The SFRS comprises three separate components: the Syphon Ureteric Access Sheath (SAUS), a Dual Action Pump (DAP), and an Agitator incorporated into one system (see Fig. 1).

This novel ureteric access sheath (UAS), previously described by Lazarus [3] and Yekani [19] incorporates a syphon mechanism at the outlet of an otherwise traditional UAS, improves irrigant outflow, and reduces IRP compared to a traditional UAS [19].

This is a low-volume, user-controlled (foot or hand) pumping unit that enables the urologist to deliver fluid boluses of no more than 2mL, into the upper urinary tract as the unit is compressed, while the same volume of fluid is drawn out. The greater the compression, the larger the bolus volume; the quicker the compression, the faster the bolus is jetted into the upper urinary tract. The delivery of a bolus during DAP compression momentarily renders the small stone fragments waterborne. These fragments are expelled from the collecting system simultaneously by aspiration of the same volume of bolus from the kidney through the descending arm of the SAUS. Bolus delivery and bolus aspiration are synchronised in time to prevent changes in the IRP. The DAP is released to prime the pump to deliver the next bolus. Repetitive, careful compression and release can be performed in quick succession to quickly clear away debris. Flushed-out fragments are collected in a sieve inside the syphon box, allowing easy retrieval.

This thin (6 Fr) steerable catheter enables urologists to direct fluid boluses in the direction of stone fragments in the kidney. The Agitator is steered in the same way as a standard flexible ureterorenoscope by moving the thumb lever up and down and rotating the Agitator. A radio-opaque marker band at the tip of the Agitator enables the surgeon to navigate under fluoroscopic guidance. At the entrance to each calyx, a few boluses are delivered to make any stone fragments in the calyx waterborne to allow them to be aspirated out, with the action of the DAP.

The syphon ureteric access sheath (upper left), Dual action pump (upper right) and 6 French agitator (bottom left and bottom right)

To evaluate safety, an experimental model was established according to the layout shown in Fig. 2. Notably, a fully validated phantom kidney, three-dimensionally printed from a high-resolution MRI scan (Max Planck Institute for Intelligent Systems, Stuttgart, Germany), was used to assess the safety of the device [4]. This kidney has demonstrated the ability to accurately mimic the anatomy and tensile/compliance properties of the human kidney, and would therefore provide an accurate reflection of the IRP that one expected in vivo. These IRP measurements were obtained using a fibre optic pressure sensor calibrated to atmospheric pressure and inserted into the renal pelvis through a micro-nephrostomy. To avoid leakage from the micro-nephrostomy, a Tuohy-Borst adapter was employed, and the entry point of the micro-nephrostomy was further sealed with silicon. A FISO FOP-M200 model with a diameter of 200 μm, equivalent to 0.008 inch (FISO Technologies Inc., Quebec City, Canada) was used. The FISO pressure metre has a sampling rate of 250 Hz.

A conventional cysto-irrigation set was connected to the DAP irrigation arm inlet positioned at a column height of 110 cm (81 mmHg), while the DAP suctioning arm outlet was suspended 40 cm below the phantom kidney model (see Fig. 2). The air was removed from the phantom kidney, an f-URS (Flex X2, Karl Storz, Tuttlingen, Germany) was inserted into the kidney, and gravitational irrigation commenced.

For the experiment, with the SFRS connected, the IRP was measured constantly for 30 s, from which the mean IRP was calculated. Second, the irrigant flow volume was measured for 1 min. Lastly, five DAP boluses were administered 1 s apart, and the maximum IRP was recorded. To establish a control, the above procedure was repeated without the use of SFRS employing a conventional UAS, where the DAP was only capable of delivering the irrigant.

A separate translucent kidney model (Max Planck Institute for Intelligent Systems, Stuttgart, Germany), made from silicone and moulded based on a high-resolution MRI image, was used to determine the efficacy of the SFRS. The selection of this model was based on its translucency, which enabled clear visualisation of the stone fragment removal process.

Small sandstone fragments, which were utilised to stimulate kidney stone fragments, were filtered through a sieve to ensure that the size of all fragments ranged from 0.8 to 1.3 mm. Dried, weighted (0.30 g) stone fragments were measured using a calibrated Mettler analytical scale and inserted into the collecting system of the clear kidney model using a syringe with fluid and an open-ended catheter. During the experiment, the SFRS was connected, and the Agitator was inserted through the SUAS into the kidney model. Irrigation flow was then started. The DAP was activated, and the Agitator was steered under fluoroscopic guidance to extract as many stone fragments as possible from the collecting system. Short breaks were periodically taken to inspect the collecting system. Following a period of 180 s of DAP, all actions were halted. The small expelled stone fragments were collected, dried, and weighed. The weight of the recovered fragments was compared to the initial 0.30 g introduced before intervention and expressed as a percentage. For the control, the same procedure was repeated with a conventional UAS and manual pump directing fluid boluses to the stone fragments. Similarly, the weight of the collected fragments in the control was also compared to the initial 0.30 g introduced before intervention and expressed as a percentage. The investigator was blinded during the procedure and was unaware of whether the SFRS was connected.

The study was approved by the Surgical Departmental Research Committee of the University of Cape Town (reference: 2023/127). Statistical analysis was performed using the GraphPad Prism version 5.03 software. Paired t tests were used. Statistical significance was set at p < 0.05.

The small fragment removal system (SFRS) schematic. It shows the sub-devices and how irrigant flows from a suspended bag through a flexible ureterorenoscope to the upper urinary tract (phantom kidney). The fluid is then removed via the Syphon UAS. Irrigation and removal of fluid is augmented if the Dual Action Pump (DAP) is activated by the foot pump