Unilateral below-knee amputees commonly experience asymmetrical gait patterns and develop comorbidities in their intact (non-amputated) and residual (amputated) legs, with the mechanisms leading to these asymmetries and comorbidities being poorly understood. Prosthetic feet have been designed in an attempt to minimize walking asymmetries by utilizing elastic energy storage and return (ESAR) to help provide body support, forward propulsion and leg swing initiation. However, identifying the influence of walking speed and prosthetic foot stiffness on amputee gait mechanics is needed to develop evidence-based rationale for prosthetic foot selection and treatment of comorbidities. In this research, experimental and modeling studies were performed to identify the influence of walking speed and prosthetic foot stiffness on amputee walking mechanics. The results showed that when asymptomatic and relatively new amputees walk using clinically prescribed prosthetic feet across a wide range of speeds, loading asymmetries exist between the intact and residual knees. However, knee intersegmental joint force and moment quantities in both legs were not higher compared to non-amputees, suggesting that increased knee loads leading to joint disorders may develop in response to prolonged prosthesis usage or the onset of joint pathology over time. In addition, the results showed that decreasing ESAR foot stiffness can increase prosthesis range of motion, mid-stance energy storage, and late-stance energy return. However, the prosthetic foot contributions to forward propulsion and swing initiation were limited due to muscle compensations needed to provide body support and forward propulsion in the absence of residual leg ankle muscles. A study was also performed that integrated design optimization with forward dynamics simulations of amputee walking to identify the optimal prosthetic foot stiffness that minimized metabolic cost and intact knee joint forces. The optimal stiffness profile stiffened the toe and mid-foot while making the ankle less stiff, which decreased the intact knee joint force during mid-stance while reducing the overall metabolic cost of walking. These studies have provided new insight into the relationships between prosthetic foot stiffness and amputee walking mechanics, which provides biomechanics-based rationale for prosthetic foot prescription that can lead to improved amputee mobility and overall quality of life. / text

While amputee gait has been studied in great detail, the influence of prosthetic foot sagittal and coronal plane stiffness on amputee walking biomechanics is not well understood. In order to investigate the effects of sagittal and coronal plane foot stiffness on amputee walking, a framework for manipulating the stiffness of a prosthetic foot needs to be developed. The sagittal and coronal plane stiffness of a low profile carbon fiber prosthetic foot was manipulated through coupling with selective-laser-sintered prosthetic ankles. The carbon fiber foot provided an underlying non-linear stiffness profile while the ankle modified the overall stiffness of the ankle-foot combination. A design of experiments was performed to determine the effect of four prosthetic ankle dimensions (keel thickness, keel width, space between the ankle top and bottom faces, and the location of the pyramid connection) on ankle-foot sagittal and coronal plane stiffness. Ankles were manufactured using selective laser sintering and statically tested to determine stiffness. Two of the dimensions, space between the ankle top and bottom faces and the location of the pyramid connection, were found to have the largest influence on both sagittal and coronal plane stiffness. A third dimension, keel thickness, influenced only coronal plane stiffness. A number of prosthetic ankle-foot combinations were created that encompassed a range of sagittal and coronal plane stiffness levels that were lower than that of the low profile carbon fiber foot alone. To further test the effectiveness of the framework to manipulate sagittal and coronal plane stiffness, two ankle-foot combinations, one stiffer than the other in the sagittal and coronal planes, were used in a case study analyzing amputee walking biomechanics. Differences in stiffness were large enough to cause noticeable changes in amputee kinematics and kinetics during turning and straight-line walking. Future work will expand the range of ankle-foot stiffness levels that can be created using this framework. The framework will then be used to create ankle-foot combinations to investigate the effect of sagittal and coronal plane stiffness on gait mechanics in a large sample of unilateral transtibial amputees. / text

Unilateral, below-knee amputees have altered gait mechanics, which can significantly affect mobility. For example, amputees often have asymmetric leg loading as well as higher metabolic cost and an increased risk of falling compared to non-amputees. Below-knee amputees lose the functional use of the ankle muscles, which are critical in non-amputee walking for providing body support, forward propulsion and leg-swing initiation. The ankle muscles also regulate angular momentum in non-amputees, which is important for providing body stability and preventing falls. Thus, compensatory mechanisms in amputee walking are developed to accomplish the functional tasks normally provided by the ankle muscles. In Chapters 2 and 3, three-dimensional forward dynamics simulations of amputee and non-amputee walking were generated to identify compensatory mechanisms and their effects on joint loading and metabolic cost. Results showed that the prosthesis provided body support, but did not provide sufficient body propulsion or leg-swing initiation. As a result, compensations by the residual leg gluteus maximus, gluteus medius, and hamstrings were needed. The simulations also showed the intact leg tibio-femoral joint contact impulse was greater than the residual leg and that the vasti and hamstrings were the primary contributors to the joint impulse on both the intact and residual legs. The amputee simulation had higher metabolic cost than the non-amputee simulation, which was primarily due to prolonged muscle activity from the residual leg gluteus maximus, gluteus medius, hamstrings, vasti and intact leg vasti and ankle muscles. In Chapter 4, whole-body angular momentum in amputees and non-amputees was analyzed. Reduced residual leg propulsion resulted in a smaller range of sagittal plane angular momentum in the second half of the gait cycle. Thus, to conserve angular momentum, reduced braking was needed in the first half of the gait cycle. Decreased residual leg braking appears to be an important mechanism to regulate sagittal plane angular momentum in amputee walking, but was also associated with a greater range of angular momentum that may contribute to reduced stability in amputees. These studies have provided important insight into compensatory mechanisms in below-knee amputee walking and have the potential to guide rehabilitation methods to improve amputee mobility. / text

