Chapter 6 Occlusion
Occlusion is the way in which the maxillary and mandibular teeth come together. This definition conjures up a static relationship; however, in function the teeth move across one another and this articulation or dynamic occlusion is equally important. These tooth contacts cannot be looked at in isolation as the masticatory system also involves the periodontium, the skeletal components (including the temporomandibular joints) and the neuromusculature.
For some the study of occlusion is shrouded in mysticism and for others there is a conviction in philosophy that has seen occlusal adjustments and rehabilitations carried out solely to ensure a patient’s occlusion fits the perfect ideology. Such conviction has been based upon subjective workings of various individuals and little sound evidence. Every patient is an individual and their occlusal management should be customized accordingly. However, when extensive rehabilitation of the dentition is required, some of the traditional teachings are useful.
It is important when restorations are placed that they are in functional harmony with the masticatory system to ensure a comfortable functioning apparatus. Failure to do so has been implicated in the aetiology of temporomandibular disorders and bruxism.
Bruxism is a parafunctional activity which involves the clenching and grinding of teeth. This can occur consciously when awake (awake bruxism) or at night when asleep (sleep bruxism). Awake bruxing is more common in females, has been linked to anxiety and stress, and is thought to affect 20% of the population. The prevalence of sleep bruxism decreases with age, with it being reported in 14–18% of children, 8% of adults and only 3% of elderly. Sleep bruxism is frequently noisy and reported by partners, and individuals may wake up with stiffness and aching of the jaws. Complaints of headaches are also common. Occlusal interferences were once thought to provoke sleep bruxism, but it is now thought of as a sleep-related movement disorder.
Most bruxism is mild and non-damaging to tooth tissue; as such, no treatment is required. However, for some, tooth wear and fracture may occur or the noise of grinding is unacceptable for partners and treatment is sought. For these patients a correctly adjusted hard acrylic splint can be supplied for night-time or daytime wear to protect the teeth. Such splints are referred to as stabilization splints and can be worn in the maxillary (Michigan splint, Figure 6.1; see also Chapter 4) or mandibular (Tanner appliance) arch.
Figure 6.1 A Michigan splint being fitted and occlusion being checked to ensure all lower teeth occlude with the splint (A, B). Manufacture using casts mounted on an articulator reduce the amount of adjustment. The splint should have a canine rise in lateral excursions (C, D).
An alternative splint is the localized occlusal interference splint (LOIS appliance, Figure 6.2). This consists of an acrylic plate retained by suitable clasps and two ball-ended wires which are placed between opposing teeth to interfere with the occlusion. Occlusal loading of the ball interferences leads to stimulation of periodontal mechanoreceptors, afferent feedback and reduction in occlusal loading. When used for bruxing habits, both splints act as a ‘habit breaker’ and patients can be gradually weaned off them.
Figure 6.2 Localized occlusal interference splint (LOIS appliance). The ball-ended wires on the occlusal surface of the canine–premolar teeth interfere with the occlusion and are aimed at breaking a bruxism habit.
(Courtesy of Dr John Radford)
If tooth destruction has taken place as a result of bruxism and restorative management is required, damage to any restorations can occur and it is important that the patient is aware of this. Damage to new restorations can be reduced by correct choice of dental materials, namely metal occlusal surfaces where possible and protection of restorations with a stabilization splint which can be worn at night for sleep bruxism and during the day if necessary for awake bruxism.
Within the dental literature there seems to be no agreement as to a definition for temporomandibular disorders (TMD). What is clear is that TMD covers a number of complex conditions with common signs and symptoms. The conditions are defined in the glossary of prosthodontics terms as those producing ‘abnormal, incomplete or impaired function of the temporomandibular joint(s)’. The most common conditions making up these disorders, together with their signs and symptoms, can be seen in Figure 6.3. A number of terms have been given to the signs and symptoms allotted to pain dysfunction syndrome, some of which include myofascial pain, craniomandibular dysfunction and mandibular dysfunction. Rarer causes of TMD are rheumatoid arthritis, psoriatic arthritis, developmental defects, infection, neoplasia and ankylosis.
Epidemiological data indicate that signs and symptoms of TMD are relatively common (50–75% and 20–25%, respectively) and are evenly distributed between men and women; however, only 3–4% of the population actually seek treatment and these are more likely to be women, hence it is frequently and incorrectly cited that women are affected more than men.
Many theories on the aetiology of TMD have been put forward and, to a degree, management has been dictated by these thoughts. Whilst bruxism is a separate entity, it has been linked with TMD, possibly as one of the aetiological or exacerbating factors. Historically the occlusion (malocclusions, interferences and non-working side contacts) was also incriminated in causing TMD but as more is known about the condition it is realized that it has a complex and multifactorial origin and that occlusion plays a relatively minor role. Occlusal adjustments have been shown in numerous studies neither to prevent nor treat TMD and should therefore not be considered. TMD patients have been shown to have higher levels of stress, anxiety, depression and aggressive behaviour and as such it is thought that psychological factors may be more important.
