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.

Design and Evaluation of a Knee Flexion Assistance Exoskeleton for People with Transtibial Amputation.

People with below-knee amputation walk with asymmetric gaits that over time can lead to further musculoskeletal disorders and decreased quality of life. While prosthesis technology is improving, prosthetic ankles may be fundamentally limited in their ability to restore healthy walking patterns because they do not assist the residual knee joint. The knee on the residual limb has muscular deficits due to the loss of the gastrocnemius, a biarticular muscle that crosses both the ankle and knee. Here we present the design, development, and preliminary evaluation of a robotic knee exoskeleton for people with transtibial amputation. The device is intended to restore gastrocnemius-like flexion moments to the knee on the residual limb. The exoskeleton uses a custom offboard actuation and control system to allow for a simple and lightweight design with high torque capabilities. A preliminary walking experiment with one person with transtibial amputation was conducted. The exoskeleton provided a range of knee flexion torque profiles and had an RMS tracking error of 1.9 Nm across four assistance conditions. This device will be used in future studies to explore the effects of providing knee flexion assistance to people with transtibial amputation during walking. Long term, findings from studies with this exoskeleton could motivate future assistive device designs that improve walking mechanics and quality of life for people with limb loss.

Evaluation of novel plantar pressure-based 3-dimensional printed accommodative insoles - A feasibility study.

BACKGROUND: Custom insoles are commonly prescribed to patients with diabetes to redistribute plantar pressure and decrease the risk of ulceration. Advances in 3D printing have enabled the creation of 3D-printed personalized metamaterials whose properties are derived not only from the base material but also the lattice microstructures within the metamaterial. Insoles manufactured using personalized metamaterials have both patient-specific geometry and stiffnesses. However, the safety and biomechanical effect of the novel insoles have not yet been tested clinically. METHODS: Individuals without ulcer, neuropathy, or deformity were recruited for this study. In-shoe walking plantar pressure at baseline visit was taken and sensels with pressure over 200 kPa was used to define offloading region(s). Three pairs of custom insoles (two 3D printed insoles with personalized metamaterials (Hybrid and Full) designed based on foot shape and plantar pressure mapping and one standard-of-care diabetic insole as a comparator). In-shoe plantar pressure measurements during walking were recorded in a standardized research shoe and the three insoles and compared across all four conditions. FINDINGS: Twelve individuals were included in the final analysis. No adverse events occurred during testing. Maximum peak plantar pressure and the pressure time integral were reduced in the offloading regions in the Hybrid and Full but not in the standard-of-care compared to the research shoe. INTERPRETATION: This feasibility study confirms our ability to manufacture the 3D printed personalized metamaterials insoles and demonstrates their ability to reduce plantar pressure. We have demonstrated the ability to modify the 3D printed design to offload certain parts of the foot using plantar pressure data and a patient-specific metamaterials in the 3D printed insole design. The advance in 3D printed technology has shown its potential to improve current care.

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.

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.

Characterization of performance and disinfection resilience of nonwoven filter materials for use in 3D-printed N95 respirators.

The COVID-19 pandemic has caused a high demand for respiratory protection among health care workers in hospitals, especially surgical N95 filtering facepiece respirators (FFRs). To aid in alleviating that demand, a survey of commercially available filter media was conducted to determine whether any could serve as a substitute for an N95 FFR while held in a 3D-printed mask (Stopgap Surgical Face Mask from the NIH 3D Print Exchange). Fourteen filter media types and eight combinations were evaluated for filtration efficiency, breathing resistance (pressure drop), and liquid penetration. Additional testing was conducted to evaluate two filter media disinfection methods in the event that the filters were reused in a hospital setting. Efficiency testing was conducted in accordance with the procedures established for approving an N95 FFR. One apparatus used a filter-holding device and another apparatus employed a manikin head to which the 3D-printed mask could be sealed. The filter media and combinations exhibited collection efficiencies varied between 3.9% and 98.8% when tested with a face velocity comparable to that of a standard N95 FFR at the 85 L min-1 used in the approval procedure. Breathing resistance varied between 10.8 to >637 Pa (1.1 to > 65 mm H2O). When applied to the 3D-printed mask efficiency decreased by an average of 13% and breathing resistance increased 4-fold as a result of the smaller surface area of the filter media when held in that mask compared to that of an N95 FFR. Disinfection by dry heat, even after 25 cycles, did not significantly affect filter efficiency and reduced viral infectivity by > 99.9%. However, 10 cycles of 59% vaporized H2O2 significantly (p < 0.001) reduced filter efficiency of the media tested. Several commercially available filter media were found to be potential replacements for the media used to construct the typical cup-like N95 FFR. However, their use in the 3D-printed mask demonstrated reduced efficiency and increased breathing resistance at 85 L min-1.

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.

Inactivation of Severe Acute Respiratory Coronavirus Virus 2 (SARS-CoV-2) and Diverse RNA and DNA Viruses on Three-Dimensionally Printed Surgical Mask Materials.

