Comparison Between the Hyperelastic Behavior of Fresh and Frozen Equine Articular Cartilage in Various Joints

Fresh and frozen cartilage samples of the fetlock, carpus, and stifle were collected from 12 deceased horses. Half were measured immediately following extraction, and half were frozen for seven days and then measured. Seven indentations (various normalized displacements) were implemented with an indention rate of 0.1 mm/s. Solid phase aggregate modulus (Es), hyperelastic material constant (α), and fluid load fraction (F′) of equine articular cartilage were assessed using the Ogden hyperelastic model. The properties were statistically compared in various joints (fetlock, carpus, and stifle), and between fresh and frozen samples using various statistical models. There was no statistical difference between the fetlock and carpus in the aggregate modulus (p = 0.5084), while both were significantly different from the stifle (fetlock: p = 0.0017 and carpus: p = 0.0406). For the hyperelastic material constant, no statistical differences between joints were observed (p = 0.3310). For the fluid load fraction, the fetlock and stifle comparison showed a difference (p = 0.0333), while the carpus was not different from the fetlock (p = 0.1563) or stifle (p = 0.3862). Comparison between the fresh and frozen articular cartilage demonstrated no significant difference among the joints in the three material properties: p = 0.9418, p = 0.7031, and p = 0.9313 for the aggregate modulus, the hyperelastic material constant, and the fluid load fraction, respectively.

Viscoelastic Properties of Ovine Adipose Tissue Covering the Gluteus Muscles

Pressure-related deep tissue injury (DTI) is a life-risking form of pressure ulcers threatening immobilized and neurologically impaired patients. In DTI, necrosis of muscle and enveloping adipose tissues occurs under intact skin, owing to prolonged compression by bony prominences. Modeling the process of DTI in the buttocks requires knowledge on viscoelastic mechanical properties of the white adipose tissue covering the gluteus muscles. However, this information is missing in the literature. Our major objectives in this study were therefore to (i) measure short-term (HS) and long-term (HL) aggregate moduli of adipose tissue covering the glutei of sheep, (ii) determine the effects of preconditioning on HS and HL, and (iii) determine the time course of stress relaxation in terms of the transient aggregate modulus H(t) in nonpreconditioned (NPC) and preconditioned (PC) tissues. We tested 20 fresh tissue specimens (from 20 mature animals) in vitro: 10 specimens in confined compression for obtaining the complete H(t) response to a ramp-and-hold protocol (ramp rate of 300mm∕s), and 10 other specimens in swift indentations for obtaining comparable short-term elastic moduli at higher ramp rates (2000mm∕s). We found that HS in confined compression were 28.9±14.9kPa and 18.1±6.9kPa for the NPC and PC specimens, respectively. The HL property, 10.3±4.2kPa, was not affected by preconditioning. The transient aggregate modulus H(t) always reached the plateau phase (less than 10% difference between H(t) and HL) within 2min, which is substantially shorter than the times for DTI onset reported in previous animal studies. The short-term elastic moduli at high indentation rates were 22.6±10kPa and 15.8±9.4kPa for the NPC and PC test conditions, respectively. Given a Poisson’s ratio of 0.495, comparison of short-term elastic moduli between the high and slow rate tests indicated a strong deformation-rate dependency. The most relevant property for modeling adipose tissue as related to DTI is found to be HL, which is conveniently unaffected by preconditioning. The mechanical characteristics of white adipose tissue provided herein are useful for analytical as well as numerical models of DTI, which are essential for understanding this serious malady.

Tensile and Compressive Properties of the Medial Rabbit Meniscus

Quantification of the material properties of the meniscus is of paramount importance, creating a ‘gold-standard’ reference for future tissue engineering research. The purpose of this study was to determine the compressive and circumferential tensile properties in the rabbit meniscus. Creep and recovery indentation experiments were performed on the meniscus using a creep indentation apparatus and analysed via a finite element optimization method to determine the compressive material properties at six topographical locations. Tensile properties of samples taken circumferentially from the rabbit meniscus were also examined. Results show that the femoral side of the anterior portion exhibits the highest aggregate modulus (510 ± 100 kPa) and shear modulus (240 ± 40 kPa), while the lowest aggregate modulus (120 ± 30 kPa) and shear modulus (60 ± 20 kPa) were found on the femoral side of the posterior location. Values of 156.6 ± 48.9 MPa for Young's modulus and of 21.6 ± 7.0 MPa for the ultimate tensile strength of were found from the tensile samples, which are similar to the values found in other animal models. These baseline values of material properties will be of help in future tissue engineering efforts.

