The sideways movement of the mandible is mainly enforced by the lateral pterygoid muscle, which runs from the neck of the mandible to the lateral lamella of the wing of the sphenoid bone. The mandible will not only rotate around the working condyle (rotating condyle) during a lateral movement, but it is additionally moved to the side as a whole by the musculature. This lateral movement of the whole mandible is known as Bennett movement (Fig 7-18).This sideways shift is generally no greater than 2 mm. Bennett movement is measured between the resting position and the displacement after a completed lateral movement of the mandible, in which the rotating condyle can perform different movements. Bennett movement can follow a uniform course, or it may be more pronounced at the start of the mandibular movement. A distinction is made between the initial and the integrated Bennett movement (Fig 7-19).
While intraorally measuring condylar paths for complete dentures, Danish dentist Carl Christensen (1857-1921) observed a separation in coplanar occlusion rims and steep condylar paths. The phenomenon by which localized gaps appear between the rows of teeth during lateral or protrusive movements while partial functional contacts are maintained is named after him: Christensen's phenomenon (Fig 7-13).This phenomenon denotes the functional separation into working side and nonworking side, or selective tooth contact. In a fully dentate dentition, this is a necessary step to avoid premature contacts with unused teeth, which can lead to periodontal damage.
Mandibular movements are determined by three guidance factors (Fig 7-8):
In centric occlusion, the teeth are in uniform contact on all sides with the antagonists; in this position, the
periodontal tissues are centrically loaded (Fig 7-3a). The mandible is at its closest to the maxilla in this position. The condyle of the mandible lies without pressure deep in the articular fossa. Centric occlusion is normally adopted as a reflex to a wide opening position, so that this position can also be regarded as the habitual occlusion or intercuspation. An opening movement of approximately 10 mm can be made out of centric occlusion as a pure hinge movement (Fig 7-3b). However, the mandible has to be forcibly held back to do this. For wider jaw opening, the mandibular condyle slides forward and downward on the oblique condylar path; thus, a gliding movement is added to the pure hinge movement.
The positional changes of the mandible in comparison with the maxilla can be observed and measured at three places (Fig 7-1): between the condyle and the articular fossa, between the jawbones, and between the occlusal surfaces of the teeth.
The musculature of the tongue is divided into two halves by the septum of the tongue (septum linguae), a sheet of connective tissue at the mid-line.The position of this septum can be seen from the superior surface of the tongue as the median sulcus (sulcus medianus linguae).Two groups of tongue muscles can be identified by their course, namely those that originate from parts of the skeleton and end in the tongue and those that have their origin and ending in the tongue itself. The first group are known as extrinsic (originating outside the tongue), while the muscles entirely within the tongue are referred to as intrinsic.
The tongue (lingua; Greek, glotta or glossa) is an oval muscular organ covered with mucosa that is mainly made up of highly developed striated musculature with extremely variable mobility. The tongue is highly perfused with blood and carries nerves for the senses of taste and touch. It helps in eating and during chewing, sucking, and swallowing, and it is used for the purposes of speech because it is highly mobile.The tongue almost completely fills the oral cavity and extends at the rear as far as the epiglottis. The body of the tongue can be divided into the root, dorsum, and apex (or tip), while the surfaces and edges are described as the superior surface (facies superior), the lateral margins (margines laterales), and the inferior surface (facies inferior). Lifting up the tongue reveals the anterior apex and lateral margins on the underside, while the central part at the root is seen to be fused to the floor of the mouth over a wide area.
The mandibular denture does not rest on a large bony foundation, unlike the support provided by the palate in the maxilla. Owing to the small area available (only the alveolar ridge with the dorsally positioned trigones), the retaining effect is also considerably reduced.The bony foundation in the mandible is highly variable in its atrophied forms. From a relatively well-developed, high, sharp alveolar ridge to a totally flat alveolar ridge that may even lie inferior to the floor of the mouth, all forms are possible.
As a result of resorption of the alveolar processes due to tooth loss, the maxillary alveolar ridge line is narrowed because the ridges are resorbed in the direction of inclination. The setup of teeth for complete dentures can interfere with the statics. The gap from the vestibular fornix to the ridge line will vary in size, depending on the degree of bone reduction. Where the ridges are highly developed, there may be vestibular undercut areas that are suitable as mechanical retentions for the denture base. There is usually a firm, immobile, poorly compressible mucosa on the rounded alveolar ridge.The mobile mucosa of the cheek and lip area extends in the arch as far as the
alveolar process and forms the vestibular fornix.
The whole of the palatal area can be occupied by a denture base wherever there is a bony support underneath. In the area of the palatal folds, the palatal mucosa is firm and, without submucosa, is fused to the bone over a wide area; it is permeated by a latticework of collagen fibers and elastic fibers, which gives the palatal mucosa a high degree of strength and deformability.