NMDA Journal Summer 2018

15 nmdental.org bounded by two lines originating from the mesial surface of the mand i bu l a r c an i ne and extending poste- riorly to the lingual and buccal aspect of the retromolar pad. This area has been called Pound’s triangle. (Fig 3) Pound’s theory is applicable in posterior teeth arrange- ment for the vast majority of patients. Residual ridge resorption is time-dependent and an irreversible bone loss process. In the lower jaw, a larger proportion of bone loss tends to occur on the lingual side. The result is an arch form that is frequently not on the residual ridge. The occlusal centric contacts follow the guideline of lingualized occlusion described by SH Payne (1941), and Pound and GA Murrel (1971). In lingualized articulation, only the lingual cusps of the posterior maxillary teeth are in contact during centric occlusion. It could be concluded that the primary contact is the lingual cusp of the maxillary teeth rather than the buccal cusp of the mandibular teeth. This primary contact is acting as an additional stabilizing factor for the maxillary denture, directing forces closer to the residual ridge. The retromolar pad is the anatomic landmark used most frequently as a reference for tooth placement for it applies in three dimensions—vertically, laterally, and anteroposteriorly. It guides the height of the occlusal plane. In a monoplane artificial-tooth arrangement, the occlusal plane lies in the middle of the pad. Laterally, the retromolar pad guides the buccolingual position of the posterior teeth. Pound states that the lingual cusps of the mandibular posterior teeth lie within a triangle formed by the buccal and lingual borders of the retromolar pad and the mesial surface of the mandibular canine. Anteroposteriorly, artificial teeth are not placed posterior to the anterior boundary of the pad. Regarding the anatomic landmarks used for locating the lower molar teeth, Pound reported that the positions of the lower molars are situated between two lines projected from the buccal and lingual aspects of the retromolar pad to the mesial aspect of the cuspid. HR Ortman (1971) stated that the lingual cusps of the natural molars are approximately in vertical alignment with the mylohyoid ridge, and thus the mylohyoid ridge is a reliable guide for determining the lingual limit of the artificial lower teeth. The lower posterior teeth must never be arranged lingually to this ridge. Both are used as guides to approximate the position occupied by the natural teeth. The lingual surfaces of the lower molars are situated between two lines projected from the buccal and lingual aspects of the retromolar pad to the mesial aspect of the cuspid. In the final tooth arrangement, the posterior form of the arch will be determined largely by the “neutral zone” between the cheeks and tongue. This zone is the space resulting from the removal of the posterior teeth and the loss of bone from the residual ridges. Therefore, the final arrangement of the arch must be developed with respect to the tongue and cheek. The solution to the problem is to position the teeth along a line extending from the tip of the canine to the middle of the retromolar pad. This arbitrary line should pass through the central fossa of the mandibular premolars and molars. The basic principle for the buccolingual positioning of posterior teeth is that they should be positioned over the residual ridge. The canine and retromolar pad should provide guides for this arrangement. The distance between the distal of the mandibular canine and the mesial of the retromolar pad is measured to determine the total anteroposterior space to be covered. Central (centroid) Bearing Plate This is not used like your “father’s old gothic arch tracing” that we learned in dental school. In dentistry, a device that provides a central point of bearing, or support, between upper and lower record bases; it consists of a contacting point that is attached to one base and a plate attached to the other that provides the surface on which the bearing point rests or moves. Fig 4—Central (centroid) bearing plate. The central bearing plate can be used at the “wax try-in appointment” to refine the occlusion. (Fig 4) After the aesthetics, vertical, and phonetics, such as bilabial fricatives and sibilants are satisfied, place the striking plate in the centroid of the maxillary denture with wax. The set screw and an MS hex screw is mounted in the centroid of the mandibular denture. Open the vertical minimally, i.e., just to the point of eliminating the occlusal interferences. Direct the patient to an acquired/ comfortable/functional maxillary and mandibular relationship and record this position. Remount the case and refine the occlusion before processing. Fig 3—Pound’s Triangle. continues on next page 