AN EVALUATION OF THE NON-LINEAR VISCOELASTIC PROPERTIES OF THE HEALING MEDIAL COLLATERAL LIGAMENT

Injuries to knee ligaments are frequent, demanding an increased understanding of the healing process. Clinically, the injured medial collateral ligament (MCL) has been found to heal without surgical intervention. However, laboratory studies have shown that, even one year after injury, the biomechanical properties, biochemical composition, and histomorphological appearance of the healing MCL remains suboptimal. While research has focused on the changes in mechanical properties (i.e. stress-strain behavior) of the healed MCL, studies on its viscoelastic properties are limited. Yet, this knowledge is critical to determine the overall kinetic response of the knee joint.The quasi-linear viscoelastic (QLV) theory proposed by Professor Y.C. Fung has been frequently used to model the viscoelastic properties of the MCL. This theory was developed based on an idealized step-elongation during a stress relaxation test. As this is experimentally impossible, the constants of the theory may not be representative when they are determine based on experiments that utilize finite strain rates. Thus, the overall objectives of this dissertation were to 1) develop and validate a novel experimental and analytical approach that accounts for finite strain rates and provides an accurate determination of the viscoelastic properties of the normal MCL, 2) apply this new approach to describe the viscoelastic behavior of the healing MCL, and 3) to determine whether the new approach can describe the response of the MCL to harmonic oscillations.This work demonstrated that a newly developed approach could be utilized to determine the constants of the quasi-linear viscoelastic theory and successfully describe the viscoelastic behavior of both the normal and healing MCLs. Interestingly, the healing ligaments display a lower initial slope of the stress-strain curve and a greater propensity to dissipate energy, suggesting other structures within the knee would have to play a compensatory role in knee function. It was also found that the mechanisms governing the viscoelastic response of the MCL to harmonic oscillations may not be the same as that which governs stress relaxation behavior. Thus, a more general theory may be necessary to describe both phenomena

Biomechanics of knee ligaments: injury, healing, and repair

Abstract Knee ligament injuries are common, particularly in sports and sports related activities. Rupture of these ligaments upsets the balance between knee mobility and stability, resulting in abnormal knee kinematics and damage to other tissues in and around the joint that lead to morbidity and pain. During the past three decades, significant advances have been made in characterizing the biomechanical and biochemical properties of knee ligaments as an individual component as well as their contribution to joint function. Further, significant knowledge on the healing process and replacement of ligaments after rupture have helped to evaluate the effectiveness of various treatment procedures. This review paper provides an overview of the current biological and biomechanical knowledge on normal knee ligaments, as well as ligament healing and reconstruction following injury. Further, it deals with new and exciting functional tissue engineering approaches (ex. growth factors, gene transfer and gene therapy, cell therapy, mechanical factors, and the use of scaffolding materials) aimed at improving the healing of ligaments as well as the interface between a replacement graft and bone. In addition, it explores the anatomical, biological and functional perspectives of current reconstruction procedures. Through the utilization of robotics technology and computational modeling, there is a better understanding of the kinematics of the knee and the in situ forces in knee ligaments and replacement grafts. The research summarized here is multidisciplinary and cutting edge that will ultimately help improve the treatment of ligament injuries. The material presented should serve as an inspiration to future investigators.

The Impact of Boundary Conditions on Surface Curvature of Polypropylene Mesh in Response to Uniaxial Loading

Exposure following pelvic organ prolapse repair has been observationally associated with wrinkling of the implanted mesh. The purpose of this study was to quantify the impact of variable boundary conditions on the out-of-plane deformations of mesh subjected to tensile loading. Using photogrammetry and surface curvature analyses, deformed geometries were accessed for two commercially available products. Relative to standard clamping methods, the amount of out-of-plane deformation significantly increased when point loads were introduced to simulate suture fixation in-vivo. These data support the hypothesis that regional increases in the concentration of mesh potentially enhance the host׳s foreign body response, leading to exposure

Effects of Cell Seeding and Cyclic Stretch on the Fiber Remodeling in an Extracellular Matrix–Derived Bioscaffold

The porcine small intestine submucosa, an extracellular matrix–derived bioscaffold (ECM-SIS), has been successfully used to enhance the healing of ligaments and tendons. Since the collagen fibers of ECM-SIS have an orientation of ± 30°, its application in improving the healing of the parallel-fibered ligament and tendon may not be optimal. Therefore, the objective was to improve the collagen fiber alignment of ECM-SIS in vitro with fibroblast seeding and cyclic stretch. The hypothesis was that with the synergistic effects of cell seeding and mechanical stimuli, the collagen fibers in the ECM-SIS can be remodeled and aligned, making it an improved bioscaffold with enhanced conductive properties. Three experimental groups were established: group I (n = 14), ECM-SIS was seeded with fibroblasts and cyclically stretched; group II (n = 13), ECM-SIS was seeded with fibroblasts but not cyclically stretched; and group III (n = 8), ECM-SIS was not seeded with fibroblasts but cyclically stretched. After 5 days' experiments, the scaffolds from all the three groups (n = 9 for group I; n = 8 for groups II and III) were processed for quantification of the collagen fiber orientation with a small-angle light scattering (SALS) system. For groups I and II, in which the scaffolds were seeded with fibroblasts, the cell morphology and orientation and newly produced collagen fibrils were examined with confocal fluorescent microscopy (n = 3/group) and transmission electronic microscopy (n = 2/group). The results revealed that the collagen fiber orientation in group I was more aligned closer to the stretching direction when compared to the other two groups. The mean angle decreased from 25.3° to 7.1° (p < 0.05), and the associated angular dispersion was also reduced (37.4° vs. 18.5°, p < 0.05). In contrast, groups II and III demonstrated minimal changes. The cells in group I were more aligned in the stretching direction than those in group II. Newly produced collagen fibrils could be observed along the cells in both groups I and II. This study demonstrated that a combination of fibroblast seeding and cyclic stretch could remodel and align the collagen fiber orientation in ECM-SIS bioscaffolds. The better-aligned ECM-SIS has the prospect of eliciting improved effects on enhancing the healing of ligaments and tendons