Stones that fill the greater part of the collecting system are called staghorn because they resemble the antlers of a male deer. These stones usually fill the pelvis of the kidney, the infundibula, and most of the calices. Partial staghorns fill a lesser portion of the collecting system, but lack of agreement as how to define these stones prevents a more exact definition. The caliceal, infundibular, and pelvic portions of the stone usually are all connected, but the stone occasionally has been identified before these separate portions have had time to coalesce.

Urologists assume that most staghorn stones are composed of struvite (magnesium ammonium phosphate). This crystal forms only in the presence of bacteria that produce the enzyme urease; nevertheless, other stones can assume a staghorn configuration: cystine, calcium oxalate monohydrate, and uric acid all may grow to the point where they fill the collecting system. With experience, it often is possible to identify the composition of a staghorn stone on plain film, particularly when armed with the patient's clinical history.

Most staghorns occur in women because urinary tract infections are more common in women. If the patient's infections have been caused by Escherichia coli only, for example, an organism that does not elaborate the enzyme urease, then one should be suspicious that the stone is not formed of struvite. Struvite forms in the presence of Proteus species and Klebsiella species, both of which produce urease and make stones that tend to be lightly calcified and relatively less dense on plain radiograph. Pseudomonas, some species of Streptococcus, and Staphylococcus are weakly positive for urease, and stones formed in the presence of these bacteria are usually relatively dense on radiograph, reflecting their greater percentage of calcium. Cystine staghorns have the typical, uniform, ground-glass appearance on radiograph, and their dendritic extensions are rounded rather than sharp-pointed. These stones are more likely to be associated with multiple, round satellite stones. One's level of suspicion should be raised by a urine positive for cystine. Calcium oxalate monohydrate stones are very dense, rarely fill all the calyces, and do not require urea-splitting bacteria to form.

Regrettably, there is no widely accepted system for measuring the size of a staghorn. Such a system should recognize that staghorns vary widely in the amount of stone they contain and that the collecting systems also vary. A system should facilitate comparing results of treatment. Ackermann et al,<sup>1 Ackermann D., Griffith D.P., Dunthorn M. , e al. Calculation of stone volume and urinary stone staging with computer assistance J Endourol 1989 ;  3 : 355  [cross-ref]

Cliccare qui per andare alla sezione Riferimenti</sup> Di Silverio et al,<sup>6 DiSilverio F., Gallucci M., Alpi G. Staghorn calculi of the kidney: Classification and therapy Br J Urol 1990 ;  65 : 449-452

Cliccare qui per andare alla sezione Riferimenti</sup> Griffith and Valiquette,<sup>10 Griffith D.P., Valiquette L. PICA/burden: A staging system for upper tract urinary stones J Urol 1987 ;  138 : 253-257

Cliccare qui per andare alla sezione Riferimenti</sup> and Rocco et al<sup>18 Rocco F., Mandressi A., Larcher P. Surgical classification of renal calculi Eur Urol 1984 ;  10 : 121

Cliccare qui per andare alla sezione Riferimenti</sup> all devised systems to address these points, but none has been widely accepted. Lam et al<sup>13 Lam H.S., Lingeman J.E., Barron M. , e al. Staghorn calculi: Analysis of treatment results between initial percutaneous nephrostolithotomy and extracorporeal shock wave lithotripsy monotherapy with reference to surface area J Urol 1992 ;  147 : 1219-1225

Cliccare qui per andare alla sezione Riferimenti</sup> used a digitizer attached to a desktop computer to measure the outline of a stone. The area of the stone then could be computed, which it turns out correlates reasonably well with the volume of the stone. If this technique meets with wide acceptance, it should facilitate evaluation of treatment considerably.