Toughness as a metric for bone tissue engineering scaffold success

Objectives: The goal of this study was to determine the strength and toughness of novel tissue engineering scaffolds needed for large defects in crainofacial and maxillofacial applications. Methods: Novel high porosity hydroxyapatite scaffolds^[1] mimicking trabecular architecture were prepared by coating polymer templates and sintering. Three designs of mean pore size 450, 340 and 250 µm were prepared. Scaffolds were tested to failure under monotonic compression hydrated at 37°C or after 21 days of static in vitro culture with osteoblast precursor cells. Scaffold porosity was also measured using pycnometry and interconnectivity established using Micro-CT. Results: The 450, 340 and 250 µm pore size scaffolds had no significant difference in macroscale porosity with mean porosity of 87.8%. The ultimate strength of the control scaffolds was measured at 0.24, 0.51 and 0.59 MPa and elastic modulus was 103, 44 and 59 MPa respectively (n=6). After cell seeding, the ultimate strength was 0.13, 0.29 and 0.38 MPa and the elastic modulus was 40, 68 and 76 MPa respectively. The greatest significant change was observed in terms of toughness where control samples had toughness values of 0.6, 5.8 and 5.4 KPa while after 21 days cell culture, the values significantly increased to 4.0, 17.1 and 21.0 KPa respectively. Conclusion: Cells colonizing hydroxyapatite scaffolds lay down extra-cellular matrix that significantly increases the toughness or strain energy to failure of the constructs. The compressive strength and elastic modulus values were comparable to reported trabecular bone in the mandible^[2]. Given the large variations in the elastic moduli and the dependence of ultimate strength on the weakest link in a random construct, it is suggested that toughness is a more reliable metric for predicting bone graft performance.

[1] S. Oh, et al. Key Eng. Mater. Vols. 2001; 192:91-94.

[2] C.E. Misch, et al. J Oral Maxillofac Surgery 1999; 57:700-06.