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Stone Burden in an Average Swedish Population of Stone Formers Requiring Active Stone Removal: How Can the Stone Size Be Estimated in the Clinical Routine?
European Urology
European Urology 43 (2003) 275±281
Stone Burden in an Average Swedish Population of Stone Formers Requiring Active Stone Removal: How Can the Stone Size Be Estimated in the Clinical Routine? Hans-GoÈran Tiseliusa,b,*, Annika Anderssonc a
Department of Urology, Huddinge University Hospital, SE-141 86 Stockholm, Sweden Division of Urology, Center for Surgical Sciences, Karolinska Institutet, Stockholm, Sweden c Department of Urology, LinkoÈping University Hospital, LinkoÈping, Sweden b
Accepted 26 December 2002
Abstract Objective: To get information on the distribution of stone burdens in an average and representative group of Swedish stone forming patients requiring active removal of stones from the kidneys or ureters and to compare different methods for assessing the stone burden. Methods: A computerised device was used to measure the total stone surface area (Ameasured) of 599 stone situations in kidneys and ureters in a consecutive group of patients referred to active stone removal. These measurements were compared with the large and short transverse diameters of the greatest stone, the sum of the largest diameters of the stones, the arithmetically calculated surface area (Acalculated) as well as with the stone-types (A±F) previously described. Result: There were 483 stone situations with one and 116 with more than one stone. The stones were found in 407 men and 192 women. In 343 cases were the stones on the left side and in 256 on the right side. There were 34 staghorn stones. Of the examined stone situations 250 were in the kidney and 349 in the ureter. An Ameasured above 300 mm2 was recorded in 7% of all stone situations. The corresponding numbers for Ameasured above 200 mm2, 500 mm2 and 700 mm2 were 13%, 4% and 3%, respectively. When staghorn stones were excluded, good correlations were recorded for all variables but the best correlation was found between Ameasured and Acalculated. A revision of the previously published stone-type subgroups is suggested based on the following limits for the stone surface area: A 30 mm2, B 31 300 mm2, C 301 700 mm2 and D > 700 mm2. Conclusion: The distribution of stone situtations with different stone burden in an average Swedish population is described. With the exception of staghorn stones and stones with extremely irregular form an acceptable estimate of the stone surface are can be arithmetically derived from the length and the width of the stone. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Epidemiology; Stone surface area; Urolithiasis; Categories 1. Introduction Appropriate conclusions on the ef®cacy of various forms of stone removing procedures cannot be made without detailed information on the speci®c stone situation in the kidney or ureter. Numerous technique independent factors in¯uence the treatment result such *
Corresponding author. Tel. 46-8-5858-7762; Fax: 46-8-5858-7760. E-mail address: [email protected] (H.-G. Tiselius).
as the anatomy of the renal collecting system, the position of the stone or stones, the habitus and mobility of the patient as well as the chemical composition of the stone. It is also generally held that the efforts and results of active stone removal to a large extent is determined by the size of the stone [1±6]. The latter aspect particularly came into focus when the modern non-invasive or low-invasive methods for stone removal were introduced, but irrespective of which surgical procedure that is selected for a speci®c stone problem it is essential to
0302-2838/03/$ ± see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0302-2838(03)00006-X
276
H.-G. Tiselius, A. Andersson / European Urology 43 (2003) 275±281
be able to predict the outcome and precautions that are necessary to optimise the treatment. Several methods have been reported in the literature in order to describe the stone burden and to categorise patients with regard to the stone burden. Common estimates thereby have been the largest [6±8] or transverse diameters of the largest stone [9,10], the sum of the largest diameters of all stones, the stone surface area as measured on the plain ®lm [11,12] and a calculated estimate of the stone volume [13]. For a clinical subgrouping of patients we have, in our department, applied a system based on the largest stone diameters and the number of stones [14]. Although the latter method is easy to use in the daily clinical work it is not always suf®ciently accurate for describing the stone burden. With modern computerised imaging techniques [9,10,15,16] it will certainly be possible to get exact estimates of the stone volume, but probably not as a routine procedure. Although the stone burden and indirectly the volume can be derived from measurements on the plain X-ray ®lm such procedures are usually too time consuming to be attractive in the routine work. Moreover it is desirable to have a method for estimating the stone burden that can be applied in every place where patients with urinary tract stones are treated. Thereby it should be possible to compare different groups of patients and different techniques in a global perspective. The investigation in this paper was undertaken to get information on the stone burden in a consecutive series of patients requiring active removal of stones from the kidney or the ureter. A comparison was subsequently made between different clinical estimates of the stone burden and the directly measured stone surface area in order to get a reliable and routinely feasible procedure for describing the stone situation in the individual kidney or ureter. 