Track 2. Musculoskeletal Mechanics-Joint ISB Track

6779 We-Th, no. 6 (P56) Application of acrylic split-hopkinson-bar method for determining mechanical properties of articular cartilage at high strain rate K. Kobayashi 1 , M. Sakamoto 1 , '~ Tanabe 2, T. Kikuchi 3, G. Omori 4, '~ Koga 5. 1Dept. of Health Sciences, Niigata University School of Medicine, Niigata, Japan, 2 Dept of Mechanical Engineering, Niigata University, Niigata, Japan, 3Department of Orthopaedic Surgery, Niigata University School of Medicine, Niigata, Japan, 4Center for Transdisciplinary Research, Niigata University, Niigata, Japan, 5Department of Orthopaedic Surgery, Niigata Kobari Hospital, Niigata, Japan

The modified split-Hopkinson pressure-bar (SHPB) technique with the use of acrylic circular bars was applied for measuring a precise stress-strain curve of the articular cartilage at a high strain rate. The effects of attenuation and dispersion on the stress wave profiles were first corrected based on the viscoelastic wave propagation theory, since an acrylic bar is a viscoelastic ma- terial. Osteochondral plugs of 7.2 mm diameter were cored from bovine distal femora. Articular cartilage specimens were obtained by removing underlying subchondral bone. The water content of the specimen was determined from its wet and dry weights. Application of this technique to the articular cartilage demonstrated that the clear transmitted wave profile was able to be captured in the output bar. The velocity of the striker bar was chosen to be 2 m/s, and the corresponded strain rate was approximately 3600s -1. In this study, Young's modulus, approximately 60 MPa, was higher than the static Young's modulus of approximately 5 MPa. No correlation between the water content and Young's modulus was found. The fluid in articular cartilage is immobile at high rate of strain, resulting in the dramatic increase in Young's modulus.

7636 We-Th, no. 7 (P56) Composition based finite element analysis of micropipette aspiration reveals changes in the pericellular matrix during cartilage degeneration P. Julkunen 1 , W. Wilson 2, J. Jurvelin 1 , R. Korhonen 3. 1Department ef Applied Physics, University of Kuopio, Finland, 2Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, Netherlands, 3Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Alberta, Canada

Introduction: Previously, the mechanical properties of chondrocyte and peri- cellular matrix (PCM), composing a chondron, have been assessed by applying micro-aspiration tests. However, the changes in PCM composition leading to the altered mechanical behaviour of the chondron during cartilage degener- ation are not known. This study aimed for the assessment of compositional changes in articular cartilage PCM during cartilage degeneration, by combin- ing microscopic mechanical testing, micro-aspiration, and compositional finite element (FE) analysis [1]. Methods: A FE model [1] for a single chondron was constructed in a mi- cropipette aspiration geometry [2]. Composition and mechanical properties of the chondrocyte were taken from literature [2,3]. Micropipette aspiration experiments were performed by increasing the aspiration suction pressure gradually from 0kPa to 15 kPa and measuring the ratio between aspiration length and radius of the aspiration pipette [2]. Each suction pressure was kept constant until equilibrium was reached and no fluid flow was allowed in or out of the chondron. Model parameters (fixed charge density, solid matrix shear modulus and water content) of PCM were fitted to the published experimental micro-aspiration results of non-osteoarthritic and osteoarthritic chondron 2. Results and Conclusions: The non-osteoarthritic chondron had a fixed charge density of 0.122 mEq/ml, water content of 88.9% and solid matrix shear modulus of 122.1 kPa. The osteoarthritic chondron showed similar fixed charge density (0.122 mEq/ml), higher water content (92.3%) and lower solid matrix shear modulus (98.8 kPa) as the osteoarthritic chondron. As the permeability for the chondrons has already been published for this specific experimental data [2], in the present study, only equilibrium properties were examined. This study suggests that the changes in the composition of PCM during the development of osteoarthritis can be simulated and assessed using the present FE model [1]. This kind of compositional data (fixed charge density, water content) for normal and osteoarthritic chondrons has not been available earlier.

References [1] Wilson. BiomechModelMechanobiol, in press, 2006. [2] Lexopoulos. J biomech. 2005; 38: 509-517. [3] Lai. Biorheology 2002; 39: 39-47.

