A theoretical discussion of mandibular kinetics during incision and
mastication has been presented.1 The observations detailed in the
aforementioned study should initiate critical discussion of occlusal treatment
based only on the patient's execution of protrusion and lateral movements. The
clinical differences between the classical and the functional occlusal
approaches are compared to assess their impact on the occlusal equilibrium and
the treatment of the temporomandibular disorders.
If the intercuspal position of the patient is unsteady or
disrupted by a dental interference on the closure path, the neuromuscular
system is compelled to address the disruption in order to function. The system
adapts a path of closure to avoid the obstacle and establishes a steady
functional intercuspal position, or the mandibular rest position is modified in
accordance with the new intercuspal position. If these adjustments exceed the
adaptive threshold of the masticatory system, craniomandibular disorder may
While the premature contacts may be "hidden" by
the patient as he or she avoids the antagonizing movement, several techniques
can be utilized to detect their presence. Manipulation and swift, repetitive
movements are often used in this manner, although the authors prefer to utilize
an anterior deprogramming device. Interposing a jig along the closure path for
several minutes allows the clinician to cancel avoidance and adaptive reactions
generated by the central nervous system (CNS) of the patient.2,3 The
jig, which is fabricated of resin, promotes the symmetric relaxation of the
patient's masticatory muscles.
Once the patient has been relaxed and deprogrammed with the
jig, he or she occludes directly against the premature contact that was
previously "hidden."3 Following the completion of occlusal
correction using progressive subtraction, the dental contacts must be
reestablished to reflect simultaneous and equal balance on a maximum of teeth
During the process of mastication, several dissimilarities
are evident between the incision and protrusion movements. Due to the activity
of the elevator muscles, the occlusal contacts and guidance are significantly
more important during incision than protrusion. When exposed to the activity of
the antagonist muscular groups,1 the spatial position of the
mandible is slightly different and the dental paths of the two movements are
During incision, the contacts and guidance are well marked
between the edge of the mandibular incisors and the palatal concavity and
cingulum of the maxillary incisors, whereas during protrusion they seem to
"fly over" the anatomical landmarks (Figure 1).
During incision, the posterior maxillary and mandibular
teeth are in closer proximity, while they are well separated during protrusion.
The posterior inferior teeth slightly contact the superior teeth for the
majority of the incision process in order to provide proprioceptive information
to the CNS.
When utilizing solely protrusion movement to examine
functional anterior guidance, the clinician may neglect the overguidance
contacts on the functional path of incision, since they are often not evident
during protrusion. In the natural dentition, this phenomenon may be compensated
for in part by the slight displacement of the incisors, due to their extensive
physiological mobility. Conversely, lateral and/or axial mobility is reduced
with conventional bridges and implant-supported restorations (Figures 2-3-4-5). As a consequence, overguidance contacts may become significantly more
important,4 as the mechanisms of avoidance can be impeded or
canceled. At each incision or cycle of mastication, the teeth collide on the
overguidance contacts. This collision occurs without the awareness of the
patient and is consequently unavoidable.
Additional concerns must be addressed when examining
functional anterior guidance. The attending clinician must provide sufficient
anteroposterior functional freedom in the area of the maximum intercuspation
contacts. This freedom is necessary to allow posterior trituration without
anterior interferences or overguidance on the anterior teeth. Furthermore, a
posterior interference may be evident during incision that cannot be detected
by protrusive movement.
Trituration Versus Lateral Movement
Lateral movement is generally guided by the cuspids on the
corresponding side. During the mastication cycle, the spatial position of the
mandible is slightly different in centripetal orientation, and the dental
guidance is harmoniously distributed on the entire occlusal faces of the
cuspidated teeth. Performing the lateral movement only to check the posterior
functional equilibrium results in the following complications:
The clinician is not able to determine functional posterior
contacts and guidance during the dental entry ("cycle in") and the
dental exit ("cycle out").1
Detection of important interferences and overguidance
contacts is possible; however, the functional guidance may be compromised.
Overguidance, underguidance, or insufficient functional freedom may result from
this complication (Figures 6-7-8).
If the classical concept locating maximum intercuspation in
centric relation is rigorously applied, the retrusion movement can be
suppressed. On the masticating side, however, the "centripetal return
movement" from a lateral retrusive position is a kinetic component of the
dental "cycle in."1 To deprive a patient of a part of this
functional limit may cause the muscular pathways to alter the masticating cycle
mesially to restore functional centric freedom. If the intercuspation contacts
are too rigidly engaged, they may become overguidance contacts or functional
interferences. Bruxism may result from the elimination of these complications.
