In Vitro Evaluation of Ureteral Perforation Forces

Mechanical characteristics of the ureter and clinical implications

The ureteric wall is a complex multi-layered structure. The ureter shows variation in passive mechanical properties, histological morphology and insertion forces along the anatomical length. Ureter mechanical properties also vary depending on the direction of tensile testing and the anatomical region tested. Compliance is greatest in the proximal ureter and lower in the distal ureter, which contributes to the role of the ureter as a high-resistance sphincter. Similar to other human tissues, the ureteric wall remodels with age, resulting in changes to the mechanical properties. The passive mechanical properties of the ureter vary between species, and variation in tissue storage and testing methods limits comparison across some studies. Knowledge of the morphological and mechanical properties of the ureteric wall can aid in understanding urine transport and safety thresholds in surgical techniques. Indeed, various factors alter the forces required to insert access sheaths or scopes into the ureter, including sheath diameter, safety wires and medications. Future studies on human ureteric tissue both in vivo and ex vivo are required to understand the mechanical properties of the ureter and how forces influence these properties. Testing of instrument insertion forces in humans with a focus on defining safe upper limits and techniques to reduce trauma are also needed. Last, evaluation of dilatation limits in the mid and proximal ureter and clarification of tensile strength anisotropy in human specimens are necessary.

What Guidewire Is the Best for Bypassing an Impacted Ureteral Stone?

Introduction and Objectives: To determine the optimal guidewire for bypassing an impacted ureteral stone. Materials and Methods: Three different benchtop models of varying impaction (300, 362, and 444 mm Hg pressure) were used to compare the ability of 13 different guidewires to bypass an impacted ureteral stone. In the first and second models, we recorded the maximum force required to bypass the stone. In the first model (300 mm Hg) 10 new wires for each of the 13 types were advanced past a ureteral stone using a series 5 digital force gauge. In the second model (362 mm Hg), the top 5 performing guidewires were similarly tested. In the third model (444 mm Hg), 5 attending urologists and 5 urology residents (blinded to wire type) compared the 13 guidewires and rated the wire performance using a Likert scale. Statistical analysis was performed with analysis of variance and the chi-square test. Results: In the first model, there was a significant difference between wires (p < 0.001) with the lowest mean force to bypass a stone seen in the Glidewire (0.117 ± 0.02 lbs) and HiWire (0.130 ± 0.01 lbs). Of the five wires tested in the second model, the Glidewire (0.24 ± 0.09 lbs) and UltraTrack (0.40 ± 0.35 lbs) both required less force than the other three wires (p = 0.018). In the third model, only two wires (Roadrunner and Glidewire) bypassed the impacted stone in 100% of trials. When comparing standard, hybrid, and hydrophilic wires, the hydrophilic had the highest success rate (standard = 0%, hybrid = 36.67%, and hydrophilic = 70.67%; p = 0.000) and Likert score (standard = 1.03, hybrid = 2.38, and hydrophilic = 3.24; p = 0.000). Hydrophilic wires required the least time to bypass the stone (hybrid = 82.81 seconds vs hydrophilic = 45.37 seconds, p = 0.000). Conclusions: In this benchtop study, standard wires performed poorly and hybrid wires were not as effective as hydrophilic wires. The Glidewire required the least force, the shortest insertion time, and had the highest surgeon satisfaction rating.

In vitro study of age-related changes in human ureteral failure properties according to region, direction, and layer

