the success rate of eswl in the treatment of ureteral stones is about 80 to 90% . some patients whose stones may not be fragmented by repeated eswl will require other surgical interventions . the success rate is influenced by stone factors ( stone size , location , composition , degree of obstruction ) , clinical factors ( symptom severity , patient 's expectations , associated infection , solitary kidney , abnormal ureteral anatomy ) , and technical factors ( available equipment , cost ) . however , it is not certain which factors influence the outcome of eswl [ 5 - 8 ] . recently , several studies have reported the incidence of secondary signs and their influence in eswl treatment [ 9 - 12 ] . however thus , we investigated the predictive factors of eswl outcome , including secondary signs , in the treatment of ureteral stones . a total of 153 patients who underwent eswl between july 2006 and july 2009 for ureteral stones that were diagnosed by non - enhanced spiral computed tomography ( nesct ) were included in this retrospective study . the inclusion criteria were stone size > 4 mm and stones that were solitary and radiopaque on the pretreatment plain radiography . patients with urinary tract infections , blood coagulation disorders , ureteral stricture , neurogenic bladder , or polycystic kidney were excluded . we divided the patients into two groups ( group a , stone size 10 mm ; group b , stone size > 10 mm ) and assessed age , sex , body mass index ( bmi ) , stone size , laterality , location , skin - to - stone distance ( ssd ) , hounsfield unit ( hu ) , and presence of secondary signs ( hydronephrosis , renal enlargement , perinephric fat stranding , and tissue rim sign ) in each group ( table 1 ) . the bmi was measured for each patient by dividing the patient 's weight in kilograms by height in meters squared . the ssd was measured on nesct by three distances from the stone to the skin ( 0 , 45 , and 90 ) . the average ssd was measured from these values and was recorded as the representative ssd for each stone . non - enhanced helical images were obtained by using a 5-mm collimation width from the top of the kidneys to the level of the pubic symphysis . three regions of interest ( roi ) were analyzed in the images showing the stones in the largest dimension . secondary signs included the presence or absence of hydronephrosis , renal enlargement , perinephric fat stranding , and tissue rim sign ( fig . renal enlargement was detected as an increase in the thickness of the renal parenchyma or an increase in the length of the kidney in acute obstruction . tissue rim sign was recognized as the observation of the annular soft tissue caused by edematous ureteral wall surrounding the stones . eswl was performed by using an electroconductive lithotriptor ( sonolith praktis , edap tms , vaulx - en - velin , france ) . a maximum of 3,000 shockwaves ( range , 1,500 to 3,000 ; median , 2,500 ) were delivered to a maximal power of 20 to 24 kv at 120 shocks / min during one session . the stones were fragmented under fluoroscopic guidance , and the number of shockwaves and energy level were recorded . the result of treatment was evaluated by plain radiography at 1 or 2 weeks after each eswl . when there was a large fragment with a long diameter > 4 mm , the failure of eswl was defined as remnant stones larger than 4 mm at 3 months after the first session . univariate analysis was used to individually assess the association between the various factors and outcomes . thereafter , the significantly associated variables were tested with multivariate logistic regression analysis to identify the independent predictors of treatment failure . the patients ' mean age was 54.511.78 years ( range , 34 to 74 years ) in group a and 55.513.28 years ( range , 32 to 79 years ) in group b. the mean stone size was 6.11.36 mm ( range , 4.1 to 10.0 mm ) and 14.34.65 mm ( range , 10.1 to 22.0 mm ) in each group . the overall success rate was 83.0% ( n=127 ) and the failure rate was 17.0% ( n=26 ) . twenty - five patients with remnants after eswl underwent ureteroscopic removal of stone and 1 patient underwent open ureterolithotomy ( table 2 ) . we observed a complication rate of 3.2% ( n=5 ) , including acute pyelonephritis ( n=3 , 1.9% ) that was successfully treated by percutaneous nephrostomy catheterization with antibiotics and a subcapsular hematoma ( n=2 , 1.3% ) that resolved spontaneously . a static steinstrasse was observed in 3 patients and analgesics were required in 5 patients