Original Full Length ArticleThe plastic nature of the human bone-periodontal ligament-tooth fibrous joint

•An adapted human bone-periodontal ligament (PDL)-tooth fibrous joint was highlighted by a narrowed PDL-space and presence of adapted bone.•Bony protrusion(s) or adaptations identified as bundle bone with structural, biochemical, and mechanical heterogeneities, were dissimilar from lamellar bone.•The observed physicochemical heterogeneities could be the functional cues for an original PDL-bone to serve as the genesis for bony protrusions.•Heterogeneous constructs contained physicochemical discontinuities in the bone-PDL-tooth fibrous joint could be “markers” indicative of pathological adaptation of alveolar bone.•Bony adaptations or protrusions within the complex can perpetuate functional demands and shift the organ to a pathological regime.

This study investigates bony protrusions within a narrowed periodontal ligament space (PDL-space) of a human bone-PDL-tooth fibrous joint by mapping structural, biochemical, and mechanical heterogeneity. Higher resolution structural characterization was achieved via complementary atomic force microscopy (AFM), nano-transmission X-ray microscopy (nano-TXM), and microtomography (MicroXCT™). Structural heterogeneity was correlated to biochemical and elemental composition, illustrated via histochemistry and microprobe X-ray fluorescence analysis (μ-XRF), and mechanical heterogeneity evaluated by AFM-based nanoindentation. Results demonstrated that the narrowed PDL-space was due to invasion of bundle bone (BB) into PDL-space. Protruded BB had a wider range with higher elastic modulus values (2-8 GPa) compared to lamellar bone (0.8-6 GPa), and increased quantities of Ca, P and Zn as revealed by μ-XRF. Interestingly, the hygroscopic 10-30 μm interface between protruded BB and lamellar bone exhibited higher X-ray attenuation similar to cement lines and lamellae within bone. Localization of the small leucine rich proteoglycan biglycan (BGN) responsible for mineralization was observed at the PDL-bone interface and around the osteocyte lacunae. Based on these results, it can be argued that the LB-BB interface was the original site of PDL attachment, and that the genesis of protruded BB identified as protrusions occurred as a result of shift in strain. We emphasize the importance of bony protrusions within the context of organ function and that additional study is warranted.