Specifications of 11 currently used lithotripters were used from published references (Ref 1).

The distribution of pressure of a lithotripter is centered at the focal point F2 and includes all points whose pressure is between 100% ( f2) and 50% of the Peak Power( 6 dB).

The shape of this focal volume is approximately an ellipsoid (a "cigar" or "watermelon" shape) (Ref 2).
This ellipsoid volume is obtained by rotating the focal ellipse around the long axis.

The geometric specifications of the focal area are the Long Dimension (LD) and Short Dimension (SD) of the ellipse and ellipsoid. The (a) Long radius and (b) Short Radius equal half of the previous values respectively.

1) Focal Area Cross Section (FACS)

The easiest way of visualizing how much of the stone is subjected to pressure is to look at the Cross Section of the ellipsoid at F2 (Like "cutting" the ellipsoid/cigar at F2 and looking at the circle that originated).
We can calculate the Cross Section area using the formula of the circle area

b = Focal Short Radius =SD/2
SD=Focal Short Dimension (diameter)

3) Ellipse Focal Area (EFA)

Using the ellipse Long Radius a , and the Short Radius b, we can calculate the full Ellipse area using the formula

4) Truncated Ellipse Focal Area (TEFA)

Using the Long Radius a, and the Short Radius b, we can calculate the Truncated Ellipse area using the formula

5) Ellipsoid Focal Volume (EFV)

6) Truncated Ellipsoid Focal Volume (TEFV)

RESULTS

Calculations and graphs were made using the Excel (Microsoft) Program 1)

1) Focal Area Cross Section

Focal Cross Sections at F2
Circle #	Manufacturer	Model	Diameter Short DimensionSD (mm)	Cross Section Area(mm 2)
1	1 Dornier	Doli S	5	20
2	Siemens	Lithostar	5	20
3	Edap	Praktis	5	20
4	Storz	Modulith	6	28
5	Siemens	Modularis	6	28
6	Dornier	Compact Delta	7.7	47
7	Healthronics	Lithotron	8	50
8	Medispec	Econolith	13	133
9	Direx	Tripter Compact	13.5	143
10	Medstone	STS-T	15	177
11	Dornier	HM-3	15	177
	Average Large Focal Areas		157
	Standard Deviation		23
	Average Small Focal Areas		30
	Standard Deviation		13
Small Focal Areas			Large Focal Areas
Table and Graph # 1

Truncation of Focal Ellipses

Table and Graph # 2

3) Truncated Ellipse Focal Area

Table and Graph # 3

4) Truncated Ellipsoid Focal Volume

Table and Graph # 4

Analyzing the results shown in Tables #1 through #4, we may distinguish two categories of Lithotripters:


a) Large Focus: 4 lithotripters are in this category:
Dornier HM-3, Medstone STS-T, Direx Tripter Compact, and Medispec Econolith.

b) Small Focus: 7 Lithotripters are in this category:
Storz Modulith, Dornier Doli S, Dornier Compact Delta, Siemens Lithostar, Siemens Modularis (All electromagnetic lithotripters), Edap Praktis, and the Healthronics Lithotron (Spark Gap).

• The 2 categories of Lithotripters are clearly differentiated, the ration of their Cross Sections, Areas and Volumes are between 2.35 and 5.3.
  
• The Large Focus group is more homogeneous (Standard Deviation 8 to 15 %) , whereas the Small Focus is less (Standard Deviation 21% to 44 %). This is due to the fact that the Dornier Delta and Healthronics Lithotron have relatively bigger dimensions than the rest of the group, but still far form the Large Focus group.
  
• ALL Large Focus Lithotripters use the Spark Gap technology.
  
  ALL Electromagnetic units fall into the Small Focus category.
  
• Two Spark Gap units are also in the Small Focus category: Edap Praktis and Healthronics Lithotron.

The Edap Praktis, although basically a Spark Device, uses a variation of what is called the Electroconductive Technology.

The purpose of this technology is to reduce the pressure fluctuation between shocks. In order to achieve this, the system uses a special electrode in a highly conductive liquid, with a very small gap and as a result, the focal volume is much smaller than conventional Spark Gap devices.

It can be seen on Graph # 1, that the Large Focus Lithotripters will " cover" most of the stone areas at F2 ( diameter 13 to 15 mm) whereas the Small Focus ones will cover only a fraction of the typical stone.

