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The ability to engineer anatomically correct pieces of viable and functional human bone would have tremendous potential for bone reconstructions after congenital defects, cancer resections, and trauma. We report that clinically sized, anatomically shaped, viable human bone grafts can be engineered by using human mesenchymal stem cells (hMSCs) and a "biomimetic" scaffold-bioreactor system. We selected the temporomandibular joint (TMJ) condylar bone as our tissue model, because of its clinical importance and the challenges associated with its complex shape. Anatomically shaped scaffolds were generated from fully decellularized trabecular bone by using digitized clinical images, seeded with hMSCs, and cultured with interstitial flow of culture medium. A bioreactor with a chamber in the exact shape of a human TMJ was designed for controllable perfusion throughout the engineered construct. By 5 weeks of cultivation, tissue growth was evidenced by the formation of confluent layers of lamellar bone (by scanning electron microscopy), markedly increased volume of mineralized matrix (by quantitative microcomputer tomography), and the formation of osteoids (histologically). Within bone grafts of this size and complexity cells were fully viable at a physiologic density, likely an important factor of graft function. Moreover, the density and architecture of bone matrix correlated with the intensity and pattern of the interstitial flow, as determined in experimental and modeling studies. This approach has potential to overcome a critical hurdle-in vitro cultivation of viable bone grafts of complex geometries-to provide patient-specific bone grafts for craniofacial and orthopedic reconstructions.biomimetic | bioreactor | craniofacial regeneration | mesenchymal stem cells | temporomandibular joint | tissue engineering

Introduction: Standard morphological analyses of trabecular architecture lack explicit segmentations of individual trabecular plates and rods. In this study, a complete volumetric decomposition technique was developed to segment trabecular bone microstructure into individual plates and rods. Contributions of trabecular typeassociated morphological parameters to the anisotropic elastic moduli of trabecular bone were studied. Materials and Methods: Seventy-one human trabecular bone samples from the femoral neck (FN), tibia, and vertebral body (VB) were imaged using CT or serial milling. Complete volumetric decomposition was applied to segment trabecular bone microstructure into individual plates and rods. The orientation of each individual trabecula was determined, and the axial bone volume fractions (aBV/TV), axially aligned bone volume fraction along each orthotropic axis, were correlated with the elastic moduli. The microstructural type-associated morphological parameters were derived and compared with standard morphological parameters. Their contributions to the anisotropic elastic moduli, calculated by finite element analysis (FEA), were evaluated and compared. Results: The distribution of trabecular orientation suggested that longitudinal plates and transverse rods dominate at all three anatomic sites. aBV/TV along each axis, in general, showed a better correlation with the axial elastic modulus (r 2 ‫ס‬ 0.95∼0.99) compared with BV/TV (r 2 ‫ס‬ 0.93∼0.94). The plate-associated morphological parameters generally showed higher correlations with the corresponding standard morphological parameters than the rod-associated parameters. Multiple linear regression models of six elastic moduli with individual trabeculae segmentation (ITS)-based morphological parameters (adjusted r 2 ‫ס‬ 0.95∼0.98) performed equally well as those with standard morphological parameters (adjusted r 2 ‫ס‬ 0.94∼0.97) but revealed specific contributions from individual trabecular plates or rods. Conclusions: The ITS-based morphological analyses provide a better characterization of the morphology and trabecular orientation of trabecular bone. The axial loading of trabecular bone is mainly sustained by the axially aligned trabecular bone volume. Results suggest that trabecular plates dominate the overall elastic properties of trabecular bone.

High-resolution peripheral quantitative computed tomography (HR-pQCT) is a newly developed in vivo clinical imaging modality. It can assess the 3D microstructure of cortical and trabecular bone at the distal radius and tibia and is suitable as an input for microstructural finite element (µFE) analysis to evaluate bone's mechanical competence. In order for microstructural and image-based µFE analyses to become standard clinical tools, validation with a current gold standard, namely, high-resolution micro-computed tomography (µCT), is required. Microstructural measurements of 19 human cadaveric distal tibiae were performed for the registered HR-pQCT and µCT images, respectively. Next, whole bone stiffness, trabecular bone stiffness, and elastic moduli of cubic subvolumes of trabecular bone in both HR-pQCT and µCT images were determined by µFE analysis. The standard HR-pQCT patient protocol measurements, derived bone volume fraction (BV/TVd), trabecular number (Tb.N*), trabecular thickness (Tb.Th), trabecular spacing (Tb.Sp), and cortical thickness (Ct.Th), as well as the voxel-based direct measurements, BV/TV, Tb.N*, Tb.Th*, Tb.Sp*, Ct.Th, bone surface-to-volume ratio (BS/BV), structure model index (SMI), and connectivity density (Conn.D), correlated well with their respective gold standards, and both contributed to µFE-predicted mechanical properties in either single or multiple linear regressions. The mechanical measurements, although overestimated by HR-pQCT, correlated highly with their gold standards. Moreover, elastic moduli of cubic subvolumes of trabecular bone predicted whole bone or trabecular bone stiffness in distal tibia. We conclude that microstructural measurements and mechanical parameters of distal tibia can be efficiently derived from HR-pQCT images and provide additional information regarding bone fragility. © 2010 American Society for Bone and Mineral Research.