Regional and temporal changes in the acoustic properties of fracture callus in secondary bone healing

Electrical impedance spectroscopy - a potential method for the study and monitoring of a bone critical-size defect healing process treated with bone tissue engineering and regenerative medicine approaches

The development of strategies of bone tissue engineering and regenerative medicine has been drawing considerable attention to treat bone critical-size defects (CSDs). Notably, new strategies and/or treatment approaches always require appropriate tools to track the healing process so as to evaluate their success. In this paper, we present the development of a novel approach for the non-invasive, yet real-time, monitoring and assessment of bone CSDs treated with biomaterials and biomedical approaches. For this, we employed the technique of electrical impedance spectroscopy (EIS) to quantitatively monitor and assess the changes in electrical impedance, and thus the regeneration process. In our in vitro tests, we examined the biochemical changes of the fracture area and investigated the influence of collagen and hydroxyapatite on the changes in electrical impedance by EIS, thus inferring the changes in bone regeneration and structure. Based on this success, we further demonstrated, in real time, the process of regeneration of the traumatic area in an in vivo rabbit model. Our electrical-impedance data of the experiment groups, i.e., the ones treated with natural coral and bone morphogenetic protein-2 (BMP-2), revealed that each group has its unique impedance graph characteristics, which are directly associated with the degree of regeneration. For comparison, we also employed radiography, gross anatomy, and histological analyses in examination. Our results illustrate that EIS holds considerable potential as a non-invasive tool for monitoring, in real time, the healing of bone CSDs by allowing for quantitatively characterizing the changes of both hydroxyapatite and collagen.

Quantitative phenotyping of bone fracture repair: a review

Fracture repair is a complex process that involves the interaction of numerous molecular factors, cell lineages and tissue types. These biological processes allow for an impressive feat of engineering: an elastic soft callus is progressively replaced by a more rigid and mineralized callus. During this reparative phase, the healing bone is exposed to a risk of re-fracture. Bone volume and bone quality are the two major factors determining the strength of the callus. Although both factors are important, often only bone volume is analyzed and reported in preclinical studies. Recent developments in techniques for examining bone quality in the callus will enable the rapid and detailed analysis of its material properties and its microstructure. This review aims to give an overview of the methods available for quantitatively phenotyping the bone callus in preclinical studies such as Raman spectroscopy, nanoindentation, scanning acoustic microscopy, in vivo micro-computed tomography (micro-CT) and high-resolution micro-CT. Consolidated and emerging experimental methods are described with a focus on their applicability, and with examples of their utilization.

Spatial-temporal mapping of bone structural and elastic properties in a sheep model following osteotomy

The course of bone healing in animal models is conventionally monitored by morphologic approaches, which do not allow the determination of the material properties of the tissues involved. Mechanical characterization techniques are either dedicated to the macroscopic evaluation of the entire organ or to the microscopic evaluation of the tissue matrix. The latter provides insight to regionally specific alterations at the tissue level in the course of healing. In this study, quantitative scanning acoustic microscopy was used at 50 MHz to investigate microstructural and elastic alterations of mineralized callus and cortical tissue after transverse osteotomy in sheep tibiae. Analyses were performed after 2, 3, 6 and 9 weeks of consolidation with stabilization by either a rigid or a semi-rigid external fixator. Increased stiffness and decreased porosity were observed in the callus tissue over the course of the healing process, which was dependent on the fixator type. In the adjacent cortical tissue, stiffness decreased during the first 3 weeks. Cortical porosity increased over time but the time-dependence was different between the two fixator types. The changes of stiffness of cortical and callus tissues were measured with respect to the distance to the periosteal cortex-callus boundary. Stiffness of cortex and callus tissue smoothly decreased as a function of the distance from the inner cortical region. The data obtained in this study can help to understand the processes involved in tissue maturation during endogenous bone healing.

Role of local insulin augmentation upon allograft incorporation in a rat femoral defect model

Each year, over one million orthopedic operations are performed which a bony defect is presence, requiring the use of further augmentation in addition to bony fixation. Application of autogenous bone graft is the standard of care to promote healing of these defects, but several determents exist in using autogenous bone graft exist including limited supply and donor site morbidity. Prior work has demonstrated that local insulin application to fracture sites promote fracture healing, but no work has been performed to date in its effects upon defect healing/allograft incorporation. The goal of this study was to examine the potential role of local insulin application upon allograft incorporation. Microradiographic, histologic, and histomorphometric analysis outcome parameters showed that local insulin significantly accelerated new bone formation. Histological comparisons using predetermined scoring systems demonstrated significantly greater healing in femora treated with insulin compared to control femora (p < 0.001). Quantitatively more bone production was also observed, specifically in areas of endosteal (p = 0.010) and defect (p = 0.041) bone in femora treated with local insulin, compared to control femora, 6 weeks after implantation. This study demonstrates the potential of local insulin as an adjunct for the treatment of segmental defect and allograft incorporation.