INFLUENCE OF PRE-CONDITIONING LOADS ON BOVINE ARTICULAR CARTILAGE STRESS RELAXATION BEHAVIOR IN CONFINED COMPRESSION

Articular Cartilage is a load bearing tissue whose microarchitecture, electrochemical composition, and fluid interactions afford it unique mechanical properties. It consists of an extracellular matrix (ECM) interspersed with a sparse population of chondrocytes, varying in density by depth. The structure and mechanical properties of this highly specialized tissue also vary depending on depth from the articular surface; with three specialized zones, each with unique material properties. Typically this tissue is mechanically modeled as a biphasic material, consisting of a solid phase and a fluid phase, which can redistribute itself under loading, altering hydrostatic pressure within the material. Thus, articular cartilage exhibits a time-dependent viscoelastic behavior when subjected to constant loading or deformation, and will reach an equilibrium via stress relaxation and creep behavior. The objective of this study was to test a custom designed confined compression chamber. We characterize the ability of the test chamber to generate curves capable of quantifying the stress relaxation level and equilibrium state in bovine articular cartilage, and to show the preliminary results of a comparison between the equilibrium aggregate modulus (HA) obtained from pre- conditioned and non-conditioned tissues. Using fresh bovine articular cartilage samples, stress relaxation tests were conducted in compression, obtaining equilibrium stress and HA through a linear relation between the initial strain and the equilibrium stress. The test specimens were divided into two groups, one with a pre-conditioning load and the other without. The tests resulted in equilibrium stresses of 0.015 ± 0.0067 MPa for the non-conditioned and 0.067 ± 0.012 MPA for the pre-conditioned, and HA values of 0.205 ± 0.100 MPa for the unconditioned group and 0.878 ± 0.160 MPa in the pre-conditioned group. Our confined compression chamber successfully produced the stress relaxation curve characterizing the mechanical behavior of articular cartilage, defining both the equilibrium stress and HA. Our results suggest that pre-conditioning correlates with a higher equilibrium stress and aggregate modulus based on the fact that pre-loading the specimens reduces the effects of viscoelasticity.

THE USE OF ENTIRE FRESH PATELLAR ALLOGRAFT FOR ARTICULAR CARTILAGE REPLACEMENT IN RABBITS: A LONG-TERM INTERDISCIPLINARY STUDY

Fresh patellar allograft without violating the continuum of the articular cartilage was evaluated in rabbits. Twenty-four skeletally immature New Zealand White rabbits underwent resurfacing of the patella with fresh allografts and 92% (22/24) of the allografts survived. These specimens were analyzed to assess the geometric parameters of the patellofemoral joint anatomy as well as the biomechanical and histological properties of the patellar articular cartilage at 12 (n=8), 26 (n=7) and 52 weeks (n=7) postoperatively. Despite incomplete restoration of the patellofemoral joint geometry, both the biomechanical and histologic results showed excellent preservation of the articular cartilage at 26 and 52 weeks. From the biomechanical testing, the aggregate modulus (Ha) and the permeability (k) of the transplanted cartilage for the 26- and 52-week groups showed no difference between the experimentals and the controls. For the 26-week group, the aggregate modulus was 0.70±0.07 MPa and 0.72±0.19 MPa for the experimental and control, respectively (p>0.5) and the permeability was (0.97±0.13)×10-15 m4/N-s and (1.17±0.33)×10-15 m4/N-s for the experimental and control, respectively (p>0.5). For the 52-week group, the aggregate modulus was 0.93±0.14 MPa and 1.03±0.06 MPa for the experimental and control, respectively (p>0.5) and the permeability was 2.32±0.57)×10-15 m4/N-s and 2.12±0.85)×10-15 m4/N-s for the experimental and control, respectively (p>0.5). This study clearly demonstrates the long-term viability of articular cartilage in entire osteochondral patellar allograft in rabbits.