Cellular mechanisms of bone repair

The Effect of Low-Intensity Pulsed Ultrasound on Bone Regeneration and the Expression of Osterix and Cyclooxygenase-2 during Critical-Size Bone Defect Repair

Low-intensity pulsed ultrasound (LIPUS) is a form of ultrasound that utilizes low-intensity pulsed waves. Its effect on bones that heal by intramembranous ossification has not been sufficiently investigated. In this study, we examined LIPUS and the autologous bone, to determine their effect on the healing of the critical-size bone defect (CSBD) of the rat calvaria. The bone samples underwent histological, histomorphometric and immunohistochemical analyses. Both LIPUS and autologous bone promoted osteogenesis, leading to almost complete closure of the bone defect. On day 30, the bone volume was the highest in the autologous bone group (20.35%), followed by the LIPUS group (19.12%), and the lowest value was in the control group (5.11%). The autologous bone group exhibited the highest intensities of COX-2 (167.7 ± 1.1) and Osx (177.1 ± 0.9) expression on day 30. In the LIPUS group, the highest intensity of COX-2 expression was found on day 7 (169.7 ±1.6) and day 15 (92.7 ± 2.2), while the highest Osx expression was on day 7 (131.9 ± 0.9). In conclusion, this study suggests that LIPUS could represent a viable alternative to autologous bone grafts in repairing bone defects that are ossified by intramembranous ossification.

The Role of Mesenchymal Stem Cell Secretome (Extracellular Microvesicles and Exosomes) in Animals' Musculoskeletal and Neurologic-Related Disorders

The advances in regenerative medicine are very important for the development of medicine and the discovery of stem cells has shown a greater capacity to raise the level of therapeutic quality while their use becomes more accessible, especially in their mesenchymal form. In veterinary medicine, it is not different. The use of those cells, as well as recent advances related to the use of their extracellular vesicles, demonstrates a great opportunity to enhance therapeutic methods and ensure more life quality for patients, which can be in clinical or surgical treatments. Knowing the advances in these modalities and the growing clinical and surgery research and demands for innovations in orthopedic and neurology medicines, this paper aimed to review the literature about the methodologies of use and applications such as the pathways of action and the advances that were postulated for microvesicles and exosomes derived from mesenchymal stem cells in veterinary medicine, especially for musculoskeletal disorders and related injuries.

The effects of mechanical instability on PDGF mediated inflammatory response at early stage of fracture healing under diabetic condition

BACKGROUND AND OBJECTIVE

Mechanical stability plays an important role in fracture healing process. Excessive interfragmentary movement will continuously damage the tissue and newly formed capillaries at the fracture site, which leads to overproduction of platelet-derived growth factor (PDGF) that attracts more macrophages into fracture callus, ultimately persistent and enhanced inflammatory response happens. For diabetic condition, the impact of mechanical instability of fracture site on inflammatory response could be further compliciated and the relevant research in this field is relatively limited.

METHODS

Building on previous experimental studies, this study presents a numerical model consisting of a system of reactive-transport equations representing the transport as well as interactions of different cells and cytokines within the fracture callus. The model is initially validated by available experimental data, and then implemented to investigate the role of mechanical stability of fracture site in inflammatory response during early stage of healing. It is assumed that there is an increased release of PDGF due to the rupture of blood vessels resulting from mechanical instability, which leads to increased production of inflammatory cytokines (i.e., TNF-α). The bone healing process under three different conditions were investigated, i.e., mechanically stable condition with normal inflammatory response (Control, Case 1), mechanically unstable condition with normal inflammatory response (Case 2) and mechanically unstable condition with diabetes (Case 3).

RESULTS

Mechanical instability can promote the macrophage infiltration and thus induce an enhanced and prolonged inflammatory response, which could impede the MSCs proliferation during the early fracture healing stage (e.g., compared with the control condition, the MSCs concentration in unstable fracture with normal inflammatory response can be reduced by 3.2% and 5.2% on day 2 and day 10 post-fracture, respectively). Under diabetic condition, the mechanical instability of fracture site could lead to a significant increase of TNF-α concentration in fracture callus (Case 3) in comparison to control (Case 1) (e.g., three-fold increase in TNF-α concentration compared to control). In addition, the results show that the mechanical instability affects the cell differentiation and proliferation in fracture callus in a spatially dependent manner, e.g., for diabetic fracture patients, the mechanical instability could potentially decrease the concentration of MSCs, osteoblasts and chondrocytes by around 39%, 30% and 29% in cortical callus, respectively, in comparison to control.

CONCLUSION

The mechanical instability together with diabetic condition can significantly affect the natural resolution of inflammation during early stage of healing by turning acute inflammation into chronic inflammation which is characterized by a continuously upregulated TNF-α pathway.

Immunohistochemical comparison of lateral bone augmentation using a synthetic TiO2 block or a xenogeneic graft in chronic alveolar defects

OBJECTIVES

To evaluate osteogenic markers and alveolar ridge profile changes in guided bone regeneration (GBR) of chronic noncontained bone defects using a nonresorbable TiO₂ block.

MATERIALS AND METHODS

Three buccal bone defects were created in each hemimandible of eight beagle dogs and allowed to heal for 8 weeks before GBR. Treatment was assigned by block randomization: TiO₂ block: TiO₂ -scaffold and a collagen membrane, DBBM particulates: Deproteinized bovine bone mineral (DBBM) and a collagen membrane, Empty control: Only collagen membrane. Bone regeneration was assessed on two different healing timepoints: early (4 weeks) and late healing (12 weeks) using several immunohistochemistry markers including alpha-smooth muscle actin (α-SMA), osteopontin, osteocalcin, tartrate-resistant acid phosphatase, and collagen type I. Histomorphometry was performed on Movat Pentachrome-stained and Von Kossa/Van Gieson-stained sections. Stereolithographic (STL) models were used to compare alveolar profile changes.

RESULTS

The percentage of α-SMA and osteopontin increased in TiO₂ group after 12 weeks of healing at the bone-scaffold interface, while collagen type I increased in the empty control group. In the defect area, α-SMA decreased in the empty control group, while collagen type I increased in the DBBM group. All groups maintained alveolar profile from 4 to 12 weeks, but TiO₂ group demonstrated the widest soft tissue contour profile.

CONCLUSIONS

The present findings suggested contact osteogenesis when GBR is performed with a TiO₂ block or DBBM particulates. The increase in osteopontin indicated a potential for bone formation beyond 12 weeks. The alveolar profile data indicated a sustained lateral increase in lateral bone augmentation using a TiO₂ block and a collagen membrane, as compared with DBBM and a collagen membrane or a collagen membrane alone.

Investigation of the process intergrowth of bone tissue into the hole in titanium implants (Experimental research)

INTRODUCTION

Despite the use of modern implants, complications such as nonunion and avascular necrosis of the femoral head are observed in femoral neck fractures (FNF). We have created a new perforated I-beam implant for FNF osteosynthesis and developed a new osteosynthesis philosophy based not only on the mechanical and biomechanical interaction of the bone-implant system, but also on the interaction of the biological properties of the bone and the implant. The purpose of the work is to study the interaction of the biological process of the bone - its regeneration (germination) of bone tissue into the holes of the implant.

