Publications by authors named "Victor Birman"

16 Publications

  • Page 1 of 1

Adhesive-based tendon-to-bone repair: failure modelling and materials selection.

J R Soc Interface 2019 04;16(153):20180838

6 NSF Science and Technology Center for Engineering Mechanobiology, Department of Mechanical and Aerospace Engineering, Washington University , St Louis, MO 63130 , USA.

Surgical reattachment of tendon to bone is a procedure marked by high failure rates. For example, nearly all rotator cuff repairs performed on elderly patients with massive tears ultimately result in recurrence of tearing. These high failure rates have been attributed to stress concentrations that arise due to the mechanical mismatch between tendon and bone. Although recent studies have identified potential adhesives with mechanical properties tuned to alleviate these stress concentrations, and thereby delay the onset of failure, resistance to the progression of failure has not been studied. Here, we refined the space of adhesive material properties that can improve surgical attachment by considering the fracture process. Using cohesive zone modelling and physiologically relevant values of mode I and mode II adhesive fracture toughnesses, we predicted the maximum displacement and strength at failure of idealized, adhesively bonded tendon-to-bone repairs. Repair failure occurred due to excessive relative displacement of the tendon and bone tissues for strong and compliant adhesives. The failure mechanism shifted to rupture of the entire repair for stiffer adhesives below a critical shear strength. Results identified a narrow range of materials on an Ashby chart that are suitable for adhesive repair of tendon to bone, including a range of elastomers and porous solids.
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http://dx.doi.org/10.1098/rsif.2018.0838DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6505561PMC
April 2019

Enhanced tendon-to-bone repair through adhesive films.

Acta Biomater 2018 04 8;70:165-176. Epub 2018 Feb 8.

Department of Orthopedic Surgery, Columbia University, New York, NY 10032, United States; Department of Biomedical Engineering, Columbia University, New York, NY 10027, United States. Electronic address:

Tendon-to-bone surgical repairs have unacceptably high failure rates, possibly due to their inability to recreate the load transfer mechanisms of the native enthesis. Instead of distributing load across a wide attachment footprint area, surgical repairs concentrate shear stress on a small number of suture anchor points. This motivates development of technologies that distribute shear stresses away from suture anchors and across the enthesis footprint. Here, we present predictions and proof-of-concept experiments showing that mechanically-optimized adhesive films can mimic the natural load transfer mechanisms of the healthy attachment and increase the load tolerance of a repair. Mechanical optimization, based upon a shear lag model corroborated by a finite element analysis, revealed that adhesives with relatively high strength and low stiffness can, theoretically, strengthen tendon-to-bone repairs by over 10-fold. Lap shear testing using tendon and bone planks validated the mechanical models for a range of adhesive stiffnesses and strengths. Ex vivo human supraspinatus repairs of cadaveric tissues using multipartite adhesives showed substantial increase in strength. Results suggest that adhesive-enhanced repair can improve repair strength, and motivate a search for optimal adhesives.

Statement Of Significance: Current surgical techniques for tendon-to-bone repair have unacceptably high failure rates, indicating that the initial repair strength is insufficient to prevent gapping or rupture. In the rotator cuff, repair techniques apply compression over the repair interface to achieve contact healing between tendon and bone, but transfer almost all force in shear across only a few points where sutures puncture the tendon. Therefore, we evaluated the ability of an adhesive film, implanted between tendon and bone, to enhance repair strength and minimize the likelihood of rupture. Mechanical models demonstrated that optimally designed adhesives would improve repair strength by over 10-fold. Experiments using idealized and clinically-relevant repairs validated these models. This work demonstrates an opportunity to dramatically improve tendon-to-bone repair strength using adhesive films with appropriate material properties.
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http://dx.doi.org/10.1016/j.actbio.2018.01.032DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5871607PMC
April 2018

Toughening of fibrous scaffolds by mobile mineral deposits.

Acta Biomater 2017 08 19;58:492-501. Epub 2017 May 19.

