RESUMEN
Considerable progress has been made in understanding implant wear and developing numerical models to predict wear for new orthopaedic devices. However any model of wear could be improved through a more accurate representation of the biomaterial mechanics, including time-varying dynamic and inelastic behaviour such as viscosity and plastic deformation. In particular, most computational models of wear of UHMWPE implement a time-invariant version of Archard's law that links the volume of worn material to the contact pressure between the metal implant and the polymeric tibial insert. During in-vivo conditions, however, the contact area is a time-varying quantity and is therefore dependent upon the dynamic deformation response of the material. From this observation one can conclude that creep deformations of UHMWPE may be very important to consider when conducting computational wear analyses, in stark contrast to what can be found in the literature. In this study, different numerical modelling techniques are compared with experimental creep testing on a unicondylar knee replacement system in a physiologically representative context. Linear elastic, plastic and time-varying visco-dynamic models are benchmarked using literature data to predict contact deformations, pressures and areas. The aim of this study is to elucidate the contributions of viscoelastic and plastic effects on these surface quantities. It is concluded that creep deformations have a significant effect on the contact pressure measured (experiment) and calculated (computational models) at the surface of the UHMWPE unicondylar insert. The use of a purely elastoplastic constitutive model for UHMWPE lead to compressive deformations of the insert which are much smaller than those predicted by a creep-capturing viscoelastic model (and those measured experimentally). This shows again the importance of including creep behaviour into a constitutive model in order to predict the right level of surface deformation on a tibial insert. At high compressive loads, inelastic deformation mechanisms (creep and plasticity) dominate the mechanical response of UHMWPE components by altering the surface geometry (penetration depth and so contact area) and therefore the contact pressure. Although generic creep models can provide a good first approximation of material characteristics, for best accuracy both viscous and plastic effects must be captured, and model parameters must be founded upon specific experimental test data. Such high-fidelity numerical creep models will provide a better foundation for the next generation of more robust and accurate in-silico wear prediction tools.
Asunto(s)
Análisis de Elementos Finitos , Ensayo de Materiales , Polietilenos , Prótesis e Implantes , Elasticidad , Presión , Propiedades de Superficie , ViscosidadRESUMEN
Our group has shown that numerous factors can influence how tissue engineered tendon constructs respond to in vitro mechanical stimulation. Although one study showed that stimulating mesenchymal stem cell (MSC)-collagen sponge constructs significantly increased construct linear stiffness and repair biomechanics, a second study showed no such effect when a collagen gel replaced the sponge. While these results suggest that scaffold material impacts the response of MSCs to mechanical stimulation, a well-designed intra-animal study was needed to directly compare the effects of type-I collagen gel versus type-I collagen sponge in regulating MSC response to a mechanical stimulus. Eight constructs from each cell line (n=8 cell lines) were created in specially designed silicone dishes. Four constructs were created by seeding MSCs on a type-I bovine collagen sponge, and the other four were formed by seeding MSCs in a purified bovine collagen gel. In each dish, two cell-sponge and two cell-gel constructs from each line were then mechanically stimulated once every 5 min to a peak strain of 2.4%, for 8 h/day for 2 weeks. The other dish remained in an incubator without stimulation for 2 weeks. After 14 days, all constructs were failed to determine mechanical properties. Mechanical stimulation significantly improved the linear stiffness (0.048+/-0.009 versus 0.015+/-0.004; mean+/-SEM (standard error of the mean ) N/mm) and linear modulus (0.016+/-0.004 versus 0.005+/-0.001; mean+/-SEM MPa) of cell-sponge constructs. However, the same stimulus produced no such improvement in cell-gel construct properties. These results confirm that collagen sponge rather than collagen gel facilitates how cells respond to a mechanical stimulus and may be the scaffold of choice in mechanical stimulation studies to produce functional tissue engineered structures.
