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Pushability

The Pushability tab evaluates how pushable a catheter design is: how much force the tip can deliver before the shaft bows out of the way. It runs a quasi-static 2D push test of the free (unsupported) distal length of the catheter and reports the whole force/stroke curve (the stiff pre-buckling rise, the buckling knee, and the post-buckling plateau) together with an animated deflected shape and the contact force at the tip.

The tool is the Pushability tab, reachable from the tab bar or from Tools → Pushability. It requires a built model (the per-section bending and axial stiffness it pushes with come from those analysis results), and until there is one the tab shows a short "build the model first" state. The scene is drawn as soon as you open the tab and redraws as you change inputs, so you can see the catheter, the target wall, and the vessel before running anything.

Pullwires are ignored

The shaft is pushed as a passive elastic rod. Tendon tension, actuated pre-curve, and pullwire stiffness contributions are not included: the simulation represents the shaft as delivered, before any deflection is commanded.

Initial Conditions​

  • Free length (from tip): The unsupported length being pushed, measured back from the tip, the length sticking out of the guide sheath or past the last point of vessel support. Defaults to 3 in (76 mm), or the whole catheter if it is shorter. Sections and partial sections inside the free length are stacked as a stepped column, each with its own EI and EA (weak-axis EI on anisotropic multi-lumen cores); rigid sections stay straight. The proximal end is clamped (position and tangent held, as if emerging from a sheath), and the clamp is drawn as a pair of opposed triangles that travel forward with the push. When the free length is shorter than the catheter, the shaft behind the clamp is shown as a fading tail to remind you it is not part of the model.
  • Restraint: How the tip is held. The four ideal Euler end conditions are available for textbook-style comparisons: Free (K = 2), Pinned (K = 0.7), Guided (K = 1), and Clamped (K = 0.5), and they span roughly an 8× range in deliverable force, which is why the choice is yours rather than a hidden assumption. The default, Wall contact, is the physical case: the tip pushes against a rigid wall, and whether it digs in or skids away emerges from the approach angle, tip shape, and friction below.
  • Tip shape (wall contact): Round (hemispherical): the contact force acts through the hemisphere's centre, so a rotating tip rolls on the wall; Conical / dilator: a point contact at the apex that concentrates load and digs in; Flat (square cut): the leading corner carries the load, and its lever arm resists tipping over. The chosen shape is drawn on the end of the catheter.
  • Approach angle: The angle between the catheter axis and the wall normal, 0–45°. 0° is a square, head-on hit. On an angled wall the tip sticks when tan(angle) is below the tip friction μ and skids otherwise.
  • Tip friction μ: Coulomb friction between the tip and the target surface.

Vessel Walls​

  • Enable vessel walls: Governs the whole section. Unchecked by default: an unsupported push in an open chamber (a heart chamber, the aortic root), which is the conservative reading. Checking it reveals the inputs below.
  • Lumen diameter: The vessel's inner diameter, defaulting to 4× the catheter OD; typical lumens for reference are coronary ~3 mm, iliac/femoral ~8 mm, aorta ~25 mm. Contact is with the catheter's outer surface, not its centreline: the shaft is drawn at true width, and its edge is what touches the lumen. Inside a vessel the shaft picks up wall support after it first buckles and the transmissible force climbs again, so a supported shaft delivers far more than an open-chamber one.
  • Shaft friction μ: Coulomb friction between the shaft and the vessel wall. Typical values: hydrophilic-coated ~0.03, uncoated Pebax/nylon ~0.15.

Push​

  • Stroke: How far the clamped end is advanced, capped at half the free length; past that the shaft is folding double rather than being pushed. Solver resolution is chosen automatically: the mesh is sized so the buckle wavelength inside a vessel is always resolved, and a run takes from a few seconds (open chamber) to ~30 s (frictional wall inside a tight vessel).
  • Run Simulation: Solves the push step by step, drawing each stroke increment the moment it lands: the deflected shape, the force curves, and the progress bar advance together.

While it runs​

This is a tab, not a dialog, so a run does not tie up the application:

  • Start it and walk away. Switch to another tab and the run keeps going. A dot on this tab pulses while it is working and turns solid when it finishes, and the status bar posts a short note, so you can carry on designing and come back to it.
  • Cancel stops the run cleanly and leaves the inputs as they were.
  • Rebuilding the model while results are on screen does not delete them. They are marked as belonging to the previous build, with a banner offering to clear them and start again; you decide whether they are still representative.
  • Add the results to a report by ticking this tool's block in Report → Generate Report.... See Reports.

