Subir Banerji and Shamir B. Mehta
In this chapter, the means by which the occlusal surfaces of the mandibular cast relate to those of the maxillary cast will be described, thereby permitting the mounting of a set of casts against each other on the articulator. In order to accomplish this, there is a need to attain an accurate intra-occlusal record. Inter-occlusal records may either provide a record of the intercuspal position (ICP, synonymously termed the position of maximum intercuspation or MI) or of the centric relation (CR).
The CR record aims to record the relationship of the mandibular arch to that of the maxillary arch when the condyles are seated in their most anterior–superior positions in the glenoid fossae. In this position, opening and closing movements of the mandible take place in a rotational manner (as opposed to translation) for the first few millimetres and would correspond to rotational movements occurring at the dental articulator’s condylar housing.
Unlike the intercuspal record, which records the position where the antagonistic occlusal surfaces are maximally meshed together, the CR record is attained regardless of any given position of tooth contact. The CR record is often referred to as being a fixed and reproducible record in the literature; fixed in this context is in reference to a fixed anatomical position (independent of the occlusal surfaces), which is reproducible between that of the patient’s condyles and the condylar housing of the dental articulator, where rotational movements of the condyles will take place against the corresponding articular eminences.
Lateral excursive and/or protrusive mandibular records may also be taken in conjunction with the CR record in order to programme the condylar guides on the articulator, which would relate to the anatomical limits of the movements of the condyles in their glenoid fossae.
An intercuspal record is generally taken when mounting a set of working casts where the occlusal scheme is stable, often where relatively simple restorations are being considered; hence the need to conform to the existing occlusal scheme. The use of casts mounted in CR for this purpose may culminate in an undesirable occlusal interference.1
The CR record, in contrast, is advocated when fabricating study casts, thereby permitting evaluation of the retruded contact point (RCP), also referred to as the centric relation contact position (CRCP), which is the first point of tooth contact while the mandible is in CR. Evaluation of the RCP may otherwise be very difficult in the presence of the patient’s soft tissues and protective neuro-muscular reflexes.
Approximately 90% of patients have a slide between RCP and ICP; casts mounted in CR can readily elucidate this. In this case, any premature tooth contacts, also termed deflective contacts (which will guide the patient’s mandible from CRCP to ICP), can be noted, and the need for any occlusal correction determined prior to embarking on complex prosthodontic treatment plans.
For patients with pathological tooth wear, where active restorative intervention is being considered, the discrepancy between RCP and ICP may result in a level of inter-occlusal clearance. This may be utilised to place restorative materials when re-organising the occlusion without a need for increasing the occlusal dimension or subtractive tooth preparation to accommodate future restorations.2