Registration of the Jaw Relationship
The aim of this chapter is to describe the stages involved in the recording of the jaw relationship for an edentulous patient.
At the end of this chapter, the practitioner should understand the need to prescribe occlusal rim construction with respect to the individual patient’s requirements. It should be recognised that the manipulation of occlusal rims to record the jaw relationship and to indicate the desired tooth position must be undertaken in a sequential manner with reference to the treatment plan. The clinician should also understand the rationale for the choice of articulator used to mount working casts for setting up trial denture teeth.
Once the working impressions of the patient’s denture-bearing area have been made, the clinician should prescribe the construction of occlusal rims. The prescription features should include details of the dimension of the wax rims and the material from which the base should be constructed.
Regarding the rim dimensions, the clinician should refer to the treatment plan. If no major changes are to be made to the vertical height of the existing dentures, then the rims should be of similar vertical and horizontal dimensions. When changes are planned then the size of the rims should reflect these changes and prescribed accordingly. Given an accurate prescription, clinical time will be saved in reducing or building up rims at the chairside.
There are a number of materials available for the construction of the bases:
self-cured acrylic resin
heat-cured acrylic resin
non-precious metal alloy
thermoplastic materials (shellac).
Self-cured acrylic resin, wax and thermoplastic materials are considered as “temporary bases” in that they are not incorporated into the final denture. They are easily manipulated and the bases can be constructed quickly. However, these materials are not dimensionally stable, and are poorly adapted to the underlying tissues. This can make them difficult to control in the patient’s mouth. Heat-cured bases (Fig 4-1) are considered “permanent bases” as they are ultimately incorporated into the finished denture. When the working impressions have been cast, the base is kid processed, deflasked and polished. A wax rim is then added to the processed base and it is returned to the clinician on a duplicate model. If the working impressions were satisfactory, the bases should be stable and retentive in the patient’s mouth. This, in turn, should make the process of recording the jaw relationship much easier, particularly when there has been significant resorption of the residual alveolar ridge. There are no absolute contraindications to using these bases, but there are some potential disadvantages, such as:
The base will have to go through a second curing cycle when processing the teeth to the base. In theory, this could cause warpage of the base.
They consume a greater amount of technical time and therefore are more expensive.
If there is little inter-ridge space, or if the labial aspect of the anterior maxilla is prominent (i.e. in a skeletal Class 2 case), then room for the wax rim is reduced. In this situation, reduction of the wax rim may leave little wax to retain teeth, and the clinician or technician may have to remove some of the base to add teeth for a trial set-up.
The working cast is destroyed to make the base.
Metal bases can be made prior to the jaw registration stage, but it is prudent to wait until the trial denture stage has been completed. The rationale for this is that the position of the teeth helps to determine the extent of the base, and therefore an acceptable tooth position should be determined first. They are indicated when there has been a history of denture fracture. However, it should be remembered that they are difficult to adjust and tend to be heavier than acrylic bases. In addition, other alternatives such as high-impact acrylic should also be considered.
The terminology used throughout the remainder of this text is that contained in the Glossary of Prosthodontic Terms. In a dentate patient, the position of maximum intercuspation (ICP) or of first contact in a retruded arc of closure – the retruded contact p/>