Interdigitation and the development of the dento-facial complex. An experimental study in growing macaca fascicularis.
A scientific essay in Medical Sciences
DOCTORAL THESIS defended in public on 13th of December 1995
The aim of the present investigation was to study the role of interdigitation in the coordination of the maxillary and mandibular growth and the development of their dentition. As an experimental animal, the Macaca fascicularis monkey was used, in which the interdigitation was eliminated by grinding of the cusps of the deciduous canines and molars and the first permanent molars. Dental and skeletal changes over time were analyzed using series of standardized occlusal and lateral radiographs, series of dental casts, and histological techniques.
Chapter 1 presents a review of the literature dealing with the coordination mechanism for maxillary and mandibular growth and the development of the dentition. It is mainly focused on the role of mechanical stimuli originating from the dentition in this mechanism. The cone-funnel mechanism is generally accepted as a major factor in the establishment of a proper occlusion. The opinions on the possible role of interdigitation in skeletal and occlusal development, however, are conflicting. The importance of interdigitation is emphasized, on a hypothetical level, by several authors, and also in clinical orthodontics a contribution of interdigitation to maxillo-facial development is often assumed. However, in experimental approaches so far, the original craniofacial development has been disturbed, which limits the extrapolation of the findings to normal growing systems. To meet this shortcoming, in the present study the contribution of interdigitation to the development of the maxillo-mandibular complex will be investigated using an experimental set-up in which growth centres are not directly affected. The use of M. fascicularis in this field of research is advocated and the chapter ends with questioning the rail mechanism hypothesis as formulated by Van der Linden (1986).
Chapter 2 describes normal developmental characteristics for the dentition of juvenile M. fascicularis as derived from radiographs and relates them to chronological age. As tooth development is a progressive and continuous intra-osseous process which is independent from environmental factors, these data can be used to estimate the age of young M. fascicularis monkeys with an unknown date of birth.
Chapter 3 gives a description of the dento-facial growth and development in the juvenile M. fascicularis using standardized lateral cephalometric radiographs. Growth data were obtained by analyzing the changes in relation to the frontal bone implant and the Anterior Cranial Base line and also in relation to implants placed in the jaws. The results suggest that the dento-facial growth and development in the juvenile M. fascicularis and in humans have many points in common, and therefore M. fascicularis appears to be a good model for further studies in the regulation of the processes involved.
Chapter 4 deals with the role of the interdigitation in sagittal growth of the maxillo-mandibular complex using standardized lateral cephalometric radiographs. It appeared that elimination of interdigitation resulted in a deviating antero-posterior relationship between the jaws and in a significant inhibition of the vertical displacement of the maxilla in the second half of the experimental period, while the total facial height was not noticeably affected. A more prognathic skeletal relationship and a more mesial occlusion developed as a result. It can be concluded that interdigitation plays a role in the regulation of vertical and antero-posterior growth and development of the maxillo-mandibular complex in M. fascicularis monkeys.
In chapter 5 standardized occlusal radiographs are studied in order to evaluate the role of interdigitation on transverse maxillary growth and dental arch development. Midpalatal sutural growth appeared to be independent of interdigitation. The maxillary dental arch, however, showed locally a greater increase in width under experimental conditions. The maxillary dental arch can be considered to be guided by that of the mandible through interdigitation of the posterior teeth.
Chapter 6 deals with the study of the development of the dentition on a series of dental casts and at the histological level by the use of vital staining techniques. The finding that the maxillary dental arch broadened faster in the experimental group was confirmed by this part of the study. The results seemed to be contradictory to the situation in humans, where lack of interdigitation in most cases leads to a narrowing of the maxillary dental arch. However, differences in the proportional relationship between both jaws in humans and M. fascicularis can explain this discrepancy. The mechanism itself is the same in both species, but the direction of its action, and therefore the outcome of the mechanism is the reverse. As the mandibular dental arch development appeared to be not affected by the experimental interference, it could be considered to be independent of interdigitation. Experimental intervention also led to less prevalence of anterior open bites, which is a normal feature in the developing M. fascicularis.
Chapter 7 analyzes earlier findings and extrapolates them to the human situation leading to theoretical reflections on clinical orthodontics. The Appendix describes the mathematical analysis used to detect possible unequal transverse development of both maxillary halves, which could dissimulate eventually experimentally induced differences. This analysis revealed that asymmetries occurred at random, and therefore it was concluded that they did not interfere with the results.