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.