In dentistry, the term occlusion refers to the way that the upper and lower teeth come together. Originally, the term "occlusion" meant just the way in which each individual tooth met with it's mate in the opposite arch. Thus courses in dental school that dealt with occlusion were most concerned with such concepts as which cusp of an upper molar occluded (came together) with which particular groove in the lower molar that it touched when the patient closed his teeth together.
Indeed, the very first course in occlusion that I took as a dental student involved exclusively such minutia as "The mesiobuccal cusp of the maxillary first molar occludes with the mesial buccal groove of the mandibular first molar...." and so on throughout the mouth. It is unknown whether anyone ever stayed awake throughout a full lecture period. This sorry state of dental education did not begin to change until the late 1970's or early 80's.
Fortunately for the dental students of the world, and especially fortunately for the dental patients of the world, the study of occlusion came into its own as research showed that in order for the upper and lower teeth to meet, the teeth had to be attached to something! And whatever it was that they were attached to had to have a mechanism that allowed them to come together. Much more research showed that the teeth were embedded in human jaws and that the jaws were attached to a joint in front of the ear. (Whoda thunk it?)
In fact, occlusion is one of the most important factors in dentistry because the success or failure of practically everything a dentist does in a patient's mouth depends upon its ability to operate within the boundaries of the patient's physiologic occlusion. Even a simple filling that changes the way a patient bites can cause untold agony for the patient. See my page on TMJ for a full rundown of the disease states that are associated with occlusal factors.
This page is concerned principally with an explanation of the relationship between the way the teeth come together, and the resulting configuration of the temperomandibular joint. It is of interest mostly to dental students, hygienists and assistants who want a simple, practical straightforward understanding of occlusion as it relates to their patient without having to deal with the technical minutia and professional infighting that has become the daily fare of the dentist or physician who wants to specialize in this field.
By the way
For a thorough understanding of glass and porcelain, Students and dental professionals should consult my five page course "Dental Ceramics for the beginner"
The first factor is the minute relationship of the upper and lower teeth when they come together. This generally coincides with the most common definition of the patient's "bite", but also includes the specifics of which cusp on a specific tooth contacts which groove on the opposing tooth. It is also concerned with how the teeth contact during lateral excursions (The way that the upper and lower teeth contact during side to side movements of the lower jaw). This is discussed in detail below.
The second factor is the exact relationship of the components of the temperomandibular joint (the TMJ). (See the highlighted spot on the image of the skull at the top of this page to get your bearings before looking at the diagram at the right.) The TMJ is the ball and socket joint that allows the lower jaw to swing open and closed. The components of the TMJ are as follows :
This is the "ball" in the joint. It is a part of the mandible (lower jaw), and is covered in a layer of cartilage which allows for smooth motion within the joint assembly. The condyle is the part of the lower jaw around which the lower teeth pivot. Click the image for a larger view.
The glenoid fossa:
The fossa is the "socket", or depression in which the condyle sits. It is located in the temporal bone of the skull. The glenoid fossa is also covered with a layer of cartilage which allows smooth activity in the joint. The back of the fossa is steep bone, and the condyle of the mandible sits fairly snugly up against it and can move only slightly backwards from its normal position in the fossa. The front of the fossa is a more gentle slope of bone called the articular eminence. The eminence is also covered with cartilage. The condyle is able to "translate" forward over this eminence of bone and does so whenever the mouth is opened wide, moves side to side, or whenever the patient protrudes his jaw.
The articular disk:
The articular disc is also called the meniscus. It is made of hyaline cartilage. The meniscus has an indentation on the bottom side to accommodate the head of the condyle. The articular disc is really part of a larger structure composed of the cartilage disc plus fibrous ligaments on either side and behind it. The ligament behind the meniscus is called the retrodiscal pad in deference to its function as a shock absorber for the condyle when the lower jaw is drawn back as far as it will go. These ligaments are all connected to the condyle only at their periphery so that there is a thin "potential" space filled with synovial fluid both above and below the articular disc. (A potential space is a collapsed space like the one between a rubber glove and a hand. It is present, but not immediately apparent, and it could potentially get wider if air or water were introduced under the glove.) The articular disc remains between the condyle and the fossa and acts as a shock absorber. The majority of physical derangements of the TM Joints involve damage to the articular disc and/or displacements of fragments of the articular disc.
The joint capsule:
The joint capsule is the covering of the TM joint. Think of it as a bag that contains the joint. It isolates the contents of the joint and allows free movement of the condyle and articular disk within a small "swimming pool" of synovial fluid. The capsule has lots of blood vessels and nerves as well as connective tissue. Inflammation of the capsule (capsulitis) is a factor in much of the pain from TMJ disorders. All major joints in the body are surrounded by a synovial capsule.
The third factor is the neuromuscular system: This involves the muscles of mastication which open and close the jaw, as well as the brain and the cranial nerves which give sensory and motor innervation to the muscles. The muscles of mastication are discussed later in this piece. The brain is important in the concept of occlusion because it is the source of both the voluntary muscular activity which operates the system, as well as unconscious habits such as bruxing (grinding and clenching) which can lead to some of the most serious disease states of occlusion. This subject is discussed in detail below.