Functions and Reactions of the PDL
- Mechanisms of tooth support
- Influence of occlusion on the periodontium
- Bruxism and occlusal therapy
Mechanisms of tooth support
The relative importance of the principal fiber groups, the vascular elements, and the ground substance of the PDL in mediating tooth mobility and providing support has been a source of considerable discussion. Three hypotheses have become the center of discussion.
 Fig 2.7 Horizontal displacement of a maxillary central incisor of a Rhesus monkey. Force of 0.5 N applied for 2 sec. Adapted from Muhlemann30. |
The first of the three most recent hypotheses can be termed the tensional mechanism of tooth support. The initial low resistance portion of the force-displacement curve has been associated with the gradual unfolding of the fiber bundles before taking up tension. (Fig. 2-7) In the second, slower phase, the fiber bundles straighten, transmitting forces to the alveolar bone. In the final phase of movement, the straightened fibers deliver increasing load to the alveolar bone which is then transmitted to the basal bone of the jaws.47 Most investigators would now agree that a fiber tension theory is insufficient to explain all of the available experimental evidence. |
The second type of hypothesis would consider the PDL to act as a viscoelastic system. Loading of a tooth is accompanied by extracellular fluid passing from the PDL into the vascular spaces of the marrow through tiny holes in the wall of the alveoli. The rate of fluid passage is dependent upon the rate of loading. A second phase of tooth motion occurs as the result of the fiber bundles absorbing the slack and tightening, thereby leading to constriction of the vessels of the PDL. This constriction leads to arterial back pressure, which, in turn, replenishes the extracellular fluid.
The third hypothesis is somewhat radical compared to the others. This hypothesis considers the PDL to be a collagenous thixotropic gel and that periodontal fibers are histologic artifacts.48,49
Although the tensional mechanism of tooth support is clearly inadequate, both the viscoelastic and thixotropic viewpoints can explain the present experimental data concerning the force-displacement characteristics of teeth. Most investigators prefer the viscoelastic theory since a radical change in our ideas concerning the structure and function of the PDL would be necessary if it is thixotropic.
Influence of occlusion on the periodontium
The structural and metabolic stability of the periodontal ligament and alveolar bone depend on the mechanical stimulation of occlusal forces. With increased functional demand the periodontium endeavors accommodation. The adaptive capacity varies in different persons and in the same person at different times. The effect of occlusal forces on the periodontium is influenced by their magnitude, direction, duration, and frequency.50 An increase in the magnitude of occlusal force leads to a widening of the PDL.
The duration and frequency of occlusal forces affect the bone response. Constant pressure causes resorption, whereas intermittent force favors bone formation. Recurrent forces over short time spans have a similar resorbing effect as constant pressure.50 When occlusal forces exceed the adaptive capacity of the tissues, injury results.51-53 This injury is called trauma from occlusion. The histologic characteristics of the lesion have been well documented. An occlusion that produces such injury is called a traumatic occlusion.50
Trauma from occlusion may be caused not only by alteration in occlusal forces but also by the reduced capacity of the periodontium to withstand occlusal forces or by a combination thereof. Primary trauma from occlusion exists when the alteration in the periodontium is caused solely by occlusion. A high restoration or orthodontic movement of teeth into functionally unacceptable positions would be common examples. Secondary trauma from occlusion occurs when the adaptive capacity of the periodontium has been reduced by previous bone loss or systemic disorders.50
The influence of trauma from occlusion on the initiation, progression, and management of periodontal disease is controversial. Most experts agree that primary occlusal trauma produces morphologic alterations in the PDL and alveolar bone. Furthermore, these alterations are reversible when either the force is discontinued or the tooth moves away from the influence of the force. Moreover, these changes occur without affecting the level of connective tissue attachment, and pocket formation is not initiated. 54-60 The probable explanation for failure of trauma from occlusion to initiate periodontal pockets is the presence of the supracrestal fibers that prevent the apical migration of the junctional epithelium.61
