Table of Contents
- 1 Factors that affect Occlusion
- 1.1 How the position of the teeth determines the position of the components in the joints.
- 1.2 Group function versus canine guidance:
- 1.3 How a normal jaw opens and closes
- 1.4 How the Articular Disc behaves during normal jaw opening?
Factors that affect Occlusion
How the position of the teeth determines the position of the components in the joints.
Centric occlusion is the term used to describe the position of the lower jaw when the teeth are fully occluded (together). This varies from person to person depending upon the number and position of teeth in each jaw. In the image of the skull at the top of the first page in this course, all 32 teeth are present and occluding (biting) in an absolutely normal class I relationship. This position can change throughout a person’s life depending on such factors as the loss of teeth (with the consequent shifting of the teeth that takes place after teeth are removed), fractures of the jaw, orthodontic movement of the teeth to new positions, or the shifting of the teeth due to the constant pressures from bruxing (generally unconscious habits of grinding of clenching). When the teeth are fully occluded, the condyle is forced into a specific position within the glenoid fossa which is called centric occlusion. While a patient’s centric occlusion may be physiologic, meaning that the joint is placed into a comfortable, if not ideal position, the position of the teeth when they are occluding may force the joints into a pathologic, eccentric position. This may produce organic joint dysfunction and pain on movement of the jaw.
In the image of the skull at the top of the first page of this course, the TMJ has been lightened to show the general anatomy. In this case, the condyle is approximately centered in the glenoid fossa. More specifically, the condyle is slightly closer to the top and the back of the glenoid fossa. This relaxed, centralized position of the condyle within the glenoid fossa is called centric relation. If the TM Joints are in a state of health, they tend to approximate this position whenever the teeth are slightly separated and the muscles of mastication are relaxed. Ideally, this position of the joints should also be approximated when the teeth are brought together into the patient’s centric occlusion. The image on the right shows another dried skull with the teeth in centric occlusion and the joint in centric relation. Click on this image to see an enlargement. In life, the articular disc would be resting in the natural space between the condyle and the glenoid fossa without much pressure placed upon it. This is a normal, healthy situation, and in an ideal world, everyone would have a centric occlusion that would allow the condyle to remain in centric relation. It is considered the ideal joint configuration, and it is also the configuration that all dentists strive to produce in patients in which a new centric occlusion must be recreated from scratch.
Unfortunately, when a patient places his teeth together in centric occlusion, the condyles on either side of the jaw do not always line up within the glenoid fossa in centric relation. Even if the tooth-to-tooth position is a perfect class I centric occlusion as shown above, the condyle could be forced into abnormal positions within the fossa. It could (depending upon the growth patterns the patient has experienced throughout early life) actually be jammed up hard against the top of the glenoid fossa. Or it could be located considerably forward of the ideal position, with a consequent tendency toward sliding backwards up the articular eminence. It might be jammed hard against the fibrous connective tissue at the back of the articular disc at the back of the fossa. Situations like these often lead to pain in the joint with frequent headaches and referred pain that is perceived as earaches and neckaches. They also seem to lead to bruxing which further exacerbates the pain.
A discrepancy between centric occlusion and centric relation can also develop in later life if the patient loses teeth, is injured in an accident or has orthodontics in which not enough consideration was given to joint position in the finished case.
Does a discrepancy between centric occlusion and centric relation always cause trouble for the patient?
NO! Even a centric occlusion that causes serious misalignment of the TM joint may cause no noticeable joint dysfunction or pain. The reason that serious misalignment of the joints when the teeth are closed together may be of no significance is that there is no physiologic reason for the patient to keep their teeth together at all. When the teeth are nearly touching, but not occluded, healthy TM joints will automatically fall into a comfortable configuration approaching normal centric relation, even if forceful occlusion would ordinarily force them out of this position. Even while chewing food, the teeth rarely contact at all. (The next time you eat, take notice.) The only time during a normal day when the teeth come together for normal physiological processes is during swallowing, and even then, it is only necessary for light contact to take place on a relatively few teeth…UNLESS…the patient has a bruxing habit!
Bruxing is a nervous habit of grinding the teeth for prolonged periods during the day. To get an idea of the full extent of the pain and agony caused by bruxing habits, read my pages on TMJ. If there is no bruxing habit, even seriously dysfunctional occlusions can remain perfectly physiologic and comfortable.
