Taking measurements
The Measurement section of the sidebar holds Line, Circle, TPA, CORA, Norberg and Tangent (the common tangent). Two lines that cross also read the angle between them.
Line, Circle and TPA need a calibrated radiograph. CORA and Norberg read only angles, which is why they work on a film with no scale set. The common tangent’s angle works without one too; its advancement is in millimetres and needs a scale. All of them scale automatically with zoom and rotation.
Measurement modes
How do I measure a length?
Distance between two points. Used for fragment length, plate span, bone width, and any general linear measurement.
Arm Line and click the film once: a line drops there, starting where you clicked. Drag each end onto its landmark. Dragging on the film instead of clicking moves the film, not the line.
How do I measure an angle?
Draw two lines that cross, one along each axis or edge, each from landmark to landmark. Where they cross, the film reads the angle between them in degrees. The lines must actually cross on the film, so lengthen one if they do not reach — lock its angle first and it cannot tip while you do.
Two crossing lines make four angles. Click inside the one you want and it becomes the reading, or click the number to move it round to the next. With either line selected, the sidebar offers the angle and the one beside it, acute or obtuse. The choice is saved with the case.
The angle is listed in the Items list under the second of the two lines, and printed in the surgical report. Each line also reads its own length, so lines need a calibrated radiograph.
How do I lengthen a line without changing its angle?
Select the line and press Lock angle, under its length in the sidebar. Its ends then slide along the line, so dragging either one makes it longer or shorter and nothing else, however your hand wanders — the way to stretch an axis until it crosses another line without disturbing the angle. The arrow keys move a locked end along the line too.
A padlock beside the length on the film shows the line is locked. It cannot be moved by its middle while it is; press Lock angle again to free it. The lock is saved with the case.
How do I measure a circle?
Diameter, radius or circumference of a circular feature — joint surfaces, medullary canals, or bone fragments.
Arm Circle and click once: a circle drops, centred where you clicked. Drag its three handles onto the edge of what you are measuring; the circle always runs through all three. They need not be spread evenly, so all three can sit on bone you can see even where the rest of the rim is hidden — a femoral head in its socket, say. Drag inside the circle to move it. It reads the radius, the diameter and the circumference — in full in the sidebar, and as R, Ø and C on the film.
A radius runs from the circle’s top handle to its centre, and ends exactly on the centre. Move that handle round the edge to turn the radius. Press Lock radius in the sidebar and its direction holds: the top handle then only makes the circle bigger or smaller along the radius, and the other two handles reshape the circle without turning it. Where a line crosses the radius, the two read the angle between them, just as two crossing lines do.
TPA (tibial plateau angle)
A two-line construction used in TPLO planning: the mechanical axis of the tibia, and the tibial plateau line. The TPA reading appears in degrees, just past the caudal end of the plateau line, so it stays off the joint whichever way the film faces.
On a mediolateral view that reaches the talus, pressing TPA places it on the four points for you, ready to adjust — see TPLO planning.
Refer to your standard TPLO planning references for target angles.
Where is the CORA for a CBLO?
It is an option on the TPA. Select the TPA and turn on CORA in its panel, and two lines join it. A solid line runs up the middle of the tibia, from the marrow cavity halfway down, through Gerdy’s tubercle — where a TTA cut starts — and 15 mm past it. It is placed for you, with a handle at each end to drag. Where the tubercle cannot be found, the line runs through the middle of the marrow cavity a third of the way down instead. A dashed line, the proximal axis, runs from the top of the TPA’s axis at 90° less the target TPA to the plateau: 80° for the usual CBLO target of 10°. Where the two lines cross is the centre of rotation a CBLO is planned from. A short dashed line labelled TBL, the transverse bisecting line, crosses the bone through that point.
The panel reads the Magnitude, the angle between the two lines: how far a CBLO centred there turns the plateau. The Target TPA starts at 10°, the usual target for a CBLO, whatever your TPLO default in Settings; type another and press Enter. It is the CBLO’s target too — change it here or in the CBLO’s panel, and both show the same figure, the CORA moves, and a CBLO planned from it moves with it. If the app cannot place the shaft line, it starts on the TPA’s own axis and the panel asks you to drag its two handles onto the marrow cavity and Gerdy’s tubercle. Turn the option off to take both lines away. The Surgical report and the plan summary on the film list the magnitude and its target TPA under the TPA.
CORA
Centre of rotation of angulation. Draw the proximal and distal axes; the construct reads the deformity angle and marks the CORA where the axes cross.
