FACTORS OF FRAGMENT RETENTION AFTER EXTRACORPOREAL SHOCKWAVE LITHOTRIPSY (ESWL)
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JAN H. RÜFFER, LADISLAV PRIKLER, DANIEL K. ACKERMANN

Department of Urology, Kantonsspital St. Gallen, St. Gallen, Switzerland

ABSTRACT

     Purpose: Fragment retention is a common problem after sufficient disintegration of urinary tract calculi. In this paper we review and discuss factors that impede the excretion of fragments after ESWL.
     Materials and Methods: We reviewed the literature using Knowledge Finder® and MEDLINE.
     Results: Stone factors, renal anatomy, metabolic factors, non-urological patients’ factors and non-patients’ factors are known to influence outcome and natural passage of fragments after ESWL.
     Conclusions: ESWL is the treatment of choice for most urinary tract stones. Particularly adverse factors for stone clearance are high stone burden, stone location in the lower pole and anatomical passage hindrance due to unfavorable anatomy, strictures or malformations.

Key words: kidney calculi; lithotripsy; ESWL; treatment outcome
Braz J Urol, 28: 3-9, 2002

INTRODUCTION

     Extracorporeal Shockwave Lithotripsy (ESWL) has become the favored treatment for most urinary tract stones. Overall stone free rates after ESWL vary from 50% to 87% (1 ,2 ) depending on many factors affecting the overall success rate.
     Basically, success arises from the effective fragmentation and clearance of stones. ESWL of renal stones generally results in stone fragmentation. The clearance of stone fragments after ESWL; takes time and is influenced by numerous factors. Successful treatment is measured as being stone free rate three months after ESWL, however, successful treatment remains lower than the disintegration rate.
     Several factors have been identified limiting the clearance of stone fragments after ESWL.

STONE BURDEN AND STONE NUMBER

     It is generally accepted that stone burden plays a major role in treatment outcome after ESWL. Politis & Griffith (3 ) have shown that kidneys with small stones (£ 15 mm in longest diameter) become stone free in 78% of the cases, whereas kidneys with large stones (> 15 mm) are stone free in 66% of the cases. Furthermore, small stones had significantly lower complication rates and needed fewer auxiliary procedures such as ureteral stenting (Table-1).



     Similarly, Gupta et al. (4 ) have found increasing stone free rates with smaller stones. Stone free rates varied from 38.7 to 72.1 % depending on the stone size (Table-2).



     Because of unsatisfactory success rates, patients with high stone burden are often treated with alternative procedures such as ureterorenoscopy or percutaneous nephrolitholapaxy.
     Although stone size is a proven factor in the outcome of ESWL, stone free rates after ESWL are adversely affected by the multiplicity of stones. In small stones the stone burden was found to be of little prognostic influence (5 -7 ).

STONE COMPOSITION

     The chemical composition of urinary stones determines their fragility to shock waves. Therefore stone composition influences success after ESWL (8 ). Best disintegration rates are observed in calcium oxalate dihydrate (COD) and struvite stones followed by uric acid and apatite. The lowest disintegration rates are observed in with calcium oxalate monohydrate (COM), which is very hard, and in cystine, which is elastic and therefore more resistant to shock waves (Table-3).


STONE LOCATION

     Since its initial application in 1980 (9 ) the indication of ESWL has rapidly extended from kidney stones to almost all urinary stones. Coz et al. (2) analyzed the outcome of ESWL according to site in 2016 urinary tract stones. Stone free rates of lower caliceal stones and upper or iliac ureteral stones are lower than the overall stone free rate. Best stone free rates are observed in stones in the pelvic ureter, the renal pelvis and the upper or middle calix (Table-4). Stone free rates for ureteral and renal stones are influenced by different factors: ureteral stones seem to be more resistant to disintegration whereas lower pole stones tend to fragment retention after sufficient fragmentation. Ureteral stones, especially if located in the upper ureter can be pushed back into the renal pelvis before disintegration. The best disintegration rate in the renal pelvis compares with the need for a more invasive procedure when pushing back the stone and stenting the ureter.



     Lower caliceal stones show a relatively poor stone free rate due to fragment retention after sufficient disintegration. Stone free rates are also related to the stone size. Stone free rates drop down below 30% in stones bigger than 20 mm. In stones smaller than 11 mm, stone free rates rise above 60% (10 ) (Table-5).



     Coz et al. (2) found the outcome of lower caliceal stones to be related to size: the success rate was 83% in stones smaller than 24 mm in diameter and dropped sharply as the size increased. Similar data were published by Chen and Streem (11 ). After ESWL of lower pole calculi, the authors found a stone free rate of 54% in patients with stones £ 20 mm2 and of 13% in patients with stones > 20 mm2.
     After disintegration, the clearance of fragments depends upon their location. Successful ESWL is usually defined as stone free three months after treatment. Residual fragments larger than 5 mm in diameter are generally considered a failure of the ESWL session. If the residual fragments are between 1 and 4 mm in size, asymptomatic and noninfected they, are susceptible to be cleared spontaneously.
     The natural history of these residual fragments depends upon the initial location three months after ESWL. Residual fragments located in the renal pelvis have a spontaneous clearance rate of 66%. The spontaneous clearance rate of residual fragments in the middle calyx is 50%, and residual fragments in the inferior calyx 37% (12 ).

