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Journal of Oral Rehabilitation 2008 35; 446–453




Review Article
Dental occlusion: a critical reflection on past, present and
future concepts
J. C. TURP*, C. S. GREENE† & J. R. STRUB‡
       ¨                                                                       *Dental School, University of Basel, Switzerland, †College of
Dentistry, University of Illinois, Chicago, IL, USA and ‡School of Dentistry, University of Freiburg, Germany




SUMMARY For nearly a century, the diversity of                           been challenged. As a result, the acceptance of
concepts about ‘normal’ and ‘ideal’ dental occlusal                      morphological and functional variability of the
relationships has led to confusion in trying to                          stomatognathic system has gained increasing sup-
describe the occlusion of any individual patient. In                     port, and this change has important consequences
addition, a similar controversy arises when trying to                    for modern dental practice. In this article, the past,
formulate treatment plans for patients who need                          present and future of the subject of occlusion will be
extensive dental restorations or orthodontic treat-                      considered.
ment. And finally, the application of occlusal con-                       KEYWORDS:    dental occlusion, centric relation,
cepts to patients with temporomandibular pain and                        centric occlusion, temporomandibular disorders,
dysfunction has created a third area of debate. Over                     gnathology
the past few decades, however, an appreciable part
of the tenacious dogmatic heritage of this topic has                     Accepted for publication 16 April 2007




                                                                         relationships between all components of the mastica-
Introduction
                                                                         tory system – not just teeth and their supporting
Since the emergence of modern dentistry, the study of                    tissues but also the neuromuscular system, the tem-
dental occlusion has been a subject of major interest                    poromandibular joints (TMJs) and the craniofacial
(1–4). This is not surprising because knowledge about                    skeleton (8–10).
occlusion-related issues is essential to good clinical                      As most dentists are aware, debates and controversies
practice in all dental disciplines (5). Indeed, after a long             about issues related to dental occlusion have been
journey throughout the history of dental practice and                    prevalent for more than a century. These controversies
science, occlusion has attained its current role as ‘the                 have affected dental practice at every level, beginning
medium of dentistry’ (6).                                                with orthodontic issues in childhood and progressing to
   In a narrow connotation, the term ‘occlusion’                         occlusal evaluations of ordinary adults in the dental
relates to the arrangement of maxillary and mandib-                      office. In addition, the need to perform complex
ular teeth and to the way in which teeth contact.                        restorative dentistry requires some organizing concepts
Thus, a simple definition of occlusion includes such                      for delivery of optimal results – yet, dentists cannot
terms as ‘the static relationship’ (7) or ‘any contact’                  seem to agree on what those concepts should be, nor
(8) ‘between the incising or masticating surfaces of                     even on what constitutes a good outcome. Further-
the maxillary and mandibular teeth or tooth ana-                         more, for those patients who were unlucky enough to
logues’ (7). In a broader context, however, the                          develop pain and dysfunction in the masticatory system
definition of the term ‘occlusion’ is not limited to                      (temporomandibular disorders, TMDs), there has been
morphological tooth contact relationships. Rather, it                    a high probability that their problems would be anal-
embraces the dynamic morphological and functional                        ysed and treated within some occlusal paradigm.

ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd                               doi: 10.1111/j.1365-2842.2007.01820.x
DENTAL OCCLUSION                   447


   All three of these occlusion topics have been under-              Confusion in occlusion
going dynamic (and mostly positive) changes during
                                                                     In spite of its obvious clinical importance – or maybe
the past few decades. The general thrust of these
                                                                     because of it – the topic of occlusion has been
changes has been in the direction of broader definitions
                                                                     characterized by a certain degree of confusion among
for acceptable natural and therapeutic occlusal rela-
                                                                     dental students and practitioners. Among a variety of
tionships, based more firmly on biological principles. In
                                                                     possible explanations, three main reasons may be
addition, the recognition that occlusal variables are
                                                                     identified for this phenomenon:
only infrequently significant in the development of
                                                                     1 the growing diversity of concepts about occlusion;
TMDs (11–13) has led to a more medical model for the
                                                                     2 shifts in the definition of important occlusion-related
management of those disorders.
                                                                     terms;
   The aim of the present article therefore was to reflect
                                                                     3 insufficient education about occlusal principles, as
on the past, present and future of occlusal concepts. The
                                                                     well as diversity within school faculties on these topics.
authors hope that the clinical implications of these
concepts for every practising dentist will become
apparent.                                                            The growing diversity of concepts about occlusion

                                                                     Over the years, a plethora of ideas, hypotheses, theories
The importance of the occlusion                                      and practical concepts about occlusion-associated topics
                                                                     have been accumulated and propagated in the dental
Besides knives and forks, teeth are the most important
                                                                     literature. Suggestions were proposed by a great variety
tools with which humans are cutting their food. Yet,
                                                                     of dentists, anatomists, anthropologists, dental techni-
the tasks associated with the dental occlusion reach far
                                                                     cians and others, often based on self-confident convic-
beyond masticatory tools (Table 1). The important
                                                                     tions and claims without scientific support. The
biological and psychosocial role of the dental occlusion
                                                                     remarkable diversity of occlusion-related clinical pro-
becomes most obvious when teeth are missing. Tooth-
                                                                     cedures and treatment approaches that were used over
less, occlusion-free jaws are subject to a variety of
                                                                     decades for the diagnosis and management of patients
functional, aesthetic, psychological and social impair-
                                                                     worldwide has left many clinicans – and patients –
ments. However, in striking contrast to other mammals,
                                                                     unsure about what the ‘right’ approach should be.
where edentulousness is more or less equivalent to a
death sentence, modern humans do not need teeth for
survival.                                                            Shifts in the definition of important occlusion-related terms

