Text I. Occlusal interferences and occlusal harmony

The mandible is unique in that it exists as a single bone crossing the midline, with symmetrical articulating com­ponents, at each end. This, in combination with the unique nature of these articulating components, leads to a com­plicated three-dimensional geometry of movement. Secondary to the primary articulation of the temporo-mandibular joints (TMJs) is the articulation of the upper and lower teeth.

The relationship of the maxillary and mandibular teeth when in contact is called the occlusal relationship and the teeth are said to be in 'occlusion'. The term occlusion defines no precise or specific tooth contact nor does it infer any particular quality of interarch contact; it defines nothing more than the existence of a state of interarch tooth contact. The teeth out of contact are said to be in 'disclusion’. In the absence of any interventive therapy such as orthodontic care, the adult interarch relationship occurs principally as a result of the individual's genetic status, but can be influenced by habitual, pathological and iatrogenic factors with the result that an individual's occlusal rela­tionship is unique. As such, the term normal occlusion is used to define an occlusal relationship generally consid­ered to satisfy the requirements of function and aesthetics, to which the individual has adapted satisfactorily, but which may exhibit minor irregularities of harmonious tooth contact. Malocclusion defines an occlusal relationship that does not fulfil the ideal harmony of an ideal occlusion. In the absence of a pathological component, malocclusion may be considered to be normal.

The deflective contact has been presented as an example of an occlusal interference. An occlusal interference may be defined as any occlusal contact that gives rise to disharmony in the free gliding movements of the mandible whilst it maintains occlusion with the maxilla. If the cusp-fossae pattern is altered, e.g. by tooth migration, over-eruption or inappropriate restoration the functioning cusps will collide, the resultant contact being described as an occlusal interference.

It has been previously stated that the majority of indi­viduals exhibit some degree of occlusal interference to which they adapt satisfactorily with no significant prob­lems. Indeed, normal occlusion has been described as a malocclusion in which the requirements of function and aesthetics are satisfied but the individual has adapted satisfactorily to minor occlusal interferences.

Occlusal interferences are only considered significant if they are associated with a degenerative or pathological condition, such as tooth fracture, mobility or excessive wear. It is generally accepted that an individual's ability to adapt to the presence of occlusal interferences is strongly influenced by emotional and psychological state.

An occlusal contact on the non-working side that causes either disclusion of the teeth on the working side or displacement or pathology of teeth on the the non-working side is termed a non-working interference. An occlusal contact on the working side that disrupts the smooth harmonious movement of the functioning tooth contacts is termed a working interference.

Text II. Facial clefts

Aberrations in embryonic facial development lead to a wide variety of defects. Although any step may be impaired, defects of primary and secondary palate development are most common.

Most cases of clefts of the lip with or without associated cleft palate appear to form a group etiologically different from clefts involving only the secondary palate. For example, when more than one child in family has facial clefts, the clefts are almost always found to belong only to one group.

There is some evidence that underdevelopment (small size) of the medial or lateral nasal prominences is involved in primary palate clefting, so the contact at the site of fusion is either prevented or inadequate. Other data present the evidence that many cases of cleft lip result from a combination that together reduce the mesenchyme in the primary palate at the point of fusion by an amount sufficient to prevent normal contact and fusion. About two thirds of patients with clefts of the primary palate also have clefts of the secondary palate. Studies of experimental animals suggest that excessive separation of jaw segments as a result of the primary palate cleft prevents the palatal shelves from contacting after elevation. The degree of clefting is highly variable. Clefts may be either bilateral or unilateral and complete or incomplete. Most of this variation results from differing degrees of fusion.

Clefts involving only the second palate constitute the second most frequent facial malformation in humans (after clefts involving the primary palate). Cleft palate can also be produced in experimental animals with a wide variety of chemical agents or other manipulations affecting the embryo. Usually, such agents retard or prevent shelf elevation. In other cases, although elevation occurs, the shelves are too small to make contact. There are also some proofs that the state develops after the application of some environmental agents.

Less frequently, other types of facial clefting are observed. In most instances they can be explained by failure of fusion between facial prominences of reduced size, and similar clefts can be produced experimentally. Examples include failure of merging and fusion between the maxillary prominence and the lateral nasal prominence, leading to oblique facial clefts, or failure of merging of the maxillary prominence and mandibular arch, leading to lateral facial clefts (macrostomia). Other rare facial malformations (including oblique facial clefts) may also result from abnormal pressures or fusions with folds in the fetal (e.g., amniotic) membranes.