2. Materials and methods The stone situations were considered in a consecutive series of 599 patients with stones in the kidneys or ureters and who were referred for active stone removal. All together 407 male and 192 female kidneys and ureters radio-opaque stones were considered. For each stone identi®ed on a plain X-ray ®lm (KUB) the maximal length (lmax) and the maximal width perpendicular to l (wmax) were measured with a ruler and expressed in mm. No correction was made for the magni®cation caused by the distance between stone and ®lm. The sum of the largest diameters of all stones (Slmax) was obtained by addition of lmax of each stone. The surface area of the stone (Ameasured) was measured as follows: the circumference of the stones was followed with a Wacom styler connected to a Macintosh computer. The software
Table 1 Principles for subgrouping of non-staghorn renal stones Stone-type
One stone
Two to three stones
Four or more stones
A B C D
5 mm 6±20 mm 21±30 mm 31 mm
± 10 mm 20 mm >20 mm
± ± 10 mm >10 mm
used to analyse the surface area was the NIH Image program (version 1.49). Before use on the Wacom table the contours of the stones were transferred by a pencil to a semitransparent paper. Ameasured was expressed in mm2. Assuming an ellipsoid shape of the stone an arithmetical estimate of the stone surface area (Acalculated) was accordingly derived Acalculated p
lmax wmax 0:785 lmax wmax : 2 2
Subgrouping of the various stone situations into stone-types was made according to the principles in Table 1 [14]. Stone-type E represents complete or partial staghorn stones and stone-types Fp, Fm and Fd proximal, mid and distal ureteral stones irrespective of size. Statistical analysis was carried out with StatisticaTM, StatSoft, Tulsa, OK, USA.
3. Results 3.1. Epidemiological observations The cumulative frequency distribution of the total Ameasured in all 599 stone situations considered is shown in Fig. 1. An Ameasured above 300 mm2 was recorded in only 7% of the renal units. There were Ameasured-values above 200 mm2 in 13% and above 500 mm2 in 4% of the cases. Fig. 1 also shows the distribution of Ameasured-values when the stone situations in kidneys and ureters were considered separately. For stones situated in the kidney only 12%, 10% and 8% had Ameasured above 200 mm2, 300 mm2 and 500 mm2, respectively. These values of Ameasured correspond to projections on the X-ray ®lm of circular stones with diameters of 16.0 mm, 19.5 mm and 25.2 mm. It needs to be emphasised that only 3% of all stones in the kidney had an Ameasured of 1000 mm2 or more. In terms of ureteral stones the overall distribution disclosed that in 97% Ameasured did not exceed 200 mm2. An Ameasured up to 100 mm2 was recorded in 83% and above 300 mm2 in only 1% of the cases. At the time of treatment 41.5% of the ureteral stones were found in the proximal ureter, 15.5% in the middle and 43.0% in the distal ureter. The mean (S.D.) Ameasured for all ureteral stones was 73 mm2 (104) with a range of 10±1708 mm2. The recordings for proximal, mid and distal ureteral stones are given in Table 2.
H.-G. Tiselius, A. Andersson / European Urology 43 (2003) 275±281
277
Table 3 The measured surface area (Ameasured; mm2) of stones in men an women and in the left and right kidney/ureter
Fig. 1. Cumulative frequency distributions of Ameasured in all stone situations (triangle), stones in the kidney (circle) and stones in the ureter (square). Table 2 The measured surface area (Ameasured; mm2) of stones situated in 349 ureters at the time of active stone removal Position of stones
Number
Proximal ureter 145 Mid ureter 54 Distal ureter 150
Mean (mm2)
S.D. (mm2)
Minimum (mm2)
Maximum (mm2)
86 72 53
144 41 41
11 21 10
1626 196 244
Subgroup of patients
Number
Mean (mm2)
S.D. (mm2)
Minimum (mm2)
Maximum (mm2)
Men Women Left system Right system
407 192 343 256
147 228 173 171
364 416 385 381
10 5 5 9
4210 2600 4210 3876
It is of note that in 343 cases (57%) the stones were found on the left side and in 256 (43%) on the right side. There were 407 male and 192 female kidneys and ureters that contained stones. The mean (S.D.) Ameasured for these subgroups are shown in Table 3. The stone burden was apparently similar in the right an left renal units, but Ameasured was greater in women than in men (p < 0:02). Table 4 The measured surface area (Ameasured; mm2) for stones in the kidney of types A±E (see Section 2) Stone-type
Number
Mean (mm2)
S.D. (mm2)
Minimum (mm2)
Maximum (mm2)
A B C D E
6 135 42 33 34
13 70 209 601 1204
6 50 139 477 923
5 10 21 85 237
21 258 721 2245 4209
Fig. 2. Cumulative frequency distributions of Ameasured in kidney stone-types A (open square), B (®lled square), C (®lled triangle) and D (open triangle).
