Horizontal position is secured in clasp dentures by the rigid parts of cast clasps, such as clasp bodies, shoulders, and upper arms. They secure the denture against horizontal displacement and twisting and, in interaction with the rigid denture frames, evenly transfer masticatory pressure to other segments of the dental arch. They produce a splinting effect for the abutment teeth.
Respiratory disorders with partial or complete displacement of the upper airways during sleep are associated with snoring noises that arise from vibration of the soft palate (Figs 11-28 and 11-29). Snoring is normally not harmful to health and does not require treatment. However, health may be impaired if respiratory arrest occurs for more than 10 seconds, despite breathing effort, because the upper airways are displaced. This arrest causes a decrease in oxygen saturation of the blood and triggers a waking reaction (Fig 11-30). Depending on the severity of this disorder, such respiratory arrest events followed by a waking reaction may occur between 5 and 40 times an hour (up to 100 times a night).The repeated waking reactions and interruption of sleeping phases cause increased drowsiness during the day. This condition is known as obstructive sleep apnea (OSA).
Progressive destruction of the masticatory system leads to functional disorders of the muscles, TMJs, and periodontal tissues
. The vertical distances and horizontal relationships of the jaws are altered. The masticatory system adapts to the function that has been altered by loss of teeth or occlusal surfaces, but the malocclusions usually lead to hyperactivity of the muscles of mastication, which means that the normal neuromuscular program is disrupted. This results in neuromuscular coordination problems with avoidance or adaptive programs, giving rise to muscle diseases (myopathies) that also cause diseased change to the joint tissues.
Repositioning splints and distraction (decompression) splints may be described as types of "eccentric splint" (ie, out of centric relation).
The Michigan splint was developed in the 1950s at the University of Michigan. It is a hard, transparent acrylic splint that covers all the maxillary teeth and has flat, occlusal centric stops for the mandibular teeth so that, on jaw closure, there are even and simultaneous occlusal contacts of the mandibular teeth on the splint surface. This centric relation splint can be fabricated in an adjustable articulator and is fitted with a canine guidance that has a sufficiently steep inclination to prevent both working and nonworking contacts as well as anterior guidance contacts.
Centric relation splints (or simply centric splints) initiate self-centering of the condyles in the fossae, as established via a centric record. With the physiologic (relieved) joint position, neuromuscular relaxation is achieved, a pain-free therapeutic position is created in the case of intra-articular disorders, or pretreatment is undertaken for extensive occlusal correction (eg, in an abraded dentition).
Joint diseases caused by disorders in dentition closure or stress-induced parafunctions can be treated with reflex splints, which interrupt the imprinted, pathologic movement pattern. This is done by raising the occlusal position and reducing muscle activity with a bruxism splint (see Fig 11-7). Neuromuscular diseases originating from psychologic stress and parafunctions can be treated with this kind of reflex splint. These are also used for acute joint pains as immediate treatment as well as for pretreatment prior to centric relation splints (to achieve centric capability).
Splint therapy, as a biomechanical form of treatment, is used for a variety of functional disorders of the masticatory system. The treatment is carried out with occlusal splints and is indicated for:
Diseases of the TMJs are mainly intra-articular disorders (Fig 11-3), with distinctions drawn between reversible microtrauma and irreversible macrotrauma and between congenital and acquired developmental disorders. Inflammatory microtraumas are joint noises (eg, clicking) occurring occasionally or constantly that result from rough joint surfaces caused by compression. If the compression is removed by means of a muscle relaxation splint (centric relation splint), the microtrauma is also eliminated.
The orofacial system includes the maxilla and the mandible with the sets of teeth, the masticatory muscles, the temporomandibular joints (TMJs), and the neuronal control with motor end plates. Functional diseases of this system develop as a result of multifactorial mechanisms. A functionally impaired masticatory system will display monotonous movement sequences, for instance, whereas variable movement patterns are seen in a healthy system (Fig 11-1).