Strain-guided mineralization in the bone–PDL–cementum complex of a rat periodontium
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OBJECTIVE: The objective of this study was to investigate the effect of mechanical strain by mapping physicochemical properties at periodontal ligament (PDL)-bone and PDL-cementum attachment sites and within the tissues per se. DESIGN: Accentuated mechanical strain was induced by applying a unidirectional force of 0.06N for 14 days on molars in a rat model. The associated changes in functional space between tooth and bone, mineral formation and resorbing events at the PDL-bone and PDL-cementum attachment sites were identified by using micro-X-ray computed tomography (micro-XCT), atomic force microscopy (AFM), dynamic histomorphometry, Raman microspectroscopy, AFM-based nanoindentation technique, and were correlated with histochemical stains specific to low and high molecular weight GAGs, including biglycan, and osteoclast distribution through tartrate-resistant acid phosphatase (TRAP) staining. RESULTS: Unique chemical and mechanical qualities including heterogenous bony fingers with hygroscopic Sharpey's fibers contributing to a higher organic (amide III - 1240 cm-1) to inorganic (phosphate - 960 cm-1) ratio, with lower average elastic modulus of 8 GPa versus 12 GPa in unadapted regions were identified. Furthermore, an increased presence of elemental Zn in cement lines and mineralizing fronts of PDL-bone was observed. Adapted regions containing bony fingers exhibited woven bone-like architecture and these regions rich in biglycan (BGN) and bone sialoprotein (BSP) also contained high-molecular weight polysaccharides predominantly at the site of polarized bone growth. CONCLUSIONS: From a fundamental science perspective the shift in local properties due to strain amplification at the soft-hard tissue attachment sites is governed by semiautonomous cellular events at the PDL-bone and PDL-cementum sites. Over time, these strain-mediated events can alter the physicochemical properties of tissues per se, and consequently the overall biomechanics of the bone-PDL-tooth complex. From a clinical perspective, the shifts in magnitude and duration of forces on the periodontal ligament can prompt a shift in physiologic mineral apposition in cementum and alveolar bone albeit of an adapted quality owing to the rapid mechanical translation of the tooth.