Abstract
Introduction:
Retrograde intrarenal surgery (RIRS) is one of the most common procedures in urology. During RIRS, ureteral access sheath (UAS) insertion with forces exceeding 8 Newtons (N) poses a risk of transmural ureteral injury. 1 To monitor force during UAS passage, our institution developed a simple and cost-effective force sensor (Air Force #1) based on the specific amount of force required to compress room air in an occluded 1.0 mL syringe (Boyle’s Law). 2 This video illustrates the assembly, usage, and initial clinical evaluation of the Air Force #1 sensor.
Method:
The materials required include a 1.0 mL Luer lock syringe, an airtight Luer tip cap (both from Becton Dickinson, Franklin Lakes, NJ), sterile adhesive (e.g., Dermabond), a 20 mL syringe with the plunger removed, and a Kelly clamp. The plunger of the 1.0 mL syringe is drawn to the 1.0 mL mark, and adhesive is applied to the inner and lateral edges of the Luer tip cap. The syringe’s Luer tip is then occluded with the adhesive-coated tip cap, placed in a vertical position, and allowed to dry for 1 minute. Next, the 1.0 mL syringe is inserted into the barrel of the 20 mL syringe and secured to the flange of the 20 mL barrel using a Kelly clamp. Assembly time is approximately 2 minutes.
Following retrograde guidewire placement into the renal collecting system using a 0.035” Amplatz Super-Stiff wire (Boston Scientific, Marlborough, MA), a 12 Fr ureteral access sheath (Flexor®, Cook Medical, Bloomington, IN), followed by the Air Force #1 force sensor, was advanced over the wire. The guidewire was threaded through the Luer tip of the 20 mL syringe barrel, and the barrel’s Luer tip was connected to the UAS obturator’s Luer port, establishing a linear force transmission system.
The UAS is advanced by pushing only on the plunger of the occluded 1.0 mL syringe. Plunger displacement from 1.0 mL to 0.30, 0.20, and 0.15 mL corresponds to forces of 4, 6, and 8 Newtons, respectively. Post-RIRS, ureteral injury is assessed during withdrawal of the ureteroscope and UAS. After validation on the benchtop, we subsequently evaluated this device clinically in 21 patients enrolled in our RIRS IRB-approved database.
Result:
The Air Force #1 enables real-time monitoring of UAS insertion forces. In this video, the volume on the 1.00 mL syringe never fell below 0.20 mL (i.e., 6 N) during passage of a 16 Fr UAS to the renal pelvis; post-RIRS ureteroscopy showed no ureteral injuries. Air Force #1 has been used in 21 patients at our institution, facilitating safe UAS passage to the renal pelvis/proximal ureter in 20 patients (95.2%), as shown in Table 1. In one patient, 8 N was reached while attempting to pass a 12 Fr UAS beyond the distal ureter; a stent was placed and the procedure postponed. No patient experienced a transmural ureteral tear (PULS 3).
Baseline Demographics and Intraoperative Details for RIRS/PCNL Patients Undergoing UAS Insertion with the Air Force #1 Force Sensor The bolded value indicates that no injury occurred while using the force sensor.
Demographics
Age, years (SD)
65.33 ± 9.48
BMI, kg/m2 (SD)
29.63 ± 7.52
Gender
Male
15 (71.4%)
Female
6 (28.6%)
Intraoperative
Procedure
URS
12 (57.1%)
PCNL
9 (42.8%)
Laterality
Left
13 (61.9%)
Right
8 (38.1%)
Pre-stented
Yes
7 (33.3%)
No
14 (66.6%)
UAS size (Fr)
No UAS
1 (4.8%)
12
8 (38.1%)
14
6 (28.6%)
16
6 (28.6%)
Highest PULS Grade
0
2 (10.0%)
1
17 (85.0%)
2
1 (5.0%)
3
Insertion Force (N)
≤4
41.18 %
>4–≤6
47.06 %
>6–<8
11.76 %
Conclusion:
The Air Force #1 sensor is a simple, easily constructed, and cost-effective tool—approximately $4—for ensuring safe UAS insertion. Among 21 patients, its use was associated with zero high-grade ureteral injuries.
Author Disclosure Statement:
No conflicts of interest or commercial associations to report.
Author(s) have received and archived patient consent for video recording/publication in advance of video recording of the procedure.
Runtime of video: 5 mins and 21 secs.
Video Category: Urolithiasis/Endourology.
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