Occlusion (dentistry)
Occlusion, in a dental context, means simply the contact between teeth. More technically, it is the relationship between the maxillary (upper) and mandibular (lower) teeth when they approach each other, as occurs during chewing or at rest. Static occlusion refers to contact between teeth when the jaw is closed and stationary, while dynamic occlusion refers to occlusal contacts made when the jaw is moving. The masticatory system also involves the periodontium, the TMJ (and other skeletal components) and the neuromusculature, therefore the tooth contacts should not be looked at in isolation, but in relation to the overall masticatory  system. Anatomy of Masticatory System. One cannot fully understand occlusion without an in depth understanding of the anatomy including that of the teeth, TMJ, musculature surrounding this and the skeletal components. The Dentition and Surrounding Structures The human dentition consists of 32 permanent teeth and these are distributed between the alveolar bone of the maxillary and mandibular arch. Teeth consist of two parts: the crown, which is visible in the mouth and lies above the gingival soft tissue and the roots, which are below the level of the gingiva and in the alveolar bone. The periodontal ligament unites the  cementum on the outside of the root and the alveolar bone. This bundle of connective tissue fibres is vital in dissipating forces that are applied to the underlying bone during the contact of teeth in function. The teeth are highly specialised and different teeth are involved in specific functions. The masticatory system is largely influenced by these intra and inter-arch relationships and a wider understanding of the anatomy can greatly benefit those who want to understand occlusion.Skeletal Components The maxilla forms a crucial aspect of the upper facial skeleton. Two irregularly shaped bones fuse at the intermaxillary suture  during development forming the upper jaw. This forms the palate of the oral cavity and also supports the alveolar ridges that hold the upper teeth in place. The lower facial skeleton on the other hand, is formed of the mandible, a U shaped bone, which supports the lower teeth and also forms part of the TMJ. The mandibular condyle and the squamous portion of the temporal bone, at the base of the cranium articulate with one another. TMJ The TMJ is formed from the temporal bone of the cranium, specifically the glenoid fossa and articular tubercle and the condyle of  the mandible, with a fibrocartilaginous disc lying in between. It is classified as a ginglymoarthrodial joint and can perform a range of gliding and hinge type movements. The disc, which lies in between is composed of dense fibrous tissue and is predominantly avascular and lacking nerves. Muscles There are various muscles that contribute to occlusion of the teeth including the muscles of mastication and other accessory muscles. The temporalis, masseter, medial and lateral pterygoids are the muscles of mastication and these contribute to the elevation, depression, protrusion and retraction of the mandible. The anterior and posterior belly of the digastric  are also involved in the depression of the mandible and elevation of the hyoid bone and are therefore relevant to the masticatory system. Ligaments There are various ligaments associated with the TMJ and these limit and restrict border movements by acting as passive restraining devices. They do not contribute to joint function, rather exert a protective role. The key ligaments relevant to the TMJ are: Development of occlusion. As the primary (baby) teeth begin to erupt at 6 months of age, the maxillary and mandibular teeth aim to occlude with one another. The erupting teeth are moulded into position by  the tongue, the cheeks and lips during development. Upper and lower primary teeth should be correctly occluding and aligned after 2 years whilst they are continuing to develop, with full root development complete at 3 years of age. Around a year after development of the teeth is complete, the jaws continue to grow which results in spacing between some of the teeth (diastema). This effect is greatest in the anterior (front) teeth and can be seen from around age 4 – 5 years. This spacing is important as it allows space for the permanent (adult) teeth to erupt into the correct  occlusion, and without this spacing there is likely to be crowding of the permanent dentition. In order to fully understand the development of occlusion and malocclusion, it is important to understand the premolar dynamics in the mixed dentition stage. The mixed dentition stage is when both primary and permanent teeth are present. The permanent premolars erupt ~9–12 years of age, replacing the primary molars. The erupting premolars are smaller than the teeth they are replacing and this difference in space between the primary molars and their successors (1.5mm