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Mechanical characteristics of the ureter and clinical implications

Nature Reviews Urology volume 21, pages 197–213 (2024)Cite this article

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Abstract

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.

Key points

  • The ureter shows variation in passive mechanical properties along the anatomical length. The proximal and distal ureter have the highest and lowest compliance, respectively.

  • The uniaxial tensile strength of the ureter is anisotropic between axial and circumferential directions; however, results vary between studies, and further experiments are needed in human tissues.

  • The passive mechanical properties of the ureter change with age, probably owing to the remodelling of the ureter wall.

  • Various factors affect the forces required to insert ureteroscopes and ureteral access sheaths into the ureter, including sheath diameter, medications and the use of safety wires.

  • Excessive insertion and withdrawal forces are associated with substantial ureteral trauma, and further testing of insertion forces in humans with a focus on defining a safe upper limit are required.

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Fig. 1: Ureter anatomy and histology.
Fig. 2: Mechanical testing of ureteric specimens.

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Acknowledgements

Funding for this review was provided by the MD StAR programme of the Royal College of Surgeons of Ireland.

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Authors and Affiliations

  1. Department of Surgery, Royal College of Surgeons of Ireland (RCSI), Dublin, Ireland

    Sorcha O’Meara, Stefanie M. Croghan & Niall F. Davis

  2. Department of Urology, Blackrock Clinic, Blackrock, Co., Dublin, Ireland

    Sorcha O’Meara, Stefanie M. Croghan & Niall F. Davis

  3. Biomaterials Cluster, Bernal Institute, University of Limerick, Limerick, Ireland

    Eoghan M. Cunnane, Connor V. Cunnane & Michael T. Walsh

  4. School of Engineering, University of Limerick, Limerick, Ireland

    Eoghan M. Cunnane, Connor V. Cunnane & Michael T. Walsh

  5. Tissue Engineering Research Group, Department of Anatomy and Regenerative Medicine, Royal College of Surgeons in Ireland, Dublin, Ireland

    Fergal J. O’Brien

  6. Trinity Centre for Biomedical Engineering, Trinity College Dublin, Dublin, Ireland

    Fergal J. O’Brien

  7. Advanced Materials and Bioengineering Research Centre (AMBER), RCSI and TCD, Dublin, Ireland

    Fergal J. O’Brien

  8. Department of Urology and Transplant Surgery, Beaumont Hospital, Dublin, Ireland

    Niall F. Davis

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Contributions

S.O.M. and S.M.C. researched data for the article. S.O.M., E.M.C., M.T.W. and N.F.D. contributed substantially to discussion of the content. S.O.M., E.M.C. and N.F.D. wrote the article. S.O.M., E.M.C., C.V.C., F.J.O.B. and N.F.D. reviewed and/or edited the manuscript before submission.

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Correspondence to Sorcha O’Meara.

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Nature Reviews Urology thanks Dimitrios Sokolis, Dirk Lange and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Glossary

Anisotropy

Variation in mechanical properties depending on the direction of testing.

Balloon dilatation

Strictures or orifices might be dilated using a balloon. This procedure is usually carried out under radiological guidance using specific devices and can be performed in the urinary tract and other organ systems such as the gastrointestinal tract.

Benchtop testing

Performing experiments in the laboratory setting, typically used to evaluate the correctness of a technique or model.

Circumferential strain

Change in length along the circumferential axis of the structure (ureter).

Compliance

The ease with which a structure elastically deforms in response to force.

Contrast extravasation

The leakage of contrast on imaging. In the ureter, contrast extravasation might suggest perforation or rupture.

Distensibility

Capacity of the ureter to dilate in response to intraluminal pressure.

Elastic modulus

A unit of measurement that describes the ability of a tissue to withstand elastic deformation under stress sand maintain its shape.

Hydronephrosis

Dilatation of the ureter and renal pelvis.

Laplace’s law

A physiological concept that describes the relationship between wall tension, intraluminal pressure and radius. Wall tension Is equal to intraluminal pressure multiplied by radius, and is correlates inversely with wall thickness.

Newton

Amount of force required to make a mass of 1 kilogram accelerate at a rate of 1 metre per second square; in practical terms, this force can be compared with that exerted by an average sized apple on the hand of a person who is holding it.

Reference state

Position from which testing should ideally be performed to ensure that all external and internal forces have been removed.

Safety wire

The use of a wire placed in the renal pelvis during ureterorenoscopy to enable the placement of a guidewire or insertion of instruments in a safe manner.

Serial dilatation

Dilatation of an orifice or structure by starting with a small size of dilator and steadily increasing the size of dilator in increments.

Sirius red staining

A standard staining technique to assess the organization of collagen fibres in tissues.

Traxer

A visual five-point scale used to measure ureteral wall injury. One and five refer to the least and most severe injury, respectively.

Ureteral access sheath insertion force

The force required by a surgeon or a device user to insert an access sheath into the ureter.

Ureteric access sheath

Commonly used devices that are inserted into the ureter through the urethra over a guidewire to enable easy access to the proximal ureter and kidney and to ensure continuous irrigation throughout flexible ureterorenoscopy. These devices are removed at the end of the procedure. The French (Fr) system is used for the sizing, with the first number referring to the size of the inner dilator, and the second number referring to the size of the outer sheath (for example, 12/14 Fr).

Ureterotomy

Cutting into or opening of a ureter.

Wall tension

The reaction force through which the ureter wall resists the forces attempting to expand it.

Zero-stress state

The state with which all external and internal forces are removed. This state provides the reference state from which the stress–strain response can be examined.

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O’Meara, S., Cunnane, E.M., Croghan, S.M. et al. Mechanical characteristics of the ureter and clinical implications. Nat Rev Urol 21, 197–213 (2024). https://doi.org/10.1038/s41585-023-00831-1

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