The complete process of bioresorption and bone replacement using devices made of forged composites of raw hydroxyapatite particles/poly l-lactide (F-u-HA/PLLA)

Here we document the complete process of bioresorption and bone replacement of rods made of forged composites of unsintered hydroxyapatite particles/poly l-lactide (F-u-HA/PLLA) implanted in the femoral medullary cavities of rabbits. Bioresorption, osteoconductive bioactivity and bone replacement were compared in three implantation sites. In the first site, the end of the rod was located near the endosteum in the proximal medullary cavity. In the second, the rod was located at the centre of the bone marrow space without contacting the endosteum. In the third, the rod was in direct contact with cancellous bone within the distal femoral condyle. Micro-computerised tomography, scanning electron microscopy and photomicrographs of stained sections were used to document the complete process of bioresorption and bone replacement. At the first implantation site, the rod was completely resorbed and unbound u-HA particles were detected in and around the endosteum 5–6 years after implantation. At the second site, the rod showed significant shrinkage 4–5 years after implantation due to the release of almost all the PLLA, although a contracted cylindrical structure containing a few u-HA persisted even after ∼6 years. At the third site, u-HA particles were almost completely replaced with bone after 5–6 years. Conversely, PLLA-only rods showed little bone conduction, and small amounts of degraded PLLA debris and intervening some tissue persisted even after long periods. Namely, the u-HA/PLLA composites were replaced with bone in the distal femoral condyle, where they were in direct contact with the bone and new bone formation was anatomically necessary. By contrast, composite rods were resorbed without replacement in the proximal medullary cavity, in which new bone growth was not required. We therefore conclude that the F-u-HA30/40 composites containing 30 wt%/40 wt% u-HA particles are clinically effective for use in high-strength bioactive, bioresorbable bone-fixation devices with the capacity for total bone replacement.