for maxillary, 2.5mm for mandibular), termed Leeway Space. This allows  the permanent molars to drift mesially into the spaces and develop a Class I occlusion. Incisor and molar classification. Classification of occlusion and malocclusion plays an important role in diagnosis and treatment planning in orthodontics. In order to describe the relationship of the maxillary molars to the mandibular molars, the Angle’s classification of malocclusion has commonly been used for many years. This system has also been adapted in an attempt to classify the relationship between the incisors of the two arches. Incisor Relationship When describing the relationship between maxillary and mandibular incisors, the following categories make up Angle's  incisal relationship classification: Molar Relationship When discussing the occlusion of the posterior teeth, the classification refers to the first molars and may be divided into three categories: Any deviation from the normal relation of teeth (Class I) is considered a malocclusion. Class I relationships are thought to be “ideal”, however this classification does not take into consideration the positions of the two TMJ’s . Class II and III molar and incisor relationships are thought to be forms of malocclusion, however not all of these are severe enough to require orthodontic treatment. The Index of Orthodontic Treatment Need is a system  that attempts to rank malocclusions in terms of significance of various occlusal traits and perceived aesthetic impairment. The index identifies those who would benefit most from orthodontic treatment and onward referral to an orthodontist. Occlusal terminology. Intercuspal Position (ICP), also known as Habitual Bite, Habitual Position or Bite of Convenience , is defined at the position where the maxillary and mandibular teeth fit together in maximum interdigitation. This position is usually the most easily recorded and is almost always the occlusion the patient closes into when they are asked to 'bite together'. This is the occlusion that the patient is accustomed  to, hence sometimes termed the Habitual Bite. Centric relation (CR) describes a reproducible jaw relationship (between the mandible and maxilla) and is independent of tooth contact. This is the position in which the mandibular condyles are located in the fossae in an antero-superior position against the posterior slope of the articular eminence. It is said that in CR, the muscles are in their most relaxed and least stressed state. This position is not influenced by muscle memory, but rather by the ligament which suspend the condyles within the fossa. Therefore it is the position that dentists use to create  new occlusal relationships as for example, while making maxillary and mandibular complete dentures. When the mandible is in this retruded position, it opens and closes on an arc of curvature around an imaginary axis drawn through the centre of the head of both condyles. This imaginary axis is termed the "terminal hinge axis". The first tooth contact that occurs when the mandible closes in the terminal hinge axis position, is termed Retruded Contact Position (RCP). RCP can be reproduced within 0.08mm of accuracy due to the non-elastic TMJ capsule and restriction by the capsular ligaments, thus it can  be considered a ‘border movement’ in Posselt’s envelope. Centric Occlusion (CO) is a confusing term, and is often incorrectly used synonymously with RCP. Both terms are used to define a position where the condyles are in CR, however RCP describes the initial tooth contact on closure, however this may be an interference contact. On the other hand, CO refers to the occlusion where the teeth are in maximum intercuspation in CR. Posselt (1952) determined that only in 10% of natural tooth and jaw relationships does ICP = CO (maximum intercuspation in CR) and so the term RCP is more  appropriate when discussing the occlusion that occurs when the condyles are in their retruded position. CO is a term that is more relevant to complete denture application or where multiple fixed unit prosthodontics are provided, where the occlusion is arranged so that when the mandible is in CR, the teeth are interdigitating. Posselt's Envelope of Border Movements. Posselt’s Envelope of Border Movement (often referred to as the 'border movements of the mandible') is a schematic diagram of the maximum jaw movement in three planes (sagittal, horizontal and frontal). This encompasses all movements away from RCP, and includes: Guidance , natural teeth and function. Mandibular movements are guided by two different systems; the ‘posterior’ guidance system and the ‘anterior’ guidance system Posterior guidance system. Posterior guidance refers to TMJ articulations and associated structures . It is the condyles within the fossa and the associated muscles and ligaments together with its neuromuscular link that determines mandibular