Full body musculoskeletal model for simulations of gait in persons with transtibial amputation.

This paper describes the development, properties, and evaluation of a musculoskeletal model that reflects the anatomical and prosthetic properties of a transtibial amputee using OpenSim. Average passive prosthesis properties were used to develop CAD models of a socket, pylon, and foot to replace the lower leg. Additional degrees of freedom (DOF) were included in each joint of the prosthesis for potential use in a range of research areas, such as socket torque and socket pistoning. The ankle has three DOFs to provide further generality to the model. Seven transtibial amputee subjects were recruited for this study. 3 D motion capture, ground reaction force, and electromyographic (EMG) data were collected while participants wore their prescribed prosthesis, and then a passive prototype prosthesis instrumented with a 6-DOF load cell in series with the pylon. The model's estimates of the ankle, knee, and hip kinematics comparable to previous studies. The load cell provided an independent experimental measure of ankle joint torque, which was compared to inverse dynamics results from the model and showed a 7.7% mean absolute error. EMG data and muscle outputs from OpenSim's Static Optimization tool were qualitatively compared and showed reasonable agreement. Further improvements to the muscle characteristics or prosthesis-specific foot models may be necessary to better characterize individual amputee gait. The model is open-source and available at (https://simtk.org/projects/biartprosthesis) for other researchers to use to advance our understanding and amputee gait and assist with the development of new lower limb prostheses.

A novel workflow to fabricate a patient-specific 3D printed accommodative foot orthosis with personalized latticed metamaterial.

Patients with diabetes mellitus are at elevated risk for secondary complications that result in lower extremity amputations. Standard of care to prevent these complications involves prescribing custom accommodative insoles that use inefficient and outdated fabrication processes including milling and hand carving. A new thrust of custom 3D printed insoles has shown promise in producing corrective insoles but has not explored accommodative diabetic insoles. Our novel contribution is a metamaterial design application that allows the insole stiffness to vary regionally following patient-specific plantar pressure measurements. We presented a novel workflow to fabricate custom 3D printed elastomeric insoles, a testing method to evaluate the durability, shear stiffness, and compressive stiffness of insole material samples, and a case study to demonstrate how the novel 3D printed insoles performed clinically. Our 3D printed insoles results showed a matched or improved durability, a reduced shear stiffness, and a reduction in plantar pressure in clinical case study compared to standard of care insoles.

A Biofabrication Strategy for a Custom-Shaped, Non-Synthetic Bone Graft Precursor with a Prevascularized Tissue Shell.

Critical-sized defects of irregular bones requiring bone grafting, such as in craniofacial reconstruction, are particularly challenging to repair. With bone-grafting procedures growing in number annually, there is a reciprocal growing interest in bone graft substitutes to meet the demand. Autogenous osteo(myo)cutaneous grafts harvested from a secondary surgical site are the gold standard for reconstruction but are associated with donor-site morbidity and are in limited supply. We developed a bone graft strategy for irregular bone-involved reconstruction that is customizable to defect geometry and patient anatomy, is free of synthetic materials, is cellularized, and has an outer pre-vascularized tissue layer to enhance engraftment and promote osteogenesis. The graft, comprised of bioprinted human-derived demineralized bone matrix blended with native matrix proteins containing human mesenchymal stromal cells and encased in a simple tissue shell containing isolated, human adipose microvessels, ossifies when implanted in rats. Ossification follows robust vascularization within and around the graft, including the formation of a vascular leash, and develops mechanical strength. These results demonstrate an early feasibility animal study of a biofabrication strategy to manufacture a 3D printed patient-matched, osteoconductive, tissue-banked, bone graft without synthetic materials for use in craniofacial reconstruction. The bone fabrication workflow is designed to be performed within the hospital near the Point of Care.

Evaluation of a quasi-passive biarticular prosthesis to replicate gastrocnemius function in transtibial amputee gait.

