‹ Knowledge base

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 turn it; the wedge, straight-cut and Reposition fragments can be carried as well, while a TPLO fragment only turns. 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.

The Osteotomy section

Open Osteotomy and three buttons appear: Straight, Wedge and TPLO, with CBLO, TTA and CCWO below them. Opening the section arms nothing — click a sub-tool to arm it, then click the film. TTA is the exception: it needs no click (see below).

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.

TPLO can place itself. Press it on a film with a TPA (or a side view of the knee the app can read) and the cut drops centred on the tibial axis just below the eminence, with the blade that leaves about 30% of the tuberosity in front of it (never under 25%), its ends on the back and front of the tibia, and the TT line it measured that from — no click needed. See TPLO planning workflow.

Blade Size — what does it set?

Blade Size appears under the 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 → General → 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.

How do I start a cut again?

Press Reset in the cut’s panel — Reset Cutting on a wedge. The cut goes back to where it was placed, uncut: the trace, the turn or the closure all come off together, ready to adjust and cut again. Undo takes back only the last step.

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 anywhere inside the blade circle, or on the circle itself, 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 Fragment — keeps a movable arc: drag the circle or its centre ring 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 Fragment — which am I about to get?

The Current TPA box decides what is under Plateau Levelling:

• Cut and Rotate and Trace Fragment, side by side — the box has an angle in it, taken from a TPA you measured on this film or typed in yourself. Cut and Rotate makes the cut, traces the fragment round the tibia and turns it to the target; Trace Fragment lets you draw the outline yourself instead.
• Trace Fragment alone — 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 → General → Planning Defaults → Default Target TPA. Pressing Enter in either box does what the button does. If the Target TPA box is empty, Cut and Rotate is greyed rather than hidden, and a line under it says so. An empty Current TPA box is not a fault — it is what makes the button Trace Fragment.

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.

What does Cut and Rotate’s fragment take with it?

The proximal tibia, traced round its own edge from one end of the cut to the other: the plateau, the eminence, and the head of the fibula, which is tied to the tibia and moves with the plateau in surgery. The outline sits about 2 mm outside the bone, so the whole cortex turns with it, and it stops short of the femur. Where the film has no TPA to trace from, or cannot be read, the fragment is a standard shape built from the arc instead.

The fragment’s outline is wrong — how do I fix it?

Drag it. With the cut selected, the pointer becomes a crosshair over the dashed outline: take hold where it is wrong and drop it where the edge should be. The app traces the fragment again through that point, following the bone there, and leaves the rest of the outline as it was. One undo puts it back.

If it is wrong all over, press Trace Fragment beside Reset Auto and draw the outline yourself. The plan’s rotation stays.

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.

Letting go never finishes a trace — on this cut, the wedge or the straight cut. It lifts the pen, and you can press on the wire’s tip to carry on. 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.

If the plan came from a TPA measured on this film, turning it away from the planned angle makes the panel say where the plateau actually sits: Turned by hand: the plateau sits at 5.2° — and warn if you have gone past level. Without a measurement on the film there is nothing to read the plateau from, so the Plateau Levelling heading shows only how far the fragment has turned.

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; click the film and a line lands there, starting where you clicked. Drag its ends onto the two landmarks and the tile reads it. Changed your mind before clicking the film? Right-click the film (or press Escape) and the tile goes out again. Alt-click or right-click a tile clears its line.

Draw the TT line first — from the tibial tuberosity across to the back of the bone — and D1 and D2 fill in by themselves. On a side view of the knee, pressing TT thickness draws it for you — from where the patellar ligament inserts, across to the caudal cortex — ready to drag into place; on any other view, or once the TPLO has a TT line, it waits for your click as before. The first time, the app downloads the model that finds the line, about 44 MB, once. It runs on your computer, so the film never leaves it. D1 runs from the tuberosity square to the tibia’s long axis — the axis of your TPA measurement — until it meets the cut; D2 runs along the front edge of the bone to where the cut comes out of it. Both start at the TT line’s first point, follow the cut as you move it, and say auto on their tile. The three readings sit in a column in front of the tuberosity, each with a faint dashed thread: TT thickness points at the insertion, D1 and D2 at their own lines. They have no grips. Want one somewhere else? Click its tile and draw it like any other; your line replaces the automatic one until you delete it. With no TPA on the film, D1 runs to the nearest point of the cut instead, and if it meets no part of the cut its tile stays blank until you draw it by hand.

Only one TT line shows its grips at a time: the one you drew last, or last took hold of, and its tile is tinted to match. Click any other line to give it the grips — or just drag the end of any line, grips or not. Drag along a line, away from its ends, to slide the whole line. With a line in hand, Delete removes just that line; the TPLO stays. Taking hold of the TPLO’s own grips takes the grips off the lines. Collapsing the section hides the lines on the film as well.

