Abstract
Introduction; In contrast to hydroxyapatite (HA), carbonate substituted hydroxyapatite (CHA) is resorbed by osteoclasts, and is more osteoconductive in vivo. On bone, osteoclastic resorption results not only in topographical changes, but also changes in the proteinaceous matrix within the resorption pit to which osteoblasts respond [1]. This study sought to investigate a possible link between the different bioresorptive properties of these biomaterials and subsequent bone formation on their surfaces, analogous to the coupling seen in normal bone remodelling.
Methods; Phase-pure HA and 2.7wt% CHA were prepared by aqueous precipitation methods [2] and processed into dense sintered discs for cell culture. Human osteoclasts derived from CD14+ precursors were cultured for 21 days on discs of HA and CHA; subsequently, cells and the proteinaceous layer were removed from some discs leaving a topographically altered surface (assessed by SEM and profilometry), whilst in others the proteinaceous layer was left intact. Control (unresorbed) discs were also prepared. The discs were then seeded with human osteoblasts (HOBs) which were cultured for up to 28 days, in some cases in the presence of hydrocortisone and â-glycerophosphate. Proliferation (MTS assay), collagen synthesis (3-H Proline incorporation), and the formation of mineralised nodules (tetra-cycline labelling [3] and SEM) were assessed.
Results; Osteoclasts altered the ceramic surfaces. Large pits were seen on CHA in contrast to limited erosion of the HA surface, accompanied by a greater increase surface roughness (Ra) (p< 0.05). After 6 days of culture, proliferation of HOBs was increased on resorbed discs provided the proteinaceous layer resulting from osteoclastic activity was left intact. At 28 days, cells had formed confluent sheets and there were no significant differences in their number. At 6 days, collagen synthesis by HOBs on CHA was increased on resorbed surfaces, and further increased if the proteinaceous layer was left intact. A similar response was seen on HA, but not until 28 days. Mineralised nodules formed after 28 days of culture in the presence of hydrocortisone and â-glycerophosphate on tissue culture plastic, but not in their absence. By contrast on the ceramics there was no evidence of mineralised nodule formation on any of the discs, although globular accretions were present in small amounts throughout the collagenous matrix regardless of the presence or absence of supplements.
Conclusion; Prior osteoclastic activity on HA and CHA affects subsequent proliferation and collagen production by HOBs. The effects of topographical alteration and matrix conditioning appear synergistic, and are apparent at an earlier time-point on a more resorbable ceramic. Osteoclastic activity may be important in the osteoconductive properties of biomaterials.
Correspondence should be addressed to Mr Carlos Wigderowitz, Honorary Secretary BORS, University Dept of Orthopaedic & Trauma Surgery, Ninewells Hospital & Medical School, Dundee DD1 9SY.
None of the authors have received anything of value from a commercial or other party related directly or indirectly to the subject of the presentation
References
1 IR Gibson, W Bonfield. J Biomed Mater Res2002;15:697–708. Google Scholar
2 J Scheu et al. J Biol Chem2003;278:438–43. Google Scholar
3 E Parker et al. In: Bone Engineering , Ed. JE Davies. Em squared, Toronto, (2001). Google Scholar
