Mechanobiology of mandibular distraction osteogenesis: experimental analyses with a rat model

Finite element analysis of distraction osteogenesis with a new extramedullary internal distractor

Distraction osteogenesis (DO) is a bone regenerative maneuver, which is conventionally done with external fixators and, more recently, with telescopic intramedullary nails. Despite the proven effectiveness, external approaches are intrusive to the patient's life while intramedullary nailing damages the growth plates, making them unsuitable for pediatric patients. An internal DO plate fixator (IDOPF) was developed for pediatric patients to address these limitations. The objective of this study was to test the hypothesis that the IDOPF can withstand a partial weight bearing scenario and create a favorable mechanical microenvironment at the osteotomy gap for bone regeneration as the device elongates. A finite element model of a surrogated long bone diaphysis osteotomy fixation by means of the IDOPF was created and subjected to axial compression, bending and torsion. As the osteotomy gap increased from 2 mm to 20 mm, under compression, The average axial interfragmentary strains decreased from 2.33% to 0.35%. Stress increased from 179 MPa to 281 MPa at the contact interfaces of the telescopic compartments, which exceeded the endurance limit of stainless steel (270 MPa) but was below its yield limit (415 MPa). These results demonstrate, that the IDOPF can withstand a partial load bearing scenario and provide a stable biomechanical environment conductive to bone healing. However, high contact stresses at the telescopic interfaces of the device are likely to cause wear, as is frequently reported in telescopic fixators. This study is a step towards refining the IDOPF design for clinical use.

One-Year Treatment Outcome of Profile Changes After Transcutaneous Maxillary Distraction Osteogenesis in Growing Children With Cleft Lip and Palate

OBJECTIVE

To evaluate the long-term stability of LeFort I osteotomy followed by distraction osteogenesis with a transcutaneous rigid external device for the treatment of severe maxillary hypoplasia in patients with cleft lip and palate.

PATIENTS AND METHODS

Nine patients with cleft lip and palate underwent rigid external distraction after a LeFort I osteotomy for maxillary advancement. Lateral cephalometric films were analyzed for assessment of treatment outcome and stability in 1 month, 6 months, and 1 year after distraction.

RESULTS

Significant maxillary advancement was observed in the horizontal direction, with the anterior nasal spine (ANS) distance of the maxilla increasing by an average of 20.5 ± 5.1 mm after distraction. The ANS relapse rates in 6 months and 1 year were 8.7% and 12.8%, respectively. The mean inclination of upper incisors to the palatal plane was almost unchanged (before: 109.8° ± 6.6°; after: 108.9° ± 7.5°). The movement ratios at the nasal tip/ANS, soft tissue A point/A point, and the upper vermilion border/upper incisor edge were 0.36:1, 0.72:1, and 0.83:1, respectively.

CONCLUSION

Considerable maxillary advancement was achieved with less change of incisors inclination after distraction. Moreover, the relapse rate after 1 year was minimal. The concave facial profile was improved as well as the facial balance and aesthetics.

Evolution of callus tissue behavior during stable distraction osteogenesis

Multiple studies have sought to characterize the mechanical behavior of callus tissue in vivo during distraction osteogenesis. The aims of such studies are to understand the mechanobiology of distraction and elucidate the complex viscoelasticity and evolution of the tissue. The former objective has direct clinical relevance to surgical technique and process control while the latter is necessary for the calibration and validation of the predictive healing models. Such models seek to reduce the researcher's dependence on animal studies and prospectively allow improved surgical planning. To date, no study has been capable of controlling the mechanical conditions sufficiently enough to decouple the distraction process from the secondary mechanical stimulation associated with the finite stiffness of the fixation constructs employed. It is the goal of this work to understand the mechanobiology of pure distraction as well as characterize viscoelastic tissue behavior under precisely defined mechanical conditions. This is achieved using a novel lateral distraction model. The structural integrity of the bone is maintained, allowing the collection of force relaxation data due to a stepwise distraction process without the superimposed influence of secondary mechanical stimulation. The average instantaneous modulus increases from approximately 2 kPa to approximately 1100 kPa while the equilibrium modulus increases from approximately 0 kPa to 200 kPa over the distraction period.

Mechanical Stimulation of Adipose-Derived Stem Cells for Functional Tissue Engineering of the Musculoskeletal System via Cyclic Hydrostatic Pressure, Simulated Microgravity, and Cyclic Tensile Strain

It is critical that human adipose stem cell (hASC) tissue-engineering therapies possess appropriate mechanical properties in order to restore function of the load bearing tissues of the musculoskeletal system. In an effort to elucidate the hASC response to mechanical stimulation and develop mechanically robust tissue engineered constructs, recent research has utilized a variety of mechanical loading paradigms including cyclic tensile strain, cyclic hydrostatic pressure, and mechanical unloading in simulated microgravity. This chapter describes methods for applying these mechanical stimuli to hASC to direct differentiation for functional tissue engineering of the musculoskeletal system.

Fabrication of novel high surface area mushroom gilled fibers and their effects on human adipose derived stem cells under pulsatile fluid flow for tissue engineering applications

The fabrication and characterization of novel high surface area hollow gilled fiber tissue engineering scaffolds via industrially relevant, scalable, repeatable, high speed, and economical nonwoven carding technology is described. Scaffolds were validated as tissue engineering scaffolds using human adipose derived stem cells (hASC) exposed to pulsatile fluid flow (PFF). The effects of fiber morphology on the proliferation and viability of hASC, as well as effects of varied magnitudes of shear stress applied via PFF on the expression of the early osteogenic gene marker runt related transcription factor 2 (RUNX2) were evaluated. Gilled fiber scaffolds led to a significant increase in proliferation of hASC after seven days in static culture, and exhibited fewer dead cells compared to pure PLA round fiber controls. Further, hASC-seeded scaffolds exposed to 3 and 6dyn/cm(2) resulted in significantly increased mRNA expression of RUNX2 after one hour of PFF in the absence of soluble osteogenic induction factors. This is the first study to describe a method for the fabrication of high surface area gilled fibers and scaffolds. The scalable manufacturing process and potential fabrication across multiple nonwoven and woven platforms makes them promising candidates for a variety of applications that require high surface area fibrous materials.

