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Welcome to the musculoskeletal biomechanics lab


The general goal of our laboratory is to understand normal and pathological effects of mechanical factors on the structure and function of human tissues, with an emphasis on the musculoskeletal system. The research conducted in the Musculoskeletal Biomechanics Lab, under the direction of Prof. Amit Gefen, is unified under the umbrella of integrated biomechanics, and utilizes state-of-the-art experimental, analytical, and numerical simulation techniques. Work in the laboratory has yielded fundamental information about adaptation and damage phenomena in bone and soft tissues.

Focus

The aetiology of chronic wounds:
Cell-level, tissue-level and organ-level studies


Pressure ulcers under the bony prominences of the pelvis girdle, e.g. the ischial tuberosities and sacrum, are a common complication in patients confined to a wheelchair or bed for prolonged periods. Patients with a spinal cord injury (SCI) or a neuromuscular disease, for example, are susceptible populations.

For a number of years now, there are hypotheses in the literature that elevated deformations in soft tissues warped between bony prominences and external support surfaces (e.g. mattress, cushion) induce cell death over time, and that muscle tissue is particularly susceptible to deformation damage. Our research work provided convincing evidence that supports these hypotheses and in particular, it has strengthen the theory that internal tissue loads, rather than contact pressures between the body and external surfaces, should be used to predict tissue damage.

 

Measurement of internal soft tissue loads

We have introduced, for the first time, a method to determine internal tissue loads in vivo in weight-bearing soft tissues of human subjects, by combining MRI scans and biomechanical finite element (FE) computer models. This method allows for calculation of spatial distributions of tissue deformations, mechanical strains and stresses. To date, we have employed this method to study internal tissue loads in the buttocks of sitting and lying subjects post SCI versus controls, as well as for studying soft tissue loads in residual limbs of subjects post transtibial amputation.

 

Constitutive properties of soft tissues involved in chronic wounds

The development and improvement of the MRI-FE method requires extensive experimental work to characterize the viscoelastic mechanical properties of the soft tissues involved in the wounds, and the contact mechanics between such soft tissue layers, for accurate representation of these properties in the models. Hence, we measure these viscoelastic/contact properties in loading configurations that mimic situations where rigid bones locally compress soft tissues.

 

Identifying risk factors for chronic wounds using computational models

We are developing computational models for identifying risk factors for chronic wounds, to support epidemiological studies in this field. Such models were developed in the lab, for example, to demonstrate that obesity is a risk factor for severe pressure ulcers, after some other literature suggested that obesity might have a protective effect. Obesity is common in subjects with SCI, who are most probable to manage a sedentary lifestyle. Likewise, SCI cause muscle atrophy and some changes to the shapes of bone surfaces, which, as we have shown, further induce elevated localized tissue loads in muscle and fat tissues.

 

Monitoring internal soft tissue loads during everyday life

Our studies are consistently showing that the individual anatomy of the patient (e.g. the sharpness of bony prominences and thickness of the muscle and fat layers) has critical influence on the internal loading conditions in soft tissues, and thereby, on the individual's risk for chronic wounds. Accordingly, we developed a method and experimental system for patient-specific monitoring of internal mechanical loads in soft tissues under weight-bearing bony prominences, in real-time. The system was employed to determine the internal soft tissue loads in the buttocks of a group of patients with SCI, and was able to indicate tissue stresses and exposures to stress over time in a manner applicable to bedside or home monitoring, that is, without interrupting the patient's lifestyle and daily activities. These technologies are of enormous potential, both for basic science and for medical practice.

Accordingly, the applications were extended to develop similar systems for subjects who underwent limb amputations in order to protect the soft tissues overlying the truncated bones from pressure ulcers and deep tissue injury. A dedicated real-time, clinically oriented experimental system was used in patient studies in a hospital and was shown to be practical and clinically feasible. We also recently modified and miniaturized the system so that it can be used by amputation patients at their daily environment, to protect their residual limb from injury.

Other types of systems were developed to monitor internal loads in the plantar soft tissues of the foot continuously and in real-time, in order to protect the feet of patients with diabetic neuropathy from ulcers. A clinical study was conducted to compare internal foot tissue loads between subjects with diabetes and controls, in framework of a research project funded by the Chief Scientist of the Israeli Ministry of Health, called the "Smart Shoes". The objective of this project grant program was to develop a shoe that measures internal loads in the soft tissues of the foot (e.g. compression and shear stresses), in order to ultimately prevent diabetic neuropathic foot ulcers.

 

The biological tolerance of cells and tissues to sustained mechanical loads

Pressure ulcers and deep tissue injuries are associated with excessive soft tissue deformations and inadequate perfusion over critical time durations, as well as with ischemia-reperfusion cycles and deficiency of the lymphatic system. Skeletal muscle shows the lowest tolerance to deformation injuries, compared with more superficial tissues. We are using animal (rat) models of deep tissue injury to monitor the microstructural changes and viability in skeletal muscle tissue/cells over time in response to mechanical loads. One particularly important contribution of our research group to the body of knowledge in this field is a new mathematical sigmoidal characterization of the pressure-time threshold for cell death based on our experimental data from these animal models. This mathematical formulation of the damage law defines the cell and tissue tolerances at short periods of exposure to mechanical load, versus those at longer exposure periods where the tolerance gradually decreases. The sigmoidal function is useful for design of studies in animal models of pressure ulcers and deep tissue injury, and in computer simulations (e.g. finite element simulations) of development of these wounds, where there is often a need to extrapolate from mechanical loads in cells/tissues, to biological damage. Other than animal models, we are also developing and employing experimental cellular mechanics and tissue engineering models of chronic wounds to determine damage thresholds of cells and tissues subjected to sustained deformations.