Asunto(s)
Tendones , Resistencia a la Tracción/fisiología , Ingeniería de Tejidos/métodos , Andamios del Tejido , Animales , Materiales Biocompatibles/química , Materiales Biocompatibles/metabolismo , Células Cultivadas , Colágeno Tipo I/química , Elasticidad , Femenino , Geles/química , Trasplante de Células Madre Mesenquimatosas , Células Madre Mesenquimatosas/química , Células Madre Mesenquimatosas/citología , Células Madre Mesenquimatosas/metabolismo , Conejos , Estrés Mecánico , Tendones/química , Tendones/citología , Tendones/metabolismo , Ingeniería de Tejidos/instrumentación , TransductoresRESUMEN
Abnormal joint kinematics and loads induced after soft tissue injuries are assumed to contribute to long-term degenerative joint disease and osteoarthritis. Controlling abnormal kinematics after repair and reconstruction of these injured structures would seem to be important for limiting wear of the articular cartilage surfaces. In this paper, we propose to expand the paradigm of functional tissue engineering to more fully characterize normal joint function and to establish design parameters for soft tissue repair and reconstruction to ultimately protect joint surfaces after surgery. Structure-function relationships are examined for tissues of increasing complexity, from tendons to menisci. Emphasis is placed on understanding normal in vivo function of tissues by conducting biomechanical experiments in vitro that better mimic in vivo conditions. This process yields nine classes of functional tissue engineering parameters: differential fiber length, in vivo force and displacement, variations in relative attachment site locations, loading from adjacent structures, fiber interactions, types of insertion, regional variations in material properties, nonparallel fiber orientations, and complex loading within the structure. These functional tissue engineering parameters are useful not only for understanding the function of normal tissues but for more effectively designing their repair and replacement. This paper concludes with a discussion of research directions that investigators might take to establish tissue-specific functional tissue engineering parameters for improving joint function and reducing articular surface degradation and osteoarthritis.
Asunto(s)
Osteoartritis/cirugía , Ingeniería de Tejidos/métodos , Humanos , Ligamentos Articulares , TendonesRESUMEN
Despite various attempts to repair and replace injured tendon, an understanding of the repair processes and a systematic approach to achieving functional efficacy remain elusive. In this review the epidemiology of tendon injury and repair is first examined. Using a traditional paradigm for repair assessment, the biology and biomechanics of normal tendon, natural healing, and repair are then explored. New treatment strategies such as functional tissue engineering are discussed, including a functional approach to treatment that involves the development of in vivo functional design parameters to judge the acceptability of a repair outcome. The paper concludes with future directions.
Asunto(s)
Fenómenos Biomecánicos/métodos , Tendones/patología , Ingeniería de Tejidos/métodos , Cicatrización de Heridas , Humanos , Modelos Biológicos , Traumatismos de los TendonesRESUMEN
Collagen gels were seeded with rabbit bone marrow-derived mesenchymal stem cells (MSCs) and contracted onto sutures at initial cell densities of 1, 4, and 8 million cells/ml. These MSC-collagen composites were then implanted into full thickness, full length, central defects created in the patellar tendons of the animals providing the cells. These autologous repairs were compared to natural repair of identical defects on the contralateral side. Biomechanical, histological, and morphometric analyses were performed on both repair tissue types at 6, 12, and 26 weeks after surgery. Repair tissues containing the MSC-collagen composites showed significantly higher maximum stresses and moduli than natural repair tissues at 12 and 26 weeks postsurgery. However, no significant differences were observed in any dimensional or mechanical properties of the repair tissues across seeding densities at each evaluation time. By 26 weeks, the repairs grafted with MSC-collagen composites were one-fourth of the maximum stress of the normal central portion of the patellar tendon with bone ends. The modulus and maximum stress of the repair tissues grafted with MSC-collagen composites increased at significantly faster rates than did natural repairs over time. Unexpectedly, 28% of the MSC-collagen grafted tendons formed bone in the regenerating repair site. Except for increased repair tissue volume, no significant differences in cellular organization or histological appearance were observed between the natural repairs and MSC-collagen grafted repairs. Overall, these results show that surgically implanting tissue engineered MSC-collagen composites significantly improves the biomechanical properties of tendon repair tissues, although greater MSC concentrations produced no additional significant histological or biomechanical improvement.