Osteotomy planning on the radiograph: PSO, SPO/Ponte, cages and resections
In ScolioSim’s Plan tab the radiograph itself is cut and moved as you mark it, to show the alignment on the real image while the parameters update beside it. It works on the standing lateral film (sagittal plan) and on the standing frontal film (coronal plan). The two plans are independent, and the bendings have no plan.
The film is moved as marked. The plan does not foresee how the patient will stand afterward. The pelvis turns only as far as you tell it to.
How the plan works
- Cuts are ordered from the pelvis upward, and each piece of film moves with every cut below it. Each cut can be switched on or off by itself.
- Switch “Show the plan” on to see the film cut and moved. “Animate” runs the change in half a second over twelve frames. While the plan is shown, the film is drawn in pieces and the spinopelvic marks travel with their piece.
- The Film / Plan table updates live: pelvic incidence, pelvic tilt, sacral slope, lordosis L1–S1 and L4–S1, PI − LL, kyphosis T5–T12, T1 pelvic angle, global tilt, SVA, degenerative shape and GAP. On the frontal film it shows what you drew with the measuring tools (Cobb angles, C7 against the sacrum, shoulders), which travels with its piece of film.
- The PDF carries the table and a page with the film cut and moved, in negative.
The tools
| Tool | What it does |
|---|---|
| PSO / osteotomy | A closing wedge: a hinge point, an angle closed (0° to 20°) and which line of the wedge ends up level (upper cut, middle or lower cut), with a tilt for that level (−20° to +20°). The film is split along the bisector; behind the hinge the wedge is removed and what is above turns on the hinge until it closes. On the lateral film it is the pedicle subtraction osteotomy (PSO), with its hinge on the front of the vertebral body. On the frontal film you say on which side the wedge is taken. |
| SPO/Ponte | Lateral film. Smith-Petersen or Ponte osteotomies at one disc or at several, marked together as one entry of the plan. At each disc what is above only turns, about a hinge inside the disc, without sliding: the back ends of the two endplates come 25% closer and the front ends go 5% further apart. The card gives the lordosis each disc adds, and their sum. No scale is needed. |
| Interbody cage | Lateral film. Mark the two endplates of the disc. The cage fixes the lordosis between them (up to 20° either way; negative is kyphosis) and the height, up to 15 mm, which is always that of its highest side. Without a scale only the lordosis is applied. |
| Asymmetric cage | Frontal film, for a disc closed on one side. You choose only how many millimeters the closed side is opened, up to 15 mm; ScolioSim finds the closed side and turns what is above about the middle of the two ends on the open side. It needs a scale. |
| Resection | A vertebral column resection, or a hemivertebra taken out. Two cut lines; the film between them is removed and an implant put in its place: its base rests on the lower cut, its height is that of its highest side, and its top platform makes an angle with the base, up to 20° or as far as leaves its short wall 3 mm high, whichever comes first. The upper cut is seated on the platform. An implant of 0 mm is bone on bone and needs no scale. |
| Slide | A line along the disc. What is above slides along it by the millimeters given, backward positive on the lateral film (the reduction of an anterior listhesis). Without a scale it does not move. |
| Femur (lateral) | Mark the axis of the femoral shaft. Its angle with the vertical is measured (positive with the knee in front of the hip). Switched on, the whole film turns about the hip axis until the shaft is at the target angle: it undoes the false uprightness of a patient who bends the knees. |
| Pelvic obliquity (frontal) | Draw a line across the pelvis (the tops of the iliac crests or of the femoral heads) and level it by turning the whole film, as a lift would. |
| Pelvis turn (lateral) | Turns the pelvis and everything on it about the hip axis, the femurs staying still, by the amount you set; positive means more pelvic tilt. It is set by hand: it does not calculate how the pelvis would come forward. |
The SPO/Ponte figures (25% closer behind, 5% further apart in front, a hinge inside the disc, no sliding) and the limits of the tools (osteotomy closure 0° to 20°, level tilt ±20°, cage and implant angles up to 20°, cage height up to 15 mm) are the software author’s working figures. They are not taken from a publication.
Femoral obliquity: the 1.5° reference
The default target for the femoral shaft is 1.5° with the vertical, knee in front of the hip, and it can be edited. The reference is 1.5° ± 2.8°, the femoral obliquity angle of the control group in table 1 of Cheng 2017; the measured angle is flagged in yellow above the mean plus two standard deviations (7.1°). (Cheng 2017)
That control group is 40 adults with degenerative lumbar pathology and PI − LL of 0 or less, mean age 56 years. They are not healthy volunteers.
Where the level of a PSO acts on the lordosis
In a published series of 126 lumbar pedicle subtraction osteotomies, one at L5 increased the lower arc of the lordosis and lowered the apex; at L4 it increased both arcs; at L3 or above it increased the upper arc, raised the apex and the inflection, and added about two vertebrae to the lordosis. (Pizones 2020)
This describes what the level of the osteotomy does to the shape. It is not a result for a given patient. The arcs, the apex and the inflection are explained in the Roussouly classification guide.
Try a plan
- Load the standing lateral or frontal film and, for lengths in millimeters, set its scale from a known length.
- For the lateral film, mark the spinopelvic landmarks first (see spinopelvic parameters).
- Open Simulate, 2D, Plan; add the cuts with the buttons under the cuts and set them. The film moves and the table changes.
Measure, plan and simulate on your own radiographs, in the browser. Nothing about a patient or a case is stored on a server.
Try it in ScolioSim — freeReferences
- Pizones J, et al. (2020). Effect of lumbar pedicle subtraction osteotomy level on lordosis distribution and shape. Eur Spine J. doi:10.1007/s00586-020-06421-8
- Cheng (2017). Gait Posture. doi:10.1016/j.gaitpost.2017.04.041