Which line is D1, and which is D2?

Vets number these two both ways round, so the first time you open the panel it asks which one you call D1. To change your answer later, drag the D1 tile onto D2 (or D2 onto D1) — on a touchscreen, hold the tile a moment first — or use Settings → General → Planning Defaults → TT measurements: D1 is. Only the names swap: the lines, the figures and the report stay the same, and whichever you call D1 is listed first.

What does “Thin tuberosity” mean?

The cut leaves less than a quarter of the TT line in front of it, and the panel says so: Thin tuberosity — below 25%; fracture risk. Move the cut more caudal. It goes by the percentage alone, however many millimetres that is. A TPLO cut the app places for you aims to leave 30%.

The TTA

How do I plan a TTA?

Press TTA on the mediolateral view of the stifle. It is planned from the common tangent: if the film has one, it uses it; if not, on a side view of the knee it places the common tangent for you first. With no click on the film, it drops a straight cut behind the tibial tuberosity and turns the tuberosity forward about the bottom of the cut until the patellar ligament is at 90° to the common tangent. A cage is drawn in the gap near the top of the cut.

Where does the TTA cut go?

Where an experienced veterinary surgeon plans it: the app learned the cut from that surgeon’s own TTA plans on real knee films. The top of the cut is on the proximal tibia just in front of the tibial plateau, so the whole tuberosity goes with the piece. The bottom is the hinge, inside the bone near where the tibial crest finishes. The TTA appears once it has been placed on the bone, ready to adjust — check the hinge in particular, and drag either end if it is not where you would cut.

The piece that turns is traced round the tuberosity and down the crest, with a little soft tissue to spare, as the TPLO’s is.

On a film where the knee is not found, click to place the common tangent, fit it to the bone, then press TTA again. See Taking measurements for the common tangent.

What does the TTA panel show?

• PTA — the patellar ligament angle, as the common tangent reads it now.
• Target PTA — the angle the tuberosity is brought forward to. It starts at 90°. Type another to re-plan.
• Advancement — how far the tibial tuberosity comes forward, along the common tangent.
• Cage — the width of the gap the cage holds open, where it sits near the top of the cut.

The film shows the ligament as planned, dashed, beside the one the common tangent measured.

Can I move the TTA cut?

Yes. Drag either end of the cut, or the cut itself, and the plan follows: how far the piece turns is worked out again as you move it, and when you let go the piece is traced round the bone again for the cut where you left it. Moving any handle of the common tangent re-plans the TTA too. You cannot turn the piece by hand, because the plan decides the turn. To change it, change the target or the cut.

A longer cut (the hinge further down the crest) needs less turn for the same advancement. If the target cannot be reached within 35° of turn, the panel says so.

Can I move the TTA cage?

Yes. Drag the cage up or down the cut. Its size is read where it sits: lower down, nearer the hinge, the gap is narrower for the same advancement. One undo puts it back.

The TTA piece’s outline is wrong — how do I fix it?

Drag it, as on the TPLO. With the TTA selected, the pointer becomes a crosshair over the dashed outline of the piece: take hold where it is wrong and drop it where the edge should be. The app traces the piece again through that point, following the bone there, and leaves the rest of the outline as it was. The cut and the cage stay where they are. One undo puts it back.

The CBLO cut

Where is the CBLO?

CBLO sits on the second row of Osteotomy, beside TTA.

How do I plan a CBLO?

Put a TPA on the side view of the knee first — pressing TPA finds one for you. Then press CBLO. The app turns on the TPA’s CORA option if it is off, and places the cut by the CORA, with no click on the film. The cut runs over the top of the circle, from the cranial cortex round to the caudal cortex, and the piece that turns is the whole top of the tibia above it. On a side view with no TPA, the TPA the app finds is placed with it. The cut is selected and ready to adjust, and one undo takes it all back. When the whole film is on screen, the app also zooms in, keeping the TPA and the CORA’s lines in view.

How does the app choose the CBLO blade?

It starts at the CORA, then moves the blade’s centre a little and picks the blade so the cut runs out along the cranial cortex and cuts across the caudal cortex, as surgeons draw it, taking the fit closest to the CORA. The cut’s two ends are where it leaves the tibia. From CORA in the panel says how far the centre moved, and Snap to CORA puts it on the CORA. Change the blade on the Blade Size slider. The app never changes the blade size for you once the cut is placed.

How far does a CBLO level the plateau?

Target TPA starts at 10°, the usual target for a CBLO, whatever your TPLO default in Settings. Set it in the CBLO’s panel or in the TPA’s, and press Enter or click away: it is one target, the CORA’s, so both panels show the same figure, the CORA moves when you change it, and the blade moves with it.

What happens to the CBLO when I move the CORA?