278
H.-G. Tiselius, A. Andersson / European Urology 43 (2003) 275±281
The Ameasured-values recorded for the stone situations referred to stone-types A±E are summarised in Table 4. The cumulative frequency distributions for Ameasured in kidneys with stone-types A±D are demonstrated in Fig. 2 and as expected there is a considerable overlapping of Ameasured for the latter four stone-types. It is of note that whereas an Ameasured above 300 mm2 was recorded in 76% of the stone situations referred to type D, 14% of type C stones had an Ameasured above that limit and 5% of type C stones Ameasured was above 650 mm2. In 70% of type C stones Ameasured was 150 mm2 or more compared with 8% in stone-type B.
Stone situations comprising only one stone was most commonly encountered and recorded in 496 cases (83%) whereas in 103 cases (17%) more than one stone was identi®ed. 3.2. Comparison between different expressions of the stone burden The correlations between Ameasured and lmax, wmax, Slmax and Acalculated for stone situations comprising one or more than one stone, are summarised in Table 5. Although there was a good correlation between Ameasured and all the other considered variables the
Fig. 3. Relationship between Ameasured and Acalculated in patients with non-staghorn stones in the kidney (above) and stones in the ureter (below).
H.-G. Tiselius, A. Andersson / European Urology 43 (2003) 275±281 Table 5 Coef®cients of correlation between Ameasured, Acalculated, lmax, wmax and Slmax Compared variables
Number lmax vs. Ameasured wmax vs. Ameasured Slmax vs. Ameasured Acalculated vs. Ameasured
Coef®cients of correlation All stone situations
Situations with one stone
Situations with two or more stones
599 0.82 0.87 0.77 0.89
483 0.88 0.88 0.88 0.93
116 0.62 0.83 0.79 0.79
Table 6 Stone-types of 250 stone situations in the kidney according to the old and new system for subgrouping Stone-type
Old method
New method Ameasured
New method Acalculated
A B C D E
6 135 42 33 34
31 155 31 33 ±
48 152 21 29 ±
best correlation was obtained between Ameasured and Acalculated. Fig. 3 shows the relationship between Ameasured and Acalculated. 3.3. Modi®ed system for subgrouping of stone situations In order to get a better correspondence between the stone-types A±E and Ameasured, the following ranges of Ameasured were chosen: A 30 mm2, B 31±300 mm2, C 301±700 mm2, D > 700 mm2. Because of the small number of large stones, the stone-type E was omitted. Inasmuch as stones with staghorn shape (previously designated E) had an Ameasured between 249 mm2 and 4420 mm2 it is obviously better to indicate in the various surface intervals if the stone has a complete or partial staghorn con®guration. Table 6 shows the distribution of stone-types with the old and new systems for subgrouping of the stone burden in kidneys. When Acalculated was used for this classi®cation the result was very similar to that obtained with Ameasured. 4. Discussion It is today well recognised that information on the stone burden is essential for an appropriate planning of stone removal from the urinary tract [1±6]. The need to describe the stone situation in terms of size of the stone(s) became particularly apparent when non-inva-
279
sive or low-invasive methods replaced open surgery. Moreover, conclusions on the ef®cacy of different stone-disintegrating devices cannot be made unless data on treated stone burden are given in the report. The most common way to express the size of stones is to give the longest diameter of the largest stone [6±8]. The results presented above show that the longest diameter well correlates with the measured stone surface area at least when only one stone is considered. The explanation for this outcome is that the majority of patients presented with only one stone in the kidney or in the ureter. This observation is thus in good agreement with the SWL-results that recently were presented by Buchholz and co-workers [17]. As expected, however, the correlation was less good when more than one stone constituted the stone situation. Previous reports on this matter have clearly emphasised the usefulness of measuring the surface area on a plain X-ray ®lm. In this way it is possible to make a much more precise description of the stone burden and the clinical problem. In addition it has been shown that knowledge of the stone burden enables us to derive an approximate estimate of the stone volume [13]. With spiral CT-examinations and advanced principles for image analysis it is certainly possible to get an exact measure of the stone volume. This, however, cannot be achieved without considerable efforts and de®nitely not as a standard clinical routine procedure. Neither is it possible to easily measure the surface area on a plain X-ray ®lm without special tools. It is therefore reassuring that method for arithmetically assessing the stone surface area might be used as a reasonable substitute. Moreover dif®culties have been reported in the accuracy of measuring the vertical diameters of stones on CT-images [15]. It needs to be pointed out in this regard that no correction was made for the ampli®cation of the stone that occurs as a result of distance between stone and the ®lm. The correlation between Ameasured and Acalculated was acceptable and although deviations occur, Acalculated can certainly be used for most clinical situations and as a rough description of the treated stone situations when different methods should be compared. It stands to reason, however, that the method is not applicable when stones with a pronounced irregular shape are encountered. Therefore, when the stone morphology deviates very much from the ellipsoid geometry, other methods for assessing the stone surface area are necessary. This is particularly the case for complete and partial staghorn stones. When Ameasured and Acalculated were used to classify the various renal stone situations in the stone-type categories A±D, there was a very good agreement
