There is only one moving joint in the head of the horse: the temporomandibular joint, or TMJ. This is a powerful hinge which also glides.
When the TMJ is out of alignment, a condition called TMD (the D stands for dysfunction) occurs. The misalignment can be caused by bits that don’t fit, riders pulling too hard, lack of proper dental work, tight nose bands, or an atlas (first vertebrae at top of neck) that is not properly aligned.
Tight muscles will restrict movement of the jaw. There are massage techniques to loosen a tight jaw joint. It should only be done if all dental work is up to date so as to not cause discomfort to the horse. I always massage the masseter (large muscle of the cheek) to help loosen the jaw.
If your horse is dropping a lot of feed, if you can hear clicking when he chews, if he is having difficulty flexing at the poll, have a qualified massage therapist loosen up the major muscles of the skull and jaw.