A major biomedical application of additive manufacturing is prosthetics, which replace damaged tissue with nonliving material (usually a metal alloy) in a form that at least approximates natural function. Examples include dental crowns and bridges, heart valves and stents, and knee and hip replacements.
Hip prosthetics consist of a stem that fits into the upper end of the femur and an acetabular cup that fits into the pelvis. These meet through a low-friction polyethylene interface. Typically, conventional, mass-production casting or forging processes form the two parts out of titanium and cobalt-chrome alloys. But because the range of sizes is consequently limited, orthopedic surgeons must modify the upper femural cavity (in a seemingly barbaric way with rasps) to accommodate the best-fit hip stem available. The resulting fit may not be exact, which leads to variable cement gaps and a weaker implant. This problem has prompted considerable work on adapting additive manufacturing processes such as electron-beam melting (EBM) to the production of prostheses exactly tailored to the patient’s geometry, as determined by CAT or MRI data.
Titanium and cobalt-chrome alloys are biocompatible and have more than enough strength to carry the structural loads of a hip joint over the patient’s lifetime. Unfortunately, the density and stiffness of these materials are much higher than those of the bone being replaced, so future replacement surgeries are often needed. Again, additive-manufacturing techniques can be used to provide material distributions that match actual bone — that is, a dense structure in the outer region (cortical bone tissue) and a latticelike internal structure (trabecular region) — leading to weight distribution and flexibility that more nearly match that of an actual hip joint.
Both stem and cup components of hip prostheses must integrate with their mating bone structures through tissue ingrowth at their surfaces to ensure long implant lifetime and prevent painful repair operations. EBM produces acetabular cups having an optimized “lacy” surface with deep porosity. The number of successful applications using this method is approaching several thousand.
In contrast to the replacement therapies described above, regenerative therapies use porous scaffolds seeded with stem cells and growth factors to provide a platform for the growth of new tissue. After being seeded, the scaffolds are placed in a bioreactor to acclimate the implant in-vitro to the temperature and fluid-flow rate it will experience in-vivo. The scaffold is then placed in the damaged area of the bone, and natural cellular interaction takes place between the scaffold and surrounding sound tissues for a seamless repair.
Additive manufacturing plays a significant role in regenerative medicine because it can generate scaffolds from scan data to produce the precise shape needed to fill damaged areas. It can also generate controlled pore morphologies that provide comfortable attachment points for the seeded cells. Current efforts focus on additive methods for producing bone scaffolds from hydroxyapatite and polymer-ceramic combinations.
A National Science Foundation-funded program is also developing a magnesium alloy for scaffolds. Magnesium is bioresorbable and provides greater strength than hydroxyapatite for load-bearing applications. The objective of the program is to determine compositions that control resorption at the same rate as new tissue formation. Additive manufacturing of magnesium alloys for scaffolds has already been shown. — Howard A. Kuhn
Howard A. Kuhn is R&D Director of The Ex One Co. and is responsible for developing and implementing direct digital manufacturing and tooling technologies. Kuhn is also an adjunct professor at the Univ. of Pittsburgh, School of Engineering, where he is involved in the digital manufacturing of scaffolds for regenerative medicine. Edited by Leslie Gordon