Articulation has been defined as the static and dynamic relationship between the occlusal surfaces of the teeth during function. Management of articulation in the dentition is one of the most important aspects of modern dentistry. Failure to manage articulation can result in problems for the patient and for the restorations placed by the dentist.
The articulator is an often-neglected tool at the dentist’s disposal. An articulator is any mechanical instrument that represents the temporomandibular joints and jaws, to which maxillary and mandibular casts can be attached to simulate some or all mandibular movements. With over a century of development, the modern dental articulator is a very useful tool in the dentist’s armamentarium. The articulator has three basic functions:
- To study the static and dynamic relationships of the patient’s teeth, allowing the dentist to diagnose problems not readily apparent clinically.
- To plan treatment that involves the interrelationship of the teeth.
- To fabricate a prosthesis, extra-orally in the dental laboratory, in harmony with the patient’s dentition.
There are four basic articulator types available to the dentist. These have been classified as:1
- Class I: A simple holding instrument capable of accepting a single static registration – vertical motion is possible (Figure 1).
- Class II: An instrument that permits horizontal and vertical motion but does not orient the motion to the temporomandibular joints (Figure 2).
- Class III: An instrument that simulates condylar pathways by using averages or mechanical equivalents for all or part of the motion, often referred to as ‘semi’ adjustablearticulators. These instruments allow for orientation of the casts relative to the joints (Figure 3).
- Class IV: An instrument that will accept three-dimensional dynamic registrations, often referred to as ‘fully’ or ‘highly’ adjustable articulators. These instruments allow orientation of the casts to the temporomandibular joints and simulate mandibular movements (Figure 4).
In dental practice, the articulator classes that are most often used are the Class II for simple restorations such as a single crown, and the Class III, which can be used for more complex reconstructions, up to and including complete dentures or full mouth fixed reconstruction. Examples of common situations where a Class III articulator can be of value are: 1) where canine guidance is lacking; and, 2) fixed restorations that involve the most distal teeth in an arch. A Class III instrument is not as programmable as the Class IV; however, if its limitations are acknowledged and it is accepted that a degree of adjustment may be required when the prosthesis is placed, then its use is acceptable for all but the most complex cases.
Use of the facebow
For Class III, semi-adjustable articulators, positioning of the maxillary cast on the upper member is the first step in programming the articulator. This is done with the aid of a facebow. A facebow is a caliper-like instrument that records the spatial relationship of the maxilla to anatomic reference points and transfers this relationship to the articulator. The anatomic reference points are usually the mandibular condyles’ transverse horizontal axis and one other selected anterior point, usually referred to as the third point of reference.2 In many facebows this point is infra-orbital. There are two types of facebow: 1) a kinematic facebow, which uses the previously located transverse horizontal axis or the ‘true axis’ (Figure 5); and, 2) an average axis facebow, which uses an estimated position, based on prior, researched measurements, of the true horizontal hinge axis of the mandible (Figure 6). Average axis facebows are normally used with Class III articulators.
While not as accurate as kinematic facebows, research has indicated that average axis facebows are sufficiently accurate for most clinical purposes. For example, Schalhorn3 noted that a facebow with an arbitrary axis was within 5mm of the true kinematic value in 98% of the population. A 5mm error in the location of the hinge axis results in a 0.2mm error at the second molar. An error of this magnitude can be easily adjusted intra-orally.4
Modern facebows allow for quick and easy recording and transfer to the laboratory. Figures 7, 8 and 9 illustrate the sequence for making a record with a Whipmix® facebow. This facebow is designed to be used by a single operator, but utilisation of the dental assistant to position the earpieces can make the process extremely efficient. Utilisation of a mounting jig ensures that only a portion of the facebow, not the cumbersome earpieces, need to be sent to the laboratory (Figures 10, 11 and 12).
Attaching the mandibular cast
Once the maxillary cast is placed, the second step is to attach the mandibular cast to the lower member of the articulator. The mandibular cast may be mounted in centric relation (CR) or maximum intercuspation (MIP) depending on the specifics of the case. A jaw relation record must be made. Mounting in CR requires that the condyles articulate with the thinnest avascular portion of their respective disks, with the complex in the anterior-superior position against the shapes of the articular eminencies. This position is independent of tooth contact.1 A variety of techniques is available to position the condyles correctly. Once the condyles have been placed correctly, the interocclusal record is made at the correct vertical dimension (Figure 13). For most cases where the new restoration is being fabricated to conform to the current occlusion scheme, a maximum intercuspation record is made. One of the most accurate methods to do this is to locate occluding reference points intraorally, mark these on the models and use these reference points when mounting (Figure 14).
To complete the mounting, the third and last step is to use check bites to program the adjustable components of the articulator. This is done using protrusive and lateral records made intraorally.
Accurately mounted casts in an articulator provide the dentist and dental technician with an excellent tool to analyse, plan and fabricate dental restorations. Articulator utilisation can help to avoid problems and allow for minimal adjustment, saving chairtime and preserving thickness of restorative material when a new prosthesis is placed.
1. The glossary of prosthodontic terms. J Prosthet Dent 2005; 94: 10-92.
2. Wiens, J. Fundamentals of Occlusion. American College of Prosthodontists, 2015: 73.
3. Schalhorn, R.G. A study of the arbitrary center and kinematic center of rotation for facebow mounting. J Prosthet Dent 1957; 7: 162-169.
4. Weinberg, L.A. Evaluation of the face-bow mounting. J Prosthet Dent 1961; 11: 32-42.
Dr Paul Quinlan
Specialist practice limited to prosthodontics and periodontics,
Quinlan Dental Care, Dublin. www.quinlandentalcare.com