MATERIALS AND METHODS

The experiment was carried out on fourteen Chinchilla rabbits in accordance with all international standards. A perforated implant specially made of titanium (ChM, Poland) was surgically implanted into the proximal femur. The implant measurements were as follows: length - 6 mm, width - 3 mm, thickness - 2 mm, 2 holes with a diameter of 2 mm. The 14 rabbits were divided into 7 groups. After 1, 2, 3, 4, 5, 10 and 12 weeks the animals were withdrawn from the experiment according to the standard rules in sequential order. The preparations were placed in a formalin solution and sent to the pathomorphology laboratory (CITO, Russia) for histological studies.

RESULTS

Weekly histopathological studies revealed a gradual transition from the organization of a hematoma to the formation of mature bone tissue in the holes of the implants. The titanium implant is bioinert and did not cause any visible reactions from the bone tissue. Simultaneous integration of vascular proliferation and newly formed bone tissue into the implant holes were revealed. On 10-12-week preparations, the formation of trabecular structures of mature bone tissue was revealed in the holes of the implants and elements of adipose and bone marrow tissue were observed. Macroscopic examination of 4-5-week preparations showed almost complete filling of the holes with bone tissue. On 10-12-week preparations, the bone tissue in the holes of the implants did not differ from the bone tissue surrounding the implant. The processes of formation of mature bone tissue in the holes of the implants were similar to the processes of physiological bone healing (regeneration) at the fracture site.

CONCLUSIONS

The obtained results show the following: 1.The titanium implant is bioinert and does not cause any visible reactions from the bone tissue; 2. There is a gradual process of formation of new vessels, and then the formation of new bone tissue in the holes of the implant instead of the one damaged during implantation. Thus, the results of this experiment indirectly confirm our assumption that a perforated implant for FNF osteosynthesis will participate not only in the mechanical and biomechanical interaction of the bone-implant system, but will also include the 3rd element in this system - the biological properties of the bone itself. We assume that these properties of the new implant will increase blood flow in the femoral neck and partially replenish the volume of bone tissue destroyed during osteosynthesis which does not occur with FNF osteosynthesis by any of the known implants.

Local dual delivery therapeutic strategies: Using biomaterials for advanced bone tissue regeneration

Bone development is a complex process involving a vast number of growth factors and chemical substances. These factors include transforming growth factor-beta, platelet-derived growth factor, insulin-like growth factor, and most importantly, the bone morphogenetic protein, which exhibits excellent therapeutic value in bone repair. However, the spatial-temporal relationship in the expression of these factors during bone formation makes the bone repair a more complicated process to address. Thus, using a single therapeutic agent to address bone formation does not seem to provide a clinically effective option. Conversely, a dual delivery approach facilitating the co-delivery of agents has proved to be a dynamic alternative since such a strategy can provide more efficient spatial-temporal action. Such delivery systems can smartly target more than one pathway or differentiation lineage and thus offer more efficient bone regeneration. This review discusses various dual delivery strategies reported in the literature employed to achieve improved bone regeneration. These include concurrent use of different therapeutic agents (including growth factors and drugs), enhancing bone formation and cell recruitment, and improving the efficiency of bone healing.

CSBD Healing in Rats after Application of Bovine Xenogeneic Biomaterial Enriched with Magnesium Alloy

Xenogeneic biomaterials Cerbone^® and OsteoBiol^® are widely used in oral implantology. In dental practice, xenogeneic biomaterial is usually combined with autologous bone to provide bone volume stability needed for long-term dental implants. Magnesium alloy implants dissolve and form mineral corrosion layer that is directly in contact with bone tissue, allowing deposition of the newly formed bone. CSBD heals by intramembranous ossification and therefore is a convenient model for analyses of ostoconductive and osteoinductive properties of different type of biomaterials. Magnesium alloy-enriched biomaterials have not yet been applied in oral implantology. Therefore, the aim of the current study was to investigate biological properties of potentially new bovine xenogeneic biomaterial enriched with magnesium alloy in a 5 mm CSBD model. Osteoconductive properties of Cerabone^®, Cerabone^® + Al. bone, and OsteoBiol^® were also analyzed. Dynamics of bone healing was followed up on the days 3, 7, 15, 21, and 30. Calvary bone samples were analyzed by micro-CT, and values of the bone morphometric parameters were assessed. Bone samples were further processed for histological and immunohistochemical analyses. Histological observation revealed CSBD closure at day 30 of the given xenogeneic biomaterial groups, with the exception of the control group. TNF-α showed high intensity of expression at the sites of MSC clusters that underwent ossification. Osx was expressed in pre-osteoblasts, which were differentiated into mature osteoblasts and osteocytes. Results of the micro-CT analyses showed linear increase in bone volume of all xenogeneic biomaterial groups and also in the control. The highest average values of bone volume were found for the Cerabone^® + Mg group. In addition, less residual biomaterial was estimated in the Cerabone^® + Mg group than in the Cerabone^® group, indicating its better biodegradation during CSBD healing. Overall, the magnesium alloy xenogeneic biomaterial demonstrated key properties of osteoinduction and biodegradidibility during CSBD healing, which is the reason why it should be recommended for application in clinical practice of oral implantology.

Levels of Biological Markers of Nitric Oxide in Serum of Patients with Mandible Fractures

Background: Nitric oxide is a small gaseous molecule with significant bioactivity. It has been observed that NO may have a dual role dependent on its production and concentrations in the bone microenvironment. The objective of the study was to assess the concentration of total nitric oxide malonyldialdehyde, nitrotyrosine, and asymmetric dimethylarginine in the serum of patients with mandibular fractures and to understand the relationship between these compounds, in order to expand the knowledge base of the role of nitric oxide and its activity indicators in the process of bone fracture healing. Material and Methods: The study included 20 patients with mandibular fractures who were undergoing inpatient and outpatient treatments and a control group of 15 healthy people. Results were analyzed with respect to the measurement time. Total nitric oxide concentration in the blood serum was determined according to the Griess reaction, while the concentration of malonyldialdehyde, nitrotyrosine, and asymmetric dimethylarginine was estimated using the immunoenzymatic method (i.e., enzyme-linked immunosorbent assay). Results: Before the procedure, as well as on the first day and 2 and 6 weeks after the procedure, higher concentrations of total nitric oxide and lower concentrations of malonyldialdehyde were observed in the blood serum of patients with mandibular fractures compared to the control group. No statistically significant differences were found in nitrotyrosine concentrations in the blood serum of patients throughout the measurement period. However, a significantly higher asymmetric dimethylarginine concentration was observed in the patient serum before the procedure and on the first day of operation as compared with the control group. Analysis of the results observed in patient serum with respect to the number of fractures within the mandible demonstrated the same trend of concentrations for the tested compounds for the entire study group. Conclusions: In summary, our results revealed that the intensity of local processes resulting from mandibular fractures is associated with the concentration of nitric oxide, confirming its significant role, as well as that of its indicators, in the process of bone fracture healing in this patient population.

