New bone and connective tissue ingrowth in a hydroxyapatite block repairing a rabbit skull defect

Standardized 11 mm rabbit skull trephine defects were implanted with 9 mm diameter hydroxyapatite (HA) blocks. New bone and connective tissue ingrowth was studied histologically and by X-ray after 8, 12 and 16 weeks. Empty defects without HA served as controls. Increasing amounts of new bone and fibrous connective tissue appeared inside the pores of the HA block with time. The interface area was constituted at 8 weeks predominantly by fibrous connective tissue which changed gradually at 12 weeks and definitely showed a solid bony union between the HA block and the host bone at 16 weeks. Movement of the implant was a possible reason for the prolonged healing time. However, trabecular new bone ingrowth appeared slowly even though a contact between the host bone and the HA block was not primarily provided. Comparison between a calvarial defect inserted with an HA block and an empty defect is not critically valuable, although the amount of new bone increased and that of connective tissue decreased in the empty defect during 16 weeks of observation. At the end of the experiment the HA-filled defect was seen to be totally repaired while the empty control defect was repaired with new bone to about half of its area.