Knowledge of the capacity of the ureteral wall to withstand urodynamic or external stresses is essential to understand ureteral injury and rupture that mostly occur following trauma, but may also be secondary to obstructive uropathy. It has clinical significance as well in the prevention of iatrogenic injury, for example, during ureteroscopy, but no information is available with regard to the age-related failure properties and regional differences have not been systematically described. Uniaxial tensile testing was performed on 166 ureteral rings and strips from 21 humans free of overt urologic disease; histological evaluation was performed. The degree of layer participation to the intact wall failure stress (=tissue strength), peak elastic modulus (=stiffness), and failure stretch (=extensibility) was assessed by examining layer-specific ruptures in the stress-stretch data. Failure stress at and peak elastic modulus before the first (muscle/adventitial) rupture correlated inversely less with age ( p < 0.05 in few regions/directions) than failure stress at the second (mucosal) rupture ( p < 0.05 in the middle and lower ureter), consistent with the decreased mucosal thickness in ≥50-year-old subjects. Failure stretch at both ruptures did not correlate with age ( p > 0.05 in most regions/directions), paralleling elastin content. Correlations with age were more significant in females than males. Failure stress at the second rupture point was higher ( p < 0.05) distally in <50-year-old but not in ≥50-year-old subjects, justified by the increased collagen distally in the former. Directional differences in failure stretches ( p < 0.05 at all ages/regions/genders) were justified by preferentially axial collagen reinforcement. The presented results may establish the foundation for computational models of iatrogenic/accidental ureteral trauma.

Tipless Nitinol Stone Baskets: Comparison of Penetration Force, Radial Dilation Force, Opening Dynamics, and Deflection

OBJECTIVE

To evaluate 5 commercially available tipless nitinol baskets (2.2F) in 4 performance factors: penetration force, radial dilation force, opening dynamics, and deflection limitation.

MATERIALS AND METHODS

The 2.2F Coloplast Dormia No-Tip, 1.5F Sacred Heart Medical Halo, 2.2F Cook NCircle Nitinol Tipless Stone Extractor, 1.9F Bard SkyLite Tipless Nitinol Basket, and 1.9F Boston Scientific Zero Tip Nitinol Stone Retrieval Basket were tested for penetration force (safety metric), radial dilation force (functional metric for ureteral calculi), and opening or closing dynamics. Limitation of deflection (functional metric) was tested by measuring the difference in maximal upward and downward angle of deflection of a ureteroscope with and without a basket in place.

RESULTS

The Sacred Heart Medical Halo 1.5F basket had the highest mean force required to perforate the foil at 0.676N ± 0.117 (P < .0001). The Sacred Heart Medical Halo 1.5F basket also had the highest mean radial dilation force at 3.04 g ± 0.15 (P < .0001). The Cook NCircle Nitinol Tipless Stone Extractor 2.2F had the most linear pattern of opening, whereas the Coloplast Dormia No-Tip 2.2F and the Sacred Heart Medical Halo 1.5F exhibited exponential opening dynamics. The Cook NCircle Nitinol Tipless Stone Extractor 2.2F limited scope deflection the most with a decrease in 4° downward and 10° upward. The Sacred Heart Medical Halo 1.5F had the least influence on deflection with a decrease in 3° downward and 5° upward.

CONCLUSION

The penetration force, radial dilation force, opening dynamics, and resistance to deflection varied between 5 commonly available tipless nitinol stone baskets. A small diameter 1.5F basket is capable of providing optimal performance while sacrificing linear opening.

Evaluation of the tensile strength of the human ureter--preliminary results

BACKGROUND AND PURPOSE

Ureteral injuries such as avulsion are directly related to mechanical damage of the ureter. Understanding the tensile strength of this tissue may assist in prevention of iatrogenic injuries. Few published studies have looked at the mechanical properties of the animal ureter and, of those, none has determined the tensile strength of the human ureter. Therefore, the purpose of this work was to determine the tensile strength of the human ureter.

METHODS

We harvested 11 human proximal ureters from patients who were undergoing nephrectomy for either kidney tumors or nonfunctioning kidney. The specimens were then cut into multiple circumferentially and longitudinally oriented tissue strips for tensile testing. Strips were uniaxially stretched to failure in a tensile testing machine. The corresponding force and displacement were recorded. Finally, stress at failure was noted as the tensile strength of the sample. Circumferential tensile strength was also compared in the proximal and distal regions of the specimens.

RESULTS

The tensile strength of the ureter in circumferential and longitudinal orientations was found to be 457.52±33.74 Ncm(-2) and 902.43±122.08 Ncm(-2), respectively (P<0.001). The circumferential strength in the proximal portion of the ureter was 409.89±35.13 Ncm(-2) in comparison with 502.89±55.85 Ncm(-2) in the distal portion (P=0.08).