BACKGROUND: Personal protective equipment (PPE) is a critical need during the coronavirus disease 2019 (COVID-19) pandemic. Alternative sources of surgical masks, including 3-dimensionally (3D) printed approaches that may be reused, are urgently needed to prevent PPE shortages. Few data exist identifying decontamination strategies to inactivate viral pathogens and retain 3D-printing material integrity. OBJECTIVE: To test viral disinfection methods on 3D-printing materials. METHODS: The viricidal activity of common disinfectants (10% bleach, quaternary ammonium sanitizer, 3% hydrogen peroxide, or 70% isopropanol and exposure to heat (50°C, and 70°C) were tested on four 3D-printed materials used in the healthcare setting, including a surgical mask design developed by the Veterans' Health Administration. Inactivation was assessed for several clinically relevant RNA and DNA pathogenic viruses, including severe acute respiratory coronavirus virus 2 (SARS-CoV-2) and human immunodeficiency virus 1 (HIV-1). RESULTS: SARS-CoV-2 and all viruses tested were completely inactivated by a single application of bleach, ammonium quaternary compounds, or hydrogen peroxide. Similarly, exposure to dry heat (70°C) for 30 minutes completely inactivated all viruses tested. In contrast, 70% isopropanol reduced viral titers significantly less well following a single application. Inactivation did not interfere with material integrity of the 3D-printed materials. CONCLUSIONS: Several standard decontamination approaches effectively disinfected 3D-printed materials. These approaches were effective in the inactivation SARS-CoV-2, its surrogates, and other clinically relevant viral pathogens. The decontamination of 3D-printed surgical mask materials may be useful during crisis situations in which surgical mask supplies are limited.

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.

A smart cane with vibrotactile biofeedback improves cane loading for people with knee osteoarthritis.

Nine million adults have symptomatic knee osteoarthritis (OA) in the U.S. and almost half of those people have a walking aid such as a cane. Proper cane loading (e.g. 15% body weight [BW]) can reduce knee loading and may slow OA progression. The purpose of this study was to investigate the efficacy of a novel smart cane with vibrotactile biofeedback that aims to facilitate increased cane loading. Ten subjects with knee OA performed a 50 m hallway walk test under four conditions: 1) naïve, 2) conventional cane with verbal instruction, 3) smart cane, and 4) conventional cane post smart cane. The cane load (% BW; mean ± 1 standard deviation) for the four conditions was 9.0 ± 1.9 (naïve), 12.7 ± 2.6 (conventional cane), 17.6 ± 2.4 (smart cane), and 15.6 ±3.1 (conventional cane post smart cane). These results indicate that the smart cane's vibrotactile biofeedback helped the users achieve the target cane loading of 15% BW or more as compared to naïve or verbal instruction alone. After using the smart cane, conventional cane loading was higher than the naïve and verbal instruction conditions demonstrating a potential smart cane training effect. Long term increased cane loading may reduce knee pain and improve joint function.

A pediatric robotic thumb exoskeleton for at-home rehabilitation: the Isolated Orthosis for Thumb Actuation (IOTA).

In this paper, we present the design of a thumb exoskeleton for pediatric at-home rehabilitation. Pediatric disorders, such as cerebral palsy (CP) and stroke, can result in thumb in palm deformity greatly limiting hand function. This not only limits children's ability to perform activities of daily living but also limits important motor skill development. Specifically, the device, dubbed IOTA (Isolated Orthosis for Thumb Actuation) is a 2-DOF thumb exoskeleton that can actuate the carpometacarpal (CMC) and metacarpalphalangeal (MCP) joints through ranges of motion required for activities of daily living. The device consists of a lightweight hand-mounted mechanism that can be custom secured and aligned to the wearer. The mechanism is actuated via flexible cables that connect to a portable control box. Embedded encoders and bend sensors monitor the two degrees of freedom of the thumb and flexion/extension of the wrist. Using this platform, a number of control modes can be implemented that will enable the device to be intuitively controlled by a patient to assist with opposition grasp, fine motor control, and ultimately facilitate motor recovery. We envision this at-home device augmenting the current in-clinic therapy and enabling tele-rehabilitation where a clinician can remotely monitor a patient's usage and performance.

Second metatarsal length is positively correlated with increased pressure and medial deviation of the second toe in a robotic cadaveric simulation of gait.