Osteoarthritis (OA) is a debilitating disease affecting roughly 31 million Americans. The incidence of OA is significantly higher for persons who have suffered a transtibial amputation. Abnormal cartilage stress can cause higher OA risk, however it is unknown if there is a connection between exercise type and cartilage stress. To help answer this, a tibiofemoral FEA model was created. Utilizing linear elastic isotropic materials and non-linear springs, the model was validated to experimental cadaveric data. In a previous study, 6 control and 6 amputee subjects underwent gait and cycling experiments. The resultant knee loads were analyzed to find the maximum compressive load and the respective shear forces and rotation moments for each trial, which were then applied to the model. Maximum tibial contact stress values were extracted for both the medial and lateral compartments. Only exercise choice in the lateral compartment was found to be a significant interaction (p<0.0001). No other interactions in either compartment were significant. This suggests that cycling reduces the risk for lateral OA regardless of amputation status and medial OA risk is unaffected. This study also developed a process for creating subject-specific FEA models.

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Millions of individuals around the globe are impacted by osteoarthritis, which is the prevailing type of arthritis. This condition arises as a result of gradual deterioration of the protective cartilage that safeguards the ends of the bones. This is especially true of transtibial amputees, who have a significantly higher incidence of osteoarthritis of the knee in their intact limb than non-amputees. Engaging in regular physical activity, managing weight effectively, and undergoing specific treatments can potentially slow down the advancement of the disease and enhance pain relief and joint function. Nevertheless, the relationship between the type of exercise and its impact on cartilage stress remains uncertain. In order to address this question, tibiofemoral finite element analysis (FEA) models were developed. The models incorporated more realistic material properties for cartilage, hexahedral elements, and non-linear springs for ligaments. To ensure their accuracy, the models were validated against experimental data obtained from cadaveric studies. The contact loads and flexion angles of two individuals with amputations and one individual without amputation, which were obtained in a previous study conducted at Cal Poly, were implemented in the FEA models for gait, cycling, and elliptical exercises. The FEA models were used to extract the maximum stress values experienced in the tibial contact areas, specifically in the medial and lateral compartments of the knee. In cycling, the normalized contact pressure on the tibial articular cartilage, relative to body weight, was generally higher for the two participants with amputations compared to the control participant, except for the medial compartment. Furthermore, when comparing different exercises, cycling resulted in the lowest contact pressure values, with elliptical and walking exercises producing similar maximum values. The findings indicated that individuals with amputations are at a greater risk of developing OA, regardless of the type of exercise performed. However, among the different exercises studied, cycling was found to exert the lowest levels of compression stress on the tibial cartilage.

Several studies have been devoted to the metabolic costs of amputees walking on prostheses with different masses added to their components. However, limited study has been directed at quantifying the mass differences of the actual materials available to amputees and the metabolic and mechanical work required to walk on these materials. The energetics of two materials currently used in the design of lower extremity prosthetics were examined in an attempt to determine if mass differences had an effect on amputee walking. A total of fifteen, unilateral amputees (8 transfemoral and 7 transtibial) performed treadmill walking on prostheses assembled from titanium and stainless steel components. Standardized components (knees, pylons, adapters, feet) made from each material were added below the level of the socket. Submaximal oxygen consumption {W/kg} and mechanical power allowing transfers within and between segments {W/kg} were calculated as subjects walked at self-selected velocities until steady state was achieved. Results show that despite significant mechanical differences [F(1,12)= 4.85, p<.048], the decreased mass associated with the use of titanium materials does not have an effect on the metabolic costs [F(1,14)=1.45, p<.249] of the subjects in this study. In addition, stride rate and stride length showed little differences when walking with both materials. Further division of subjects by age and experience walking on a prosthesis do suggest that older amputees and established walkers do benefit most from the use of titanium, both metabolically and mechanically. The choice of materials for use in every day walking will display differences in the mechanical work of amputees however, these differences are not great enough to realize metabolic consequences. / Thesis / Master of Science (MS)

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