Department of Orthopedic Surgery, Columbia University, New York, NY, USA; Department of Biomedical Engineering, Columbia University, New York, NY, USA. Electronic address:

Partially mineralized fibrous tissue situated between tendon and bone is believed to be tougher than either tendon or bone, possibly serving as a compliant, energy absorptive, protective barrier between the two. This tissue does not reform following surgical repair (e.g., rotator cuff tendon-to-bone re-attachment) and might be a factor in the poor outcomes following such surgeries. Towards our long-term goal of tissue engineered solutions to functional tendon-to-bone re-attachment, we tested the hypotheses that partially mineralized fibrous matrices can derive toughness from mobility of mineral along their fibers, and that in such cases toughness is maximized at levels of mineralization sufficiently low to allow substantial mobility. Nanofibrous electrospun poly(lactic-co-glycolic acid) (PLGA) scaffolds mineralized for prescribed times were fabricated as model systems to test these hypotheses. Tensile tests performed at varying angles relative to the dominant fiber direction confirmed that mineral cross-linked PLGA nanofibers without adhering to them. Peel tests revealed that fracture toughness increased with mineralization time up to a peak value, then subsequently decreased with increasing mineralization time back to the baseline toughness of unmineralized scaffolds. These experimental results were predicted by a theoretical model combining mineral growth kinetics with fracture energetics, suggesting that toughness increased with mineralization time until mineral mobility was attenuated by steric hindrance, then returned to baseline levels following the rigid percolation threshold. Results supported our hypotheses, and motivate further study of the roles of mobile mineral particles in toughening the tendon-to-bone attachment.

Statement Of Significance: Effective surgical repair of interfaces between tendon and bone remains an unmet clinical need, in part due to a lack of understanding of how toughness is achieved in the healthy tissue. Using combined synthesis, experiment, and modeling approaches, the current work supported the hypothesis that toughening of a fibrous scaffold arises from brittle mineral particles that crosslink the fibers, but only if the particles are free to slide relative to the fibers. In the case of the tendon-to-bone interface, this suggests that partially mineralized tissue between tendon and bone, with mobile mineral but relatively low stiffness, may serve as a compliant, energy-absorbing barrier that guards against injury. These results suggest an opportunity for fabrication of tough and strong fibrous scaffolds for tissue engineering applications.
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http://dx.doi.org/10.1016/j.actbio.2017.05.033DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5537019PMC
August 2017

Micro-mechanical properties of the tendon-to-bone attachment.

Acta Biomater 2017 07 11;56:25-35. Epub 2017 Jan 11.

School of Engineering, University of Portsmouth, Portsmouth, UK. Electronic address:

The tendon-to-bone attachment (enthesis) is a complex hierarchical tissue that connects stiff bone to compliant tendon. The attachment site at the micrometer scale exhibits gradients in mineral content and collagen orientation, which likely act to minimize stress concentrations. The physiological micromechanics of the attachment thus define resultant performance, but difficulties in sample preparation and mechanical testing at this scale have restricted understanding of structure-mechanical function. Here, microscale beams from entheses of wild type mice and mice with mineral defects were prepared using cryo-focused ion beam milling and pulled to failure using a modified atomic force microscopy system. Micromechanical behavior of tendon-to-bone structures, including elastic modulus, strength, resilience, and toughness, were obtained. Results demonstrated considerably higher mechanical performance at the micrometer length scale compared to the millimeter tissue length scale, describing enthesis material properties without the influence of higher order structural effects such as defects. Micromechanical investigation revealed a decrease in strength in entheses with mineral defects. To further examine structure-mechanical function relationships, local deformation behavior along the tendon-to-bone attachment was determined using local image correlation. A high compliance zone near the mineralized gradient of the attachment was clearly identified and highlighted the lack of correlation between mineral distribution and strain on the low-mineral end of the attachment. This compliant region is proposed to act as an energy absorbing component, limiting catastrophic failure within the tendon-to-bone attachment through higher local deformation. This understanding of tendon-to-bone micromechanics demonstrates the critical role of micrometer scale features in the mechanics of the tissue.

Statement Of Significance: The tendon-to-bone attachment (enthesis) is a complex hierarchical tissue with features at a numerous scales that dissipate stress concentrations between compliant tendon and stiff bone. At the micrometer scale, the enthesis exhibits gradients in collagen and mineral composition and organization. However, the physiological mechanics of the enthesis at this scale remained unknown due to difficulty in preparing and testing micrometer scale samples. This study is the first to measure the tensile mechanical properties of the enthesis at the micrometer scale. Results demonstrated considerably enhanced mechanical performance at the micrometer length scale compared to the millimeter tissue length scale and identified a high-compliance zone near the mineralized gradient of the attachment. This understanding of tendon-to-bone micromechanics demonstrates the critical role of micrometer scale features in the mechanics of the tissue.
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http://dx.doi.org/10.1016/j.actbio.2017.01.037DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5575850PMC
July 2017

The concentration of stress at the rotator cuff tendon-to-bone attachment site is conserved across species.

