Microporosity enhances bioactivity of synthetic bone graft substitutes

Abstract

This paper describes an investigation into the influence of microporosity on early osseointegration and final bone volume within porous hydroxyapatite (HA) bone graft substitutes (BGS). Four paired grades of BGS were studied, two (HA70-1 and HA70-2) with a nominal total porosity of 70% and two (HA80-1 and HA80-2) with a total-porosity of 80%. Within each of the total-porosity paired grades the nominal volume fraction of microporosity within the HA struts was varied such that the strut porosity of HA70-1 and HA80-1 was 10% while the strut-porosity of HA70-2 and HA80-2 was 20%. Cylindrical specimens, 4.5 mm diameter x 6.5 mm length, were implanted in the femoral condyle of 6 month New Zealand White rabbits and retrieved for histological, histomorphometric, and mechanical analysis at 1, 3, 12 and 24 weeks. Histological observations demonstrated variation in the degree of capillary penetration at 1 week and bone morphology within scaffolds 3-24 weeks. Moreover, histomorphometry demonstrated a significant increase in bone volume within 20% strut-porosity scaffolds at 3 weeks and that the mineral apposition rate within these scaffolds over the 1-2 week period was significantly higher. However, an elevated level of bone volume was only maintained at 24 weeks in HA80-2 and there was no significant difference in bone volume at either 12 or 24 weeks for 70% total-porosity scaffolds. The results of mechanical testing suggested that this disparity in behaviour between 70 and 80% total-porosity scaffolds may have reflected variations in scaffold mechanics and the degree of reinforcement conferred to the bone-BGS composite once fully integrated. Together these results indicate that manipulation of the levels of microporosity within a BGS can be used to accelerate osseointegration and elevate the equilibrium volume of bone.

Bone tissue engineering: A review in bone biomimetics and drug delivery strategies

“…A literature review indicates that a pore size in the range of 10-400 lm may provide enough nutrient and osteoblast cellular infusion, while maintaining structural integrity. [185][186][187][188][189][190] A wide variety of fabrication techniques have been investigated to recreate the microscale porosity and special organization of native bone. Some examples of well developed techniques include: micromachining, photolithography, calcium-phosphate sintering, rapid prototyping, melt extrusion, salt leaching, emulsion templating, phase separation, fiber bonding, membrane lamination, and polymer demixing.…”

Medical Application of Calcium Orthophosphate Bioceramics

“…Thus, there is no need to create calcium orthophosphate bioceramics with very big pores; however, the pores must be interconnected [95,384,397,398]. Interconnectivity governs a depth of cells or tissue penetration into the porous bioceramics, as well as it allows development of blood vessels required for new bone nourishing and wastes removal [455,456].…”

“…Bioceramic microporosity (pore size < 10 μm), which is defined by its capacity to be impregnated by biological fluids [455], results from the sintering process, while the pore dimensions mainly depend on the material composition, thermal cycle and sintering time. The microporosity provides both a greater surface area for protein adsorption and increased ionic solubility.…”

“…For example, an essential mean pore interconnection size of ~ 10 μm was found to be necessary to allow cell migration between the pores [655]. As such, both porosity and general architecture are critical in determining the rate of fluid transport through porous bioceramics, which, in turn, determines the rate and degree of bone ingrowth in vivo [115,455,456,656].…”

The effect of an alginate carrier on bone formation in a hydroxyapatite scaffold

“…Bone formation begins with the formation of an apatite layer which enhances protein adsorption resulting in the differentiation of osteoprogenitor cells into osteoblasts leading to the deposition of bone (25). In order for the apatite layer to form, the bone graft material must first come into contact with blood, which facilitates the dissolution of the graft and its re-precipitation onto the surface (14,24,25). In addition, blood is an abundant source of proteins, which would further enhance this process.…”