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

Viscoelastic spectrum analysis and the identification of a fung viscoelastic material

Despite its many limitations, the Fung “quasi-linear viscoelastic” constitutive model continues to serve as a workhorse of the biomechanics community. A central challenge in applying the model is that it requires a specific form for the relaxation spectrum that is difficult to relate to easily obtained experimental spectra such as a generalized Maxwell relaxation spectrum. Here, we present a simple and general technique for obtaining a from relaxation data a viscoelastic spectrum appropriate to the Fung model. We apply the model to identify several biomaterials that are modeled reasonably by a Fung model, and many more that are not

Characterizing the Maternal Adaptations of Pregnancy and Recovery Following Vaginal Delivery in the Rodent Model

ABSTRACT Pelvic organ prolapse and urinary incontinence are common conditions in women that significantly diminish quality of life. Vaginal delivery and maternal birth injury are the number one risk factors for the development of pelvic floor disorders. The goal of this study was to characterize maternal adaptations throughout pregnancy and recovery after vaginal delivery in terms of the passive quasi-static mechanical properties of the vagina using a rodent model. Virgin (n=8), mid-pregnant (n=7, day 15-16), late-pregnant (n=7, day 20-21), immediate postpartum (n=8, <2 hours post delivery), and 4 week postpartum (n=6) Long-Evans female rats were utilized in this study. The mechanical properties (tangent modulus, tensile strength, ultimate strain, and strain energy density) were quantified by testing longitudinal sections of vaginal tissue to failure. The tangent modulus of virgin animals (25.1±5.1 MPa) was significantly higher compared to mid-pregnant (11.7±7.7 MPa, p=0.003), late-pregnant (7.9±4.0 MPa, p<0.001), and immediate postpartum (8.5±4.7 MPa, p=0.001) animals. A similar trend was also observed in the tensile strength, whereas the ultimate strain increased throughout pregnancy until the time of vaginal delivery. Recovery was observed four weeks postpartum as no significant difference was found from virgin animals for any of the parameters. This study has shown a significant decrease in the tangent modulus and tensile strength along with an increase in the ultimate strain of longitudinal sections of vaginal tissue throughout pregnancy. These maternal adaptations are likely to increase the overall distensibility of the vagina and allow for vagina delivery with minimal injury. This process appears to be effective in the rodent model as the properties recovered to virgin levels by 4 weeks. In the future, we hope to alter these adaptations or exceed them in order to study the risk and impact of birth injury in this model

A Comparison of the In Vivo Contact Pressure at the Tibiotalar Joint During Walking and Running

Category: Ankle Introduction/Purpose: Knowledge of cartilage pressure distribution in healthy ankle joints during gait is important for understanding the loading environment of articular cartilage and for providing a basis for comparison to evaluate how ankle pathology and surgical procedures affect cartilage loading. Finite element models of the ankle have been developed to examine in vitro loads at the tibiotalar joint during simulated standing in healthy and injured ankle joints [1, 2]. However, there are currently no in vivo studies of tibiotalar cartilage pressure during dynamic loading activities. The goal of this study was to develop a subject-specific finite element model of the tibiotalar joint to estimate contact pressure during walking and running. Methods: Informed consent was obtained from one healthy male, age 23 yrs., BMI 27 kg/m2). Synchronized biplane radiographs of the ankle were acquired at 100 and 150 frames per second during the support phase of overground walking and running, respectively, at a self-selected pace (1.5 m/s and 3.0 m/s, respectively). CT-based bone models of the tibia and talus were matched to the stereoradiographic images to precisely track the three-dimensional bone movement [3]. Six degrees-of-freedom joint kinematics were calculated for each bone model, and used to position bone models in the finite element analysis. Cartilage volumes for the distal tibia and proximal talus were created in Geomagic software by extruding the articulating bone surface. Bones were modeled as rigid bodies and cartilage was modeled as deformable bodies with uniform thickness of 1.3 mm [4-7]. Simulations were performed using FEBio software. The primary outcome parameter was peak cartilage pressure in the tibiotalar joint. Results: On average, peak tibiotalar cartilage pressure was approximately 25% greater during the midstance phase of running in comparison to walking (Figure 1). During walking, peak contact pressure occurred on the lateral-central region of the tibiotalar cartilage throughout the entire stance phase. During the early support phase of running, the location of peak contact pressure was also on the lateral-central region of the tibiotalar cartilage. During running push-off, pressure increased in the medial-central cartilage region and the overall peak cartilage pressure increased. Conclusion: A novel finding of this study is that the peak pressure in tibiotalar cartilage moves from the lateral to medial side of the joint during running, but remains on the lateral side throughout the support phase of walking. This suggests that the location and magnitude of the loads seen by tibiotalar joint cartilage are activity dependent, even in the healthy ankle joint. Future work will investigate cartilage loading in pathologic ankles before and after surgical intervention, as well as during other common athletic activities

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.

Anisotropy of the passive and active rat vagina under biaxial loading

Pelvic organ prolapse, the descent of the pelvic organs from their normal anatomical position, is a common condition among women that is associated with mechanical alterations of the vaginal wall. In order to characterize the complex mechanical behavior of the vagina, we performed planar biaxial tests of vaginal specimens in both the passive (relaxed) and active (contracted) states. Specimens were isolated from virgin, female Long-Evans rats (n = 16) and simultaneously stretched along the longitudinal direction (LD) and circumferential direction (CD) of the vagina. Tissue contraction was induced by electric field stimulation (EFS) at incrementally increasing values of stretch and, subsequently, by KCl. On average, the vagina was stiffer in the CD than in the LD (p < 0.001). The mean maximum EFS-induced active stress was significantly higher in the CD than in the LD (p < 0.01). On the contrary, the mean KCl-induced active stress was lower in the CD than in the LD (p < 0.01). When comparing the mean maximum EFS-induced active stress to the mean KCl-induced active stress, no differences were found in the CD (p = 0.366) but, in the LD, the mean active stress was much higher in response to the KCl stimulation (p < 0.001). Collectively, these results suggest that the anisotropic behavior of the vaginal tissue is determined not only by collagen and smooth muscle fiber organization but also by the innervation