123 Healthy dentition
Left: Microscopic view of the histology of a periodontal space: cementum (1), collagen fibers (2), alveolar bone (3).
Right: Simultaneous tooth contact is the foundation for neuromuscular guidance. The mechanoceceptors in the periodontium react most to lingual loading of the tooth (Trulsson et al. 1992). The reaction and adaptation of a mechanoreceptor depends among other things, upon the speed of the mechanical stimulus (Linden and Millar 1988b, Linden et al. 1994, Millar et al. 1989).
124 Incisors, premolars, molars
Left: Relation of the incisors in protrusive position. The periodontal membranes of the incisors have the highest concentration of mechanoreceptors. Furthermore, their threshold of stimulation is significantly lower than that of the molars.
Center: Extensive abrasion is not necessarily indicative of occlusal trauma (Green and Levine 1996).
Right: Apparently intact occlusal surfaces are no guarantee of physiological tooth loading.
125 Excessive attrition
Advanced attrition can result in specific changes in the distribution and micro morphology of the mechanoreceptors (Sodeyama et al. 1996) and suppress the guiding and protective mechanisms of periodontal perception.
Condylar Positions
The centric condylar position has been a subject of controversy in dentistry for many decades. However, if one considers mainly the anatomical and neurophysiological fundamentals and relegates the artificial, mechanistic concept of a rigid, unalterable condylar position to the background, the problem becomes less confusing. According to our current state of knowledge, four basic condylar positions can be distinguished: centric, habitual, therapeutic, and adapted. Of these, only the centric condylar position is not necessarily dictated by the occlusion.
Centric condylar position describes the ideal arrangement of a temporomandibular joint (van Blarcom 1994, Lotzmann 1999). Because many deviations from this ideal condition are seen in everyday practice, Dawson (1995) introduced the term adapted condylar position. This applies to all fossa-disk-condyle relationships that deviate from the ideal norm within the individual’s range of adaptation. These condylar positions are the ones most frequently encountered. The habitual and therapeutic condylar positions will be discussed in more detail in later chapters.
126 Centric condylar position
This is defined as the anterosuperor position of both condyles in a physiological condyle-disk relationship with no lateral shift and physiological loading of the involved tissues. The centric condylar position is totally independent of the occlusion of the patient’s teeth. Therefore it can be determined clinically only while there is no tooth contact. Correct anterosuperior positioning of the condyles is achieved only through the patient’s neuromuscular system. Any manipulation will produce a deviation.
127 Habitual condylar position
This is the position usually assumed by the condyle on the articular eminence. The habitual condylar position is determined exclusively by the patient’s static occlusion with no influence from the fossa or the position of the disk. In an ideal case, the habitual condylar position would coincide with the centric condylar position. For its clinical determination and its differentiation from the adapted condylar position, see pages 125ff and 206ff.
128 Therapeutic condylar position
The therapeutic condylar position is a position selected arbitrarily by the dentist to correct a problem. The purpose may be to re-establish the functional unity of an anteriorly displaced disk and its condyle or to temporarily relieve the pressure on the bilaminar zone within an inflammed capsule. Usually this is accomplished by inserting a splint with indentations for the opposing molar cusps and retrusion planes in the premolar and canine regions.
Static Occlusion
Because the condylar positions are primarily independent of the occlusion, there are analogous definitions for the corresponding tooth contacts. Occlusion itself is defined as “every contact of teeth of the maxilla with those of the mandible” (van Blarcom 1994, Lotzmann 1999). Static occlusion means tooth contact with no movement of the mandible. Static occlusions can be divided into centric, habitual, and maximal occlusion. Centric occlusion is defined as those tooth contacts that occur with the condyles in centric condylar position. The term can apply to one single contact (premature contact) or maximum contact at many points. Habitual occlusion is the static occlusion the patient usually assumes. It determines the habitual condylar position. In some cases the habitual occlusion may be the same as centric occlusion. In this situation the habitual condylar position would likewise be the same as the centric condylar position. Maximal occlusion describes a static occlusion in which there is the maximum number of contacts. The number of contacts aimed for in treatment depends upon which concept of occlusion is utilized.
129 Centric occlusion
The traditional definition of centric occlusion has been expanded to include not only maximum intercuspation, but also any tooth contacts that the patient can make with the condyles in centric position (left).
130 Habitual occlusion
This indicates the most commonly assumed tooth contacts. These can be many or, with a malocclusion, few. Habitual occlusion determines the habitual condylar position. With the teeth in habitual occlusion the condyles may be displaced in the sagittal plane either posteriorly (yellow) or anteriorly (gray) from the centric condylar position. Lateral displacement is also possible. All the joint structures may adapt successfully to this condition.
131 Maximal occlusion
This describes the presence of a maximum number of contact points. The concept of maximal occlusion is not dependent upon the position of the condyles. Closing into maximal occlusion may displace the condyles along any of the three spatial coordinates. Ideally, however, maximal occlusion would occur simultaneously with centric condylar position (gray). There is no compelling medical reason to treat a condylar shift in maximal occlusion if the joint structures are well adapted.
Dynamic Occlusion
The term “dynamic occlusion” covers all tooth contacts that occur during movements of the mandible (Lotzmann 1981, van Blarcom 1994). The term “articulation” is now obsolete. There are three important concepts for eccentric occlusion:
• Anterior/canine protected occlusion: The incisors and canines provide disclusion of all other teeth. Anterior/canine guidance is the occlusal concept most easily achieved clinically and in the laboratory.
• Unilateral balanced occlusion (group function): guidance on all the teeth of the laterotrusive side with disclusion of the opposite side.
The