The blade follows it. Drag the CORA’s shaft line, move the TPA or change the target, and the blade’s centre moves by the same amount, keeping its size and its distance from the CORA, with its ends put back where the circle meets the bone. Snap to CORA puts the blade’s centre on the CORA. Once the cut is made, the blade stays where it was cut.

What are D1 and D2 on a CBLO?

The two distances that carry the planned cut onto the bone in theatre. D1 runs from the patellar tendon’s insertion to where the cut leaves the cranial cortex, and D2 from the caudal edge of the plateau to where it leaves the caudal cortex. The app works both out once the cut is placed, finding the insertion on the film. They show as tiles marked auto under D1 and D2, and as lines on the film while the cut is selected. To move where one starts, point at its top end until a white handle shows, and drag it: D1’s is the insertion, D2’s the edge of the plateau, and the TPA stays where it is; the other end stays on the cut. Or click a tile, then the film, to draw your own instead. Each tile’s eye hides its line. If the app cannot find the insertion, draw D1 yourself.

What do the CBLO’s figures mean?

Levelling is how far the top of the tibia turns. Current TPA comes from the TPA. Tibial Length is how much the tibia lengthens (plus is longer), and Axis Shift is how far its mechanical axis tilts. Chord and Arc are how far the cut face travels, straight across and along the cut: the chord is the distance to set between two marks either side of the cut. From CORA is how far the blade’s centre is from the CORA.

How do I make the CBLO cut?

Press Cut and Rotate. The top of the tibia is traced round the bone, cut along the arc and turned to the target. If the outline is wrong, drag it on the film, as on the TPLO. Reset puts the cut back. The Surgical report prints the blade, the turn, the chord, the arc, D1, D2 and the plan it was aimed from.

The CCWO

Where is the CCWO?

CCWO sits on the second row of Osteotomy, beside TTA.

How do I plan a CCWO?

Put a TPA on the side view of the knee first — pressing TPA finds one for you. Then press CCWO. The app places a closing wedge planned from that TPA, with no click on the film: the upper cut leaves the cranial cortex 5 mm below the patellar tendon’s insertion, which the app finds on the film, and runs back to the apex. On a side view with no TPA, the TPA the app finds is placed with it. The wedge is selected and ready to adjust, and one undo takes it back. When the whole film is on screen, the app also zooms in on the knee.

How big is the CCWO wedge?

The app works the angle out: the wedge is as big as it needs to be for the plateau to read the Target TPA against the tibia’s new axis once it is closed. Closing tips the tibia’s mechanical axis forward, so the wedge is cut a little bigger than the TPA alone asks, and Axis shift shows by how much. Tibial Length shows how much the tibia shortens or lengthens (plus is longer).

What are Standard, Hinged and Neutral?

Three ways to cut a CCWO, chosen at the top of its panel. Switching re-plans the same wedge, so the three can be compared on one film, and the next CCWO opens on the one you used last. Whichever you choose, the angle is worked out the same way, to land on the target.

• Standard: the apex on the caudal cortex, and two cuts of one length, so the cranial cortices meet as it closes.
• Hinged: its apex inside the bone, leaving a thin caudal hinge. Caudal gap shows how far the bone behind the apex opens.
• Neutral: cut so the tibia keeps nearly all of its length. Caudal step shows how far the top piece’s caudal cortex stands out behind the lower one once closed.

How do I close the CCWO?

Press Close Wedge. The top of the tibia is traced round the bone and swung shut on the apex. Reset opens it again.

Can a CCWO share the levelling with another cut?

Yes. Turn on With TPLO in the CCWO’s panel: a TPLO is added above the wedge and the levelling is shared between the two, and the slider sets how much of it the wedge does. You can plan them in either order.

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).

The V lands open at about 23°, so both read straight away. They show a dash only if an arm is dragged back onto the apex, where it has no direction and so no angle to report.

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

Can the app place the closing wedge for me?

Yes, from a CORA on the same film. With a CORA placed, pressing Wedge drops a closing wedge straight away, without a click on the film: the app follows the edges of the bone the CORA is drawn on, finds where the cut level meets its two cortices, and lands the wedge with its apex on the concave cortex, its angle set to the CORA’s correction, and its arms reaching across the bone to just past the convex cortex. It arrives pinned to the TBL and aimed from the CORA, so the panel already reads Apex on TBL: 2.4 mm concave · closing, Pinned to TBL and Aimed from CORA 1, 12.0°. It is selected and ready to adjust, and one undo takes it back.

The CORA may sit just outside the bone, beyond its convex cortex — that is where the two axes of a bowed bone cross. The wedge is seated where the cut level crosses the bone either way.

Each CORA gets one wedge this way. With two CORAs on the film, the first without a wedge is used, and the wedge’s panel names it; once every CORA has its wedge, Wedge arms as it always did and a click places the ordinary V. To seat a wedge again after moving the CORA, delete it and press Wedge once more.

