Fundamentals of Fixed Prosthodontics. James C. Kessler
system, producing mandibular movement that avoids premature contacts. This guides the mandible into a position of maximal intercuspation with the condyle in a less-than-optimal position. The result will be either some hypertonicity of nearby muscles or trauma to the TMJ, but it is usually well within most people’s physiologic capacity to adapt and will not cause discomfort.
However, the patient’s ability to adapt may be influenced by the effects of psychologic stress and emotional tension on the central nervous system.32 An increase in the patient’s stress level will frequently increase parafunctional jaw activity such as clenching or bruxing, and a normal occlusion can become a pathologic one33–37 (Fig 2-15). Simple muscle hypertonicity may give way to muscle fatigue and pain, with chronic headaches and localized muscle tenderness, or TMJ dysfunction may occur. Pathologic occlusion can also manifest itself in the physical signs of trauma and destruction. Heavy facets of wear on occlusal surfaces, fractured cusps, and tooth mobility often are the result of occlusal disharmony. There is no evidence that occlusal trauma will produce a primary periodontal lesion. However, when occlusal trauma is present, there will be more severe periodontal breakdown in response to local factors than there would be if only the local factors were present.38
Habit patterns may develop in response to occlusal disharmony and emotional stress. Bruxism and clenching, the cyclic rubbing together of opposing occlusal surfaces, will produce even greater tooth destruction and muscle dysfunction.
When the acute discomfort of a patient with a pathologic occlusion has been relieved, changes that will prevent the recurrence of symptoms must be effected in the occlusal scheme. Care must also be taken when providing occlusal restorations for a patient without symptoms. The dentist must not produce an iatrogenic pathologic occlusion.
In the placement of restorations, the dentist must strive to produce an occlusion that is as nearly optimum as his or her skills and the patient’s oral condition will permit. The optimum occlusion is one that requires minimal adaptation by the patient. The criteria for such an occlusion have been described by Okeson39:
In closure, the condyles are in the most superoanterior position against the discs on the posterior slopes of the eminences of the glenoid fossae. The posterior teeth are in solid and even contact, and the anterior teeth are in slightly lighter contact.
Occlusal forces are along the long axes of the teeth.
In lateral excursions of the mandible, working-side contacts (preferably on the canines) disocclude or separate the nonworking teeth instantly.
In protrusive excursions, anterior tooth contacts will disocclude the posterior teeth.
In an upright posture, posterior teeth contact more heavily than do anterior teeth.
Organization of the Occlusion
The collective arrangement of the teeth in function is quite important and has been subjected to a great deal of analysis and discussion over the years. There are three recognized concepts that describe the manner in which teeth should and should not contact in the various functional and excursive positions of the mandible: (1) bilateral balanced occlusion, (2) unilateral balanced occlusion, and (3) mutually protected occlusion.
Bilateral balanced occlusion
Bilateral balanced occlusion is based on the work of von Spee40 and Monson.41 It is a concept that is not used as frequently today as it has been in the past. It is largely a prosthodontics concept that dictates that a maximum number of teeth should contact in all excursive positions of the mandible. This is particularly useful in complete denture construction, in which contact on the nonworking side is important to prevent tipping of the denture.41 Subsequently, the concept was applied to natural teeth in complete occlusal rehabilitation. An attempt was made to reduce the load on individual teeth by sharing the stress among as many teeth as possible.42 It was soon discovered, however, that this was a very difficult type of arrangement to achieve. As a result of the multiple tooth contacts that occurred as the mandible moved through its various excursions, there was excessive frictional wear on the teeth.43
Unilateral balanced occlusion
Unilateral balanced occlusion, which is also commonly known as group function, is a widely accepted and used method of tooth arrangement in restorative dental procedures today. This concept had its origin in the work of Schuyler44 and others who began to observe the destructive nature of tooth contact on the nonworking side. They concluded that inasmuch as cross-arch balance was not necessary in natural teeth, it would be best to eliminate all tooth contact on the nonworking side.
Therefore, unilateral balanced occlusion calls for all teeth on the working side to be in contact during a lateral excursion. On the other hand, teeth on the nonworking side are contoured to be free of any contact. The group function of the teeth on the working side distributes the occlusal load. The absence of contact on the nonworking side prevents those teeth from being subjected to the destructive, obliquely directed forces found in nonworking interferences. It also saves the centric holding cusps (ie, the mandibular facial cusps and the maxillary palatal cusps) from excessive wear. The obvious advantage is the maintenance of the occlusion.
The functionally generated path technique, originally described by Meyer,45 is used for producing restorations in unilateral balanced occlusion. It has been adapted by Mann and Pankey for use in complete-mouth occlusal reconstruction. 46,47
Mutually protected occlusion
Mutually protected occlusion is also known as canine-protected occlusion or organic occlusion. It had its origin in the work of D’Amico,48 Stuart,49,50 Stallard and Stuart,27 and Lucia51 and the members of the Gnathological Society. They observed that in many mouths with a healthy periodontium and minimum wear, the teeth were arranged so that the overlap of the anterior teeth prevented the posterior teeth from making any contact on either the working or the nonworking side during mandibular excursions. This separation from occlusion was termed disocclusion. According to this concept of occlusion, the anterior teeth bear the entire load, and the posterior teeth are disoccluded in any excursive position of the mandible. The desired result is an absence of frictional wear.
The position of maximal intercuspation coincides with the optimal condylar position of the mandible. All posterior teeth are in contact with the forces being directed along their long axes. The anterior teeth either contact lightly or are very slightly out of contact (approximately 25 microns), relieving them of the obliquely directed forces that would be the result of anterior tooth contact. As a result of the anterior teeth protecting the posterior teeth in all mandibular excursions and the posterior teeth protecting the anterior teeth at the intercuspal position, this type of occlusion came to be known as a mutually protected occlusion. This arrangement of the occlusion is probably the most widely accepted because of its ease of fabrication and greater tolerance by patients.
However, to reconstruct a mouth with a mutually protected occlusion, it is necessary to have anterior teeth that are periodontally healthy. In the presence of anterior bone loss or missing canines, the mouth should probably be restored to group function (unilateral balance). The added support of the posterior teeth on the working side will distribute the load that the anterior teeth may not be able to bear. The use of a mutually protected occlusion is also dependent on the orthodontic relationship of the opposing arches. In either a Class II or a Class III malocclusion (Angle), the mandible cannot be guided by the anterior teeth. A mutually protected occlusion cannot be used in a situation of reverse occlusion, or crossbite, in which the maxillary and mandibular facial cusps interfere with each other in a working-side excursion.
Fig