A novel radiographic grid aiming device for percutaneous renal puncture

This in-vitro study aims to assess the ecacy of a novel radiographic grid aiming device as compared to the conventional “bull’s eye” puncture technique for percutaneous renal puncture. This new device uses two radiopaque rings for targeting, somewhat similar to the concept of gun sights. The device is designed to improve the ease and accuracy of the “bull’s eye” puncture technique. We compared this device to the conventional “bull’s eye” puncture technique using a porcine kidney inside a chicken carcass as the study model. Two rst-year residents used both the radiographic grid aiming device in turn, Group A, and the conventional bull’s eye technique, Group B, to perform punctures to reach the target porcine calyces in order. Related data were recorded and analyzed.


Results
23 renal calyceal punctures were performed per group. There were no statistical differences in the overall puncture success rate, puncture time, or radiation exposure time and dose between the two groups. The number of punctures required to achieve success was higher in group B and the distance between the puncture site to the center of papillary fornix was shorter in group A, all P < 0.05.

Conclusions
The novel radiographic grid aiming device showed no advantage in the overall success rate for renal puncture. However, it offered the advantages of more precise puncture with fewer puncture attempts. It may serve as a useful tool for training. It may also offer some bene t to surgeons who are novices at percutaneous renal puncture.

Background
Percutaneous nephrolithotomy (PCNL) is the commonly accepted treatment method for larger renal stones. Precise calyceal puncture is the rst and key step in PCNL. An ideal calyceal puncture should ensure the least trauma for nephrostomy tract creation, complete removal of the calculi, and minimal complications. The puncture should be aimed at the center of the papillary of the targeted calyx.
Conversely, an inaccurate puncture may result in injuries to the kidney and excessive bleeding, and thus impact the overall surgical success and patient outcome. The "bull's eye" technique usually is the basic technique taught to novices [1]. Any of the percutaneous puncture techniques generally has a fairly steep learning curve. One of our guest faculty, Professor Shaw P. Wan, M.D., developed a novel puncture device using two radiographic grids with same-size radiopaque targeting rings for aiming the puncture needle.
Professor Wan has asked us at the Translational Laboratory of the Fifth A liated Hospital of the Guangzhou Medical University to independently evaluate this device. We feel that this device may improve the accuracy of percutaneous renal puncture and shorten the learning curve; therefore, we consented to undertake this task. We used a porcine kidney inside a whole chicken carcass as the study model for this experiment.
Materials And Methods 1. Preparation of ex vivo porcine kidney in chicken carcass model This study was approved by the Ethics Committee of the Fifth A liated Hospital of the Guangzhou Medical University. Whole chickens were purchased from the supermarket. They were completely disemboweled and cleaned to ensure adequate space for insertion of porcine kidneys. The porcine kidneys with ureters ve centimeters or longer were purchased directly from the slaughterhouse. Any kidney with anatomical abnormality or obvious hydronephrosis was discarded. The ureter was carefully dissected free from the surrounding adipose tissue. The renal capsule was stripped to remove any uid that might extravasate between the kidney and the capsule during the puncture trials. The accumulated uid might give a false notion of reaching the targeted calyx. A ve French ureteral catheter was inserted into the renal pelvis through the ureter and tied in place. The porcine kidney was then placed inside the chicken carcass with the chicken in a supine position (the abdomen of the chicken faced upward). The kidney, however, was rotated to a position that mimicked the kidney in a human body in prone position. Radiolucent sponges were used to x the kidney in place when necessary. The skin of the chicken was closed with silk suture to complete the study model ( Figure 1).

Radiographic grid aiming device
The device consists of two circular radiographic grids. The grids are constructed with radiolucent material with radiopaque centrally located rings. Thus, the radiographic images of the grids show mostly the central rings. The grids are designated as proximal and distal grids. The distal grid is placed on the surface of the object to be punctured. The proximal grid is held near the image intensi er of the uoroscopy machine. Both grids and their central rings are the same size. There is also a central aperture in each of the grids for passage of the puncture needle. The distal grid is constructed with soft silicone material. The distal grid aperture allows the passage of the puncture needle. The proximal grid is made of hard clear plastic. The proximal grid aperture is constructed with a luer lock mechanism to allow the puncture needle to be detachably secured to the proximal grid. In addition, the proximal grid has peripheral direction markers to aid in aligning the grids. A handle can be attached to the proximal ring so that the operator's hand can be outside the imaging eld during the puncture (Figure 2). On the uoroscopic image, the distal ring appears to be smaller than the proximal ring. However, as the puncture advances through tissue, i.e. the chicken and porcine kidney, both rings begin to approximate each other's sizes.