7254 We-Th, no. 8 (P56) One-step-insertion technique for osteochondral transplantation: An alternative over tapping? V.K. Shekhawat, T. Kressner, M. Salineros, M.A. Wimmer. Rush University Medical Center, Chicago, USA

Osteochondral AIIograft Transplantation (OAT) typically involves tapping proce- dures to anchor the donor tissue in the host bone. This process is potentially

2.1 Cartilage Biomechanics S477

traumatic to cartilage tissue and chondrocyte death has been reported [1]. In this study we tested the biomechanical feasibility of swift one-step-insertion of grafts into their bone bed. Mechanical properties being sensitive indicators for early degeneration of cartilage [2]; compressive stiffness was therefore chosen as the endpoint. Using Arthex ® OAT-system, 30 grafts were transplanted. Anatomical locations for donor and recipient femoral condyles of two bovine calves were matched. Grafts were inserted flush to the surrounding recipient cartilage surface. A surgical mallet was used to insert 15 randomly chosen grafts (group-A) while, one-step-insertion was used to insert 15 grafts (group-B) with one swift displacement-controlled, one-step-insertion from a load frame. Artscan ® stiffness probe [3] was used for stiffness measurements, obtained for intact joints, retrieved grafts, and post transplants from days 0 through 5. Group-A grafts experienced compressive stresses of 1.9 ± 0.35 MPa for 20 ± 4 strikes of approximately 10ms each. Group-B grafts saw mean compressive stresses of 6 .7±2 .6MPa over 1.1 seconds. While group-A grafts recovered 98% of their original stiffness on day 1; group-B grafts recovered only 89% of their original stiffness (p < 0.05). These observations did not change until day 5 for both groups. It appears that adequately tapped donor grafts are able to (nearly) regain original stiffness in their host bed and are superior over one-step-insertion plugs. Since all plugs were transplanted using unconfined boundary conditions, the relatively long compressive time period for the swift-one-step-insertion plugs may have caused excessive matrix deformation, water loss and chon- drocyte damage to the tissue [4]. Additional studies are necessary to link the biomechanical observations to cell survival and metabolism.

References [1] Pylawka T. J. Knee Surg., in press [2] Caldwell III. Orthop. Clin. North. Am. 2005; 36: 459. [3] Lyyra T. Med. Eng. Phy. 1995; 17:5 [4] Milentijevic D. J. Biomech 2005; 38: 493.

6268 We-Th, no. 9 (P56) Deformation analysis of articular cartilage during impaction loading S. Mueller 1 , U.-J. Goerke 1 , H. Guenther 2, M.A. Wimmer 3. 1Institute of Mechanics, Chemnitz University of Technology, Chemnitz, Germany, 2TBZ-Pariv, Chemnitz, Germany and AO Research Institute, Davos, Switzerland, 3Department of Orthopedic Surgery, Rush University Medical Center, Chicago, USA

Osteochondral grafting is one of the possible procedures to repair cartilage defects. Clinically, the re-implantation of osteochondral plugs results in con- siderable loads during impaction [1]. Due to the complex structure of articular cartilage the local stress-strain relationship is not readily accessible. In this study, the non-linear, rate-dependent behavior of cartilage in unconfined compression, similar to the clinical situation, was studied. Using a high speed camera system with 2000 frames per second allowed the observation of local tissue deformation at various test speeds (up to 100 mm/s) using a material testing machine. Extracted frames of the videos were utilized to calculate deformation fields based on grey-scale correlation analyses. The dynamic modulus reached up to 36MPa at higher loading rates and thus was remarkably higher than the equilibrium modulus. During compression the cylindrical cartilage layer expanded radially changing its shape to a combi- nation of ton and frustum. At lower testing speeds the deformed shape was more like a ton while at higher it adapted the form of a frustum. The measured radial displacements at 20% compression and 100 mm/s test velocity revealed 0.18 mm for the superficial zone and 0.05 mm for the deep zone. The results of this study expose the inhomogenity of material properties of articular cartilage in dependence of the loading rate and will be used as a basis for parameter identification using a finite element material model.

References [1] Pylawaka, Wimmer, et al. Impaction affects cell viability in osteochondral tissues.

J Knee Surg, in press.

