Additively manufactured custom load-bearing implantable devices: grounds for caution

• Published in: Australasian Medical Journal (Online) Vol. 10; no. 8; p. 694
• Main Authors: Zanetti, Elisabetta M, Aldieri, Alessandra, Terzini, Mara, Calì, Michele, Franceschini, Giordano, Bignardi, Cristina
• Format: Journal Article
• Language: English
• Published: Floreat Australasian Medical Journal 01.01.2017
• Subjects: 3-D printers; Additive manufacturing; Biomechanics; Biomedical materials; Cell adhesion & migration; Composite materials; Custom design; Fractures; Joint surgery; Laser etching; Load; Medical device industry; Medical equipment; Orthodontics; Prostheses; Titanium; Topography; Transplants & implants
• ISSN: 1936-1935
• DOI: 10.21767/AMJ.2017.3093

First applications were finalised for educational purposes, pre-operative planning, and design of surgical guides; recent applications also encompass the production of implantable devices where 3D printing can bring substantial benefits such as customization, optimization, and manufacturing of very complex geometries. First of all mechanical tests require long times since usual tests run at few hundred Hertz and at least 10 million cycles must be performed25 when a fatigue limit is supposed to exist (this is not the case of laser additive manufactured massif Ti6Al4V according to the above cited article34), otherwise the number of cycles in dynamic tests should replicate the expected implant life, reaching 30-40 million cycles for a lower limb prosthesis lasting 20 years. [...]this property has a sensible statistical variability; therefore at least 15 specimens need to be tested.25 Finally, meta-materials usually exhibit a marked anisotropic behaviour due to both the geometry of the unit cell and build orientation during the selective melting process; as a consequence, different combinations of multiaxial loads must be studied to fully characterize material behaviour. [...]the fatigue mechanism appears to be an interaction between cyclic ratcheting and fatigue crack initiation and growth;26 fatigue failure usually takes place in correspondence with micro-defects due, for example to gaseous bubbles entrapped during the powder fusion process; the number and frequency of these flaws is likely to depend not only on process parameters (defining the 'material' in a wide sense), but also on printed component size, leading to a relevant 'scale effect', which depends on the printed component itself and its morphology rather than on material properties.