STATEMENT OF SIGNIFICANCE

We report here for the first time the successful fabrication of novel high surface area gilled fiber scaffolds for tissue engineering applications. Gilled fibers led to a significant increase in proliferation of human adipose derived stem cells after one week in culture, and a greater number of viable cells compared to round fiber controls. Further, in the absence of osteogenic induction factors, gilled fibers led to significantly increased mRNA expression of an early marker for osteogenesis after exposure to pulsatile fluid flow. This is the first study to describe gilled fiber fabrication and their potential for tissue engineering applications. The repeatable, industrially scalable, and versatile fabrication process makes them promising candidates for a variety of scaffold-based tissue engineering applications.

Photocrosslinkable and elastomeric hydrogels for bone regeneration

Nanocomposite biomaterials are extensively investigated for cell and tissue engineering applications due their unique physical, chemical and biological characteristics. Here, we investigated the mechanical, rheological, and degradation properties of photocrosslinkable and elastomeric nanocomposite hydrogels from nanohydroxyapatite (nHAp) and gelatin methacryloyl (GelMA). The addition of nHAp resulted in a significant increase in mechanical stiffness and physiological stability. Cells readily adhere and proliferate on the nanocomposite surfaces. Cyclic stretching of cells on the elastomeric nanocomposites revealed that nHAp elicited a stronger alignment response in the direction of strain. In vitro studies highlight enhanced bioactivity of nanocomposites as determined by alkaline phosphate (ALP) activity. Overall, the elastomeric and photocrosslinkable nanocomposite hydrogels can be used for minimally invasive therapy for bone regeneration.

Nonvascular transport distraction osteogenesis in bone formation and regeneration. Is it an accidental phenomenon?

PURPOSE

To explore the osteogenic mechanism of nonvascular transport distraction osteogenesis (NTDO) by constructing mandibular defects in dogs.

METHODS

Sixty adult dogs were randomly divided into three groups with 20 dogs in each group. Canine mandibular defect models of NTDO were constructed. Animals were euthanized 1, 4 and 12 weeks after distraction, and the transport disc and surrounding tissue were collected and fixed. Histochemical staining using hematoxylin and eosin (H&E) and electron microscopic observations were used to examine bone regeneration.

RESULTS

Distraction bone regeneration was observed in the distraction gap and around the transport disc, and osseous connections had formed between new bone and the transport disc after one week. Osteoclasts gathered around the transport disc, and bone absorption pit formation could be seen. After 4 weeks of distraction, the new bone around the transport disc was close to maturity with thick sclerostin on the middle of the transport disc. After 12 weeks the new bone and the transport disc were fully integrated, and were difficult to distinguish by H&E staining and electron microscopy.

CONCLUSIONS

Canine mandibular defects were successfully repaired by NTDO resulting in ideal new bone formation and fully recovered mandibular physiological function. The surrounding tissues, including musculoskeletal tissues, the periosteum and other soft tissues and the nonvascular transport disc, together contribute to bone regeneration and neovascularization in NTDO.

Effect of cyclic mechanical stimulation on the expression of osteogenesis genes in human intraoral mesenchymal stromal and progenitor cells

We evaluated the effects of mechanical stimulation on the osteogenic differentiation of human intraoral mesenchymal stem and progenitor cells (MSPCs) using the Flexcell FX5K Tension System that mediated cyclic tensile stretch on the cells. MSPCs were isolated from human mandibular retromolar bones and characterized using flow cytometry. The positive expression of CD73, CD90, and CD105 and negativity for CD14, CD19, CD34, CD45, and HLA-DR confirmed the MSPC phenotype. Mean MSPC doubling time was 30.4 ± 2.1 hrs. The percentage of lactate dehydrogenase (LDH) release showed no significant difference between the mechanically stimulated groups and the unstimulated controls. Reverse transcription quantitative real-time PCR revealed that 10% continuous cyclic strain (0.5 Hz) for 7 and 14 days induced a significant increase in the mRNA expression of the osteogenesis-specific markers type-I collagen (Col1A1), osteonectin (SPARC), bone morphogenetic protein 2 (BMP2), osteopontin (SPP1), and osteocalcin (BGLAP) in osteogenic differentiated MSPCs. Furthermore, mechanically stimulated groups produced significantly higher amounts of calcium deposited into the cultures and alkaline phosphatase (ALP). These results will contribute to a better understanding of strain-induced bone remodelling and will form the basis for the correct choice of applied force in oral and maxillofacial surgery.

Cyclic tensile strain enhances osteogenesis and angiogenesis in mesenchymal stem cells from osteoporotic donors

We have shown that the uniaxial cyclic tensile strain of magnitude 10% promotes and enhances osteogenesis of human mesenchymal stem cells (hMSC) and human adipose-derived stem cells (hASC) from normal, nonosteoporotic donors. In the present study, MSC from osteoporotic donors were analyzed for changes in mRNA expression in response to 10% uniaxial tensile strain to identify potential mechanisms underlying the use of this mechanical loading paradigm for prevention and treatment of osteoporosis. Human MSC isolated from three female, postmenopausal osteoporotic donors were analyzed for their responses to mechanical loading using microarray analysis of over 47,000 gene probes. Human MSC were seeded in three-dimensional collagen type I constructs to mimic the organic extracellular matrix of bone and 10% uniaxial cyclic tensile strain was applied to promote osteogenesis. Seventy-nine genes were shown to be regulated within hMSC from osteoporotic donors in response to 10% cyclic tensile strain. Upregulation of six genes were further confirmed with real-time RT-PCR: jun D proto-oncogene (JUND) and plasminogen activator, urokinase receptor (PLAUR), two genes identified as potential key molecules from network analysis; phosphoinositide-3-kinase, catalytic, delta polypeptide (PIK3CD) and wingless-type MMTV integration site family, member 5B (WNT5B), two genes with known importance in bone biology; and, PDZ and LIM domain 4 (PDLIM4) and vascular endothelial growth factor A (VEGFA), two genes that we have previously shown are significantly regulated in hASC in response to this mechanical stimulus. Function analysis indicated that 10% cyclic tensile strain induced expression of genes associated with cell movement, cell proliferation, and tissue development, including development in musculoskeletal and cardiovascular systems. Our results demonstrate that hMSC from aged, osteoporotic donors are capable of enhanced osteogenic differentiation in response to 10% cyclic tensile strain with significant increases in the expression of genes associated with enhanced cell proliferation, musculoskeletal development, and angiogenesis. Surprisingly, cyclic tensile strain of magnitude 10% not only enhanced osteogenesis in hMSC from osteoporotic donors, but also enhanced expression of angiogenic factors. Better understanding and methodologies to promote osteogenesis in hMSC from elderly, osteoporotic donors may greatly facilitate achieving long-term success in bone regeneration and functional bone tissue engineering for this ever-growing patient population.