Recent Developments in Polyurethane-Based Materials for Bone Tissue Engineering

To meet the needs of clinical medicine, bone tissue engineering is developing dynamically. Scaffolds for bone healing might be used as solid, preformed scaffolding materials, or through the injection of a solidifiable precursor into the defective tissue. There are miscellaneous biomaterials used to stimulate bone repair including ceramics, metals, naturally derived polymers, synthetic polymers, and other biocompatible substances. Combining ceramics and metals or polymers holds promise for future cures as the materials complement each other. Further research must explain the limitations of the size of the defects of each scaffold, and additionally, check the possibility of regeneration after implantation and resistance to disease. Before tissue engineering, a lot of bone defects were treated with autogenous bone grafts. Biodegradable polymers are widely applied as porous scaffolds in bone tissue engineering. The most valuable features of biodegradable polyurethanes are good biocompatibility, bioactivity, bioconductivity, and injectability. They may also be used as temporary extracellular matrix (ECM) in bone tissue healing and regeneration. Herein, the current state concerning polyurethanes in bone tissue engineering are discussed and introduced, as well as future trends.

Early osteoimmunomodulatory effects of magnesium-calcium-zinc alloys

Today, substantial attention is given to biomaterial strategies for bone regeneration, and among them, there is a growing interest in using immunomodulatory biomaterials. The ability of a biomaterial to induce neo vascularization and macrophage polarization is a major factor in defining its success. Magnesium (Mg)-based degradable alloys have attracted significant attention for bone regeneration owing to their biodegradability and potential for avoiding secondary removal surgeries. However, there is insufficient evidence in the literature regarding the early inflammatory responses to these alloys in vivo. In this study, we investigated the early body responses to Mg-0.45wt%Zn-0.45wt%Ca pin-shaped alloy (known as ZX00 alloy) in rat femora 2, 5, and 10 days after implantation. We used 3D micro computed tomography (µCT), histological, immunohistochemical, histomorphometrical, and small angle X-ray scattering (SAXS) analyses to study new bone formation, early macrophage polarization, neo vascularization, and bone quality at the implant bone interface. The expression of macrophage type 2 biological markers increased significantly after 10 days of Mg alloy implantation, indicating its potential in stimulating macrophage polarization. Our biomineralization results using µCT as well as histological stained sections did not indicate any statistically significant differences between different time points for both groups. The activity of alkaline phosphatase (ALP) and Runt-related transcription factor 2 (Runx 2) biological markers decreased significantly for Mg group, indicating less osteoblast activity. Generally, our results supported the potential of ZX00 alloy to enhance the expression of macrophage polarization in vivo; however, we could not observe any statistically significant changes regarding biomineralization.

Bone and Cartilage Interfaces With Orthopedic Implants: A Literature Review

The interface between a surgical implant and tissue consists of a complex and dynamic environment characterized by mechanical and biological interactions between the implant and surrounding tissue. The implantation process leads to injury which needs to heal over time and the rapidity of this process as well as the property of restored tissue impact directly the strength of the interface. Bleeding is the first and most relevant step of the healing process because blood provides growth factors and cellular material necessary for tissue repair. Integration of the implants placed in poorly vascularized tissue such as articular cartilage is, therefore, more challenging than compared with the implants placed in well-vascularized tissues such as bone. Bleeding is followed by the establishment of a provisional matrix that is gradually transformed into the native tissue. The ultimate goal of implantation is to obtain a complete integration between the implant and tissue resulting in long-term stability. The stability of the implant has been defined as primary (mechanical) and secondary (biological integration) stability. Successful integration of an implant within the tissue depends on both stabilities and is vital for short- and long-term surgical outcomes. Advances in research aim to improve implant integration resulting in enhanced implant and tissue interface. Numerous methods have been employed to improve the process of modifying both stability types. This review provides a comprehensive discussion of current knowledge regarding implant-tissue interfaces within bone and cartilage as well as novel approaches to strengthen the implant-tissue interface. Furthermore, it gives an insight into the current state-of-art biomechanical testing of the stability of the implants. Current knowledge reveals that the design of the implants closely mimicking the native structure is more likely to become well integrated. The literature provides however several other techniques such as coating with a bioactive compound that will stimulate the integration and successful outcome for the patient.

Radiologic results of additional single screw fixation with lateral locking plate after hybrid closed-wedge high tibial osteotomy

Background

Hybrid closed-wedge high tibial osteotomy (hybrid CWHTO) is an effective surgical treatment for medial compartment osteoarthritis of the knee. Our study investigated whether the combination of a lateral locking plate and a single medial screw promoted bone union after hybrid CWHTO.

Methods

The study cohort consisted of 30 patients (15 men and 15 women) who underwent hybrid CWHTO for medial compartment osteoarthritis or spontaneous osteonecrosis of the knee. Sixteen knees were fixed with a lateral locking plate (LP group), and 17 were fixed with both a lateral locking plate and a cannulated cancellous screw on the medial side of the tibia (LPS group). The times to bone union, radiolucency, and callus formation at the osteotomy site were evaluated radiographically.

Results

The mean postoperative time to radiographic confirmation of bone union was 5.5 ± 2.6 months in the LP group and 3.4 ± 1.5 months in the LPS group. Radiolucency at the osteotomy site and excess callus formation on the posterior side of the tibia were lower in the LPS group than in the LP group.

Conclusions

This modified hybrid CWHTO combining a lateral locking plate and a cannulated cancellous screw on the medial side of the tibia improves the stability of the osteotomy site and shortens the period of bone union.

[Revision surgery after failed (partial-) arthrodesis of the wrist]

(Partial) arthrodeses of the wrist have been proven cornerstones to treat many lesions for decades, especially in the case of revision surgery. Four-corner, scapho-trapezo-trapezoidal (STT), radio-scapho-lunate (RSL) and total wrist fusions are very common techniques in hand surgery. However, even these proven surgical procedures have significant non-fusion rates. Prior to revising a failed arthrodesis, it is essential to analyse the latter failure precisely. A technically adequate revision is only feasible when based on a correct and meticulous analysis. The understanding of the biological processes and technical aspects of the implants are the basis for solving this issue.

Osteogenesis of 3D printed macro-pore size biphasic calcium phosphate scaffold in rabbit calvaria

To investigate the osteogenesis of macro-pore sized bone scaffolds, biphasic calcium phosphate scaffolds with accurately controlled macro-pore size (0.8, 1.2, and 1.6 mm) and identical porosity of 70% were fabricated by the 3D printing technology. Eight New Zealand rabbits were selected in the present study, while four 8-mm-diameter calvarial defects were created in each rabbit to place BCP scaffolds with different macro-pore size. The harvested specimens of four and eight weeks were used to evaluate the bone forming ability by micro CT and histological examination. All 3D-printed BCP scaffolds exhibited excellent mechanical properties and had better bone-forming ability than the control at both four and eight weeks. Among them, scaffold with 0.8 mm pore size was superior for initial bone formation and maturation, resulting in the highest value of total bone formation.

