Dominique P. Pioletti

Viscoelastic constitutive law in large deformations: application to human knee ligaments and tendons

Traction tests on soft tissues show that the shape of the stress strain curves depends on the strain rate at which the tests are performed. Many of the constitutive models that have been proposed fail to properly consider the effect of the strain rate when large deformations are encountered. In the present study, a framework based on elastic and viscous potentials is developed. The resulting constitutive law is valid for large deformations and satisfies the principles of thermodynamics. Three parameters -- two for the elasticity and one for the viscosity -- were enough to precisely fit the non-linear stress strain curves obtained at different strain rates with human cruciate ligaments and patellar tendons. The identification results then in a realistic, three-dimensional viscoelastic constitutive law. The developed constitutive law can be used regardless of the strain or rotation values. It can be incorporated into a finite element program to model the viscoelastic behavior of ligaments and tendons under dynamic situations.

Bone regeneration and stem cells

•  Introduction •  Bone fracture healing and healing problems •  Biomaterial scaffolds and tissue engineering in bone formation ‐  Bone tissue engineering ‐  Biomaterial scaffolds ‐  Synthetic scaffolds ‐  Micro‐ and nanostructural properties of scaffolds ‐  Conclusion •  Mesenchymal stem cells and osteogenesis ‐  Bone tissue ‐  Origin of osteoblasts ‐  Isolation and characterization of bone marrow derived MSC ‐  In vitro differentiation of MSC into osteoblast lineage cells ‐  In vivo differentiation of MSC into bone ‐  Factors and pathways controlling osteoblast differentiation of hMSC ‐  Defining the relationship between osteoblast and adipocyte differentiation from MSC ‐  MSC and sex hormones ‐  Effect of aging on osteoblastogenesis ‐  Conclusion •  Embryonic, foetal and adult stem cells in osteogenesis ‐  Cell‐based therapies for bone ‐  Specific features of bone cells needed to be advantageous for clinical use ‐  Development of therapeutic biological agents ‐  Clinical application concerns ‐  Conclusion •  Platelet‐rich plasma (PRP), growth factors and osteogenesis ‐  PRP effects in vitro on the cells involved in bone repair ‐  PRP effects on osteoblasts ‐  PRP effects on osteoclasts ‐  PRP effects on endothelial cells ‐  PRP effects in vivo on experimental animals ‐  The clinical use of PRP for bone repair ‐  Non‐union ‐  Distraction osteogenesis ‐  Spinal fusion ‐  Foot and ankle surgery ‐  Total knee arthroplasty ‐  Odontostomatology and maxillofacial surgery ‐  Conclusion •  Molecular control of osteogenesis ‐  TGF‐β signalling ‐  FGF signalling ‐  IGF signalling ‐  PDGF signalling ‐  MAPK signalling pathway ‐  Wnt signalling pathway ‐  Hedgehog signalling ‐  Notch signalling ‐  Ephrin signalling ‐  Transcription factors regulating osteoblast differentiation ‐  Conclusion •  Summary

Technical NoteViscoelastic constitutive law in large deformations: application to human knee ligaments and tendons

Traction tests on soft tissues show that the shape of the stress strain curves depends on the strain rate at which the tests are performed. Many of the constitutive models that have been proposed fail to properly consider the effect of the strain rate when large deformations are encountered. In the present study, a framework based on elastic and viscous potentials is developed. The resulting constitutive law is valid for large deformations and satisfies the principles of thermodynamics. Three parameters -- two for the elasticity and one for the viscosity -- were enough to precisely fit the non-linear stress strain curves obtained at different strain rates with human cruciate ligaments and patellar tendons. The identification results then in a realistic, three-dimensional viscoelastic constitutive law. The developed constitutive law can be used regardless of the strain or rotation values. It can be incorporated into a finite element program to model the viscoelastic behavior of ligaments and tendons under dynamic situations.

The cytotoxic effect of titanium particles phagocytosed by osteoblasts

The cytotoxic effect of different concentrations of titanium particles on osteoblasts was studied in vitro. It was found that the viability of the osteoblasts was inversely proportional to the particle concentration. Phagocytosis of particles by the osteoblasts was evident and was demonstrated to be responsible for cell necrosis. Moreover, during and after phagocytosis, the osteoblasts released products that were cytotoxic for other osteoblasts, as established with a conditioned medium assay. The titanium particles thus had both a direct and an indirect effect on osteoblast viability. It also was observed that the titanium particles induced a process of programmed cell death (apoptosis) when co-cultured with osteoblasts. The results of this study suggest that not only is the amount of wear debris generated important, but the local accumulation of the debris also may have a significant impact on bone cell function.

The effects of calcium phosphate cement particles on osteoblast functions

Calcium phosphate cements (CPC) are increasingly used in the orthopedic field. This kind of cement has potential applications in bone defect replacements, osteosynthetic screw reinforcements or drug delivery. In vivo studies have demonstrated a good osteointegration of CPC. However, it was also observed that the resorption of CPC could create particles. It is known from orthopedic implant studies that particles can be responsible for the peri-implant osteolysis. Biocompatibility assessment of CPC should then be performed with particles. In this study, we quantified the functions of osteoblasts in the presence of beta-TCP, brushite and cement particles. Two particle sizes were prepared. The first one corresponded to the critical diameter range 1-10 microm and the second one had a diameter larger than 10 microm. We found that CPC particles could adversely affect the osteoblast functions. A decrease in viability, proliferation and production of extracellular matrix was measured. A dose effect was also observed. A ratio of 50 CPC particles per osteoblast could be considered as the maximum number of particles supported by an osteoblast. The smaller particles had stronger negative effects on osteoblast functions than the larger ones. Future CPC development should minimize the generation of particles smaller than 10 microm.

