Similar to other tissues in the body, the dentoalveolar complex is constantly subjected to functional loads (physiological, parafunctional, and therapeutic). As loads are applied, teeth undergo micromotion within respective alveolar sockets. The micromotion results in local deformations within the softer vascularized and innervated periodontal ligament (PDL), and subsequently within the surrounding harder alveolar bone and mineralized tissues of a tooth. Over prolonged loading, the resulting adaptation to local strains is thought to be due to an activation of a cascade of biological events that occur at multiple length scales. These biological events identified as modeling and remodeling processes within respective tissues subsequently guide the local tissue morphology/architecture and material properties, and in turn the overall biomechanics of the dentoalveolar complex. The cascade of biological events is a part of a local feedback loop or autoregulation and does not reach a plateau in its activity level until an optimization in biochemical and physicochemical properties is achieved to accommodate the functional demands.
In this study, load-mediated adaptation of the vascularized and innervated complex was investigated from a mechanics and materials perspective. This approach was sought as particularly the complex contains both hard-soft and hard-hard tissue interfaces all of which are uniquely designed to transmit mechanical forces while minimizing fracture and failure particularly in bone and tooth. Of note are the functionally graded entheses, which are the insertion sites of the PDL into the adjacent alveolar bone and cementum tissues and serve as focal regions for strain concentrations and hotspots for the cellular processes leading to modeling and remodeling of all tissues related to the bone-PDL-tooth complex.
The central objective of this dissertation was to investigate, from a multiscale perspective, the shift in functional adaptation of the bone-PDL-tooth fibrous joint in response to reduced functional load by using a small-scale animal model. It was hypothesized that function-related strains at the bone-PDL and cementum-PDL interfaces stimulate cells to form or resorb mineral by creating a localized micro-niche. To test this hypothesis, two specific aims were formed: Specific Aim 1: Test the hypothesis that the adaptation of the bone-PDL-tooth fibrous joint to reduced functional loads is temporal and is tissue-specific (mineralized and/or unmineralized). Specific Aim 2: Test the hypothesis that organ level joint mechanics directly influence physiological drift of teeth through the mechanobiological response to site-specific deformations within the functional space. To investigate temporal adaptation to reduced functional loads in Aim 1, approach taken was focused on mapping biomechanics, and tissue related physicochemical properties at different length scales (multiscale) using complementary methods including: X-ray microscopy, micro/nanoindentation, histology and immunohistochemistry, and transmission electron microscopy (TEM) techniques. Organ-level adaptations at a macroscale were characterized by analyzing shift in joint morphology and were correlated to joint biomechanics. Tissue-level adaptation was measured by mapping shifts in deformations of respective tissues which were evaluated using digital volume correlation (DVC). Tissue-level adaptations were also determined by using indentation methods to measure changes in local tissue hardness and elastic modulus. In specific Aim 2, the organ-level biomechanics was correlated with localized biochemical shifts within tissues and at the hard-soft tissue interfaces by staining for tartrate resistance acid phosphatase (TRAP), alkaline phosphatase (ALP), and immunogold labelling for bone sialoprotein (BSP) and osteocalcin (OC) macromolecular localization. Overall, the information gathered from this multidisciplinary approach was used to correlate the biomechanical events at an organ level with localized adaptations at the tissue and cellular levels.
Results specific to Aim 1 highlighted the importance of age by indicating that the shift in biomechanics due to reduced functional loads adaptation of the fibrous joint in younger mammals is significant compared to older mammals. Rats subjected to reduced functional loads illustrated that adaptations within the PDL resulted in a significant reduction in functional space and an increase in joint stiffness at younger ages. At an older age, differences included alveolar bone adaptations in the form of a decrease in bone volume fraction (form) and a decrease in elastic modulus (material properties). In line with the classical theories on the functional history of mineralized tissues, results from this study demonstrated that the observed temporal adaptations within the dentoalveolar complex are registered as a functional history in tissues and joints. Overall, these results highlight an optimization paradigm within the context of joint biomechanics, in that, adaptation observed at a macroscale is due to the coupled effect of a change in organ-level morphology as a result of a change in functional shape (hypothesized to be primarily driven through the modulation of cementum) and shifts in physicochemical properties of tissues including that of alveolar bone within the dentoalveolar complex. Specific to Aim 2, the use of in situ imaging and modeling of the changes in the PDL-space in an intact oral and craniofacial complex revealed the effect of a natural tilt of the tooth in the distal direction within a single mastication cycle. Within the functional space, compressive and shear strains were computationally determined and found to be primarily concentrated on the distal side of the tooth. Both these strains correlated spatially with sites of increased alveolar bone resorption as determined through TRAP staining and TEM. Of particular interest was that the site-specific distal TRAP activity was reduced when the animals were given softer diets. These results collectively lead to the hypothesis that the physiological distal drift in rodents in part is driven as a response to mastication forces through the site-specific resorption and apposition, thus sculpting the alveolar bone socket to accommodate functional demands.
Overall, the findings using a multiscale approach to highlight load-mediated adaptions of the bone-PDL-tooth fibrous joint fit into the larger continuum of functional adaptation of the oral and craniofacial complex. This was enabled through development of methodologies specific to in situ mechanical testing coupled with experimental mechanics to correlate organ-level biomechanics with tissue-level strains. Tissue-level strains specific to the PDL and alveolar bone were correlated with local biochemical expressions. Additionally, insights from this study indicated that by correlating organ-level biomechanics with tissue-level mechanobiology, functional loads are needed to sculpt the bony tissues. The insights gathered included that the growth of a load bearing organ not limited to the oral and craniofacial complex can be modulated by shaping tissues and this in turn is done by regulating magnitude, frequency of load and the age at which these stimuli were given. Lastly, the validated technology can be adapted to develop experimental models, however under ex vivo conditions, to highlight the local effects within tissues as a result of therapeutic treatments or parafunctional habits on the bone-PDL-tooth fibrous joint within the larger biomechanical continuum containing the temporomandibular joint (TMJ).