E6 Mechanical Balance

Topic

The mechanical equilibrium of the stomatognathic system describes the balance of forces and moments that maintains the mandibular resting position and keeps the teeth within their alveoli.

At physiological rest, the mandibular elevator muscles—the masseter, temporalis, and pterygoids—balance against the weight of the mandible itself and the tone of the opposing depressor muscles; this equilibrium among elevator muscles, mandibular weight, and depressor muscles establishes a free-way space (interocclusal clearance) of two to four millimeters between the upper and lower teeth.

Within each alveolus, the periodontal ligament acts as a viscoelastic spring system that cushions occlusal loads received by the tooth and transfers them to the alveolar bone in a distributed rather than concentrated manner—performing the same cushioning role it plays regarding tooth mobility and load distribution around an implant.

Dental occlusion achieves its most stable equilibrium in the position of maximum intercuspation—the position where the upper and lower teeth make the greatest possible number of contacts, distributed across multiple bilateral points rather than concentrated on just one or two.

Dental restorations must precisely replicate this occlusal equilibrium found in maximum intercuspation: if a restoration creates a premature contact, that contact places a localized overload on the restored tooth—just as a premature contact overloads any tooth in a malocclusion—and this imbalance manifests as sensitivity in the newly restored tooth, fractures, or hypertrophy of the masticatory muscles compensating for the overload.

To anticipate where stress might concentrate before a restoration, prosthesis, or implant overloads the dental system, a numerical method is used that divides the dental system into many small elements and calculates, element by element, how the load is distributed across the dental enamel, dentin, root cementum, and alveolar bone. Thus, the balance between the elevator and depressor muscles at rest, the cushioning effect of the periodontal ligament, the balance of contacts in the position of maximum intercuspation, and the absence of premature contacts collectively maintain the mechanical equilibrium of the stomatognathic system described at the beginning.

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