Next generation orthopaedic implants by additive manufacturing using electron beam melting

Abstract

This paper presents some examples of knee and hip implant components containing porous structures and fabricated in monolithic forms utilizing electron beam melting (EBM). In addition, utilizing stiffness or relative stiffness versus relative density design plots for open-cellular structures (mesh and foam components) of Ti-6Al-4V and Co-29Cr-6Mo alloy fabricated by EBM, it is demonstrated that stiffness-compatible implants can be fabricated for optimal stress shielding for bone regimes as well as bone cell ingrowth. Implications for the fabrication of patient-specific, monolithic, multifunctional orthopaedic implants using EBM are described along with microstructures and mechanical properties characteristic of both Ti-6Al-4V and Co-29Cr-6Mo alloy prototypes, including both solid and open-cellular prototypes manufactured by additive manufacturing (AM) using EBM.

Figure 1

Schematic view of the Arcam A2 electron beam melting (EBM) system.

Figure 2

Examples of open-cellular structure CAD models for additive manufacturing using EBM. (a) Dode-thin unit cell/element lattice-mesh structure. (b) Foam structure. (c) Bone unit cell/element lattice structure (http://www.pro-fit.de/).

Figure 3

Monolithic cellular/solid structures. (a) CAD model for porous foam core surrounded by less porous (higher density) foam structure. (b) Ti-6Al-4V components fabricated by EBM. (c) Alternative, perspective view of (b). Arrow shows a thin, solid tube core surrounded by foam structure. The arrow for the solid cylinder in (b) shows the build direction parallel to the cylinder axis.

Figure 4

X-ray image (a) and incision-suture photograph for total knee replacement (b) (Courtesy of Patricia Murr).

Figure 5

Co-29Cr-6Mo alloy femoral knee implant prototype fabricated by EBM and HIPed according to ASTM-F75 standard. The insert illustrates the porous inner surface zone (p) composing the monolith.

Figure 6

Stiffness-density design plot composed of experimental data [6]. The data field for Ti-6Al-4V and Co-29Cr-6Mo is shown shaded. The shaded regime follows a fitted slope, n = 2.1. Examples for prototype designs discussed in the text are noted.

Figure 7

Comparative microstructures for Co-29Cr-6Mo components. (a) Optical micrograph showing columns of Cr₂₃C₆ precipitates in the build direction (arrow). (b) TEM image of Cr₂₃C₆ precipitate columns in (a). (c) Optical micrograph showing HIP-annealed grain structure. (d) TEM image for intrinsic stacking faults in (c).

Figure 8

Tibial (knee) Ti-6Al-4V stem, monolithic prototype development example. (a) CAD model orientation with respect to the solid stem (core) axis. Mesh structure is based on dode-thin element in Figure 2(a). (b) EBM-fabricated prototype using CAD model in (a). (c) CAD model in (a) rotated 45°.

Figure 9

Comparison of Ti-6Al-4V solid, cylindrical component microstructures. (a) EBM (z) component α-phase (acicular) plates. (b) EBM (x, y) component fine α + β structure. (c) α + α′ martensite structure for mesh component as in Figure 8(b). Corresponding Vickers microindentation hardness values are (a) 3.5 GPa; (b) 4.1 GPa; (c) 4.5 GPa.

Figure 10

TEM image comparison for dislocation substructures in EBM-fabricated Ti-6Al-4V components as in Figure 9(a). (a) Low dislocation density and shallow β wall thickness surrounding α-phase grains. (b) High dislocation density and thicker β phase surrounding α-grains. Hardness in (a) and (b) was HV 3.6 and 3.9 GPa, respectively.

Figure 11

Total hip replacement components and examples of commercial hip stem implants. Small arrows illustrate porous coating areas using sinter technologies. Commercial examples (lower left) courtesy of DiSanto, Inc.

Figure 12

Hip X-ray image showing total replacement at left (right side) (from Wikipedia).

Figure 13

EBM-fabricated Ti-6Al-4V acetabular cup with outer porous-mesh structure region for hip bone (socket) ingrowth. (a) and (b) show rotated views while (c) and (d) show magnified sections illustrating the porous-mesh structure (sample courtesy of Arcam).

Figure 14

Section view of acetabular cup in Figure 13 showing porous-mesh surface region.

Figure 15

Upper femur with simulated intramedullary rod insert fabricated from Ti-6Al-4V by EBM in (a). (b) shows a through-section cutaway view illustrating inner, more porous foam core surrounded by more dense foam structure for stiffness compatibility.

Figure 16

CAD-model examples of intramedullary rod or hip stem prototypes. (a) Section view for foam of higher density (and stiffness) surrounding foam of lower density (and stiffness) as in Figure 15(b). (b) Section view for foam of low-density core surrounded by high-density mesh (bone element in Figure 2(c)) structure.