Occlusion (dentistry)

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:

• Protrusive movements: When the mandible moves forward from centric relation, this is considered as protrusion.[12] In Class I occlusion, the predominant contacts occur on the incisal and labial surfaces of the mandibular incisors and the incisal edges and palatal fossa areas of the maxillary incisors.[6]
• Lateral movements: When the mandible moves to the left or right, the mandibular posterior teeth move laterally across the opposing teeth. For example, when the mandible moves to the right, the right mandibular teeth move laterally across their opponents and this is termed the working side of the mandible (the side towards which the mandible is moving). In contrast, the left mandibular teeth move medially, downwards and anteriorly across their opposing posteriors and this is called the non-working side (the side to which the mandible is moving away from).
• Retrusive movements: This is when the mandible moves posteriorly from ICP. Compared with protrusive and lateral movements, retrusive movements are generally considerably smaller with a range of movement around 1 or 2 mm due to restriction by the ligamentous structures.[2]

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)

• The maximum lateral movement of the mandible to the left or right side is approximately 10-12mm[2]
• The primary movement in lateral excursions occurs on the non-working side (NWS) condyle (also called the balancing or orbiting condyle). The NWS condylar head moves in a downward, forward and medial direction. This movement is defined against two separate planes, the vertical and horizontal plane
  • Bennet Angle : the angle of medial movement on NWS condyle relative to the vertical plane
  • Condylar Angle : the angle of downwards movement of the NWS condyle relative to the horizontal plane
• The working side (WS) condyle (also called the rotating condyle) undergoes an immediate, non-progressive lateral shift. This movement is called the Bennet movement (however this may be confused with the Bennet Angle), so this is more commonly referred to as an Immediate Side Shift. The condyle is seen to rotate with a slight lateral shift in the direction of movement[6]

Protrusive movements

• The condylar heads predominantly translate forwards and downwards along the distal face of the articular face in the glenoid fossa. Protrusive movements are restricted by the ligamentous structures to a maximum of ~8-11mm (depending on skull morphology and size of subject)[2]

Retrusive movements

• As for retrusion, this movement is restricted by the ligamentous structures and the maximum retrusive limit is usually ~1mm however 2-3mm is rarely seen in some patients.[2]

We can not influence the posterior guidance system through dental restorative treatment.

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 during right lateral excursions (Institute of Dentistry University of Aberdeen)

Canine Guidance

• Dynamic occlusion that occurs on the canines (on the working side) during lateral excursions of the mandible.
• These teeth are best suited to accept horizontal forces in eccentric movements due to their long roots and good crown/root ratio
• It is easy for the dental technicians during wax up and construction of restoration to provide this

Group Function

• Multiple contacts between the maxillary and mandibular teeth in lateral movements on the WS whereby simultaneous contact of several teeth acts as a group to distribute occlusal forces.
• It is preferable for this guidance to be as anterior as possible e.g. premolars rather than molars, as there is increased force applied when the contacts are closer to the TMJ.

Incisal Guidance

• The influence of the contacting surfaces of the mandibular and maxillary incisors on mandibular movements[12] is characterised by the overbite and overjet of the maxillary incisors.

In restorative treatment, it is possible to manipulate and design anterior guidance systems that is in harmony with the masticatory system.

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.

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.[15] 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.[16]

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[17] 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.[18]

Mutually protected occlusion - Posterior disocclusion of teeth as the mandible is protruded (Institute of Dentistry Aberdeen University)

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’[12]

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.[19] 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.[20]

1) Facial Appearance[26]

The facial symmetry of the patient should be observed.

The skeletal relationship of the patient should then be identified and noted.

• Class I: The maxilla and mandible are in harmony and coincide
• Class II: The maxilla lies anterior to the mandible and is retrognathic
• Class III: The maxilla lies posterior to the mandible and is prognathic[27]

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.[28] 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.[29] Temporomandibular dysfunction commonly presents with muscular tenderness,[25] 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.[28] 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.[25]

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).[25] Abfraction, faceting and possible vertical enamel fracture lesions should also be noted if present.[30]

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[2] and an overjet of 2-3mms are considered to be within the range of normal.[13]

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.

• During ICP, most opposing teeth should be contacting[2]
• Close examination of these contacts marked by the articulating paper help to identify the nature of the tooth contacts
• Good stable contacts often appear as small and not very prominent markings when articulating paper is used and there are multiple contacts on each tooth
• Broad and rubbing contacts identified in ICP may be associated with disturbances in function and may indicate occlusal instability[25]
• These contacts can be verified using Shimstock (a 12.5μm thick mylar strip) and the stability of the contacts can be checked
• The operator should pull the Shimstock through the teeth, whilst the patient is biting together
• This should be carried out for each set of teeth and will highlight if there is adequate contact to hold the Shimstock
• This material is appropriate as it is thinner and will eliminate any false contacts that may occur with even thin articulating papers that are roughly 20μm thick
• One is also able to pull shim stock through when patients are biting together unlike other articulating paper, which will tear

6) RCP

The patient may be guided into CR using one of the follow methods;

• Bimanual manipulation- manipulating the patient's condyles so they are in CR
• The operator should lightly rest their fingers along the inferior border of the mandible and their thumbs should lie lightly on the anterior aspect of the chin
• When the patient is relaxed place light downward pressure on the chin and light upward pressure under the angle of the mandible
• Deprogramme the jaw by guiding the opening and closing of the jaw and once the patient is relaxed, ask them to close gently and stop when they feel teeth first contacting
• Chin point guidance- one hand is used to apply pressure to the chin guiding the chin posteriorly with some force

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.[25] The slide should be smooth and the direction should be recorded.[30] 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.[25] The RCP-ICP slide for most dentate patients tends to be roughly 1–2 mm in an anterior and upward direction.[30] 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.[25]

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.[28] 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[25]

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.[31]

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.

Involves simply grinding down involved cusps or restorations and may be indicated after careful examination when:

• Overloading of occlusal forces has resulted in pain, tooth fracture or mobility
• Interocclusal space is required for restoration provision (e.g. in the case of an overerupted tooth where occlusal plane corrections required)

May be required in more severe circumstances and some examples of these include:

• Elimination of an anterior thrust causing pain, wear, drifting or mobility
• To provide space for anterior restorations
• Management of bruxism (however, uncommon)
• The elimination of a temporomandibular joint disorder (however, as previously mentioned, occlusion is rarely an aetiological factor for TMD so there should be significant evidence to support this before alteration of the occlusion is pursued)

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.[25] 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.

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).[25] 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.

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.