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between the two variables. The stone-type system was introduced to enable a clinical comparison between patients with stone burdens of different size. This system which is very approximate gives a considerable overlapping between the different types and it is an improvement if this subgrouping instead is based on the stone surface area. In this respect Acalculated provides a both acceptable and easily derived estimate. Thereby we found it reasonable to omit the previously used stone-type E. All staghorn stones are better referred to those subgroups that correspond to their total surface area. The surface area for staghorn stones always has to be assessed with more direct methods than the arithmetical calculation. Information on the distribution of stone situations in the population is dif®cult to extract from the literature. The result is presented in different ways usually with various size ranges of the largest stone diameter. Most of these reports are given in association with stone removing procedures, usually shock wave lithotripsy. In most of these studies the interpretation is hampered by the fact that the group of patients is not representative for a given population. Patients from the same geographical area might either have been treated in different institutions or referred to alternative forms of stone removal. Moreover there might in some departments have been an over-representation of dif®cult cases. Inasmuch as the 599 patients presented in this report represented a consecutive series of patients from a population in which all patients requiring active stone removal were referred to the same centre the observations in this study are of great epidemiological interest.
The results show that only 7% of all stone situations in the kidney had a surface area exceeding 300 mm2. According to the EAU guidelines for stone treatment [6], this is thus the fraction of patients in whom methods alternative to shock wave lithotripsy should be considered. This means that in a population of 1 million, with an annual incidence of 2000 and a need for stone removing procedures in 500, not more than approximately 35 patients should be considered for other stone removing procedures than shock wave lithotripsy. This is thus the situation for stones in which the surface area correspond to a largest diameter of approximately 20 mm. If this limit is extended to 700 mm2 or 30 mm2, the corresponding number of patients will be 15 (3%). It should be noted that only those patients were considered who were candidates for active stone removal. The stones were not measured in those patients who spontaneously passed their stones. This group comprises around 75% of all patients that annually are af¯icted by stone disease. The results presented in this paper give interesting epidemiological information. The results moreover show that an arithmetic calculation of the stone surface area using the length and width of the stones on the plain X-ray ®lm for most stone situations is a useful measure of the stone surface area. Only the surface area of irregular and staghorn stones needs to be measured differently. Finally an estimate of the stone surface area is obviously a more appropriate basis for subgrouping of patients with stones than the diameters and numbers of stones.
References [1] Ackermann D, Claus R, Zehnter C, Schreiber K. Extracorporeal shock wave lithotripsy for large renal stones. Eur Urol 1988;15:5±8. [2] Anderson PAM, Norman RW, Awad SA. Extracorporeal shock wave lithotripsy experience with large renal calculi. J Endourol 1989;3:31±6. [3] Gleeson MJ, Shabsigh R, Grif®th DP. Outcome of extracorporeal shock wave lithotripsy in patients with multiple renal calculi based on stone burden and location. In: Lingeman JE, Newman DM, editors. Shock wave lithotripsy: state of the art. New York: Plenum Press; 1988. p. 131±6. [4] Lingeman JE. Relative roles of extracorporeal shock wave lithotripsy and precutaneous nephrostolithotomy. In: Lingeman JE, Newman DM, editors. Shock wave lithotripsy 2. Urinary and biliary lithotripsy. New York: Plenum Press; 1989. p. 303±7. [5] Murray MJ, Chandoke PS, Berman CJ, Sankey NE. Outcome of extracorporeal lithotripsy monotherapy for large renal calculi: effect of stone and collecting system surface areas and cost effectiveness of treatment. J Endourol 1995;9:9±13. [6] Tiselius H-G, Ackermann D, Alken P, Buck C, Conort P, Gallucci M. Guidelines on urolithiasis. Eur Urol 2001;40:362±71. [7] Gor®t ON, Poda D, Meretyk S, Katz G, Shapiro A, Golijanin D, et al. Is the pediatric uereter as ef®cient as the adult ureter in transporting
[8]
[9] [10]
[11] [12] [13]
fragments following extracorporeal shock wave lithotripsy of renal stones larger than 10 mm? J Urol 2001;166:1862±4. Lingeman JE, Smith LH, Woods JR, Newman DM. Nonstaghorn renal calculi. Staghorn calculi. In: Lingeman JE, Smith LH, Woods JR, Newman DM, editors. Urinary calculi: ESWL, endourology and medical therapy. Philadelphia: Lea and Febiger; 1989. p. 149±91. Coll DM, Varanelli MJ, Smith RC. Relationship of spontaneous passage of ureteral calculi to stone size an location as revealed by unenhanced helical CT. Am J Roentgenol 2002;178:101±3. Fielding JR, Silverman SG, Samuel S, Zou KH, Loughlin KR. Unenhanced helical CT of ureteral stones a replacement of excretory urography in planning treatment. Am J Roentgenol 1998;171: 1051±3. Chandoke PS. Cost-effectiveness of different treatment options for staghorn calculi. J Urol 1996;156:1567±71. Lam HS, Lingeman JE, Russo R, Choa GT. Stone surface area determination techniques: a unifying concept of staghorn stone burden assessment. J Urol 1992;148:1026±9. Ackermann D, Grif®th D, Dunthorn M, Newman R, Finlayson B. Calculation of stone volume and urinary stone staging with computer assistance. J Endourol 1989;3:355±60.
