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Osteotomy and repositioning tools

Four tools cut or move bone on a radiograph: Straight, Wedge and TPLO in the Osteotomy section of the sidebar, and the fragment tools in Reposition.

This article is about the tools themselves — what each control does and what you can change by hand after a cut is placed. For the start-to-finish TPLO sequence see TPLO planning workflow; for how the pieces fit together in a fracture case see Fracture planning. The app describes geometry only. Which cut to use, and what to correct to, are yours.

Before any of it works

Why are the Straight, Wedge and TPLO buttons greyed out?

Because the radiograph has no scale yet. Every one of these cuts is drawn at a real size — a saw blade of so many millimetres, a wedge of so many degrees across a bone of so many millimetres — so none of them can be placed without one. The same is true of every Reposition tool, because a moved fragment reports how far it moved in millimetres.

The buttons are dark rather than lit-and-then-refused, and the panel says so: Osteotomy planning needs a calibrated radiograph. Calibrate the film first — see Calibrating radiographs.

The four tools are one machine with four pivots

All four cut or outline a piece of bone and then let you carry it and turn it. What differs is the point each one turns about, and that difference is the clinical one:

The three osteotomies turn about a point you choose and leave — the cortical hinge you intend to keep intact. A repositioned fragment has no hinge: it is already free, so it turns about the middle of its own outline and can be carried anywhere. If you want a piece of bone to swing on a hinge, use an osteotomy tool; if you want it to travel, use Reposition.

Both sections only appear on orthopaedic films.

The Osteotomy section

Open Osteotomy and three buttons appear: Straight, Wedge and TPLO. Opening the section arms nothing — click a sub-tool to arm it, then click the film.

All three are one-click tools. You do not drag out a shape: one click on the film drops a whole cut, and the tool disarms. You then adjust it with its grips and its panel.

Blade Size — what does it set?

Blade Size appears under the three buttons while TPLO is armed and nothing is placed yet. There it sets the radius a new arc opens at — the same figure as Settings → Planning Defaults → Default Blade Size, so changing it here changes it for the next film too.

Once an arc is on the film and selected, the same control in the same place sizes that arc instead, and the heading reads its radius. The slider offers the catalogue radii only: 8, 10, 12, 15, 18, 20, 21, 24, 27, 30 and 33 mm.

It stops being adjustable once the cut has been made. The slider stays on screen, greyed, so you can still read which blade the cut was made with against the scale: Fixed — the cut has been made with this blade. Undo the cut to choose another.

The TPLO cut

Can I move the crescentic cut after I have placed it?

Yes, until you commit it — and the answer differs between the two ways of committing.

While the arc is still uncut:

• Drag the arc, or its centre ring, to move the whole cut.
• Drag either of the two grips on the arc’s ends to change how far round the circle the cut runs.
• Use the Blade Size slider to change the radius.

Once Cut and Rotate has made the cut, the arc is fixed: the centre ring and both end grips stop responding, so the blade cannot be re-aimed or re-sized. What you can still do is turn the fragment, with the rotation grip on the film or by retyping the figures in the panel. To change the cut itself, undo it (Ctrl/Cmd + Z).

A cut made the other way — Trace and Rotate — keeps a movable arc: drag it and the outline you traced travels with it. Its two end grips have gone, though, so how far round the circle the cut runs is settled once you start tracing.

Cut and Rotate or Trace and Rotate — which am I about to get?

There is one button under Plateau Leveling, and what it says is which. The Current TPA box decides:

• Cut and Rotate — the box has an angle in it, taken from a TPA you measured on this film or typed in yourself. The app makes the cut, works out the fragment and turns it to the target.
• Trace and Rotate — the box is empty. You draw the fragment’s outline round the bone yourself, then turn it with its rotation grip.

Both figures stay editable afterwards. Target TPA starts at the figure in Settings → Planning Defaults → Default Target TPA. Pressing Enter in either box does what the button does. If a figure is missing the button is greyed rather than hidden, and a line under it says which one to type.

Beside the two typed angles the panel shows Chord and Arc — the straight-line travel of the cut face for the planned rotation, and its travel along the curve.

How do I finish a hand trace?

The trace runs from one end of the cut round the cortex to the other. Start it by pressing on the ring at the near end of the cut and dragging; it closes itself when you bring the wire back to the far end.

If you let go somewhere else, the trace is left open. Finish Trace appears where the commit button was, and closes the outline onto the cut for you.

Turning the fragment by hand

Once cut, the fragment carries a rotation grip on a stalk. Drag it round the blade centre to turn the plateau; drag it nearer or further to shorten or lengthen the stalk itself. A longer stalk moves the fragment less for the same movement of your hand, which is how you land on a particular angle rather than near it. The length is saved with the plan.