CONCLUSIONS

The circumferential tensile strength of the ureter was found to be significantly lower than the longitudinal strength. Circumferential tensile strength was also lower with more proximal parts of the ureter. This information may be important for the design of "intelligent" devices and simulators to prevent complications.

Significance of Extraction Forces in Kidney Stone Basketing

INTRODUCTION

Ureteroscopic stone extraction devices are effective tools in the management of urolithiasis, but on occasion, their improper use can cause injury to the ureter. Avulsion and perforation of the ureter as a result of excessive forces on the extraction device are some of the more serious complications of this treatment.

MATERIALS AND METHODS

In this article, avulsion and perforation forces were measured by two different test setups. Eleven clinicians were asked to apply three ranges of forces (safe, cautious, and dangerous).

RESULTS

The output force measurements were recorded and plotted for further analysis. The maximal average perforation forces were 7.13±2.36 N in the benchtop tests and 7.07±2.20 N in the ex-vivo porcine tests (P=0.54). The maximal average avulsion forces were measured to be 10.14±2.01 N in the benchtop tests. Although the average forces were similar in the proximal and distal parts of the ureter (P=0.27), higher values were recorded for the distal part. The operative time was noted to be significantly different in the safe and cautious force regions (P=0.006). The average forces were higher in the benchtop tests compared with the porcine ureter tests. The extraction forces were measured and were noted to be significantly different for attending physicians and residents. The results suggest the need for force feedback training for residents.

CONCLUSION

The findings can be used to design a "smart device" that can provide visual force feedback to clinicians while they are operating, leading to improved patient outcome.

Xenx (Xenolith): preliminary considerations of a new "all-in-one" ureteral guidewire and anti-repulsion device

A new anti-retropulsion device for the endoscopic treatment of ureteral stones was evaluated for safety and potential efficacy. The Xenx™ (Rocamed, Monaco-Montecarlo) is an anti-retropulsion device that operates as a normal hydrophilic guidewire when "closed" and as a nitinol ureteral mesh when "open". We performed semirigid ureterorenoscopy and Ho:YAG Laser lithotripsy in 15 patients, with a single ureteral stone. For each procedure, the papilla was negotiated with the Xenx™, the radiopaque markers were positioned over the stone via direct visualization and the device was opened under fluoroscopic control. The ureteroscope was then retracted and reinserted beside the Xenx™. At the end of the procedure, the Xenx™ was closed, and a ureteral catheter was coaxially placed and left for 24-48 h post-operation. We evaluated device positioning success with respect to pushability, ease of deployment, full expansion and fitting with the ureteral walls, kink resistance and stone retention capabilities during lithotripsy and device retrieval. Operative time, post-operative complications (Clavien-Dindo scale), ultrasound kidney stone-free rate and the hydronephrosis grade, were also recorded. At 4 weeks post-operation, the stone-free rate was assessed via non-contrast computed tomography with 1-mm slices. All procedures were successfully accomplished without complications according to the Clavien-Dindo scale. In all the cases, Xenx™ was inserted successfully past the stone and opened over it. In 4/15 cases (27%) some difficulties in pushability and kink resistance were recorded. In no case stone fragment basketing was performed. The median operative time was 24 min. At hospital discharge, 14/15 patients (93%) were kidney stone-free. At 4 weeks, the NCCT stone-free rate was 100%. This study demonstrated that the Xenx™ is safe and effective in terms of the stone-free rate, complications and operating time. Moreover, use of the Xenx™ allows the use of a basket or guide wire to be avoided.

Tissue damage from ultrasonic, pneumatic, and combination lithotripsy

PURPOSE

To conduct a comparative evaluation of ultrasonic, pneumatic, and dual ultrasonic (DUS) lithotripsy to predict the safety of probes on urinary tract tissue.

MATERIALS AND METHODS

The Swiss Lithoclast Ultra (ultrasonic-only [US] and ultrasonic-pneumatic combination [US+P]) and the Gyrus ACMI Cyberwand (DUS) were evaluated. Fresh porcine ureter, bladder, and renal pelvis tissues were used with a hands-free setup to vertically apply 0, 400, or 700 g of force with each probe for a duration of 3 seconds, 5 seconds, or 3 minutes (or until perforation occurred). Data collection included whether perforation occurred and time to perforation. Histological analysis of nonperforated samples was used to compare the anatomical depth to which damage occurred.

