Abstract
Orthopaedic reconstruction procedures to combat osteoarthritis, inflammatory arthritis, metabolic bone disease and other musculoskeletal disorders have increased dramatically, resulting in high demand on the advancement of bone implant technology. In the past, joint replacement operations were commonly performed primarily on elderly patients, in view of the prosthesis survivorship. With the advances in surgical techniques and prosthesis technology, younger patients are undergoing surgeries for both local tissue defects and joint replacements. This patient group is now more active and functionally more demanding after surgery. Today, implanted prostheses need to be more durable (load-bearing), they need to better match the patient's original biomechanics and be able to survive longer.
Additive manufacturing (AM) provides new possibilities to further combat the problem of stress-shielding and promote better bone remodelling/ingrowth and thus long term fixation. This can be accomplished by matching the varying strain response (stiffness) of trabecular or subchondral bone locally at joints. The purpose of this research is therefore to determine whether a porous structure can be produced that can match the required behaviour and properties of trabecular bone regardless of skeletal location and can it be incorporated into a long-term implant.
A stochastic structure visually similar to trabecular bone was designed and optimised for AM (Figure 1) and produced over a range of porosities in multiple materials, Stainless Steel 316, Titanium (Grade 23 – Ti6Al4V ELI) and Commercially Pure Titanium (Grade 2) using a Renishaw AM250 metal additive manufacturing system. Over 150 cylindrical specimens were produced per material and subjected to a compression test to determine the specimens' Elastic Modulus (Stiffness) and Compressive Yield Strength. Micro-CT scans and gravimetric analysis were also performed to determine and validate the specimens' porosity. Results were then graphed on a Strength vs. Stiffness Ashby plot (Figure 2) comparing the values to those of trabecular bone in the tibia and femur.
It was found that AM can produce porous structures with an elastic modulus as low as 100 MPa up to 2.7 GPa (the highest stiffness investigated in this study). Titanium structures with a stiffness <500MPa had compressive strengths towards the bottom range of similar stiffness trabecular bone. Between 500 MPa − 1 GPa Titanium AM porous structures match the compressive strength of equivalent stiffness trabecular bone and from 1 GPa − 2 GPa the Ti structures exceed the strength of equivalent stiffness trabecular bone up to ∼2.5 times and consequently increase by a power law.
These results show that AM can produce structures with similar stiffness to trabecular bone over a range of skeletal locations whilst matching or exceeding the compressive strength of bone. The results have not yet taken into account fatigue life with the fatigue life of these types of structures tending to be between 0.1 – 0.4 of their compressive strength. This means that a titanium porous structure would need to be 2.5 – 10 times stiffer or stronger than the portion of trabecular bone it is replacing. This data is highly encouraging for AM manufactured, bone stiffness matched implant technology.