This may explain why the electromagnetic devices typically require significantly more shocks to adequately fragment kidney stones and also may result in higher retreatment rates.

Recently, concerns have been raised ( Ref 5) regarding the fact that some new Electromagnetic Lithotripters that have very small focal areas and extremely high peak positive pressures are reporting higher clinically significant hematoma rates of 3 to 12% (Ref 6,7 and 8). A trend that is worrisome.

It is becoming clear that the electromagnetic devices with very long and thin focal area/volumes are not suited to fragment stones.

The Truncated Areas and Volumes are intended to advance the discussion relative to the effectiveness of various lithotripters.

2) Page 9

…." In addition, the newer generation lithotriptors that have very small focal areas and extremely high peak positive pressures are reporting higher clinically significant hematoma rates of 3 to 12% (Kohrmann et al, 1995; Stefan et al, 1998; Piper et al, 2001; Ueda et al, 1993), a trend that is worrisome."

3) Page 10

…." In addition, Roessler et al (1996) determined the size of lesion induced by an electromagnetic vs. electrohydraulic lithotriptor and found a much larger lesion with the electrohydraulic machines. However, the electromagnetic lithotriptor produced complete cellular destruction at F2, which may explain a higher rate of subcapsular hemorrhage for electromagnetic lithotriptors."


4) Page 69

…." Newer generation machines have not improved outcomes for patients. The introduction of new lithotriptors that generate extremely high pressures and tight focal zones does not appear to be much of an improvement, as the need for re-treatment and the incidence of adverse effects with these devices appears to be higher with older machines."

Introduction: obesity is considered as an important risk factor for illnesses such as the arterial hypertension, diabetes, coronary and metabolic disorders, etc., increasing considerably the surgical risk. By means of ESWL we treated an interesting number of patients with the purpose to avoid the prolonged and after surgery complications, due patients volume.
The patients had been selected by use of the Index of Mass Corporal (IMC) of Quetelet:

		25 < IMC < 27	27 < IMC < 30	IMC > 30
		medium overweight	important overweight	obese

Material and methods: From April 2000 to August 2001 we have treated 100 obese patients with kidney and urethral stones, using a Nova Ultima Extra Corporal Lithotripter from Direx Medical Systems.
A neuroleptanalgesic had been done to a 92% of the patients and narconeuroleptanalgesic to an 8% of them. The 85% stayed 24 hours in the hospital and the 15% were ambulatories. Treated patients: 63 men with more than 90 kg weight (90-141) and 37 women with more than 80 kg weight (80-118). Age average: 42 years old (20-72). Treatment on: 77 kidney stones, 12 pelvic stones, and 7 urethral stones. An 80% had been localized by means of an ultrasound scan (more than 50% in lateral posture) and 20% by means of a fluoroscopic. The stone size average was 12 mm. In a 62% of cases the localization was easy and in a 38% it was difficult, principally the urethral and pelvic stones. All the patients received 3000 shoots at KV 21-22, in a high energy with a time average of 35 minutes every session.

Lateral posture Ultrasound scanner Localization	Dorsal posture Fluoroscopy Localization	Lateral posture Ultrasound scanner Localization	Dorsal posture Fluoroscopy Localization	Lateral posture Ultrasound scan Localization

The lateral posture permits an excellent ultrasound scans image avoiding abdominal and suprarenal folds and also an improvement to the skin contact with the membrane.

Results: The results in a three months period had been:

The best results obtained were with the pelvic and lumbar urethral stones (82%)

We carried out a supplementary session on 24 patients, two sessions on 6 patients and three sessions on 4 patients (voluminous stones)

Lower calix (before & after lithotripsy)

Lumbar urethral                 UIV               After lithotripsy

Lower calix (before & after lithotripsy)

Discussion: For the kidney and urethral stones treatment on obese patients the lithotripter features are very important and the operator experience too. The knowledge and application of the localization systems permits to the specialist to obtain a better percentage of stone free. These techniques are important for these patients due to the posture on the table, which is fundamental to obtain a good result (50% in lateral posture with ultrasound scan).

Conclusion: The indication of using ESWL on obese patients is a good choice but we always have to be respectful with the criteria of selection. The percentage of stone free in a three-month period has been satisfactory and the complications are similar to the ones published by other authors.

XIV National Conference of Lithiasis and Urinary Endoscopy,
January 28-29-30, 2002, Sig?enza (Guadalajara, SPAIN)