H.-G. Tiselius, A. Andersson / European Urology 43 (2003) 275±281 [14] Tiselius HG, Pettersson B, Hellgren E, Carlsson P. Classi®cation of patients subjected to extracorporeal shock wave lithotripsy. Scand J Urol Nephrol 1988;22:65±70. [15] Naripalem N, Sundaram CP, Boridy IC, Yan Y, Heiken JP, Clayman RV. Comparison of helical computerized tomography and plain radiography for estimating the stone size. J Urol 2002;167:1235±8.
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[16] Takahashi A, Kawashina A, Ernst RD, Boridy IC, Goldman SM, Benson GS, et al. Ureterolithiasis: can clinical outcome be predicted with unenhanced helical CT? Radiology 1998;208:97±102. [17] Buchholz NP, Rhabar MH, Talati J. Is measurement of stone surface area necessary for SWL treatment of nonstaghorn calculi? J Endourol 2002;16:215±20.
European Urology 43 (2003) 275±281
Stone Burden in an Average Swedish Population of Stone Formers Requiring Active Stone Removal: How Can the Stone Size Be Estimated in the Clinical Routine? Hans-GoÈran Tiseliusa,b,*, Annika Anderssonc a
Department of Urology, Huddinge University Hospital, SE-141 86 Stockholm, Sweden Division of Urology, Center for Surgical Sciences, Karolinska Institutet, Stockholm, Sweden c Department of Urology, LinkoÈping University Hospital, LinkoÈping, Sweden b
Accepted 26 December 2002
Abstract Objective: To get information on the distribution of stone burdens in an average and representative group of Swedish stone forming patients requiring active removal of stones from the kidneys or ureters and to compare different methods for assessing the stone burden. Methods: A computerised device was used to measure the total stone surface area (Ameasured) of 599 stone situations in kidneys and ureters in a consecutive group of patients referred to active stone removal. These measurements were compared with the large and short transverse diameters of the greatest stone, the sum of the largest diameters of the stones, the arithmetically calculated surface area (Acalculated) as well as with the stone-types (A±F) previously described. Result: There were 483 stone situations with one and 116 with more than one stone. The stones were found in 407 men and 192 women. In 343 cases were the stones on the left side and in 256 on the right side. There were 34 staghorn stones. Of the examined stone situations 250 were in the kidney and 349 in the ureter. An Ameasured above 300 mm2 was recorded in 7% of all stone situations. The corresponding numbers for Ameasured above 200 mm2, 500 mm2 and 700 mm2 were 13%, 4% and 3%, respectively. When staghorn stones were excluded, good correlations were recorded for all variables but the best correlation was found between Ameasured and Acalculated. A revision of the previously published stone-type subgroups is suggested based on the following limits for the stone surface area: A 30 mm2, B 31 300 mm2, C 301 700 mm2 and D > 700 mm2. Conclusion: The distribution of stone situtations with different stone burden in an average Swedish population is described. With the exception of staghorn stones and stones with extremely irregular form an acceptable estimate of the stone surface are can be arithmetically derived from the length and the width of the stone. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Epidemiology; Stone surface area; Urolithiasis; Categories 1. Introduction Appropriate conclusions on the ef®cacy of various forms of stone removing procedures cannot be made without detailed information on the speci®c stone situation in the kidney or ureter. Numerous technique independent factors in¯uence the treatment result such *
Corresponding author. Tel. 46-8-5858-7762; Fax: 46-8-5858-7760. E-mail address: [email protected] (H.-G. Tiselius).
