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Mastication is the first phase of the digestive process. It serves to reduce the size ofsolid food and to mix it with saliva so that it can be swallowed. The degree to which food is reduced in size depends on various factors, such as the masticatory force generated by the jaw muscles, the number and form of the (pre-)molars, the neuro-muscular control of the movements of the mandible and the quantity and composition of the saliva. If the mastication is to operate efficiently, form and function have to be properly balanced. Disturbances in function and abnormalities inform can lead to complaints and are often accompanied by a diminished fragmentation of the food. By investigating the fundamental mechanisms of the functional process, abnormalities can clinically be better understood.

Premature occlusal contacts may force the mandible into a not optimally functional intercuspal position, a so-called forced bite. When the mandible is forced laterally, it is called a lateral forced bite, more prevalent in children than in adults. In the PhD thesis (1983) 'Mandibular movement patterns: a methodological and clinical investigation of children with a lateral forced bite', open-close-clench cycles had been studied in 12 children with a lateral forced bite and in a control group of 6 children. Their mandibular movements were registered using an opto-electronic registration technique. Only in the children with a lateral forced bite, the mandible appeared to be displaced laterally into the direction of the forced bite side both during cycle series into intercuspal position and, although to a lesser extent, when occlusal contact was eliminated using flat occlusal splints. Apparently, in children with a lateral forced bite the neuromuscular co-ordination of the temporomandibular joint is disturbed. Subsequent research projects demonstrated similar findings. Consequently, a lateral forced bite in children should be eliminated as early as possible, in order to enable normal growth and development of the stomatognathic system.

In a pilot study, the masticatory function of patients with juvenile idiopathic arthritis was studied. The chewing efficiency and maximum bite force were measured in five adult patients and compared with a control group consisting of healthy individuals. The chewing efficiency of the patients with juvenile idiopathic arthritis was statistically significantly compromised compared to that of the control group. The maximum bite force was not statistically significantly smaller within this small group. The results of this pilot study support the hypothesis that the masticatory function of patients with juvenile idiopathic arthritis is compromised. We concluded that a more extensive study is necessary to investigate the masticatory function of patients with juvenile idiopathic arthritis and to evaluate the consequences with regard to the quality of life.

Bruxism (tooth grinding and clenching) is generally considered a contraindication for dental implants, although the evidence is usually based on clinical experience only. So far, studies to the possible cause-and-effect relationship between bruxism and implant failure do not yield consistent and specific outcomes. This is partly due to the large variation in the technical and the biological aspects of the investigations. Although there is still no proof that bruxism causes overload of dental implants and their suprastructures, a careful approach is recommended. Practical advices as to minimize the chance of implant failure are given. Besides the recommendation to reduce or eliminate bruxism itself, these advices concern the number and dimensions of the implants, the design of the occlusion and articulation patterns, and the use of a hard nightguard.

One of the functions of the masticatory system is to prepare food for swallowing by crushing it into small pieces to be moistened with saliva. The degree of fragmentation of the food particles depends on factors like the chewing force generated by the closer muscles, the jaw movement, and the morphological aspects of the teeth. The jaw movement is the result of a precise neuromuscular control of the various chewing muscles. Experimental research showed that the muscle activity needed to crush the food particles exists of two components: an anticipating component and a component evoked by the food resistance. The anticipating muscle activity is observed only if food resistance is expected. The muscle activity evoked by the food starts on average 25 ms after food contact. The amount of peripherally induced muscle activity linearly increases as a function of the food resistance. This part of the muscle activity is controlled by sensory information of the masticatory system.

Studies focussed on the interactions between masticatory function and the variation in craniofacial height have received an important impulse by the availability of non-invasive imaging techniques like CT and MRI. These techniques allow for in vivo determination of the cross-sectional area and spatial orientation of the human jaw muscles. In recent MRI studies it has been established that the jaw muscles of long-face subjects are up to 30% smaller than those of normal individuals, while the position of the muscles was fairly comparable in both groups. The maximum bite force of long-face subjects is roughly half that of normals. The observed variation of the in vivo data of normal and long-face jaw muscle geometry does explain only half of the difference in their average maximum bite force. The jaw muscles of long-face and normal subjects are presumably different with respect to their force generating capacity per unit of cross-sectional area, which may be attributed to a different muscle fiber type composition.