Pressing CORA places one on the main long bone of the film for you, ready to adjust: a joint centre on each joint, each axis drawn from its joint centre through the middle of its half of the shaft, and the CORA where the two axes cross. If the bone has no bend to find, the CORA is placed halfway along it, reading 1°, and a note says so: move its handles to the bend you see. If it finds no bone on the film, a note says so and you place the CORA by clicking, as before. If the film already has a CORA, pressing CORA again lets you place another by clicking, as before: a second bend is placed with a second CORA. Pressing CORA only ever places one on the view you have selected, so to put a CORA on the view beside it, select that view first.
The angle is the whole bend between the two axes as drawn. Where a normal bone is not straight in that view — a radius on a lateral, with its cranial bow — type that bow into the Normal bow box on the wedge’s panel, not the CORA’s: the wedge then takes out the angle less the normal bow; see Why does the wedge aimed from a CORA look like too much?
Norberg
The Norberg angle, both hips at once. Drop both femoral head centres and both craniolateral acetabular rims; it reads each hip live, with 105° or more shown as normal.
On a ventrodorsal view of the hips with the hips extended, pressing Norberg places it for you — both head circles and both rims — ready to adjust. If the hips are small on screen, the film zooms in on them. If another view is selected, it goes on the hip film, and that film becomes the selected one. On any other film, or if you click first, it drops where you click as before.
Each head centre is found from a circle: drag its three handles onto the edge of the femoral head, and the centre follows. Until you touch one, the second head’s circle copies the size of the first. If you would rather lay one ring over a head and size it, as with a transparency, press Ring for that hip in the panel.
The L and R labels follow the patient, not the screen. A lead side marker on the film, where the app has read one, decides which hip is which. Without one, the app goes by how the view is usually hung, and mirroring the radiograph tells it the film had been stored back to front. Flipping the film top to bottom never changes the labels — see How do I flip an X-ray?.
How do I measure the common tangent for a TTA?
Press Tangent on a mediolateral view of the stifle, ideally with the stifle at about 135°. On a side view of the knee it is placed for you, with no click on the film: a circle on the femoral condyles, a circle on the tibial condyles, the cranial edge of the patella at its distal end, and the patellar ligament’s insertion on the tibial tuberosity. On any other film, click the stifle joint to place it.
Either way, drag its handles to fit the bone: three on each circle’s outline, the patella and the insertion. If the two condyles do not line up, put each circle halfway between the two outlines. The tangent runs at right angles to the line between the two circles’ centres, where the circles meet.
It reads the PTA, the angle between the patellar ligament and the common tangent, and the Advancement: how far the tibial tuberosity has to come forward, along the tangent, for the PTA to be 90°.
Is a measurement accurate if the calibration is only estimated?
An angle is. Norberg, CORA, the TPA, the angle between two lines and the common tangent’s PTA are worked out from where the points sit on the film, not from its scale, so they read the same whether the scale is estimated, measured off a ball, or — for Norberg and CORA — not set at all. What decides an angle’s accuracy is where the points are.
A length is only as good as the scale. With Estimated showing beside the px/mm figure, the scale is the DICOM’s own pixel size, and a Line, a Circle’s diameter, an advancement in millimetres and an implant’s size can be 10–25% out. Calibrate off a ball before trusting those — see What does “Estimated” beside the scale mean?.
How do I place the points accurately?
- Magnification loupe. Toggle it from the top bar for precise endpoint placement on bony landmarks.
- Snapping. When you place a new measurement, clicking within a few pixels of an existing endpoint or centre starts it exactly on that point, and a ring shows when it will. Dragging an endpoint afterwards does not snap.
- Arrow keys. Rest the mouse on a point — or drag it roughly into place and let go — then tap the arrow keys to walk that point one pixel at a time (Shift + arrow for ten). Its ring turns heavier while the arrow keys are holding it.
How do I hide the measurements on the film?
The ruler button in the top bar hides every reading on the film at once — the millimetres and degrees — if they are cluttering the view; the lines stay. On a narrow window it is Readings on the film in the menu. To take a measurement’s lines off the picture as well, use the eye on its row in the Items list. Nothing is deleted either way, and the surgical report still lists them.
Do measurements stay on the film?
- They remain on the radiograph until you delete them, and the case auto-saves.
- They live in image coordinates, not screen coordinates — they zoom, pan, and rotate with the radiograph.
- As many as you like can sit on the same radiograph.
How do I move or delete a measurement?
- Move an endpoint: click and drag the small handle at either end.
- Move the whole measurement: click and drag the line body. A line whose angle is locked moves only by its ends.
- Move its number out of the way: drag the reading itself. It stays where you put it through zoom, rotation and flipping, and when the case is opened again, with a dotted line back to what it measures. Double-click it to put it back.
- Delete: select and press Delete or Backspace.