Mandibular movement

Why this matters

Every clinical thing you will ever do to a tooth happens inside a moving system. A restoration that fits in one jaw position and fouls in another is a remake; a denture built on a recorded joint position is only as good as the recording; a jaw that will not move, or will not stop moving, is a referral. This lesson is where the joint you learned in Y1-03.07 meets the occlusion you learned in Y1-06.09, and where you learn to ask of every jaw-movement number the three questions that make it usable: which point was measured, against which reference plane, and by what method.

The big picture

One rigid bone, two joints, and every movement a mixture of turning and travelling. Name the directions - protrusive, retrusive, lateral - then name the two condyles, then meet the instruments built to copy those movements outside the mouth, and find that the instruments encode a model of the joint rather than a measurement of it.

The 15 cards in this lesson

1. 01Rotation and translation are components of every jaw movement, not two phases of one
2. 02Between closed and wide open the condyle leaves the fossa, and the disc goes with it
3. 03The 20 to 25 millimetres of pure rotation is a figure with no measurement behind it
4. 04The envelope of motion is a measured shape, and the only complete three-dimensional measurement of it is one person
5. 05Protrusion is a large movement, retrusion is a 2 mm one, and only one of them has a usable adult denominator
6. 06Every angle on a lab prescription is measured off a trace against a stated reference line
7. 07Two condyles, several names each, and the glossary marks three of the old names obsolete
8. 08At the teeth the same excursion is a question about two guidances that do not predict each other
9. 09The working-side condyle does not simply spin, and much of what you measure there is your choice of point
10. 10The articulator presets are not averages: they are deliberate errors in the direction that shortens cusps
11. 11The corner every diagram draws as a sharp point is a recording, not a landmark
12. 12The immediate side shift may not exist, and the progressive one is simply the Bennett angle
13. 13An articulator is a model of the joints, and its class tells you which movements it can even attempt
14. 14The hinge axis cannot be located to better than about a millimetre, and the methods that find it cannot refute the theory behind them
15. 15The anchor: two motions, two condyles, two settings - and every angle carries its plane

You have got it when

You can now: (1) say why rotation and translation are components of every movement, not two phases; (2) define protrusion, retrusion and lateral excursion, and say which has a good adult denominator; (3) name the two condyles in current and obsolete vocabulary, and say which one the Bennett angle is measured on; (4) explain why the articulator presets are deliberate offsets, not averages; and (5) demand a reference…