                                                                     Another source of confusion has been the change of the
Table 1. Various tasks of dental occlusion in modern Homo sapiens
                                                                     meaning of some long-established terms, such as centric
                                                                     relation and centric occlusion, for which different authors
Primary tasks
  Food intake: biting ⁄ cutting (anterior teeth)                     and clinicians have used various, sometimes contradic-
  Mastication: food crushing (posterior teeth)                       tory definitions. These inconsistencies have not helped
  Ensuring the vertical dimension between upper and                  to clarify the study of occlusion. For example, the
   lower jaw                                                         current version of the Glossary of Prosthodontic Terms
  Ensuring the minimal distance between mandible and maxilla
                                                                     (7) lists (partly for historical and educational reasons)
   necessary for optimum functioning of the masseter and
                                                                     seven different definitions of the term centric relation
   medial pterygoid muscles
  Speech articulation: particularly anterior teeth: alveolar         (CR). According to the Glossary, centric relation is
   fricatives |s| and |z|, interdental fricatives |h| and |d
                                                           |        currently defined as a condylar position that is located
  Jaw clenching and tooth grinding (as a consequence of stress       in an anterior–superior position against the slopes of
   and as part of sleep physiology or phenomenology)                 the articular eminences. Conversely, some decades
  Tool (teeth as ‘third hand’) and weapon (sharpness of
                                                                     before, CR was described as the uppermost and
   anterior teeth)
Secondary tasks, shaped by socio-cultural influences                  rearmost position of the mandibular condyles, which
  Aesthetics (anterior teeth; indirectly also posterior teeth)       implies a completely different location of the mandible,
  Cosmetics (‘white teeth’; dental adornment)                        and some clinicians still use this latter definition today.
  Erotic function                                                    Johnston (14), an orthodontist, once remarked that the


ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
448             ¨
      J . C . T U R P et al.


      progressive modification in the definition of CR,               the incorporation of the theory into dentistry only
      i.e. from an extremely retruded to a more forward             added mysticism and confusion (...)’. Regarding the
      position, ‘has done more to eliminate centric slides than     curve of Spee, the anthropologist Brace (19) remarked
      20 years of grudging acquiescence to the precepts of          that the ideal of ‘spherical articulation’ maintained its
      gnathology’. In other words, the mere change of the           popularity far into the 1940s, particularly in prosthetic
      definition of CR brought about a significant reduction          dentistry. ‘Now as we look back we are struck by the
      of the magnitude of centric slides.                           fact that these (geometric; added by the authors) views
         Similar confusion exists regarding the term centric        were offered and accepted in the virtual absence of
      occlusion: once, it was a tooth-determined position,          supporting evidence’ (19).
      identical with maximum intercuspation of opposing                Later, theory-driven gnathological authorities estab-
      teeth and irrespective of condylar position (3), and it is    lished mechanical views of and strong opinions about
      still defined in that way by some authors (15). Now-           the masticatory system in general and occlusion in
      adays, however, the term centric occlusion is mostly used     particular. Rigid occlusal standards based on pre-deter-
      to characterize an occlusion of opposing teeth when the       mined, predominantly morphological criteria were
      condyles are located anterosuperiorly (16), i.e. in           established (8); however, Walther (21) concluded that
      centric relation as currently defined. This position,          ‘they were taught and applied without prior scientific
      however, may or may not coincide with maximum                 investigation’. Thus, gnathology developed on rather
      intercuspation (7), so it is no surprise if many dentists     weak scientific grounds (22). Later, neurophysiological
      are confused by this shifting terminology.                    and biomechanical aspects were considered, but many
                                                                    arguments were advanced in a simplistic way, as
                                                                    indicated by a comparison with current literature (23,
      Where do we come from? The
                                                                    24).
      gnathological heritage

      The authoritarian foundation                                  On the search of the ideal occlusion

