Most restorative procedures affect the shape of the occlusal surfaces. Proper dental care ensures that functional occlusal contact relationships are restored in harmony with both dynamic and static conditions. Maxillary and mandibular teeth should contact uniformly onclosing to allow optimal function, minimize trauma to the supporting structures, and allow for uniform load distribution throughout the dentition. Positional stability of well aligned teeth is crucial if arch integrity and proper fuunction are to be maintained over time.
Most dentitions deviate from optimal alignment and occlusion. Many patients adapt well to less than optimal occlusion, but malocclusion may be associated with undesirable changes to the teeth, the musculature, the temporomandibular joints (TMJs), or the periodontium. As an aid to the diagnosis of occlusal dysfunction, it is helpful to evaluate the condition of specific anatomic featurres and functional aspects of a patients occlusion with reference to a concept of (optimum) or (ideal) occlusion.
Deviation from this concept can then be measured objectively and may prove to be a useful guide during treatment planning and active treatment phases.
Over time, many concepts of (ideal) occlusion have been proposed. In the literature, the concepts of what is (ideal,) (acceptable,) and (harmful) continue to evolve.
ANATOMY
Temporomandibular Joints
The major components of the TMJs are the cranial base, the mandible, and the muscles of mastication with their innervation and vascular supply. The TMJs are ginglymoarthrodial, meaning that they are capable of both a hinging and a gliding articulation. An articular disk separates the mandibular fossa and the articular tubercle of the temporal bone from the condylar process of the mandible.
The articulating surfaces of the condylar processes and fossae are covered with avascular fibrous tissue (in contrast to most other joints, which have hyaline cartilage). The articular disk consists of dense connective tissue; it also is avascular and devoid of nerves in the area where articulation normally occurs. Posteriorly , it is attached to loose highly vascularized and innervated connective tissue: the retrodiscal pad or bilaminar zone (called bilaminar because it consists of two layers: an elastic superior layer and a collagenous inelastic inferior layer). The retrodiscal pad connects to the posterior wall of the articular capsule surrounding the joint. Medially and laterally, the articular disk is attached firmly to the poles of the condylar process. Anteriorly, it fuses with the capsule and with the superior lateral pterygoid muscle. Superior and inferior to the articular disk are two spaces: the superior and inferior synovial cavities. These are bordered peripherally by the capsule and the synovial membranes and are filled with synovial fluid. Because of its firm attachment to the poles of each condylar process, the articular disk follows condylar movement during both hinging and translation, which is made possible by the loose attachment of the posterior connective tissues.
Ligaments
The body of the mandible is attached to the base of the skull by muscles and three paired ligaments: the temporomandibular (also called the lateral), the sphenomandibular, and the stylomandibular ligaments.
Ligaments cannot be stretched significantly, and thus joint movement is limited. The temporomandinular ligaments restrict rotation of the mandible, limit border movements, and protect the structures of the joint. The sphenomandibular and stylomandibular ligaments limit separation between the condylar process and the articular disk; the stylomandibular ligaments also limit forward (protrusive) movement of the mandible.
Musculature
Several muscles are responsible for mandibular movements. These can be grouped as the muscles of mastication and the suprahyoid muscles. The former include the temporal, masseter, and medial and lateral pterygoid muscles; the latter are the geniohyoid, mylohyoid, and digastric muscles. Their respective origins, insertions, innervation, and vascular supply.
Muscular Function
The functions of the mandibular muscles are well coordinated and complex. Three paired muscles of mastication provide elevation and lateral movement of the mandible: the temporal, masseter, and medial pterygoid muscles. The lateral pterygoid muscles eaach have two bellies that function as two separate muscles, which contrct in the horizontal plane during opening and closing; the inferior belly (inferior lateral pterygoid muscle) is active during protrusion, depression, and lateral movement of the mandible; the superior belly (superior lateral pterygoid muscle) is active during closure. Because the superior belly has been shown to attach to the articular disk and the neck of the condyle, it is thought to assist in maintaining the integrity of the condyle-articula disk assembly by pulling the condylar process firmly against the articular disk.
The suprahyoid muscles have a dual function: They can elevate the hyoid bone or depress the mandible. The movement that results when they contract depends on the state of contraction of the other muscles of the neck and mandibular region. When the muscles of mastication are in a state of contraction, the suprahyoid muscles elevate the hyoid bone. However, if the infrahyoid muscles (which anchor the hyoid bone to the sternum and clavicle) are contracted, the suprahyoid muscles depress and retract the mandible. The geniohyoid and mylohyoid muscles initiate the opening movements, and the anterior belly of the digastric muscle completes mandibular depression. Although the stylohyoid muscle (which also belongs to the suprahyoid group) may contribute indirectly to mandibular movement through fixation of the hyoid bone, it does not play a significant role in mandibular movement.