Turn it away from the planned angle and the panel says where the plateau actually sits: Turned by hand: the plateau sits at 5.2° — and warns if you have gone past level.

Level Plateau re-levels a fragment that is already cut, from the two figures in the panel. Reset Auto (or Reset) clears the rotation, the fragment outline and the plan’s figures, taking it back to an uncut arc.

TT Measurements

The TPLO panel also carries four tuberosity readings — TT thickness, D1, D2 and D3 — each one both a readout and the button that measures it. Click a tile and it lights to say it owns your next click; draw that line on the film and the tile reads it. Alt-click or right-click a tile clears its line, and each line’s two ends stay grips on the film.

Collapsing the section hides the lines on the film as well. If the tuberosity is thin the panel says so: Thin tuberosity — below 25% and 10 mm; fracture risk. Move the cut more caudal.

The wedge

How do I place a wedge?

One click drops a whole V — an apex at the click and two arms reaching out from it. All three points then drag: the apex, and either arm’s tip. The panel reads Angle (the included angle, what you are taking out of the bone) and Base Width (across the mouth of the V).

Both read a dash rather than a number until each arm has been dragged clear of the apex. An arm that has not moved has no direction, so there is no angle to report.

The apex is the hinge. It is the corner the fragment swings on when the wedge closes.

Can a measurement aim the wedge for me?

Yes, from a CORA construct on the same film. Two buttons appear on the wedge’s panel while it is still open, and they do different halves of the job:

• Snap to CORA — moves the wedge onto the CORA: apex on the centre of rotation, V opening at the convex cortex. It keeps your angle and your arm lengths.
• Aim Wedge — sets the angle to the deformity the CORA measures, and leaves the apex and the way the V faces alone.

Both name the CORA they will use, because on a film holding two the app picks the nearest one and that guess should be visible. Both are greyed with no CORA on the film, and Aim Wedge is also greyed on a CORA reading past 90°, where the panel shows its complement instead.

There is a third route from the other direction: a placed CORA’s own panel has an Add Wedge button, which drops a wedge already aimed and seated on that CORA. See Taking measurements for placing a CORA.

Once the wedge is closed these controls go, because the fragment has moved and re-seating the V underneath it would describe a cut that is no longer the one made.

Apex on TBL: closing, neutral or opening

The transverse bisecting line runs across the shaft at the level of the cut. With the angle settled, sliding the apex along that line is the one remaining choice, and it has a name:

• on the CORA — the neutral wedge
• toward the concave cortex — a closing wedge
• away from it — an opening one

The Apex on TBL readout gives that as a signed distance in millimetres plus the word: 2.4 mm concave · closing, or Neutral.

Pin to TBL holds the apex on that line. Pinned, it slides along the cut level and nowhere else, and follows the construct if that moves — and because a pinned apex is re-seated by carrying the whole V rather than the apex alone, the angle you aimed at survives the slide untouched. Click it again (it reads Pinned to TBL) to free it; free, the apex drags anywhere, which changes the angle as well as the seat.

If the CORA is edited after you aimed from it, the panel says so rather than letting the wedge go quietly stale: Aimed at 12.0°, but CORA now reads 14.5°. Aim again to follow it.

Closing the wedge: the two ways

An open wedge is the same triangle whichever way you are going to close it, so the choice is made when you close it, not when you place it. Two buttons sit side by side:

• Close Wedge — closes it now. Everything beyond the cut swings on the apex, with nothing to trace.
• Trace Fragment — you draw the fragment’s own outline on the film first, from arm 1 round to the apex, then a second Close Wedge closes the wedge onto what you drew.

Trace when the far cortex is where the shape actually matters.

Once closed, the fragment has a move grip at the hinge and a rotation stalk beside it, so you can nudge it off the exact closure. Reset Cutting puts the wedge back to an uncut V — the trace, the closure and any nudge all come off together, so closing it a second time is the same cut as the first. The arms are left where they are: you are resetting the cut, not the measurement.

The straight cut

How do I place and aim a straight cut?

One click lands a horizontal line centred on the click. Drag either end to aim and lengthen it, or drag the line itself to slide the whole cut.

It lands as a bare line and nothing moves yet.

What does Swap Hinge do?

Swap Hinge, on the straight cut’s panel, chooses which end of the line the fragment pivots on — that is the cortical hinge you intend to leave intact, and it is the only thing on this tool that decides which way the bone swings. Press it and the hinge moves to the other end; nothing else about the cut changes.