Effect of different Ti-6Al-4V surface treatments on osteoblasts behaviour

The purpose of the present work was to examine the effect of different Ti-6Al-4V surface treatments on osteoblasts behaviour. Previous work in this laboratory has demonstrated that an ageing treatment reduces metal ion release from this alloy compared to standard passivation procedures. In this study. human osteosarcoma MG-63 were used in short-term in vitro tests to assay for cell viability and cell proliferation at 12, 24 and 72 h while SaOS-2 were used in long-term in vitro tests to assay for osteonectin, osteopontin, osteocalcin gene expression, total protein amount (TP). alkaline phosphatase activity (ALP) and fibronectin production (FN) for 1-4 weeks. Epifluorescence microscopy was used to observe SaOS-2 cell morphology. After 24h, there was no difference in MG-63 cell viability proliferation or in SaOS-2 cell morphology between the different surface treatments. For the long-term tests, the aged Ti-6Al4V induced significantly higher cell proliferation than the control Ti-6Al-4V at 72h. At week 1, no difference in the osteonectin, osteopontin, and osteocalcin gene expression was found between samples. The peak of ALP activity appeared earlier at week 2 for the control surface compared with the passivated and aged surfaces. The early increase in ALP activity for the control sample could be a compensatory effect of decreased osteoblasts proliferation. There was no difference in the expression of FN for the different surface treatments. Our present results showed that the different surface treatments, which induced different metal ion release kinetics and surface properties, influenced the cell proliferation and ALP activity of osteoblast cells. Aluminium ions release kinetics as well as presence of vanadium ions may play a major role in influencing the osteoblasts behaviour in the present study.

Non-linear viscoelastic laws for soft biological tissues

The development of a conceptual framework to test different viscoelastic constitutive laws is presented. This framework has the advantage of satisfying a priori the thermodynamic restrictions and is valid for large deformations. In addition, the different mechanical contributions are separated according to the time scale of their effects. As an illustration of its ability to model the immediate, short time memory and long time memory contributions, the framework is used to identify mechanical tests performed on human patellar tendons. The resulting viscoelastic law is then proposed to model the soft biological tissues as these tissues present an important viscoelastic behaviour.

Human fetal bone cells associated with ceramic reinforced PLA scaffolds for tissue engineering

Fetal bone cells were shown to have an interesting potential for therapeutic use in bone tissue engineering due to their rapid growth rate and their ability to differentiate into mature osteoblasts in vitro. We describe hereafter their capability to promote bone repair in vivo when combined with porous scaffolds based on poly(l-lactic acid) (PLA) obtained by supercritical gas foaming and reinforced with 5 wt.% beta-tricalcium phosphate (TCP). Bone regeneration was assessed by radiography and histology after implantation of PLA/TCP scaffolds alone, seeded with primary fetal bone cells, or coated with demineralized bone matrix. Craniotomy critical size defects and drill defects in the femoral condyle in rats were employed. In the cranial defects, polymer degradation and cortical bone regeneration were studied up to 12 months postoperatively. Complete bone ingrowth was observed after implantation of PLA/TCP constructs seeded with human fetal bone cells. Further tests were conducted in the trabecular neighborhood of femoral condyles, where scaffolds seeded with fetal bone cells also promoted bone repair. We present here a promising approach for bone tissue engineering using human primary fetal bone cells in combination with porous PLA/TCP structures. Fetal bone cells could be selected regarding osteogenic and immune-related properties, along with their rapid growth, ease of cell banking and associated safety.

On the independence of time and strain effects in the stress relaxation of ligaments and tendons

The hypothesis of variables separation, namely the time and the strain separation in the relaxation function, is widely used in soft tissue biomechanics. Although this hypothesis is central to several biomechanical models, only few experimental works have tried to verify it. From these studies, contradictory results have been found. Moreover, it has recently been noted that no such experimental verification has been performed for ligament tissues. In this paper, an experimental method is developed to test the hypothesis of variables separation. This method is then used with human cruciate ligaments and patellar tendons. It is shown that the use of the variables separation hypothesis is justified at least for strain values lower than 16% for anterior cruciate ligament, lower than 12% for posterior cruciate ligament and lower than 6% for patellar tendon. The method presented in this paper could be used to verify the validity of variables separation for other tissues.

Nanofibrillated cellulose composite hydrogel for the replacement of the nucleus pulposus

The swelling and compressive mechanical behavior as well as the morphology and biocompatibility of composite hydrogels based on Tween® 20 trimethacrylate (T3), N-vinyl-2-pyrrolidone (NVP) and nanofibrillated cellulose (NFC) were assessed in the present study. The chemical structure of T3 was verified by Fourier transform infrared spectroscopy and proton nuclear magnetic resonance, and the degree of substitution was found to be around 3. Swelling ratios of neat hydrogels composed of different concentrations of T3 and NVP were found to range from 1.5 to 5.7 with decreasing concentration of T3. Various concentrations of cellulose nanofibrils (0.2-1.6wt.%) were then used to produce composite hydrogels that showed lower swelling ratios than neat ones for a given T3 concentration. Neat and composite hydrogels exhibited a typical nonlinear response under compression. All composite hydrogels showed an increase in elastic modulus compared to neat hydrogel of about 3- to 8-fold, reaching 18kPa at 0% strain and 62kPa at 20% strain for the hydrogel with the highest NFC content. All hydrogels presented a porous and homogeneous structure, with interconnected pore cells of around 100nm in diameter. The hydrogels are biocompatible. The results of this study demonstrate that composite hydrogels reinforced with NFC may be viable as nucleus pulposus implants due to their adequate swelling ratio, which may restore the annulus fibrosus loading, and their increased mechanical properties, which could possibly restore the height of the intervertebral discs.