this is a prospective , observational study approved by the institutional review board , and the study was conducted according to the tenets of the declaration of helsinki . axial length ( al ) and anterior chamber depth ( acd ) were measured using an iolmaster ( carl zeiss meditec , jena , germany ) . central corneal thickness ( cct ) was measured using a specular microscope sp-2000p ( topcon , tokyo , japan ) . individuals were excluded if they met any of the following criteria : ( 1 ) preoperative iop > 20 mmhg ; ( 2 ) trabecular meshwork invisible in any quadrant on static gonioscopy ; ( 3 ) refractive error exceeding 6 diopters ( d ) spherical equivalent or 3 d of astigmatism ; ( 4 ) history of ocular disease or trauma ; ( 5 ) previous ocular surgery except uncomplicated cataract surgery of the contralateral eye ; or ( 6 ) glaucomatous optic disc changes such as excavation , thinning or notching of the neuroretinal rim , or disc hemorrhages . individuals were excluded from the analysis if they had surgical complications during or after phacoemulsification , such as posterior capsular rupture , intraocular lens ( iol ) placement in the ciliary sulcus , or postoperative iop spike . unexpected cataract surgery of the contralateral eye within 1 month was also considered a reason for exclusion . patients were treated with topical 0.5% levofloxacin and 0.1% diclofenac sodium for four times a day starting 3 days before surgery . surgery was performed by a single surgeon at a single surgical center from may 2014 to september 2014 . after making a 2.2-mm incision in the temporal clear cornea a single - piece acrylic foldable posterior chamber iol was implanted in the capsular bag in all cases . then , the ophthalmic viscoelastic agent was removed and the incision site was closed by stromal hydration with no suture . a single observer measured iop using a single tono - pen avia ( reichert inc . , iop readings were always obtained after calibration of the tono - pen and in the right eye first . the iop value for each posture was the mean of two consecutive measurements that were within 2 mmhg and that had < 5% error as indicated on the tono - pen or the median of three measurements if the first two differed by 3 mmhg or more . the patients underwent iop measurements in different body postures from 7 to 11 a.m. on the day of surgery . to assign a randomized sequence of iop measurements in different body postures ( sitting , supine , or ld ) , every patient was asked to pick up one card , where a different body posture was written on the back , one after another until all the three cards were opened . first , each patient was asked to sit on a chair under dim light conditions and undergo iop measurement with a goldmann applanation tonometer ( gat ) . then , the patient was asked to assume and maintain each of the body postures in the randomized sequence predetermined in an aforementioned manner . all participants rested their head on a soft pillow to keep the head parallel to the bed and they were instructed to neither turn their heads nor bury their eyes into the pillow . a pilot study revealed that the standard deviation ( sd ) of the iop reduction after cataract surgery of the operated eye was 2.8 mmhg in ld position ( 2.4 mmhg for supine position and 1.8 mmhg for sitting position ) . a sample size calculation , using g*power software ( version 3.0.10 ; universitt kiel dusseldorf , germany ) with  = 0.05 , determined that 28 patients would be required to detect an iop difference of > 1.6 mmhg in the operated eye at an sd of 2.8 mmhg with a power of 80% . accounting for the loss of follow - up to be about 20% , we estimated a final sample of 35 patients in this study . wilcoxon signed - rank test was used to compare iop changes after cataract surgery in different body postures . the amount of iop changes after surgery with different body postures was compared using the friedman test . we performed spearman 's correlation analysis to evaluate the relationship between iop change and ocular biometric parameters , with the change of iop as a dependent parameter and the ocular biometric parameters as independent parameters . p values were considered statistically significant at values < 0.05 unless the bonferroni correction method for multiple comparisons was used , in which case a p < 0.017 was considered statistically significant . a pilot study revealed that the standard deviation ( sd ) of the iop reduction after cataract surgery of the operated eye was 2.8 mmhg in ld position ( 2.4 mmhg for supine position and 1.8 mmhg for sitting position ) . a sample size calculation , using g*power software ( version 3.0.10 ; universitt kiel dusseldorf , germany ) with  = 0.05 , determined that 28 patients would be required to detect an iop difference of > 1.6 mmhg in the operated eye at an sd of 2.8 mmhg with a power of 80% . accounting for the loss of follow - up to be about 20% , we estimated a final sample of 35 patients in this study . wilcoxon signed - rank test was used to compare iop changes after cataract surgery in different body postures . the amount of iop