Heterogeneity within Human Alveolar Bone; a Functional Necessity

Objective:  This study highlights differences between lamellar bone (LB) and bundle bone (BB) within human alveolar bone (AB).  Results indicate “quality” of AB is maintained through distinct heterogeneous regions necessary to maintain overall biomechanical function of the bone-tooth fibrous joint. 

Method:  X-ray attenuation and elemental spatial variations on 1-2µm sections from alveolar bone were identified using transmission X-ray microscopy (TXM, 5.4 keV) and microprobe X-ray fluorescence imaging (µ XRF, 12 keV). Mechanical resistance of specific sites in BB, lamellae and interlamellar spaces of AB were investigated by displacing load at a rate of 100nm/sec using site-specific indentation technique. 

Result: Two distinct types of bones marked by differences in X-ray attenuation, calcium (Ca), phosphorus (P), and zinc (Zn) content were identified in AB.  Bone with radial fibers, commonly known as bundle bone (BB) was identified as a higher X-ray attenuating material and X-ray micro-fluorescence measurements yielded higher counts of Ca, P, and Zn.  Zn is also known to participate in both function- and disease-induced biomineralization of AB.  Higher X-ray attenuating bands were observed in inter-lamellae regions of LB.  AFM wet scans illustrated hygroscopic lamellae relative to inter-lamellae regions. Interestingly, radial PDL fibers in BB, changed their orientation into circumferential in LB within a junction of 10-30µm illustrating an elastic discontinuity.   Chemical and structural heterogeneities were further complemented by elastic modulus values of 23.9 ± 2.8 GPa for lamellae, 33.2 ± 0.4 GPa for inter-lamellae, with significant elastic modulus differences between lamellae and inter-lamellae confirmed via Student’s t-test (P< 0.05).

Conclusion:  Higher packing density due to differential collagen fiber orientation, not necessarily mineral changes, within BB and LB could contribute to a higher attenuation. These heterogeneous regions represented by radial-fiber bone and lamellar-bone could be adaptive features to maintain the “functional quality” of AB in the fibrous joint.