      Long before the foundation of the Gnathological Soci-         Since the mid-1920s, many concepts of a ‘perfect’ or
      ety of California in 1926 by Beverly B. McCollum and          ‘ideal’ occlusion have been proposed (3, 25–28). Yet, an
      coworkers (17), important occlusion-related morpho-           ‘ideal’ occlusion as described by various authorities is
      logical and functional features had been observed by          rarely found in real life (29), and it is by no means
      anatomists and dentists. From early on, however,              synonymous with a physiologically acceptable occlu-
      authoratitive beliefs dominated occlusion-related top-        sion (30). Instead, theoretically ‘ideal’ concepts of
      ics. Typical for this way of thinking was a subjective and    occlusion are the result of human contemplation, and
      biased view, a lack of clear evidence and a tendency to       as such they are subject to personal interpretation (31).
      resort to black-and-white conclusions (18). Assump-           As pointed out by Huffman et al. (27), the word ideal
      tions were taken for granted, despite the fact that they      implies ‘something like infinity in that it can be
      were often based on a mixture of personal conviction          approached but never actually reached’. Hence, it is
      and plausibility, but without evidence from adequate          worthwhile to recall the patient-friendly statement of
      research in support of the assertions made. For exam-         Becker and Kaiser (32) that ‘it is presumptuous to state
      ple, neither the well-known Bonwill triangle nor the          nature’s intent for an ideal occlusion, and therefore it is
      equally recognized curve of Spee exists today in the          recommended to avoid occlusal therapy for individuals
      way they were originally described and propagated for         who appear to be functioning in health, even if their
      decades. Bonwill believed that the mandible comprised         occlusal scheme does not fit a concept of optimum
      an equilateral triangle running from condyle to condyle       occlusion’.
      and to a point between the lower central incisors. He
      saw in this equilaterality a proof of the existence of God
                                                                    Therapeutic occlusion: deviations from mother nature
      (19). As early as 1921, however, Wilson (20) noted that
      ‘Dr. Bonwill was obsessed with the idea of an equilat-        In order to attain the goal of a theoretically ‘ideal’
      eral triangle’, but as he correctly pointed out ‘an           occlusion, numerous therapeutic occlusal schemes for
      equilateral triangle was not nature’s ideal, and that         doing major reconstructive dentistry have been intro-

                                                              ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
DENTAL OCCLUSION                   449


duced into dental practice, for instance, by Peter                   of vertical dimension’ from occlusal dental wear is
K. Thomas, Harry Lundeen, Everitt Payne and Michael                  usually compensated to some degree by continuous
Polz. However, Mohl (33) noted that a therapeutic                    eruption).
occlusion may include some structural modifications                      While clinicians should not necessarily regard Bey-
that are not necessarily found in nature, and he                     ron’s observations as being the final word on what a
cautioned that such concepts should not be applied to                ‘successful occlusion’ should look like, it is pretty clear
functioning patients who do not need extensive dental                that the above parameters are fundamental elements of
work. Mechanically based views of the masticatory                    healthy functional occlusal relationships.
system often fostered rigid occlusal treatment concepts
that were not always tolerated by patients – regardless
                                                                     Recents developments
of the astonishing adaptive potential of the human
masticatory system. For example, some full-mouth
                                                                     The ‘occlusion wave’: new perspectives on occlusion
reconstruction patients had locking intercuspation in
centric relation (‘point centric’), with no room for                 The past two decades have seen a renaissance of the
anterior or lateral ‘freedom’.                                       interest in occlusion-related topics. This is evidenced,
   There is no argument about the fact that, in order to             for instance, by the publication of a variety of new
give clinicians and dental technicians a guide, certain              textbooks (38–41) and by the fact that prestigious
recommendations for establishing a therapeutic occlu-                dental journals have devoted special issues to the topic
sion are required. Interestingly, a concept of a func-               of occlusion (42, 43). Obviously, talking about occlu-
tionally optimal occlusion as proposed 50 years ago by               sion is en vogue again; however, this time more people
Beyron (34, 35) has recently been appreciated as a                   are approaching this topic with a critical mind. Char-
recommendable basis for the design of therapeutic                    acteristic features of the new look have been:
occlusal schemes (36, 37). Instead of creating new and               1 a critique of the rigid traditional views that con-
rigid definitions of ideal mandibular positions, tooth                stricted the dental occlusion within narrow mechanistic
relationships or functional guideline patterns, these                concepts, instead of allowing for a multidimensional
authors have adapted the empirical observations of                   freedom of tooth contact in closure as well as in all jaw
Beyron about natural dentitions. After studying the                  excursions;
occlusion in hundreds of successful subjects, he was                 2 a focus towards a biological understanding of man-
able to characterize the essential features of those                 dibular function as well as dysfunction;
patients’ occlusions as follows.                                     3 a recognition of the adaptive potentials of many
1 Maximum number of bilateral centric stops during                   components of the masticatory system – and especially
closing in maximum intercuspation, regardless of any                 the TMJs themselves.
specific jaw relationship.                                               As a result of this new awareness, a growing part of
2 As far as possible, axial loading of posterior teeth for           the dental community has been acknowledging that
optimal force distribution within the alveolus.                      morphological and functional variation of the mastica-
3 Freedom in the retrusive range of occlusal contact                 tory system is a normal biological principle rather than
(up to 1 mm), because this ensures that the mandible is              a sign of dysfunction or disease. Both animal and
not being forced into a border position as the teeth are             human research has shown that a ‘physiological occlu-
meeting in centric occlusion.                                        sion’ develops through a natural process of growth and
4 Multidimensional freedom of occlusal contact move-                 maturation, and as a result, intra- and interindividual
ment, with group function during laterotrusion, and                  variabilities are both normal and continuously chang-
anterior tooth contacts during protrusion. This provides             ing. Although the occlusion of any individual patient
the most favourable force distribution for each type of              may deviate from a pre-conceived ideal, it does not
functional movement.                                                 automatically require therapeutic intervention (9).
5 Adequate vertical dimension of occlusion. (Note: No                Furthermore, because of local environmental factors
clear definitions for this term have ever been pro-                   and intra-individual biological processes (adaptations),
posed, even by the gnathologists – but it has been                   the dental occlusion is changing constantly throughout
discussed in terms of speaking, chewing, swallowing,                 life. For too long, this insight has not been recognized
freeway space, etc. As Beyron noted, the so-called ‘loss             by many members of the dental profession. Instead, as