Clinically, it is not possible to determine whether the histologic lesions of occlusal trauma are or have been present. The clinician must use the presence of increased mobility and radiographic evidence of a widened PDL. These signs in conjunction with the absence of periodontal pockets indicate the probable existence of traumatic forces. If the magnitude of the mobility is constant, adaptation by the periodontium to the increased force has likely occurred. If tooth mobility is progressively increasing, or patient function and comfort are compromised, occlusal adjustment or fixed splinting is indicated.62
Whether occlusal trauma accelerates the progression of established marginal periodontitis remains controversial. Numerous investigations have concluded that inflammation and occlusal trauma are codestructive factors in initiating and perpetuating periodontal breakdown, and superimposition of trauma on existing periodontitis accelerates periodontal destruction, resulting in angular, infrabony pockets.53,63-66
Human studies agree with animal investigations in regard to the management of periodontal disease through the resolution of inflammation.61,62,67-73 There is also supporting evidence for bone regeneration once inflammation has been controlled even if hypermobility persists.74-76 And in spite of persisting hypermobility, there is no further breakdown of the attachment apparatus.77-80 Moreover, increased tooth mobility does not alter the connective tissue attachment level during healing after pocket elimination.81
In summary, control of inflammation is the primary requisite in the management of periodontitis alone or in combination with occlusal trauma. Any residual mobility, whether due to just bone loss or combined with secondary trauma from occlusion, will not initiate further periodontal breakdown. The management of hypermobility is therefore based on preventing increasing mobility and change in tooth position as well as retaining the patient's masticatory function and comfort.66,83
Bruxism and occlusal therapy
Given the current controversy involving trauma from occlusion and periodontal destruction, the relative importance of bruxism and occlusal therapy is also contended. Some researchers and clinicians have concluded from the studies of occlusal trauma in association with periodontal disease that after inflammation has been controlled, any residual tooth hypermobility only needs management if the patient's masticatory function or comfort is impaired. They feel that few cases eventually require some form of occlusal management for successful treatment of marginal periodontitis.62 Others have concluded from the literature and clinical observation that only extremely traumatic situations will lead to an increase in the rate of periodontal destruction and the formation of intrabony defects. Otherwise, adaptation to the excessive load will take place without any effect on the spread of inflammation toward the deeper periodontal structures.82 Many clinicians conclude that until the controversy is settled, it seems prudent to adjust the occlusal forces on teeth that have moderate to advanced periodontal bone loss.83
Unless an occlusion has been adjusted or restored, occlusal interferences are always present. It is not possible with our present knowledge to predict when occlusal interferences will produce injury to the supporting tissues of the teeth.82 Individuals with minimal psychic tension or a high capacity for a adaptation will generally suffer no pathologic effect from interferences.82,84,85
As previously discussed, the occlusal contacts resulting from bruxism are of increased frequency and duration compared to functional contacts.27-29 It has been observed that bruxism can produce an almost uninterrupted, mainly horizontal load over periods of minutes and several times during a night's sleep.25 Although forces of short duration are well tolerated by the tissues, the forces generated during bruxism may exceed the adaptive capacity of the periodontium and result in breakdown. Occlusal adjustment has been shown to decrease the duration of tooth contacts and increase the interval between contacts.25
A review of the literature and clinical observation may result in the following conclusions82:
- Periodontal tissues may be affected simultaneously by inflammatory changes and traumatic changes.
- Unless a traumatic injury is diagnosed, the occlusion should not be altered as part of the periodontal treatment. There is no place for prophylactic treatment of occlusion.
- Occlusal adjustment, occlusal bite splints, orthodontic treatment, and restorative dentistry are all means to treat occlusion.
- Occlusal treatment is generally performed after control of the inflammatory periodontal changes by root preparation and oral hygiene.
- The aim of occlusal therapy should be the establishment and maintenance of stable occlusal relationships and restoration of an optimal occlusal function.
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