New functions for the proprioceptive system in skeletal biology

Muscle spindles and Golgi tendon organs (GTOs) are two types of sensory receptors that respond to changes in length or tension of skeletal muscles. These mechanosensors have long been known to participate in both proprioception and stretch reflex. Here, we present recent findings implicating these organs in maintenance of spine alignment as well as in realignment of fractured bones. These discoveries have been made in several mouse lines lacking functional mechanosensors in part or completely. In both studies, the absence of functional spindles and GTOs produced a more severe phenotype than that of spindles alone. Interestingly, the spinal curve phenotype, which appeared during peripubertal development, bears resemblance to the human condition adolescent idiopathic scoliosis. This similarity may contribute to the study of the disease by offering both an animal model and a clue as to its aetiology. Moreover, it raises the possibility that impaired proprioceptive signalling may be involved in the aetiology of other conditions. Overall, these new findings expand considerably the scope of involvement of proprioception in musculoskeletal development and function.This article is part of the Theo Murphy meeting issue 'Mechanics of development'.

The Proprioceptive System Regulates Morphologic Restoration of Fractured Bones

Successful fracture repair requires restoration of bone morphology and mechanical integrity. Recent evidence shows that fractured bones of neonatal mice undergo spontaneous realignment, dubbed "natural reduction." Here, we show that natural reduction is regulated by the proprioceptive system and improves with age. Comparison among mice of different ages revealed, surprisingly, that 3-month-old mice exhibited more rapid and effective natural reduction than newborns. Fractured bones of null mutants for transcription factor Runx3, lacking functional proprioceptors, failed to realign properly. Blocking Runx3 expression in the peripheral nervous system, but not in limb mesenchyme, recapitulated the null phenotype, as did inactivation of muscles flanking the fracture site. Egr3 knockout mice, which lack muscle spindles but not Golgi tendon organs, displayed a less severe phenotype, suggesting that both receptor types, as well as muscle contraction, are required for this regulatory mechanism. These findings uncover a physiological role for proprioception in non-autonomous regulation of skeletal integrity.

Stimulating Fracture Healing in Ischemic Environments: Does Oxygen Direct Stem Cell Fate during Fracture Healing?

Bone fractures represent an enormous societal and economic burden as one of the most prevalent causes of disability worldwide. Each year, nearly 15 million people are affected by fractures in the United States alone. Data indicate that the blood supply is critical for fracture healing; as data indicate that concomitant bone and vascular injury are major risk factors for non-union. However, the various role(s) that the vasculature plays remains speculative. Fracture stabilization dictates stem cell fate choices during repair. In stabilized fractures stem cells differentiate directly into osteoblasts and heal the injury by intramembranous ossification. In contrast, in non-stable fractures stem cells differentiate into chondrocytes and the bone heals through endochondral ossification, where a cartilage template transforms into bone as the chondrocytes transform into osteoblasts. One suggested role of the vasculature has been to participate in the stem cell fate decisions due to delivery of oxygen. In stable fractures, the blood vessels are thought to remain intact and promote osteogenesis, while in non-stable fractures, continual disruption of the vasculature creates hypoxia that favors formation of cartilage, which is avascular. However, recent data suggests that non-stable fractures are more vascularized than stable fractures, that oxygen does not appear associated with differentiation of stem cells into chondrocytes and osteoblasts, that cartilage is not hypoxic, and that oxygen, not sustained hypoxia, is required for angiogenesis. These unexpected results, which contrast other published studies, are indicative of the need to better understand the complex, spatio-temporal regulation of vascularization and oxygenation in fracture healing. This work has also revealed that oxygen, along with the promotion of angiogenesis, may be novel adjuvants that can stimulate healing in select patient populations.

Osseointegration improves bone-implant interface of pedicle screws in the growing spine: a biomechanical and histological study using an in vivo immature porcine model

PURPOSE

Implant failure is a frequent complication in corrective surgery for early onset scoliosis, since considerable forces are acting on small and fragile vertebrae. Osseointegration showing biomechanical and histological improvement in bone-implant interface (BII) after dental implant placement has been well investigated. However, there are no studies regarding osseointegration in immature vertebral bone. The purpose was to evaluate the timecourse of biomechanical and histological changes at BII after pedicle screw placement using in vivo immature porcine model.

METHODS

Ten immature porcine were instrumented with titanium pedicle screws in the thoracic spine. After a 0-, 2-, 4-, and 6-month survival periods, the spines were harvested at the age of 12 months. Histological evaluation of BII was conducted by bone volume/tissue volume (BV/TV) and bone surface/implant surface (BS/IS) measurements. Bone mineral density (BMD) measurement and biomechanical testing of BII were done.

RESULTS

Contact surface and bone volume around the screw threads were significantly increased over the time. BV/TV and BS/IS were improved with statistically significant differences between 0- and ≥4-month (p ≤ 0.001) periods. BMD in all subjects was determined to be the same (p ≥ 0.350). Pullout strength was also increased over time with significant differences between 0- and ≥2-month (p ≤ 0.011) periods.

CONCLUSION

Improved stability at BII caused by osseointegration was confirmed by in vivo immature porcine model. A two-stage operation is proposed based on the osseointegration theory, in which an implant is installed in advance in the vertebrae at the first stage and deformity correction surgery is performed after sufficient stability is obtained by osseointegration at a later stage.

TiO2 nanotube platforms for smart drug delivery: a review

Titania nanotube (TNT) arrays are recognized as promising materials for localized drug delivery implants because of their excellent properties and facile preparation process. This review highlights the concept of localized drug delivery systems based on TNTs, considering their outstanding biocompatibility in a series of ex vivo and in vivo studies. Considering the safety of TNT implants in the host body, studies of the biocompatibility present significant importance for the clinical application of TNT implants. Toward smart TNT platforms for sustainable drug delivery, several advanced approaches were presented in this review, including controlled release triggered by temperature, light, radiofrequency magnetism, and ultrasonic stimulation. Moreover, TNT implants used in medical therapy have been demonstrated by various examples including dentistry, orthopedic implants, cardiovascular stents, and so on. Finally, a future perspective of TNTs for clinical applications is provided.

NaOCl-mediated biofunctionalization enhances bone-titanium integration

The aim of this study was to examine the effect of NaOCl pretreatment on the biomechanical fixation of implant at the early healing stage of a rat model. Polished titanium cylindrical implants and disks were prepared, and one-half of these samples were dual acidetched. Then, one-half of both surfaces were chemically-cleaned by pretreatment with 5% NaOCl solution for 24 h. Morphological analyses showed that there was no significant difference between before and after NaOCl treatment. The wettability measurement demonstrated that NaOCl treatment secondarily converted both titanium surfaces from hydrophobic to superhydrophilic, accompanied by the removal of hydrocarbons from the titanium surfaces. Biomechanical push-in test indicated that the bone-titanium integration strength of the NaOCl-treated implants were significantly greater than that of the untreated implants (p<0.05). These results showed that NaOCl pretreatment enhanced the osseointegration capability of titanium, indicating its potential for a simple chemical chair-side pretreatment method.

