Custom-made composite scaffolds for segmental defect repair in long bones

Abstract

Current approaches for segmental bone defect reconstruction are restricted to autografts and allografts which possess osteoconductive, osteoinductive and osteogenic properties, but face significant disadvantages. The objective of this study was to compare the regenerative potential of scaffolds with different material composition but similar mechanical properties to autologous bone graft from the iliac crest in an ovine segmental defect model. After 12 weeks, in vivo specimens were analysed by X-ray imaging, torsion testing, micro-computed tomography and histology to assess amount, strength and structure of the newly formed bone. The highest amounts of bone neoformation with highest torsional moment values were observed in the autograft group and the lowest in the medical grade polycaprolactone and tricalcium phosphate composite group. The study results suggest that scaffolds based on aliphatic polyesters and ceramics, which are considered biologically inactive materials, induce only limited new bone formation but could be an equivalent alternative to autologous bone when combined with a biologically active stimulus such as bone morphogenetic proteins.

Fig. 1

MicroCT 3-D reconstructions of a PDLLA-TCP-PCL (A) and mPCL-TCP scaffold (B) (height 20 mm, diameter 18 mm). Compressive stiffness values averaged 446 N/mm (SD = 66.3) for mPCL-TCP and 418 N/mm for PLDLLA-TCP-PCL (SD = 88.1) scaffolds (C), the elastic modulus 22.17 MPa (SD 3.0) and 24.70 MPa (SD = 3.3) (D), respectively. Scaffold porosity was determined to be 70.55% for mPCL-TCP (SD = 3.78) scaffolds and 43.76% for PLDLLA-TCP-PCL (SD = 10.02) scaffolds (E) as determined by microCT analysis. Error bars represent standard deviations, n = 6

Fig. 2

Tibial segmental bone defect of 2 cm length stabilised with a limited contact locking compression plate (LC-LCP, Synthes) and filled with a PDLLA-TCP-PCL (A) and mPCL-TCP scaffold (C). Prior to scaffold insertion, the periosteum (B), which is in close proximity to the neurovascular bundle, was entirely removed within the defect area (D)

Fig. 3

X-ray images in the anterior-posterior plane of 2-cm tibial segmental bone defects left untreated (A), reconstructed with ABG (B), a mPCL-TCP scaffold (C) or a PDLLA-TCP-PCL scaffold (D) 12 weeks after surgery. The images show the subcritical nature of the defect as bridging was observed in the empty control defect. Homogeneous bone formation and bridging throughout the defect was observed when treated with ABG. Less bone and discontinuous bridging occurred in the scaffold groups

Fig. 4

Von Kossa/van Gieson staining on PMMA embedded specimens (A–D) showed extensive bone formation (black) and bridging in the autograft group (B, F), considerably less bone formation and non-union in the empty control (A, E) and the scaffold groups (mPCL-TCP: c, g; PDLLA-TCP-PCL: D, H). Histology results correlated well with 3-D microCT reconstructions of the defects (E–H). Movat’s pentachrome staining (I–K) suggested endochondral bone formation in all groups (I, cartilage, green) with subsequent osteoid formation (J). In the autograft group, first signs of bone remodelling were evidenced by osteoclasts and giant cells within the defect area (K)

Fig. 5

Torsion testing data showed highest values for defect reconstructed with autograft (18.67%) when compared to untreated controls (11.19%) or defects treated with mPCL-TCP (4.96%) or PDLLA-TCP-PCL (14.41%) scaffolds (A). A similar trend was found for the relative bone volume formed within the defect (B) and the trabecular number (C) with highest values for autografts (47.64%; 2.28/mm³) followed by defects treated with PDLLA-TCP-PCL (30.34%; 1.14/mm³) scaffolds, empty controls (31.76%; 0.73/mm³) and mPCL-TCP-treated defects (11.96%; 0.39/mm³). No difference in bone mineral density was found (D). Mechanical testing data are presented as box plots and the microCT data as bar graphs with error bars representing standard deviations