for 1 day for post - eswl pain . in the univariate analysis , age , sex , bmi , laterality , location , and hu were not significantly different in terms of outcome of eswl ( table 3 ) . the mean stone size in the success and failure groups , respectively , was 5.91.27 mm vs. 7.51.43 mm in group a ( p<0.001 ) and 12.22.15 mm vs. 19.14.15 mm in group b ( p<0.001 ) . the mean ssd in the success and failure groups , respectively , was 102.412.88 mm vs. 110.85.66 mm in group a ( p=0.043 ) and 97.812.97 mm vs. 107.913.02 mm in group b ( p=0.013 ) ( table 3 ) . all the secondary signs showed statistically significant differences in terms of outcome of eswl ( table 3 ) . in the multivariate logistic regression , stone size ( odds ratio [ or ] , 50.005 ; 95% confidence interval [ ci ] , 6.207 to 402.852 ) was an independent predictive factor for failure of eswl in group a. perinephric fat standing ( or , 77.634 ; 95% ci , 1.349 to 446.558 ) and stone size ( or , 19.718 ; 95% ci , 1.600 to 243.005 ) were independent predictive factors in group b ( table 4 ) . since its introduction in the early 1980s , eswl has become a safe and accepted treatment modality for ureteral stones . the success rate of eswl in the treatment of ureteral stones is about 80 to 90% . many studies have demonstrated that the consistency , size , shape , location , and hu of ureteral stones and bmi may be predictors of the outcome of eswl [ 4 - 8 ] . concerning stone size , a larger size is associated with a higher risk for failure of treatment of urinary tract stones . recent studies have shown that stone size is an independent predictor of the stone - free rate after eswl [ 14 - 17 ] . it has been reported that patients with stones > 10 mm more frequently fail to be rendered stone - free by eswl . in the 2007 american urological association / european association of urology guideline at all locations in the ureter , the treatment outcome for stones less than 10 mm was superior to that for stones greater than 10 mm . specifically , for stones in the distal ureter , the stone - free rate for stones less than 10 mm treated by swl was 86% , compared with a 74% stone - free rate for those greater than 10 mm . in our series , we divided the patients into two groups and evaluated their outcomes and predictive factors . the success rates were 90.2% in the group with a stone size 10 mm ( group a ) and 68.6% in the group with a stone size > 10 mm ( group b ) . the multivariate analysis revealed that stone size was an independent predictor of failure of eswl in each group . bmi and ssd are certainly interrelated , but body fat distribution varies between by gender and race   and can not be reliably used as a surrogate marker for ssd . for example , pareek et al .   found bmi to be a significant predictor of success . conversely , in another study , bmi failed to predict successful eswl outcomes , whereas ssd remained a significant predictor . in our series , we suggest that the effect of bmi is probably related to the distance of the stone from the skin , which reflects the shockwave path in the body . bmi may not truly reflect central body fat distribution , which is probably the main factor affecting ssd . therefore , ssd is probably a more direct measurement of the effect of body build on eswl outcome than is bmi . wiesenthal et al .   suggested that ssd was a significant predictor of lithotripsy success for ureteral stones . in the multivariate analysis , ssd > 110 mm ( or , 0.49 ; 95% ci , 0.31 to 0.78 ) was a significant predictor of outcome . they reported that ssd of < 9 cm ( or : 2.8 ; 95% ci : 1.1 - 7.2 ) can predict swl success . in our series , ssd was a significant predictor of treatment outcome in the univariate analysis ( < 1.0 cm group , p=0.043 ; > 1.0 cm group , p=0.013 ) , but was not a significant predictor in the multivariate analysis ( < 1.0 cm group , p=0.082 ; > 1.0 cm group , p=0.110 ) . we may not have had enough data to distinguish the body fat distribution ( range , 81.4 to 124.1 mm ) , and additional evidence from larger trials may reveal the importance of ssd in the management of ureteral stones . the attenuation value of ureteral stones on axial computed tomography images has been studied as a predictor of fragmentation efficiency . however , because these values differ according to the size of the roi and portion in the same stone , the use of these values in the management of ureteral stones is still problematic . in our series , we suggest that the small sample size and short