movements. Lateral, protrusive and repressive excursions of the mandible are guided by the posterior system. "Lateral excursions" It is important to define the movement of the condyles in lateral excursions: - Working condyle: This is the condyle closest to the side which the mandible is moving (e.g.  if the mandible moves laterally to the right, the right condyle is the working side condyle) - Non-working condyle: This is the side to which the mandible is moving away from (e.g. if the mandible moves laterally to the right, the left condyle is the non-working side condyle) "Protrusive movements" "Retrusive movements" We can not influence the posterior guidance system through dental restorative treatment. Anterior guidance system. Anterior guidance refers to the influence of contacting teeth on the paths of mandibular movements. The tooth contacts may be anterior, posterior tooth contacts or both - however termed anterior guidance as these  contacts are still anterior to the TMJ. This can be further classified into: "Canine Guidance" "Group Function" "Incisal Guidance" In restorative treatment, it is possible to manipulate and design anterior guidance systems that is in harmony with the masticatory system. Clinical relevance of guidance. Tooth contact involved in guidance is particularly important as these occlude a vast number of times per day and so need to be able to resist both heavy and non-axial occlusal loads. When restoring the anterior guidance system should be compatible with the posterior guidance system. This means that excessive strain should not be applied  on the posterior guidance system which is limited by the ligamemtous structures. Upon restoration of the occlusal surfaces of teeth, it is likely to change the occlusion and therefore guidance systems. It is unlikely the TMJ will adapt to these changes in occlusion, but rather the teeth adapt to the new occlusion through tooth wear, tooth movement or fracture. For this reason, it is important to consider these guidance concepts when providing restorations. Guidance should also be considered before restorations as it should not be expected for a heavily restored tooth to provide guidance alone as this leaves the tooth  vulnerable to fracture during function. Organisation of the occlusion. The arrangement of teeth in function is important and over the years three recognised concepts have been developed to describe how teeth should and should not contact: Bilateral balanced occlusion. This concept is based on the curve of Spee and curve of Wilson and is becoming outdated for the restored natural dentition. However, it still finds application in removable prosthodontics. This scheme involves contacts on as many teeth as possible (both on the working and non-working side) in all excursive movements of the mandible. This is especially important in the  case of complete denture provision as contacting teeth on the NWS help stabilise the denture bases in mandibular movement. It was believed in the 1930s that this arrangement was ideal for the natural dentition when providing full occlusal reconstruction in order to distribute the stresses. However, it was found that the lateral forces placed on the restored posterior teeth produced damaging effects on the restorations. Unilateral balanced occlusion. On the other hand, unilateral balanced occlusion is a widely used tooth arrangement that is used in current dentistry and is commonly known as group function. This concept is based on the  observation that NWS contacts were destructive and therefore the teeth on the NWS should be free of any eccentric contacts, and instead the contacts should be distributed on the WS thus sharing the occlusal load. Group function is used when canine guidance cannot be achieved and also in the Pankey-Mann Schuyler (PMS) approach where it was deemed better than canine guidance as it distributed the loading on the WS better. Mutually protected occlusion. "The Journal of Prosthetic Dentistry" (2017) defines mutually protected occlusion as ‘an occlusal scheme in which the posterior teeth prevent excessive contact of the anterior  teeth in maximal intercuspal position, and the anterior teeth disengage the posterior teeth in all mandibular excursive movements’ In eccentric movements, damaging forces are applied to the posterior teeth and the anteriors are best suited to receiving these. Therefore during protrusive movements, the contact or guidance of the anteriors should be adequate to disocclude and protect the posterior teeth. In contrast, the posterior teeth are more suited to accept the forces that are applied during closure of the mandible. This is because the posteriors are positioned so the forces are applied directly along the long axis of the tooth and  are able to dissipate them efficiently whereas the anteriors cannot accept these heavy forces as well due to their labial positioning and angulation. It