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      J . C . T U R P et al.


      noted by Brace (19), ‘since the dawn of modern                recent years, based on a large body of clinical studies.
      dentistry, as it were, the idea of the perfect occlusion      Thorough reviews of the literature as well as several
      has shimmered in the imagination of the dental                epidemiological investigations were unable to show that
      profession somewhat like the Holy Grail of Arthurian          naturally occurring features such as centric, balancing,
      legend – the unattainable height of earthly aspiration’.      working or protrusive occlusal ‘interferences’, various
      According to Ross (44), the rare occurrence of a              occlusal guidance patterns, missing teeth and oral ⁄ den-
      so-called ‘perfect’ or ‘ideal’ occlusion in natural denti-    tal parafunctions are meaningfully associated with TMD
      tions indicates that ‘nature does not require such            signs and symptoms (47–49). However, it has been
      perfection’. And finally, as we know today, occlusal           found that the presence of painful TMDs may influence
      variations and deviations are not necessarily associated      mandibular positions and movements, thus possibly
      with any specific health risks such as periodontal             leading to occlusal disturbances (50). Similarly, degen-
      diseases or TMDs (45).                                        erative changes in the TMJs can lead to changes in
         In this context, a few words on the term ‘malocclu-        occlusal relationships (51). This means that many of the
      sion’ are warranted. Malocclusion is defined as ‘a dental      occlusal ‘imperfections’ observed in these patients may
      occlusion typified by variation from ideal form ⁄ mor-         be consequences rather than causes of TMDs. As such,
      phology’ (36). However, about 70% of North American           they should not be analysed or adjusted until symptoms
      youths have some form of malocclusion, mostly crowd-          abate (if at all). Furthermore, TMD patients who also
      ing of teeth within an Angle class I occlusion (36).          require prosthodontic rehabilitation should have that
      Because of the large prevalence of ‘‘mal’’occlusions,         treatment postponed until the pain condition has been
      most of which are not bad at all, Palla (30) has recently     resolved (52).
      suggested to erase the term ‘malocclusion’ from the              Other interesting relationships between TMDs and
      dental literature. Obviously, both the biological under-      occlusion have come from the results of experimental
      standing and the semantic labelling of various occlusal       studies. For example, in a double-blind randomized
      patterns need to be updated as new information                crossover study, Michelotti et al. (13) demonstrated that
      emerges.                                                      experimental occlusal interferences do not increase the
                                                                    habitual activity in the jaw muscles of asymptomatic
                                                                    volunteers and do not lead to TMD signs and ⁄ or
      Therapeutic occlusal concepts
                                                                    symptoms. On the other hand, the results of a study
      Without doubt, for restorative purposes, some scientif-       by Le Bell et al.(53) indicate that subjects with no TMD
      ically based recommendations are desirable for reaching       history may respond differently to experimental inter-
      an acceptable occlusion. Okeson (4) noted that the            ferences than individuals with a former TMD history: in
      most favourable occlusal concept should ‘be the least         a randomized double-blind clinical investigation, these
      pathogenic for the greatest number of patients over the       clinicians found that subjects without a TMD history
      longest time’, but he did not provide any specific             showed fairly good adaptation to artificial occlusal
      recommendations for achieving that goal. Today, there         interferences, while individuals with a TMD history
      is an increasing tendency for caring clinicians to choose     were characterized by a significant increase in clinical
      simpler therapeutic occlusal concepts and procedures          signs compared with the other groups. It should be
      that are limited to what has traditionally been con-          noted that the increase related only to clincal signs of
      ceived to be necessary, e.g. the type of functional           TMDs but not to symptoms. Based on these findings, it
      occlusion precepts advocated by Beyron (46). Those            appears feasible that individuals with a history of TMDs
      dentists who insist on utilizing complex occlusal con-        may indeed have a greater risk of developing TMD signs
      cepts and protocols must be viewed with skepticism            in response to occlusal changes from dental work, so
      unless they can prove the superiority of their personal       their dentists should be aware of that possibility.
      concepts.

                                                                    Where are we heading?
      Occlusion and temporomandibular disorders
                                                                    Lately, prosthodontics has been appreciated as ‘a
      The importance of occlusal features as aetiologic factors     biologically based discipline with variable but
      for the genesis of TMDs has been de-emphasized in             profoundly significant psychosocial, functional and

