Fig. 2.6 Horizontal displacement of a human maxillary central incisor after sudden application and removal of a 0.05 N horizontal force. (Adapted from Berkovitz et al12, p. 271).
There is no disagreement concerning the role of the PDL in providing support for the teeth, but there is considerable controversy involving the mechanism by which this function is accomplished. The precise sequence of events that occurs in the PDL when force is applied to a tooth is not known. Before the possible mechanisms of tooth support are discussed, an examination of the source and nature of axial and horizontal loads withstood by the PDL is necessary.
Five muscles are concerned with mastication: (1) the temporal, (2) the masseter, (3) the medial pterygoid, (4) the upper lateral pterygoid and (5) the lower lateral pterygoid. They act in all planes to create the forces necessary to resist gravity, close the jaws against the resistance of food, and maintain tonus and position. They are controlled by the fifth cranial nerve. The actions of these muscles are involved in almost every functional movement of the stomatognathic system. They are directly responsible for application of stress to the facial skeleton. The muscles that control the movements of the mandible and soft tissues of the orofacial system are complex and interrelated. Some muscles, such as the masseter muscle, act primarily as mandibular elevators. Others, such as the lateral pterygoid muscles, act as mandibular positioners. Other muscles have multiple functions; the temporal muscle, for example, has elevating and positioning functions.
The highest masticatory forces are generated when the maxillary and mandibular teeth are in contact. Normally these periods of tooth-to-tooth contact are very short, lasting about 100 milliseconds during chewing strokes and longer during swallows. These periods add up to 15 to 30 minutes of actual contact loading each day. However, those individuals with parafunctional habits, such as bruxism, may subject the teeth, periodontal ligament (PDL), alveolar bone, muscle attachments, and TMJ to loads of up to 11 N for several hours per day. It is this type of abuse that results in degenerative changes.
During mastication, occlusal contacts occur on almost every chewing cycle once the bolus has been chewed through.16 The functional range of occlusal contacts includes frequent contacts in maximum intercuspation or habitual occlusion and lateral gliding in both the initial opening and final closing phases of the masticatory stroke. Due to the nature of these contacts, both axial and horizontal loads are produced during chewing. This combination of loads produces tooth movement in all directions.17,18
The functional forces generated during mastication have been measured using transducers incorporated within fixed and removable prostheses. These devices allow measurements of the three components of the force vector.19,20 In humans, maximum biting forces of 800 N** have been measured in the molar region, and 100 to 200 N has been measured in the incisor region. Most investigators have shown higher values for men than women, but this sex difference is less than might be expected from their difference in general muscle force. At 5 years of age, children show a maximum bite force of 400 N.21 Higher levels of bite force have been recorded on those who brux than for those who do not brux.22 The state of the dentition also effects maximum bite force. Individuals with fixed prostheses have similar values as individuals with a full natural dentition. Complete denture wearers have values that are one third to one fourth of those with a complete natural dentition. Patients with implant borne dentures have a stronger maximum bite force than those with conventional complete dentures.21
Occlusal forces that occur during mastication are considerably lower than maximum bite forces. Maximum axial loads of 70 to 150 N have been recorded during chewing and swallowing a variety of foods.23,24 In most instances, chewing forces do not exceed 10N.
Axial loads also may be applied to teeth during swallowing, speech, clenching, and bruxism. Although the total cumulative time of daily tooth contact is relatively small (approximately 18 minutes, with 9 minutes due to swallowing25), significant contribution to loading of the periodontium can occur. It has been shown that axial forces generated during swallowing are in the range of 70 to 150 N.26
In contrast, the frequency of tooth contacts for bruxers showed four times the number of contacts during 8 hours of sleep compared with nonbruxers.27 Studies on the duration of contacts in bruxism show a longer time of tooth contact. The mean duration of bruxism per hour of sleep was 40 seconds, with an average contact duration of 9 seconds.28,29 Adequate studies involving tooth contacts during speech are not available.