as the anatomy of the renal collecting system, the position of the stone or stones, the habitus and mobility of the patient as well as the chemical composition of the stone. It is also generally held that the efforts and results of active stone removal to a large extent is determined by the size of the stone [1±6]. The latter aspect particularly came into focus when the modern non-invasive or low-invasive methods for stone removal were introduced, but irrespective of which surgical procedure that is selected for a speci®c stone problem it is essential to
0302-2838/03/$ ± see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0302-2838(03)00006-X
276
H.-G. Tiselius, A. Andersson / European Urology 43 (2003) 275±281
be able to predict the outcome and precautions that are necessary to optimise the treatment. Several methods have been reported in the literature in order to describe the stone burden and to categorise patients with regard to the stone burden. Common estimates thereby have been the largest [6±8] or transverse diameters of the largest stone [9,10], the sum of the largest diameters of all stones, the stone surface area as measured on the plain ®lm [11,12] and a calculated estimate of the stone volume [13]. For a clinical subgrouping of patients we have, in our department, applied a system based on the largest stone diameters and the number of stones [14]. Although the latter method is easy to use in the daily clinical work it is not always suf®ciently accurate for describing the stone burden. With modern computerised imaging techniques [9,10,15,16] it will certainly be possible to get exact estimates of the stone volume, but probably not as a routine procedure. Although the stone burden and indirectly the volume can be derived from measurements on the plain X-ray ®lm such procedures are usually too time consuming to be attractive in the routine work. Moreover it is desirable to have a method for estimating the stone burden that can be applied in every place where patients with urinary tract stones are treated. Thereby it should be possible to compare different groups of patients and different techniques in a global perspective. The investigation in this paper was undertaken to get information on the stone burden in a consecutive series of patients requiring active removal of stones from the kidney or the ureter. A comparison was subsequently made between different clinical estimates of the stone burden and the directly measured stone surface area in order to get a reliable and routinely feasible procedure for describing the stone situation in the individual kidney or ureter. 2. Materials and methods The stone situations were considered in a consecutive series of 599 patients with stones in the kidneys or ureters and who were referred for active stone removal. All together 407 male and 192 female kidneys and ureters radio-opaque stones were considered. For each stone identi®ed on a plain X-ray ®lm (KUB) the maximal length (lmax) and the maximal width perpendicular to l (wmax) were measured with a ruler and expressed in mm. No correction was made for the magni®cation caused by the distance between stone and ®lm. The sum of the largest diameters of all stones (Slmax) was obtained by addition of lmax of each stone. The surface area of the stone (Ameasured) was measured as follows: the circumference of the stones was followed with a Wacom styler connected to a Macintosh computer. The software
Table 1 Principles for subgrouping of non-staghorn renal stones Stone-type
One stone
Two to three stones
Four or more stones
A B C D
5 mm 6±20 mm 21±30 mm 31 mm
± 10 mm 20 mm >20 mm
± ± 10 mm >10 mm
used to analyse the surface area was the NIH Image program (version 1.49). Before use on the Wacom table the contours of the stones were transferred by a pencil to a semitransparent paper. Ameasured was expressed in mm2. Assuming an ellipsoid shape of the stone an arithmetical estimate of the stone surface area (Acalculated) was accordingly derived Acalculated p
lmax wmax 0:785 lmax wmax : 2 2
Subgrouping of the various stone situations into stone-types was made according to the principles in Table 1 [14]. Stone-type E represents complete or partial staghorn stones and stone-types Fp, Fm and Fd proximal, mid and distal ureteral stones irrespective of size. Statistical analysis was carried out with StatisticaTM, StatSoft, Tulsa, OK, USA.
3. Results 3.1. Epidemiological observations The cumulative frequency distribution of the total Ameasured in all 599 stone situations considered is shown in Fig. 1. An Ameasured above 300 mm2 was recorded in only 7% of the renal units. There were Ameasured-values above 200 mm2 in 13% and above 500 mm2 in 4% of the cases. Fig. 1 also shows the distribution of Ameasured-values when the stone situations in kidneys and ureters were considered separately. For stones situated in the kidney only 12%, 10% and 8% had Ameasured above 200 mm2, 300 mm2 and 500 mm2, respectively. These values of Ameasured correspond to projections on the X-ray ®lm of circular stones with diameters of 16.0 mm, 19.5 mm and 25.2 mm. It needs to be emphasised that only 3% of all stones in the kidney had an Ameasured of 1000 mm2 or more. In terms of ureteral stones the overall distribution disclosed that in 97% Ameasured did not exceed 200 mm2. An Ameasured up to 100 mm2 was recorded in 83% and above 300 mm2 in only 1% of the cases. At the time of treatment 41.5% of the ureteral stones were found in the proximal ureter, 15.5% in the middle and 43.0% in the distal ureter. The mean (S.D.) Ameasured for all ureteral stones was 73 mm2 (104) with a range of 10±1708 mm2. The recordings for proximal, mid and distal ureteral stones are given in Table 2.