A distinct regulatory region of the Bmp5 locus activates gene expression following adult bone fracture or soft tissue injury

Bone morphogenetic proteins (BMPs) are key signaling molecules required for normal development of bones and other tissues. Previous studies have shown that null mutations in the mouse Bmp5 gene alter the size, shape and number of multiple bone and cartilage structures during development. Bmp5 mutations also delay healing of rib fractures in adult mutants, suggesting that the same signals used to pattern embryonic bone and cartilage are also reused during skeletal regeneration and repair. Despite intense interest in BMPs as agents for stimulating bone formation in clinical applications, little is known about the regulatory elements that control developmental or injury-induced BMP expression. To compare the DNA sequences that activate gene expression during embryonic bone formation and following acute injuries in adult animals, we assayed regions surrounding the Bmp5 gene for their ability to stimulate lacZ reporter gene expression in transgenic mice. Multiple genomic fragments, distributed across the Bmp5 locus, collectively coordinate expression in discrete anatomic domains during normal development, including in embryonic ribs. In contrast, a distinct regulatory region activated expression following rib fracture in adult animals. The same injury control region triggered gene expression in mesenchymal cells following tibia fracture, in migrating keratinocytes following dorsal skin wounding, and in regenerating epithelial cells following lung injury. The Bmp5 gene thus contains an "injury response" control region that is distinct from embryonic enhancers, and that is activated by multiple types of injury in adult animals.

Effect of nicotine on RANKL and OPG and bone mineral density

AIM

The signaling pathway OPG/RANK/RANKL is a key in maintaining the balance between the activity of osteoblasts and osteoclasts in order to prevent bone loss. In this study, our aim was to assess the effects of long-term nicotine exposure on plasma RANKL and OPG levels, tissue RANKL and OPG immunoreactivities, and bone mineral density (BMD) scores in rats.

MATERIALS AND METHODS

Thirty-six Swiss Albino rats weighing 70 ± 10 g were divided into three groups. While the controls (n = 12) were only given normal drinking water, for low-dose nicotine (LDN) group (n = 12) 0.4 mg/kg/day; for high-dose nicotine (HDN) group (n = 12), 6.0 mg/kg/day nicotine was added to drinking water for a year. At the end of 12th month, BMD scores were measured using an X-ray absorptiometry and bone turnover was assessed by measuring plasma RANKL and OPG levels and RANKL and OPG immunoreactivities in tail vertebrae of the rats.

RESULTS

There was no statistically significant difference in BMD scores of lumbar spine and femoral regions of the nicotine groups in comparison to controls. Plasma OPG levels were found to be significantly higher in HDN group, in comparison to the controls and LDN groups (p = .001) unlike plasma RANKL levels. Tissue RANKL and OPG immunoreactivities decreased significantly in the LDN and HDN groups (p < .001, p < .01, respectively).

CONCLUSIONS

The results of this study show that nicotine is not primarily responsible for the decrease in BMD frequently seen in smokers. Measuring plasma RANKL and OPG levels did not reflect tissue immunoreactivities.

Modulation of macrophage activity during fracture repair has differential effects in young adult and elderly mice

OBJECTIVES

Advanced age is a factor associated with altered fracture healing. Delays in healing may increase the incidence of complications in the elderly, who are less able to tolerate long periods of immobilization and activity restrictions. This study sought to determine whether fracture repair could be enhanced in elderly animals by: (1) inhibiting macrophage activation, (2) blocking the M-CSF receptor c-fms, and (3) inhibiting monocyte trafficking using CC chemokine receptor-2 (CCR2) knockout mice.

METHODS

Closed unstable tibial shaft fractures were produced in mice aged 4, 12, and 78 weeks. Mice were then fed a diet containing PLX3397 or a control diet from days 1-10 after injury. Fractures were similarly made in CCR2 mice aged 78 weeks. The fracture callus was collected during fracture healing and was assessed for its size and the presence of macrophages, both of which were evaluated using the Mann-Whitney U test.

RESULTS

PLX3397 treatment resulted in a decrease in the number of macrophages in the fracture callus at day 5. Calluses in juvenile mice trended toward being smaller compared with those in elderly mice (P = 0.08). There was also a trend toward larger callus size and increased bone formation in PLX3397-treated elderly animals when compared with those of the control animals (P = 0.12). Similar increases in bone formation (P = 0.013) and decreases in cartilage within the callus (P = 0.03) were seen at day 10 in CCR2 mice.

CONCLUSIONS

The inhibition of macrophages in elderly mice may lead to an acceleration of fracture healing. Altering macrophage activation after fracture may represent a therapeutic strategy for preventing delayed healing and nonunion in the elderly.

Drug-releasing implants: current progress, challenges and perspectives

This review presents the different types and concepts of drug-releasing implants using new nanomaterials and nanotechnology-based devices.

Perioperative glucocorticosteroid treatment delays early healing of a mandible wound by inhibiting osteogenic differentiation

AIM

The purpose of this study is to investigate the effects of dexamethasone on repair of a critical size defect of the mandible in male Sprague-Dawley rats.

MATERIALS AND METHODS

Fifty rats were divided into 2 groups: saline control and dexamethasone-treated groups. A 1 mm × 3 mm full-thickness bone defect was created at the inferior border of the mandible. Saline or dexamethasone was administered once a day for 5 days after postoperative palinesthesia. On days 1, 3, 6, 10 and 17, after cessation of drug administration, 5 samples from each group were analysed. The bone defect healing process was examined and analysed by stereology, radiology, histology and histochemical staining for total collagen, tartrate-resistant acid phosphatase staining for osteoclasts and immunohistochemical staining for the COX-2, RUNX2 and osteocalcin antigens.

RESULTS

The dexamethasone-treated rats exhibited significantly lower radiopacity properties compared to the control rats. Histological staining revealed that the osteogenic differentiation and maturation of a callus in the defect region was significantly delayed from day 1 to day 10 in the dexamethasone group after cessation of drug administration compared to the control group. Consistent with the histological data, the level of total collagen protein was significantly lower in the dexamethasone group than in the control group. However, there was no significant difference between the 2 groups at day 17. Immunohistochemical analysis of COX-2, RUNX2 and osteocalcin expression showed that, at day 1, COX-2 and RUNX2 expression in the dexamethasone group was significantly lower than in the control group. There was no significant difference in osteocalcin expression between the two groups at each time point. There was no significant difference in the number of osteoclasts between the two groups.

