Biomimetic orthopaedic implants: harnessing bone’s remodelling response with additive manufacturing

Abstract

The long-term healing of bone is inherently coupled with the strain it experiences, as described by numerous laws governing bone’s mechanobiology and natural remodelling process. Bone regeneration can therefore be accelerated by the use of porous structures in orthopaedics which have a mechanical modulus akin to the bone being replaced. Following design and regulatory requirements, a further consideration is that yield strength and fatigue strength must be suitable for the implants applications. Conventional orthopaedic implants are solid metal so therefore have excessive strength and stiffness. This means that they cause regions of bone to be underloaded, leading to bone resorption which complicates surgery. This PhD explored optimising additively manufactured (AM) porous structures for application as bone growth stimulating orthopaedic implants. The mechanical modulus, strength and anisotropy of bone and AM structures was characterised in a workflow which uses manufacturing and imaging methods commonly used in industry. Following this, individual AM structures were designed and manufactured based on computed tomography (CT) scans of bone to control the strain experienced. Orthopaedic implants were then manufactured which replicated the loading and strain environment experienced in healthy bone. Mechanical mappings of bone and AM structures were shown to predict properties to within 5.6% and 2%, while custom AM implants could control bone strain from -15% to +15% of homeostasis (corresponding to an extension of -4500 µm to 4500 µm). Conventional solid counter parts underloaded up to 70% of peri-prosphethic bone, whereas AM implants closely replicated the loaded experienced by bone prior to implantation (achieving mechanical environments which were 68-78% similar). This thesis has emphasis on understanding bone’s mechanical properties and the effect of implants on mechanical stimulus, which will allow for more accurate testing of the mechanical behaviour of bone in response to implanted porous scaffolds and for better translation from in vivo studies to commercial implants.