RENAL ANATOMY

     The configuration of the pelvicaliceal system influences stone clearance after ESWL. Several abnormalities like polymegakalikosis, medullary sponge kidney, horseshoe kidneys, strictures, diverticula, cysts and dilatation adversely influence the outcome of ESWL (13 ,14 ).
     Within a regular renal anatomy, clearance of renal stone fragments after ESWL depends on the stone location, being lowest for stones in the lower calix. Poorer clearance for lower pole fragments is considered to be due to gravity.
     Sampaio & Aragao (15 ) first described the inferior pole collecting system anatomy and its role in the outcome of ESWL.
     Width, length and infundibulopelvic anatomy has been shown to be relevant for stone clearance after ESWL of lower pole stones (16 ). Stone clearance has been shown to be poorer for an acutely angled than for an obtusely angled inferior calix, and better for a shorter calix with a wider infundibulum than for a longer calix with a narrower one (4) (Table-6).



     Elbahnasy (17 ) found only a 17% success rate in patients with all three unfavorable factors. Much the same are the data published by Gupta et al. (4): an obtuse infundibulopelvic angle was most closely associated with stone free status following ESWL. Caliceal length was found not to be significant for stone clearance after ESWL (Table-7).



     Keeley et al. (6) found the infundibulopelvic angle to be the only significant factor predicting stone free status after ESWL of lower pole stones (Table-8).



     There are several aspects of the problem of measuring renal anatomy. Measurement of infundibular width and length is easy on pyelography but the x-ray might be influenced by technique and renal obstruction. The best technique for measuring the infundibulopelvic angle is still under discussion. Several methodologies of measurement for the infundibulopelvic angle exist:
 - infundibular and renal pelvic axis
- infundibular and ureteropelvic axis
- infundibular and vertical ureteral axis
     In addition, a considerable inter- and intra-observer variation is found within the same methodology (6).
     As the measured angle between the pelvis and the lower pole infundibulum appeared to be difficult to reproduce the caliceal pelvic height (CPH) was suggested as a factor that may impede stone clearance from the lower pole. CPH is the distance between the lower lip of the renal pelvis and the bottom of the calix containing the stone. If the CPH is below 15 mm clearance rates reach 92%, whereas a CPH of 15 mm or above shows clearance rates of only 52% (18 ).
     To sum up, the data suggests that clearance of fragments after ESWL of lower caliceal stones is adversely influenced by an obtusely angled inferior calix with a long and narrow infundibulum. But methodology and accuracy of measurement is under discussion as well as clinical weighting. Anatomical measurements of the lower pole anatomy should not be used to withdraw treatment from patients.

METABOLIC FACTORS

     Urolithiasis is known to be related to certain metabolic disorders (19 ). Elevated serum calcium was found to influence the outcome of ESWL adversely (5).
     Elevated urinary excretion of citrate was found to be positively associated with stone clearance (20 ).

BODY MASS INDEX (BMI)

     The BMI is a non-urological patient factor that influences the outcome of ESWL (5,21,22 ). Obesity may cause difficulty in imaging and in placement of the calculus at the shock wave focal point. The best chance for successful ESWL was found in patients with a BMI of 20 to 28 (5). Consistent with this, Robert et al. (22) found patients with a BMI > 25 as having significantly deeper urinary calculi and a worse outcome after ESWL.

DOCTORS’ EXPERTISE, PLAN OF TREATMENT

     The first analysis of interoperator variation at the same institution in success following ESWL was published by Logarakis et al. (1). Comparable to other surgical procedures the outcome of ESWL differed depending on the urologist. The best results were obtained by the urologist who treated the greatest number of patients, used a high number of shocks and had the longest fluoroscopy time.
     Some hospitals favor several consecutive ESWL treatments (“multiple session therapy”) (23 ), which may influence outcome. Repeated treatments lead to a cumulative stone free rate. The number of ESWL treatments performed before alternative modalities are used is still under discussion (24 ).
     The outcome rate varies according to additional procedures as well: ESWL retreatment of completely fragmented but persistent stone debris (“stir-up”) has been shown to promote the passage of residual debris after previous ESWL therapy (25 ).
     Several other procedures have been used to enhance clearance after ESWL such as forced diuresis, vibration massage with the patient in upside down position (26 ), inversion therapy (27 ), a cobra catheter for directed irrigation of the stone containing inferior calix (28 ) and percutaneous caliceal irrigation (29 ). However, none of these methods has gained widespread favor.
     Another important factor regarding the clearance of stone fragments after ESWL is time, as the clearance of the fragments produced by ESWL is not immediate. A few days after treatment 85% of patients have radiological evidence of residual fragments in the kidney (30 ). Fragments with sizes between 1 mm and 4 mm remaining in the kidney three months after the treatment are usually considered residual fragments. Without any treatment, the spontaneous clearance rate of residual fragments three months after ESWL is between 23% and 38% (31 ,12).

QUALITY OF LITHOTRIPTORS

     With comparable technical principles, various lithotriptors have been developed. Commonly used lithotriptors use electrohydraulic, electromagnetic and piezoelectric means of shock wave generation. Exact comparison of lithotriptors is quite difficult as many variables must be considered. Overall treatment results with different lithotriptors seem to be similar (32 ,33 ).

CONCLUSION

     The prerequisite for stone clearance is effective disintegration, which depends mainly on stone composition but also on stone location, BMI, metabolic factors and doctors’ expertise as well.
     Passage of disintegrates is depends on anatomical conditions. Urinary tract obstruction distal to the calculi is a contraindication to ESWL. Best results can be achieved in a solitary renal pelvis stone. The calculi should be 2 cm or less in diameter. In lower pole calculi a wide, short and obtuse angled infundibulum is desirable.

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____________________
Received: July 25, 2001
Accepted: August 22, 2001

_______________________
Correspondence address:
Dr. Jan H. Rüffer
Department of Urology
Kantonsspital St. Gallen
CH-9007 St. Gallen, Switzerland
Fax: + + (41) (71) 494-2891
E-mail: jan.rueffer@kssg.ch