Biological adaptation and normative values.

The occlusal interface is often considered in terms of a structural entity, and to reflect several structural components of the masticatory system: alveolar and cranial bone, the tem-poromandibular joint (TMJ) including its disc and associated ligaments, the dentition (both pulp and periodontium), and the masticatory muscles (including ligaments and tendons and other connective tissues as well as contractile skeletal muscle fibers). However, a number of intrinsic physiologic processes act upon the masticatory system and collectively determine its parameters of function and thereby the occlusal interface. As shown in the working schema (Fig 1) provided for discussions at the symposium, these physiologic determinants include cardiovascular, endocrine, immunologic, metabolic, and neural (both peripheral [eg, receptors, nerve fibers] and central nervous system [CNS]) influences. 1–6 These intrinsic determinants are themselves acted upon by an array of environmental and genetic factors that thereby influence the function of the masticatory system. Environmental influences include mechanical stress and trauma, disease, nutrition, and " life events " associated with occupational loading of tissues, psychologic stress, socioeconomic conditions, etc. Genetic factors encompass gene-based determinants of development and aging, gender , and homeostatic processes. As noted in Fig 1, some of these factors (eg, mechanical stress, trauma) can act directly on components of the masticatory system. Most times, however, these determinants act indirectly through their influence on the intrinsic processes; they can also influence each other (eg, genetic abnormalities resulting from environmental insults). The masticatory system has a remarkable capacity to adapt to these various influences, and it has considerable functional reserve. Figure 1 outlines some examples of such " positive " responses to mechanical loading, infection, or trauma of craniofacial tissues; they include TMJ remodel-ing, reparative dentin, a host of protective reflexes (eg, no-ciceptive jaw opening, airway maintenance, tongue-pro-trusion reflexes), and acute pain and associated reversible nociceptive changes in the CNS. But several factors have been demonstrated or invoked to be risk or predisposing factors that can compromise these adaptive responses, and as a consequence, the functional reserve of the tissues may be exceeded. The level of evidence, however, varies in the extent to which these factors do compromise the mastica-tory system. Nonetheless, such influences identified in the literature include aging, disease, genetic abnormalities, gender, parafunctions, psychologic stress and neurologic dysfunction, nutritional inadequacies, and macro-or mi-crotrauma and overloading of tissues. The threshold of the adaptive potential of the tissues may be exceeded and be associated with a number …