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. guidance, natural teeth and function bennet angle - the tv (orbiting) condyle, as the mandible moves (progressive side shift). (institute of dentistry aberdeen university) 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) the maximum lateral movement of the mandible to the left or right side is approximately 10-12mm 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 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) 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. 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 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 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. 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 unilateral balanced occlusion mutually protected occlusion 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 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’ 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). non-working side interference (photograph) detectable with articulating paper or plastic shimstock, as the mandible moves to the left (working side). (institute of dentistry aberdeen university) the occlusal interferences may be classified as follows: working side interference: when there is a heavy or early tooth contact between the maxillary and mandibular teeth on the side that the mandible is moving towards, and this contact may or may not discludes the anteriors. non-working side interference: an occlusal contact on the side the mandible is moving away from that prevents harmonious movement of the mandible. these have the potential to be more destructive in comparison to ws interferences due to the obliquely directed forces. protrusive interference: contacts that occur between the distal aspects of the maxillary posterior teeth and the mesial aspect of the mandibular posterior teeth. these interferences are potentially very damaging and may even cause an inability to incise properly due to the close proximity of the interference to the muscle. 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. '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) there should be even and simultaneous contacts of all posterior teeth when the mouth is closed and the condyles are lying in their most