The purpose of this clinical study is to evaluate BackStop, a polymer-based device that is intended to be used during ureteroscopic lithotripsy to prevent retrograde stone migration. It is a water soluble polymer with reverse thermosensitive properties; the polymer exists as a liquid at low temperature (below 17 C) and rapidly transitions to a high viscosity gel at body temperature (i.e. in the ureter). BackStop is injected above the stones in the ureter and is intended to prevent retrograde migration of stones during ureteroscopic lithotripsy. Upon completion of the lithotripsy procedure, BackStop dissolves naturally or by irrigation.

The study hypothesis is that a greater proportion of patients will experience no retropulsion of a kidney stone when BackStop is used versus no anti-retropulsion device when undergoing intracorporeal lithotripsy.

Inclusion Criteria:

• Patient with a solitary, radio-opaque ureteral stone in the proximal ureter.
• Clinical indication for treatment by ureteroscopic lithotripsy.
• Minimum 18 years old (or any institutional age limits for participation in clinical study).
• Patients must be willing and able to participate in any follow-up visits, as required.
• Patients must provide informed consent.

Exclusion Criteria:

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