Comparison of the two puncture techniques
The study was conducted by two rst-year residents (Xu, Zhu) who had minimal exposure to PCNL and had not performed any percutaneous puncture on actual patients. All the procedures were performed using C-arm uoroscope by two residents, randomly. 46 quali ed chicken-porcines were divided into two groups of equal numbers and performed by two rst-year residents in turn. Group A models were punctured using the radiographic grid aiming device. Group B used the conventional "bull's eye" technique. Contrast material mixed with methylene blue was injected into the porcine kidney in retrograde fashion to image the pyelocalyceal system as well as to create a mild arti cial hydronephrosis. The most direct anterior upper pole calyx was selected as the target.
3-1 Puncture using the radiographic grid aiming device-Group A The distal grid was placed on the abdomen of the chicken with the central ring over the targeted calyx.
Static images were used for adjusting the position of the grid. A modi ed two-part trocar puncture needle (with luer lock connection) was secured to the proximal grid through the locking mechanism. The handle was then attached to keep the operator's hand outside the imaging eld. The needle was inserted through the central aperture of the distal grid. Using continuous uoroscopic images, the two central rings were aligned, and the needle was advanced. This was accomplished with the C-arm in an approximately 30 o angle. After about two centimeters of advancement, the uoroscope was moved to 0 o away from the operator to check the depth of penetration (Figure 3). Successful puncture of the targeted calyx was de ned by rst observing the previously-instilled contrast material draining through the needle hub, followed by injection of fresh contrast material through the needle for further image con rmation. Next, a 0.035" guide wire was passed through the puncture needle and advanced down the ureter. The puncture needle was removed to nish the experiment. The time required for a successful puncture, number of trials to achieve a successful puncture, and uoroscopy exposure time and dose were recorded. Failure was de ned by failing to reach the selected calyx after six attempts. The successfully punctured porcine kidney was next retrieved with the guide wire in place and dissected to expose the punctured calyx. The distance between the needle puncture site and the center of the calyceal was measured and recorded. We used a scoring system to de ne the distance between the needle entry point and the center of renal papilla (NEP-CRP Score). If the distance was between 0 and 1mm, we gave a of score 1; for >1mm to 2mm, we gave a score of 2; and so on; thus, >8mm to 9mm would have a score of 9.

3-2. Conventional bull's eye puncture technique -control
An 18-gauge standard two-part trocar puncture needle was used. A long straight Kelly clamp was used to hold the needle shaft and keep the operator's hand outside the imaging eld. The tip of the needle was placed directly over the targeted calyx using static and uoroscopic images. The needle was then rotated 90 o so that it was now in a straight line with the image receptor to create a "bull's eye" image. The needle was advanced downward under continuous uoroscopic images. After about two centimeters of advancement, the uoroscope was angled to 0 o away from the operator to check the depth of penetration. Successful puncture of the targeted calyx was similarly de ned by rst observing the previously instilled contrast material draining through the needle hub followed by injection of fresh contrast material through the needle for image con rmation. Failure to reach the target after six attempts was deemed failure. Relative data were recorded. The porcine kidney was then retrieved and the distance between the needle puncture site and the center of the calyceal was measured and recorded.

Statistical analysis
Statistical analysis was performed using the SPSS version 19.0. software. Continuous variables were analyzed using parametric tests. P<0.05 was deemed statistically signi cant.
Results 46 porcine kidneys were punctured. As shown in Table 1, there was no statistical difference in the overall success for punctures between Group A and Group B (77.8% versus 83.3%; P=0.457). There were also no statistical differences in the puncture time or radiation time and dose for successful puncture between the two groups (P>0.05). However, the number of punctures required to achieve success was higher in group B (P=0.01), and the distance from the puncture site to the center of papillary (the NEP-CRP Score) was shorter in group A.

Discussion
PCNL is a demanding endourological procedure. Precise puncture is the rst step and one of the most crucial steps in PCNL. It can be challenging even in the most experienced hands. The puncture can be performed under either uoroscopic, ultrasonographic, or combined guidance. Currently, uoroscopicguided puncture remains the mainstay technique. The most commonly used uoroscopic-guided puncture techniques are the "bull's eye" and the triangulation techniques [1,2]. Precise percutaneous renal puncture has a fairly steep learning curve; thus, it deters many urologists from performing this procedure.
The radiographic grid aiming device has been developed with the goals to improve the accuracy of renal puncture and to shorten the learning curve.
Whether papillary puncture is necessary for PCNL is still being debated [3][4][5][6]. Renal access through the center of the papilla is thought to be ideal. Reasons for papillary puncture include the assumption that it is the shortest tract through the renal parenchyma [5,7,8] and anatomical study suggests that punctures through the papillary pose the least injury to the kidney and its vasculature [9].
Historically, renal access for PCNL has been performed mostly by interventional radiologists. According to the literature, 69.7% of urologists perform PCNL, but only 11%-27% obtain their own access despite training in percutaneous surgery during their residencies [10,11]. Evidence has shown that percutaneous access performed by urologists themselves is associated with a statistically signi cant improvement in the overall stone-free rate [12,13]. Published literature suggests that the learning curve for PCNL is about 60 cases to derive competence [14,15]. The novel radiographic grid aiming device may shorten this learning curve, as proven by the fact that even in inexperienced hands, a stable and precise outcome can be achieved (Coe cient of Variation for NEP-CRPF Score: 0.393 vs. 0.521 for Group A and Group B respectively).
The weaknesses of this study include that it is a single institution study. Moreover, Professor Wan is one of our guest faculty. Even though there is no nancial con ict of interest, there is the potential of unintentional bias favoring him. Further studies may be bene cial once this device is commercialized.

Conclusion
The radiographic grid aiming device offers the advantages of more precise puncture and fewer puncture trials to achieve successful puncture. It may serve as a useful training tool. It may also offer some bene ts to surgeons who are novices at percutaneous renal puncture.   Two circular radiographic grids with handle. The central aperture of the proximal grid is constructed with a luer lock mechanism to allow the puncture needle to be detachably secured to the proximal grid. Figure 3