COMPANION NOTES: the jaw, the ear, and the mesh

In the previous edition of Field Notes, we traced the jaw from its embryological beginnings, through the primitive reflexes of rooting, sucking and swallowing, into the postural patterns of the adult body. We arrived at the idea that your jaw is the first joint of your spine. Now, we can move our explorations from how the jaw supports spinal organisation to how it is one of the most frequently used joints in your body - just think how much speaking and chewing you do in an average day.

THE MOTION OF THE JAW

Your jaw is a bilateral structure that articulates simultaneously at the two temporomandibular joints (TMJs), one on each side of the skull.

Place your index fingers about a centimetre in from your earlobes and you will feel a small indentation. Open and close your mouth a few times and you will feel your jaw joint in motion.

This feeling of your mouth opening and closing can seem like the mandible (lower jaw bone) is simply dropping and rising, but the movement inside the joint space is much more complex than this ‘hinging’ experience. In fact, the two bones don’t fit together very well and are classified as incongruent joints. In fully opening the jaw, the mandible both rotates and translates forward, the condylar heads rolling and gliding along the articular disc and the posterior slope of the temporal bone. The disc moves with the condyle, absorbing load and maintaining fluency through the range of motion in what is, anatomically speaking, a somewhat awkward joint space.

Your TMJs are able to move through depression, elevation, protrusion, retrusion, rotation, translation, and lateral excursion, which makes it much more expressive than the hinge joint of your knees, for example. But even this isn’t the whole story. The maxilla (upper jaw bone), which houses your upper teeth, forms the floor of the nasal cavity and the hard palate, and integrates with the cranial base, is usually overlooked or regarded as a fixed structure against which the mandible moves. This is only partly true. Its relationship with the whole cranial structure means that when you explore movement of the upper jaw, you cannot but sequence into the architecture of the skull (and spine) itself.