Table of Contents
The Muscles of Mastication
The teeth could not occlude or disocclude without the five (paired) muscles of mastication that make it all possible. The relationship between centric occlusion and centric relation obviously influences the way the muscles behave (the way the patient moves them during the course of a normal day). It is very important to note, however, that the reverse is also true. The use of the muscles may, over time, heavily influence the relationship between centric occlusion and centric relation. Just as important, the growth patterns of the structures in the joint, the occlusion itself, and the shape and length of the muscles of mastication all influence each other while the child grows. A majority of the pain and headaches patients experience with temperomandibular dysfunctions (TMD) comes from muscle splinting (cramps) in these powerful muscles.
The temporalis is one of three muscles that close the jaw and clench the teeth. It’s origin is from the periosteum (covering of the bone) of the temporal fossa. It forms a thick tendon which passes under the zygomatic arch and inserts into the the medial surface (the inside surface) and the anterior border of the coronoid process. The way the muscle is leveraged gives it a great amount of power, and splinting in the temporalis can cause serious headaches.
The image above is of the masseter muscle. The medial pterygoid muscle is leveraged in the same way as the masseter, only on the medial (inside) surface of the mandible.
The masseter has an origin on both the outside and inside of the zygomatic process of the maxilla and the zygomatic arch. The masseter inserts into a broad part of the lower jaw, along the lateral surface of the coronoid process, the ramus and the angle of the mandible. This is also a powerfully leveraged muscle, and overuse of this muscle can produce a “square jaw” appearance to the face.
The medial pterygoid muscle arises from the medial (inside)surfaces of the lateral pterygoid plate which is attached to the undersurface of the temporal bone. In lay terms, the attachment is on the undersurface of the skull just behind the last upper tooth. The fibers of the medial pterygoid are directed downward and backward, just like the masseter (pictured above), only on the inside of the mandible. The insertion of this muscle is to the inside of the lower border and angle of the mandible. Click on the thumbnail to the left to see a large cutaway diagram of the medial pterygoid.
The masseter and medial pterygoid act like a contractile “hammock” in which the lower jaw rests. These two muscles are more or less “twins”, the masseter acting on the outside of the lower jaw and the medial pterygoid on the inside.
The lateral pterygoid muscle is an incredibly important muscle. It is responsible for drawing the jaw forward when both the right and left muscles are equally active. It is also responsible for moving the lower jaw from side to side when the right or left lateral pterygoid is active separately. Contraction of the right lateral pterygoid muscle moves the jaw to the left, and contraction of the left draws the jaw to the right. It is also responsible, in combination with the digastric muscle for opening the lower jaw during the translation phase of opening.
The image above shows the lateral pterygoid muscle partially obscured by the coronoid process and part of the zygomatic arch. Click on the thumbnail to the left to see a large cutaway diagram of the lateral pterygoid muscle. It is actually shaped a bit like a partly unfolded fan. The wide end of the fan, it’s origin, originates from a small, finlike projection under the skull called the lateral pterygoid plate. The narrow end of the fan inserts into the anterior surface of the coronoid process. Using your imagination, you can see how contraction of this muscle draws the condyle–(and the lower jaw) forward.
This muscle is composed of two parts. The upper belly inserts into the articular disk inside the TMJ (as noted previously). The lower belly inserts into the neck of the condyle. The two bellies may work independently, but usually in concert to keep the articular disk always situated between the closest points of contact between the condyle and the glenoid fossa during both the rotational phase of jaw opening and the translational phase.
Overuse of the lateral pterygoid during bruxism–remember that the lateral pterygoid is responsible for lateral movements of the lower jaw–causes stretching of the ligaments that hold the articular disk in place over the head of the condyle. This in turn can cause the two heads of the lateral pterygoid to begin to function out of sync which causes even more stretching of the ligaments. This causes the articular disc too much latitude and allows the disc to displace anteriorly. This further exacerbates the asynchronization of the two heads which causes further anterior displacement…and so on until the disc becomes traumatized. The huge forces placed on the condyle by the masseter, temporalis and the medial pterygoid during bruxing will “mash” the articular disc if it is improperly situated between the condyle and the glenoid fossa.
Overuse of the lateral pterygoid also causes cramps in the muscle which manifests as an earache. If the lateral pterygoid is sore, pain can be stimulated by sticking the fingers in the ear and pressing forward on the tragus.
The digastric muscle is the muscle most responsible for opening the lower jaw (in combination with the coordinated contraction of the lateral pterygoid muscles). It is actually composed of two muscles connected in the middle by a strong intermediate tendon. The tendon loops under the hyoid bone which is the only bone in the human body not directly connected to at least one other bone by ligaments.
The hyoid bone is supported in the neck at the level of the Adams apple by a large number of strap-like muscles, all of which brace each other using the hyoid bone as an anchor point.
The front half of the digastric is called the anterior belly. The half of the digastric behind the hyoid bone is called the posterior belly. The intermediate tendon that joins the two bellies actually slides forward and backward under the hyoid bone. Both bellies of the digastric muscle are colored purple in the diagram below.
The digastric’s relatively small bulk, and leveraging under a bone not directly connected to the rest of the skeleton makes it quite a weak muscle when compared with the tremendous upward pressure that can be exerted on the jaws by the combined force of the temporalis, masseter and medial pterygoid which oppose it. This accounts for the inability of a patient to open his mouth against spasms of any of the three closing muscles. The inability to open the mouth is a condition called trismus.