Reading the results​

  • Deflected shape: Each section in its Catheter Setup colour, at true width, with the tip contact-force vector, its head on the actual contact point (the hemisphere surface, cone apex, or loaded corner), its shaft running back out through the target so it never lies over the catheter. The arrow's direction is the dig-in-vs-skid signal: force normal to the wall pushes into the target, tangential force slides along it. Scrub the Stroke step slider, or use Play / Stop (with Loop) to animate the push.
  • Force curves: Push force (measured at the clamp) and tip normal force against stroke. The difference between them is what friction along the walls absorbs.
  • Wall contact curves (vessel walls enabled): one dashed curve per distinct stretch of vessel wall the shaft comes to lie against, drawn against the same force axis, so you can watch each contact build as the shaft bows into it and ease off as it slides past. A curve lifting off zero is that contact landing. Contacts are numbered in the order they are first touched, and each names where along the free length it sits on hover.
  • Wall contact vectors: the same contacts on the deflected-shape plot, one numbered arrow per contact, drawn the same way as the tip vector: head on the contact point, shaft running out through the vessel wall. Length is the load that contact carries at the stroke step you are looking at, on the same scale as the tip force vector, so the two can be read against each other directly. Each vector wears its curve's colour and number.
  • Peak push force: The largest force the clamp applied over the stroke.
  • Buckling knee: The stroke where the pre-buckling rise meets the post-buckling plateau, the practical buckling onset, read off the solved curve. Inside a vessel the curve never flattens (the wall keeps feeding the shaft support), so no knee is reported there; read the curve instead.
  • Peak tip normal force: The largest force the tip delivered into the target, the number to compare against a perforation or lesion-crossing threshold for your tip geometry and tissue.
  • Peak wall contact force: The hardest any one stretch of vessel wall is pressed over the stroke, the highest point reached by the wall contact curves. It is the total load that patch of contact delivers, summed across the shaft lying on it, which is what makes it independent of how finely the shaft is meshed.
  • Tightest bend (distal of the clamp): For each section, the tightest centreline bend radius it reached anywhere in the push. Read it against that section's first-failure radius from Kink Radius: a push that bends a section tighter than it survives is a kinked catheter, not a delivered force. The clamped end is excluded: it is held straight by its boundary condition, so its curvature belongs to the clamp rather than to the shaft.
  • Changing any input after a run clears the results; run again to refresh them. Every step is audited physically after the solve: forces and moments are summed on the solved shape, and a step counts as settled only when that free-body check passes. If any step fails it, a note says so, names the stroke where it starts, and that run's forces from there on should be treated as unresolved; when the failure begins past the buckling knee, the knee numbers themselves are still resolved. Inside a vessel the unsettled steps are usually the ones where the shaft first lands on the wall or leaves it, with the contact set changing under the solver.

Exporting​

This tool has no export buttons of its own. Tick its block in Report → Generate Report... and the report carries the run: scenario inputs and results side by side, the deflected shape at the buckling knee and at end of stroke, and the force curves. Choosing the Excel format also writes the raw curve as its own sheet: one row per stroke step, with push force, tip normal and friction force, tip force vector, tip position, and maximum lateral deflection. See Reports.

Model notes and limits​

The shaft is modelled as a planar (2D) elastic rod pushed quasi-statically, with contact and Coulomb friction at the tip and vessel walls. A small stress-free bow (L/500) stands in for real-world straightness imperfection: a perfectly straight column would balance on the knife edge instead of buckling.

  • 2D: out-of-plane and helical buckling modes inside a vessel are not represented, so a vessel-supported shaft is slightly over-predicted.
  • The tip's contact with the vessel wall follows the tip shape you chose, not the shaft's diameter. A flat tip reaches the wall by its end-face corner, a conical tip by its modelled cone, a round tip by its hemisphere, and how far each of those reaches depends on how far the tip has tilted, so a buckled tip meets the wall where its actual surface does. The drawn catheter touches the wall where the model says it does.
  • Quasi-static: no dynamics, no viscoelasticity.
  • Pullwires are ignored (see above); pre-curved tip shapes are not modelled.
  • A conical tip has a modelled cone length, set beside the tip shape and defaulting to twice the distal OD. It tapers the shaft over that length and so changes where the wall is met and what the forces are; a report records it, because a run is not reproducible without it.
  • The model reports what the shaft can deliver. Whether that perforates or crosses tissue depends on tip area and the tissue itself; that comparison is yours to make.

Treat the numbers the way the Model Accuracy page treats EI: relative comparisons between designs and scenarios are the reliable use, and the absolute forces inherit the accuracy of the underlying stiffness values.