is therefore accepted that the posterior teeth should have heavier contacts than the anteriors in ICP and act as a stop for vertical closure. Additionally, in lateral excursions either canine or group function should act to disclude the posterior teeth on the WS because, as described above, the anterior teeth are best suited to dissipate damaging horizontal forces, as well as the contact being further away from the TMJ, so the forces created are decreased  in strength. Group function or canine guidance should also provide disocclusion of the teeth on the NWS as the amount and direction of force applied to the TMJ and teeth can be destructive due to an increase in muscle activity. An absence of NWS contacts also allows smooth movement of the working side condyle as a contact may disengage the guidance of the condyle and therefore cause an unstable mandibular relationship. Deflective contacts and interferences. A deflective contact is a contact that diverts the mandible from its intended movement. An example of this is when the mandible is deflected into  ICP by the RCP-ICP slide, which path is determined by the deflective tooth contacts. This is often involved in function (e.g. chewing), however in some cases these deflective contacts can be damaging and may lead to pain around the tooth (often associated with bruxism). However, some patients may be totally unaware of similar deflective contacts suggesting that it is the patient's adaptability rather than the contact that may influence the patient's presentation. An occlusal interference is any tooth contact that prevents, or hinders harmonious mandibular movement (an undesirable tooth contact). The occlusal interferences may be classified as  follows: When the dentist is providing restorations, it is important that these do not create an interference, otherwise the restoration will receive increased loading. As for deflective contacts, interferences may also be associated with parafunction such as bruxism (although evidence is weak) and may adversely affect the distribution of heavy occlusal forces. Interferences may also cause pain in the masticatory muscles due to altering their activity, however there is large controversy and debate as to whether there is a relationship between occlusion and temporomandibular disorders. Almost all dentate individuals have occlusal interferences, and therefore they are not seen to be  an etiology of all TMDs. When there is an acute change or significant instability in the occlusal condition and subsequently represents an etiological factor for a TMD, occlusal treatment is required. Occlusal adjustment (removal of occlusal interferences) may be carried out in order to obtain a stable occlusal relationship and is achieved by selectively grinding the occlusal interferences or through wear of a hard occlusal splint to ensure true retruded relationship is established. Section::::'Ideal' occlusion. When there is an absence of symptoms and the masticatory system is functioning efficiently, the occlusion is considered normal or physiological. It is understood that  no such ‘ideal’ occlusion exists for everyone, but rather each individual has their own 'ideal occlusion'. This is not focused on any specific occlusal configuration but rather occurs when the person’s occlusion is in harmony with the rest of the stomatognathic system (TMJ, teeth and supporting structures, and the neuromuscular elements). However, an optimal functional occlusion is important to consider when providing restorations as this helps to understand what is trying to be achieved. It is defined in established texts as: 1. Centric occlusion and centric relation being in harmony (CO=CR) 2. Freedom in CO 3. Immediate  and lasting posterior disocclusion upon mandibular movement 4. Canine guidance is considered the best anterior guidance system It is necessary to understand the concepts that influence the function and health of the masticatory system in order to prevent, minimise or eliminate any breakdown or trauma to the TMJs or teeth. Patient adaptability. There are various factors that play a role in the adaptive capability of a patient with regards to changes in occlusion. Factors such as the central nervous system and the mechanoreceptors in the periodontium, mucosa and dentition are all of importance here. It is in fact, the somatosensory  input from these sources that determines whether an individual is able to adapt to changes in the occlusion, opposed to the occlusal scheme itself. Failure of adaptation to minor changes in the occlusion can occur, although rare. It is thought that patients who are increasingly vigilant to any changes in the oral environment are less likely to adapt to any occlusal changes. Psychological and emotional stress can also contribute to the patient's ability to adapt as these factors have an impact on the central nervous system. Occlusal examination. In individuals with unexplained pain, fracture, drifting, mobility and tooth wear , a full occlusal examination is vital. Similarly when complex restorative work is planned it is also essential to identify whether any occlusal changes are required prior to the provision of definitive restoration In some people even minor discrepancies in the occlusion can lead to symptoms involving the TMJ or acute orofacial pain so it is important to identify and eradicate this cause. Instruments Required The examination should be carried out using a systematic approach whilst assessing the following: Extra-oral examination. 