Long-term osseointegration of 3D printed CoCr constructs with an interconnected open-pore architecture prepared by electron beam melting

Abstract

In orthopaedic surgery, cobalt chromium (CoCr) based alloys are used extensively for their high strength and wear properties, but with concerns over stress shielding and bone resorption due to the high stiffness of CoCr. The structural stiffness, principally related to the bulk and the elastic modulus of the material, may be lowered by appropriate design modifications, to reduce the stiffness mismatch between metal/alloy implants and the adjacent bone. Here, 3D printed CoCr and Ti6Al4V implants of similar macro-geometry and interconnected open-pore architecture prepared by electron beam melting (EBM) were evaluated following 26week implantation in adult sheep femora. Despite higher total bone-implant contact for Ti6Al4V (39±4%) than CoCr (27±4%), bone formation patterns were similar, e.g., densification around the implant, and gradual ingrowth into the porous network, with more bone in the outer half (periphery) than the inner half (centre). Raman spectroscopy revealed no major differences in mineral crystallinity, the apatite-to-collagen ratio, or the carbonate-to-phosphate ratio. Energy dispersive X-ray spectroscopy showed similar Ca/P ratio of the interfacial tissue adjacent to both materials. Osteocytes made direct contact with CoCr and Ti6Al4V. While osteocyte density and distribution in the new-formed bone were largely similar for the two alloys, higher osteocyte density was observed at the periphery of the porous network for CoCr, attributable to slower remodelling and a different biomechanical environment. The results demonstrate the possibility to achieve bone ingrowth into open-pore CoCr constructs, and attest to the potential for fabricating customised osseointegrated CoCr implants for load-bearing applications. Statement of SignificanceAlthough cobalt chromium (CoCr) based alloys are used extensively in orthopaedic surgery, stress shielding due to the high stiffness of CoCr is of concern. To reduce the stiffness mismatch between CoCr and bone, CoCr and Ti6Al4V implants having an interconnected open-pore architecture were prepared by electron beam melting (EBM). After six months of submerged healing in sheep, both alloys showed similar patterns of bone formation, with densification around the implant and gradual ingrowth into the porous network. The molecular and elemental composition of the interfacial tissue was similar for both alloys. Osteocytes made direct contact with both alloys, with similar overall osteocyte density and distribution. The work attests to the potential for achieving osseointegration of EBM manufactured porous CoCr implants.