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Denture Occlusion 101

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Denture Occlusion 101

Answers About Denture Occlusion

Do you view complete dentures as a low cost, last resort prosthetic? If you do, you are not alone. In truth, the clinician and dental technician are tasked with fabricating a full-mouth rehabilitation: restoring esthetics and phonetics, as well as masticatory function. This requires a high level of commitment, knowledge, and skill from all parties involved. A properly planned and fabricated complete set of dentures can be a life-changing experience for most patients.

I frequently receive questions about denture occlusion, both by my peers, as well as the general practitioner. Therefore, I will attempt to answer some of them as comprehensively as possible below.

Anterior Contact in Centric

Patients often ask their clinicians to assure contacts in the anterior teeth in centric occlusion, especially when the patient presents with well-worn appliances featuring this type of contact.  Chances are the anterior occlusal contacts are present, but it’s not by design. They are present because of a slow loss of vertical dimension of occlusion (VDO) in the posterior over the dentures’ lifetime; the patient has simply gotten used to it (“Lettuce Leaf-Occlusion”).

Proper Amplitude of overbite and overjet

Figure 1. The image shows the proper amplitude of overbite and overjet.

It is, however, of the utmost importance to avoid this type of anterior contact in removable complete dentures. As the patient moves the mandible into protrusive excursion, the resulting anterior guidance is likely to break the seal of the maxillary denture, thereby dislodging it. This can lead to obvious functional issues and repeated sore spots on the anterior residual ridge.

The severity of this effect depends on the angle of the TMJ and, to some degree, on the cuspital angulation of the posterior teeth.

To avoid this problem, the dental technician is aiming to achieve an overbite/overjet ratio of 1 to 2mm, depending on the Angle Class and ridge relation. (Figure 1)

Positioning of the Posterior Teeth in Relation to the Ridge

Where should you position the posterior teeth in most removable appliances? The short answer is on the ridge. Or more precisely the lingual cusps of the maxillary teeth and central fissures of the mandibular teeth are aligned with the crest of the residual ridge.

While this is certainly one of the most widely researched and accepted guidelines, the answer is more complicated than this.

Quite often we encounter uneven resorption of the ridges, typically with the maxillary arch form becoming smaller, while the mandibular ridge often widens over time. The technician then is forced to set posterior teeth into a cross-bite relation in order to follow the prosthetic rules described above. If this guideline is ignored, it will make for a denture that is unstable under function, resulting in sore spots. It can also lead to repeated midline fractures of the maxillary denture base, as the masticatory forces are not absorbed by the bone, but rather are redirected into the buccal corridor.

The patient may be alarmed initially, because their natural dentition or any previous denture may have been in Angle Class I occlusion.

Functional Confines of the Posterior Set-up

Functional zone (green lines) and stop line (red)

Figure 2. Functional zone (green) and stop line (red) marked on mandibular master cast.

The well-trained dental technician will analyze and survey each master cast to determine certain landmarks used to establish the general occlusal plane. In addition, a ridge profile compass is used to transfer the contour of the mandibular posterior residual ridge onto the cast’s land-area for analysis. The dental technician will determine the lowest point in relation to the occlusal plane and will attempt to set the first molar as close to this point as possible; also known as the “functional zone”. This assures axial distribution of the masticatory forces onto the load-bearing structures of the mouth and stabilizes the denture. Additionally, the technician will mark where the alveolar ridge begins to rise significantly towards the retromolar pad; this will determine the posterior confine of the set-up (“stop line”). (Figure 2)

Ramp effect

Figure 3. Load on the posterior slope of the denture bearing structures causes dislodgement of the mandibular denture (Ramp effect)

If teeth are set distal to this stop line, dislodgement of the mandibular denture under function is extremely likely, along with repeated, painful sore spots. (Figure 3)

Because of this, the experienced dental technician will skip either a bicuspid or a second molar in the set-up. Again, this can be difficult to relate to the patient. The patient’s previous set of dentures may feature all eight posterior teeth.

 

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