H.-G. Tiselius, A. Andersson / European Urology 43 (2003) 275±281
277
Table 3 The measured surface area (Ameasured; mm2) of stones in men an women and in the left and right kidney/ureter
Fig. 1. Cumulative frequency distributions of Ameasured in all stone situations (triangle), stones in the kidney (circle) and stones in the ureter (square). Table 2 The measured surface area (Ameasured; mm2) of stones situated in 349 ureters at the time of active stone removal Position of stones
Number
Proximal ureter 145 Mid ureter 54 Distal ureter 150
Mean (mm2)
S.D. (mm2)
Minimum (mm2)
Maximum (mm2)
86 72 53
144 41 41
11 21 10
1626 196 244
Subgroup of patients
Number
Mean (mm2)
S.D. (mm2)
Minimum (mm2)
Maximum (mm2)
Men Women Left system Right system
407 192 343 256
147 228 173 171
364 416 385 381
10 5 5 9
4210 2600 4210 3876
It is of note that in 343 cases (57%) the stones were found on the left side and in 256 (43%) on the right side. There were 407 male and 192 female kidneys and ureters that contained stones. The mean (S.D.) Ameasured for these subgroups are shown in Table 3. The stone burden was apparently similar in the right an left renal units, but Ameasured was greater in women than in men (p < 0:02). Table 4 The measured surface area (Ameasured; mm2) for stones in the kidney of types A±E (see Section 2) Stone-type
Number
Mean (mm2)
S.D. (mm2)
Minimum (mm2)
Maximum (mm2)
A B C D E
6 135 42 33 34
13 70 209 601 1204
6 50 139 477 923
5 10 21 85 237
21 258 721 2245 4209
Fig. 2. Cumulative frequency distributions of Ameasured in kidney stone-types A (open square), B (®lled square), C (®lled triangle) and D (open triangle).
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The Ameasured-values recorded for the stone situations referred to stone-types A±E are summarised in Table 4. The cumulative frequency distributions for Ameasured in kidneys with stone-types A±D are demonstrated in Fig. 2 and as expected there is a considerable overlapping of Ameasured for the latter four stone-types. It is of note that whereas an Ameasured above 300 mm2 was recorded in 76% of the stone situations referred to type D, 14% of type C stones had an Ameasured above that limit and 5% of type C stones Ameasured was above 650 mm2. In 70% of type C stones Ameasured was 150 mm2 or more compared with 8% in stone-type B.
Stone situations comprising only one stone was most commonly encountered and recorded in 496 cases (83%) whereas in 103 cases (17%) more than one stone was identi®ed. 3.2. Comparison between different expressions of the stone burden The correlations between Ameasured and lmax, wmax, Slmax and Acalculated for stone situations comprising one or more than one stone, are summarised in Table 5. Although there was a good correlation between Ameasured and all the other considered variables the
Fig. 3. Relationship between Ameasured and Acalculated in patients with non-staghorn stones in the kidney (above) and stones in the ureter (below).
H.-G. Tiselius, A. Andersson / European Urology 43 (2003) 275±281 Table 5 Coef®cients of correlation between Ameasured, Acalculated, lmax, wmax and Slmax Compared variables
Number lmax vs. Ameasured wmax vs. Ameasured Slmax vs. Ameasured Acalculated vs. Ameasured
Coef®cients of correlation All stone situations
Situations with one stone
Situations with two or more stones
599 0.82 0.87 0.77 0.89
483 0.88 0.88 0.88 0.93
116 0.62 0.83 0.79 0.79
Table 6 Stone-types of 250 stone situations in the kidney according to the old and new system for subgrouping Stone-type
Old method
New method Ameasured
New method Acalculated
A B C D E
6 135 42 33 34
31 155 31 33 ±
48 152 21 29 ±
best correlation was obtained between Ameasured and Acalculated. Fig. 3 shows the relationship between Ameasured and Acalculated. 3.3. Modi®ed system for subgrouping of stone situations In order to get a better correspondence between the stone-types A±E and Ameasured, the following ranges of Ameasured were chosen: A 30 mm2, B 31±300 mm2, C 301±700 mm2, D > 700 mm2. Because of the small number of large stones, the stone-type E was omitted. Inasmuch as stones with staghorn shape (previously designated E) had an Ameasured between 249 mm2 and 4420 mm2 it is obviously better to indicate in the various surface intervals if the stone has a complete or partial staghorn con®guration. Table 6 shows the distribution of stone-types with the old and new systems for subgrouping of the stone burden in kidneys. When Acalculated was used for this classi®cation the result was very similar to that obtained with Ameasured. 4. Discussion It is today well recognised that information on the stone burden is essential for an appropriate planning of stone removal from the urinary tract [1±6]. The need to describe the stone situation in terms of size of the stone(s) became particularly apparent when non-inva-
