Upper urinary tract calculi is one of the most common diseases worldwide, which has caused serious harm to the safety of life and property of people all over the world. In the past 30 years, the incidence of upper urinary tract calculi has been increasing continuously. The overall incidence of upper urinary tract stones in China is 6.4%, among which the incidence of upper urinary tract stones in men is 6.5% and in women is 5.1%, with a male to female ratio being 1.27. The harm of upper urinary tract calculi to the human body is huge. Therefore, it is very important to understand the risk factors related to the adverse outcome of postoperative residual calculi in upper urinary tract calculi and to select appropriate clinical treatment and prevention methods, which can reduce the physical burden and the medical expenses of patients.
In previous studies, we have concluded that postoperative residual stones may have the following hazards: (1) Postoperative residual stones may cause and perpetuate the urinary tract infection; (2) New stones may form with residual stones as the core; (3) The movement of residual stones after surgery may cause obstruction and trigger new symptoms. Although postoperative residual stones have many hidden dangers, it is still difficult to completely remove all stones due to the limitations of surgical equipment, medical technology, and surgeon experience in actual clinical work. In the past, in conditions of normal upper urinary tract anatomy, no urinary tract infection or other symptoms, stones with postoperative diameter less than 4mm were called meaningless residual stones. Moreover, it was considered that these stones had a high pass rate. However, at present, the significance of these meaningless residual stones is becoming controversial. In recent studies, the probability of adverse outcomes for what were previously considered nonsignificant residual stones has also increased. In the 2020 edition of the EAU Guidelines on Urolithiasis, postoperative residual calculi greater than 2mm increase the risk of recurrence of calculi-related symptoms and require appropriate clinical management. Stones smaller than 2mm also require close clinical observation. Therefore, in the future treatment of upper urinary tract stones, removing residual stone fragments as far as possible may become a new issue to urologists. In our study, we found that there was a strong correlation between the size of postoperative residual stones and the postoperative adverse outcome. The larger the size of postoperative residual stones are, the greater the possibility of adverse outcomes is. The adverse outcome rate of upper urinary tract calculi increases nearly 1.5 times with the increase by 1mm in the diameter of postoperative residual stones, which indicates the necessity of clinical intervention for postoperative residual stones.
Although there is still controversy on the treatment of postoperative residual stones, we believed it is important for urologist to help every stone patient remove the stones completely in the future. Many researchers have done a lot of researches and developed novel techniques on the treatment of postoperative residual stones. Friedlander et al developed a novel device that captures stones in a sealed polyethylene bag in vivo to prevent dispersion of stone fragments during PCNL and found in their study that the use of this device significantly reduced the median time for stone fragmentation . Tracy et al developed a magnetic tool to improve the efficiency of lithotripsy under a ureteroscope. Compared with the traditional Ni-Ti alloy stone extraction basket, the time for magnetic stone extraction tools to extract residual stone fragments is reduced by 53%. Although the visual field of magnetic stone extraction tools is worse than that of traditional stone extraction tools, and it is more difficult to deal with residual stones larger than 3mm, this design is still under further development and improvement[16, 17]. Heinet et al proposed a new type of biocompatible adhesive. In the experiment of pig kidneys, the time of chasing stones in the experimental group was significantly less than that in the control group, and no obvious adverse reactions were found in the experiment. With the emergence, innovation and use of new technologies, the treatment of residual stones after upper urinary tract stone surgery will become much easier in the future of urology.
In the study, we found that bacterial or fungal infection in urine is an independent risk factor for the adverse outcome of residual stones after PCNL. Patients with a positive urine culture were nearly twice as likely to have an adverse outcome of postoperative residual stones as those who were negative. Among the bacterial species cultured, Escherichia coli was the most common, with 27 (55.1%) cases in the adverse outcome group and 10 (58.8%) cases in the control group. Previous studies have shown that in Escherichia coli-infected kidneys, the expression of osteopontin (OPN) is elevated in renal tubular epithelial cells, which leads to tubular epithelial cell damage and induces a renal inflammatory response that activates multiple immune response signaling pathways, thereby promoting stone formation.
Component analysis of calculi in some patients revealed that infectious stones accounted for 28.9% of all stones, with a 60.9% prevalence of infectious stones in women, similar to previous epidemiological studies. In our study, we also cultured the urine of all patients, in which a small amount of Klebsiella and Proteus were cultured. But due to the limited number of positive cases of culture, it is hard to prove whether a special bacterial infection increases the probability of an adverse outcome in patients with residual stones after the operation. The low detection rate of these urease-producing bacteria may be explained that they usually produce biofilms adhering to the urothelial tissue, thus reducing the bacterial count in the urine and making it more difficult for clinical testing. Whether there is a direct relationship between the formation of infectious stones and urease-producing bacteria may require further experiments and data analysis of stone culture, molecular analysis, or other detection methods.
In previous epidemiological studies, the incidence of upper urinary tract stones was generally higher in men than in female patients. However, studies in recent years have found that the incidence of upper urinary tract stones in female patients is also increasing with the increase in the incidence of upper urinary tract stones worldwide. It has been reported that for women between the ages of 15 and 44, their lifetime risk of developing urinary calculi increased by 45% or more . This study found that the incidence of the adverse outcome of postoperative residual stones in female patients was significantly higher than that in male patients. In the adverse outcome group, the ratio of female to male patients reached 1:1, while in the control group, the proportion of female to male patients was just 1:2. It seemed that female patients with upper urinary tract calculi have a higher probability of adverse outcome after operation. However, after binary logistics analysis, it can’t be considered that gender is an independent risk factor for adverse outcome of postoperative residual stones in patients with upper urinary tract calculi. Therefore, whether female is a risk factor associated with the adverse outcome of residual stones after PCNL deserved further study.