BIOMECHANICAL ASSESSMENT OF BONE INGROWTH IN POROUS HYDROXYAPATITE

Porous hydroxyapatite (Endobon(R)) specimens were implanted into the f emoral condyle of New Zealand White rabbits for up to 6 months. After sacrifice, specimens were sectioned for histology and mechanical testi ng, where the extent of reinforcement by bony ingrowth was assessed by compression testing and fixation was assessed by push-out testing. Fr om histological observations, it was established that the majority of bone ingrowth occurred between 10 days and 5 weeks after implantation and proceeded predominantly from the deep end of the trephined defect, with some integration from the circumferential sides. At 3 months, th e implants were fully integrated, exhibiting bony ingrowth, vasculariz ation and bone marrow stroma within the internal macropores. After 5 w eeks, the mean ultimate compressive strength of retrieved implants (6. 9 MPa) was found to be greater than that of the original implant (2.2 MPa), and by 3 months the fully integrated implants attained a compres sive strength of approximately 20 MPa. Push-out testing demonstrated t hat after 5 weeks in vivo, the interfacial shear strength reached 3.2 MPa, increasing to 7.3 MPa at 3 and 6 months.