Measurement of bone electrical impedance in fracture healing

BACKGROUND

Although external fixation is widely used for fractures and limb lengthening, evaluation of the time for removing the external fixator is dependent upon radiographic examinations and clinical findings, and a useful method has yet to be established clinically. This study aimed to measure the bone electrical impedance (Z values) non-invasively by using external fixation pins as electrodes, and clarify the relationship with bone union.

METHODS

Thirty rabbits received the external fixation at the right tibia and were assigned to a control group (group C; n = 5) and a fractured group (group F; n = 25). Z values were measured once a week following surgery. The animals of group F were assigned to 5 groups (weeks 2, 3, 4, 5, and 6 after osteotomy, each n = 5). The resistivity (rho) of the electrical property between electrodes was measured prior to euthanasia, and fracture cross-sectional area (FrA) of the conduction pathway and maximum bending stress (Bmax) were measured following excision of the tibia.

RESULTS

Although Z values in group F increased through 5 weeks after surgery, Z values in group C remained constant at 3 weeks, and significant differences were observed between groups at 4, 5, and 6 weeks. The rho values and FrA in group F decreased through 5 weeks; while Bmax increased, reaching a plateau at 5 weeks.

CONCLUSIONS

Narrowing of conduction pathway due to the decrease in the contour of fracture area accompanying bone remodeling resulted in an increase of Z values. Both Z values and Bmax in group F reached a peak at 5 weeks, this was believed to be the optimal time for removal of external fixation. These results suggest that measurement of Z values makes it possible to evaluate bone union.

Quantitative scanning acoustic microscopy compared to microradiography for assessment of new bone formation

Recently, it has been shown that quantitative scanning acoustic microscopy (SAM) is a powerful tool to image the acoustic impedance of even inhomogeneous materials like bone. Therefore, the aim of our study was to compare SAM to conventional microradiography with respect to histomorphometrical assessment of undecalcified sections of newly formed bone. Forty specimens were harvested 12 weeks after implantation of either autogenous cancellous bone graft or 5.0 mg of Osteogenic Protein-1 (BMP-7) in a critical-sized defect model in sheep. Undecalcified transverse bone sections of 500 microm thickness were investigated with conventional microradiography and SAM. Linear regression analysis was carried out to compare the measurements of the area of new bone formation within the defect sites. Both methods allowed for good discrimination between newly formed bone and cortical bone at the edges of the former defect. Images obtained with SAM revealed a better resolution and sharpness compared to that of microradiographs since SAM imaging unlike microradiography does not depend on the thickness of bone sections. The results of quantitative histomorphometric analysis obtained by both methods showed no significant differences, and it was possible to predict 90% of the variability of each method (coefficient of determination r2 = 0.90; P < 0.0001). In conclusion, SAM offers comparable quantitative histomorphometric information with a better spatial resolution than conventional microradiography. Thus, SAM is a promising new micro-visualizing technique for basic bone research.

Callus mineralization following distraction osteogenesis of the mandible monitored by scanning acoustic microscopy (SAM)

BACKGROUND

Scanning acoustic microscopy uses ultrasound to analyse histomorphology of tissues with microscopic resolution and delivers data about physical properties of the specimen.

MATERIAL AND METHODS

Bony consolidation was monitored by scanning acoustic microscopy in 12 embedded specimens of dog mandibles after distraction osteogenesis. Increasing mineralization was detected by measurements of acoustic impedance (Z).

RESULTS

There was a strong correlation between acoustic impedance and time of consolidation. Measurements of the speed of sound (v) provided specific information about non-mineralized zones of the distracted area. Distribution of density in the distracted area could be reconstructed by using the measurements of acoustic impedance and speed of sound.

CONCLUSION

The method seems suitable for studying bone remodelling qualitatively and quantitatively.

