Relative Maxillo-Mandibular Growth Expressed on the Functional Occlusal Plane

Three basic factors influence the development of dental occlusion from the usual flush terminal plane occlusion in the mixed dentition to a Class I permanent dental occlusion : 1) leeway space differential, 2) rotation of functional occlusal plane and 3) differential maxillomandibular growth along the functional occlusal plane. The purpose of the study was to analyze and differentiate maxillary from mandibular growth as expressed on the functional occlusal plane. The rationale of the study was based upon the speculation that mandibular growth out paces that of maxillary growth as expressed on the functional occlusal plane. If this were so, it would help explain why dental occlusions shift predominately toward a Class I relationship as an individual transitions from childhood to adulthood. Thirty nine untreated subjects (20 males and 19 females), 8 to 18 years, served as the basis for this longitudinal growth study wherein 406 lateral cephalometric radiographs were analyzed. The research sample came from the Mooseheart Growth Study, originating in Chicago, lllinois, and now residing at Marquette University. Each radiograph was normalized for enlargement and ten measurements were made on each lateral head x-ray using a digitizer. Two growth axis relative to sella were used for the maxilla (C-axis and CR-axis) and one for the mandible (Y-axis) from which annual increments of change were determined. The angle between the growth axis and the functional occlusal plane was recorded and the anteroposterior aspect of the growth axis was mathematically projected onto the functional occlusal plane. There was no clinically significant correlation (r\u3e0.8) found between any of the changes along functional occlusal plane nor within any of the three variables as a function of age. The functional occlusal plane to sella-nasion angle decreased 4.4 degrees from 8 to 18 years, a counterclockwise rotation. The expression of mandibular growth on the functional occlusal plane by the Y-axis totaled 13.7mm ± 1.08 per year which was, from ages 8 to 18 years, 4.8mm greater than for the maxillary C-axis expression on functional occlusal plane and 4.4mm greater than the maxillary CR-axis expression. Under the conditions of this longitudinal growth study, mandibular growth as expressed on the functional occlusal plane, was greater than the maxillary expression. This finding supports the contention that differential growth of the jaws is an important factor in accounting for dental occlusion changes in transition from childhood to adulthood