“No dentist should restore, move, or remove any tooth without proper knowledge of occlusion.”
INTRODUCTION
According to The Glossary of Prosthodontic Terms1, occlusion is the static relationship between the incisal or occlusal surfaces of maxillary and mandibular teeth and dentures. Functional occlusion is the dynamic, morphologic, and functional relationship, harmonized by the interaction between neuromuscular activity and the anatomy of the teeth and supporting structures, capable of executing the movements allowed by the temporomandibular joints (TMJs). For didactic reasons, this author uses the term “occlusion” in the sense of functional occlusion to describe both the static and dynamic relationships defined above.
Treatment approaches and clinical procedures are constantly improving and becoming more accurate and less invasive as restorative materials are improved and new products developed. However, it cannot be said that the same degree of evolution has occurred in understanding occlusion, which is one of the great pillars of long-term treatment success.
This chapter aims to contextualize occlusion from a logical and biologic basis of scientific evidence to establish a pragmatic and predictable working model. Occlusion is related to all the different dental specialties, and, in this author’s opinion, the more the dentist knows about it, the better they will be able to diagnose treatment and minimize its risks, preventing the progression of damage to the teeth or restorations and the consequent decay of the stomatognathic system. In a careful review of almost 140 years of the literature, it appears that many concepts and theories of occlusion have spread without proper scientific foundation. Several principles disseminated as universal dogmas by “experts” of the time were based on their own experiences and clinical observations. These principles had repercussions in the dental field due to the absence of appropriate studies and scientific evidence2. The teaching of occlusion has been carried out in an incomplete or compartmentalized way in most dental schools, creating difficulties for students to integrate these parcels of knowledge and understand the picture as a whole.
An idealized model of occlusion containing stereotyped biomechanical requirements was created from studies on articulators, reproducing the premise that an optimal occlusion would need certain morphologic criteria3. Individuals who deviated from this model were classified as having “malocclusions” and were recommended orthodontic treatments or extensive oral rehabilitation4. Many of these treatments may have been performed adequately and successfully, although they were not always necessary.
The stomatognathic system is a functional unit and comprises structures related to vital activities such as chewing, swallowing, phonation, breathing, and stress expression5. An intricate proprioceptive system links the central nervous system with the teeth, dentogingival complex, head and neck muscles, bones, and TMJs. These elements cannot be considered individually, as any change in one of them has the potential to interfere with the function of them all.
Today, it is understood that the behavior of the stomatognathic system is dynamic, and that morphologic variation seems to be the norm4. All patients should be considered individually at the particular moment in their lives that they present for treatment. Recent concepts concerning occlusion have been developed from recognizing individual factors of adaptation, remodeling, and neuroplasticity that were not considered by more traditional schools6–8. However, recognizing whether the patient is within the thresholds of physiologic adaptation and without pathologic manifestations requires a balanced clinical judgment based on a methodic evaluation system combined with the knowledge and clinical experience of the dentist.
The degradation of the stomatognathic system occurs mainly through the action of bacteria in caries and periodontal disease as well as due to functional and parafunctional harmful stimuli. In this regard, the magnitude, frequency, duration, distribution, and direction of forces should be considered dynamically in terms of individual adaptive responses. The stomatognathic system can also deteriorate due to abrasion, biocorrosion, previous iatrogenic treatments, or trauma. In this author’s opinion, of all these etiologic factors, occlusion is the most complex to understand, diagnose, and control.
Adaptive remodeling responses in the TMJs can influence the occlusal relationships, affecting the periodontal support and leading to dysfunction in the neuromuscular system, with multiple possible consequences. Likewise, an occlusal change in one tooth can cause a dysfunction in the neuromuscular system and generate adaptive responses in the TMJs. Thus, in clinical practice, how the individual reacts or adapts to occlusal imperfections seems more critical than their presence7–11.
Every dentist needs to know the fundamentals of occlusion and should be able to diagnose the patient’s functional status. Dentists should also make appropriate decisions regarding the type and extent of treatment, and plan individualized occlusal schemes according to the unique demands of each patient. They should also know how to control and reduce the effects of parafunctional activities and recognize the presence of temporomandibular disorders (TMDs). However, due to its complex nature, the treatment of TMDs deserves special consideration that is beyond the scope of this book.
Some fundamental concepts regarding the functional behavior of the stomatognathic system need to be clarified to develop a practical and effective line of reasoning for diagnosis.
Reference models for occlusal diagnosis and treatment
Therapeutic reference model
The therapeutic reference model defines an ideal morphologic, functional, and biomechanical pattern for intra- and interarch relationships, both static and dynamic, to optimize the distribution and direction of occlusal forces on the teeth and TMJs, providing adequate muscle function and stability to the stomatognathic system4,7,12–15.
The therapeutic reference model is a diagnostic parameter and the basis for comparing the morphologic variations. However, epidemiologic studies and clinical experience demonstrate that an “optimal,” “ideal,” or “idealized” occlusion is an abstract concept not commonly found in the natural dentition12,15,16, and that the absence of any of its principles does not imply that treatment is required17,18 [Figure 7-01].
[Figure 7-01] The therapeutic reference model provides an optimized distribution of occlusal forces and is favorable from a long-term biomechanical point of view. This does not mean, however, that those patients who do not have a particular number and distribution of occlusal contacts require occlusal treatment.