RESULTS

The total percentage of trials resulting in perforation for all tissue types, contact durations, and forces was found to be 8.5% (10/117) for US, 13.7% (16/117) for US+P, and 26.4% (31/117) for DUS. No perforations occurred with light contact (0 g) of probe force, regardless of tissue type, lithotripsy mode, or contact duration. Overall, the renal pelvis was most resistant to perforation (p=0.0004), while no difference was found between the bladder and ureter tissue (p=0.32). Force beyond 400 g and contact greater than 5 seconds increased risk for damage.

CONCLUSIONS

Mode of lithotripsy, tissue type, probe force, and probe-tissue contact duration all significantly impacted the extent of damage and likelihood for perforation to occur. All devices and tissue types provided a reasonable margin of safety for probe-tissue contact times of 3 and 5 seconds with no more than 400 g of force.

[Use of a polymeric gel to prevent retropulsion during intracorporeal lithotripsy]

INTRODUCTION

Stone retropulsion during ureteroscopic lithotripsy may lead to additional procedures needed for residual calculi. Several devices have been introduced in an attempt to reduce retropulsion. We set out to report our initial experience utilizing the new polymeric gel, BackStop.

MATERIAL AND METHODS

We prospectively collected data on 7 ureteroscopy procedures with distal ureteral calculi treated with BackStop. Perioperative data including stone size, location, operative time, stone free rate, the presence or absence of retropulsion was collected. Success was defined as no residual fragments, no retropulsion, and no additional procedures required.

RESULTS

All of the patients were rendered stone free after URS and no retropulsion occurred. There were no intraoperative complications nor gel migration or problems with dissolving the gel.

CONCLUSIONS

BackStop is a new promising therapy to prevent retropulsion during ureteral intracorporeal lithotripsy. It is safe, easy to apply and very effective in preventing stone fragment migration. BackStop has the potential to reduce operative time.

Full-length ureteral avulsion caused by ureteroscopy: report of one case cured by pyeloureterostomy, greater omentum investment, and ureterovesical anastomosis

The aim of this report was to look for a good solution to full-length ureteral avulsion. This report retrospectively analyzed the data of the patient. The patient underwent ureteroscopic management. Full-length avulsion of ureter occurred during ureteroscopy. Pyeloureterostomy plus greater omentum investment outside the avulsed ureter and ureterovesical anastomosis were performed immediately. The patient was followed-up 25 months. After the first operation, the patient developed hydronephrosis because of press of fibrosis tissue outside ureter and anastomotic atresia of ureter-bladder. When we finished the second operation, the renal function recovered well. Full-length avulsion of ureter is a severe complication. Pyeloureterostomy plus greater omentum investment outside the avulsed ureter and ureterovesical anastomosis are probably a good choice.

Complications of stone baskets: 14-year review of the manufacturer and user facility device experience database

PURPOSE

We categorized trends in failure of the stone baskets as reported in the United States Food and Drug Administration Manufacturer and User Facility Device Experience database.

MATERIALS AND METHODS

We queried the online database using the code for stone baskets (FFL) from January 1996 to December 2009. Variables extracted were the type of basket, malfunction and treatment, and patient outcome.

RESULTS

We identified 556 adverse events related to stone baskets. The device configuration was tipped in 48% of cases, tipless in 36%, forceps in 8% and the Stone Cone™ in 8%. Malfunction type included detachment of a portion of the basket in 49% of cases, breakage without detachment in 39% and inability to withdraw the basket in 12%. Compared to the early period studied (1996 to 2004) there was a 3-fold increase in adverse events from 2005 to 2007 and a 6-fold increase from 2008 to 2009. Of adverse events 79% and 11% were managed by endoscopy and open surgery, respectively. Of the patients 42 experienced serious complications requiring major surgery, including ureteral reconstruction in 7, reimplantation in 4 and nephrectomy in 7.

CONCLUSIONS

With the increased use of stone baskets in the upper collecting system the number of adverse events has increased. Urologists should remain vigilant to prevent, recognize and manage these events.