Lower limb amputees experience gait impairments, in part due to limitations of prosthetic limbs and the lack of a functioning biarticular gastrocnemius (GAS) muscle. Energy storing prosthetic feet restore the function of the soleus, but not GAS. We propose a transtibial prosthesis that implements a spring mechanism to replicate the GAS. A prototype Biarticular Prosthesis (BP) was tested on seven participants with unilateral transtibial amputation. Participants walked on an instrumented treadmill with motion capture, first using their prescribed prosthesis, then with the BP in four different spring stiffness conditions. A custom OpenSim musculoskeletal model, including the BP, was used to estimate kinematics, joint torques, and muscle forces. Kinematic symmetry was evaluated by comparing the amputated and intact angles of the ankle, knee, and hip. The BP knee and ankle torques were compared to the intact GAS. Finally, work done by the BP spring was calculated at the ankle and knee. There were no significant differences between conditions in kinematic symmetry, indicating that the BP performs similarly to prescribed prostheses. When comparing the BP torques to intact GAS, higher spring stiffness better approximated peak GAS torques, but those peaks occurred earlier in the gait cycle. The BP spring did positive work on the knee joint and negative work on the ankle joint, and this work increased as BP spring stiffness increased. The BP has the potential to improve amputee gait compensations associated with the lack of biarticular GAS function, which may reduce their walking effort and improve quality of life.

A novel walking cane with haptic biofeedback reduces knee adduction moment in the osteoarthritic knee.

Knee osteoarthritis is a leading cause of ambulatory disability in adults. The most prescribed mobility aid, the walking cane, is often underloaded and therefore fails to reduce knee joint loading and provide symptomatic relief. For this study, a novel walking cane with haptic biofeedback was designed to improve cane loading and reduce the knee adduction moment (KAM). To determine; 1) the short-term efficacy of a novel walking cane using haptic biofeedback to encourage proper cane loading and 2) the effects of the novel cane on KAM. Cane loading and KAM, peak knee adduction moment (PKAM), and knee adduction angular impulse (KAAI)) while walking were calculated under five conditions: 1) naïve, 2A) after scale training (apply 20%BW to cane while standing, using a beam scale), 2B) scale recall (attempt to load the cane to 20%BW), 3A) after haptic training (vibrotactile biofeedback delivered when target cane load achieved), and 3B) haptic recall (attempt to load the cane to 20%BW with vibrotactile biofeedback delivered). Compared to the naïve condition all interventions significantly increased cane loading and reduced PKAM and KAAI. No differences between haptic recall and scale recall condition were observed. The haptic biofeedback cane was shown to be an effective and simple way to increase cane loading and reduced knee loading. Haptic biofeedback and scale training were equally effective at producing immediate short-term improvements in cane loading and knee loading. Future studies should examine the long-term effects of scale training and canes with haptic biofeedback on knee joint health, pain, and osteoarthritis disease progression.

Design and Development of a Quasi-Passive Transtibial Biarticular Prosthesis to Replicate Gastrocnemius Function in Walking.

Lower-limb amputees experience many gait impairments and limitations. Some of these impairments can be attributed to the lack of a functioning biarticular gastrocnemius (GAS) muscle. We propose a transtibial prosthesis that implements a quasi-passive spring mechanism to replicate GAS function. A prototype biarticular prosthesis (BP) was designed, built, and tested on one subject with a transtibial amputation. They walked on an instrumented treadmill with motion capture under three different biarticular spring stiffness conditions. A custom-developed OpenSim musculoskeletal model, which included the BP, was used to calculate the work performed and torque applied by the BP spring on the knee and ankle joints. The BP functioned as expected, generating forces with similar timing to GAS. Work transfer occurred from the ankle to the knee, with stiffer springs transferring more energy. Driven mostly by kinematics, the quasi-passive design of the BP consumed very low power (5.15 W average) and could lend itself well to future lightweight, low-power designs.

A Model-Based Method for Minimizing Reflected Motor Inertia in Off-board Actuation Systems: Applications in Exoskeleton Design.

