Mechanical and morphologic investigation of the tensile strength of a bone-hydroxyapatite interface

For load-bearing calcium-phosphate biomaterials, it is important to understand the relative contributions of direct physical-chemical bonding vs. mechanical interlocking to interfacial strength. In the limit of a perfectly smooth hydroxyapatite (HA) surface, a tensile test of the bone-HA interface affords an opportunity to isolate the bonding contribution related to HA surface chemistry alone. This study measured the bone-HA interfacial tensile strength for highly polished (-0.05 µm alumina) dense HA disks (5.25 mm in diameter, 1.3 in mm thickness) in rabbit tibiae. Each of five rabbits received four HA disks, two per proximal tibia. Pull-off loads ranged from 3.14 ± 2.38Nat 55 days after implantation to 18.35 ± 11.9Nat 88 days; nominal interfacial tensile strengths were 0.15 ± 0.11 MPa and 0.85 ± 0.55 MPa, respectively. SEM of failed interfaces revealed failures between HA and bone, within the HA itself and within adjacent bone. Tissue remnants on HA were identified as mineralized bone with either a lamellar or trabecular structure. Oriented collagen fibers in the bone intricately interdigitated with the HA surface, which frequently showed breakdown at material grain boundaries and a rougher surface than originally implanted. Mechanical interlocking could not be eliminated as a mode of tissue attachment and contribution to bone-HA bonding, even after implanting an extremely smooth HA surface. © 1997 John Wiley & Sons, Inc.