The effects of blood glucose control upon fracture healing in the BB Wistar rat with diabetes mellitus

Several clinical series, analyzing fracture healing in patients with diabetes mellitus (DM). demonstrated significant incidence of delayed union, non-union, and pseudarthrosis. In this study, analysis was performed to evaluate the effects of blood glucose (BG) control on fracture healing in the DM BB Wistar rat, a rat strain that represents a close homology to Type I DM in man. Our study showed decreased cell proliferation at the fracture site as well as decreased mechanical stiffness and bony content in the poorly controlled DM rats. To determine the effect of BG control, DM rats were treated with insulin sufficient to maintain physiologic BG levels throughout the course of the study. Values of cellular proliferation, biomechanical properties and callus bone content in tightly controlled DM animals were not significantly different from values of non-DM control values. This study suggests that insulin treatment with resultant improved BG control will ameliorate the impaired early and late parameters of DM fracture healing.

Repair of bone allograft fracture using bone morphogenetic protein-2

Long-term clinical data have shown that reconstruction using bone allografts provide adequate function after extensive tumor surgery. Complications such as nonunion of allograft-host interface, infection, and allograft fracture often require major revision surgeries. Allograft fractures usually do not induce the same repair process that is seen in normal fracture healing. The authors did an experimental study to test whether bone morphogenetic protein-2 can induce and achieve osseous repair in an allograft osteotomy model. Recombinant human bone morphogenetic protein-2 was applied at femoral intercalary allograft osteotomy sites in 20 rats. Forty additional rats served as controls (carrier alone and sham). Specimens in all groups were examined histologically and radiographically at 4 and 8 weeks. Specimens in the control groups showed only fibrosis by 8 weeks. In contrast, none of 10 specimens in the experimental group showed radiographic union at 8 weeks. New bone formation and integration with underlying allografts were seen in the experimental group as early as 4 weeks. These data suggest that fracture repair in the allograft bone can be triggered by a biologic regulator that is expressed during normal fracture healing.

Demineralized bone matrix as a biological scaffold for bone repair

Experimental models were created in rat fibula to represent impaired bone healing so that biological deficiencies that cause bone repair to fail or to be delayed may be investigated. These models consist of a 4-mm-long segmental defect, created in rat fibula by osteotomy, and fitted with a 7-mm-long tubular specimen of demineralized bone matrix (DBM) over the cut ends of the fibula. The experiments in this study involved various modifications of the DBM scaffold designed to reduce its osteoinductive activity: steam sterilization (sDBM), ethylene oxide sterilization (eoDBM), trypsin digestion (tDBM), and guanidine hydrochloride extraction (gDBM). Bone healing was evaluated by bending rigidity of the fibula and mineral content of the repair site at 7 weeks post-surgery. The sDBM scaffolds resorbed completely by 7 weeks and hence this model was a nonhealing negative control. Rigidities in the unmodified DBM and tDBM groups were comparable, whereas in the gDBM and eoDBM groups it was significantly reduced. Histologically, in the 4-mm defects repaired with unmodified DBM, direct and endochondral bone formation in the scaffold and the defect resulted in a neocortex consisting of woven and lamellar bone uniting the broken bone by 7 weeks post-surgery. We conclude that the eoDBM and gDBM groups represent failure or delay of the bone repair process when compared with the unmodified DBM group in which the process is analogous to normal bone healing.

Repair of segmental bone defects in the rat: an experimental model of human fracture healing

Bone repair models in animals may be considered relevant to human fracture healing to the extent that the sequence of events in the repair process in the model reflect the human fracture healing sequence. In the present study, the relevance of a recently developed segmental defect model in rat fibula to human fracture healing was investigated by evaluating temporal progression of rigidity of the fibula, mineral content of the repair site, and histological changes. In this model, a surgically created 2-mm-long defect was grafted with a 5-mm-long tubular specimen of demineralized bone matrix (DBM) by inserting it over the cut ends of the fibula. The temporal increase in rigidity of the healing fibula demonstrated a pattern similar to biomechanical healing curves measured in human fracture healing. This pattern was characterized by a short phase of rapidly rising rigidity during weeks 4-7 after surgery, associated with a sharp increase in the mineral content of the repair tissue. This was preceded by a phase of nearly zero rigidity and followed by a phase of slow rate of increase approaching a plateau. Histologically, chondroblastic and osteoblastic blastema originating from extraskeletal and subperiosteal (near fibula-graft junction) regions, infiltrated the DBM graft during the first 2 weeks. The DBM graft assumed the role of a "bridging callus." By weeks 6-8, most of the DBM was converted to new woven and trabecular bone with maximal osteoblastic activity and minimal endochondral ossification. Medullary callus formation started with direct new bone formation adjacent to the cortical and endosteal surfaces in the defect and undifferentiated cells in the center of the defect at 3 weeks. The usual bone repair process in rodents was altered by the presence of the DBM graft to recapitulate the sequential stages of human fracture healing, including the formation of a medullary callus, union with woven and lamellar bone, and recreation of the medullary canal.