CONCLUSION

In a model of bone healing of a mandible defect, dexamethasone-treated rats exhibited impaired osteogenic differentiation and maturation due to the inhibition of COX-2, osteogenic gene, RUNX2 and collagen protein expression, which resulted in delayed bone repair. Although perioperative short-term therapy did not exhibit long-term effects on wound healing of the maxillofacial bone, the application of glucocorticoids should be cautiously considered in the clinic.

Site specific effects of zoledronic acid during tibial and mandibular fracture repair

Numerous factors can affect skeletal regeneration, including the extent of bone injury, mechanical loading, inflammation and exogenous molecules. Bisphosphonates are anticatabolic agents that have been widely used to treat a variety of metabolic bone diseases. Zoledronate (ZA), a nitrogen-containing bisphosphonate (N-BP), is the most potent bisphosphonate among the clinically approved bisphosphonates. Cases of bisphosphonate-induced osteonecrosis of the jaw have been reported in patients receiving long term N-BP treatment. Yet, osteonecrosis does not occur in long bones. The aim of this study was to compare the effects of zoledronate on long bone and cranial bone regeneration using a previously established model of non-stabilized tibial fractures and a new model of mandibular fracture repair. Contrary to tibial fractures, which heal mainly through endochondral ossification, mandibular fractures healed via endochondral and intramembranous ossification with a lesser degree of endochondral ossification compared to tibial fractures. In the tibia, ZA reduced callus and cartilage formation during the early stages of repair. In parallel, we found a delay in cartilage hypertrophy and a decrease in angiogenesis during the soft callus phase of repair. During later stages of repair, ZA delayed callus, cartilage and bone remodeling. In the mandible, ZA delayed callus, cartilage and bone remodeling in correlation with a decrease in osteoclast number during the soft and hard callus phases of repair. These results reveal a more profound impact of ZA on cartilage and bone remodeling in the mandible compared to the tibia. This may predispose mandible bone to adverse effects of ZA in disease conditions. These results also imply that therapeutic effects of ZA may need to be optimized using time and dose-specific treatments in cranial versus long bones.

Heterotopic ossification in complex orthopaedic combat wounds: quantification and characterization of osteogenic precursor cell activity in traumatized muscle

BACKGROUND

Heterotopic ossification frequently develops following high-energy blast injuries sustained in modern warfare. We hypothesized that differences in the population of progenitor cells present in a wound would correlate with the subsequent formation of heterotopic ossification.

METHODS

We obtained muscle biopsy specimens from military service members who had sustained high-energy wartime injuries and from patients undergoing harvest of a hamstring tendon autograft. Plastic-adherent cells were isolated in single-cell suspension and plated to assess the prevalence of colony-forming cells. Phenotypic characteristics were assessed with use of flow cytometry. Individual colony-forming units were counted after an incubation period of seven to ten days, and replicate cultures were incubated in lineage-specific induction media. Immunohistochemical staining was then performed to determine the percentage of colonies that had differentiated along an osteogenic lineage. Quantitative real-time reverse-transcription polymerase chain reaction was used to identify changes in osteogenic gene expression.

RESULTS

Injured patients had significantly higher numbers of muscle-derived connective-tissue progenitor cells per gram of tissue (p < 0.0001; 95% confidence interval [CI], 129,930 to 253,333), and those who developed heterotopic ossification had higher numbers of assayable osteogenic colonies (p < 0.016; 95% CI, 12,249 to 106,065). In the injured group, quantitative real-time reverse-transcription polymerase chain reaction performed on the in vitro expanded progeny of connective-tissue progenitors demonstrated upregulation of COL10A1, COL4A3, COMP, FGFR2, FLT1, IGF2, ITGAM, MMP9, PHEX, SCARB1, SOX9, and VEGFA in the patients with heterotopic ossification as compared with those without heterotopic ossification.

CONCLUSIONS

Our study suggests that the number of connective-tissue progenitor cells is increased in traumatized tissue. Furthermore, wounds in which heterotopic ossification eventually forms have a higher percentage of connective-tissue progenitor cells committed to osteogenic differentiation than do wounds in which heterotopic ossification does not form. The early identification of heterotopic ossification-precursor cells and target genes in severe wounds not only may be an effective prognostic tool with which to assess whether heterotopic ossification will develop in a wound, but may also guide the future development of individualized prophylactic measures.

A scaffold-free multicellular three-dimensional in vitro model of osteogenesis

In vitro models of osteogenesis are essential for investigating bone biology and the effects of pharmaceutical, chemical, and physical cues on bone formation. Osteogenesis takes place in a complex three-dimensional (3D) environment with cells from both mesenchymal and hematopoietic origins. Existing in vitro models of osteogenesis include two-dimensional (2D) single type cell monolayers and 3D cultures. However, an in vitro scaffold-free multicellular 3D model of osteogenesis is missing. We hypothesized that the self-inductive ossification capacity of bone marrow tissue can be harnessed in vitro and employed as a scaffold-free multicellular 3D model of osteogenesis. Therefore, rat bone marrow tissue was cultured for 28 days in three settings: 2D monolayer, 3D homogenized pellet, and 3D organotypic explant. The ossification potential of marrow in each condition was quantified by micro-computed tomography. The 3D organotypic marrow explant culture resulted in the greatest level of ossification with plate-like bone formations (up to 5 mm in diameter and 0.24 mm in thickness). To evaluate the mimicry of the organotypic marrow explants to newly forming native bone tissue, detailed compositional and morphological analyses were performed, including characterization of the ossified matrix by histochemistry, immunohistochemistry, Raman microspectroscopy, energy dispersive X-ray spectroscopy, backscattered electron microscopy, and micromechanical tests. The results indicated that the 3D organotypic marrow explant culture model mimics newly forming native bone tissue in terms of the characteristics studied. Therefore, this platform holds significant potential to be used as a model of osteogenesis, offering an alternative to in vitro monolayer cultures and in vivo animal models.

Influence of cyclic bending loading on in vivo skeletal tissue regeneration from periosteal origin

INTRODUCTION

Periosteum osteogenic and chondrogenic properties stimulate the proliferation then differentiation of mesenchymal precursor cells originating from its deeper layers and from neighboring host tissues. The local mechanical environment plays a role in regulating this differentiation of cells into lineages involved in the skeletal regeneration process.

HYPOTHESIS

The aim of this experimental animal study is to explore the influence of cyclic high amplitude bending-loading on skeletal tissue regeneration. The hypothesis is that this mechanical loading modality can orient the skeletogenesis process towards the development of anatomical and histological articular structures.

MATERIAL AND METHODS

A vascularised periosteal flap was transferred in close proximity to each knee joint line in 17 rabbits. On one side, the tibiofemoral joint space was bridged and loading occurred when the animal bent its knee during spontaneous locomotion. On the other side, the flap was placed 12 mm distal to the joint line producing no loading during bending. Tissue regeneration was chronologically analyzed on histologic samples taken from the 4th day to the 6th month.

