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The aims of this chapter are to consider the importance of anterior guidance to normal function and to describe how restorations may be vulnerable to repetitive loading, especially where they are incorrectly contoured. We recommend that readers view animations B–F to bring to life the figures marked with DVD logos.
At the end of this chapter, the clinician should:
appreciate how teeth function during mastication
realise that weakened and restored teeth are vulnerable to repeated stresses and strains
understand that “guidance teeth” are important to guide the mandible to and from the intercuspal position (ICP) but will be subject to repetitive loading
understand the terms associated with lateral excursions: canine guidance, group function, working side and non-working side, and protrusion
comprehend how posterior teeth move across each other in lateral excursions
realise the potentially catastrophic consequences of interfering with anterior guidance.
Even in normal function, occlusal loads are repetitive and can be heavy. Although natural teeth may be able to take this load for a lifetime, teeth weakened by disease and restoration may not. Therefore, dentists need to take care to ensure that teeth are not subjected to damaging loads during normal day-to-day function. To understand how such damage may happen, it is important to understand how the teeth are involved in function. In the previous chapter, we looked at how the teeth meet in ICP, in which they transmit maximum force during eating. It is easy to appreciate how a molar weakened by a large mesio-occluso-distal restoration (or worse still, root-canal treated with incipient cracks) may suffer fracture under heavy forces developed during chewing hard foods; for example, those containing whole grains. However, even with a soft refined diet, under repeated loading teeth are at risk from excursive contacts. Understanding the nature of these contacts and how to work with them is fundamental to successful restorative dentistry.
Teeth have evolved for one major functional purpose: eating. The shape and position of the crowns and roots of teeth are no accident and have taken millions of years of evolution to acquire their current form and relationship. Practitioners, despite working on dentitions every day, sometimes forget how the masticatory system works, in particular, how harmonious function relies on teeth that guide mandibular movement into and away from ICP, providing what is termed “anterior guidance”. To identify which teeth are the guidance teeth, simply ask the patient to slide their jaw out to the left or the right with the teeth in light contact and note which teeth provide “gliding” (guiding) contacts. Now repeat this with the jaw pushed forward in protrusion. It is worth remembering that the movement you are asking the patient to perform (sliding out from ICP) is the reverse of what happens during biting or chewing. It is also worth emphasising that this anterior guidance may involve both anterior and posterior teeth.
Guidance teeth are important because they come into functional contact hundreds of times each day. As such, they need to be sufficiently robust to resist the wear and tear associated with heavy and often non-axial occlusal loads.
During biting, the function of the incisors is to incise, or cut, the food in an edge-to-edge position and then guide the mandible in towards ICP along a protrusive path of excursion (“incisal guidance”). Clearly, the potential to provide such guidance will depend on the amount of overlap between the upper and lower anterior teeth. Where these teeth do not contact during protrusion, for example in a Class III incisor relationship, other teeth – canines, premolars or molars – provide the guidance.