Micromotion-induced strain fields influence early stages of repair at bone-implant interfaces

Implant loading can create micromotion at the bone-implant interface. The interfacial strain associated with implant micromotion could contribute to regulating the tissue healing response. Excessive micromotion can lead to fibrous encapsulation and implant loosening. Our objective was to characterize the influence of interfacial strain on bone regeneration around implants in mouse tibiae. A micromotion system was used to create strain under conditions of (1) no initial contact between implant and bone and (2) direct bone-implant contact. Pin- and screw-shaped implants were subjected to displacements of 150 or 300 μm for 60 cycles per day for 7 days. Pin-shaped implants placed in five animals were subjected to three sessions of 150 μm displacement per day, with 60 cycles per session. Control implants in both types of interfaces were stabilized throughout the healing period. Experimental strain analyses, microtomography, image-based displacement mapping, and finite element simulations were used to characterize interfacial strain fields. Calcified tissue sections were prepared and Goldner trichrome stained to evaluate the tissue reactions in higher and lower strain regions. In stable implants bone formation occurred consistently around the implants. In implants subjected to micromotion bone regeneration was disrupted in areas of high strain concentrations (e.g. >30%), whereas lower strain values were permissive of bone formation. Increasing implant displacement or number of cycles per day also changed the strain distribution and disturbed bone healing. These results indicate that not only implant micromotion but also the associated interfacial strain field contributes to regulating the interfacial mechanobiology at healing bone-implant interfaces.

Comparison of osteogenic medium and uniaxial strain on differentiation of endometrial stem cells

BACKGROUND

Mechanotransduction plays a pivotal role in remodeling and repair of skeletal tissues. This mechanism has been widely used in bone tissue engineering especially under in vitro conditions. To date, various stem cells have been used for this purpose. The present study was the first to evaluate the effect of mechanical loading on differentiation of human endometrial stem cells (hESCs) to osteoblasts.

MATERIALS AND METHODS

Adhesion of endometrial stem cells after isolation and culture on a silicone membrane covered with collagen was evaluated under scanning electron microscope (SEM). Twenty-four hours after cell culture on the membrane and ensuring appropriate cell adhesion, a group of cells in a conventional culture medium received 3% static uniaxial strain. In the positive control group, cells cultured on the membrane were placed in an osteogenic medium without receiving any mechanical strain. The negative control group was placed in a regular medium and received no strain either. Two weeks later, cultured cells were evaluated for expression of osteogenic markers using immunofluorescence staining and real-time polymerase chain reaction (PCR). Data of real-time PCR was analyzed by ANOVA. P < 0.05 was considered statistically significant.

RESULTS

SEM analysis revealed adequate cell adhesion to the membrane after 24 h. Two weeks after loading, expression of markers in the positive control group was significantly higher compared to test group.

CONCLUSION

We can conclude that static uniaxial strain exerted on hESCs results in their differentiation to osteoblasts. However, this magnitude of static strain in the tested time period cannot yield excellent differentiation when compared to the osteogenic medium.

EFFECTS OF AGING ON THE LATENCY PERIOD IN MANDIBULAR DISTRACTION OSTEOGENESIS: A COMPUTATIONAL MECHANOBIOLOGICAL ANALYSIS

Mandibular symphyseal distraction osteogenesis is a clinical procedure utilized in orthodontics for solving problems of dental overcrowding on the mandibular arch. A critical issue is to evaluate the optimal duration of the latency period between the osteotomy and the first aperture of distraction device. In fact, the latency period should change with the patient's age. To this end, a computational mechanobiological model has been developed in order to find optimal durations of latency period for young, adult, and elder patients. The model is implemented in a finite element framework simulating the process of tissue differentiation in the bone callus formed after osteotomy. The biophysical stimulus regulating the tissue differentiation process is hypothesized to be a function of the octahedral shear strain and interstitial fluid flow velocity. The resulting spatial distribution of stiffness properties in the callus region is analyzed in order to assess the risk of premature bone union of osteotomy edges. The three-dimensional (3D) finite element model (FEM) of human mandible is reconstructed from computed tomography (CT) scans and also includes a tooth-borne device. Under unilateral occlusion, the mandible is submitted to full mastication loading or to mastication forces reduced by 70%. The results show that optimal durations of the latency period for preventing premature bone union are about 5–6 days for the young patient, 7–8 days for the adult patient, and 9–10 days for the elder patient. These durations seem rather insensitive to the magnitude of mastication forces. Finally, distraction force values predicted by the present mechanobiological model are in good agreement with data reported in the literature.

Comparison of gene expression of tissue inhibitor of matrix metalloproteinase-1 between continuous and intermittent distraction osteogenesis: a quantitative study on rabbits

BACKGROUND

Distraction osteogenesis is a controlled surgical procedure that initiates a regenerative process and uses mechanical strain to enhance the biological responses of the injured tissues to create new bone. To explore the effect of high-frequency mechanical traction on the expression of tissue inhibitor of matrix metalloproteinase-1 (TIMP-1), we compared the gene expression of TIMP-1 between continuous and intermittent distraction osteogenesis using a rabbit model of mandibular lengthening.