Dentition
The relative positions of the maxillary and mandibular teeth influence mandibular movement. Many (ideal) occlusions have been described. In most of these, the maxillary and mandibular teeth contact simultaneously when the condylar processes are fully seated in the mandibular fossae, and the teeth do not interfere with harmonious movement of the mandible during function. Ideally, in the fully bilateral seated position of the condyle-articular disk assemblies, the maxillary and mandibular teeth exhibit maximum intercuspation. This means that the maxillary lingual and mandibular buccal cusps of the posterior teeth are evenly distruted and in stable contact with the opposing occlusal fossae. These functional cusps can then act as stops for vertical closure without excessively loading any one tooth, while left and right TMJs concurrently are in an unstrained position.
Howevver, in many patients, maximal intercuspal contact occurs with the condyles in a slightly translated position. This position is referred to as maximum intercuspation, which is defined as the complete intercuspation of the opposing teeth, independent of condylar position; this is sometimes considered the best fit of the teeth regardless of condylar position.
If the mesiobuccal cusp of the maxillary first molar is aligned with the buccal groove of the mandibular first molar, the orthodontic relationship is considered Angle classI; this is considered normal occlusion. In such a relationship, the anterior teeth overlap both horizontally and vertically. This position is defined as the dental relationship in which the anteroposterior relationship of the jaws is normal, as indicated by correct intercuspation of maxillary and mandibular molars, Orthodontic textbooks have traditionally described an arbitrary 2-mm horizontal overlap and 2-mm vertical overlap as being ideal. For most patients, however, greater vertical overlap of the anterior teeth is desirable for preventing undesirable posterior tooth contact. Mandibular flexing during mastication also may contribute to such undesirable contact. Empirically, dentitions with greater vertical overlap of the anterior teeth appear to have a better long-term prognosis than do dentition with minimal vertical overlap.
CENTRIC RELATION
Centric relation is defined as the maxillomandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective articular disks with the complex in the anterosuperior position against the shapes of the articular eminences. This position is independent of tooth contact. It is also clinically discernible when the mandible is directed superior and anterior and is restricted to a purely rotary movement about the transverse horizontal axis.
Centric relation is considered a reliable and reproducible reference (and treatment) position. If maximum intercuspation coincides with the centric relation position, restorative treatment is often straightforward. When maximum intercuspation dose not coincide with centric relation, it is necessary to determine whether corrective occlusal therapy is needed before restorative treatment is initiated.
MANDIBULAR MOVEMENT
As any other movement in space, complex three-dimensional mandibular movement can be divided into two basic components: translation, in which all points within a body have identical motion, and rotation, in which the body is turning about an axis. Every possible three-dimensional movement can be described in terms of these two components. It is easier to understand mandibular movement when the components are described as projections in three perpendicular planes: sagittal, horizontal, and frontal.
Reference Planes
Sagittal plane
In the sagittal plane, the mandible is capable of a purely rotational movement, as well as translation. Rotation occurs around the terminal hinge axis, an imaginary horizontal line through the rotational centers of the left and right condylar processes. The rotational movement is limited to abbout 12 mm of incisor separation before the temporomandibular ligaments and structures anterior to the mastoid process force the mandible to translate. The initial rotation or hinging motion occurs between the condylar process and the articular disk. During translation, the inferior lateral pterygoid muscle contracts and moves the condyle-articular disk assembly forward along the posterior incline of the tubercle. Condylar movement is similar during protrusive mandibular movement.
Horizontal Plane
In the horizontal plane, the mandible is capable of rotation around several vertical axes. For example, lateral movement consists of rotation around an axis situatef in the working (laterotrusive) condylar process with relatively little concurrent translation. A slight lateral translation of the condyle on the working side in the horizontal plane-known as laterotrusion, Bennett movement, or mandibular side shift is frequently present. This may be in a slightly forward direction (lateroprotrusion) or slightly backward direction (lateroretrusion). The orbiting (nonworking) condyle travels forward and medially as limited by the medial aspect of the mandibular fossa and the temporomandibular ligament. In addition, the mandible can make a straight protrusive (anterior) movement.
Frontal plane
In a lateral movement in the frontal plane, the nonworking (mediotrusive) condyle moves down and medially, whereas the working (laterotrusive) condyle rotates around the sagittal axis perpendicular to this plane. Again, as determined by the anatomy of the medial wall of the mandibular fossa on the mediotrusive side, transtrusion may be observed; as determined by the anatomy of the medial wall of the mandibular fossa on the laterotrusive side, this movement may be lateral and upward (laterosurtrusion) or lateral and downward (laterodetrusion). A straight protrusive movement observed in the frontal plane, with both condylar processes moving downward as they slide along the tubercular eminences.
