Mastication

Now that we have studied both the bones of the skull and the muscles we can look at the interaction between them as we use our mouths to process our food to make it easier to swallow and digest. While you may have been told that humans are omnivores in reality we are fructivores although both have similar dentitions. Fructivores have canines that do not interfere with side to side motion of the jaw but do have some cusps to allow cutting and tearing which is necessary as many foods we eat have a hard to penetrate covering over the softer, more delicious inner portion. Humans use both vertical movement to cut and tear and horizontal movement to grind.

With that introduction we can look at both cases. The easier case is the cutting and tearing motions, in general we use the anterior teeth for this function. The first movement then is to position the lower jaw so that the incisors will align which is protrusion assuming normal tooth relationships and the jaw in resting position which is slightly open. People with open bites or with Class III malocclusion are anatomically prohibited from having the incisors become edge to edge and people with very deep bites will have to open the mouth far beyond normal rest position prior to protruding the mandible. The lateral pterygoids generally both contract to move the mandibular condyle and articular cartilage anteriorly down the articular eminence. This will increase the space between the upper and lower teeth. If that increase is not enough to fit around the object, we are stuffing into our mouths, other muscles come into play to rotate the TMJ without the translation of the lateral pterygoids. These are mostly in the suprahyoid group and include the digastric, mylohyoid and geniohyoid muscles directly and the infrahyoids indirectly to fix the hyoid in place so these muscles work on the mandible rather than the hyoid. Once we are lined up the mandibular elevators take the lead as the masseter, temporalis and medial pterygoid go into action and forcibly close the teeth on the object while the suprahyoids relax. The lateral pterygoids will stay involved though to stabilize the mandible in a slightly protruded position so that the incisal edges line up. Tearing with the canines sometimes is done just be putting the food into the mouth at an angle and sometimes, if it looks like you need to be canine tip to tip one pterygoid will predominate and the mandible will deviate towards that side enough to get the food directly between the sharp cusps of the canines.

Grinding will follow biting in order to increase the surface area of the food, soften it with the addition of saliva and thus prepare it to be swallowed. This requires coordination of a number of muscles. The tongue will take the food that sits in the anterior part of the mouth and by tilting move it onto the occlusal surfaces of the posterior teeth. Obviously, all the muscles of mastication are involved in the chewing stroke but at different times. The three elevators are all used to create force on the food as the lateral pterygoids take turns protracting the jaw on one side and then the other to create a side to side chewing motion. This motion is also aided by contractions of the posterior portion of the temporalis muscle that retracts the jaw. You can feel the actions of the temporalis and masseter by placing fingers on them as you chew but the action of the pterygoids is not discernible in this fashion as they are deep to the ramus. The force of the contractions and the direction of the lateral movements are controlled by feedback from pressure receptors in the periodontal ligament (PDL) surrounding the tooth roots that sense the force put on the teeth and by proprioceptive receptors in the muscles and their associated tendons themselves which can tell the length of the muscle and the amount of tension it is creating. Greater forces will be applied if the muscle does not shorten and less pressure will be applied if the sensors in PDL reveal that the teeth are being over stressed.

The role of the PDL sensors cannot be overemphasized as the amount of force on the teeth changes the speed and strength of the contraction which is what makes popcorn such a dangerous food. As the majority of popcorn is soft and requires considerable muscular pressure to break up but is soft so cushions the force on the teeth meaning the muscles contract hard and they increase the lateral movement of the jaw to work through the fibers. Insert an un-popped kernel into this equation and the great force is no longer cushioned and the tooth is overstressed which can easily result in tooth fractures or damage to the PDL.

In addition, the buccinator and the muscles of the tongue work in concert with them to force the food to stay on the occlusal surfaces of the teeth. Lateral movements are guided by the anatomy of the posterior teeth. In addition to the jaw movements the hyoid and the soft palate move in concert with the chewing stroke to control the placement of the food being chewed so that it stays in the oral cavity and on the back teeth.

Feedback from the PDL indicating interfering cusps will change the contractions of the lateral pterygoids to avoid hitting them. This is why many malocclusions overstress the lateral pterygoids leading to TMD. Like most repeated motions in the body chewing becomes a patterned motion based in the cerebellum. This makes it similar to a golf or tennis swing in that it is fairly consistent over time unless consciously worked on to change. In the rare instances where these carefully coordinated feedback loops are disrupted damage can result to the teeth, the cheek, or the tongue. Dental work that changes the height or position of the cusps or the vertical dimension of occlusion will often result in damage as the habitual pattern of the muscles when chewing is disrupted. Like changing your golf swing, (If you don’t play golf ask a golfer about this) it takes time and much training. During that retraining period things often go awry, cusps can be broken, teeth can become mobile, the muscles can get sore and, in some cases, the TMJ itself sustains damage. None of these are a good outcomes to an avoidable situation.