MATERIALS AND METHODS

Forty adult New Zealand white rabbits were randomly assigned to the intermittent and continuous distraction groups. A unilateral mandibular osteotomy was performed and a custom-designed manual-driven or auto-driven distractor was bridged over the osteotomy segments. Animals were sacrificed at day-6, day-10, day-14 and day-21 after osteotomy. Samples were examined with real-time polymerase chain reaction (PCR).

RESULTS

Real-time PCR examination showed significantly higher mRNA levels of TIMP-1 under continuous distraction than that under intermittent distraction at day-6 and day-10. No significant differences were found at day-14 and day-21.

CONCLUSION

High-frequency traction provides a good mechanical environment for accelerating bone formation by up-regulating TIMP-1.

[Autologous tissue engineering by means of distraction osteogenesis: Biomechanical considerations]

Tissue engineering consists in producing functional replacement tissue. Distraction osteogenesis is a tissue engineering technique that uses the mechanical environment of cells to induce tissue regeneration, without need for exogenous biochemical factors. A better understanding of the optimal mechanical conditions of distraction callus stretching may reduce the duration, discomfort, and even social impact of distraction protocols, and complications and failures. We present the current state of knowledge in this field by addressing the fundamentals of elongating bone tissue biomechanics, the influence of rhythm and rate of distraction, and that of vectors and stability. Finally, we present the innovations currently studied, which may modify our clinical protocol in the short term.

Adipose-derived stem cells in functional bone tissue engineering: lessons from bone mechanobiology

This review aims to highlight the current and significant work in the use of adipose-derived stem cells (ASC) in functional bone tissue engineering framed through the bone mechanobiology perspective. Over a century of work on the principles of bone mechanosensitivity is now being applied to our understanding of bone development. We are just beginning to harness that potential using stem cells in bone tissue engineering. ASC are the primary focus of this review due to their abundance and relative ease of accessibility for autologous procedures. This article outlines the current knowledge base in bone mechanobiology to investigate how the knowledge from this area has been applied to the various stem cell-based approaches to engineering bone tissue constructs. Specific emphasis is placed on the use of human ASC for this application.

A model for tissue engineering applications: femoral critical size defect in immunodeficient mice

Animal models for preclinical functionality assays lie midway between in vitro systems such as cell culture and actual clinical trials. We have developed a novel external fixation device for femoral critical size defect (CSD) in the femurs of immunodeficient mice as an experimental model for studying bone regeneration and bone tissue engineering. The external fixation device comprises four pointed rods and dental acrylic paste. A segmental bone defect (2 mm) was created in the midshaft of the mouse femur. The CSD in the femur of the mice were either left untreated or treated with a bone allograft, a cell-scaffold construct, or a scaffold-only construct. The repair and healing processes of the CSD were monitored by digital x-ray radiography, microcomputed tomography, and histology. Repair of the femoral CSD was achieved with the bone allografts, and partial repair of the femoral CSD was achieved with the cell scaffold and the scaffold-only constructs. No repair of the nongrafted femoral CSD was observed. Our results establish the feasibility of this new mouse femoral model for CSD repair of segmental bone using a simple stabilized external fixation device. The model should prove especially useful for in vivo preclinical proof-of-concept studies that involve cell therapy-based technologies for bone tissue engineering applications in humans.

Nanoscale surface modifications of medically relevant metals: state-of-the art and perspectives

Evidence that nanoscale surface properties stimulate and guide various molecular and biological processes at the implant/tissue interface is fostering a new trend in designing implantable metals. Cutting-edge expertise and techniques drawn from widely separated fields, such as nanotechnology, materials engineering and biology, have been advantageously exploited to nanoengineer surfaces in ways that control and direct these processes in predictable manners. In this review, we present and discuss the state-of-the-art of nanotechnology-based approaches currently adopted to modify the surface of metals used for orthopedic and dental applications, and also briefly consider their use in the cardiovascular field. The effects of nanoengineered surfaces on various in vitro molecular and cellular events are firstly discussed. This review also provides an overview of in vivo and clinical studies with nanostructured metallic implants, and addresses the potential influence of nanotopography on biomechanical events at interfaces. Ultimately, the objective of this work is to give the readership a comprehensive picture of the current advances, future developments and challenges in the application of the infinitesimally small to biomedical surface science. We believe that an integrated understanding of the in vitro and particularly of the in vivo behavior is mandatory for the proper exploitation of nanostructured implantable metals and, indeed, of all biomaterials.

Computational simulation of spontaneous bone straightening in growing children

Periosteal surface pressures have been shown to inhibit bone formation and induce bone resorption, while tensile strains perpendicular to the periosteal surface have been shown to inhibit bone resorption and induce new bone deposition. A new computational model was developed to incorporate these experimental findings into simulations of spontaneous bone straightening in children with congenital posteromedial bowing of the tibia. Three-dimensional finite element models of the periosteum were used to determine the relationships between the defect angle and the distribution of bone surface pressures and strains due to growth-generated tensile strains in the periosteum. These relationships were incorporated into an iterative simulation to model development of a growing, bowed tibia with an initial defect angle of 27 degrees. When periosteal loads were included in the simulation, the defect angle decreased to 10 degrees after 2 years, and the bone straightened by an age of 25 years. When periosteal loads were not included in the simulation, the defect angle decreased to 23 degrees after 2 years, and a defect angle of 9 degrees remained at an age of 25 years. A "modeling drift" bone apposition/resorption pattern appeared only when periosteal loads were included. The results suggest that periosteal pressures and tensile strains induced by bone bowing can accelerate the process of bone straightening and lead to more complete correction of congenital bowing defects. Including the mechanobiological effects of periosteal surface loads in the simulations produced results similar to those seen clinically, with rapid straightening during the first few years of growth.

Comparison of gene expression of osteogenic factors between continuous and intermittent distraction osteogenesis in rabbit mandibular lengthening

OBJECTIVES

This study aimed to evaluate the effect of distraction frequency on the gene expression of osteogenic mediators in mandibular distraction osteogenesis.

