Elastic discontinuity due to ectopic calcification in a human fibrous joint

Abstract

Disease can alter the natural ramp-like elastic gradients to steeper step-like profiles at soft-hard tissue interfaces. Prolonged function can further mediate mechanochemical events that alter biomechanical response within diseased organs. In this study a human bone-tooth fibrous joint was chosen as a model system, in which the effects of bacterial-induced disease, i.e. periodontitis, on natural elastic gradients were investigated. Specifically, the effects of ectopic biomineral, i.e. calculus, on innate chemical and elastic gradients within the diseased cementum-dentin complex in comparison to a healthy complex, both of which are fundamental parameters to load-bearing tissues, will be discussed. Complementary techniques for mapping changes in physicochemical properties as a result of disease, included micro-X-ray computed tomography, microprobe micro X-ray fluorescence imaging, transmission electron and atomic force microscopy (TEM, AFM) techniques, and AFM-based nanoindentation. Results demonstrated primary effects as derivatives of ectopic mineralization within the diseased fibrous joint. Ectopic mineralization with no cementum resorption, but altered cementum physicochemical properties with increasing X-ray attenuation, exhibited stratified concretion with increasing X-ray fluorescence counts of calcium and phosphorus elements in the extracellular matrix. These were correlated to decreased hygroscopicity, indenter displacement, and apparent strain relieving characteristics. Disease progression identified as concretion through the periodontal ligament (PDL)-cementum enthesis and sometimes the originally hygroscopic cementum-dentin junction, resulted in a significantly increased indentation elastic modulus (3.16±1.19 GPa) and a shift toward a discontinuous interface compared to healthy conditions (1.54±0.83 GPa) (Student’s t-test, p<0.05). The observed primary effects could result in secondary downstream effects, such as compromised mechanobiology at the mechanically active PDL-cementum enthesis that can catalyze disease progression.

Mineral Density Volume Gradients in Normal and Diseased Human Tissues

“…The mineral density of normal cementum (1240–1340 mg/cc) was also identified. In addition, higher Ca content of the surface layer of cementum was observed by XRF, comparable to the densely mineralized surface layer observed by our previous studies[ 8 ]. Cementum is often affected by concretion due to cascade of biochemical and physicochemical effects as a result of yet another acerbated stimulus, such as periodontitis.…”

“…Supra- and subgingival[ 6 ] calculi are thought to arise from supersaturation of ions in saliva and/or bacteria-induced mineralization[ 7 ]. Given the disparate ways in which calculi form, it may mix with adapted cementum resulting in its concretion and varying mineral forms[ 8 ].…”

The Adaptive Nature of the Bone-Periodontal Ligament-Cementum Complex in a Ligature-Induced Periodontitis Rat Model

“…Additionally, strains could be altered at the PDL-bone and PDL-cementum interfaces [71] in ligated groups. These altered three-dimensional strain profiles are of special interest because deviations from normal physiological strain can lead to pathological function (aberrant loads/function), and compromised mechanotransduction over time [5, 72, 73] including the soft-hard tissue interfaces where multiple cell types reside.…”

Periodontal ligament entheses and their adaptive role in the context of dentoalveolar joint function

“…A gradual gradient in stiffness between a soft and hard tissue can adapt to an abrupt gradient to accommodate physiologic or non-physiologic demands on joints. It is this philosophy that was observed to highlight adaptation/regeneration in regions farther away from the site of injury/insult (which normally occurs closer or on the crown) within the context of joint function (33, 43). …”

“…The importance of a gradual transition permitted by multiple zones is highlighted specifically when it is contrasted with a single zone, indicated by a sharp increase or decrease in organic to inorganic ratio (54, 55). These single zone regions are observed as elastic discontinuities within load-bearing systems with impaired function (Figure 2) (43). …”