LINGUALIZED" OCCLUSION AS AN ALTERNATIVE OCCLUSAL ADJUSTMENT IN SEVERE ALVEOLAR BONE RESORPTION IN EDENTULOUS PATIENTS

Introduction
Payne originally developed the "lingualized" occlusion concept, which was later imitated or modified by many others. The aim of this concept is to combine the advantages of the anatomical posterior set up (easy to arrange and adjust) and the monoplane set up. In this paper the principles and practical implementation of an arrangement in "lingualized" occlusion will be discussed.

Principles of the "lingualized" occlusion
On the basis of the fact that the movements of the mandibula are dictated by the temporo-mandibula joint during function, a good stability in the dentures can be obtained with a balanced occlusion concept. This is achieved by arranging the teeth in such a way that on both the working (active) side and the balancing (passive) side there are simultaneous bilateral balancing contacts. Bite forces are thereby distributed over the largest possible area of the edentulous jaw. On the basis of a normal class I jaw relationship, this means that with a "conventional" atomical occlusion concept, during lateral movement both the buccal and lingual cusps of the upper and lower denture come into contact on the working (active) side, which means a large number of contacts during excentric jaw movements. Just as in the natural dentition, there is therefore more than one contact per antagonistic pair of teeth. In comparison with this conventional anatomical concept, the number of occlusal contacts is reduced considerably with the "lingualized" occlusion concept, only the lingual cusps of the posterior teeth in the upper denture make contact in centric relation in the central fossae of the lower posteriors. The buccal cusps are out of contact. There is therefore only one "centric stop" between upper and lower antagonistic pair of posterior teeth.

With the help of anatomically formed upper posterior teeth, an aesthetically pleasing result is achieved, while in the lower jaw semi (or non) anatomical posteriors molars can be used to give mechanical benefits. Although any type of anatomically formed posteriors molars can be made suitable for the "lingualized" occlusion concept by means of grinding, special teeth have been developed for this occlusion concept over the years (Sosin ® , Condyloform ® ). In the nineteen fifties, Professor DR. A. Gerber (Zürich) made an important modification to the "lingualized" occlusion concept. In order to obtain better chewing efficiency, the contact relation of the first premolar was changed in the sense that the cusp of the first lower premolar comes in contact with the mesial fossa of the first upper maxillary premolar. The result of this is that particularly during functioning, the cuspid and the first premolar guarantee a good "scissors" effect which improves chewing efficiency.
The "lingualized" occlusion concept needs no special class or anteriors teeth arrangement. It is always possible that the mandibular cuspids are arranged in a special way and a specific position, before the other anterior teeth and premolars are arranged. During lateral movement in the "lingualized" occlusion concept, contact only occurs on the working (active) side between the lingual cusps of the upper and lower posteriors with the exception of the first premolar; on the balancing side there is contact between the lingual cusps of the upper denture and the lingual inner curve of the buccal cusps of the lower denture, such as is normal with the "conventional" occlusion concept.

In order to guarantee a balanced occlusion, a compensation curve is arranged during the set-up of the mandibular posterior teeth. No posterior teeth are placed in the ascending part of the mandible in order to prevent protrusive dislocation of the lower denture. In most cases this results in a reduction of the number of teeth by leaving out the second premolar or molar, depending on the space available. Protrusive balanced contacts may only occur between the maxillary lingual cusps and the mandibular posteriors; in other words, even during functioning the anterior teeth are clear and not under pressure. If anterior interference occurs, either move or grind the lower offending anterior teeth slightly or increase the distal incline on the lower molars by increasing the compensating curve. In this way, a balanced occlusion "in all directions" is created from the "centric stops" of approximately 3 mm freedom of centric.