If the cortex cannot be read — no bone could be followed at the CORA, or the film itself could not be read — the wedge lands on the CORA instead, the neutral seat that Snap to CORA gives, and an amber note over the film says so: slide the apex along the cut level to the concave cortex for a closing wedge.

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, while its handles have not yet been dragged onto their landmarks, or on a CORA reading past 90° — a reading that means one of its axes was drawn back to front, and the panel says which two handles to swap.

So the whole route by hand is: place the CORA, drop a wedge anywhere on the same film, then Snap to CORA and Aim Wedge. Pressing Wedge with the CORA already on the film does the whole thing at once, seated on the concave cortex (see above). The CORA’s own panel used to carry an Add Wedge button that did all three at once; it was taken off in September 2026, because the wedge’s own two buttons already do the job where the wedge is. See Taking measurements for placing a CORA.

Aim Wedge aims at the wedge’s Correction: the CORA’s angle less whatever is typed in the wedge’s Normal bow box. With nothing typed the two are the same number.

Why does the wedge aimed from a CORA look like too much?

Because the CORA measures the whole bend between the two axes you drew, and closing a wedge of that angle takes the whole bend out, so the two axes end up in a straight line. On a view where a normal bone is straight, that is the correction. On a lateral view of the radius it is not: a normal radius keeps a cranial bow there, so straightening it fully over-corrects by exactly that bow.

Type the bend a normal bone has in this view into the wedge’s Normal bow box, beside its angle. The Correction tile drops to the CORA’s angle less that, and a wedge already aimed from the CORA follows at once; otherwise Aim Wedge aims at the correction. The panel says so: Aimed from CORA 1, 12.0° (32.0° less a 20.0° normal bow). If the normal bow you type is the whole reading or more, there is nothing left to cut, the panel says so, and Aim Wedge waits until it is lowered. The surgical report carries the CORA’s angle, the wedge’s angle and the normal bow.

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.

The readout and its pin appear on a wedge seated by Snap to CORA, and on one dropped by pressing Wedge with a CORA on the film: each puts the apex on the TBL and pins it there, so the button already reads Pinned to TBL. Pinned, the apex 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 the button to free it (it then reads Pin to TBL); free, the apex drags anywhere, which changes the angle as well as the seat. Aim Wedge on its own does not tie the wedge to the line, so neither the readout nor the pin appears until you snap.

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: start at the apex, go round the fragment, and bring the line back to the base of the wedge. The wedge reads which of its two cuts the fragment sits against from the outline you drew, and closes onto it when the line reaches the ring at the base, or when you press Finish Trace. Letting go only lifts the pen; press on the wire’s tip to carry on.

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. Letting go only lifts the pen: the trace stays open, and you can pick it up again from its own tip.

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

• Finish Trace — closes the outline onto the cut, with a straight line back to its far end. Greyed until you have drawn enough of one for it to be an outline rather than a twitch of the hand.
• Cancel Trace — throws away what you have just drawn and puts the cut back as it was: after a Retrace, the fragment you had, where you had moved it; on a first trace, the bare line.

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, and Cancel Trace brings the old outline back.

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.

How do I remove a segment of bone, such as an ulnar ostectomy?

Place a straight cut at one end of the piece you want out, then press Remove Segment on its panel. A second cut lands a centimetre away — down the film, or on the fragment’s side if you have already traced one — and the bone between the two cuts turns black: that bone is gone.

• Drag the ends of either cut onto the cortices.
• Drag a cut’s line to slide that cut on its own. That is how you set the length.
• Drag the black between the cuts to slide both along the bone.

The panel reads Segment removed, and the same figure is written on the film, in the Items list and in the surgical report. It is measured along each side of the gap: one figure when the cuts are parallel, the shorter and the longer side when one is oblique (9.5–14.0 mm). Put Segment Back takes the second cut away and the bone comes back.

Does the removed segment move with the bone?

Yes, if its cut was on that bone when the other cut was made. Close a wedge, trace a fragment or outline a Reposition piece over a straight cut, and the cut and its segment go wherever that piece of bone goes. A straight cut you place afterwards stays where you put it, on the bone as it lies by then.

To shorten a bone, take the segment out on a cut whose fragment you have traced. The segment comes out of the fragment, and sliding the fragment back across the gap closes it.

A cut that is riding bone another cut has moved cannot be traced — Trace Fragment is greyed and says so. Reset the other cut first.

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. Bring the line back into the dashed ring at the start and the outline closes itself. If you stop short, click inside that ring or press Enter to close it from where you stopped. 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:

• The engine gives you one outline. To change it, drag a corner of the box to re-frame it, add + Include or − Exclude clicks, or change Detail.
• 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.

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.