Redoing Anatomic Alignments and Physiology
Written by Dr. Duane C. Keller, DMD
After 40 years, some of the things regarding head and neck function have become clearer. There has been a misunderstanding regarding the importance of the way the head and the neck relate and how compromises affect structural and functional interrelationships. Dentists have learned about the importance of dental function, and how the teeth only meet about 10 minutes each day. We did not appreciate the relationship of the jaw to the head and neck the other 23 hours and 50 minutes. It is important for stability of the upper quadrant that form and function coincide during all 24 hours.
The picture to the left shows the primary muscles used to chew and move the jaw. There are two jaw positions: one where the teeth touch and the other one where the teeth do not touch. The muscles and joints determine the relationship of the lower jaw to the top jaw when the teeth do not contact. The lower jaw rotates at the temporomandibular joint for the teeth to be able to touch. But there is another way to close.
Bringing the teeth together involves the entire upper quadrant (head and neck muscles). The lower jaw movement starts from the muscle and joint position. There should be no joint noise; there should be a full opening without deviation or deflection. You cannot move your lower jaw without also affecting the muscles of your head/neck. The following will help illustrate the importance of this association and the importance of the head/neck function.
Place your elbow on the table and rest your jaw in the palm of your hand. Your lower jaw is now restricted in opening by the table on which you are resting. Remember you cannot have a problem with your jaw joint without also having a neck compensation. You can open without moving your jaw. All you have to do is rotate your neck and you can open. This image helps show how your neck function and jaw position are related.
There is not only one way to open your mouth. You have a second center of jaw movement: your neck. Any compromise to the jaw or joint muscles necessitates a compromise in neck and base of the skull. Just as a jaw joint problem can cause a neck problem, a neck problem like a whiplash accident can compromise jaw function. A lot is involved in dysfunction.
Neck compromise and TMJ issues can be related. Treatments that are successful must take into account all of these anatomic and physiological factors. We do this when we evaluate our patients for treatment.
The temporomandibular joint is a unique joint in the body. It is a fibrocartilaginous joint, whereas other joints are hyaline cartilage. This force vector coincides with most of the primary muscle force vectors shown in the diagram. There is a harmony of form and function, unless this harmony is disrupted and the tissues become compromised. It is also important to understand this joint is generally not loaded, which is why it can be fibrocartilaginous in nature. The jaw joint is held in place by muscles, not your typical ball and socket joint.
The lower jaw “hangs” by the primary muscles of mastication. There is a minor, but significant muscle activity to hold the mandible in this position, but sensors placed inside the joint register a negative pressure when the lower jaw is not in use. While chewing, the forces increase inside the joint, but the following diagrams will show how quickly the pressure returns to a negative pressure. This joint is not designed to be loaded. This joint is designed to function without pressure, unless something causes the teeth to touch, causing the joints to be loaded. Even after chewing the pressure returns to a negative value in a matter of seconds/minutes.
The following graph shows how the pressure inside the joint increases with use, but almost immediately returns to normal. If the joint goes back in the socket the pressure increases and does not return to normal.
One reason this structural arrangement becomes injured is because the form can be compromised by function. The primary muscles for chewing exert a forward force on the jaw as demonstrated on the right. This coincides with the anatomy of the joint. The condyle (top of jaw joint) rests against the disc (soft tissue) against the front of the socket. The jaw is guided in movement by the front teeth which function in harmony with the joint anatomy.
This is a crowded case without adequate dental arch space. What is most important is why the arch is so crowded? Multiple treatments exist. Arch expansion may be very difficult, especially with wisdom teeth. Perhaps it would be best to remove teeth and then align the teeth? Arch development or extraction?
There are many factors to consider when making treatment decisions. Looking at the above case it would be simple to consider extraction mechanics. A decision to remove teeth would not be wrong, but there are ramifications in this type of treatment. Most of the time it would work adequately, but there are things to consider.
There is a question of how the upper and lower teeth fit together and when they meet what forces are applied to the jaw joint. It is also important to remember that the upper jaw is also the floor of the nose. Taking out upper teeth, and closing tooth spaces and will decrease the arch dimensions. Making the upper arch smaller would decrease the nasal airway.
The photograph at the left is of a wax up where teeth were removed from the model. Many things have happened:
1. The upper arch is smaller
2. The front teeth have moved back into the arch
3. This causes the lower jaw to move back
4. The airway is smaller
5. There is less lip and cheek support
6. This affects both the head and neck
The anatomy of the temporomandibular joint is unique. What keeps the disc in place is thick tissue in the back of the jaw joint. There are no muscles behind the disc to “pull” it back into position. The anatomy of form keeps the disc functioning properly. When the jaw moves too far back in the socket, over a period of time it will flatten the tissue and compromise the function of the disc. This joint is designed to function forward and back. A posterior force is pathologic (Ide and Nakazawa – Anatomic Atlas of the TMJ). When taking out teeth we must take into consideration how the lower jaw relates to the top jaw and how the joint functions.
Making the maxilla smaller and moving the jaw back also decreases the airway. It increases upper airway resistance. In Jerald H Simmons MD and Ronald Prehn’s DDS: Nocturnal bruxism as a protective mechanism against obstructive breathing during sleep; they state that people grind their teeth to move the jaw forward to open the airway and improve breathing. Most TMJ patients also have an airway restriction. You cannot adequately advance a dislocated joint to open the airway without treating the joint dysfunction.
Duane C Keller DMD