Denture tooth set up

Setting an ideal position of  teeth for a lower full denture is of the utmost importance due to the almost universal instability of the lower full denture. One of my pet peeves is how the technician wants to set everything with an "anatomic" class 1 relationship. Tooth position can be in a wide range in the natural dentition but this is anchored in bone with periodontal ligament support. Because of this, natural dentition tooth position can exist outside the zone of soft tissue influence a la the neutral zone concept. The influence of external factors on tooth position becomes more obvious as there is loss of periodontal support in diseased states which allows buccal and lingual soft tissues as well as the occlusion to move the teeth resulting in flared teeth and diastemas. 

In contrast to the natural dentition, full dentures are at the mercy of the movement of soft tissues during speech and function which can affect their success if denture base extensions are improperly designed as well as if teeth and polished surfaces encroach outside the neutral zone. Denture base adaptation, sufficient peripheral exension and the availability of attached mucosa is important in determining "support". Maintenance of the peripheral seal by avoiding overextension of the denture bases into the muscle attachments is the most important determinant of denture stability but improper tooth placement can destabilise even a properly extended denture. This can be due to occlusal forces on an inclined plane causing lateral movement of the denture as well as the action of the soft tissues on teeth placed outside the neutral zone.

The pattern of resorption in the maxilla tends to be on the buccal and labial aspect which causes a reduction of the arch circumference and retraction of the maxilla. Resorption on the mandible tends to be on both the buccal and lingual with the crest of the ridge tending not to change position as severely and the posterior aspect of the arch appearing to widen. The concurrent resorption patterns result in a pseudo class 3 appearance to the patient mainly due to marked resorption of the maxillary alveolar bone and stability of the chin position as it is not alveolar bone. On restoring the form of the teeth, bone and soft tissues, the temptation is to set the teeth where they were pre-extraction. This may be possible most of the time in the maxilla provided there is good retention of the upper denture. This is to ensure that the denture is not destabilised as the teeth are set more anteriorly into the upper lip zone. Even very good retention can be overcome with tipping forces and if the clinician wishes to set the upper anterior teeth forward to improve the lip support, they must avoid any anterior contact of the dentures to avoid the tipping forces which would destabilise the denture. Conversely, if the clinican wants to allow anterior contact and avoid such loss of stability, they must compromise on the aesthetics and set the teeth further back over the ridge where the occlusal forces will be better supported and the soft tissue forces less apparent.

                                  

To minimise destabilising forces on the denture, the following strategies can be employed:

- Set the lower teeth with the occlusal edges over the ridge crest. This means the incisal edges and central ridges have to be over the ridge. The labial surface of the teeth can be modified by altering the angulation of the tooth but where the incisal edge is where the force will pass through the denture and this must be over the ridge. 

-Upper teeth can go buccal to the ridge as some suction can be obtained to counteract the tipping forces but the more you desire stability, the closer the incisal edges will approach the ridge crest. 

-If the lower incisal edges are labial to ridge then the pressure of the mentalis will tip the dentures. If you think about the muscle attachments in this zone, the mylohyoid is on the lingual and the mentalis is on the labial. There is no way that you can avoid encroaching on either of these muscles if the incisal edge is not set over the ridge. 

-The polished surfaces of the denture may be modified with a neutral zone technique but the incisal edge position will likely remain unchanged.

- To check the orientation of the denture teeth in relation to the ridge. dray a pencil line as an extension from the ridge onto the land area in the anterior land and retromolar land area. This gives you a good way to confirm the position of the ridge when the denture base is on the model. The same goes with the position of the anterior ridge. dray a horizontal line on the land area to denote the anterior limit of where you would set your lower incisal edge.

In the case below, the lower full denture try in was tipping the upper denture try in as the technician had placed the incisors way too far to the labial to make incisal contact and force a class 1 relationship. There is no issue with class 2 relationship in dentures, in fact it is important to avoid any anterior contact as it will destabilise both dentures. No matter how much I emphasise to set the lower incisal edge over the ridge and no matter how many diagrams I draw, I seem to get variable results where the teeth are always forward of the ridge. If you carve your upper wax rim to an ideal tooth position, the lower ridge position is likely to be posterior to the upper teeth in class 1 and 2 patients. The technician is trained to set the teeth to class 1 no matter what the jaw relationship and this training is hard to get around. The difficulty comes because soft tissues aren't possible to visualise on an articulator. If you imagine the attachment of the mentalis as well as the position of the lower lip, it becomes obvious that a tooth set up like the one below will be tipped posteriorly with even the slightest of lip movements and stability with these dentures would be an impossibility.

 

 

 

To remedy this, I reset the lower incisors back but they ended up still labial to ridge. As the posterior tooth set up is done after the anteriors, there is a limit to how far you can tip the anterior teeth back without having to reset the posteriors. In this case I could have removed the lower first premolar and set the canine in this position as the initial set up had left me no room to reset the anteriors. In this case, I  as lower had some suction, I left them slightly anterior to the ridge as the retention was sufficient having moved the lower incisors back from the lower lip. Also considering that doing this removed any possibility of anterior tooth contact, I was more confident that the denture would be stable during function. In hindsight, it would be more wise to stick to the principles at hand and set the incisal edges completely over the ridge but there was some element of laziness as I didn't want to reset the posteriors or send the job back to be reset at the lab. 

I needed space for the incisors to be set back as making a smaller arch automatically ensures that the teeth selected are too large. Simply tipping the teeth back will cause space issues jsut as it would in a natural dentition. To get around this, I narrowed the teeth slightly with a bur at the interproximal areas and added in some crowding in the lower incisal area. In the anterior region, it doesn't matter what the configuration is because it is mainly aesthetic. If there is no anterior contact, the denture will function just as well with no anterior teeth as with anterior teeth. The only real consideration to limit anterior tooth position is phonetics and a lower incisor position too high or too far back may hinder the "s" sound as it may constrict tongue movement. Tucking the teeth back and introducing crowding is a good way to deal with the space issue and create a natural appearance of the dentures as most older patients will have more lower incisor display. Crowded, set back incisors were much more natural in appearance than the massively proclined, aligned set that was provided to me. Rotations are ideal around the long axis of the tooth but it is often unnatural and stands out if you tip the teeth mesiodistally. Try to avoid incisal plane cants if possible.