STUDY DESIGN

Forty adult New Zealand white rabbits were randomly assigned to the continuous and intermittent distraction groups. Unilateral mandibular osteotomy was performed and custom-designed manual-driven or autodriven distractor was bridged over the osteotomy segments. Animals were humanely killed at day 6, day 10, day 14, and day 21 after osteotomy. mRNA expression of the osteogenic mediators in the distraction regenerate was examined by real-time polymerase chain reaction.

RESULTS

The expression of transforming growth factor-beta(1) was significantly higher at day 6, and the expression of the bone morphogenetic protein-2 was significantly higher from day 6 to day 14, in the continuous distraction group.

CONCLUSION

High-frequency traction up-regulates the expression of osteogenic mediators contributing to the enhanced bone formation.

Mesenchymal stem cell-seeded collagen matrices for bone repair: effects of cyclic tensile strain, cell density, and media conditions on matrix contraction in vitro

Type I collagen is the most abundant extracellular matrix protein in bone and contains arginine- glycine-aspartic acid sequences that promote cell adhesion and proliferation. We have previously shown that human mesenchymal stem cells (hMSCs) seeded in three-dimensional (3D) collagen gels upregulate BMP-2 mRNA expression in response to tensile strain, indicative of osteogenesis. Therefore, collagen could be a promising scaffold material for functional bone tissue engineering using hMSCs. However, high contraction of the collagen gels by hMSCs poses a challenge to creating large, tissue-engineered bone constructs. The effects of cyclic tensile strain, medium (with and without dexamethasone), and hMSC seeding density on contraction of collagen matrices have not been investigated. hMSCs were seeded in 3D collagen gels and subjected to cyclic tensile strain of 10% or 12% for 4 h/day at a frequency of 1 Hz in osteogenic-differentiating or complete MSC growth media for up to 14 days. Viability of hMSCs was not affected by strain or media conditions. While initial seeding density affected matrix contraction alone, there was a high interdependence of strain and medium on matrix contraction. These findings suggest a correlation between hMSC proliferation and osteogenic differentiation on collagen matrix contraction that is affected by media, cell-seeding density, and cyclic tensile strain. It is vital to understand the effects of culture conditions on collagen matrix contraction by hMSCs in order to consider hMSC-seeded collagen constructs for functional bone tissue engineering in vitro.

Osteogenic effects of rest inserted and continuous cyclic tensile strain on hASC lines with disparate osteodifferentiation capabilities

We investigated the effects of two types of cyclic tensile strain, continuous and rest inserted, on osteogenic differentiation of human adipose-derived adult stem cells (hASCs). The influence of these mechanical strains was tested on two hASC lines having different mineral deposition potential, with one cell line depositing approximately nine times as much calcium as the other hASC line after 14 days of culture in osteogenic medium on tissue culture plastic. Results showed that both continuous (10% strain, 1 Hz) and rest inserted cyclic tensile strain (10% strain, 1 Hz, 10 s rest after each cycle) regimens increased the amount and rate of calcium deposition for both high and low calcium depositing hASC lines as compared to unstrained controls. The response was similar for both types of tensile strain for a given cell line, however, cyclic tensile strain had a much stronger osteogenic effect on the high calcium depositing hASC line, suggesting that mechanical loading has a greater effect on cell lines that already have an innate ability to produce bone as compared to cell lines that do not. This is the first study to investigate the osteodifferentiation effects of cyclic tensile strain on hASCs and the first to show that both continuous (10%, 1 Hz) and rest inserted (10%, 1 Hz, 10 s rest) cyclic tensile strain accelerate hASC osteodifferentiation and increase calcium accretion.

Histologic study of the cellular events during rat mandibular distraction osteogenesis

OBJECTIVE

The cellular events, underlying bone regeneration through rat mandibular distraction osteogenesis (DO) was examined using micro computerized tomography (microCT), histology, and histochemistry.

STUDY DESIGN

After 5-day latency, mandibles were distracted at 0.2 mm/12 h for 10 days, and fixed at latency 5 days (L5D), distraction 3, 6, 10 days (D3D, D6D, D10D), and consolidation 1, 3, 6, 10 weeks (C1W, C3W, C6W, C10W).

RESULTS

The microCT demonstrated radiopacity at the distraction gap (DG) during C1W, which was filled with new bone at C6W and C10W. At D3D, collagen fibers were aligned along the axis of the distraction vector. At D6D, alkaline phosphatase-positive osteoblasts and intramembranous ossification was observed. Collagen bundles became thicker with new bony trabeculae at D10D. Type II collagen-immunopositive areas first appeared at C1W. At C3W, cartilage tissue and endochondral ossification were found. By C6W, the entire DG had been bridged by new bone. The C10W specimens showed mature lamellar bone.

CONCLUSION

Mandibular DO produces bone through both intramembranous and endochondral ossification.

Finite element modeling of 3D human mesenchymal stem cell-seeded collagen matrices exposed to tensile strain

The use of human mesenchymal stem cells (hMSCs) in tissue engineering is attractive due to their ability to extensively self-replicate and differentiate into a multitude of cell lineages. It has been experimentally established that hMSCs are influenced by chemical and mechanical signals. However, the combined chemical and mechanical in vitro culture conditions that lead to functional tissue require greater understanding. In this study, finite element models were created to evaluate the local loading conditions on bone marrow-derived hMSCs seeded in three-dimensional collagen matrices exposed to cyclic tensile strain. Mechanical property and geometry data used in the models were obtained experimentally from a previous study in our laboratory and from mechanical testing. Eight finite element models were created to simulate three-dimensional hMSC-seeded collagen matrices exposed to different levels of cyclic tensile strain (10% and 12%), culture media (complete growth and osteogenic differentiating), and durations of culture (7 and 14 days). Through finite element analysis, it was determined that globally applied uniaxial tensile strains of 10% and 12% resulted in local strains up to 18.3% and 21.8%, respectively. Model results were also compared to experimental studies in an attempt to explain observed differences between hMSC response to 10% and 12% cyclic tensile strain.