J Mech Behav Biomed Mater 2016 09 23;62:24-32. Epub 2016 Apr 23.

Department of Mechanical Engineering & Materials Science, Washington University, St. Louis, MO 63130, USA. Electronic address:

The tendon-to-bone attachment site integrates two distinct tissues via a gradual transition in composition, mechanical properties, and structure. Outcomes of surgical repair are poor, in part because surgical repair does not recreate the natural attachment, and in part because the mechanical features that are most critical to mechanical and physiological functions have not been identified. We employed allometric analysis to resolve a paradox about how the architecture of the rotator cuff contributes to load transfer: whereas published data suggest that the mean muscle stresses expected at the tendon-to-bone attachment are conserved across species, data also show that the relative dimensions of key anatomical features vary dramatically, suggesting that the amplification of stresses at the interface between tendon and bone should also vary widely. However, a mechanical model that enabled a sensitivity analysis revealed that the degree of stress concentration was in fact highly conserved across species: the factors that most affected stress amplification were most highly conserved across species, while those that had a lower effect showed broad variation across a range of relative insensitivity. Results highlight how micromechanical factors can influence structure-function relationships and cross-species scaling over several orders of magnitude in animal size, and provide guidance on physiological features to emphasize in surgical and tissue engineered repair of the rotator cuff.
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http://dx.doi.org/10.1016/j.jmbbm.2016.04.025DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4955721PMC
September 2016

Effective elastic properties of a composite containing multiple types of anisotropic ellipsoidal inclusions, with the application to the attachment of tendon to bone.

J Mech Phys Solids 2015 Sep;82:367-377

Department of Mechanical Engineering & Materials Science, Washington University, St. Louis, MO 63130, USA.

Estimates of the effective stiffness of a composite containing multiple types of inclusions are needed for the design and study of functionally graded systems in engineering and physiology. While excellent estimates and tight bounds exist for composite systems containing specific classes and distributions of identical inclusions, these are not easily generalized to complex systems with multiple types of inclusions. For example, three-point parameters are known for only a few inclusion shapes and orientations. The best estimate available for a composite containing multiple classes of inclusions arises from the Kanaun-Jeulin approach. However, this method is analogous to a generalized Benveniste approach, and therefore suffers from the same limitations: while excellent for low volume fractions of inclusions, the Kanaun-Jeullin and Benveniste estimates lie outside of three-point bounds at higher volume fractions. Here, we present an estimate for composites containing multiple classes of aligned ellipsoidal inclusions that lies within known three-point bounds at relatively higher volume fractions of inclusions and that is applicable to many engineering and biological composites.
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http://dx.doi.org/10.1016/j.jmps.2015.05.017DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4785846PMC
September 2015

Shear lag sutures: Improved suture repair through the use of adhesives.

Acta Biomater 2015 Sep 25;23:229-239. Epub 2015 May 25.

Department of Orthopaedic Surgery, Washington University, St Louis, MO 63110, United States; Department of Biomedical Engineering, Washington University, St Louis, MO 63130, United States; Department of Mechanical Engineering and Materials Science, Washington University, St Louis, MO 63130, United States. Electronic address:

Suture materials and surgical knot tying techniques have improved dramatically since their first use over five millennia ago. However, the approach remains limited by the ability of the suture to transfer load to tissue at suture anchor points. Here, we predict that adhesive-coated sutures can improve mechanical load transfer beyond the range of performance of existing suture methods, thereby strengthening repairs and decreasing the risk of failure. The mechanical properties of suitable adhesives were identified using a shear lag model. Examination of the design space for an optimal adhesive demonstrated requirements for strong adhesion and low stiffness to maximize the strength of the adhesive-coated suture repair construct. To experimentally assess the model, we evaluated single strands of sutures coated with highly flexible cyanoacrylates (Loctite 4903 and 4902), cyanoacrylate (Loctite QuickTite Instant Adhesive Gel), rubber cement, rubber/gasket adhesive (1300 Scotch-Weld Neoprene High Performance Rubber & Gasket Adhesive), an albumin-glutaraldehyde adhesive (BioGlue), or poly(dopamine). As a clinically relevant proof-of-concept, cyanoacrylate-coated sutures were then used to perform a clinically relevant flexor digitorum tendon repair in cadaver tissue. The repair performed with adhesive-coated suture had significantly higher strength compared to the standard repair without adhesive. Notably, cyanoacrylate provides strong adhesion with high stiffness and brittle behavior, and is therefore not an ideal adhesive for enhancing suture repair. Nevertheless, the improvement in repair properties in a clinically relevant setting, even using a non-ideal adhesive, demonstrates the potential for the proposed approach to improve outcomes for treatments requiring suture fixation. Further study is necessary to develop a strongly adherent, compliant adhesive within the optimal design space described by the model.
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http://dx.doi.org/10.1016/j.actbio.2015.05.002DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4522236PMC
September 2015

Nonlocal Effect on Stiffness Measurements of a Collagen Molecule.

