Porous titanium bases for osteochondral tissue engineering

Abstract

Tissue engineering of osteochondral grafts may offer a cell-based alternative to native allografts, which are in short supply. Previous studies promote the fabrication of grafts consisting of a viable cell-seeded hydrogel integrated atop a porous, bone-like metal. Advantages of the manufacturing process have led to the evaluation of porous titanium as the bone-like base material. Here, porous titanium was shown to support the growth of cartilage to produce native levels of Young's modulus, using a clinically relevant cell source. Mechanical and biochemical properties were similar or higher for the osteochondral constructs compared to chondral-only controls. Further investigation into the mechanical influence of the base on the composite material suggests that underlying pores may decrease interstitial fluid pressurization and applied strains, which may be overcome by alterations to the base structure. Future studies aim to optimize titanium-based tissue engineered osteochondral constructs to best match the structural architecture and strength of native grafts.

Statement of significance: The studies described in this manuscript follow up on previous studies from our lab pertaining to the fabrication of osteochondral grafts that consist of a bone-like porous metal and a chondrocyte-seeded hydrogel. Here, tissue engineered osteochondral grafts were cultured to native stiffness using adult chondrocytes, a clinically relevant cell source, and a porous titanium base, a material currently used in clinical implants. This porous titanium is manufactured via selective laser melting, offering the advantages of precise control over shape, pore size, and orientation. Additionally, this manuscript describes the mechanical influence of the porous base, which may have applicability to porous bases derived from other materials.

Keywords: Articular cartilage; Osteochondral grafts; Porous titanium; Tissue engineering.

Fig. 1

(A) Representative stereoscopic images of 10 mm porous titanium disks with uniform pore distribution; (B) Top: Strut thickness (^) and pore side length (*) shown on a 1200 μm unit cell pore size base; Bottom: The same base processed for area fraction measurements. Scale bars in mm.

Fig. 2

Representative stereoscopic images of 10 mm diameter 600 μm randomly oriented porous titanium and 4.7 mm diameter porous tantalum. Scale bars in mm.

Fig. 3

Annotated photo of shear testing apparatus with gel contrast enhanced in blue for visualization.

Fig. 4

(A) Schematic: mechanically testing E_Y of acellular agarose hydrogels on an impermeable surface (E_chondral, left) and atop a porous titanium base (E_{osteochondral}, right). Relationship between E_chondral and E_{osteochodnral} using (B) 600; (C) 900; (D) and 1200 μm pore bases using 2%, 4%, 6% w/v agarose. Curve-fit slopes (R₂ > 0.9) are indicated above graphs. For each scatterplot, n≥14.

Fig. 5

A representative composite of transmitted light and fluorescent light images, showing 4% agarose gel compressed against a cut 1200 μm pore titanium base (20% applied strain, cells seeded for speckle) with overlaid compressive strain map (left). Open pores abutting gel are marked with # and struts abutting gel are marked with ^. Compressive strain across the solid white line plotted (right) indicating increased compressive strain over struts (^) compared to pores (#).

Fig. 6

(A) Equilibrium Young’s modulus (E_Y); (B) and dynamic modulus (G*) measured at 0.01 Hz across days 0, 14, 28, and 35 (n = 3–4 for days 0, 14, 28; n = 8 for day 35); (C) glycosaminoglycan content per wet weight (GAG/WW); (D) and collagen content per wet weight (Collagen/WW) at day 35 (n≥7); (E) construct photos (chondral, top; osteochondral, bottom) at day 35. Data are expressed as mean ± standard deviation. Indicates significantly different groups (p ≤ 0.05).

Fig. 7

Day 35 representative cross-sectional images for (A) live/dead (n = 1), (B) Alcian blue (n = 3), and (C) Picrosirius red (n = 3) staining for chondral-only control (top) and osteochondral (bottom) constructs. OC constructs are positioned with the gel’s top surface facing right. Scale bar indicates 1 mm.

Fig. 8

(A) Schematic of microscope-mounted depth-dependent testing setup, adapted from [44]. (B) Representative strain map of control construct showing “U-shape” profile with bins demarcated. Average strain profiles normalized by peak compressive strain for (C) chondral-only controls; and for osteochondral constructs grown on (D) 600; and (E) 1200 μm pore bases. Indicates significantly different groups (p ≤ 0.05). For each group, n = 3–4. (F and G) Respective average strain profiles contextualized on schematics of chondral-only and osteochondral constructs. Data are expressed as mean ± standard deviation.

Fig. 9

Schematic (left) and gross photo (right) of an OC construct with a multilayered bases consisting of a cell-seeded chondral region (a), a gel-integrating region (b), an impermeable interface (c), and a bone-integrating region (d).