RESULTS

The structure and mechanical behavior of regenerating tissue evolved over time. As a result of the cyclic bending-loading regimen, cartilage tissue was maintained in specific areas of the regenerating tissue. When loading was discontinued, final osteogenic and fibrogenic differentiation occurred in the neoformed cartilage. Fissures developed in the cartilage aggregates resulting in pseudo-gaps suggesting similar processes to embryonic articular development. Ongoing mesenchymal stem cells stimulation was identified in the host tissues contiguous to the periosteal transfer.

DISCUSSION

These results suggest that the pseudarthrosis concept should be reconsidered within the context of motion induced articular histogenesis.

[Vascularized periosteum and bone regeneration]

The osteogenic potential of periosteum is widely recognized. During development, it plays a prominent role in the radial growth of long bones. Similarly, it has a key role in the consolidation of fractures. The physiological function of periosteum in the healthy, mature skeleton remains relatively subtle; however, its detachment from the bone surface reactivates its potential for fibrogenic and osteochondrogenic regeneration. This discreet anatomical structure is actually a reservoir of mesenchymal progenitor cells capable of proliferating and differentiating, by reinitializing cellular and molecular cascades of embryogenesis in mesenchymal tissues. However, given the hitherto limited knowledge of the quantitative potential of periosteum and of the pathways regulating tissue differentiation during regeneration, human applications have remained anecdotal. The findings of several in vivo and in vitro experiments indicate that the maintenance of the periosteum's vascularization stimulates its quantitative potential. The structural organization of the regenerated material in vivo is governed by locoregional biological and mechanical regulatory mechanisms that serve to make it capable of performing its new functions. The increasing awareness of periosteum's potential is stimulating active research in the fields of cellular biology and tissue engineering. The demonstration of its regenerative potential in animals gives reason to believe that strips of vascularized periosteum could become part of the developing armamentarium of regenerative medicine.

Modelling the propagation of ultrasound in the joint space of a human knee

There is strong evidence to support the clinical use of low-intensity pulsed ultrasound (LIPUS) to augment fracture healing. A previous experimental study showed that ultrasound can propagate in the joint space of a single human cadaveric knee. A full experimental investigation of this propagation is not possible due to poor reproducibility, the scarcity of human cadaveric tissues and the practical difficulties in making ultrasound measurements in the knee. The aim of the present work is to investigate whether a computer simulation (Wave2000 Pro®; Cyberlogic Inc., New York, NY, USA) can give a good representation of the experimental model. The simulations provided a good agreement with the experimental data, giving some confidence in the application of this computer simulation method as a means of determining whether ultrasound can propagate through different anatomical regions where bone is present.

VEGF and BMP-6 enhance bone formation mediated by cloned mouse osteoprogenitor cells

New strategies such as combined utilization of growth factors may provide a better treatment for difficult fractures. We have demonstrated enhanced angiogenesis and osteogenesis through the actions of vascular endothelial growth factor (VEGF) and bone morphogenetic protein-6 (BMP-6) on the osteogenic differentiation of a cloned mouse osteoprogenitor cell in vitro and ectopic bone formation in vivo. Human VEGF and BMP-6 genes expressed together produced a significant increase in alkaline phosphatase activity, expression of the RunX2 and osteocalcin genes and mineralization. Microcomputed tomographic analysis of subcutaneous implants consisting of cells transfected with VEGF and BMP-6 cDNA and delivered on a 3D poly (lactic-co-glycolic acid) scaffold confirmed the additive effects between VEGF and BMP-6. Ectopic bone formation in the VEGF plus BMP-6 group was greatest compared to that in either VEGF or BMP-6 alone. This is the first study that demonstrates osteogenesis in vitro and in vivo through the additive effects of VEGF and BMP-6.

The control of fracture healing and its therapeutic targeting: improving upon nature

Fracture repair is a complex process involving timed cellular recruitment, gene expression, and synthesis of compounds that regenerate native tissue to restore the mechanical integrity, and thus function of injured bone. While the majority of fractures heal without complication, this takes time and a subset of patients ( approximately 10%) experience healing delays, extending their morbidity and treatment costs. Consequently, there is a need for efficacious therapeutics for the intervention of fracture healing. Recent studies into the molecular control of fracture repair and advances in the understanding of the skeleton as a whole have resulted in the identification of numerous novel targets and compounds for such intervention. These include traditional agents such bone morphogenetic proteins and other growth factors, but also relatively newer compounds such as parathyroid hormone and modulators of the Wnt signaling pathway. These agents, along with others, are discussed in the current article in terms of their investigative status and potential for clinical implementation. Hopefully, these agents, as well as others yet to be discovered, will demonstrate sufficient clinical utility for successful intervention of fracture healing. This may have significant implications for the duration of morbidity and costs associated with traumatic bone fractures.

Vascularization in bone tissue engineering: physiology, current strategies, major hurdles and future challenges

The lack of a functional vascular supply has, to a large extent, hampered the whole range of clinical applications of 'successful' laboratory-based bone tissue engineering strategies. To the present, grafts have been dependent on post-implant vascularization, which jeopardizes graft integration and often leads to its failure. For this reason, the development of strategies that could effectively induce the establishment of a microcirculation in the engineered constructs has become a major goal for the tissue engineering research community. This review addresses the role and importance of the development of a vascular network in bone tissue engineering and provides an overview of the most up to date research efforts to develop such a network.

Effects of photodynamic therapy on the structural integrity of vertebral bone

STUDY DESIGN

This study investigates the effects of photodynamic therapy (PDT) on the structural integrity of vertebral bone in healthy rats.

OBJECTIVE

To determine the short-term (1 week) and intermediate term (6 weeks) effects of a single PDT treatment on the mechanical and structural properties of vertebral bone.

SUMMARY OF BACKGROUND DATA

Spinal metastasis develops in up to one-third of all cancer patients, compromising the mechanical integrity of the spine and thereby increasing the risk of pathologic fractures and spinal cord damage. PDT has recently been adapted to ablate metastatic tumors in the spine in preclinical animal models. However, little is known about the effects of PDT on the structural integrity of vertebral bone.

METHODS

A single PDT treatment was administered to healthy Wistar rats at photosensitizer and light doses known to be effective in athymic rats bearing human breast cancer metastases. At both 1 and 6 weeks posttreatment, changes in trabecular architecture, global stiffness and strength of vertebrae were quantified using micro-CT stereological analysis and axial compression testing.

RESULTS

At 6 weeks, there was a significant increase in bone volume fraction (to 55.7 +/- 11.1% vs. 38.5 +/- 6.4%, P < 0.001) and decrease in bone surface area-to-volume ratio (16.9 +/- 5.0/mm vs. 22.8 +/- 4.5/mm, P = 0.001), attributed to trabecular thickening (130 +/- 40 microm vs. 90 +/- 20 microm, P < 0.001). Similar trends were found at 1 week after PDT. There was a significant increase in stiffness from control (306 +/- 123 N/mm) at 1 week (399 +/- 150 N/mm, P = 0.04) and 6 weeks (410 +/- 113 N/mm, P = 0.05) post PDT. There was a positive trend toward increased ultimate stress at 1 week, which became statistically significant at 6 weeks compared with control (39.3 +/- 11.3 MPa vs. 27.5 +/- 9.5 MPa control, P = 0.002).

