The use of disposable digital flexible ureteroscopes is becoming widespread in response to the fragility issue seen with flexible ureteroscopes. Their operability has been shown to be comparable to that of reusable ureteroscopes.1–4 However, due to differences in medical systems among countries, there is no consensus concerning their cost-effectiveness.5–7 In countries that have adopted a national health insurance system such as Japan, when using a disposable ureteroscope, it is necessary to calculate the scope’s market price and the number of cases that are likely to require the scope at a hospital so that a deficit does not develop. A hybrid method is therefore applied, wherein a disposable ureteroscope is used for cases in which the scope might be easily broken, while a conventional reusable ureteroscope is used in other cases.6
LithoVue® was launched in 2017 as the first disposable digital flexible ureteroscope in Japan. Since then, we have been using LithoVue® scopes in selected cases where the ureteroscope might be easily broken, including cases with large renal stones of ≥ 15 mm. Therefore, in this study, we divided patients with large renal stones of 15–40 mm in size who underwent RIRS into two groups: the reusable-scope group and the disposable-scope group; we then compared the surgical outcomes.
When the variables were compared between the two groups, the stone diameter and stone volume of the disposable-scope group were significantly larger in comparison to the reusable-scope group. The stone-free rates of the two groups were equivalent, although the mean operative time of the disposable-scope group was significantly longer as the stone size grew. After matching, the disposable-scope group was found to have the same stone-free rate as the reusable-scope group. Considering these results, a disposable ureteroscope performed comparably, not superior, to a current reusable ureteroscope. However, using a disposable ureteroscope, we were able to successfully treat larger renal stones than we previously treated using with a reusable ureteroscope. We think the advantages of a disposable ureteroscope influenced the results. The surgeons were freed from the anxieties about breaking the scope. Furthermore, surgeons were able to perform longer operations with less fatigue because of the light weight of the disposable ureteroscope.8 These advantages have increased the size of renal stones for which RIRS is indicated.
In this study, the mean operative time of the disposable-scope group tended to be longer than that of the reusable-scope group even after matching. This trend was also shown in a recently reported meta-analysis comparing disposable and reusable flexible ureteroscopes for RIRS.9 The benchtop and in-vivo studies have demonstrated that LithoVue® is not inferior to reusable ureteroscopes when comparing image quality and manoeuvrability.10,11 However, in clinical practice, the different image quality associated with LithoVue® may be responsible for this longer operative time, although the time is closely linked with the experience of the surgeon.
Conventionally, percutaneous nephrolithotomy is the first treatment option for renal stones of > 20 mm in size according to the guidelines.12 However, in clinical practice, many reports have suggested that ureteroscopy can be used to treat stones of up to 25 or 30 mm in size, including in two-stage operations.13,14 Indeed, in this study, the cut-off value of stone diameter for predicting a stone-free status after one RIRS session was 26.0 mm in the disposable-scope group, although we could not determine a significant cut-off value in the reusable-scope group.
Generally, the stone volume can be calculated by the conversion formula (π/6 × length × width × height). However, when the stone size becomes quite large, the stone volume can no longer be calculated accurately by this formula.15,16 Some authors hold the opinion that the stone volume -rather than the stone diameter- should be used as a selection criterion for the treatment of large renal stones.17 We can now measure the stone volume accurately using a volume analyzer, even if the stone has a complicated shape. Indeed, in this study, we were able to determine more accurate cut-off values with the stone volume than with the stone diameter. Moreover, the cut-off value was 2.9 cc in the disposable-scope group, which was quite precise (AUC 0.94). In the future, the selection of treatments for large stones should be based on the stone volume rather than the stone diameter.
The present study was associated with several limitations. Firstly, this was a retrospective study, although we used a propensity-score matching to derive a result as close to a prospective study as possible. Secondly, several disposable ureteroscopes are currently available on the market. Although we do not have a case series using other disposable ureteroscopes, the LithoVue® is considered to be show excellent operability and durability. We have never experienced failure of a LithoVue® ureteroscope during surgery. Thirdly, there were differences in the experience of the surgeons, although all surgeries were performed under the supervision of one experienced surgeon. Conversely, this study showed the general advantages of using a disposable ureteroscope, irrespective of the surgeon’s experience.