Numbers on a CT report are abstract until you can feel them. Below is a live model of a glenoid baseplate. Drag retroversion and inclination and watch two things move: the baseplate on the scapula, and a dot climbing a modelled failure-risk curve. It is a teaching cartoon, not a planning tool — but the shape of the relationship is real, and the shape is the whole point.
The interesting feature is the non-linearity. Under about 10–15° of combined deformity the curve is nearly flat — small errors barely matter. Past it, the curve climbs steeply (the shaded band marks roughly where guidelines start recommending augmentation or navigation). Sixteen degrees is not a magic number; it is simply where small errors in placement start to cost a great deal.
How to read this
Two takeaways survive the simplification:
- The relationship between deformity and risk is not a straight line. Averages hide this; the tail is where the surgery gets hard, and where tools like navigation earn their keep.
- Correcting towards neutral is the goal, but over-correction has its own costs — which is why moving either slider away from a stable seat pushes the dot up the curve.
For the clinical argument behind the cartoon, see the companion post on live navigation.