1) Facial Appearance The facial symmetry of the patient should be observed. The skeletal relationship of the patient should  then be identified and noted. The facial height of the patient should be considered and it should be noted where there may have been a loss. 2) Muscles Begin by simply palpating the muscles concerned with the occlusion of the teeth. These muscles include the muscles of mastication and other muscles within the head and neck area, such as the supra-hyoid muscles. It is best to palpate the muscles simultaneously and bilaterally. The temporalis, masseter, medial and lateral pterygoids, geniohyoid, mylohyoid and digastric muscles alongside the trapezius, posterior cervical muscles, occipitalis muscle and the sternocleidomastoid should all be checked  for any signs of wasting or tenderness. Temporomandibular dysfunction commonly presents with muscular tenderness, but pain or palpable soreness associated with the muscles can also be linked to parafunctional activity. 3) TMJ TMJ disorders can be detected through occlusal examination. Ask the patient to open and close whilst placing two fingers over the space of the TMJ. Opening of less than 35mm in the mandible is considered to be restricted and such restriction may be associated with intra-capsular changes within the joint. Following this, ask the patient to move their jaw to the right and following this, to  the left. Note any clicking, crepitus, pain or deviation. Intra-oral examination. 4) Maxillary / Mandibular Arch Assess each arch and identify whether there are any signs of occlusal disharmony, overloading, tooth migration, wear, craze lines, cracking or mobility (not due to periodontal causes). Abfraction, faceting and possible vertical enamel fracture lesions should also be noted if present. 5) Contacts in ICP Begin by assessing the incisor and molar relationship as described above. Similarly examine the overbite and overjet. An overbite of 3-5mm and an overjet of 2-3mms are considered to be within the range of normal. To  look at the ICP, articulating paper should be placed on the occlusal surface and the patient asked to bite together, which will mark their occlusal contacts. It is best to check these whilst the teeth are dry. 6) RCP The patient may be guided into CR using one of the follow methods; In some patients it may be difficult to guide the mandible into CR, for example in those with muscle tension, muscle splinting, occlusal disharmony or parafunctional habit. For these patients a Lucia Jig or deprogramming appliance can be constructed at chair-side. Mark RCP tooth contacts using articulating  paper, note the teeth which are contacting and identify whether this RCP position is causing problems related to the occlusion. For example if there is a heavy contact or interference in RCP this may be the cause of occlusal disturbance. It is important to be able to guide the patient into RCP, as a registration may need to be taken in this position particularly if the occlusion is being reorganised, the OVD is being changed or even just for diagnostic and treatment planning purposes. 7) RCP-ICP Slide The patient should be supine and relaxed. They should be placed into  RCP by the operator and then asked to bite together “normally”, this is moving them from RCP into their position of maximum intercuspation (ICP). Ask the patient to feel the slide and identify whether this is small or large. The slide should be smooth and the direction should be recorded. The operator should evaluate from both the side of the patient and the front of the patient, how far the mandible travels both forward and laterally (however this is difficult and it may be easier to observe by mounting casts onto an articulator). This can be done by observing the  maxillary and mandibular incisors during the slide. The RCP-ICP slide for most dentate patients tends to be roughly 1–2 mm in an anterior and upward direction. A deflective RCP-ICP slide, can have some relation to an anterior thrust. An anterior thrust, which is likely to be associated with the anterior teeth or other teeth involved in guidance such as canine teeth, often causes the teeth to exhibit fremitus. 