Examining the Impact of Different Properties of Ureteral Access Sheaths in Reducing Insertion Force during Retrograde Intrarenal Surgery: An In Vitro Study

Peak Force of Insertion During Ureteral Access Sheath Placement In An Ex-Vivo Experimental Model With Different Commercially Available Access Sheaths

OBJECTIVES

To measure the force necessary to win the resistance during insertion of ureteral access sheaths (UAS) in an experimental homemade model and to compare the peak force of insertion (PFOI) of different commercially available UASs.

METHODS

Three investigators (2 novice and 1 expert) inserted the UASs into 2 different adapters with diameters of 10 Fr and 8 Fr. The force of insertion was continuously measured with a digital force gauge connected to the UAS during each insertion. Four different brands of UAS with different diameters, totally 11 different UASs were used for the experiment. The PFOI of each UAS was compared among each other and adapter diameters.

RESULTS

The mean PFOI in adapters 1 and 2 were 1.85 N and 5.32 N, respectively. All of the mean PFOIs were significantly lower in adapter 1 compared to adapter 2, regardless of the novice vs expert surgeons and the UASs. (p<0.001) In adapter 1, the mean PFOI was lowest with the UAS-1 and highest with the UAS-8. In adapter 2, the mean PFOI was lowest with the UAS-3 and highest with the UAS-9. For adapters 1 and 2, no statistical difference was found when comparing an expert and the two novice surgeons.

CONCLUSION

The PFOI during UAS placement is not solely correlated with UAS thickness and adapter diameters. Other factors such as hydrophilic coating, UAS flexibility, inner dilator properties, UAS smoothness and the actual measured external diameter of UASs should be taken into consideration. The clinical relevance and ureteral injury risk of the UAS PFOI needs to be studied.

The relationship between the force applied and perceived by the surgeon during ureteral access sheath placement: ex-vivo experimental model

PURPOSE

To define a peak force of insertion (PFOI) threshold for ureteral damage during ureteral access sheath (UAS) placement on an experimental ureteral orifice model.

METHODS

A specially designed water tank using 2 laparoscopic 5 mm ports and 2 different size (10 Fr and 8 Fr) sealing cap adaptors (SCA) as ureteral orifices was used to perform the test. A 10-12 Fr UAS was fixed to a load cell and the force of insertion (FOI) was continuously recorded with a digital force gauge.13 experts in the field of endourology who participated performed 3 UAS insertions. The FOI was recorded initially with 10 Fr followed by 8 Fr SCA. On the final insertion, the orifice was obstructed, leaving a 5 cm length to insert the UAS. The experts were asked to "Stop at the point they anticipate ureteral damage, and they would not proceed in real life".

RESULTS

Using 10 Fr SCA the PFOI was 2.12 ± 0.58 Newton (N) (range:1.48-3.48) while 8 Fr SCA showed a PFOI 5.76 ± 0.96 N (range:4.05-7.35). Six of the experts, said they would stop proceeding when they reached above 5.1 N. Three experts had PFOI < 5.1 N and the other 4 stated they would go with PFOIs of 5.88, 6.16, 6.69 and 7.35 N when using SCA of 8 Fr.The highest load they would stop proceeding had a PFOI of 6.09 ± 1.87 N (range: 2.53-10.74).

CONCLUSION

The PFOI threshold for ureteral damage inserting UAS of the experts is variable. Although FOI is a subjective perception, experience suggests that ureteral injury may occur at an average of 6.05 N perceived by surgeons' tactile feedback. In-vivo measurement of UAS PFOI may confirm a threshold.

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.