The temporalis and masseter muscles have a complex architectural design with large attachment areas. As a consequence, each of these muscles is capable of producing a large number of mechanical effects. In addition, within each muscle the muscle fibres and sarcomeres undergo different excursions during jaw movements. This leads to intramuscular differences in the possible excursion range and the capacity to produce force. Depending on the desired motor task, the nervous system is able to vary both the magnitude and direction of muscle force by selective activation of specific muscle portions. The anterior temporalis and the superficial and deep masseter are capable of generating large forces, e.g. during biting and chewing. The posterior temporalis and the anterior and posterior deep masseter are suited to establish a precise adjustment of forces and movements during laterotrusion and protrusion/retrusion.

The masticatory muscles are able to produce forces. These forces may cause movements of the lower jaw. Furthermore, they can be applied by the teeth for the generation of bite or chewing forces. During these kind of processes the temporomandibular joints will be loaded also. The interaction between forces and movements in the masticatory system is complex but obeys the relatively simple laws of mechanics. By application of these laws the development of joint loading, force patterns and movements during masticatory function and dysfunction can be understood. This is illustrated by a few examples of both statical and dynamical masticatory performance.

The masticatory muscles in human beings and in other mammals show a number of specific adaptations. Their muscle fibres contain at least four different isoforms of myosin heavy chain (MHC) and many fibres express more than one kind simultaneously. This implicates a continuous range of fibre contraction speeds for these muscles. Most or all fibres have a high oxidative capacity and consequently are very resistant against fatigue. The fast muscle fibres of the masticatory muscles appear to have smaller calibers than the slow ones. This condition is particular and points to mild atrophy. The motor units of the jaw muscles are characterised by large fibre numbers per unit, but concentrated into small subvolumes of muscle. Because of the anatomical heterogeneity of the muscles, motor units are capable of generating quite a variety of force directions. The fibres of jaw muscle motor units often belong to different fibre types, as far as their MHC-composition is concerned. For this reason, the units cannot be subdivided into clear-cut types, but show a continuous range of contraction times.

The anatomical design of the jaw-closing and jaw-opening muscles is of importance for their capability to produce forces and displacements. The length of the sarcomeres is a major determinant for both force and shortening range. The maximal work, force and shortening range each muscle is capable of producing, are proportional to the architectural parameters volume, physiological cross-sectional area and fiber length, respectively. Compared to the jaw openers, the jaw closers are characterized by shorter sarcomeres at the closed jaw, larger masses of contractile and tendinous tissue, larger physiological cross-sectional areas, shorter fibers and shorter moment arms. In addition, architectural features vary across the muscles of the same functional group and in some muscles significant intramuscular differences are found. In general, the jaw closing-muscles have architectural features that suit them for force production, whereas the jaw-opening muscles are better designed to produce velocity and displacement.

Masticatory movements in man are almost cyclic and mainly vertical. The precise movements depend on many factors, such as the amount of food, its consistency and the morphology of the occlusal planes. In normal function chewing forces range from 10 to 50 N. For solid food the chewing performance can be characterized by determining the particle size of the comminuted food. Chewing movements and chewing forces result from a coordinated pattern of muscle activities. The basic pattern results from a central pattern generator in the brain stem. The control of the precise timing and intensity of the muscle activity necessary for commuting the food is a result of anticipation and of sensoric information from the masticatory system. During chewing the tongue plays an important role in transporting the food.

The problems involved in the loss of anterior teeth differ essentially from those in the premolar/molar region. Firstly, missing anterior teeth result in unacceptable changes in appearance and speech, making treatment mandatory. Secondly, the dentist has to consider specific aspects, such as aesthetics, resorption of the alveolar bone, loading of the abutment teeth, form of the clinical crown and incisal guidance. Treatment options comprise orthodontic closure, periodontally supported removable and fixed appliances and appliances supported by implants. The advantages and disadvantages of the different options are indicated.

The masticatory system has the capability to produce biteforces in many directions, the magnitude of the maximum possible biteforce varies with the biteforce direction and depends on several parameters, such as the length-tension relationship, the physiological cross-section and the orientation of the jaw muscles, the shape of the temporomandibular joint and the location of the bite point. The loading of the left and right temporomandibular joint varies, among others, with the bite-force direction.