Female athletes have significantly higher incidence of anterior cruciate ligament (ACL) injuries than males participating in similar sports [1]. To date, no clear explanation has emerged for this disparity. However, hormonal differences may provide an explanation because some ACL injuries have been linked to physiologic fluctuations in estrogen levels over the menstrual cycle [2]. Receptors for estrogen have been identified in rabbit and human ACLs and medial collateral ligaments (MCLs) [3]. Increased estrogen levels caused decreased fibroblast proliferation and collagen synthesis in cell cultures from human and rabbit ACLs [4]. Since fibroblasts maintain collagen production and degradation in ligaments and collagen is the major load-bearing component of ligaments, estrogen may affect knee ligament mechanical properties.

We hypothesize that the mechanical properties of posterior sclera play a central role in the development of glaucomatous vision loss.

The purpose of this research was to characterize the strains in the rat tibia induced by controlled muscle contractions of the lower limb. The muscle contractions were generated unilaterally by electrical stimulation of the sciatic nerve in anesthetized animals. The technique was developed by muscle biologists as a way of simulating resistive exercise, and it was used in the current research as a countermeasure for rats undergoing hindlimb unloading to simulate musculoskeletal effects of space flight. The intensity of muscle contraction is quantified by measuring the torque generated at the ankle joint. A wide range of torques can be generated by varying the frequency of the electrical stimulating signal. Three specific questions were addressed in the current study. First, what is the relationship between strain and torque? Second, does the relationship between strain and torque change during the period of hindlimb unloading with exercise? Third, are the strain magnitudes in the range generally regarded to be osteogenic? This project is part of a larger effort aimed at assessing the efficacy of the muscle stimulation method as a countermeasure for the negative musculoskeletal effects of mechanical unloading.

The glenohumeral joint is the most dislocated major joint in the body and the axillary pouch of the glenohumeral capsule is the primary stabilizer at the extreme ranges of external rotation. [1] Procedures to repair the capsule following dislocation result in 12–25% of patients still experiencing pain and instability. [2] Studies performing clinical exams have found inconsistent data on differences between males and females. Increased laxity in the glenohumeral joint of females has been found as well as overall hypermobility when compared to males. [3,4] However, others have found no differences in overall joint stiffness between genders. [5] These findings suggest that a difference in the mechanical properties might exist between genders. Therefore, the objective of this study was to determine the effects of gender on the mechanical properties of the axillary pouch during tensile loading. A combined experimental and computational approach was used to evaluate the properties of the tissue. This data could potentially be utilized to improve surgical procedures and necessitate gender-specific repair techniques.

Traumatic brain injury is a leading cause of disability and fatality in the United States. Approximately two million traumatic brain injury cases occur every year [1]. Motor vehicle crashes are a primary source [2]. Both clinical and laboratory studies have been conducted to understand injury mechanisms and establish injury thresholds [3, 4]. Physical models have also been used to investigate injury biomechanics [5, 6]. Angular acceleration is considered as a major cause of diffuse brain injuries (DBI) [7, 8], while the angular velocity is chosen as a suitable load descriptor for a diffuse brain injury criterion [4]. The present study is focused on the effect of angular acceleration duration on brain strains due to lateral impact.

A very important function of the human subcutaneous fat layer is to act as a mechanical cushion. However, prolonged loading may result in damage such as pressure ulcers. Depending on the severity and origin of the ulcer, skin, subcutaneous fat and muscle can be affected. The aetiology of pressure ulcers is still poorly understood; it is not even clear whether wounds start to develop in skin, in the fat layer or even in deeper layers [1]. One of the tools used to better understand the way mechanical loading affects tissues is mechanical modeling. The success of a mechanical model strongly depends on the constitutive equations that are used to describe the mechanical properties obtained with experimental work. For skin and muscle much is already known, but a tremendous lack of data is found regarding the properties of adipose tissue. In the case of the subcutaneous fat tissue, very few of the mechanical properties have been determined experimentally.

Rotator cuff tears are a common injury, affecting 30–40% of the population over age 60 and representing a tremendous source of pain and disability. Rotator cuff tears are often repaired surgically, but a significant percentage (perhaps 50% or more) of large cuff tears fail after surgery and result in long-term shoulder disability. The outcome of rotator cuff repair surgery is typically assessed by measuring muscle strength, joint range of motion, pain, and other subjective indicators of function. Although these types of measurements are used extensively, they provide only an indirect assessment of tendon function. We have recently developed new experimental techniques that allow us to use a biplane x-ray system to directly assess in-vivo tendon function. The purpose of this study was to assess the feasibility of using a biplane x-ray system to quantify changes in in-vivo tendon function following tendon repair in a canine rotator cuff model. We hypothesized that tendon function will improve over the healing period (as reflected by changes in measures of repair stiffness and contracture) and approach normal tendon function by 17 weeks.