                                                              ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
DENTAL OCCLUSION                        451


aesthetic implications for each patient’ (54). As a                   2. Washburn HB. History and evolution of the study of occlu-
consequence, dental education needs to shift its focus                   sion. Dent Cosmos. 1925;67:223–237,331–342.
                                                                      3. Ramfjord SP, Ash MM. Occlusion. Philadelphia: Saunders;
from predominantly technical aspects towards the
                                                                         1966:130.
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occlusion as ‘the medium of dentistry’ (6) is greatly                    and Occlusion. 5th ed. St Louis: Mosby; 2003:109–126.
affected by this paradigm shift.                                      5. Wilson N. Foreword. In: Klineberg I, Jagger R, eds. Occlusion
   The same is true for TMDs, which have lately been                     and clinical practice – an evidence-based approach. Edin-
characterized as a ‘genetic vulnerability disorder with                  burgh: Wright; 2004:v.
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                                                                         Dent. 1969;21:39–59.
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and signs are hypothesized as being related to three                     10–92.
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                                                                         Science and practice of occlusion. Chicago, IL: Quintessence;
1 genetically determined vulnerabilities because of
                                                                         1997:306–322.
(i) a deficiency within genes that are responsible for                 9. Mohl ND. Introduction to occlusion. In: Mohl N, Zarb GA,
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variants (57), which make the affected individual more                   IL: Quintessence; 1988:15–23.
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                                                                         I, Jagger R, eds. Occlusion and clinical practice – an evidence-
2 behavioural risk-conferring factors, such as stress-
                                                                         based approach. Edinburgh: Wright; 2004: vii–viii.
related jaw clenching or tooth grinding (12, 58, 59);
                                                                     11. Gesch D. Association of malocclusion and functional occlusion
3 environmental risk factors, i.e. psychosocial traits and               with signs of temporomandibular disorders in adults: results of
states.                                                                  the population-based study of health in Pomerania. Angle
   In such a construct, there is not much room left for                  Orthod. 2004;74:512–520.
occlusion to be considered as a significant aetiologic                12. Gesch D, Bernhardt O, Mack F, John U, Kocher T, Alte D.
                                                                         Association of malocclusion and functional occlusion with
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   Finally, from a clinical viewpoint, one of the most                   of Health in Pomerania (SHIP). Angle Orthod. 2005;75:183–
important objectives in providing any form of dental or                  190.
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decision-making and informed partnership is gaining                      Effect of occlusal interference on habitual activity of human
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ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd

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Dental occlusion a critical reflection on past present and