range of hu ( n=153 , 617.2 to 982.5 hu ) may have prevented accurate statistical comparison of the stone composition with the attenuation value . a correlation between stone attenuation and stone fragility was first demonstrated in vitro . as the attenuation value of calcium stones increases , likewise , in a clinical retrospective review , patients with calculi > 750 hu were 10.5 times as likely to need 3 swl sessions as were those with calculi of 750 hu . several investigators have since shown that eswl is more likely to fail for patients with renal calculi > 750 to 1,000 hu and these patients should be considered for other treatment modalities [ 6 - 8,24 ] . whether secondary signs in patients with ureteral stones affect outcome in eswl treatment remains controversial . ege et al .   reported the incidence of secondary signs and their influence on patient management . the most reliable signs indicating necessary endoscopic removal or lithotripsy were hydroureter , hydronephrosis , periureteral edema , and unilateral renal enlargement . on the other hand , boulay et al .   evaluated 99 patients and retrospectively analyzed the presence , size , and location of ureteral stones and the presence and severity of secondary signs of obstruction . a significant difference in stone size was found between patients who were treated conservatively and those who underwent a urological procedure ( 3.31.3 mm vs. 7.06.2 mm , p<0.1 ) . the presence and severity of secondary signs of obstruction were not significantly different between the two groups and did not affect treatment . those authors concluded that stone size alone correlated with the patient 's treatment and outcome . in another study , seitz et al .   reported secondary signs before laser ureterolithotripsy preoperatively did not correlate these with stone - free rates . the presence of secondary signs ( renal enlargement , perinephric stranding , ureteral dilation , ureteral rim sign ) could not predict intraoperatively verified stone impaction or stone - free rates . perinephric fat stranding is defined as linear areas of soft tissue attenuation in the perinephric space , which can result from any acute process or injury to the kidney . the changes in the perinephric space in the presence of urinary stones are thought to result from adaptation of the kidney to obstruction . immediately after acute ureteral obstruction , the intraluminal pressure of the collecting system increases and reaches 3 to 5 times normal pressure . the escape of urine into the renal interstitium ( pyelotubular backflow ) , across the renal pelvis into the renal sinus ( pyelosinus backflow ) , or into the lymphatic system ( pyelolymphatic backflow ) or the renal venous system ( pyelovenous backflow ) plays important roles in decompression of the intraluminal pressure . as proposed by kunin , lymphatic flow in the perinephric space results from elevated intrarenal venous pressure , and venous stasis , pyelosinus , and pyelovenous backflow may contribute to perinephric stranding . unilateral perinephric stranding is probably the manifestation of increased pressure in the collecting system in the early phase of ureteral obstruction . in our series , perinephric fat stranding was an independent predictor in group b ( stone size > 10 mm ) . all of the secondary signs are caused by ureteral obstruction ; however , not all of these represent the degree of ureteral obstruction . we suggest that the perinephric fat stranding reflects a condition to maintain the balance between pressure in the collecting system and ureteral wall resistance around the stones . despite increased pressure in the collecting system , the degree of obstruction was too severe in group b. in this condition , the outcomes of eswl were probably less successful than in the group without perinephric fat stranding . although the mechanism of perinephric fat stranding may be similar in patients with smaller stone sizes , perinephric fat stranding was not significant in group a. we speculate that the obstruction is more easily resolved in patients with smaller stone sizes . thus , perinephric fat stranding may be a secondary sign that more readily reflects the degree of ureteral obstruction in the case of large stones . stone size is an independent predictive factor influencing failure of eswl in the treatment of ureteral stones . in addition to stone size , the presence of perinephric fat stranding is also an independent predictive factor for larger ureteral stones ( > 10 mm ) .