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sive or low-invasive methods replaced open surgery. Moreover, conclusions on the ef®cacy of different stone-disintegrating devices cannot be made unless data on treated stone burden are given in the report. The most common way to express the size of stones is to give the longest diameter of the largest stone [6±8]. The results presented above show that the longest diameter well correlates with the measured stone surface area at least when only one stone is considered. The explanation for this outcome is that the majority of patients presented with only one stone in the kidney or in the ureter. This observation is thus in good agreement with the SWL-results that recently were presented by Buchholz and co-workers [17]. As expected, however, the correlation was less good when more than one stone constituted the stone situation. Previous reports on this matter have clearly emphasised the usefulness of measuring the surface area on a plain X-ray ®lm. In this way it is possible to make a much more precise description of the stone burden and the clinical problem. In addition it has been shown that knowledge of the stone burden enables us to derive an approximate estimate of the stone volume [13]. With spiral CT-examinations and advanced principles for image analysis it is certainly possible to get an exact measure of the stone volume. This, however, cannot be achieved without considerable efforts and de®nitely not as a standard clinical routine procedure. Neither is it possible to easily measure the surface area on a plain X-ray ®lm without special tools. It is therefore reassuring that method for arithmetically assessing the stone surface area might be used as a reasonable substitute. Moreover dif®culties have been reported in the accuracy of measuring the vertical diameters of stones on CT-images [15]. It needs to be pointed out in this regard that no correction was made for the ampli®cation of the stone that occurs as a result of distance between stone and the ®lm. The correlation between Ameasured and Acalculated was acceptable and although deviations occur, Acalculated can certainly be used for most clinical situations and as a rough description of the treated stone situations when different methods should be compared. It stands to reason, however, that the method is not applicable when stones with a pronounced irregular shape are encountered. Therefore, when the stone morphology deviates very much from the ellipsoid geometry, other methods for assessing the stone surface area are necessary. This is particularly the case for complete and partial staghorn stones. When Ameasured and Acalculated were used to classify the various renal stone situations in the stone-type categories A±D, there was a very good agreement
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between the two variables. The stone-type system was introduced to enable a clinical comparison between patients with stone burdens of different size. This system which is very approximate gives a considerable overlapping between the different types and it is an improvement if this subgrouping instead is based on the stone surface area. In this respect Acalculated provides a both acceptable and easily derived estimate. Thereby we found it reasonable to omit the previously used stone-type E. All staghorn stones are better referred to those subgroups that correspond to their total surface area. The surface area for staghorn stones always has to be assessed with more direct methods than the arithmetical calculation. Information on the distribution of stone situations in the population is dif®cult to extract from the literature. The result is presented in different ways usually with various size ranges of the largest stone diameter. Most of these reports are given in association with stone removing procedures, usually shock wave lithotripsy. In most of these studies the interpretation is hampered by the fact that the group of patients is not representative for a given population. Patients from the same geographical area might either have been treated in different institutions or referred to alternative forms of stone removal. Moreover there might in some departments have been an over-representation of dif®cult cases. Inasmuch as the 599 patients presented in this report represented a consecutive series of patients from a population in which all patients requiring active stone removal were referred to the same centre the observations in this study are of great epidemiological interest.
The results show that only 7% of all stone situations in the kidney had a surface area exceeding 300 mm2. According to the EAU guidelines for stone treatment [6], this is thus the fraction of patients in whom methods alternative to shock wave lithotripsy should be considered. This means that in a population of 1 million, with an annual incidence of 2000 and a need for stone removing procedures in 500, not more than approximately 35 patients should be considered for other stone removing procedures than shock wave lithotripsy. This is thus the situation for stones in which the surface area correspond to a largest diameter of approximately 20 mm. If this limit is extended to 700 mm2 or 30 mm2, the corresponding number of patients will be 15 (3%). It should be noted that only those patients were considered who were candidates for active stone removal. The stones were not measured in those patients who spontaneously passed their stones. This group comprises around 75% of all patients that annually are af¯icted by stone disease. The results presented in this paper give interesting epidemiological information. The results moreover show that an arithmetic calculation of the stone surface area using the length and width of the stones on the plain X-ray ®lm for most stone situations is a useful measure of the stone surface area. Only the surface area of irregular and staghorn stones needs to be measured differently. Finally an estimate of the stone surface area is obviously a more appropriate basis for subgrouping of patients with stones than the diameters and numbers of stones.
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