  • 1. Journal of Oral Rehabilitation 2008 35; 446–453 Review Article Dental occlusion: a critical reflection on past, present and future concepts J. C. TURP*, C. S. GREENE† & J. R. STRUB‡ ¨ *Dental School, University of Basel, Switzerland, †College of Dentistry, University of Illinois, Chicago, IL, USA and ‡School of Dentistry, University of Freiburg, Germany SUMMARY For nearly a century, the diversity of been challenged. As a result, the acceptance of concepts about ‘normal’ and ‘ideal’ dental occlusal morphological and functional variability of the relationships has led to confusion in trying to stomatognathic system has gained increasing sup- describe the occlusion of any individual patient. In port, and this change has important consequences addition, a similar controversy arises when trying to for modern dental practice. In this article, the past, formulate treatment plans for patients who need present and future of the subject of occlusion will be extensive dental restorations or orthodontic treat- considered. ment. And finally, the application of occlusal con- KEYWORDS: dental occlusion, centric relation, cepts to patients with temporomandibular pain and centric occlusion, temporomandibular disorders, dysfunction has created a third area of debate. Over gnathology the past few decades, however, an appreciable part of the tenacious dogmatic heritage of this topic has Accepted for publication 16 April 2007 relationships between all components of the mastica- Introduction tory system – not just teeth and their supporting Since the emergence of modern dentistry, the study of tissues but also the neuromuscular system, the tem- dental occlusion has been a subject of major interest poromandibular joints (TMJs) and the craniofacial (1–4). This is not surprising because knowledge about skeleton (8–10). occlusion-related issues is essential to good clinical As most dentists are aware, debates and controversies practice in all dental disciplines (5). Indeed, after a long about issues related to dental occlusion have been journey throughout the history of dental practice and prevalent for more than a century. These controversies science, occlusion has attained its current role as ‘the have affected dental practice at every level, beginning medium of dentistry’ (6). with orthodontic issues in childhood and progressing to In a narrow connotation, the term ‘occlusion’ occlusal evaluations of ordinary adults in the dental relates to the arrangement of maxillary and mandib- office. In addition, the need to perform complex ular teeth and to the way in which teeth contact. restorative dentistry requires some organizing concepts Thus, a simple definition of occlusion includes such for delivery of optimal results – yet, dentists cannot terms as ‘the static relationship’ (7) or ‘any contact’ seem to agree on what those concepts should be, nor (8) ‘between the incising or masticating surfaces of even on what constitutes a good outcome. Further- the maxillary and mandibular teeth or tooth ana- more, for those patients who were unlucky enough to logues’ (7). In a broader context, however, the develop pain and dysfunction in the masticatory system definition of the term ‘occlusion’ is not limited to (temporomandibular disorders, TMDs), there has been morphological tooth contact relationships. Rather, it a high probability that their problems would be anal- embraces the dynamic morphological and functional ysed and treated within some occlusal paradigm. ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd doi: 10.1111/j.1365-2842.2007.01820.x
  • 2. DENTAL OCCLUSION 447 All three of these occlusion topics have been under- Confusion in occlusion going dynamic (and mostly positive) changes during In spite of its obvious clinical importance – or maybe the past few decades. The general thrust of these because of it – the topic of occlusion has been changes has been in the direction of broader definitions characterized by a certain degree of confusion among for acceptable natural and therapeutic occlusal rela- dental students and practitioners. Among a variety of tionships, based more firmly on biological principles. In possible explanations, three main reasons may be addition, the recognition that occlusal variables are identified for this phenomenon: only infrequently significant in the development of 1 the growing diversity of concepts about occlusion; TMDs (11–13) has led to a more medical model for the 2 shifts in the definition of important occlusion-related management of those disorders. terms; The aim of the present article therefore was to reflect 3 insufficient education about occlusal principles, as on the past, present and future of occlusal concepts. The well as diversity within school faculties on these topics. authors hope that the clinical implications of these concepts for every practising dentist will become apparent. The growing diversity of concepts about occlusion Over the years, a plethora of ideas, hypotheses, theories The importance of the occlusion and practical concepts about occlusion-associated topics have been accumulated and propagated in the dental Besides knives and forks, teeth are the most important literature. Suggestions were proposed by a great variety tools with which humans are cutting their food. Yet, of dentists, anatomists, anthropologists, dental techni- the tasks associated with the dental occlusion reach far cians and others, often based on self-confident convic- beyond masticatory tools (Table 1). The important tions and claims without scientific support. The biological and psychosocial role of the dental occlusion remarkable diversity of occlusion-related clinical pro- becomes most obvious when teeth are missing. Tooth- cedures and treatment approaches that were used over less, occlusion-free jaws are subject to a variety of decades for the diagnosis and management of patients functional, aesthetic, psychological and social impair- worldwide has left many clinicans – and patients – ments. However, in striking contrast to other mammals, unsure about what the ‘right’ approach should be. where edentulousness is more or less equivalent to a death sentence, modern humans do not need teeth for survival. Shifts in the definition of important occlusion-related terms Another source of confusion has been the change of the Table 1. Various tasks of dental occlusion in modern Homo sapiens meaning of some long-established terms, such as centric relation and centric occlusion, for which different authors Primary tasks Food intake: biting ⁄ cutting (anterior teeth) and clinicians have used various, sometimes contradic- Mastication: food crushing (posterior teeth) tory definitions. These inconsistencies have not helped Ensuring the vertical dimension between upper and to clarify the study of occlusion. For example, the lower jaw current version of the Glossary of Prosthodontic Terms Ensuring the minimal distance between mandible and maxilla (7) lists (partly for historical and educational reasons) necessary for optimum functioning of the masseter and seven different definitions of the term centric relation medial pterygoid muscles Speech articulation: particularly anterior teeth: alveolar (CR). According to the Glossary, centric relation is fricatives |s| and |z|, interdental fricatives |h| and |d | currently defined as a condylar position that is located Jaw clenching and tooth grinding (as a consequence of stress in an anterior–superior position against the slopes of and as part of sleep physiology or phenomenology) the articular eminences. Conversely, some decades Tool (teeth as ‘third hand’) and weapon (sharpness of before, CR was described as the uppermost and anterior teeth) Secondary tasks, shaped by socio-cultural influences rearmost position of the mandibular condyles, which Aesthetics (anterior teeth; indirectly also posterior teeth) implies a completely different location of the mandible, Cosmetics (‘white teeth’; dental adornment) and some clinicians still use this latter definition today. Erotic function Johnston (14), an orthodontist, once remarked that the ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