Musculoskeletal injuries are a common cause of pain and disability, and result in significant health care costs. Rotator cuff injuries, which make up the majority of soft tissue injuries to the upper extremity, commonly require surgical repair to the humeral head. At the rotator cuff, for example, the recurrence of tears to surgically repaired tendons has been reported to be as high as 94% [1]. The most dramatic feature of the failed tendon-to-bone healing scenarios is the lack of a transition zone between the tendon and bone that existed before the injury [2].

Skeletal muscle exhibit non-uniform internal architectures and our research goals are to relate these to their internal mechanics during contraction to provide a better understanding of muscle function, verify muscle models and permit more sophisticated interpretation of the functional effects of injury. Our previous studies have developed a novel method to calculate 2D Lagrangian strain field of skeletal muscle from the CPC-MRI [1]. The objective of this study is to apply CPC-MRI to derive Lagrangian strain fields for the normal biceps brachii and to determine its minimum principal strain directions in vivo during contraction. We will then compare them to the muscle fiber directions or pennation angles measured by a parallel ultrasound study. If we assume the muscle force is mainly transmitted along fiber directions, we would expect that the minimum principal strain directions are the same as fiber directions. Comparisons between different regions of the muscle and flexion angles will also be made.

Glaucoma is a leading cause of blindness, and elevated intraocular pressure (IOP) characteristic of glaucoma is caused by increased aqueous humor outflow resistance. Studies have localized the bulk of outflow resistance to particular regions along the outflow pathway — namely, the inner wall endothelium of Schlemm’s canal and its underlying juxtacanalicular tissue (JCT) [1] — but the hydrodynamic details of how aqueous humor flows through these tissues and how these tissues generate outflow resistance are not well understood.

Cortical bone tissue quality is imperative in maintaining the mechanical competence of whole bones, particularly at sites of overuse and age-related fragility fractures where a considerable cortical bone component is present. (Note that cortical bone tissue is more than 80% of the bone in the body [1].)

The intervertebral disc (IVD) is the largest cartilaginous structure in human body that contributes to flexibility and load support in the spine. To accomplish these functions, the disc has a unique architecture consisting of a centrally-located nucleus pulposus (NP) surrounded superiorly and inferiorly by cartilage endplates and peripherally by the annulus fibrosus (AF). Because the disc is avascular and experiences mechanical loads, the cells in IVD tissues live in a complex physical environment. Knowledge of mechanical, chemical and electrical signals within the tissue is important for understanding mechanobiology of IVD [1]. The objective of this study was to develop a three-dimensional (3D), inhomogeneous finite element model (FEM) for human IVD for analyzing the physical environment and solute transport within the tissue under different mechanical loading conditions. A case of IVD under axial compression was simulated and reported.

The Head Injury Criterion limit, HIC 1000 T = 1000, was developed based on skull fracture data (short time intervals T ) and longer duration head translations that lead to Closed Head Injuries (CHI). However, recent results imply that the HIC limits depend on the time interval T and the victim’s age [1]. Since the HIC lacks a clear physical interpretation, it is difficult to compare (verify) the new HIC limits with data from non-translational motions, e.g., to link these limits with the Diffuse Axonal Injury (DAI) criterion developed for head rotations [2]. The recent Head Impact Power (HIP) criterion can be applied to arbitrary motions, but it requires determining six parameters experimentally [3].

Deep tissue injury (DTI) is a serious and potentially deadly type of pressure ulcers, which initiate in deep muscle tissue under bony prominences of immobilized patients, and progress outwards towards the skin with no clear visual indications of the injury at the surface of the body. It had been suggested that DTI appear in muscle tissue first, due to the dense capillary vasculature in skeletal muscles which is susceptible to obstruction and occlusion by mechanical forces [1–3]. Though mechanical forces may cause capillaries to collapse and thus induce ischemic conditions in adjacent muscle cells [2], some investigators stipulated that ischemia alone cannot explain the etiology of DTI, and so, other mechanisms, particularly excessive cellular deformations must be involved [1]. We hypothesize that physiological levels of stresses and strains in muscle tissue under bony prominences — even when muscles are highly loaded as during sitting — do not cause complete closure of muscle capillaries, and therefore, do not cause an acute ischemia in muscles. If this is indeed the case, then ischemia cannot be the only factor contributing to DTI onset. In order to test our hypothesis, we developed a finite element (FE) model of the microstructure of skeletal muscle, at the level of muscle fascicles, and employed the model to determine the stress and strain levels required for causing partial and complete closure of capillaries.

