivalent than al alone . this correlation indicated that longer eyes are not necessarily myopic and worse presenting visual acuity , including those that are long because of overall body stature . the patients in zhoukou , who had shorter al and al / cr closer to 3.0 , had the worst preoperative visual acuities . this indicated that in rural chinese population at least in the cataract patients the al / cr ratio , in other words , the spherical equivalent , had less effect on the visual acuity than |k1  k2| , the corneal astigmatism . in conclusion , this study provides normative ocular biometry in a large , representative rural chinese population . the corneal power k1 , corneal power k2 , and k ( average corneal power ) are not with normal distribution
aims . to describe and explore the distribution of ocular biometric parameters of adult cataract patients in rural china . 
 methods . 
 three lifeline express hospital eye - train missions of peking university people 's hospital in china were chosen . 
 3828 adult cataract patients aged 29 to 88 years with axial length ( al ) less than 27.0  mm were enrolled . 
 the ocular biometry including visual acuity ( va ) , intraocular pressure , al , corneal power ( k1 and k2 ) , and corneal endothelial counting ( cec ) were collected and analysis . 
 corneal radius ( cr ) was calculated from the corneal power . 
 results . 
 the participants in zhoukou of these three missions had the worse preoperative va ( p < 0.001 ) , the lowest k1 ( p < 0.001 ) , k2 ( p < 0.001 ) , and k ( p < 0.001 ) and the highest |k1  k2| ( p 
 < 0.001 ) , moreover al / cr more closely to 3.0 . the al , |k1  k2| , and al / cr were normally distributed . 
 but the k1 , k2 , k , and cec were not normal distributions . except k1 , 
 all parameters were positively skewed and peaked . conclusion . 
 our study provides normative ocular biometry in a large , representative rural chinese population . 
 the al is normally distributed with a positive skew and big kurtosis . 
 the corneal powers are not normal distribution 
 . the corneal astigmatism might have a significant effect on the visual acuity .