Occlusal Records and Articulators
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DENTURE IMPRESSION TECHNIQUES
Different philosophies for complete denture final impressions:
- Pressure/functional impressions – tissues are compressed during impression to simulate the same amount of pressure during functional loading (mastication).
- Non-pressure/mucostatic impressions – impressions with the soft tissue at rest (no pressure, passive).
- Selective pressure – pressure is selectively applied to those areas best suited for withstanding the forces of mastication using a custom tray which provides more relief over non stress-bearing areas.
The altered cast technique is used to record the edentulous alveolar ridge in function without tissue displacement. Generally impression materials cannot capture the anatomical form of the teeth and the physiological form of the soft tissue in function simultaneously. In the altered cast technique a metal framework is constructed on a cast produced by a mucostatic impression material. Base plates are then constructed on the framework in the saddle areas and border molding is carried out. A second impression is taken with the soft tissue in “functional load”, and this second impression taken to modify the original cast (hence altered cast). When properly related to the teeth, support is provided by teeth and the denture base simultaneously. The result is a potentially more stable RPD.
INTEROCCLUSAL RECORDS
Interocclusal records are used to record the relationship between the maxillary and mandibular arch. A centric relation record will allow for the cast to be mounted in (or close to) centric relation. A protrusive record registers the anterior-inferior condylar path and allows the clinician to set the condylar inclination on the articulator. A lateral excursion record allows the clinician to set the Bennet angle on the articulator.
A pantograph is an instrument that records the movement of the mandible and is used to program a fully adjustable articulator. The movement of the occlusal surfaces of one arch can be recorded using a medium attached to the opposing preparations. This is called a functionally generated path (FGP).
Posselt’s envelope of motion refers to the border movements in both the sagittal/longitudinal and coronal/frontal planes. Four things influence mandibular movement:
- Anterior guidance
- Right condylar architecture
- Left condylar architecture
- Neuromuscular control
Border molding allows for the capture of functional soft tissue movement in an impression. Recording the movement of the tongue, lips, cheeks etc. during the impression reduces the likelihood of displacement of the final prosthesis during masticatory function. Proper border molding can be assessed during the impression phase by testing for impression tray displacement in the mouth. In the case of the maxillary denture, proper border molding allows for the detailed capture of the most important zone, the mucogingival fold adjacent to the maxillary tuberosity area. An accurate impression of this area is extremely important for maximal retention. If a mandibular impression tray lifts when the patient opens their mouth, it is likely due to inadequate border molding (over-extension) in the buccal and labial area.
It can be difficult for a dental technician to design a fixed crown or bridge with perfect cuspal movement during function. The functionally generated pathway technique allows these functional movements to be recorded in wax intra-orally and transferred to an articulator using a static plaster cast as a functional index. A prerequisite for the use of this technique is the presence of an optimal occlusion, with all opposing surfaces properly restored, and no occlusal interferences before making the impression. The movement of the mandible must be directed from the eccentric to centric (not centric to eccentric).
ARTICULATORS
Articulators are mechanical devices used to simulate the movement of a patient’s temporomandibular joint. There are four types:
- Class I – non-adjustable articulators simulating vertical motion only (open and close, simple hinge). The maximum intercuspal position (MI) is the only position that can be reproduced. A facebow is not used.
- Class II – non-adjustable articulators simulating vertical and horizontal motion, but the motion does not “originate” from the TMJ. Some are full size, capable of accepting a facebow, and make provision for incisal guidance, but no adjustments of the posterior elements (condylar inclination, vertical axis of rotation, Bennet angle etc.) is possible.
- Class III – semi-adjustable articulators that use measurements to simulate the average patient’s TMJ movement. Most semi-adjustable articulators allow for some adjustments in the condylar inclination, lateral translation, side shift (Bennet angle), anterior guidance, and intercondylar distance. They utilize facebow and maxillo-mandibular records.
- Arcon – condylar elements are in the lower member and the condylar path elements on the upper member, closer to the anatomy of a real TMJ. This makes them more accurate for fabricating cast or porcelain (fixed) restorations due to correct tooth contacts during occlusion and excursions, especially when an interocclusal record is used to mount the lower cast. The angle between the condylar inclination and the occlusal plane is fixed.
- Non-arcon – condylar elements are in the upper member and the condylar path elements on the lower member. The angle between the condylar inclination and the occlusal plane is not fixed. Provides easier control when setting teeth for complete and partial dentures. An occlusal record in centric relation is needed for proper mounting. Occlusal records in protrusion and excursions are required to set the sagittal condylar guidance angles.
- Class IV – fully adjustable articulator which can accept three dimensional dynamic registration. These articulators have the potential to most accurately simulate the movement of the patient’s TMJ, capable of duplicating a wide range of mandibular movements. A pantograph is used to record the mandibular movement, and a kinematic facebow record is used to orient and articulate the maxillary cast.
The following are adjustments that may be made to articulators:
- Horizontal axis of rotation – the horizontal axis of rotation in relation to the maxillary cast.
- Intercondylar distance – the distance between the vertical axes of rotation.
- Bennet angle (Bennet movement) – simulating sideshift movement.