(Continued from page 1 )
The identification and the restoration of functional
contacts and occlusal guidance have become the principal objectives of the
clinician; consequently, a modification of conventional diagnosis and treatment
is necessary. Functional movements must be prioritized by the clinician. The
topography of the functional dental guidance and contacts must be integrated
into the identification of the occlusal anatomy. The terminology utilized to
describe the mastication cycle must include the existence of slopes guiding the
"cycle in," as well as the existence of tables guiding the
"cycle out." In addition, the occlusal subtractive or additive
techniques must be evaluated. These procedures must address:
The excessive functional guidance or overguidance corrected
The lack or the insufficient functional guidance or
underguidance corrected by addition.
The existence of interferences, which prevent functional
contacts and/or guidance, generally corrected by subtraction.
Reconstruction at the Laboratory and in Clinical Situations
In order to allow adequate reproduction of mastication
pathways, articulator setting must be determined by utilizing recordings of the
functional movements and, as required, the articular cases modified for the
same purpose. Advances in prosthetic rebuilding of occlusal surfaces, as well
as improvements in the functional simulation capacity of the articulators, allow
the clinician to go beyond anterior protective concepts and to ensure
functional equilibrated posterior sectors.
Treatment of Temporomandibular Disorders
The classical treatment of temporomandibular joint
dysfunction requires the use of an occlusal splint to sedate muscle pain and
spasms or to recapture the articular disk prior to the completion of an
occlusal equilibration. Classical anterior guidance is rendered without
addressing the effective guidance of the posterior teeth. The results of these
treatments, however, range from unpredictable to poor (from 45% to 64%
failure).5 Thus, one should carefully reconsider the role of
occlusion in temporomandibular joint dysfunction.
While the majority of these techniques consider the maximum
intercuspation contacts (swallowing functional contacts), none of the
techniques addresses the importance of the posterior dental guidance of
mastication. Therefore, the assessment of the pathological consequences of a
modification of posterior dental guidance provides several benefits. An excess
of guidance, considered as an interference by conventional techniques, causes
muscle spasms and pain. It is frequently corrected by selective grinding and
the treatment is generally successful. Alternately, a lack or an insufficient
guidance, especially in the first molar area, exhibits a less promising
clinical result. During mastication cycle, the patient experiences indirect and
then direct guidance that generally induces a widening of the cycle. This
expansion may result in several bilateral consequences:
On the triturant side, the disharmony between the
dental and articular guidance may dislocate the disk by excessive contraction
of the superior aspect of the lateral pterygoid muscle, with muscular pain or
without. On the nontriturant side, the expanding cycle may provoke interdental
contacts, which may be confused with nonworking interferences when applying
classical treatment concepts.
A new treatment modality may then be proposed,
addressing the lack of posterior guidance and suggesting its therapeutic
rehabilitation, either by the addition of composite resin material on the
occlusal slopes of natural teeth or by provisional modification of the existing
prosthetic teeth (Figures 9-10-11-12). The material must be shaped prior to
hardening and carefully equilibrated thereafter by simulation of mastication.
Completion of this procedure allows the clinician to restore the functional
equilibrium of the masticatory system and results in the achievement of
definitive prosthetic restorations according to these principles.
An occlusal adjustment is not achieved by the blind
application of a dogma or a corrective rule. Occlusal therapy must be initiated
with a complete understanding of the peculiarities of the case and their
consequences. Accurate evaluation of these consequences is essential.
Comprehensive knowledge of the determining factors of the masticatory system
(CNS, teeth, muscles, joints) must be obtained prior to the initiation of every
occlusal procedure. The successful integration of kinetic principles to the
understanding of the masticatory process and the restoration of occlusion has
become a challenge for the clinician.
JF, Le Gall MG. The function of mastication: A key determinant of dental
occlusion. Pract Periodont Aesthet Dent 1996;8(8):807-817.
V. Modern Gnathological Concepts - Updated. Carol Stream, IL:
Quintessence Publishing; 1983:449.
Guern JY. Etude exprimentale de la des contacts occlusaux sur le
chemin de fermeture lors devation mandibulaire. Int clinique. Thse
SC Odontol. 3 Cycle, Nantes,
Gall MG, Lauret J-F, Saadoun AP. Mastication forces and implant-bearing
surface. Pract Periodont Aesthet Dent 1994;6(9):37-46.
S. Les effets long terme des dsordres temporo-mandibulaires. Ralits