changes after surgery with different body postures was compared using the friedman test . we performed spearman 's correlation analysis to evaluate the relationship between iop change and ocular biometric parameters , with the change of iop as a dependent parameter and the ocular biometric parameters as independent parameters . p values were considered statistically significant at values < 0.05 unless the bonferroni correction method for multiple comparisons was used , in which case a p < 0.017 was considered statistically significant . of the 35 patients recruited in this study , 5 patients were excluded due to loss to follow - up and one patient was excluded due to unanticipated phacoemulsification of the contralateral eye within 1 month . sixteen patients were male and 13 were female , with a mean age of 64.2  9.1 years ( range : 5083 ) . baseline demographics of the patients who underwent phacoemulsification table 2 lists the iop measurements obtained in the different body postures and shows the statistical differences of iop changes before and after cataract surgery . for the operated eye , all iops measured in different body postures showed significant reduction after phacoemulsification ( p < 0.01 ) , except the iop measured by tono - pen in the sitting position ( p = 0.266 ) . however , the amount of postoperative iop change measured by tono - pen in the sitting position was significantly different between the operated and the nonoperated eyes ( 0.6  2.5 mmhg vs. 0.5  2.2 mmhg , p = 0.045 ) . neither significant difference was found for the nonoperated eye in the sitting position ( p = 0.253 ) nor in the recumbent positions . the amount of iop reduction in the operated eye was significantly different among the various body postures ( p < 0.001 , friedman test ) . although these differences were not always significant in each pair of comparisons with bonferroni correction , in which case a p < 0.017 was considered statistically significant , the amount of iop reduction was significantly larger in ld position than that in sitting position ( sitting vs. supine , p = 0.048 ; sitting vs. ld , p = 0.001 ; supine vs. ld , p = 0.028 ) . the mean postoperative iop reduction in the operated eye was 3.0  3.5 mmhg in the ld position , 1.7  2.5 mmhg in the supine position , and 0.6  2.5 mmhg in the sitting position . intraocular pressure in different body postures and biometric parameters before and after phacoemulsification when compared between the preoperative and postoperative measurements , the posture - induced iop elevations obtained by changing from the sitting to the supine or ld position were significantly reduced after the surgery [ table 3 ] . before the surgery , iop was higher ( + 2.0  1.8 mmhg ) in the supine position than in the sitting position . however , after the surgery , this posture - induced iop elevation was reduced to 0.8  2.0 mmhg ( p = 0.048 ) . such perioperative reduction in iop rise was also noted for changing body posture from the supine to the ld position ( p = 0.028 ) . posture - induced intraocular pressure changes before and after phacoemulsification table 4 shows the spearman 's correlation analysis performed to evaluate the association between ocular biometric parameters and iop reduction . higher preoperative iop was correlated with larger iop reduction after phacoemulsification in every body posture . furthermore , the iop difference in the ld position showed the strongest correlation coefficient ( spearman 's rho = 0.730 ) [ fig . 1 ] . however , preoperative acd , al , cct , and body mass index did not correlate with the iop differences in the supine and ld positions . spearman correlation analysis between preoperative ocular parameters and intraocular pressure changes before and after phacoemulsification correlation between preoperative intraocular pressure and mean decrease of intraocular pressure after phacoemulsification of the operated eye in different body postures . note that the higher preoperative intraocular pressure was correlated with larger intraocular pressure reduction in every position . a number of previous studies reported mean iop reductions ranging from 1.5 to 8.3 mmhg after phacoemulsification in eyes with or without glaucoma . recent studies also demonstrated that phacoemulsification decreased diurnal iop fluctuations in patients with pseudoexfoliation syndrome with open or occludable angles and decreased nocturnal iop fluctuation in patients with primary angle closure . however , such iop reductions may not always occur after phacoemulsification , and cataract surgery may even raise iop postoperatively . in the present study , we found that iop was reduced ( from 13.8  1.9 mmhg to 12.6  2.1 mmhg ) after phacoemulsification when measured by gat in the sitting position ( p = 0.007 ) . although the tono - pen iop readings in the sitting position did not show a significant iop reduction after cataract surgery ( p = 0.266 ) , the amount of postoperative iop change obtained with tono - pen differed significantly between the operated and the nonoperated eyes ( 0.6  2.5 mmhg vs. 0.5  2.2 mmhg , p = 0.045 ) . in addition to postoperative iop reduction in the sitting position , we also found that phacoemulsification reduced iop in the recumbent postures . to the best of our knowledge , this is the first study to look at the iop changes in the supine and ld postures after phacoemulsification in nonglaucomatous eyes . in this study , the amount of postoperative iop reductions was greater in the recumbent positions than in the sitting position ( 1.7  2.5 mmhg vs. 0.6  2.5 mmhg , p = 0.048 ; 3.0  3.5 mmhg vs. 0.6  2.5 mmhg , p = 0.001 ) and was greater in the ld position than in the supine position ( 3.0  3.5 mmhg vs. 1.7  2.5 mmhg , p = 0.028 ) . moreover , the posture - induced iop elevation was also reduced after cataract surgery . by changing from the sitting to the supine position , iop increased by 2.0  1.8 mmhg before the surgery , whereas this elevation was reduced to 0.8  2.0 mmhg at 1 month after cataract surgery ( p = 0.048 ) . similar postoperative mitigation of posture - induced iop elevation was also found between the supine and ld positions . thus , these findings suggest that uncomplicated cataract surgery may also be beneficial for iop management during the night time . the mechanisms underlying the iop reduction after cataract surgery are not yet fully understood . possible explanations include ( 1 ) increased aqueous outflow by widening of the anterior chamber angle , increased postoperative acd , and/or an increase in prostaglandin production after surgery and ( 2 ) reduction of aqueous production resulting from traction on the ciliary body due to fibrosis and contraction of the posterior lens capsule . some researchers have proposed that extraction of the cataractous lens allows the anterior lens capsule to assume a more posterior location , thereby exerting zonular traction on the ciliary body and scleral spur . the increased posterior traction on the scleral spur may expand the trabecular meshwork and schlemm 's canal , leading to enhanced aqueous outflow . as to the greater reduction of iop in the recumbent postures found in the present study , we are not able to fully explain the mechanisms . however , we speculate that the increase in outflow facility after cataract surgery may allow for greater accommodation of increased iop , helping to prevent its rise after lying down or in the decubitus position . usually , iop is elevated when the body posture is altered from the sitting to a recumbent ( supine or ld or prone ) position . such posture - induced iop alterations have been reported to be higher in glaucoma patients compared to normal controls , and these iop changes have been associated with progression of glaucoma . moreover , sawada and yamamoto also reported that successful trabeculectomy reduced the posture - induced iop changes and that measuring these changes could be a method for assessing the bleb 's filtering function . therefore , information on the iop not only in the sitting position but also in the recumbent positions may improve our understanding of the effects of cataract surgery on iop and glaucoma . a recent study investigated the effect of cataract surgery on the circadian iop pattern in primary angle closure glaucoma patients using the contact lens sensor known as triggerfish ( sensimed , lausanne , switzerland ) . this study demonstrated that the mean range of iop fluctuations during the nocturnal period was significantly decreased after cataract surgery although 24-h iop fluctuation was not significantly changed . the nocturnal iop reduction after surgery may be explained by our findings of the greater iop - reducing effects in recumbent postures . however , our results need to be verified in open - angle glaucoma patients . there are some limitations to the present study . first , the sample size of the study population was small . thus , the present findings can not be directly applicable to glaucoma patients who have a coexisting diagnosis of cataract . third , one may criticize the discrepant results between the perioperative iop readings obtained by two different tonometers in the sitting position . however , the amount of iop change between the operated and nonoperated eyes was significantly different when iop was measured using tono - pen in the sitting position . fourth , the short duration ( 10 min ) of maintaining each position may also be a limitation . hence , the current finding may not be extrapolated into the effect of longer duration supine or ld position on iop . finally , the postural variations in iop were checked only once at 1 month after cataract surgery . multiple checkups of iop variations in different postures over a longer - term postoperative period are needed . we have shown that phacoemulsification significantly reduced iop of the nonglaucomatous eye in the supine as well as in the ld body postures . in addition , our findings suggest that phacoemulsification may help to reduce posture - induced iop elevations during the night time . however , further studies are required to investigate the effect of phacoemulsification on posture - induced iop variations in eyes with glaucoma .