Elsevier

Biomaterials

Volume 21, Issue 12, June 2000, Pages 1291-1298
Biomaterials

Osteoconduction at porous hydroxyapatite with various pore configurations

https://doi.org/10.1016/S0142-9612(00)00030-2Get rights and content

Abstract

To assess the histological response and the reinforcing effects of bone ingrowth within porous hydroxyapatite (HA) implants depending on pore geometry, four kinds of cylindrical-type with parallel linear pores (φ50, 100, 300, 500 μm), one kind of sponge-type with irregular interconnecting pores (φ250 μm) and one cross-type with crossing linear pores (φ100×120 μm) of porous HA were prepared. Eighty-four rabbits were divided into six groups, and a 5×5×7 mm sized porous HA block was inserted through the medial cortical window of the proximal tibia. Histomorphological changes were examined using light and scanning electron microscopy. A biomechanical compression test was performed using material test machines. After implantation, the implants showed different histological changes depending on pore geometry. Active osteoconduction was also found in the φ50 μm sized cylindrical-type porous HA. Evidence of remodeling of new bone and bone marrow formation within porous HA was found in the larger cylindrical-types (φ300, 500 μm), and the sponge- and cross-types. The biomechanical test showed that the ultimate compressive strength increased significantly in the φ300 μm sized cylindrical-type, and in the sponge- and cross-types eight weeks after implantation. Porous HA with cylindrical pores could be a useful graft material due to its strength, osteoconductivity and the ease with which its pore geometry can be controlled.

Introduction

Hydroxyapatite (Ca10(PO4)6(OH)2), with its high biocompatibility and good bioaffinity, stimulates osteoconduction and is slowly replaced by the host bone after implantation 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. The material factors affecting the biological response to the implant in vivo include the property of HA powder, sintering property, structural configuration and the presence of secondary phases 12, 13, 14, 15, 16. The structural property of porous HA is more resorbable and more osteoconductive than dense HA, and it has been used as artificial bone graft material in many experimental and clinical trials 6, 9, 17. In these studies, it was claimed that a minimum pore size of φ100 μm was necessary for bone ingrowth into the porous implant materials. However, most of these conclusions were based on studies using implants of random pore geometry. Porous HA with random pore geometry (coral, naphthalene, polymer bead and other synthetic HA) has wide ranges of porosity and pores of various sizes with much smaller interconnecting fenestration 4, 18, 19. It is suspected that the size of interconnection is the main limiting factor of osteoconduction rather than the size of the pores themselves. Therefore, there has been no consensus regarding the optimal conditions for osteoconduction, such as pore size, shape, interconnection and the arrangement of pores 2, 15, 16, 18.

In this study, we developed a porous HA with parallel cylindrical pores of various sizes without interconnecting fenestration between adjacent pores, in order to evaluate the actual effect of pore size on osteoconduction. We also developed three types of porous HAs (cylindrical-, sponge- and cross-types) in order to evaluate the effects of interconnection and the pore arrangement.

Section snippets

HA powder

Stoichiometric HA powder (Ca/P ratio 1.67) was synthesized by solid state reaction using CaHPO4·2H2O (EP, Junsei Chemical, Tokyo, Japan) and CaCO3 (EP, Junsei Chemical, Tokyo, Japan). CaHPO4·2H2O was ignited at 1100°C to be converted to Ca2P2O7. The Ca/P ratio 1.67 of Ca2P2O7 and CaCO3 was mixed by ball-milling in water using zirconia media and calcined at 1100°C. Its composition was determined by X-ray diffraction phase analysis (Model M18XHF-SRA, Mac Sci., Japan). It was confirmed to be

Evaluation of porous HA implants

Prepared porous implants were examined by light microscopy and SEM (Fig. 1). To estimate the actual pore size of the synthesized HA blocks, the maximum and minimum diameters of a pore were measured using photos of the surface in several of the areas of each block. The average pore size was compared with the size of the fiber. There were no significant differences between the measured pore sizes and the fiber size in cylindrical-type porous HA (Table 1). The estimated total porosity of

Discussion

The ideal artificial bone demands good biocompatibility without the possibility of inflammation or foreign body/toxic reactions. Strong bonding with the host bone, active bone ingrowth into the graft, and bioabsorbability are also required 16, 20. Strength sufficient to resist the mechanical load in the implanted bone is also needed. However, none of the biomaterials that have been developed in previous studies meet all of these criteria. HA, which is one of the major inorganic materials in

Acknowledgements

This study was supported by a grant (#HMP-96-E-4-1009) from the '97 Good Health R & D Project, Korean Ministry of Health & Welfare.

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