TMD can be treated in a number of ways. Simple advice and reassurance that the condition is relatively common and not sinister may be sufficient. A simple change to a soft diet for a period of time, jaw rest and a hot pack over the tender areas can be advised. Medical management can include non-steroidal anti-inflammatory drugs which can be used topically (Mentholatum Deep Relief gel containing ibuprofen) or systemically if not contraindicated by any other medical condition such as asthma or gastritis. For the restorative dentist, the most common, time-honoured and least invasive form of treatment is to provide the patient with a stabilization splint. This treatment is often successful but why it is, is less clear. It may be: (1) a simple placebo effect; (2) the splint may disengage the occlusion, so eliminating any occlusal interferences and a reflex reduction in muscle activity; (3) the splint’s repositioning effect may allow recapture of an anteriorly displaced disc; or (4) the increased occlusal vertical dimension alone may lead to relief. It has been shown that the greater the splint thickness, the more rapid the relief of symptoms. If these simple non-invasive treatments are not successful, thought should be given to referral to a specialist where other modalities of treatment such as prescription of muscle relaxants or antidepressants can be considered.
It is advisable to avoid complex and advanced restorative work in patients who are or have suffered from TMD. Their adaptation to the smallest changes in occlusion may be poorly tolerated and signs and symptoms of TMD may be exacerbated. Such patients can also be very focused on their ‘bite’ and have unrealistic expectations. If restorative treatment is unavoidable, treatment of the TMD needs to be carried out first and great care needs to be taken to ensure that any changes to the occlusion are gradual and staged.
To enable the correct management of the occlusion, an understanding of its components and their function is important; this unfortunately is frequently overlooked in a clinical examination. This section details what should be examined and recorded.
The facial appearance can firstly be assessed for facial asymmetries and skeletal relationship. With the patient’s teeth in contact, the lower face height should be assessed in proportion to the total face height, especially in patients complaining of missing teeth and tooth wear. Loss of teeth and occlusal stops could result in an overclosed appearance but a reduction in lower face height is unusual in patients with tooth wear. This is because of dento-alveolar development or compensation which counterbalances the loss in coronal tooth tissue height. In fact it has been suggested that dento-alveolar development takes place in the absence of tooth wear and some researchers have shown an increase in face height with age. This is the mechanism which leads to overeruption of teeth when taken out of occlusion – for example, by loss of an opposing tooth (Figure 6.4).
The patient should be asked to open and close their mouth. The extent to which they can open should be measured between the incisal edges of the upper and lower incisor teeth. At maximum opening this should be more than 35 mm for women and 40 mm for men. Any limitation to opening should be recorded. Any deviation of the mandible on opening and closing from a normal vertical straight line should be observed and described. Gentle pressure over the head of the condyle should be applied when the patient opens and closes. This should be repeated with a finger in the external auditory meatus. Any tenderness, clicking or crepitus (grating sensation) should be noted and when they occur in the opening–closing cycle.
Intercuspal position can be defined as the position of the jaws when the maxillary and mandibular teeth are in maximum intercuspation. This has also been referred to as centric occlusion. Patients usually close from a rest position immediately into this position due to a conditioned path of closure. It is important to establish whether the occlusion in ICP is stable; that is, an occlusion where there is no possibility of tooth movement, namely overeruption, drifting or tilting of teeth (Figure 6.5). If teeth are extracted or lost due to other causes, or if they lose their contour (a carious lesion cavitating, tooth fracture, tooth wear), occlusal contacts and interdental contacts, teeth can move, often complicating any subsequent restorative work (Figures 6.4 and 6.6).
(Courtesy of Suzanne Blacker)
When teeth overerupt in the absence of periodontal disease, the alveolar process remodels and the gingival margin moves with the tooth (see Figure 5.3, Chapter 5). This dento-alveolar compensation can happen quickly in some patients and is a reason why temporary crowns should be placed following tooth preparation for crowns or bridges; the occlusal and proximal reduction will result in an unstable occlusion and if no temporary is placed, overeruption and drifting of teeth could occur. This would result in a prosthesis that does not seat fully and appear to be occluding prematurely or appear ‘high’ in ICP.
Centric relation describes the jaw relationship between the maxilla and mandible when the mandible is in a retruded position. Differing definitions of centric relation have focused on slightly different positions of the condyle in the glenoid fossa and some on the relationship of the head of the condyle to the interarticular disc. These are rather academic arguments as clinically the position of the condyle cannot be visualized or confirmed without complex equipment. A more pragmatic and practical definition was proposed by Christensen in 2004 which conforms to most dentists’ clinical practice. He described centric relation as the ‘most comfortable posterior location of the mandible when it is bilaterally manipulated gently backward and upward into a retrusive position’. When this is done the mandible 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 (Figure 6.7). Measured in the incisor region the arc of opening around the terminal hinge axis position takes place for about 20 mm before the condyles start to translate down onto the articular eminence (Figure 6.8). When the mandible closes in the terminal hinge axis position the first tooth contact is called the retruded contact position (RCP). The terminal hinge axis position is said to be the most reproducible jaw relationship; however, small variations from day to />