Tissue engineering in cleft palate and other congenital malformations

Contributions from multidisciplinary investigations have focused attention on the potential of tissue engineering to yield novel therapeutics. Congenital malformations, including cleft palate, craniosynostosis, and craniofacial skeletal hypoplasias represent excellent targets for the implementation of tissue engineering applications secondary to the technically challenging nature and inherent inadequacies of current reconstructive interventions. Apropos to the search for answers to these clinical conundrums, studies have focused on elucidating the molecular signals driving the biologic activity of the aforementioned maladies. These investigations have highlighted multiple signaling pathways, including Wnt, fibroblast growth factor, transforming growth factor-beta, and bone morphogenetic proteins, that have been found to play critical roles in guided tissue development. Furthermore, a comprehensive knowledge of these pathways will be of utmost importance to the optimization of future cell-based tissue engineering strategies. The scope of this review encompasses a discussion of the molecular biology involved in the development of cleft palate and craniosynostosis. In addition, we include a discussion of craniofacial distraction osteogenesis and how its applied forces influence cell signaling to guide endogenous bone regeneration. Finally, this review discusses the future role of cell-based tissue engineering in the treatment of congenital malformations.

Cyclic tensile strain increases interactions between human epidermal keratinocytes and quantum dot nanoparticles

The effects of quantum dots (QD) on cell viability have gained increasing interest due to many recent developments utilizing QD for pharmaceutical and biomedical applications. The potential use of QD nanoparticles as diagnostic, imaging, and drug delivery agents has raised questions about their potential for cytotoxicity. The objective of this study was to investigate the effects of applied strain on QD uptake by human epidermal keratinocytes (HEK). It was hypothesized that introduction of a 10% average strain to cell cultures would increase QD uptake. HEK were seeded at a density of 150,000 cells/mL on collagen-coated Flexcell culture plates (Flexcell Intl.). QD were introduced at a concentration of 3 nM and a 10% average strain was applied to the cells. After 4h of cyclic strain, the cells were examined for cell viability, QD uptake, and cytokine production. The results indicate that addition of strain results in an increase in cytokine production and QD uptake, resulting in irritation and a negative impact on cell viability. Application of physiological load conditions can increase cell membrane permeability, thereby increasing the concentration of QD nanoparticles in cells.

Uniaxial mechanical strain: an in vitro correlate to distraction osteogenesis

BACKGROUND

Distraction osteogenesis is a valuable clinical tool; however the molecular mechanisms governing successful distraction remain unknown. We have used a uniaxial in vitro strain device to simulate the uniaxial mechanical environment of the interfragmentary distraction gap.

MATERIALS AND METHODS

Using the Flexcell system, normal human osteoblasts were subjected to different levels of cyclical uniaxial mechanical strain. Cellular morphology, proliferation, migration, and the expression of angiogenic (vascular endothelial growth factor [VEGF] and fibroblast growth factor-2 [FGF-2]) and osteogenic (osteonectin, osteopontin, and osteocalcin) proteins and extracellular matrix molecules (collagen IalphaII) were analyzed in response to uniaxial cyclic strain.

RESULTS

Osteoblasts exposed to strain assumed a fusiform spindle-shaped morphology aligning parallel to the axis of uniaxial strain and osteoblasts exposed to strain or conditioned media had a 3-fold increase in proliferation. Osteoblast migration was maximal (5-fold) in response to 9% strain. Angiogenic cytokine, VEGF, and FGF-2, increased 32-fold and 2.6-fold (P < 0.05), respectively. Osteoblasts expressed greater amounts of osteonectin, osteopontin, and osteocalcin (2.1-fold, 1.8-fold, 1.5-fold respectively, P < 0.01) at lower levels of strain (3%). Bone morphogenic protein-2 production increased maximally at 9% strain (1.6-fold, P < 0.01). Collagen I expression increased 13-, 66-, and 153-fold in response to 3, 6, and 9% strain, respectively.

CONCLUSIONS

Uniaxial cyclic strain using the Flexcell device under appropriate strain parameters provides a novel in vitro model that induces osteoblast cellular and molecular expression patterns that simulate patterns observed in the in vivo distraction gap.

Tissue differentiation and bone regeneration in an osteotomized mandible: a computational analysis of the latency period

Mandibular symphyseal distraction osteogenesis is a common clinical procedure to modify the geometrical shape of the mandible for correcting problems of dental overcrowding and arch shrinkage. In spite of consolidated clinical use, questions remain concerning the optimal latency period and the influence of mastication loading on osteogenesis within the callus prior to the first distraction of the mandible. This work utilized a mechano-regulation model to assess bone regeneration within the callus of an osteotomized mandible. A 3D model of the mandible was reconstructed from CT scan data and meshed using poroelastic finite elements (FE). The stimulus regulating tissue differentiation within the callus was hypothesized to be a function of the strain and fluid flow computed by the FE model. This model was then used to analyse tissue differentiation during a 15-day latency period, defined as the time between the day of the osteotomy and the day when the first distraction is given to the device. The following predictions are made: (1) the mastication forces generated during the latency period support osteogenesis in certain regions of the callus, and that during the latency period the percentage of progenitor cells differentiating into osteoblasts increases; (2) reducing the mastication load by 70% during the latency period increases the number of progenitor cells differentiating into osteoblasts; (3) the stiffness of new tissue increases at a slower rate on the side of bone callus next to the occlusion of the mandibular ramus which could cause asymmetries in the bone tissue formation with respect to the middle sagittal plane. Although the model predicts that the mastication loading generates such asymmetries, their effects on the spatial distribution of callus mechanical properties are insignificant for typical latency periods used clinically. It is also predicted that a latency period of longer than a week will increase the risk of premature bone union across the callus.

