dilator which minimises damage to the renal pelvis. The external sheath also provides tamponade and tissue protection during what is usually a long and difficult procedure.

References Coleman, C. C. (1986). Percutaneous nephrolithotomy-dilatation techniques. In Atlas of Endourology, ed. Amplatz, K. and Lange, P. H. Chapter 15, pp. 131-148. Chicago: Year Book Medical Publishers. Mazzeo, V. P., Pollack, H. M. and Banner, M. P. (1982). Techniques for percutaneous dilatation of nephrostomy tracts. Radiology, 144, 175-176. Ware, S. (1982). Dilatation of percutaneous nephrostomy tracts. Urology, 19,311.



Requests for reprints to: W. J. Lynch, Department of Urology, The London Hospital, Whitechapel, London E l 1BB.

T I S S ~ B ~ ~ ~ ~

Fig. 2 Outer dilator holds inner dilator straight. Force is then transmitted to the tip of the thin dilator, assisting in penetration of the tissue planes.

Proximity to the kidney makes vigorous and uncontrolled methods of dividing the tough tissue more than usuallv hazardous. Fascia1 lumbotomes are an excellent means of dividing tough fascia but they are not intended for use in the perinephric area, where there is a significant risk of damage to the kidnev. Telescoping metal dilators are often used for percutaneous tract dilatation and they also cope well with dense perinephric fibrosis. One of their disadvantages is the rigidity of the tip dilator, which is associated with a higher incidence of damage to the renal pelvis, and this normal risk is further increased by the force necessary to achieve dilatation. Also, with metal dilators there is no tamponading effect of a permanent outer sheath during the pyelolithotomy and/or pyelolithotripsy. These procedures in such patients often involve multiple withdrawals and insertions of the nephroscope, as they tend to be complex cases and a onestep manoeuvre is not possible. When a thin dilator (6.5 or 8 F) encounters marked resistance from scar tissue, increased force usually results in bowing of the shaft of the dilator. By dilating down to the level of resistance rather than attempting to penetrate the scar tissue it is possible to introduce a large dilator into the more superficial part of the tract. This large bore dilator then holds the thin dilator straight while it penetrates the resistant tissue planes. Thus one achieves the rigidity necessary to perform dilatation through dense perirenal fibrosis, while maintaining the advantages of a system which uses a flexible

Pre-vesical Ureteric Cakuli: Modified Prone Position for Extracorporeal Shock Wave Lithotripsy using the Siemens Lithostar D. DALELA, R. AHLAWAT and V. K. MlSH RA, Department of Urology, Sanjay Gandhi Post Graduate Institute of Medical Sciences, Lucknow, India

Extracorporeal shock wave lithotripsy (ESWL) is an accepted treatment for renal and upper ureteric calculi and is being increasingly used to treat lower ureteric calculi, with ureteroscopy now limited to difficult or failed cases (Dretler, 1990). The main problem in treating pre-vesical calculi with ESWL lies in finding a suitable shock wave path, since the calculi are surrounded by bone and are usually treated through the greater sciatic foramen with the patient lying either supine (Holden and Rao, 1989) or in one of the other modified positions (Becht et al., 1988). When using the blast path in the supine position with a Siemens Lithostar we found it difficult to separate bone from the pathway of shock waves, resulting in pain and poor fragmentation. We considered using the pelvic brim as the entry pathway in the prone position, but calculi tended to slip behind the pubic symphysis, making fluoroscopic focusing and treatment difficult. We now use the modified prone position as described below.



Fig. 3 Modified prone position. Pelvic brim rotation has brought the calculus back into view.

physis. The calculus can then be easily focused and treated.

Comment The position described assists biplanar localisation and the fragmentation of pre-vesical calculi with higher shock wave energy and with little need of analgesics. If the patient experiences pain, this can be alleviated by the partial interposition of foam sheet between the shock wave head and the superior pubic ramus. The full bladder improves the fluoroscopic image and also provides a good path for the transmission of shock waves. Careful selection of patients is necessary because not all patients, particularly those with arthritis of the spine, can lie in the modified position for the duration of treatment. Fig. 2 True prone position. Note that calculus is behind pubic symphysis.

Technique The patient lies prone on the Lithostar table with a full bladder. A pillow is placed beneath the thighs to raise the knees by 15to 20 cm and so hyperextend the hip joints. This increases the lumbar lordosis and opens up the face of the pelvic brim to the shock wave head (Figs 1-3), bringing the calculus back into the view from behind the pubic sym-

References Becht, E., Moll, V., Neisius, D. et al. (1988). Treatment of prevesical ureteral calculi by extracorporeal shockwave lithotripsy. J. Urol., 139,916918. Dretler, S. (1990). Ureteral stone disease--options for management. Urol. Clin. North Am., 17, 217-230. Holden, D. and Rao, P. N. (1989). Ureteral stones: the results of primary in-situ extracorporeal shock wave lithotripsy. J . Urol., 142,31-39. Requests for reprints to: R. Ahlawat, Department of Urology, Sanjay Gandhi Post Graduate Institute of Medical Sciences, Post Box 375, Lucknow 226 001, India.

Pre-vesical ureteric calculi: modified prone position for extracorporeal shock wave lithotripsy using the Siemens Lithostar.

456 BRITISH JOURNAL OF UROLOGY SKIN HOLDS INNER DILATOR RIGID dilator which minimises damage to the renal pelvis. The external sheath also provide...
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