The research and development of wearable robotic devices has been accelerated by off-board control and actuation systems. While off-board robotic actuation systems provide many benefits, the impedance at the robotic joint is often high. High joint impedance is undesirable for wearable devices like exoskeletons, as the user is unable to move their joint without actively controlled motion from the motors. We propose that the impedance can be reduced substantially in off-board robotic actuation systems by minimizing the reflected inertia from the motor. We have developed a model and optimization-based methodology for selecting a motor and set of mechanical design parameters that minimize reflected inertia. This methodology was implemented in the design of an off-board knee exoskeleton as a case study. A grey-box model was developed that incorporates biomechanical knee trajectories, an experimentally determined human-device interface stiffness model, Bowden cable stiffness and friction, and a motor model. A constrained optimization routine was developed that uses the model and a library of157 candidate servo motors to select the actuator and mechanical design parameters that minimize reflected inertia at the exoskeleton joint. We found that S6 of the motors were able to carry out the necessary torque-velocity trajectories to achieve the prescribed exoskeleton joint torques and limb motions. The optimal motor was the Kollmorgen C133A-one of the largest in the library of candidate servo motors and required a 2.25 cm actuator pulley at the knee joint and a 17.5 cm cable sheave at the motor output. This methodology can be adapted by exoskeleton designers to develop more backdriveable exoskeletons and improve experimental capabilities. All code developed for the case study is open-source and freely available online.

Development of a Robotic Unloader Brace for Investigation of Conservative Treatment of Medial Knee Osteoarthritis.

Knee osteoarthritis (KOA) is a painful and debilitating condition that is associated with mechanical loading of the knee joint. Numerous conservative treatment strategies have been developed to delay time to total joint replacement. Unloader braces are commonly prescribed for medial uni-compartmental KOA, however their evidence of efficacy is inconclusive and limited by user compliance. Typical commercial braces transfer load from the medial knee compartment to the lateral knee compartment by applying a continuous brace abduction moment (BAM). We propose that brace utilization and effectiveness could be improved with a robotic device that intelligently modulates BAM in real time over the course of a step, day, and year to better protect the knee joint, improve pain relief, and increase comfort. To this end, we developed a robotic unloader knee brace ABLE (active brace for laboratory exploration) to flexibly emulate and explore different active and passive brace behaviors that may be more efficacious than traditional braces. The system is capable of modulating BAM within each step per researcher defined unloading profiles. ABLE was realized as a lightweight orthosis driven by an off-board system containing a servo motor, drive, real-time controller, and host PC. Frequency response and intra-step trajectory tracking during level-ground walking were evaluated in a single healthy human subject test to verify system performance. The system tracked BAM vs percent gait cycle trajectories with a root mean square error of 0.18 to 0.58 Nm for conditions varying in walking speed, 85-115% nominal, and trajectory peak BAM, 2.7 to 8.1 Nm. Biomechanical and subjective outcomes will be evaluated next for KOA patients to investigate how novel robotic brace operation affects pain relief, comfort, and KOA progression.

3D Printed lattice microstructures to mimic soft biological materials.

OBJECTIVE: Our group has developed a method for 3D printing mechanically-realistic soft tissue, as a building block towards developing anatomically realistic 3D-printed biomechanical testbed models. METHODS: A Polyjet 3D printer was used to print lattice microstructures, which were tested in compression to evaluate the elastic profile. Lattice properties including element diameter, element spacing (ES), element cross-sectional geometry, element arrangement, and lattice rotation were varied to determine their effect on the stress-strain curve. As a case study, a single 3D printed sample was tuned such that its elastic profile matched plantar fat. RESULTS: Element diameter and ES had the largest effect on the stress-strain profile, and rotating the lattice microstructure tends to linearize the curves. A simple cubic lattice microstructure of cylindrical elements, with 0.5 mm diameter columns and 1.2 mm spacing had a stress-strain curve the was closest to plantar fat. The elastic modulus at 10, 30, and 50% strain was 7.55, 9.50, and 252 kPa respectively. Physiologic plantar fat at the same strain values has moduli values of 1.08, 7.13, and 188 kPa. SIGNIFICANCE: We demonstrated that lattice microstructures can decrease the young's modulus of soft 3D printed materials by three orders of magnitude. By creating a method for fine-tuning the elastic profile of 3D-printed materials to behave like human soft tissue, we provide an attractive alternative to more exotic and time-consuming techniques such as molding and casting.