Page images
PDF
EPUB

Oral Motor Behaviors and Oral Prostheses

George A. Zarb

Dentists have been successful in treating patients with depleted or absent natural dentition with various dental prostheses (partial or complete dentures, implants, maxillofacial prostheses). Such prosthodontic treatment is usually prescribed for functional as well as cosmetic purposes, although the patient's motivating force in seeking treatment is most often cosmetic. The mutilating effects of tooth loss are frequently accompanied by supporting, or alveolar bone loss, and since alveolar bone forms part of the facial skeleton, its loss is irrevocable. Several features which pertain to prosthodontic treatment must be emphasized: (1) There is a dearth of information of a biologic nature about the significance of the partially and completely edentulous state; (2) Prostheses, especially removable ones, almost invariably elicit unfavorable biologic changes in the oral milieu; (3) Prosthodontic management of the disability also involves a concern with oral motor behavior responses.

The objectives of this paper are to review (1) mechanisms of dental and prosthesis support and (2) biomechanical interactions of the masticatory system's components in the context of prosthodontic interventions and oral motor behavior.

Mechanisms of support

Figure 1 is a simple diagrammatic representation of the masticatory system, and is represented as the biomechanical interaction of three components: function/para function, individual adaptive responses and temporomandibular joints. Clinical reports suggest that these components are affected by both the accompanying changes in the mechanism of support when a patient becomes partially or completely edentulous (Boucher, Hickey and Zarb, 1975; Zarb, et al., 1978) as well as by the nature of the prosthodontic service per se.

[blocks in formation]

When a patient becomes partially edentulous, the depleted masticatory system is expected to go on functioning normally. Certain clinical sequelae will clearly ensue, but it is feasible for patients to function asymptomatically even with the most conspicuously depleted dentitions, at least for a short time. When a depleted dentition is restored using a fixed or a removable tooth-supported prosthesis the objectives of cosmetic and functional replacement are usually achieved and the vast majority of patients easily adjust to their prostheses. It appears that the long-term success of this achievement is predicated on the total residual area of periodontal ligament adequately supporting the load transmitted from the "restored intact" dentition. The stage at which a restored depleted dentition fails to adapt to the dictates of the required support, is still unclear.

When patients lose several posterior teeth, removable partial denture of the dentogingivally supported variety is usually the treatment of choice; and when all the teeth are lost, complete denture treatment is prescribed. When patients undergo extensive surgical tooth and supporting tissue removal for cancer surgery, maxillofacial prostheses are prescribed. The basic problem in all three situations lies in the realization that the mechanism of support for the forces generated by functional and parafunctional activities, is now either depleted, and/or being shared by the abutment teeth's periodontal ligaments, and by the mucoperiosteum covering the edentulous residual ridge area distal to the abutments, or by the mucous membrane in toto. The unsuitability of the edentulous segment or segments for load-bearing function must be recognized immediately In the dentulous state, only light loads are placed on the mucous membrane. With tissue-supported prostheses (partially or totally), the mucous membrane and the underlying bone are forced to serve the same purpose as the periodontal ligaments.

The area of mucosa available to receive the load from complete dentures is limited when compared to the corresponding areas of support available for natural dentition. Watt (1961) has computed the mean denture-bearing area to be 22.96 cm2 in the edentulous maxillae and 12.25 cm2 in the edentulous mandible. His estimate of the areas of periodontal membrane of natural teeth is approximately 45 cm2 in each jaw--more than three and one-half times the average area of the basal seat of a mandibular complete denture. The denture-bearing area (basal seat) also becomes progressively smaller as residual ridges resorb. Furthermore, the mucosa demonstrates very little tolerance or adaptability to denture wearing, and this minimal tolerance can be reduced still further by the presence of systemic disease or as a result of radiation therapy.

It appears then that the support for the complete denture is conspicuously limited in its adaptive ability as well as its inherent capability of simulating the role of the periodontium. The mechanism of support is further complicated by the fact that complete dentures move in relation to the underlying bone during function (Arstad, 1959; Woelfel, Hickey, and Alison, 1962; Sheppard, 1963; Smith et al., 1963). This movement is related to the resiliency of the supporting mucosa and the inherent instability of the dentures during function. Conclusions regarding denture stability are usually based on clinical experience,

but denture instability has the potential of being traumatic to the supporting tissues since movement of denture bases in any direction on their basal seats can cause tissue damage.

Biomechanical interaction of masticatory system's components--function and parafunction

[blocks in formation]

In healthy dentition, teeth are not in occlusion except during the functional movements of chewing and deglutition and during the movements of para function. Graf (1969) calculated that the total time during which the teeth are subjected to functional forces of mastication and deglutition during an entire day amounts to approximately 17.5 minutes (table 1). More than half of this time is due to jaw-closing forces applied during deglutition, and Graf concluded that this total time and the range of forces seem to be well within the tolerance level of healthy periodontal tissues.

