Background and purpose: Numerous factors influence the treatment result and efforts for stone removal. To summarize the important factors, our aim was to formulate a general mathematical expression of the stone-removal procedure.
Materials and Methods: A mathematical expression (stone treatment index; STI) was developed for patients who became stone free (SF):
STISF
= NumSF
· mean(√SA · HI) · meanAgeR
· meanBMIR
· (1 + meanNumANA
) / NumSESSIONS
+ NumAUX
+ NumANE
+ NumCOMP
The variables included were the number of stone-free patients (NumSF), mean stone burden expressed as the product of the square root of the stone surface area (SA1/2) and the hardness index (HI), the mean age ratio (AgeR = age/50), the mean body mass index (BMI) ratio (BMIR = BMI/25), the mean number of patients with anatomic abnormalities (NumANA), the total number of treatment sessions (NumSESSIONS), the number of auxiliary procedures (NumAUX), the number of procedures requiring general or regional anesthesia (NumANE), and the number of complications (NumCOMP). A similar index was calculated for patients who were either stone free or had residual fragments ≤4 mm (STIDIS). For conclusions on efficacy, these indices were compared with optimal and total values of STI. The STI was calculated for 450 patients with renal and 374 patients with ureteral stones treated with shockwave lithotripsy as the primary procedure. In patients with a known BMI, this factor was used; otherwise, the BMIR was set to 1.
Results: When STI was compared with the efficiency quotient (EQ) for stones in different size intervals, STI was less sensitive to variations in stone burden. The quotients STISF/STIOPTIMAL corresponded roughly to EQ but might be more informative because of the inclusion of factors for anesthesia and complications.
Conclusion: With or without related estimates, the STI might be useful for summarizing stone-removal procedures in groups or individual patients with urinary-tract stones. The STI might be helpful because it encompasses the factors of importance for conclusions about the treatment procedure, such as the hardness of the stone, the age and body size of the patient, and the presence of anatomic abnormalities likely to influence the result. The STI might be used advantageously for comparison of different equipment and treatment strategies.