6166 We-Th, no. 10 (P56) Cartilage failure under cyclic loads N.K. Simha 1 , R. Namani 2, J.L. Lewis 1 , EH. Leo 3. 1Biomechanics Lab, Department of Orthopaedic Surgery, University of Minnesota, Minneapolis, MN, USA, 2Department of Mechanical Engineering, University of Miami, Coral Gables, FL, USA, 3Department of Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, MN, USA

Articular cartilage provides a low friction contact surface for joint articulation and is critical for joint function. Osteoarthritis (OA), a major disabling disease, is characterized by progressive degeneration of articular cartilage. In early OA swelling of cartilage is observed, whereas later stages show crack-like fibrillation, which increases with disease severity, and results in full thickness loss of cartilage exposing underlying bone. Since the physiological loading

$478 Journal of Biomechanics 2006, Vol. 39 (Suppl 1) Poster Presentations

of cartilage is cyclic and since disease is observed mainly in the elderly population, high cycle fatigue failure appears to be a potentially important mechanism for OA cartilage degeneration. Although the biochemical aspects of OA are extensively studied, little is known about the mechanical failure mechanisms. This paper develops damage and fracture mechanics based measures to quantify the mechanical failure mechanisms relevant for OA degeneration of cartilage. In vitro fatigue failure experiments on osteochondral bovine patel- lar cartilage explants subject to cyclic indentation will be described. First, we focus on early OA, control cyclic indentation to produce microstructural damage without overt cracks, and measure cartilage properties before and after cyclic loading using separate indentation tests. By examining how the measured equilibrium elastic modulus (obtained following Hayes), permeability and viscoelastic parameters (inverse FEM from relaxation data) depend on the number of loading cycles, we identify damage measures for cartilage. Second, we focus on mid/late stage OA, and measure the fatigue crack growth of an initial crack by optically measuring the crack length and calculating the nominal J-integral from measured cyclic forces using FEM. Microstructural failure mechanisms causing crack growth are identified by SEM observations of the crack tip region. The paper concludes with implications of these experiments for Osteoarthritis.

4214 We-Th, no. 11 (P56) Importance of the composition of pericellular matrix for the deformation behaviour of chondrocytes in articular cartilage P. Julkunen 1 , W. Wilson 2, J. Jurvelin 1 , R. Korhonen 3. 1Department of Applied Physics, University of Kuopio, Finland, 2Department of Biomedical Engineering, Eindhoven University of Technology, Netherlands, 3Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Canada

Introduction: Deformation of the chondrocyte in articular cartilage has been shown to relate to biosynthesis of extracellular matrix (ECM). Therefore, it is important to know the mechanical environment of cells as it influences the mechanical signals experienced by the cells. Pericellular matrix (PCM), which consists of proteoglycans and a shell of collagen fibres around the cell, protects the cell from excessive strains. The influence of the PCM composition on the cell deformation is unknown. Aim was to study the effects of the PCM on the deformation behaviour of the chondrocytes. Methods: A 3D finite element model of cartilage disc including a real- istic depth-dependent structure and composition of articular cartilage was constructed 1. The model included three chondrocytes with PCM in different cartilage zones. Information on the geometry and properties for cartilage and cells were taken from recent literature. Split-lines of the ECM were included in the model. PCM was reconstructed around the cell with collagen fibres orien- tated parallel to the chondrocyte surface. Loading protocol included swelling, 15%-compression and relaxation. Fixed charge density (FCD), fluid fraction (FF) and collagen mass fraction (CMF) in PCM were varied to determine their influence on cell deformation at equilibrium. Results and Conclusions: CMF, FCD and FF of the PCM affected significantly the cell deformation. In general, the strains of the chondrocytes decreased as CMF and FCD increased or FF decreased. Cell-strains were higher parallel to the split-lines but, as CMF increased, approached the strain values perpen- dicular to the split-lines. We conclude that composition of the PCM contributes significantly to the mechanical signals detected by the cells and potentially to the biosynthesis of the cartilage constituents. In the future, the importance of the PCM to the time-dependent cell deformation will be studied using a wider range of material parameters. Thereby, we estimate the total impact of the changes in PCM composition on the maintenance of articular cartilage integrity.

References [1] Wilson. Biomech Model Mechanobiol, in press, 2006.

5516 We-Th, no. 12 (P56) Loading rate sensitivity of articular cartilage M. Szarko, J.E.A. Bertram. University of Calgary, Calgary, Canada

