T.H.M. van den Heuvel
The periosteum of the growing rabbit mandible.
A scientific essay in Medical Sciences
DOCTORAL THESIS defended in public on 12th of February 1998
In chapter 1 the background of the study is elucidated and an extensive literature review is given regarding bone growth, and especially on the growth of the mandible. The different regulation mechanisms for both condylar and mandibular growth are emphasised. In the present study the role of the mandibular periosteum in the growth of the mandibular bone and especially the condyle, has been investigated. The study was performed in rabbits, from birth to the age of 31 weeks.
In chapter 2 the normal macroscopical structures of the mandible, the attached tissues like the masticatory muscles and the periosteum, and their way of attachment, as well as the mandibular joint are described. For this purpose rabbits, aged six weeks and young adult rabbits were used. Fresh material, and dry skulls and mandibles were studied. The attachment areas of the masticatory muscles were determined. For the way the muscles and periosteum are attached, a subdivision was made in different types of attachments. There appeared to exist different ways of attachment underneath one muscle. The mean directions of the working lines of the masticatory muscles were determined. A clear connection between the disc and the mandible was present only at the dorsocaudal side of the condyle, while this appeared to exist only of rather loose tissue on the medial, rostral, and lateral sides. There was no clear connection between the disc and the masticatory muscles. In dry skulls the bony surface structure of the mandible was studied and areas with different bony surface aspects were recorded. There appeared to be no relation between the presence of muscular tissue and the surface aspect of the bone. The surface aspect of the bone varied underneath the attachment area of one muscle, but also on places where no muscles were attached.
In chapter 3 the structure of the mandibular periosteum and the attached tissues were studied histologically in rabbits aged six weeks and 30 weeks. Within the periosteum an inner cellular (osteogenic) layer and an outer fibrous layer were discriminated. In the cellular layer different cell types were found and their activity was determined morphologically. Areas with bone deposition (osteoblasts), areas with bone resorption (osteoclasts), and so-called resting areas were distinguished. In both age groups the kind and the degree of periosteal activity appeared not to be related to the presence of muscles, and in general there was a decrease in activity in the older animals compared to the young ones. The collagenous fibres were running rostrocaudally in the rostral part of the mandible; in the caudal mandibular half they were running in the direction of the mandibular angle, while they were oriented toward the mandibular joint in the upper mandibular ramus. A relation between the direction of the fibres and the muscles could not be demonstrated. The orientation of the elastin fibres was generally in agreement with those of the collagenous fibres. However, more fibres were found running parallel to the bony surface, but perpendicular to the collagenous fibres. Furthermore, small elastin fibres were found oriented perpendicular to the bony surface. They were mainly present within the attachment areas of the masticatory muscles. The way the masticatory muscles were attached to the periosteum and to the bone itself, was studied and qualified as direct or indirect endomysial, perimysial or tendinous. The tendons appeared to be more extended in the older animals.
In chapter 4 the influence of the ramal periosteum on the condylar area was studied. For this Purpose rabbits of different ages were examined histologically. Measurements were performed on the periosteal width as well as on the osteogenic and fibrous layers separately. The mandibular joint also was studied histologically. The results were comparable with the macroscopic findings as described in Chapter 2; the discal ligaments contained both elastin and collagenous fibres. However, only the collagenous fibres continued in the mandibular periosteum and the disc. Near the condyle the periosteum was wider than in the areas situated more ventrally. The width of the osteogenic periosteal layer and that of the total periosteum in the different areas, showed a comparable pattern during growth. The results for the width of the fibrous layer were different as the values near the condyle increased during growth.
In chapter 5 the periosteal behaviour during mandibular growth has been described. For that purpose the orientation of the bony spicules was determined in rabbits perfused with India ink, and both bone markers and periosteum. markers were implanted in animals, aged six weeks. Using standardized radiographs, the displacements of the periosteum markers along the mandibular bony surface during growth were determined. The bony spicules appeared to be arranged along the bony surface according to a certain pattern. On the dorsal part of the ramus they were directed towards the condyle, while they were running rostrocaudally in the ventral part of the ramus. The periosteum markers in the rostral part of the ascending ramus moved towards the condyle. In the caudal ramal area they moved dorsocaudally, and the markers on the ventral ramal area moved caudally and ventrocaudally. During their displacement, the periosteum markers kept their same relative position on the mandible. The displacements per week appeared to be significantly different for the different areas. From these results it was concluded that a directional influence of the periosteum on mandibular growth is possible. Furthermore, the hypothesis is supported that the periosteum behaves according to the "elastic sleeve"- model.
In chapter 6 the mitotic activity in the periosteum of rabbits of different ages was determined, using _H-Thymidine, and collagen synthesis was determined with _H-Proline. In three areas on the central part of the ramus, and in four peripheral areas situated near the condyle and the caudal ramal edge, autoradiography was used for the quantification of proline labelling and the number of labelled cells, per square unit of the periosteum. In the central ramal areas both periosteal layers showed similar results, while in the peripheral areas the cellular layer showed the largest activity. This, probably indicates a uniform periosteal growth in the investigated areas, while in the peripheral areas more stretching occurs. These results give additional support to the hypothesis that the periosteum behaves according to the "elastic sleeve"- model.
In chapter 7 the results of the previous chapters are discussed and related to the literature. This leads to the conclusion that the periosteal structure and its physiological and spatial behaviour during growth strongly suggests that the periosteum influences the mandibular condylar growth both in an active and an adaptive way.