Long-term bioresorption of bone fixation devices made from composites of unsintered hydroxyapatite particles and poly-L-lactide

Osteosynthetic bone fixation devices made from composites of uncalcined and unsintered hydroxyapatite (u-HA) particles and poly-L-lactide (PLLA) are widely adopted for clinical use because of their bioresorbability and osteoconductive properties. However, how the plate systems constituting these devices change during long-term use in vivo is unknown. In this study, we present cases of two patients fitted with u- HA/PLLA devices for >5 years after surgery and evaluate the resorption process on the basis of the residual versus the resorbed material. In both cases, the majority of the degraded plates and screws had been replaced by bone. In post-operative three-dimensional (3D) CT imaging, plate and screws were maintained until two years after surgery, and then they were degraded and replaced to bone in 4-6 years after surgery. Examination of the aggregation of hydroxyapatite and decrease in molecular weight suggested that the residual material was in the final stages of resorption. The plate system examined demonstrated stable degradation without foreign body reactions in vivo. Although complete resorption is a lengthy process, it is possible to follow its progress using CT.