Managing the unstable mandibular complete denture – tooth placement and the polished surface

Placement of teeth

Conventional biometric practice advocates placing the maxillary anterior teeth labial to the ridge to compensate for the inward pattern of maxillary bone resorption. Classically, the incisive papilla is used as a reference point and the labial aspect of the central incisors placed 8–10 mm anterior of the papilla.¹² Maxillary posterior teeth should be set forward of the anterior aspect of the tuberosities.¹³ Teeth are set buccal to the ridge, ensuring that the palatal cusps contact the central fossae of the lower molars and transmit occlusal forces on to the crest of mandibular ridge.⁹ Full palatal coverage and the typically larger volume of residual ridge in the maxilla allows for greater freedom in tooth positioning without leading to instability.

In contrast, less freedom exists in the lower arch, particularly with atrophic mandibles. The frequently lower quality and reduced size of the residual ridge provides little resistance to both lateral and tipping forces produced during mastication. Various concepts and suggestions regarding tooth positioning have been proposed in order to increase stability. Commonly, it is suggested that the posterior teeth are placed perpendicular over the crest of the residual ridge in order to gain direct opposition to compression from the supporting tissues.^14,15,16 Pound suggested placing the lingual surfaces of the lower molars within a triangle formed between the mesial of the canine anteriorly and the most lingual and buccal points of the retromolar pad distally (Figure 1). The proposal being that stability is aided and the natural relationship of the teeth to the tongue and retromolar pad is maintained.¹⁷ Other variations do not lie much outside this zone, some suggesting a lingual position, whilst others suggest a lateral position over the buccal shelf to compensate for the pattern of mandibular bone resorption and provide greater tongue space.^16,18,19 Furthermore, teeth should not extend as far distal as the retromolar pad as the compressible soft tissue and curvature of the mandible may lead to instability of the denture during function. If there is insufficient space, a premolar or molar may need to be omitted from the set-up.^14,20

The lower incisors are also placed on the anterior aspect of the ridge.¹³ However, care must be taken that the teeth are not set excessively labially as this will produce a cantilever effect, may compromise lower lip activity, and lead to an anterior-posterior rock in function. Conversely, if set too far lingual, teeth will encroach upon the tongue space, resulting in a lisp.¹²

Ultimately, tooth positioning is dictated by what is functional and acceptable to the patient.

Occlusal schemes

The three main occlusal schemes are bilateral balanced, lingualized and monoplane occlusions. Bilateral balanced occlusion is commonly taught throughout the majority of UK teaching hospitals.

In this scheme, the posterior teeth are set up with the maxillary palatal cusps occluding with the opposing central fossa. The incorporation of antero-posterior and mediolateral compensating curves help provide non-working side contacts in lateral excursions and protrusion (balanced articulation), which are intended to aid stability during function and prevent Christensen's phenomenon (occurrence of a posterior open bite on mandibular protrusion).

The use of zero degree cusps in a monoplane occlusion has been suggested in the cases of the resorbed lower ridge and Class II div 1 or Class III skeletal bases to reduce instability caused by excursive interferences. At first glance this may seem sensible, but this set-up does not actually allow balanced articulation²⁰ because the lack of cusp angles prevent contact being maintained during lateral excursions.

An alternative concept is that of lingualized occlusion. The basic principle involves angling the maxillary teeth so that only the palatal cusps contact during centric occlusion and lateral excursions. The mandibular tooth is selectively ground to produce a shallow central fossa and shallow cusps; this allows both balanced working and non-working contacts²¹ (Figure 2). It is thought to improve stability of the lower denture due to the reduction of lateral stresses and dislodging forces. Theoretically, occlusal forces are shifted to the lingual side of the ridge, thus improving stability during mastication²² (the buccal flange will act to brace against any tipping motions). The frequency of prematurities in comparison to bilateral balanced occlusion is also reduced, lowering the risk of the denture being dislodged during function.¹⁵

A number of studies have investigated the potential differences in performance of different occlusal schemes. The results tentatively suggest that cusped teeth (bilateral balanced occlusion) and lingualized occlusion may improve mastication over 0 degree cusps,²³ but there was no benefit in stability or masticatory function noted between lingualized or bilateral balanced occlusions.^21,22 From a practical and clinical viewpoint, however, balanced lingualized occlusion is probably easier to achieve accurately than bilateral balanced occlusion and therefore can be recommended. One should remember in this instance though, when adjusting the articulation, that only the lower lingual cusp should be adjusted so as not to affect the occlusal vertical dimension (the upper buccal cusp will have been angled not to interfere with excursive movements).

Generally, a Class I incisal relationship is desirable but this may not be possible, depending on the individual's skeletal relationship. The opposing incisor teeth should not be in contact in the intercuspal position but will become involved in protrusion; this arrangement leads to a more stable prosthesis. Class III skeletal relationships are an exception to this, as an edge to edge relationship may be more appropriate for aesthetics and a balanced occlusion.¹⁴

Given the discrepancy between the pattern of resorption between the mandible and maxilla, ideal tooth positioning can be challenging. As the maxilla becomes narrower, the mandible becomes wider and often the posterior teeth will have to be set into crossbite to compensate for this.²⁴

Assessing the neutral zone

Assessing the neutral zone can be difficult and is not a skill that many clinicians have acquired. If the denture is stable at rest but displaces upon movement, it may be that the denture is either over-extended or lies outside of the neutral zone. The denture should be held in place and the patient asked to perform mandibular movements to assess any excessive interferences of the polished surface with the surrounding soft tissues visually; pressure indicating paste or light-bodied silicone can help here (Figure 4) (light bodied silicone will also demonstrate areas that are under-contoured). Alternatively, the denture can be seated and the direction of displacement at rest may give an idea of any neutral zone discrepancies. A common example is a supero-posterior displacement under the elastic recoil of the lower lip and modiolus region (Figure 5). This area is high risk for ‘over-bulking’ in the atrophic mandible because, following bony resorption, the mentalis attachment migrates towards the crest of the alveolar ridge.²⁷ This often makes the neutral zone more lingual in comparison to when the patient was dentate. A more technical method of assessing the neutral zone includes taking a neutral zone impression then comparing this to the existing denture (a matrix could be made if there are minor differences). It may be worth mentioning at this point that, if the discrepancies are minor, then the adjustment of these areas may negate the need for a remake.

Recording the neutral zone

The theory of recording the neutral zone is simple. A volume of material is inserted on a stable base and various movements performed by the patient to shape the material defining the limits of the neutral zone. Recording of the neutral zone has been described in a number of ways,^{26,33,38,39,40} but conventionally is recorded using a special tray at a separate stage alongside the try-in of the upper denture^26,38 (Figure 6). Table 3 and Figure 7 present the stages of recording a neutral zone impression. An alternative option is to record the neutral zone impression during registration on a lower wax rim that uses Manchester pillars¹⁰ (Figure 8). The process is the same as in Table 3, with the lower wax rim substituting for the special tray, although the number of appointments and overall cost is reduced. If desired, a second stage can be added and an impression taken of the polished surfaces alone at the try-in of the lower denture. In this technique, the teeth are set up with minimal supporting wax in the neutral zone defined by the initial impression; assuming that the tryin is suitable, an impression of the labial and lingual polished surfaces is taken on the try-in using a low viscosity silicone material. The polished surfaces are then contoured to this shape in the laboratory. If a suitable material is chosen for the initial impression, however, sufficient detail is likely to have been recorded and this stage is probably unnecessary.