In order to ensure that the supporting upper lingual cusps move completely freely in the lower central fossae, the fossae of the lower posterior teeth must be broadened, mainly by means of selective grinding. The buccal cusps are just out of contact. For this purpose the upper buccal cusps are ground when in contact to create buccal clearance. In this way the harmonizing of the working and balancing cusp contacts is simplified, since only the upper lingual cusp contacts the lower. In the Netherlands, too, a set of teeth has been introduced for this "lingualized" occlusion concept, in which the central fossae of the lower posteriors are preformed in such a way that modification and grinding during arrangement of the teeth has become (almost) unnecessary (OPTIFORM ® ). These posteriors have been developed in cooperation with Prof. Dr. W. Kalk (Nijmegen, Groningen).

Advantages of "lingualized" occlusion
Prosthodontics always have been concerned with the maintenance of alveolar bone. Many ideas concerning occlusion and it's role in preservation of the residual alveolar ridge have been proposed. It is now widely accepted that cusped teeth, if properly arranged, provide an efficient and aesthetically pleasing posterior set-up for many patients. The presence of cusps, however, does introduce potentially destructive forces.

In general it can be said that there are no contra-indications for the applications of the "lingualized" occlusion concept. This concept is indicated for patients who place high aesthetic requirements on their dentures and with whom normally an occlusion concept is indicated to minimize the horizontal component of force during mastication and parafunctional movements as a result of strong alveolar bone resorption, with flabby and knife edge ridges or abnormal jaw relation and a large inter ridge space.

The use of the "lingualized" occlusion concept brings great comfort for this patient as a result of the modified posteriors in the lower jaw, where as aesthetics are not a problem due to the use of anatomical posterior teeth in the upper jaw. This is due to the following: As a result of advanced alveolar bone resorption, a discrepancy arises between the more narrow size of the upper arch in comparison with that of the lower jaw. Since the lingual supporting cusps are active and must be situated at the highest point of the alveolar crest, the upper posterior teeth can be set fairly close to the buccal side of the mandibular ridge. This has the following results;

• The forces on the mandibular alveolar ridge are situated more centrally at the top, which increases the stability of the lower denture;
• a "cross bite" can be avoided in almost all cases;
• the facial muscles are well supported by the maxillary molars, which increases the aesthetic value even more.

Changes resulting from processing dentures
Errors in occlusion can occur even in the best laboratories, but both the amount and the frequency of the error are minimized by a standardized routine procedure and adequate quality controls. In daily practice the dentures are usually delivered "ready to wear" in a plastic bag or box, without the occlusion having been checked after finishing. Checking the occlusion in the mouth is hindered by number of factors. For example, the detection of incorrect occlusal contacts and interference's with the use of articulating paper is difficult due to the presence of saliva.
For this reason after processing, the dentures should be returned to their previous articulator positions. This is easily accomplished by using a split cast remounting procedure.

Conclusion
The "lingualized" occlusion concept is an example of a bilaterally balanced occlusion concept. The premolars and molars are arranged and modified so that only the lingual cusps of the upper posterior teeth make contact with the central fossae of the lower posterior teeth. Exceptions are the first premolars which are arranged in a more "conventional" way. The reason for this is that it enables the first mandibular premolar to perform more of the scissor function of a cuspid. The buccal cusps of the second mandibular premolar and of molars do not make contact with their antagonists.

To summarize, the advantages of the "lingualized" occlusion concept are as follows:

• The advantages of both the anatomical teeth (i.e. aesthetic and chewing capacity) and the non-anatomical teeth (i.e. less horizontal forces) are maintained, particularly in patients with severe alveolar bone resorption.
• Vertical forces are directed more centrally on the mandibular alveolar ridge, which gives more stability to the lower denture.
• The upper posteriors can be positioned more buccal of the ridge because the lingual cusps are active and should be situated at the top of the maxillary ridge. In many cases, this prevents an arrangement in cross bite and improves the aesthetic aspect of the dentures.
• Balanced occlusion with a "freedom of centric" is easy to achieve within an area of 2 to 3 mm around the centric stops.