Authors:
Victor Birman

J Appl Mech 2015 Mar;82(3):0345021-345023

Fellow ASME Engineering Education Center, Missouri University of Science and Technology , 12837 Flushing Meadows Dr., Suite 210 , St. Louis, MO 63131 e-mail: 

Accurate modeling of collagen molecules including their stiffness is essential for our understanding of mechanics of collagen fibers and tissues where these fibers play a prominent role. Studies of mechanical properties of collagen molecules employing various experimental methods and molecular dynamics (MD) simulations yield a broad range of values of the modulus of elasticity. The effect of nonlocal elasticity on the molecule stiffness derived from experiments and simulations is assessed in this brief. The estimate of the correction accounting for the nonlocal effect utilizes the exact solution of the nonlocal elasticity theory for one-dimensional elastic bars. It is demonstrated that the effect of nonlocal elasticity on the stiffness of collagen molecules can be neglected.
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http://dx.doi.org/10.1115/1.4029607DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4321113PMC
March 2015

Stochastic interdigitation as a toughening mechanism at the interface between tendon and bone.

Biophys J 2015 Jan;108(2):431-7

Department of Mechanical Engineering and Materials Science, Washington University, St. Louis, Missouri. Electronic address:

Reattachment and healing of tendon to bone poses a persistent clinical challenge and often results in poor outcomes, in part because the mechanisms that imbue the uninjured tendon-to-bone attachment with toughness are not known. One feature of typical tendon-to-bone surgical repairs is direct attachment of tendon to smooth bone. The native tendon-to-bone attachment, however, presents a rough mineralized interface that might serve an important role in stress transfer between tendon and bone. In this study, we examined the effects of interfacial roughness and interdigital stochasticity on the strength and toughness of a bimaterial interface. Closed form linear approximations of the amplification of stresses at the rough interface were derived and applied in a two-dimensional unit-cell model. Results demonstrated that roughness may serve to increase the toughness of the tendon-to-bone insertion site at the expense of its strength. Results further suggested that the natural tendon-to-bone attachment presents roughness for which the gain in toughness outweighs the loss in strength. More generally, our results suggest a pathway for stochasticity to improve surgical reattachment strategies and structural engineering attachments.
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http://dx.doi.org/10.1016/j.bpj.2014.09.049DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4302197PMC
January 2015

Stress amplification during development of the tendon-to-bone attachment.

Biomech Model Mechanobiol 2014 Oct 27;13(5):973-83. Epub 2013 Dec 27.

Department of Mechanical Engineering and Materials Science, Washington University, St. Louis, MO, 63130, USA.

Mechanical stress is necessary to sustain the mineral content of bone in adults. However, in a developing neonatal mouse, the mineralization of soft tissues progresses despite greatly reduced average mechanical stresses. In adults, these reduced loads would likely lead to bone loss. Although biochemical factors may partly explain these different responses, it is unclear how mineralization is initiated in low load environments. We present here the effect of morphometric data and initial modeling supporting a hypothesis that mechanical factors across several length scales amplify stresses, and we suggest that these stresses are of a level adequate to contribute to mechanical signaling for initiation of mineralization at the developing tendon-to-bone enthesis. A mineral gradient is evident across the insertion from the onset of mineralization. This grading maintains a constant size from early postnatal time points to adulthood. At the tissue level, this grading contributes to reduced stresses in an adult animal and to a minor elevation of stresses in a neonatal animal. At the cellular level, stress concentrations around mineralizing chondrocytes are enhanced in neonatal animals compared with adult animals. The enhancement of stresses around cells at early time points may serve to amplify and transduce low loads in order to initiate mineralization.
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http://dx.doi.org/10.1007/s10237-013-0548-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4074273PMC
October 2014

Modelling the mechanics of partially mineralized collagen fibrils, fibres and tissue.

J R Soc Interface 2014 Mar 18;11(92):20130835. Epub 2013 Dec 18.

Department of Mechanical Engineering and Materials Science, Washington University, , St Louis, MO 63130, USA.