| Regardless of the direction of loading, the force-displacement relationship for a tooth is nonlinear and time dependent (Fig. 2-6). Initially, resistance to loading is minimal, but gradually increases with time and load level. The response of the PDL to loading is viscoelastic in nature: there is an inverse relationship between rate of loading and displacement; the rate of recovery is directly related to loading rate and indirectly related to duration of load application. | Fig. 2.6 Horizontal displacement of a human maxillary central incisor after sudden application and removal of a 0.05 N horizontal force. (Adapted from Berkovitz et al12, p. 271). |
The nature of the force-displacement relationship for horizontal loads is similar to that described for axial loads. However, the magnitude of the movement for axial loads is less.30,31 A horizontal load of 1N applied for 2 seconds to a maxillary incisor leads to a displacement of approximately 150 mm. The same loading pattern in an axial direction would lead to a 15 to 20 mm displacement.
Tooth mobility is defined as movement of a tooth in a horizontal or vertical plane of space. All teeth have some degree of mobility related to the width of the periodontal ligament, root attachment area, elasticity of the alveolar process, and function of the tooth.30,32-34 Mobility must be analyzed not only in terms of being physiologic or pathologic but also in terms of being reversible or irreversible.
Teeth normally have a certain range of mobility. Single-rooted teeth are more mobile than multirooted ones, and incisors have the most mobility. These ranges have been measured in subjects with healthy periodontal conditions using various periodontometry instruments.35,36 Within individuals, the mobility of a single tooth may vary from day to day or even hour to hour.37 Mobility is normally greatest in the morning immediately on arising from sleep, and is least late in the day after the teeth have received considerable functional forces from mastication, swallowing, and fleeting tooth contacts. The upper physiologic range of horizontal tooth mobility is 0.15 mm.38
Mobility beyond this physiologic range is termed abnormal or pathologic. It is pathologic in the sense that it exceeds the limits of the normal mobility values rather than that the periodontium is necessarily diseased at the time of examination. Since mobility measurements via periodontometry are rather cumbersome, most clinicians use the method described by Miller to assess this parameter.39 The tooth is held firmly between two instruments and moved back and forth. Mobility is scored from 0 to 3. A score of 0 means no detectable movement. A score of 1 indicates barely distinguishable tooth movement. A score of 2 is any movement up to 1 mm, and 3 is given to teeth with any movement more than 1 mm or teeth that can be depressed or rotated in their sockets. A recent clinical evaluation of the Miller index showed that periodontists scored as a 2 mobility a tooth that moved approximately 0.5 mm, and a score of 3 was approximately 0.75 mm.40 These authors, therefore, suggested that the Miller index be modified accordingly.
Increased tooth mobility may be caused by a variety of factors, including pregnancy, disease status (local and systemic), trauma (including orthodontic movement), and hypofunction or hyperfunction.30,32 The two factors most often seen would be plaque induced inflammatory disease and excessive occlusal forces.
Clinically, mobility might best be thought of as either reversible or irreversible.41 Reversible mobility would be that mobility due to an abnormal force or inflammation, the removal of which results in elimination or reduction of the mobility. Irreversible mobility would be that mobility due to a reduction in periodontal support that could not be resolved short of splinting.
Periodontal receptors play a role in the discrimination of size, shape, texture, and hardness of foods and other objects placed in the mouth. These receptors are also involved in motor reflexes that affect mastication and swallowing.42 The nerve endings in the PDL, gingiva, and periosteum of the alveolar bone range from simple nonspecialized endings to Meissner-like structures and encapsulated bodies with and without myelinated fibers.
Periodontal mechanoreceptors can precisely discriminate between forces of different magnitude. Incisors can detect forces of 0.01N or less, whereas the threshold for molars is 0.1N. Force discrimination is better at lower than higher levels.43,44 Periodontal afferents also show directional sensitivity. The maxillary incisor responds mainly to incisally or labially directed forces, whereas the canine responds most to mesially directed forces.45 Directional sensitivity may be related to the direction of occlusal forces. It has been suggested that mechanoreceptors of the periodontium play a role in limiting maximum forces developed during mastication by inhibition of elevator and excitation of depressor motoneurons.46