The aim of this study was to compare the morphometric characteristics and bone mineral density of DBBM-C and LBMB graft blocks. Additionally, bone replacement, remodeling, and bone mineral density of segmental defects filled with both xenografts were compared with untreated defects following 3- or 6-months postoperative period.
Original DBBM-C and LBMB blocks had a distinct morphometric structure due to the difference in processing. DBBM-C were characterized by hydroxyapatite particles bounded by collagen, while the LBMB blocks preserved the natural trabecular structure [5, 15]. Initial DBBM-C blocks presented denser (↑BV/TV), more complex (↑BS/TV, ↑Tb.N) and connected structure (↓Tb.Pf and ↑Conn.Dn), with lower porosity (↓Po[Tot]) and smaller pore size (↓Tb.Sp) compared to LBMB.
When comparing both blocks over time, a significant impact was induced by the grafts and the follow up time in the morphometric parameters. However, current results considering the interaction between graft and follow-up time showed no significant impact in bone replacement and remodeling morphometric parameters, suggesting similar changes for the DBBM-C and LBMB grafted regions. In relation to the untreated regions, both DBBM-C and LBMB grafts resulted in an increased bone replacement and structural complexity.
In clinical practice, guided bone regeneration with bone grafts frequently precedes implant placement in cases where insufficient bone support is available for functional rehabilitation [18, 26]. Bone microarchitecture is an important component for bone quality and may influence implant stability and rehabilitation success [24]. In accordance with our results, the microarchitecture of DBBM augmented bone has been documented as consisting of residual bone graft particles in close contact with newly formed bone leading to a dense structure [27]. These residual particles may remain unresorbed for long periods, leading to high stability in the maintaining of bone volume [27, 28]. It has been demonstrated that implant stability increases with denser bone (↑BV/TV, ↑Tb.Th, ↑Tb.N, and ↓BS/TV) plate-like structure (↓SMI) and smaller marrow spaces (↓Tb.Sp) [24]. Indeed, high implant success rates have been reported in DBBM grafted sites [29, 30].
In this experiment, DBBM-C blocks presented higher connective density (↑Conn.Dn and ↓Tb.Pf) compared to LBMB. This finding might have been influenced by the micro-CT segmentation. DBBM-C blocks are formed by bovine bone particles banded to each other by porcine collagen [8, 9]. The high number of particles close to each other may lead to a miss interpretation during segmentation. In this way, connections do not reflect a real trabecular connection node. The absence of real connections explains the changes in the DBBM block shape and the significant decrease in connectivity during healing. Differently, LBMB blocks connected trabecular structure may be responsible for maintenance of connectivity and block form during bone repair.
LBMB and DBBM-C grafts presented themselves with different host-bone interfaces. In LBMB group, graft and host bone seemed to be well integrated, making it impossible to define the exact borders. In contrast, in the DBBM-C group, a hypodense line between the graft and basal cortical was often present. The formation of a bond at the graft-host interface occurs as a result of a remodeling process, and is influenced by graft osteogenic, osteoinductive properties and resorption potential, [21, 31]. The resorption rates of bovine xenografts may vary greatly among commercially available materials, influenced both by graft structure and physico-chemical properties [5, 32].
The interconnection between the graft and the host is also influenced by graft porosity. The graft structure may present sufficient porosity, pore size, and interconnectivity to allow osteoconduction [21, 33]. An open porosity above 50% and pore sizes in the range of 200 to 800 μm are pointed out to be optimal for bone tissue ingrowth [21]. Scanning electron microscopy microstructural characterization of LBMB has shown 87–963 μm pore size while DBBM presented 20–200 μm pore size [15, 34]. Current results have shown that LBMB presents higher mean porosity and trabecular separation in comparison to DBBM-C and this difference is maintained in the grafted regions.
It is possible that the selection of the VOI may have influenced the morphometric quantification. This investigation was focused on the cross-sectional evaluation of the grated regions in different post-operatory moments, hence no initial imaging of the region of the defect was available for comparison and, in some cases, the exact edge of the defect was difficult to identify. To avoid mistakes, the same volume of interest considering the region of the defect was selected for all samples. The possible differences between bone architecture in the center and in the borders of the defect were not considered. Additionally, the formation of concave cortical bone in the region of the defect and within the VOI analyzed lead to an increased BMD in the untreated group in comparison to the grafted groups. Wong et al. (2010) demonstrated that the formation of new bone occurs in the DBBM-C grafted region periphery and tends to grow across the defect [10]. A similar bone neoformation pattern has also been demonstrated for LBMB [19]. In DBBM-C group, denser regions were observed in the periphery of the graft both in T1 and T2.
The results need to be interpreted in light of some limitations. Micro-CT is a non-destructive method that provides accurate 3D imaging of bone microstructure [35]. However, bone healing is a dynamic process that is influenced by several different graft material properties that should also be considered [36]. Histological investigation of the morphometric parameters will be the focus of further investigation. Future studies may also consider an in vivo imaging follow up of the grafted regions to allow the identification of early changes.
In this investigation, a segmental defect was constructed in the rabbit’s jaw [37]. Animal models play an indispensable role to understand bone graft materials properties, such as osteoconductivity, biocompatibility, resorption, and interaction with host tissues [31]. Metabolic rate of the test animal, characteristics of the created defect, and anatomical location should be considered when evaluating the results [38]. No significant changes in bone quantity, density, and structure were observed in the non-treated defects in the interval between 3 and 6 months suggesting that segmental bone defect healing in this rabbit model have been close to completion already at 3 months.