TYPES OF OCCLUSAL FORCESThe reactio

TYPES OF OCCLUSAL FORCES
The reaction of the bone and ligament depends on the magnitude, duration and direction of the forces. Different types of occlusal forces can be recognised:
• Physiologically normal occlusal forces in chewing and
swallowing: small and rarely exceeding 5 N. They provide the positive stimulus to maintaining the periodontium and the alveolar bone in a healthy and functional condition.
• Impact forces: mainly high but of short duration. The periodontium can sustain high forces during a short
period; however, forces exceeding the viscoelastic buffer capacities of the periodontal ligament will result in fracture of tooth and bone.
• Continuous forces: very low forces (for example, ortho-dontic forces), but continuously applied in one direc-
tion are effective in displacing a tooth by remodelling the alveolus.
• Jiggling forces: intermittent forces in two different
directions (premature contacts on, for example, crowns, fillings) result in widening of the alveolus and in increased mobility.


TRAUMA FROM OCCLUSION
Trauma from occlusion has been defined as structural and functional changes in the periodontal tissues caused by excessive occlusal forces. Some of these changes are adaptive, while others should be considered pathological. Occlusal trauma can be acute if caused by external impact

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CLINICAL PRACTICE AND THE OCCLUSION

















ABc
Fig. 9.4 Clinical example of a primary occlusal trauma in a young girl with a deep bite and a persistent deciduous canine. A Frontal view showing the deep bite. B Palatal view, with the primary trauma on the palatal mucosa caused by the lower canine. When the patient closes, the lower canine is forced in between the upper canine and the central and lateral incisors. CThis traumatic occlusion causes the widening of the periodontal space, as seen on the radiograph.



forces or chronic if caused by internal occlusal factors (premature contacts, grinding). Chronic occlusal trauma can be understood as primary and secondary trauma.
Occlusal trauma is the overall process by which traumatic occlusion (that is, an occlusion that produces forces that cause injury) produces injury to the attachment apparatus.

Primary occlusal trauma
Primary occlusal trauma is caused by excessive and non-physiological forces exerted on teeth with a normal, healthy and non-inflamed periodontium. The forces may be exerted on the periodontal structures in one direction (orthodontic forces) or as 'jiggling' forces.

Forces in one direction: orthodontic forces
Forces in one direction cause tipping of the tooth in the opposite direction or tooth displacement parallel to the force resulting in a 'bodily movement'.
In the periodontal ligament, zones of compression and zones of tension are found, inducing increased resorption. The clinical result is a (temporary) increased mobility. However, there are no changes in the supracrestal fibres, no loss of periodontal attachment, or an increased probing pocket depth. The increased tooth mobility is functional adaptation to the forces exerted on that tooth. If the forces are too high and above the adaptation level, an aseptic necrosis in the tension zone of the periodontal ligament occurs, characterised by hyalinisation. In the compression zone, pressure stimulates osteoclasts in the adjacent bone and the alveolar wall is resorbed until a new connection is formed with the hyalinised bone ('undermining resorption'). In the tension zone, bone apposition and rupture of the collagen fibres occur. After removal of the force the perio-dontal ligament is reorganised and after some time

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develops a normal histological appearance. If the applied forces are too high, root resorption occurs in the middle of the hyalinised tissues. This resorption continues for a variable time, resulting in shorter roots, frequently seen after orthodontic treatment.

Jiggling forces
Jiggling forces, coming from different and opposite directions, cause more complex histological changes in the ligament. Theoretically the same events (hyalinisation, resorption) occur, however, they are not clearly separated.
There are no distinct zones of pressure and tension. Histologically, there is apposition and resorption on either side of the periodontal ligament, resulting in a widening of the periodontal space (Fig. 9.4). This may be observed on radiographs. This phenomenon explains the increased mobility without pocket formation, migration and tipping.
The clinical phenomena are not only dependent on the magnitude of the forces, but also on the crown-root relationship, the position in the arch, the direction of the long axis, and the pressure of tongue and cheek muscu-lature (Fig. 9.5). The interarch relationship (for example, deep bite) influences the extent of the trauma caused by jiggling forces. The hypermobility is found as long as the forces are exerted on the tooth: there is no adaptation. Hypermobility is therefore not a sign of an ongoing process, but may be the result of a previous jiggling force.
The long-term prognosis of teeth with increased mobility is poor, and is a complicating factor if they are used as abutment in prosthodontic reconstruction.
Successful periodontal treatment leads to healthy but reduced periodontal structures. Jiggling forces exerted on the teeth in this condition result in a pronounced increase in tooth mobility because the point of rotation (fulcrum) is closer to the apex than normal. This is uncomfortable for
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Fig. 9.5 Under jiggling forces in a healthy periodontium, the periodontal ligament space is widened, resulting in more tooth mobility but not in marginal bone resorption or attachment loss.



the patient and might be an indication for splinting of teeth (Fig. 9.6).

Secondary occlusal trauma
Secondary trauma from occlusion is defined as the trauma caused by excessive and premature occlusal forces on teeth with an inflamed periodontium. A number of animal experiments and clinical epidemiological studies investigated the role of occlusion in the pathogenesis of periodontitis. In his original studies in the 1960s, Glickman (Glickman & Smulow 1967) formulated the hypothesis that premature contacts and excessive occlusal forces could be a co-factor in the progression of periodontal disease by changing the pathway and spread of inflam-mation into the deeper periodontal tissues. Glickman hypothesised that the gingival zone was a 'zone for irritation' by the microbial plaque; the supracrestal fibres were then considered to be a 'zone of co-destruction' under the influence of a faulty occlusion (Fig. 9.7).