8) Protrusive Movements The patient is asked to move their mandible forward from ICP. This is commonly around a distance of 8-10mm and would normally be until the  lower incisors slide anterior to the incisal edges of the maxillary anteriors. Observe the contacts during this movement. Mark the contacts using coloured articulating paper alongside the ICP contacts, which should be in a different colour - any teeth providing guidance and any interferences should be noted. 9) Lateral Excursions The patient is also asked to move their lower jaw to one side. Lateral movements should be measured and measurements of 12mm are thought to be normal. Both working side and non-working side should be observed during this movement. Record any teeth that are providing guidance during this movement  and any interferences that are present (and the location of these). Smooth and unbroken contacts should be identified when these excursive movements are recorded 10) OVD If occlusal wear can be seen, a Willis gauge is used to measure the occlusal-vertical dimension and the resting vertical dimension of an individual. Take a measurement by placing two reference points on the patients face, one under the nose (usually the columella) and one under the chin. Take one measurement whilst the patient is resting (teeth should not be contacting) and one with the patient biting together i.e. in ICP and take  this measurement away from the resting measurement to give the freeway space. The normal freeway space is usually 2-4mm. Patients with considerable tooth wear may have lost occlusal vertical dimension (OVD). When restoring the dentition, it is important to be aware of the exact OVD the patient has and by how much you may be increasing this. Patient’s may not be able to adapt to a large increase in OVD and therefore this may have to be done in phases. Clinical applications of occlusion. Occlusion is a fundamental concept in dentistry yet it is commonly overlooked as  it's perceived as being not important or too difficult to teach and understand. Clinicians should have a sound understanding of the principles regarding occlusal harmony in order to be able to recognise and treat common problems associated with occlusal disharmony. Some of the advantages associated with a working knowledge of these include: Simple occlusal adjustment. Involves simply grinding down involved cusps or restorations and may be indicated after careful examination when: Complex occlusal adjustment or reorganisation. May be required in more severe circumstances and some examples of these include: Achieving a satisfactory occlusal reorganisation involves choosing a desired jaw  relationship (either conforming to existing ICP or producing a new ICP coincident with CR), deciding on the intercuspal contacts (removing deflective contacts and adjusting shapes/inclines of teeth), adjusting excursive contacts (removing interferences) and aiming for a mutually protected occlusion. This is an extremely complex process and entails a clinical occlusal examination as described above, along with detailed examination of mounted study casts and diagnostic wax-ups. Mounted study casts. It is common practice to mount mandibular and maxillary casts (impressions are made of the teeth and poured in dental stone) in an articulator in ICP when constructing restorations that  conform to the patient's existing occlusion. Casts mounted on an articulator in ICP are useful for diagnostic purposes or simple restorations, but where more extensive treatment is planned it is necessary to consider occlusal contacts relative to CR e.g. RCP -> ICP slide. Other situations a CR registration may be more appropriate than ICP include where there are plans to reorganise or adjust the existing occlusion (including changes to the occlusal vertical dimension). In these circumstances, in order to accurately stimulate mandibular movement around CR (particularly opening and closing of the mouth), using a facebow transfer, the maxillary cast  should be mounted in a semi-adjustable articulator and then the mandibular cast should be mounted using a CR registration. The patient's new occlusion is then arranged so that the new ICP occurs when patient is in CR. Diagnostic wax-ups. Wax-ups are indicated where changes to the occlusion or aesthetics are planned. Diagnostic wax-ups are when changes are made to the shapes of the teeth by methodically adding wax to the articulated stone casts representing the patient's teeth. This can be done in order to demonstrate to the patient what the planned restorations will look  like, but can also be invaluable when simulating different occlusal schemes, studying the functional occlusion as well as providing temporary coverage whilst the restoration is being constructed by the lab through use of a matrix. Once an established plan has been constructed using the wax-ups, these can be used as a tool to guide the desired outcome in the mouth and provide a useful communication tool with both the dental laboratory and the patient.