Abstract
Introduction: Joint replacement procedures such as revision impaction grafting and spinal fusion interbody operations are stretching allograft bone stocks to their limits. The need for synthetic alternatives that offer a structural and biological matrix for graft incorporation are paramount for future bone regeneration procedures. Synthetic bone graft alternatives that offer biocompatibility to the host bone (i.e. a biological response) such as hydroxyapatite/tricalcium phosphate (HA/TCP), in addition to possessing an interconnected porosity network have been shown to have a strong influence on the osteoinductive potential of these materials. The current method allows the production of calcium phosphate ceramic components (CPC) that possess an interconnected open porous network in the required size range for osteoid growth and revascularisation.
Materials and Methods: The method can be described as the reticulated foam technique, whereby two grades of calcium phosphate powder are blended together to form a HA/TCP ceramic slip. The slip is then ball milled for 24hrs with zirconia milling media. This slip is used to impregnate polyurethane (PU) foam via a mechanical plunging procedure. The impregnated foam is then held above the slip bath in order for the slip to flow and coat the struts of the foam. The impregnated foam is then dried on tissue paper and treated with high velocity compressed air to avoid the formation of any closed cells. Samples are dried at 120°C for 15hrs. The PU foams are graded as 30 and 45ppi (pores per inch). The slip viscosity ranges from 6000 – 8000 cps (measured with a Brookfield Viscometer, spindle no. 5 and at 10rpm). Samples are sintered slowly until 600°C to ensure PU burnout is complete. Sintering continues up to 1280°C to ensure densification. Image analysis was performed using optical microscopy, digital photography and SEM analysis. Mechanical testing was performed by 3 point bending using an 1122 Instron.
Results: Macroporosity in the samples varied from 40 – 70%. Typical pore sizes far exceeded 300μm (the pore size acknowledged as that needed for osteogenesis). Approx. 79% of all pores were between 150 – 450μm in area equivalent diameter. Typical strut thicknesses ranging from 100 – 500μm were also reported, as was a strut thickness-pore size-mechanical strength relationship. One hundred and twenty samples possessed a breaking stress with a 95% confidence level of 0.30MPa±0.01MPa. The low strengths reported are due to the formation of blow-out holes at triple point junctions on the interconnected struts.
Conclusions: Major requirements for replacement bone materials have been met including a wide range of interconnected porosity from 50 – 1000μm. Bioactivity combined with an excellent porosity size range suggests excellent possibility of osteogenesis. In addition, this fabrication procedure offers consistency and reliability. Future work will focus on improving the strength of these open porous calcium phosphate ceramics.
Correspondence should be addressed to Dr Carlos Wigderowitz, Honorary Secretary of BORS, Division of Surgery & Oncology, Section of Orthopaedic & Trauma Surgery, Ninewells Hospital & Medical School Tort Centre, Dundee, DD1 9SY.
Acknowledgments: The authors would like to thank the University of Bath and Stryker Howmedica Osteonics for their support.
