The formation and strengthening mechanisms of bone bonding of crystalline hydroxyapatite (HA)has been investigated using high-resolution transmission electron microscope (HRTEM)and energy-dispersive X-ray (EDX)analysis.A series of results were obtained: (i)a layer of amorphous HA, which has almost the same chemistry as the implanted HA, was formed on the surface of crystalline HA particles prior to dissolution; (ii)at 3 months a bone-like tissue formed a bonding zone between mature bone and the HA implant, composed of nanocrystalline and amorphous apatite; and (iii)at 6 months, mature bone was in direct contact with HA particles, and collagen fibres were perpendicularly inserted into the surface layer of implanted HA crystals.Findings (i)and (ii)indicated the following dissolution-precipitation process.(i)The crystalline HA transforms into amorphous HA; (ii)the amorphous HA dissolves into the surrounding solution, resulting in over-saturation; and (iii)the nanocrystallites are precipitated from the oversaturated solution in the presence of collagen fibres.A preliminary analysis indicated several conclusions: (i)the transition from crystalline to amorphous HA might be the controlling step in the bone bonding of crystalline HA; (ii)biological interdigitation (or incorporation)of collagen fibres with HA and chemical bonding of a apatite layer were both necessary to strengthen and toughen a bone bond, not only for the bonding between bone and HA at 6 months, but also for the bonding zone at 3 months, which would otherwise be very fragile due to the inherited brittleness of polycrystalline ceramics; and (iii)perpendicular interdigitation is an effective way for collagen fibres to impart their unique combination of flexibility and strength to the interface which they are keying