  • 3. 448 ¨ J . C . T U R P et al. progressive modification in the definition of CR, the incorporation of the theory into dentistry only i.e. from an extremely retruded to a more forward added mysticism and confusion (...)’. Regarding the position, ‘has done more to eliminate centric slides than curve of Spee, the anthropologist Brace (19) remarked 20 years of grudging acquiescence to the precepts of that the ideal of ‘spherical articulation’ maintained its gnathology’. In other words, the mere change of the popularity far into the 1940s, particularly in prosthetic definition of CR brought about a significant reduction dentistry. ‘Now as we look back we are struck by the of the magnitude of centric slides. fact that these (geometric; added by the authors) views Similar confusion exists regarding the term centric were offered and accepted in the virtual absence of occlusion: once, it was a tooth-determined position, supporting evidence’ (19). identical with maximum intercuspation of opposing Later, theory-driven gnathological authorities estab- teeth and irrespective of condylar position (3), and it is lished mechanical views of and strong opinions about still defined in that way by some authors (15). Now- the masticatory system in general and occlusion in adays, however, the term centric occlusion is mostly used particular. Rigid occlusal standards based on pre-deter- to characterize an occlusion of opposing teeth when the mined, predominantly morphological criteria were condyles are located anterosuperiorly (16), i.e. in established (8); however, Walther (21) concluded that centric relation as currently defined. This position, ‘they were taught and applied without prior scientific however, may or may not coincide with maximum investigation’. Thus, gnathology developed on rather intercuspation (7), so it is no surprise if many dentists weak scientific grounds (22). Later, neurophysiological are confused by this shifting terminology. and biomechanical aspects were considered, but many arguments were advanced in a simplistic way, as indicated by a comparison with current literature (23, Where do we come from? The 24). gnathological heritage The authoritarian foundation On the search of the ideal occlusion Long before the foundation of the Gnathological Soci- Since the mid-1920s, many concepts of a ‘perfect’ or ety of California in 1926 by Beverly B. McCollum and ‘ideal’ occlusion have been proposed (3, 25–28). Yet, an coworkers (17), important occlusion-related morpho- ‘ideal’ occlusion as described by various authorities is logical and functional features had been observed by rarely found in real life (29), and it is by no means anatomists and dentists. From early on, however, synonymous with a physiologically acceptable occlu- authoratitive beliefs dominated occlusion-related top- sion (30). Instead, theoretically ‘ideal’ concepts of ics. Typical for this way of thinking was a subjective and occlusion are the result of human contemplation, and biased view, a lack of clear evidence and a tendency to as such they are subject to personal interpretation (31). resort to black-and-white conclusions (18). Assump- As pointed out by Huffman et al. (27), the word ideal tions were taken for granted, despite the fact that they implies ‘something like infinity in that it can be were often based on a mixture of personal conviction approached but never actually reached’. Hence, it is and plausibility, but without evidence from adequate worthwhile to recall the patient-friendly statement of research in support of the assertions made. For exam- Becker and Kaiser (32) that ‘it is presumptuous to state ple, neither the well-known Bonwill triangle nor the nature’s intent for an ideal occlusion, and therefore it is equally recognized curve of Spee exists today in the recommended to avoid occlusal therapy for individuals way they were originally described and propagated for who appear to be functioning in health, even if their decades. Bonwill believed that the mandible comprised occlusal scheme does not fit a concept of optimum an equilateral triangle running from condyle to condyle occlusion’. and to a point between the lower central incisors. He saw in this equilaterality a proof of the existence of God Therapeutic occlusion: deviations from mother nature (19). As early as 1921, however, Wilson (20) noted that ‘Dr. Bonwill was obsessed with the idea of an equilat- In order to attain the goal of a theoretically ‘ideal’ eral triangle’, but as he correctly pointed out ‘an occlusion, numerous therapeutic occlusal schemes for equilateral triangle was not nature’s ideal, and that doing major reconstructive dentistry have been intro- ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
  • 4. DENTAL OCCLUSION 449 duced into dental practice, for instance, by Peter of vertical dimension’ from occlusal dental wear is K. Thomas, Harry Lundeen, Everitt Payne and Michael usually compensated to some degree by continuous Polz. However, Mohl (33) noted that a therapeutic eruption). occlusion may include some structural modifications While clinicians should not necessarily regard Bey- that are not necessarily found in nature, and he ron’s observations as being the final word on what a cautioned that such concepts should not be applied to ‘successful occlusion’ should look like, it is pretty clear functioning patients who do not need extensive dental that the above parameters are fundamental elements of work. Mechanically based views of the masticatory healthy functional occlusal relationships. system often fostered rigid occlusal treatment concepts that were not always tolerated by patients – regardless Recents developments of the astonishing adaptive potential of the human masticatory system. For example, some full-mouth The ‘occlusion wave’: new perspectives on occlusion reconstruction patients had locking intercuspation in centric relation (‘point centric’), with no room for The past two decades have seen a renaissance of the anterior or lateral ‘freedom’. interest in occlusion-related topics. This is evidenced, There is no argument about the fact that, in order to for instance, by the publication of a variety of new give clinicians and dental technicians a guide, certain textbooks (38–41) and by the fact that prestigious recommendations for establishing a therapeutic occlu- dental journals have devoted special issues to the topic sion are required. Interestingly, a concept of a func- of occlusion (42, 43). Obviously, talking about occlu- tionally optimal occlusion as proposed 50 years ago by sion is en vogue again; however, this time more people Beyron (34, 35) has recently been appreciated as a are approaching this topic with a critical mind. Char- recommendable basis for the design of therapeutic acteristic features of the new look have been: occlusal schemes (36, 37). Instead of creating new and 1 a critique of the rigid traditional views that con- rigid definitions of ideal mandibular positions, tooth stricted the dental occlusion within narrow mechanistic relationships or functional guideline patterns, these concepts, instead of allowing for a multidimensional authors have adapted the empirical observations of freedom of tooth contact in closure as well as in all jaw Beyron about natural dentitions. After studying the excursions; occlusion in hundreds of successful subjects, he was 2 a focus towards a biological understanding of man- able to characterize the essential features of those dibular function as well as dysfunction; patients’ occlusions as follows. 