- Incisal guidance – simulates the anterior guidance of the patient’s teeth.
- Condylar inclination – the angle of the artificial articular eminence. Can also change directional guidance of the superior, posterior and medial walls of the fossa.
The functionally generated pathway technique allows cuspal movements during function to be recorded in wax intra-orally, then transferred to the articulator in the form of a static plastic cast. The functional index becomes a static registration of all movements of the opposing cusps. This functional index requires an optimal occlusal setup. Using this technique the lab technician is able to set up a restoration that is far less likely to incorporate occlusal interferences.
FACEBOW
A facebow is an instrument used to record the maxilla-hinge axis relationship (opening and closing axis), and to transfer this information to an articulator to mount the maxillary cast. A facebow transfer is not a maxillo-mandibular record and does not provide information about the interocclusal relationship (e.g centric relation, maximal intercuspation etc.). If done properly the arc of closure on the articulator should match that of the patient. A facebow record allows the teeth to be placed within a close radius of the patient’s arc of closure, and allows for better reproduction of lateral excursions to facilitate the occlusal setup. An accurate facebow registration is very important if the dentist/technician intends to change the vertical dimension of occlusion (VDO). Evidence indicates that the face bow has nothing to do with speech, the fit of the prostheses, facial contours, ridge morphology, the color of the teeth and denture bases, the arrangement of the artificial teeth, chewing efficiency, or the psychological aspects of prosthodontic treatment.
In a perfect world, the facebow would allow the perfect duplication of the patient’s TMJ movement onto an articulator, but this is impossible. Arbitrary facebows use subjective anatomic landmarks to estimate the position of the hinge axis. There are many examples of reference points used. The condylar axis lies roughly 13mm in front of the posterior margin of the tragus, on a line from the outer canthus of the eye and the center of the tragus (Beyron point). Or 10mm anterior to the center of the spherical insert for the external auditory meatus and 7mm below the Frankfort horizontal plane (Bergstrom point). Or 13mm anterior to the margin of the external auditory meatus (Gysi point). Arbitrary facebows usually yield an error of 2mm or less for most patients.
Kinematic facebows can be used to locate the actual hinge axis, but they are more difficult and time consuming to use.
SOUNDS IN DENTISTRY
Different sounds are produced by different anatomical areas in the mouth. Each group of sounds can be used to test an aspect of a prosthesis.
Linguoalveolar (alveolar) sounds – t, d, s, sh (shunt), z (decision), ch (chair), j (job), r (rub), y (your) – made through close approximation of the tip of the tongue and the anterior part of the palate, close to the lingual surface of the maxillary incisors. These sounds bring the maxilla and mandible closer together, helping to determine the vertical length and overlap of the anterior teeth and providing information about the vertical dimension of tooth placement. To pronounce the s sound, there needs to be a slight (1-1.5mm) separation between the incisors, termed the closest speaking space. This allows an air stream to escape through a slight opening between the teeth. If maxillary and mandibular incisors and premolars contact during sibilant sounds (s, sh, z, ch) the occlusal vertical dimension is likely too great.
The anterior dorsum of the tongue forms a narrow opening near the midline that results in a slight whistling sound. A pronounced whistling sound is indicative of having a posterior dental arch form that is too narrow, and results in faulty pronunciation of consonants like d, n, l, s, and t where the lateral margins of the tongue makes contact with the palatal surface of the maxillary posterior teeth. If the space is large (wide), the “s” sound will become an “sh” sound (lisp). A thicker denture base especially at the palate would affect the clarity of alveolar sounds.
If the maxillary anterior teeth are set too far palatally (or the palate is too thick), patients attempting an “s” may pronounce it more like a “th”, and “t” may sound more like a “d”. If these teeth are set too far labially, the “d” will sound more like a “t”.
Linguavelar (velar, or guttural) sounds – k, ng, g – made with the posterior dorsal surface of the tongue raised to the soft palate. Linguopalatal consonants such as k, ng, and g are affected by incorrect post dam extensions.
Linguodental (interdental) – th, as in “this”, “that” or “those” – made with the tip of the tongue extending between the maxillary and mandibular incisal edges. This sound provides information about the labio-lingual position of the anterior teeth. If the tongue is not visible during interdental sounds the teeth are set too far forwards. If the tongue sticks out during interdental sounds the teeth are too set back.
Labiodental – f, v, ph – made between the upper incisors and the lower lip, at the wet-dry junction. The labiodental sounds like “f”, “v” are helpful in determining the antero-posterior positioning of the maxillary incisors and occlusal plane. If the clinician observes the incisal edges are contacting labial to the wet/dry junction during f or v sounds the maxillary teeth were set too far labially. If the upper anteriors are too short, “v” will sound more like “f”. If the upper anteriors are arranged below the occlusal plane (too long), “f” will sound more like “v”.
Bilabial – b, p, m – made by the contact of the lips. These test the vertical dimensions and lip support.
If a patient is whistling while speaking it could be due to inadequate vertical incisor overlap, excessive horizontal incisor overlap, or the palatal arch being too high or too narrow. Compared to recently edentulous patients, a patient who has been edentulous for many years is going to exhibit more distorted speech because of the loss of tonus of the tongue musculature.
