TTA Plan Using Common Tangent Methodology

TTA Plan Using Common Tangent Methodology in VPOP PRO

Welcome, everyone. We are going to walk through planning a Tibial Tuberosity Advancement (TTA) using the common tangent methodology. This method ensures we create a reproducible plan that can be taken to the operating room (OR) to template and achieve the perfect post-operative radiograph.


The foundation for a common tangent TTA plan is having well-positioned radiographs. Here’s what you need:

  • The stifle in extension.
  • The beam centered on the stifle and extending distally.
  • A radiographic marker level with and adjacent to the area of anatomical interest.

Why Use an Extended Stifle Mediolateral Radiograph?

To illustrate, consider superimposing two stifles—one in a neutral stance and the other in an extended stance. Both radiographs are from the same patient:

  • In a neutral position, the required advancement appears small.
  • In an extended stifle stance (mimicking the stance phase), the patella is positioned further, requiring a larger advancement.

Using a neutral stance results in under-advancement, while an extended stifle provides the correct patella position to calculate the advancement necessary for execution in the OR.

Step-by-Step Planning

Step 1: Calibration

  1. Calibrate the Radiograph:
    • Ensure you have a 25 mm spherical radiopaque marker in view.
    • Go to Menu (top right), select Image, then Calibrate.
    • Enter the numerical value (2.5 cm) for the radiographic marker and click Calibrate.

Step 2: Finding the Common Tangent

  1. Create Circles for the Femoral Condyle and Proximal Tibia:
    • Go to Tools and use the measurement line to define a radius.
    • Superimpose a circle over the femoral condyle, defining the circumference for a best fit.
    • Repeat for the proximal tibia, ensuring the radius extends to meet the caudal aspect of the tibia’s articular surface.
  2. Draw the Common Tangent:
    • Select a line from Annotations tools, choose a thin red line with double ends.
    • Draw the common tangent by connecting the intersections of the two circles.
    • Create a perpendicular line to this tangent, extending to the patellar tendon origin.
    • Alternatively, simply pan a line joining the two circle centres to the level of the patella tendon origin, since a perpendicular to a perpendicular, is a parallel. origin.

Step 3: Repositioning the Tibial Tuberosity Fragment

  1. Define and Advance the Fragment:
    • Go to Menu (top right), select Reposition Tool.
    • Start at the exit point near the tubercle of Gerdy and define the area of bone to be advanced.
    • Select a pivot point at the distal end of the osteotomy and lock it down.
    • Rotate the fragment until the patellar tendon insertion meets the target advancement line. This is a rotational advancement, not a linear one.

Step 4: Choosing and Placing the Cage

  1. Select an Appropriate Cage:
    • Go to Menu (top right), then Implants, and choose a TTA cage.
    • Test different cage sizes (e.g., 13.5 mm or 12 mm) to ensure it preserves the advancement and fits in the osteotomy void.
    • Ensure even compression is created across the cage faces without a lever arm effect on either  proximal or distal end.

Step 5: Templating for the Plate

  1. Template the Plate:
    • Go to Implants and select the appropriate plate (e.g., a six-hole long plate).
    • Manipulate the plate around the screen, adjusting the size and position to fit the template.

Step 6: Taking Measurements for Reproducibility

  1. Measure Key Distances:
    • Toggle off the implants in the Image menu.
    • Measure the following:
      • Distance from the patellar tendon insertion to the osteotomy exit (24 mm).
      • Distance to the distal exit point (47.5 mm).
      • Length of the osteotomy (63 mm).
    • Record these measurements for use in the OR.

Step 7: Annotating the Plan

  1. Annotate the Plan:
    • Toggle the implants back on.
    • Go to Tools, select Text, and begin annotating.
    • Include details such as patient name, operative side, tibial plateau angle, cage size (e.g., 13.5 mm), plate type (e.g., six-hole long), and measurements (D1, D2, D3).


This concludes our tutorial on TTA planning using the common tangent methodology in VPOP PRO. We hope you find this video useful

Thank you for your attention, and happy planning!




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