Progressive stiffening of collagen tissue by bioapatite mineral is important physiologically, but the details of this stiffening are uncertain. Unresolved questions about the details of the accommodation of bioapatite within and upon collagen's hierarchical structure have posed a central hurdle, but recent microscopy data resolve several major questions. These data suggest how collagen accommodates bioapatite at the lowest relevant hierarchical level (collagen fibrils), and suggest several possibilities for the progressive accommodation of bioapatite at higher hierarchical length scales (fibres and tissue). We developed approximations for the stiffening of collagen across spatial hierarchies based upon these data, and connected models across hierarchies levels to estimate mineralization-dependent tissue-level mechanics. In the five possible sequences of mineralization studied, percolation of the bioapatite phase proved to be an important determinant of the degree of stiffening by bioapatite. The models were applied to study one important instance of partially mineralized tissue, which occurs at the attachment of tendon to bone. All sequences of mineralization considered reproduced experimental observations of a region of tissue between tendon and bone that is more compliant than either tendon or bone, but the size and nature of this region depended strongly upon the sequence of mineralization. These models and observations have implications for engineered tissue scaffolds at the attachment of tendon to bone, bone development and graded biomimetic attachment of dissimilar hierarchical materials in general.
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http://dx.doi.org/10.1098/rsif.2013.0835DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3899858PMC
March 2014

Mechanisms of Bimaterial Attachment at the Interface of Tendon to Bone.

J Eng Mater Technol 2011 Jan;133(1)

Department of Mechanical, Aerospace, and Structural Engineering, Washington University, St. Louis, MO 63130, USA.

The material mismatch at the attachment of tendon to bone is amongst the most severe for any tensile connection in nature. Attaching dissimilar materials is a major challenge in engineering, and has proven to be a challenge in surgical practice as well. Here, we examine the material attachment schemes employed at this connection through the lens of solid mechanics. We identify four strategies to that the body adopts to achieve effective load transfer between tendon and bone: 1) a shallow attachment angle at the insertion of transitional tissue and bone, 2) shaping of gross tissue morphology of the transitional tissue, 3) interdigitation of bone with the transitional tissue, and 4) functional grading of transitional tissue between tendon and bone. We provide solutions to model problems that highlight the first two mechanisms, discuss the third qualitatively in the context of engineering practice, and provide a review of our earlier work on the fourth. We study these strategies both in terms of ways that biomimetic attachment might benefit engineering practice, and of ways that engineering experience might serve to improve surgical healing outcomes.
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http://dx.doi.org/10.1115/1.4002641DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3129865PMC
January 2011

Fibrocartilage tissue engineering: the role of the stress environment on cell morphology and matrix expression.

Tissue Eng Part A 2011 Apr 9;17(7-8):1039-53. Epub 2011 Jan 9.

Department of Orthopaedic Surgery, Washington University, St. Louis, Missouri 63110, USA.

Although much is known about the effects of uniaxial mechanical loading on fibrocartilage development, the stress fields to which fibrocartilaginous regions are subjected to during development are mutiaxial. That fibrocartilage develops at tendon-to-bone attachments and in compressive regions of tendons is well established. However, the three-dimensional (3D) nature of the stresses needed for the development of fibrocartilage is not known. Here, we developed and applied an in vitro system to determine whether fibrocartilage can develop under a state of periodic hydrostatic tension in which only a single principal component of stress is compressive. This question is vital to efforts to mechanically guide morphogenesis and matrix expression in engineered tissue replacements. Mesenchymal stromal cells in a 3D culture were exposed to compressive and tensile stresses as a result of an external tensile hydrostatic stress field. The stress field was characterized through mechanical modeling. Tensile cyclic stresses promoted spindle-shaped cells, upregulation of scleraxis and type one collagen, and cell alignment with the direction of tension. Cells experiencing a single compressive stress component exhibited rounded cell morphology and random cell orientation. No difference in mRNA expression of the genes Sox9 and aggrecan was observed when comparing tensile and compressive regions unless the medium was supplemented with the chondrogenic factor transforming growth factor beta3. In that case, Sox9 was upregulated under static loading conditions and aggrecan was upregulated under cyclic loading conditions. In conclusion, the fibrous component of fibrocartilage could be generated using only mechanical cues, but generation of the cartilaginous component of fibrocartilage required biologic factors in addition to mechanical cues. These studies support the hypothesis that the 3D stress environment influences cell activity and gene expression in fibrocartilage development.
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http://dx.doi.org/10.1089/ten.TEA.2009.0499DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3117236PMC
April 2011

Functional grading of mineral and collagen in the attachment of tendon to bone.