The Influence of Expansion Rates on Mandibular Distraction Osteogenesis: A Computational Analysis

Mandibular distraction osteogenesis is a clinical procedure used for modifying the mandibular geometry when problems of dental overcrowding and arch shrinkage occur. The objective of this study is to use a computational model of tissue differentiation to examine the influence of the rate of distraction on bone re-growth within the fracture callus of a human mandible submitted to symphyseal distraction osteogenesis. A 3D model of the mandible is reconstructed from CT scan data and meshed into finite elements. Two different mastication loadings have been investigated: a 'full' mastication load and a 'reduced' mastication load where the action of each muscle was reduced by 70%. Four different distraction rates were analyzed: 0.6, 1.2, 2, and 3 mm/day, allowing a total displacement of 6 mm. In the early stages of the distraction process it is predicted that there is a decrease in the amount of bone tissue forming within the center of the fracture gap for all distraction rates. After the initial phases of expansion, the bone tissue within the callus increases for the slower rate of distraction or continues to decrease at the faster rates of distraction. At the end of the simulated maturation period, 47% of the distracted callus was predicted to consist of bone tissue for a distraction rate of 0.6 mm/day, decreasing to 22% for a distraction rate of 3 mm/day. Significantly higher amounts of bone formation were predicted for all distraction rates for the case of reduced mastication loading. Disparities between the model predictions and what is observed in vivo were found. For instance, during the latency period, the distraction period and beyond, the model is predicting larger than expected amounts of cartilage tissue formation within the callus. This and other limitations of the proposed model are discussed and possible specific explanations for these disparities are provided in the paper. The model predicts a distraction rate of around 1.2 mm/day to be optimal as higher rates produce less bone tissue while the risk of a premature bone union is greater at slower rates of distraction because in the latter stages of the distraction process bone tissue is predicted to form between the left and right side of the bone callus.

Masticatory mechanics of a mandibular distraction osteogenesis site: interfragmentary micromovement

Micromovement at a fracture or distraction osteogenesis (DO) site may play a significant role in bone formation and healing. Mastication is an important physiological process that can cause substantial micromovement at a mandibular disjunction. The purpose of this study is to characterize and quantify the micromovement caused by mastication. Eighteen pigs, divided into three groups based on duration of consolidation, received a unilateral (right) mandibular angle distraction osteogenesis protocol. Differential variable reluctance transducers (DVRTs) and ultrasound crystals were used to measure the change of gap width as well as interfragmentary movement during mastication. Synchronized chewing video and interfragmentary movement recordings were used to determine the magnitude and direction of micromovement at different phases of the chewing cycle. The magnitude of micromovement did not increase significantly with distraction up to almost 5 mm, but did decrease gradually with consolidation. The average micromovement magnitude during the distraction phase was 0.2-0.3 mm, equaling 50,000-250,000 microstrain (muepsilon) on interfragmentary tissue. The dominant deformation pattern was bending in the sagittal plane. The most common direction of bending at the power stroke of chewing was concave dorsally, i.e., superior shortening and inferior lengthening. These findings elucidate how masticatory mechanics affect a mandibular distraction site, and the measurements may be useful for future simulation studies.

The mechanobiological effects of periosteal surface loads

We have developed an improved mechanobiological model of bone morphogenesis and functional adaptation that includes the influences of periosteum tension and pressure on bone formation and resorption. Previous models assumed that periosteal and endosteal bone deposition and resorption rates are governed only by the local intracortical daily stress or strain stimulus caused by cyclic loading. The new model incorporates experimental findings that pressures on periosteal surfaces can impede bone formation or induce bone resorption, whereas periosteal tensile strains perpendicular to bone surfaces can impede bone resorption or induce bone formation. We propose that these effects can produce flattened or concave bone surfaces in regions of periosteal pressure and bone ridges in regions of periosteal tension. The model was implemented with computer simulations to illustrate the role of adjacent muscles on the development of the triangular cross-sectional geometry of the rat tibia. The results suggest that intracortical stresses dictate bone size, whereas periosteal pressures may work in combination with intracortical stresses and other mechanobiological factors in the development of local bone cross-sectional shapes.

Osteogenic Differentiation of Human Mesenchymal Stem Cells in Collagen Matrices: Effect of Uniaxial Cyclic Tensile Strain on Bone Morphogenetic Protein (BMP-2) mRNA Expression

Human mesenchymal stem cells (hMSCs) differentiate down an osteogenic pathway with appropriate mechanical and/or chemical stimuli. This study describes the successful culture of hMSCs in 3D collagen matrices under mechanical strain. Bone marrow-derived hMSCs were seeded in linear 3D type I collagen matrices and subjected to 0%, 10%, or 12% uniaxial cyclic tensile strain at 1 Hz for 4 h/day for 7 or 14 days. Cell viability studies indicated that hMSCs remained viable throughout the culture period irrespective of the applied strain level. Real-time RT-PCR studies indicated a significant increase in BMP-2 mRNA expression levels in hMSCs strained at 10% compared to the same day unstrained controls after both 7 and 14 days. An increase in BMP-2 was also observed in hMSCs subjected to 12% strain, but the increase was significant only in the 14-day sample. This is the first report of the culture of bone marrow-derived hMSCs in 3D collagen matrices under cyclic strain, and the first demonstration that strain alone can induce osteogenic differentiation without the addition of osteogenic supplements. Induction of bone differentiation in 3D culture is a critical step in the creation of bioengineered bone constructs.

Craniofacial bone tissue engineering

Repair and reconstruction of the craniofacial skeleton represents a significant biomedical burden, with thousands of procedures per-formed annually secondary to injuries and congenital malformations. Given the multitude of current approaches, the need for more effective strategies to repair these bone deficits is apparent. This article explores two major modalities for craniofacial bone tissue engineering: distraction osteogenesis and cellular based therapies. Current understanding of the guiding principles for each of these modalities is elaborated on along with the knowledge gained from clinical and investigative studies. By laying this foundation, future directions for craniofacial distraction and cell-based bone engineering have emerged with great promise for the advancement of clinical practice.

