Imperial College London
Powder Bed Fusion of Low Stiffness Titanium Lattice Structures for Bone Regeneration
INTRODUCTION Orthopaedic implants have traditionally been machined, forged or cast solid pieces of metal that are orders of magnitude stiffer than the natural bone. Powder bed fusion (PBF) enables lattice materials with tailored stiffness profiles not previously possible for implantable devices. The aim of this study is to control trabecular bone ingrowth in response to varying strain gradients in vivo in a load bearing environment. The hypothesis is that matching the stiffness of the original bone will cause greater bone formation than an industry standard stiffer porous scaffold.
METHODS CT scanning and compression testing produced a continuous stiffness-density relationship for ovine bone. Two cylindrical implants were designed: one of a stiffness similar to typical industry-standard porous implants (6 GPa) and one which was discretized into 27 volumes, with each volume matched to the stiffness of bone it would be replacing. Implants were manufactured by PBF in CPTi and implanted in 6 skeletally mature ewes. Each hind femur contained either a stiffness matched CPTi implant as described above or a conventional ‘stiff’ porous implant implanted in the distal medial femoral condyle. Sheep were euthanised at 6 weeks. Specimens were retrieved and assessed by μCT, histology and fluorescent microscopy.
RESULTS For the stiffness matched implant, there was intimate bone ingrowth around the perimeter of the implant with an average of 14.3±3.2% bone ingrowth by volume penetrated into the centre of the implant. The smaller strut diameters generated a high surface area in contact with the bone and fluorescence imaging indicated a high level of osteoblast activity in these areas and no fibrous encapsulation was found. In the areas of bone ingrowth, histology revealed typical patterns for woven and lamellar bone at the perimeter and within the implant. For the high stiffness implant, bone ingrowth was half that of the stiffness matched implant (6.3±2.2%), with less bone penetration and observed osteoblast activity. We found scar tissue where the high stiffness implant caused low strain.
DISCUSSION The low stiffness porous PBF material tailored a strain gradient within the implant that accelerated bone formation in a load-bearing environment. This is highly exciting as it may enable orthopaedic implant designs that actively improve the quality of the bone into which they are implanted. This would be highly beneficial as the implant would improve the bone quality for any subsequent procedure that may be necessary, thus being ideal for early intervention treatments in younger patients.
Jonathan Jeffers is a Mechanical Engineer from the Department of Mechanical Engineering in Imperial College London. Prior to his current position, he worked in the orthopaedic industry where he was involved in the design and commercial launch of several hip and knee replacement implants which have been implanted in thousands of patients around the world. His research interest is orthopaedic implant design using powder bed fusion to achieve what is not possible using conventional casting or forging techniques. He holds an EPSRC early career fellowship, is on the management board of Imperial’s Musculoskeletal Medical Engineering Centre and a director of the International Society for Technology in Arthroplasty.
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