5-46. Cross-Bite Relation

a. Causes. A cross-bite is a condition in which the distance from the right to left ridge crest in the first molar areas of the mandibular arch is greater than the same distance of the maxillary arch which prevents a normal arrangement of the denture teeth. It may occur when the maxillary arch is relatively small or when the mandibular arch is relatively large. It may also be caused by the pattern of bone resorption of the edentulous mandible and maxillae. The bone of the maxillae resorbs upward and inward, while that of the edentulous mandible resorbs downward and outward. Over a period of time there is an increasing lack of parallelism between the two alveolar ridges.

b. Types of Cross-Bite Relationships. There is no uniformity in the development of a particular mouth. It may be encountered on the posterior of one side only, on the posterior of both sides, or on the posterior of both sides and in the anterior region.

c. Determination of Cross-Bite Relation. In many normal mouths, the mandibular arch is slightly wider than the maxillary but the teeth can still be set very satisfactorily using the positioning procedures previously described. In some mouths the disparity between the maxillae and the mandible is plainly evident and the requirement for a cross-bite setup is obvious. In borderline cases, the dental prosthetic specialist needs some rule of thumb to help him decide whether or not a cross-bite arrangement of the denture teeth must be used. In general, if a line is drawn between the crests of the maxillary and mandibular ridges and this line forms an angle of less than 80° with the occlusal plane, the relationship of the ridges is considered to be the cross-bite type and the denture teeth must be positioned accordingly (fig 5-88).

d. Procedure When Using Anatomic Posterior Denture Teeth. One method for handling crossbite conditions is called the "reversed" method.

(1) In this procedure, the maxillary left posterior denture teeth are set on the mandibular right side, the maxillary right denture teeth on the mandibular left, the mandibular left teeth on the maxillary right, and the mandibular right teeth on the maxillary left side.

(2) If the cross-bite relation is extreme, the facial cusps of the teeth in the mandibular arch are set directly over the crest of the ridge.

(3) The teeth in the maxillary arch are set

LESS

THAN

80°

Figure 5-88. Cross-bite relationship.

with their facial cusps fitting into the central grooves of the mandibular teeth so that the mandibular cusps overlap the maxillary teeth.

(4) In a cast where there is a normal jaw relationship in the anterior region, it is necessary to provide a cross-over point where facial cusps of the mandibular teeth which overlap the maxillary teeth cross the line of the maxillary cusps to make a smooth connection with the incisal edges of the mandibular anterior teeth. This is usually done by setting the mandibular second bicuspid in an end-to-end relation to the maxillary first and second bicuspids and completing the cross-over with the mandibular first bicuspid.

(5) Except for the preceding differences, the arrangement of the teeth is very similar to that of a normal case in characteristics such as compensating curves and principles of alinement (fig 5-89).

e. Procedure When Using Nonanatomic Posterior Denture Teeth.

(1) Some dental officers may request that the mandibular posterior teeth selected be one size larger than the maxillary set so that there will be a larger "table" or bearing surface in centric and working occlusion. If the length of the mandibular arch is short, however, the larger size of the mandibular posterior denture teeth chosen may cause these teeth to extend onto the retromolar pads on each side. In these cases it may be necessary to use mandibular posterior teeth which are the same size as the maxillary set. (2) If the cross-bite relation is extreme, the facial cusps of the mandibular posterior teeth are set directly over the mandibular ridge.

(3) The maxillary posterior teeth are set

with their facial cusps fitting into the central grooves of the mandibular posterior teeth. Since the teeth were not made to occlude in this abnormal relation, some reshaping (by grinding) will be required to obtain the proper working, balancing, and protrusive occlusions. Grinding should be limited to removing major interferences until all the teeth are positioned. Refinements in the occlusion can be made when the setup is completed.

(4) A cross-over point is provided in the same manner as in the method using anatomic teeth.

(5) Except for the above differences, the arrangement of the teeth in a cross-bite relation is very similar to that of the normal case in characteristics such as establishing a compensating curve and principles of alinement (fig 5-90).

5-47. Prognathic Ridge Relationship

a. General. Prognathism is a condition in which the mandible protrudes beyond the maxillae. The degree of protrusion may be slight and

cause little or no inconvenience. A complete denture can be fabricated for such patients with the teeth arranged to provide almost normal functioning. A greater degree of protrusion requires modification in the arrangement of the anterior teeth.

b. Selection of Teeth. The posterior teeth can be either anatomic or nonanatomic. Anatomic posterior teeth permit very little flexibility in the mesiodistal positioning of the opposing teeth. Practically all prognathic cases require that a maxillary bicuspid or molar be omitted from the setup or that a mandibular bicuspid or molar be added. This leads to problems in arranging anatomic teeth, and these problems are more easily overcome if nonanatomic teeth are used. The dental officer will prescribe which type of posterior teeth he desires for each patient.

c. Procedure.

(1) Figure 5-91, which presents views of complete dentures for a prognathic case, illustrates the principles which should be followed in arranging the teeth.