Mechanical forces due to muscle contractions play an essential role in embryonic skeletal development. In neuromuscular conditions such as congenital myotonic dystrophy, where movement of the fetus in utero is reduced or absent, the bones and joints of the newborn often show malformations [1]. In this paper, we examine the effect of muscle contractions on embryonic bone development. We propose the hypothesis that mechanical loading due to muscle contractions promotes periosteal ossification and we test this hypothesis using computational and experimental methods. A set of FE analyses were performed using anatomically realistic morphologies and loading conditions, at several timepoints during development, in order to identify biophysical stimuli active during bone formation. Avian immobilization experiments were performed to examine bone growth in the absence of skeletal muscle contractions.

Mechanical loads have been shown in numerous studies to affect the health of articular cartilage [1]. Regional variations in the local deformation within articular cartilage under both normal and abnormal loading are, however, poorly understood. Understanding these deformation patterns is important, since conditions such as anterior cruciate ligament injury have been suggested to initiate cartilage breakdown by shifting the regions of frequent loading within the joint [2]. Quantifying variations in the deformation behavior of the collagen matrix in cartilage under load will therefore provide insight into the mechanical environment within cartilage and test the feasibility of a kinematic pathway to the initiation of osteoarthritis (OA). The objective of this study was to test the hypothesis that the collagen matrix in two regions of cartilage that normally experience different applied loads will deform differently under the same applied load.

The primary goal of ACL reconstruction is to limit post-operative anterior knee laxity without over-constraining the knee. For both hamstring tendon and patella tendon grafts used for ACL reconstruction, initial graft tension is applied to limit post-operative knee laxity. Both types of graft stress relax following implantation, decreasing the graft tension. Previous studies have shown that preconditioning reduces the tension decrease due to stress relaxation for both hamstring tendons [1] and patella tendons [2]. While hamstring tendon grafts are typically preconditioned in tension on a graft board prior to implantation to limit stress relaxation, patella tendon grafts are typically implanted without preconditioning. The current study focused on characterizing the influence of preconditioning on stress relaxation for both types of graft. The authors hypothesized that the tension loss due to stress relaxation would be larger for preconditioned hamstring tendon grafts than for preconditioned patella tendon grafts.

Glaucoma is a common ocular disease that causes irreversible loss of vision. Elevated intraocular pressure (IOP) is the primary risk factor for developing glaucoma. It is believed that increased IOP causes mechanical strain on the glial cells that support the retinal ganglion cell axons and thereby causes ganglion cell apoptosis [1,2]. This damage occurs in the optic nerve head (ONH) region of the eye, and is important for understanding ONH biomechanics.

Rigid polyurethane (PU) foam is widely used as a test medium for mimicking human cancellous bone e.g. for investigating fixation of bone screws [1,2]. Whilst the solid and higher density PU foams are used as standard test materials [3], very little work has been carried out to date on modelling human osteoporotic bone. Osteoporosis (OP) is a bone disease in which bone resorption exceeds bone deposition, resulting in bone loss. Various low density, open cell rigid PU foams are available for use as OP bone models, but none of these has been standardised or universally accepted as a test material. In this study the objective was to determine whether low density PU foam might be suitable for mimicking human osteoporotic bone.

Biaxial tensile testing is the primary experiment used to functionally evaluate cardiovascular fiber-reinforced tissues [1, 2], but has not been widely applied to musculoskeletal tissues. The in situ geometry of many musculoskeletal tissues does not meet uniaxial tensile boundary conditions of freely contracting edges and large aspect ratios. In addition, biaxial tests load the sample through a larger domain of strain configurations as are experienced in situ. In contrast, uniaxial tests represent just a single path within that domain. It has been shown in cardiac tissue and grafts that model parameters determined from biaxial experiments can be used to predict uniaxial behavior, but that uniaxial tests do not predict the biaxial behavior well [1, 2]. Therefore, biaxial mechanical testing is important to address musculoskeletal tissue function.