3 a recognition of the adaptive potentials of many 1 Maximum number of bilateral centric stops during components of the masticatory system – and especially closing in maximum intercuspation, regardless of any the TMJs themselves. specific jaw relationship. As a result of this new awareness, a growing part of 2 As far as possible, axial loading of posterior teeth for the dental community has been acknowledging that optimal force distribution within the alveolus. morphological and functional variation of the mastica- 3 Freedom in the retrusive range of occlusal contact tory system is a normal biological principle rather than (up to 1 mm), because this ensures that the mandible is a sign of dysfunction or disease. Both animal and not being forced into a border position as the teeth are human research has shown that a ‘physiological occlu- meeting in centric occlusion. sion’ develops through a natural process of growth and 4 Multidimensional freedom of occlusal contact move- maturation, and as a result, intra- and interindividual ment, with group function during laterotrusion, and variabilities are both normal and continuously chang- anterior tooth contacts during protrusion. This provides ing. Although the occlusion of any individual patient the most favourable force distribution for each type of may deviate from a pre-conceived ideal, it does not functional movement. automatically require therapeutic intervention (9). 5 Adequate vertical dimension of occlusion. (Note: No Furthermore, because of local environmental factors clear definitions for this term have ever been pro- and intra-individual biological processes (adaptations), posed, even by the gnathologists – but it has been the dental occlusion is changing constantly throughout discussed in terms of speaking, chewing, swallowing, life. For too long, this insight has not been recognized freeway space, etc. As Beyron noted, the so-called ‘loss by many members of the dental profession. Instead, as ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
  • 5. 450 ¨ J . C . T U R P et al. noted by Brace (19), ‘since the dawn of modern recent years, based on a large body of clinical studies. dentistry, as it were, the idea of the perfect occlusion Thorough reviews of the literature as well as several has shimmered in the imagination of the dental epidemiological investigations were unable to show that profession somewhat like the Holy Grail of Arthurian naturally occurring features such as centric, balancing, legend – the unattainable height of earthly aspiration’. working or protrusive occlusal ‘interferences’, various According to Ross (44), the rare occurrence of a occlusal guidance patterns, missing teeth and oral ⁄ den- so-called ‘perfect’ or ‘ideal’ occlusion in natural denti- tal parafunctions are meaningfully associated with TMD tions indicates that ‘nature does not require such signs and symptoms (47–49). However, it has been perfection’. And finally, as we know today, occlusal found that the presence of painful TMDs may influence variations and deviations are not necessarily associated mandibular positions and movements, thus possibly with any specific health risks such as periodontal leading to occlusal disturbances (50). Similarly, degen- diseases or TMDs (45). erative changes in the TMJs can lead to changes in In this context, a few words on the term ‘malocclu- occlusal relationships (51). This means that many of the sion’ are warranted. Malocclusion is defined as ‘a dental occlusal ‘imperfections’ observed in these patients may occlusion typified by variation from ideal form ⁄ mor- be consequences rather than causes of TMDs. As such, phology’ (36). However, about 70% of North American they should not be analysed or adjusted until symptoms youths have some form of malocclusion, mostly crowd- abate (if at all). Furthermore, TMD patients who also ing of teeth within an Angle class I occlusion (36). require prosthodontic rehabilitation should have that Because of the large prevalence of ‘‘mal’’occlusions, treatment postponed until the pain condition has been most of which are not bad at all, Palla (30) has recently resolved (52). suggested to erase the term ‘malocclusion’ from the Other interesting relationships between TMDs and dental literature. Obviously, both the biological under- occlusion have come from the results of experimental standing and the semantic labelling of various occlusal studies. For example, in a double-blind randomized patterns need to be updated as new information crossover study, Michelotti et al. (13) demonstrated that emerges. experimental occlusal interferences do not increase the habitual activity in the jaw muscles of asymptomatic volunteers and do not lead to TMD signs and ⁄ or Therapeutic occlusal concepts symptoms. On the other hand, the results of a study Without doubt, for restorative purposes, some scientif- by Le Bell et al.(53) indicate that subjects with no TMD ically based recommendations are desirable for reaching history may respond differently to experimental inter- an acceptable occlusion. Okeson (4) noted that the ferences than individuals with a former TMD history: in most favourable occlusal concept should ‘be the least a randomized double-blind clinical investigation, these pathogenic for the greatest number of patients over the clinicians found that subjects without a TMD history longest time’, but he did not provide any specific showed fairly good adaptation to artificial occlusal recommendations for achieving that goal. Today, there interferences, while individuals with a TMD history is an increasing tendency for caring clinicians to choose were characterized by a significant increase in clinical simpler therapeutic occlusal concepts and procedures signs compared with the other groups. It should be that are limited to what has traditionally been con- noted that the increase related only to clincal signs of ceived to be necessary, e.g. the type of functional TMDs but not to symptoms. Based on these findings, it occlusion precepts advocated by Beyron (46). Those appears feasible that individuals with a history of TMDs dentists who insist on utilizing complex occlusal con- may indeed have a greater risk of developing TMD signs cepts and protocols must be viewed with skepticism in response to occlusal changes from dental work, so unless they can prove the superiority of their personal their dentists should be aware of that possibility. concepts. Where are we heading? Occlusion and temporomandibular disorders Lately, prosthodontics has been appreciated as ‘a The importance of occlusal features as aetiologic factors biologically based discipline with variable but for the genesis of TMDs has been de-emphasized in profoundly significant psychosocial, functional and ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd
  • 6. DENTAL OCCLUSION 451 aesthetic implications for each patient’ (54). As a 2. Washburn HB. History and evolution of the study of occlu- consequence, dental education needs to shift its focus sion. Dent Cosmos. 1925;67:223–237,331–342. 3. Ramfjord SP, Ash MM. Occlusion. Philadelphia: Saunders; from predominantly technical aspects towards the 1966:130. biomedical sciences and molecular biology (30). The 4. Okeson JP. Management of Temporomandibular Disorders occlusion as ‘the medium of dentistry’ (6) is greatly and Occlusion. 5th ed. St Louis: Mosby; 2003:109–126. affected by this paradigm shift. 5. Wilson N. Foreword. In: Klineberg I, Jagger R, eds. Occlusion The same is true for TMDs, which have lately been and clinical practice – an evidence-based approach. Edin- characterized as a ‘genetic vulnerability disorder with burgh: Wright; 2004:v. 6. Ricketts RM. Occlusion – the medium of dentistry. J Prosthet strong involvement of the central nervous system’ (55). Dent. 1969;21:39–59. 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