When a person bruxes his or her teeth side to side, keeping the teeth in constant contact, the cusps of the upper and lower teeth slide over each other forcing the lower jaw to drop slightly as they approach a cusp-tip to cusp-tip relationship. The number of cusps that remain in contact during lateral excursions of the lower jaw vary from person to person.
In young persons with ideal occlusal relationships, the upper and lower teeth contact evenly throughout the entire dental arch when the teeth are fully together in that person’s centric occlusion. However, as soon as he or she begins a lateral excursion, all the teeth (anterior and posterior alike) lose contact, except for the upper and lower canines on that side. In other words, the canines are situated and inclined in such a way that, while they allow full contact of all teeth in centric occlusion, they force the jaw to open as the upper and lower canines slide over each other. This disengages the cusps of all other teeth as the person begins to grind side to the side. (This phenomenon is called “cuspid rise” in deference to the fact that most articulators are hinged in such a way that the upper teeth move instead of the lower. This artificial way of mounting the models makes the upper canines appear to rise instead of the lower canines drop, which is what happens in a real mouth.) In fact, canine guidance is considered the most physiologic of all occlusal relationships because it protects the teeth from wear and tends to prevent bruxing in most persons who are likely to brux only occasionally. In the absence of chronic bruxing habits, this relationship often persists throughout life.
On the other hand, if a person is a habitual bruxer, the combination of tooth movement and cuspal wear over a period of years reduces, and eventually eliminates the prominence of the canine prematurely. This causes more and more posterior tooth cusps to remain in contact over more and more of the excursive movements. The process continues until, eventually, all the cusps of the back teeth remain in contact throughout the entire lateral excursion. This “group functioning” of all the posterior teeth now replaces the original canine rise in causing the lower jaw to drop during excursions.
An occlusion in group function is more prone to perpetuate the bruxing habit leading to greater and greater wear on all teeth. Eventually, the occlusion is worn flat, eliminating any tendency of the lower jaw to drop at all during lateral excursions. In other words, all, or most of the teeth remain in contact throughout the entire lateral excursion, and fail to disocclude, as they do in canine guided occlusions. This may cause extreme wearing of the anterior teeth as well as the posteriors. Tooth wear from bruxing is called attrition. Continual bruxing leading to continual wear of the teeth also changes the relationship of the patient’s centric occlusion to their centric relation, causing a slow, continuing protrusion of the lower jaw bringing about more and more wear on the anterior teeth. Many dentists believe that by recreating a canine guided relationship they can stop a severe bruxing habit and save a dentition otherwise doomed to “death by attrition”.
When a person opens his mouth, the lower jaw swings at the TM Joint which is located just in front of the ear at about the level of the opening of the ear canal. Place your middle fingers lightly on this spot and you can feel the condyle as it moves within the joint space. As you begin to open your mouth, at first you can feel no movement of the joint. During this early part of jaw opening, the condyle is simply rotating within the glenoid fossa. But as you continue to open your mouth wider, you can begin to feel the head of the condyle move forward. This forward movement is called translation and it is a normal part of opening the jaw wide. During translation, the condyle is slipping forward and downward as it slides over the articular eminence. The movement of the lower jaw is traced out by the red line in the image to the right. As the lower jaw begins to swing open, a point on the surface of any lower tooth traces a smooth radius around the place in the glenoid fossa where the head of the condyle rotates. As the jaw opens further, the condyle begins a smooth translation down the slope of the articular eminence. This second opening component traces out a different radius around the changing position of the condyle.
The blue arrow traces out the path of the lower jaw on closing. Notice that the jaw traces out a smooth arc on closing without the complication of rotational and translational movement seen on opening. This is because the condyle begins and ends its closing path by smoothly sliding back up the articular eminence until it comes to rest in centric relation at the end of its closing cycle. The red “broken” arc combined with the smooth blue closing arc is often referred to in dentistry as the classic “Gothic arch” due to its similarity with the architectural structure of the same name.
Note the muscle labeled “lateral pterygoid” in the image on the left above. This muscle has fibers which attach separately to the front of the articular disc, with the majority attaching to the neck of the condyle. When they contract, both the articular disc and the condyle are pulled forward in unison in order to affect translation of the condyle. When the condyle translates down the incline of the articular eminence, the articular disc follows. The disc is in red and the ligaments that attach it to the bony structures are represented in bright yellow. Notice that the disc is not rigidly attached to the head of the condyle. It remains on top of the condyle, but moves into new positions throughout the translation process. The thinnest part of the disc always remains between the closest points of contact between the articular eminence and the the condyle.