The digastric muscle is rarely involved in disorders of the TMJ or the muscular syndromes associated with bruxing. You do not get TMD by keeping your jaw open, which is the major function of the digastric. You contract these disorders by overusing the other muscles of mastication.
In the discussion above, we examined the way in which a poor occlusion, over activity in the muscles of mastication and the lack of canine guidance can cause anatomical changes in the temperomandibular joint as well as wear on the teeth themselves. This image shows how serious the wear on the teeth can be. Wear on the teeth caused by bruxing is called attrition. As the teeth wear down over time, the lower jaw tends to protrude more and more as well. Prolonged, forceful bruxing can also cause tooth movement, especially if teeth have been extracted in the arch.
Bruxing can, over the years, set up a viscous cycle involving changes in the the shape and position of the teeth, which in turn cause changes in the muscles of the muscles of mastication. People who chew or grind their teeth much more on one side than the other will tend to take on an asymmetrical facial appearance. The muscles on the side of the face in which the hyperactivity takes place tend to become larger and more bulky, while the muscles on the under utilized side tend to become atrophied.
As the muscles change in strength and length, the teeth wear unevenly on both sides causing more and more shifting in the position of the jaw’s centric relation which causes more and more pathological changes in the anatomy of the TM Joints, more bruxing and more muscle deformation.
Early in my career, I had to try to build a complete set of dentures for an old gentleman from the “old country”. He had had his dentures (the same pair) since the age of 16, and he was now 76. Needless to say, his dentures were very seriously worn.
As a denture ages, the teeth wear and the bony ridges that support the denture recede causing the space between the nose and the tip of the chin to collapse. As this process continues, the back teeth of the upper and lower dentures no longer make contact, and the patient is forced to protrude his lower jaw to get them to contact without dislodging the dentures from the ridges (gums). This can cause quite a bizarre facial appearance over time, and of course it seriously affects the patient’s ability to function. The most common effect on the wearing of the denture itself is the fact that the lower front teeth now protrude out well in front of the upper front teeth, often causing a lot of wear on the buccal (outside) surfaces of the upper front denture teeth.
In my patient’s case, the change in his centric occlusion brought about by the combination of denture wear and loss of ridge height caused not only this protrusion of the lower jaw, but a very pronounced shift of the lower jaw to the patient’s right. This caused what would ordinarily be the midlines of his upper and lower edentulous ridges (toothless gums) to be offset nearly a half inch. In addition, his nose and his chin were so close together due to loss of vertical dimension that his mouth had turned into something of a wide slit that seemed to go from ear to ear. He looked like Popeye the sailor without the squinty eye. (Actually, he did smoke a pipe which had worn a large notch in the front teeth of the denture and may have contributed to the shift in the lower jaw to the right. The notch coincided with the area where the upper and lower teeth crossed over into crossbite.) In any case, the dentures fit perfectly together in this position.
NO PROBLEMO! I can fix this! So I thought–remember it was early in my career. So I built him a nice looking upper denture with a lower denture which occluded in a perfect centric occlusion that coincided with a normal centric relation. The patient couldn’t wear it. His lower jaw kept protruding and shifting to the right, back into the position his old denture had forced it into. Nothing I did could correct this condition. So I rebuilt the denture several more times and ended up with a new version of his old denture, complete with notch in the front teeth. The patient was happy, but his wife was not impressed.
Here’s what had happened. The slow deterioration of the denture and ridge height caused a corresponding change in the shape and length of the muscles of mastication on both sides of the face. Since the back teeth no longer made contact when the patient closed his teeth together, he began to protrude his lower jaw farther out to get them to occlude. This protrusion caused further discrepancies in the ability of the back denture teeth to make contact, so the patient began to shift his lower jaw to the right to chew properly. Over the years, the muscles of mastication changed in shape and length to accommodate this unusual bite. The change in shape in the muscles was permanent and could not be reversed by a new, properly built denture. Even when forced to bite in centric relation with my first attempt at a denture, he retained a lopsided appearance with the right side of his face larger than the left. I don’t know what his joints looked like, but I suspect that there was some anatomical change there (ligament stretching and maybe even some minor bony changes) to accommodate the changed centric occlusion. This patient seemed to suffer no major joint signs or symptoms other than his pronounced tendency to shift to the right on closing. Go figure!
Pain in the joint due to disk displacement or other inflammatory changes will cause the patient to develop muscular engrams. Engrams are unconsciously memorized programmed muscle movements that happen on opening or closing the jaws. Their original purpose for the patient is to avoid placing the joint or muscle in a position which provokes pain or spasm. Thus patients will sometimes unconsciously cause their lower jaws to deviate to the right or left during particular points when opening or closing the jaws. These movements are not caused by mechanical interferences inside the joint as discussed above. They are simply reflexive memories that translate into unconscious behavior. This behavior can be quite complex with the jaw deviating right and left in complex, but completely reproducible patterns each time the patient opens or closes his mouth. Furthermore, the engramic behavior persists well after the pain that originally stimulated them has vanished. Upon occasion, engrams must be unlearned to promote healing. This is done by practice sitting in front of a mirror trying to open or close without deviation, or exercises opening against resistance such as upward pressure placed against the chin by the heel of the hand.