Distal mechanisms in robotic surgical instruments: a structured review of surgical access, miniaturisation constraints and procedure-specific performance trade-offs.

The distal end of a robotic surgical instrument is where access, dexterity, tissue interaction and procedural capability converge. In flexible, endoluminal, transoral, percutaneous, vascular and neuroendoscopic surgery, the clinical value of an instrument depends not only on kinematic complexity but also on miniaturisation, usable lumen, transmission reliability, stiffness, sensing and validation maturity. This article presents a structured narrative review of distal mechanisms for robotic surgical instruments, based on a technical-clinical synthesis of studies identified through IEEE Xplore, PubMed and Web of Science. Distal architectures are organised into discrete articulated, continuous and hybrid mechanisms and are interpreted according to surgical access route, dominant task, tissue interaction requirements and procedure-specific performance trade-offs. The review shows that no single distal architecture is universally superior. Discrete mechanisms can support local orientation and load-bearing tasks; continuous mechanisms facilitate navigation through tortuous anatomy but introduce shape-estimation and transmission challenges; and hybrid architectures seek to balance access, stability, dexterity and deployment capability. Across these families, miniaturisation should be understood as functional rather than dimensional: reduced outer diameter is clinically meaningful only when force transmission, usable lumen, stiffness, sensing and workflow compatibility are preserved. The article proposes a minimum reporting core for distal robotic mechanisms, including outer diameter, usable lumen, transmission principle, distal force, stiffness definition and validation level. Future progress will depend on procedure-specific evaluation frameworks linking mechanism design with surgical task, anatomical constraints and translational readiness.