According to Proffit and Fields17, the term “malocclusion” is etymologically inappropriate when suggesting something is wrong with the patient’s occlusion and should be updated, as 95% of the population has some degree of crowding, misalignment, migration, or tooth wear. As long as there is no pathology present, these “malocclusions” should be viewed within the concept of morphologic variability, as functional and biologic adaptive responses usually follow them during the individual’s growth and development phases7,19–21.
If the criteria of the therapeutic reference model were dogmatically applied in all treatment planning decisions, most patients would require occlusal treatment without the assessment and consideration of individual signs and symptoms. For example, one of the criteria of this model involves the relationship between the position of centric relation (CR) and the maximal intercuspal position (MIP). However, studies show that about 90% of the population experiences discrepancies between these two positions9,22–24, and that a large part of this sample does not present symptoms or evident signs of pathology. The discrepancy between CR and MIP should not be a reason for treatment but rather one for the assessment of how the TMJs, the neuromuscular system, and the teeth are reacting to the given situation, as the responses vary individually or in the same patient at different times.
Thus, in this author’s opinion, great care should be taken recommending a dental treatment in the search for an ideal therapeutic reference model. The patient’s occlusal scheme should only be altered when there are relevant structural, biologic, functional, or esthetic reasons for doing so, for example, when the patient needs to replace several teeth or has severely worn teeth [Figure 7-02A–H].
[Figure 7-02A–H] Dental arches do not need treatment just because they are morphologically different from the therapeutic reference model but because of the signs and symptoms presented by the patient. In the case shown here, in addition to esthetic concerns, problems were identified concerning the temporomandibular joints (TMJs), masticatory muscles, and relevant signs of attrition in relation to the patient’s age.
Stomatognathic system in a physiologic state
The physiologic or adapted stomatognathic system demonstrates a balance between stimuli and adaptive physiologic responses of the tooth structure, periodontium, alveolar bone, neuromuscular system, and TMJs, despite the patient being able to present morphologic patterns different from the therapeutic reference model. Patients in this state have a pattern of tooth wear compatible with their age and do not present concerns, signs, or symptoms of significant functional problems; therefore, no change in the occlusal scheme is necessary at that time4,7,9,12,15,25–28.
The stimuli are mainly related to the magnitude, frequency, duration, distribution, and direction of occlusal forces, both during function, bruxism, and parafunctional activities. Also, trauma and some diseases can act as deleterious stimuli to the system. On the other hand, adaptive capacity is influenced by internal factors (such as previous status of the TMJs, masticatory muscles, and tooth structure; facial morphology; systemic conditions; high stress levels; age; sleep quality; emotional status; neurologic disorders; individual pain threshold/modulation; and genetics) and external factors (such as, among others, lifestyle; habits; medications used; and the consumption of caffeine, alcohol, tobacco, and recreational drugs) [Figure 7-03].
[Figure 7-03] The stomatognathic system is in a physiologic state when there is a balance between stimuli and the patient’s adaptive capacity. Due to the dynamic nature of the occlusal function, the dentist should be aware of the early signs and symptoms of instability in this system through regular check-ups. The intensity of stimuli may vary according to the patient’s time of life. If they exceed their adaptive capacity, signs and symptoms of a stomatognathic system in a pathologic state are likely to appear. The patient’s adaptive capacity may also vary. Suppose, for example, the patient’s adaptive capacity is low in response to stress. In that case, there is a greater risk of signs and symptoms of a stomatognathic system in a pathologic state, even if the intensity of the stimuli is not significantly high. The dentist needs to evaluate the patient’s history and the behavior of the system as a whole; that is, to consider the function of the TMJs and masticatory muscles and not base the diagnosis only on the morphologic aspect of the patient’s dentition.
Due to the potential of the physiologic state not to remain stable over time, the key to maintaining the oral health of these patients is to be alert to early signs and symptoms of instability or pathologic changes in the stomatognathic system28.
Stomatognathic system in a pathologic state
The stomatognathic system in a pathologic state, or one that is not adapted, exhibits signs and symptoms of inadequate adaptive responses, compensations, dysfunctions, or pathologic changes in response to an increase in the stimuli that act on it, or a reduction in the patient’s adaptive capacity7,9,28–30 [Figure 7-03]. These patients may have one or more of the following signs and symptoms: pain or TMD; pain or masticatory muscle hyperactivity; concerns of uncomfortable or uneven occlusion; hypersensitive teeth; worn or fractured teeth or restorations; noncarious cervical lesions (NCCLs); bone or root resorptions; enlarged periodontal ligament; periodontal attachment loss; pulpal necrosis; tremor; hypermobility, absence of interproximal contacts, or tooth migration.
The dentist must seek to reduce the subjectivity of this diagnosis of a stomatognathic system in a pathologic state, seeking cause–effect relationships between the clinical signs and symptoms presented with the aid of complementary examinations and study models mounted on an articulator.
Patients diagnosed with a stomatognathic system in a pathologic state should be carefully informed regarding the need for treatment to control or stop the degradation process. The consequences of a lack or postponement of treatment as well as proposals for a localized or partial resolution should be clarified, as the evident instability of this system tends to perpetuate, leading to the development of more complex situations.
For these restorative treatments to take place as efficiently as possible, the patient should be physically and emotionally stable and aware of the indications, limitations, and risks of the treatment. If the condition of the TMJs and masticatory muscles is inadequate or if there is a lack of interest or collaboration on the part of the patient, treatment should not be carried out. For as long as there are concerns of chronic pain, whether systemic or orofacial, definitive treatment is not indicated.
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