Biophys J 2009 Aug;97(4):976-85

Department of Mechanical Aerospace and Structural Engineering, Washington University, St. Louis, Missouri 63130, USA.

Attachment of dissimilar materials is a major challenge because high levels of localized stress may develop at their interfaces. An effective biologic solution to this problem exists at one of nature's most extreme interfaces: the attachment of tendon (a compliant, structural "soft tissue") to bone (a stiff, structural "hard tissue"). The goal of our study was to develop biomechanical models to describe how the tendon-to-bone insertion derives its mechanical properties. We examined the tendon-to-bone insertion and found two factors that give the tendon-to-bone transition a unique grading in mechanical properties: 1), a gradation in mineral concentration, measured by Raman spectroscopy; and 2), a gradation in collagen fiber orientation, measured by polarized light microscopy. Our measurements motivate a new physiological picture of the tissue that achieves this transition, the tendon-to-bone insertion, as a continuous, functionally graded material. Our biomechanical model suggests that the experimentally observed increase in mineral accumulation within collagen fibers can provide significant stiffening of the partially mineralized fibers, but only for concentrations of mineral above a "percolation threshold" corresponding to formation of a mechanically continuous mineral network within each collagen fiber (e.g., the case of mineral connectivity extending from one end of the fiber to the other). Increasing dispersion in the orientation distribution of collagen fibers from tendon to bone is a second major determinant of tissue stiffness. The combination of these two factors may explain the nonmonotonic variation of stiffness over the length of the tendon-to-bone insertion reported previously. Our models explain how tendon-to-bone attachment is achieved through a functionally graded material composition, and provide targets for tissue engineered surgical interventions and biomimetic material interfaces.
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http://dx.doi.org/10.1016/j.bpj.2009.05.043DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2726319PMC
August 2009

Micromechanics and Structural Response of Functionally Graded, Particulate-Matrix, Fiber-Reinforced Composites.

Int J Solids Struct 2009 May;46(10):2136-2150

Department of Mechanical, Aerospace, and Structural Engineering, Washington University in St. Louis, St. Louis, MO 63130,

Reinforcement of fibrous composites by stiff particles embedded in the matrix offers the potential for simple, economical functional grading, enhanced response to mechanical loads, and improved functioning at high temperatures. Here, we consider laminated plates made of such a material, with spherical reinforcement tailored by layer. The moduli for this material lie within relatively narrow bounds. Two separate moduli estimates are considered: a "two-step" approach in which fibers are embedded in a homogenized particulate matrix, and the Kanaun-Jeulin (2001) approach, which we re-derive in a simple way using the Benveniste (1988) method. Optimal tailoring of a plate is explored, and functional grading is shown to improve the performance of the structures considered. In the example of a square, simply supported, cross-ply laminated panel subjected to uniform transverse pressure, a modest functional grading offers significant improvement in performance. A second example suggests superior blast resistance of the panel achieved at the expense of only a small increase in weight.
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http://dx.doi.org/10.1016/j.ijsolstr.2008.08.010DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3714223PMC
May 2009

Collagen fiber orientation at the tendon to bone insertion and its influence on stress concentrations.

J Biomech 2006 15;39(10):1842-51. Epub 2005 Jul 15.

Department of Orthopaedic Surgery, Washington University, 1 Barnes-Jewish Hospital Plaza, Suite 11300, Campus Box 8233, St. Louis, MO 63110, USA.

The tendon to bone insertion serves the mechanical role of transferring loads from a relatively compliant tendon to a relatively rigid bone. The details of the mechanism of load transfer are of great importance, since current surgical procedures for tendon reattachment have high failure rates. We hypothesized that the microscopic structure of the insertion is optimized to minimize stress concentrations associated with this load transfer. To explore this, collagen fiber orientation distributions were measured in the supraspinatus tendons of rats. The angular deviation of fibers was fairly uniform across the insertion, and the mean angles of the local distributions deviated mildly from the tendon axis. To explore how these observed property distributions could influence load transfer, these distributions were used to derive material properties for an idealized two-dimensional mechanical model of an insertion. Comparison between stress concentrations in this idealized model and those in three comparison models suggests that the microstructure serves to (1) simultaneously reduce stress concentrations and material mass, and (2) shield the insertion's outward splay from the highest stresses.
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http://dx.doi.org/10.1016/j.jbiomech.2005.05.021DOI Listing
December 2006
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