Tracing the fragment

Trace Fragment starts the outline at the hinge end, so you draw in the direction the saw travels away from the cortex you are levering on. Press on the ring there and drag round the bone; bringing the wire back to the far end of the cut closes it.

While a trace is open the cut is frozen — it is the thing the outline is being drawn against — and two buttons are offered:

• Close Fragment — closes the outline onto the cut. Greyed until you have drawn enough of one for it to be an outline rather than a twitch of the hand.
• Cancel Trace — throws the outline away and leaves the cut where it is.

Once traced, the cut line is fixed: dragging an end would walk the line off its own outline, so the ends stop responding and clicking the line only selects it. Retrace Fragment draws a new outline; it leaves the fragment where you have put it.

Moving the fragment

Drag the fragment body to carry it, and its rotation stalk to turn it about the hinge. The panel reads Displacement in millimetres and Rotation in degrees. Reset Position appears once either is non-zero and puts the fragment back where it was cut.

Reposition

Reposition outlines a piece of bone that is already free and lets you carry it. Its section is violet rather than the osteotomy red. Opening the section arms nothing.

What is the difference between Edge, Freehand and Segment?

Three ways to draw the same outline:

• Edge — click along the bone edge and the line snaps to it. Hold Shift for a straight line with no snapping, Backspace to undo a point, Enter to close, Esc or right-click to cancel. This is the tool for a fracture fragment.
• Freehand — drag to trace a boundary by hand. Click the start point or press Enter to close. The honest choice on a badly exposed film, where a magnetic tool has nothing to grip.
• Segment — drag a box and the computer outlines the bone inside it. Whole bones only; for a fracture fragment use Edge.

Three points are the fewest that can enclose a fragment, and the panel says so while you have fewer.

Working with a Segment outline

A box hands the model a region and the panel fills with what came back:

• A button at the top naming which outline you are looking at and how many there are — Outline 1 of 2 ▸, and so on — steps through the candidates. The engine returns whole, part and sub-part ranked by a score of its own, so the outline you want is often present but unpicked. This is the control worth reaching for first. It is not shown where the engine only ever returns one outline.
• Detail sets how many points the outline is simplified to, from 12 to 200, with Points and Max error reporting what you actually got.
• + Include and − Exclude set what your next click on the film does. Alt-click always excludes, whichever is set.
• Accept Outline turns it into a fragment. Esc or right-click discards it.

If the model cannot read the image the panel says so and points you at the two tools that still work. If its own score is low it says Edges may overlap along this outline. Check it before accepting.

Can I correct an outline after I have closed it?

Yes, in two ways, and both are offered only before you have moved the fragment — the outline’s points are still in their original position on the film, so correcting a fragment that has been carried elsewhere would chase edges nowhere near it.

• Refine Boundary arms the repulsor: a round brush that pushes the outline away from wherever you drag it, like pressing a ball against the line. The brush sizes itself to how far it is from the outline. The button becomes Done Repulsing while it is armed; press that to put the tool down.
• Snap to Edges pulls every point of the outline onto the nearest real bone edge in one press, and one undo. It tells you how many of them found an edge, and says so plainly when none did.

Once the fragment has been moved or turned, both disappear and the panel says Repulsor unavailable after fragment movement or rotation. Individual outline points are not draggable after closing — those two are the way to correct it.

Moving and turning a fragment

Drag the fragment body, or the move ring at its centre, to carry it. Drag the rotation stalk to turn it about that same centre, and drag the grip nearer or further to shorten or lengthen the stalk — a long stalk is what makes a single degree a movement your hand can actually make.

The panel reads Displacement in millimetres and Rotation in degrees, both measured from where the outline was drawn. Reset Selection puts the fragment back there.

Several fragments can be outlined and moved independently on the same film.

Why is the cut painted solid black?

Because the bone has moved and there is nothing behind it. The black stands for absent bone — the void the fragment came out of — not for styling, so it stays black in both the light and the dark theme and should not be “fixed”.

Where two moved fragments overlap, the patch reads denser than either bone alone. That is honest: on a radiograph those pixels really are two bone thicknesses summed.

What is greyed out, and why

Two things worth knowing

Hiding a cut hides the lines, not the plan. The eye beside a cut in the Items list takes away its lines, grips and readouts. The bone stays where the cut moved it, and the black stays where the bone used to be — hiding the working to look at the result is exactly when the result has to be on the film.

A cut cannot be copied to another film. The outline is traced around the bone on that particular film and is tied to the TPA or CORA it was planned from, so it would fit no bone on a second one. Plan the cut on the other film instead. See Keyboard shortcuts.