CONCLUSION

Not only may PDT be successful in ablating metastatic tumor tissue in the spine, but the positive effects of PDT on bone found in this study suggest that PDT may also improve vertebral mechanical stability.

Increased Concentrations of Prostaglandin D2 During Post-Fracture Bone Remodeling

Objective.

To test the hypothesis that increased concentrations of prostaglandin D2 (PGD2) correlate with bone remodeling. Studies using isolated bone cells indicate that PGD2 may be implicated in the regulation of bone homeostasis, with a positive influence on bone anabolism. We studied patients with traumatic fractures and age- and sex-matched healthy controls as an in vivo model of increased bone remodeling.

Methods.

Thirty-five patients with bone fracture and matched controls were recruited. Urine and sera samples were collected. Urinary 11ß-PGF2α, a PGD2 metabolite, and PGE2 metabolites (PGEM), serum lipocalin-type PGD2 synthase (L-PGDS), bone alkaline phosphatase (bone ALP), and crosslinked C-telopeptides of type I collagen (CTX) were measured.

Results.

At 5–6 weeks post-fracture, 11ß-PGF2α, L-PGDS, bone ALP, and CTX were significantly increased in the fracture patients compared to controls. PGEM levels were not different between groups. Levels of 11ß-PGF2α and bone ALP were positively correlated, suggesting that PGD2 may be implicated in fracture repair.

Conclusion.

These results support our working hypothesis that PGD2 could be implicated in the control of bone anabolism in humans.

Stem cell- and scaffold-based tissue engineering approaches to osteochondral regenerative medicine

In osteochondral tissue engineering, cell recruitment, proliferation, differentiation, and patterning are critical for forming biologically and structurally viable constructs for repair of damaged or diseased tissue. However, since constructs prepared ex vivo lack the multitude of cues present in the in vivo microenvironment, cells often need to be supplied with external biological and physical stimuli to coax them toward targeted tissue functions. To determine which stimuli to present to cells, bioengineering strategies can benefit significantly from endogenous examples of skeletogenesis. As an example of developmental skeletogenesis, the developing limb bud serves as an excellent model system in which to study how osteochondral structures form from undifferentiated precursor cells. Alongside skeletal formation during embryogenesis, bone also possesses innate regenerative capacity, displaying remarkable ability to heal after damage. Bone fracture healing shares many features with bone development, driving the hypothesis that the regenerative process generally recapitulates development. Similarities and differences between the two modes of bone formation may offer insight into the special requirements for healing damaged or diseased bone. Thus, endogenous fracture healing, as an example of regenerative skeletogenesis, may also inform bioengineering strategies. In this review, we summarize the key cellular events involving stem and progenitor cells in developmental and regenerative skeletogenesis, and discuss in parallel the corresponding cell- and scaffold-based strategies that tissue engineers employ to recapitulate these events in vitro.

The systemic angiogenic response during bone healing

INTRODUCTION

Angiogenesis is known to be a critical and closely regulated step during bone formation and fracture healing driven by a complex interaction of various cytokines. Delays in bone healing or even nonunion might therefore be associated with altered concentrations of specific angiogenic factors. These alterations might in turn be reflected by changes in serum concentrations.

METHOD

To determine physiological time courses of angiogenic cytokines during fracture healing as well as possible changes associated with failed consolidation, we prospectively collected serum samples from patients who had sustained surgical treatment for a long bone fracture. Fifteen patients without fracture healing 4 months after surgery (nonunion group) were matched to a collective of 15 patients with successful healing (union group). Serum concentrations of angiogenin (ANG), angiopoietin 2 (Ang-2), basic fibroblast growth factor (bFGF), platelet derived growth factor AB (PDGF-AB), pleiotrophin (PTN) and vascular endothelial growth factor (VEGF) were measured using enzyme linked immunosorbent assays over a period of 24 weeks.

RESULTS

Compared to reference values of healthy uninjured controls serum concentrations of VEGF, bFGF and PDGF were increased in both groups. Peak concentrations of these cytokines were reached during early fracture healing. Serum concentrations of bFGF and PDGF-AB were significantly higher in the union group at 2 and 4 weeks after the injury when compared to the nonunion group. Serum concentrations of ANG and Ang-2 declined steadily from the first measurement in normal healing fractures, while no significant changes over time could be detected for serum concentrations of these factures in nonunion patients. PTN serum levels increased asymptotically over the entire investigation in timely fracture healing while no such increase could be detected during delayed healing.

CONCLUSION

We conclude that fracture healing in human subjects is accompanied by distinct changes in systemic levels of specific angiogenic factors. Significant alterations of these physiologic changes in patients developing a fracture nonunion over time could be detected as early as 2 (bFGF) and 4 weeks (PDGF-AB) after initial trauma surgery.

Different mechanisms of spinal fusion using equine bone protein extract, rhBMP-2 and autograft during the process of anterior lumbar interbody fusion

To elucidate the molecular mechanisms of spinal fusion with different graft materials during an anterior lumbar interbody fusion, we examined the gene-expression profiles after implantation of equine bone protein extract, rhBMP-2 and autograft using microarray technology and data analysis, including hierarchical clustering, self-organizing maps (SOM), KEGG pathway and Biological process GO analyses in a porcine model. The results suggest that equine bone protein extract exhibited a more similar expression pattern with autograft than that of rhBMP-2. rhBMP-2 recruits progenitor cells, proliferation and differentiation possibly by inducing various factors including PGHS-2, IFGBP-2, VEGF and chemokines and then leads to preferable membranous ossification and bone remodeling. Conversely, equine bone protein extract results in endochondral ossification via upregulation of cartilage-related genes. Ossification by inducing direct osteoblastic differentiation and obviating the cartilaginous intermediate phases may increase spinal fusion rate.

Beta-catenin in the race to fracture repair: in it to Wnt

The Wnt/beta-catenin pathway regulates multiple biological events during embryonic development, including bone formation. Fracture repair recapitulates some of the processes of normal bone development, such as the formation of bone from a cartilaginous template, and many cell-signaling pathways that underlie bone development are activated during the repair process. The Wnt/beta-catenin signaling pathway is activated during fracture repair, and dysregulation of this pathway alters the normal bone-healing response. In early pluripotent mesenchymal stem cells, Wnt/beta-catenin signaling needs to be precisely regulated to facilitate the differentiation of osteoblasts; by contrast, beta-catenin is not needed for chondrocyte differentiation. Once mesenchymal stem cells are committed to the osteoblast lineage, activation of Wnt/beta-catenin signaling enhances bone formation. This activity suggests that the Wnt/beta-catenin pathway is a therapeutic target during bone repair. Indeed, treatments that activate Wnt/beta-catenin signaling, such as lithium, increase bone density and also enhance healing.