Rapid Growth of Cartilage Rudiments may Generate Perichondrial Structures by Mechanical Induction

Experimental and theoretical research suggest that mechanical stimuli may play a role in morphogenesis. We investigated whether theoretically predicted patterns of stress and strain generated during the growth of a skeletal condensation are similar to in vivo expression patterns of chondrogenic and osteogenic genes. The analysis showed that predicted patterns of compressive hydrostatic stress (pressure) correspond to the expression patterns of chondrogenic genes, and predicted patterns of tensile strain correspond to the expression patterns of osteogenic genes. Furthermore, the results of iterative application of the analysis suggest that stresses and strains generated by the growing condensation could promote the formation and refinement of stiff tissue surrounding the condensation, a prediction that is in agreement with an observed increase in collagen bundling surrounding the cartilage condensation, as indicated by picro-sirius red staining. These results are consistent with mechanical stimuli playing an inductive or maintenance role in the developing cartilage and associated perichondrium and bone collar. This theoretical analysis provides insight into the potential importance of mechanical stimuli during the growth of skeletogenic condensations.

Guided tissue regeneration enhances bone formation in a rat model of failed osteogenesis

BACKGROUND

Guided tissue regeneration is a technique that uses barrier materials to enhance tissue regeneration. Although previously demonstrated to be an effective way of enhancing craniofacial osteogenesis in several animal models, the ability of guided tissue regeneration to augment bone formation in the context of distraction osteogenesis is unknown. In the current study, the authors applied the principle of guided tissue regeneration to their rat mandibular distraction osteogenesis model in an attempt to enhance bone regeneration.

METHODS

Twelve (n = 6 per group) adult Sprague-Dawley rats underwent routine gradual distraction (5 days' latency, 4-mm distraction over 8 days, 4 to 6 weeks of consolidation) and acute distraction (immediate lengthening to 4 mm, 6 to 8 weeks of consolidation). An additional 10 animals underwent acute distraction followed by application of bioabsorbable Gore Resolut XT membranes (acute distraction plus guided tissue regeneration). Membranes were completely wrapped around the distraction gap. Animals were killed 6 and 8 weeks postoperatively and mandibles analyzed radiographically and histologically.

RESULTS

Quantitative histomorphometric analyses were performed to compare relative bone formation between all three groups. Gradual distraction mandibles achieved bony union by 6 weeks with 86 percent bone formation, which increased to 98 percent by 8 weeks. Acute distraction mandibles healed with a fibrous nonunion and only 37 percent bone formation by 8 weeks. In contrast, acute distraction plus guided tissue regeneration-treated mandibles formed significantly more bone than acute distraction mandibles by 6 weeks (57 percent) and achieved bony bridging by 8 weeks, with 88 percent new bone formation.

CONCLUSION

The authors' data demonstrate that guided tissue regeneration can significantly enhance bone formation in a fibrous nonunion model of mandibular distraction osteogenesis.

Relationships between tissue dilatation and differentiation in distraction osteogenesis

Mechanical factors modulate the morphogenesis and regeneration of mesenchymally derived tissues via processes mediated by the extracellular matrix (ECM). In distraction osteogenesis, large volumes of new bone are created through discrete applications of tensile displacement across an osteotomy gap. Although many studies have characterized the matrix, cellular and molecular biology of distraction osteogenesis, little is known about relationships between these biological phenomena and the local physical cues generated by distraction. Accordingly, the goal of this study was to characterize the local physical environment created within the osteotomy gap during long bone distraction osteogenesis. Using a computational approach, we quantified spatial and temporal profiles of three previously identified mechanical stimuli for tissue differentiation-pressure, tensile strain and fluid flow-as well as another candidate stimulus-tissue dilatation (volumetric strain). Whereas pressure and fluid velocity throughout the regenerate decayed to less than 31% of initial values within 20 min following distraction, tissue dilatation increased with time, reaching steady state values as high as 43% strain. This dilatation created large reductions and large gradients in cell and ECM densities. When combined with previous findings regarding the effects of strain and of cell and ECM densities on cell migration, proliferation and differentiation, these results indicate two mechanisms by which tissue dilatation may be a key stimulus for bone regeneration: (1) stretching of cells and (2) altering cell and ECM densities. These results are used to suggest experiments that can provide a more mechanistic understanding of the role of tissue dilatation in bone regeneration.

Angiogenesis is required for successful bone induction during distraction osteogenesis

The role of angiogenesis during mechanically induced bone formation is incompletely understood. The relationship between the mechanical environment, angiogenesis, and bone formation was determined in a rat distraction osteogenesis model. Disruption of either the mechanical environment or endothelial cell proliferation blocked angiogenesis and bone formation. This study further defines the role of the mechanical environment and angiogenesis during distraction osteogenesis.

INTRODUCTION

Whereas successful fracture repair requires a coordinated and complex transcriptional program that integrates mechanotransductive signaling, angiogenesis, and osteogenesis, the interdependence of these processes is not fully understood. In this study, we use a system of bony regeneration known as mandibular distraction osteogenesis (DO) in which a controlled mechanical stimulus promotes bone induction after an osteotomy and gradual separation of the osteotomy edges to examine the relationship between the mechanical environment, angiogenesis, and osteogenesis.

MATERIALS AND METHODS

Adult Sprague-Dawley rats were treated with gradual distraction, gradual distraction plus the angiogenic inhibitor TNP-470, or acute distraction (a model of failed bony regeneration). Animals were killed at the end of distraction (day 13) or at the end of consolidation (day 41) and examined with muCT, histology, and immunohistochemistry for angiogenesis and bone formation (n = 4 per time-point per group). An additional group of animals (n = 6 per time-point per group) was processed for microarray analysis at days 5, 9, 13, 21, and 41.

RESULTS AND CONCLUSIONS

Either TNP-470 administration or disruption of the mechanical environment prevented normal osteogenesis and resulted in a fibrous nonunion. Subsequent analysis of the regenerate showed an absence of angiogenesis by gross histology and immunohistochemical localization of platelet endothelial cell adhesion molecule in the groups that failed to heal. Microarray analysis revealed distinct patterns of expression of genes associated with osteogenesis, angiogenesis, and hypoxia in each of the three groups. Our findings confirm the interdependence of the mechanical environment, angiogenesis, and osteogenesis during DO, and suggest that induction of proangiogenic genes and the proper mechanical environment are both necessary to support new vasculature for bone induction in DO.