Table 1. Calculation of total time during 24 hours associated with direct occlusal force application to periodontal tissues

[blocks in formation]

When teeth contact, the lower jaw moves along a pathway determined by the shape of tooth cusps (Adams and Zander, 1964) and the force exerted by the elevator muscles during the fleeting intercuspal contacts of swallowing or mastication is distributed over a greater tooth surface area than at any other stage in the chewing cycle. Throughout the contact period the elevator muscles are working isometrically, or very nearly so.

No other movement in the body terminates with contact between two hard surfaces, which makes tooth contact during masticatory, swallowing, and parafunctional movements unique (Anderson, 1976).

Clinical experience indicates that prosthetic restorations provide patients with adequate to full recovery of masticatory function, with studies of mandibular movement patterns indicating a similarity between patients wearing complete dentures and patients with natural teeth (Sheppard and Sheppard, 1971).

Swoope and Kydd (1966) suggested that the frequency and duration of tooth contacts while swallowing may have a significant effect on deformation of a prosthesis. While their studies apply to complete dentures, their results can probably be extrapolated to the distalextension (dentogingivally supported) removable partial denture. Their work indicates that the mucosa appears to play a significant role in absorbing stress from the saddle bases during a day's functional, and presumably parafunctional, activities.

The marked differences between patients with natural teeth or toothsupported prostheses, and patients wearing dentogingivally or completely gingivally supported ones, are conspicuous in a functional or parafunctional context. These differences include:

(1) Differences in the masticatory loads recorded for natural teeth and their replacements. A recent study (Haroldsson, Karlsson, and Carlsson, 1979) compared bite force values in complete denture wearers with those in patients with natural dentition. The maximal bite force in dentate subjects was 5 to 6 times greater than in the denture wearer. In another study, Haroldsson and associates (1978) compared functional status, bite force, and postural muscle activity in patients with osseointegrated oral implant bridges to matched control patients with natural teeth, and did not find any differences.

(2) Differences in stability of the prostheses from both tooth contacts as well as dislodging forces from surrounding musculature, which may be associated with oral motor behavior disorders (e.g. dyskinesia and other conditions accompanied by involuntary persistent movement of tongue, lips, and mandible).

(3) Progressive changes in the morphology of the gingival support component, which may be increased by parafunction or other motor behaviors. These changes will in turn probably affect the stability of the prothesis.

Parafunction

Parafunctional or bruxing habits involving repeated or sustained occlusion of the teeth (table 2) and eccentric tooth positions (Ramfjord and Ash, 1971; Yemm, 1976) can bring about several changes, including tooth wear, muscle hypertrophy, asymmetrical muscle development resulting

from unilateral muscle development, damage to restorative materials and tooth-supporting tissue integrity, tooth mobility, and temporomandibular joint or muscular pain and discomfort (Rugh and Solberg, 1976).

Table 2. Direction, duration, and magnitude of the forces developed during function and parafunction

[blocks in formation]

There are no epidemiologic studies about the incidence of parafunctional occlusal stress in normal, prosthesis-wearing, or denturewearing populations. However, clinical experience indicates that bruxism is common and is a common cause of mechanical breakdown in fixed prostheses, as well as a frequent cause of hyperemic and occasionally sore mucosa under distal extension saddles supported by mucous membrane. In the latter case, parafunction habits cause additional loading upon the prosthesis-bearing tissues. The initial discomfort associated with wearing a removable prosthesis is known to evoke unusual patterns of behavior in the surrounding musculature. Frequently the complaint of a sore tongue is related to the habit of thrusting the tongue against the denture. The patient is usually unaware of the causal relationship between the painful tongue and its contact with the teeth. Similarly, patients tend to occlude the teeth of new dentures frequently at first, perhaps to strengthen confidence in retention until the surrounding muscles accommodate or because some accommodation in the chewing pattern is usually required and experimental closure of the teeth is part of the process of adaptation. A marked response to the lower lip and mentalis muscle has been observed electromyographically in long-term complete denture wearers with impaired retention and stability of the lower denture (Tallgren, 1936). It is feasible and probable that the tentative occlusal contacts resulting may trigger a greater possibility for the development of habitual nonfunctional occlusion (Zarb, 1978).

Furthermore the range of changes which result from local factors, even after short-term wear of dentures, can lead to denture instability or diminished denture control. Either predicament may elicit recurrent and excessive mandibular movements as the patient attempts to recover control of the prosthesis, thereby initiating or reinforcing a parafunctional habit. Problems with coordination and habituation to the

« PreviousContinue »