Impact loading on joints consists of two main components; load magnitude and load rate. Though occurring only once per stride cycle, the heel strike of normal gaits expose joints to loading rates in excess of 75 Hz. Such high frequency load components may jeopardize cartilage and its structural constituents be- yond consideration of load magnitude alone. The present study investigates the sensitivity of articular cartilage properties and associated subarticular tissues in the realm of these high load rates. Specific structural components of articular cartilage were selectively disrupted to determine the influence of each on the tissue properties over a spectrum of load rates. Bovine osteochondral dowels from the tibial plateau were exposed to either enzymatic digestion (Colla- genase, Worthington, USA) or chemical extraction (GuHCI, Sigma, Canada)

to target cartilage collagen and proteoglycans respectively. The viscoelastic properties of the samples were determined by nondestructive testing with low amplitude sinusoidal compression over loading rates of 0-200 Hz. The analysis system was actuated using an electrodynamic vibrator (Ling Dynamic Systems, USA) and stress and strain phase angles compared over the dynamic load range. The waveform frequencies were introduced as a 'chirp', comprising equal amplitude sine waves perfectly periodic in the time record over the full frequency spectrum. Results indicate a marked increase in phase angle after partial collagen cleavage, while proteoglycan depletion witnessed no significant changes in viscoelastic behaviour from normal cartilage. This is in contrast to the appearance of these tissues, where collagen-cleaved samples seemed remarkably normal while the robustness of proteoglycan samples was substantially diminished. The important influence of collagen disruption on the rate dependent properties of articular cartilage may suggest a particular sensitivity to higher range loading rates, a feature which may prove significant in cartilage biomechanics.

7349 We-Th, no. 13 (P56) Theoretical analysis of response of hydrogel layer bound with a rigid substrate to penetrating a rigid punch having a circular pit. Application to the novel bearing system of artificial human knee or hip replacements B. Monastyrskyy. Pidstryhach Institute for Applied Problems of Mechanics and Mathematics, Lviv, Ukraine

The present work was inspired by the papers [1,2], in which the authors suggested a novel bearing system of artificial human articulation of hip or of knee. They came up with idea to design the articular joint in such way that one of the contact surface (femoral condyle in knee articulation or femoral head in hip articulation) is constructionally introduced with microtopology (a set of micropits, so called "micropockets") and the complementary surface (tibial plateu in knee articulation or acetabular cup in hip articulation) is even and lubricated with the hydrogel substance. The theoretical analysis presented in paper [2] was performed on the basis of the one-dimensional problems, so many questions remained opened. The present contribution is an attempt of more detailed analysis. Within the framework of axially symmetric formulation the frictionless contact problem of compression of a hydrogel layer bounded to a rigid substrate by a rigid punch having a single circular pit is considered. The hydrogel is modeled as a biphasic saturated mixture consisting of a solid skeleton and interstitial fluid under the following assumptions: • both solid and fluid phases are intrinsically incompressible but the cartilage,

as a whole, is compressible due to exudation of the fluid; • the solid matrix is linearly elastic and isotropic; • the viscosity of the fluid phase is neglected; • the cartilage porosity and permeability are constant. The aim of the investigation was to trace the evolution of the pit filling, assess the filling time, study its dependence on both the geometrical parameters (height of the hydrogel layer, radius and depth of the pit) and physical properties of the hydrogel substance.

References [1] Suciu A.N., Iwatsubo T., Matsuda M. and Nishino T. Experimental study of a

artificial knee joint with PVA-hydrogel cartilage. In: Proc. 4th World Congress of Biomechanics, Calgary, Canada, CD-ROM, 2002; p.l.

[2] Suciu A.N., Iwatsubo T. and Matsuda M. Theoretical investigation of an artifi- cial joint with micro-pocket-covered component and biphasic cartilage on the opposite articulating surface. ASME J. Biomech. Eng. 2003; 125: 425-433.

7624 We-Th, no. 14 (P56) The shear stresses regulate the reconstruction processes in bone J. Danesova 1 , M. Petrtyl 1 , M. Adam 2, H. Hulejova 2. 1Czech Technica/ University, F.C. Eng., Laboratory of Biomechanics and Biomaterial Engineering, Prague, Czech Republic, 2Institute of Rheumatology, Prague, Czech Republic

Biomechanical loads and resulting biochemical processes are the basic sup- ports of tissue existence. The spherical stress tensor regulates the intensity (the rate/speed) of metabolical processes, the deviator stress tensor initiates ("starts") the metabolic processes. The bone tissue is very sensitive on the shear stresses. In the parts of bone tissue where the bone remodeling equilib- rium has been broken, i.e. where the upper "permissible" limit of bone dynamic stability has been exceeded, the bone is reconstructed and (for example) in the case of subchondral bone is formated into the granulate structure. The region of granulation formation is usually very distinct. Regions of structural changes are identified by sensors of cells that are sensitive to shear flows of liquids in the extracellular matrix. Limited levels of shear flows in disordered regions are in plane projections identical with isochromats, i.e. with the curves (in a plane) of constant shear stresses. Bone defects described (for example) by rediologists as cysts have the shape predetermined (and limited) by the