BACKGROUND: The term `crossover second toe' has been used to describe a deformity of the second metatarsophalangeal joint (MTPJ) that includes a progressive migration of the second toe in a dorsal and medial direction. The long onset and complex anatomy of the deformity has led to uncertainty about its etiology and treatment. The purpose of this study was to investigate the relationship between second metatarsal length and second MTPJ plantar pressure and joint angles during gait. We hypothesized that elongation of the second metatarsal would increase the plantar pressure underneath the second MTPJ and be associated with a deviation of the MTPJ angles in a dorsal and medial direction. METHODS: Incremental surgical elongation of the second metatarsal was performed on six cadaveric feet. A robotic gait simulator (RGS) simulated physiologic tibial motion, tendon loading, and ground reaction forces (GRF) on the cadaveric feet. We determined the peak pressure and pressure-time integral under the second MTPJ during gait, as well as the transverse and sagittal MTPJ angles. RESULTS: Second metatarsal peak pressure and pressure-time integral were positively correlated with an increase in second metatarsal length. First metatarsal peak pressure and pressure-time integral were significantly negatively associated with second metatarsal length. MTPJ transverse plane angle was positively associated with second metatarsal length but sagittal angle was not. CONCLUSION: Our results support the hypothesis that second metatarsal length is positively associated with medial deviation of the second toe and increased plantar pressure underneath the second MTPJ. CLINICAL RELEVANCE: It is biomechanically plausible that this association could lead to the joint instability seen in crossover toe patients.

A robotic cadaveric flatfoot analysis of stance phase.

The symptomatic flatfoot deformity (pes planus with peri-talar subluxation) can be a debilitating condition. Cadaveric flatfoot models have been employed to study the etiology of the deformity, as well as invasive and noninvasive surgical treatment strategies, by evaluating bone positions. Prior cadaveric flatfoot simulators, however, have not leveraged industrial robotic technologies, which provide several advantages as compared with the previously developed custom fabricated devices. Utilizing a robotic device allows the researcher to experimentally evaluate the flatfoot model at many static instants in the gait cycle, compared with most studies, which model only one to a maximum of three instances. Furthermore, the cadaveric tibia can be statically positioned with more degrees of freedom and with a greater accuracy, and then a custom device typically allows. We created a six degree of freedom robotic cadaveric simulator and used it with a flatfoot model to quantify static bone positions at ten discrete instants over the stance phase of gait. In vivo tibial gait kinematics and ground reaction forces were averaged from ten flatfoot subjects. A fresh frozen cadaveric lower limb was dissected and mounted in the robotic gait simulator (RGS). Biomechanically realistic extrinsic tendon forces, tibial kinematics, and vertical ground reaction forces were applied to the limb. In vitro bone angular position of the tibia, calcaneus, talus, navicular, medial cuneiform, and first metatarsal were recorded between 0% and 90% of stance phase at discrete 10% increments using a retroreflective six-camera motion analysis system. The foot was conditioned flat through ligament attenuation and axial cyclic loading. Post-flat testing was repeated to study the pes planus deformity. Comparison was then made between the pre-flat and post-flat conditions. The RGS was able to recreate ten gait positions of the in vivo pes planus subjects in static increments. The in vitro vertical ground reaction force was within ± 1 standard deviation (SD) of the in vivo data. The in vitro sagittal, coronal, and transverse plane tibial kinematics were almost entirely within ± 1 SD of the in vivo data. The model showed changes consistent with the flexible flatfoot pathology including the collapse of the medial arch and abduction of the forefoot, despite unexpected hindfoot inversion. Unlike previous static flatfoot models that use simplified tibial degrees of freedom to characterize only the midpoint of the stance phase or at most three gait positions, our simulator represented the stance phase of gait with ten discrete positions and with six tibial degrees of freedom. This system has the potential to replicate foot function to permit both noninvasive and surgical treatment evaluations throughout the stance phase of gait, perhaps eliciting unknown advantages or disadvantages of these treatments at other points in the gait cycle.

Foot bone kinematics as measured in a cadaveric robotic gait simulator.

The bony motion of the foot during the stance phase of gait is useful to further our understanding of joint function, disease etiology, injury prevention and surgical intervention. In this study, we used a 10-segment in vitro foot model with anatomical coordinate systems and a robotic gait simulator (RGS) to measure the kinematics of the tibia, talus, calcaneus, cuboid, navicular, medial cuneiform, first metatarsal, hallux, third metatarsal, and fifth metatarsal from six cadaveric feet. The RGS accurately reproduced in vivo vertical ground reaction force (5.9% body weight RMS error) and tibia to ground kinematics. The kinematic data from the foot model generally agree with invasive in vivo descriptions of bony motion and provides the most realistic description of bony motion currently available for an in vitro model. These data help to clarify the function of several joints that are difficult to study in vivo; for example, the combined range of motion of the talonavicular, naviculocuneiform, metatarsocuneiform joints provided more sagittal plane mobility (27.4°) than the talotibial joint alone (23.2°). Additionally, the anatomical coordinate systems made it easier to meaningfully determine bone-to-bone motion, describing uniplanar motion as rotation about a single axis rather than about three. The data provided in this study allow for many kinematic interpretations to be made about dynamic foot bone motion, and the methodology presents a means to explore many invasive foot biomechanics questions under near-physiologic conditions.

Arthrodesis of the first metatarsophalangeal joint: a robotic cadaver study of the dorsiflexion angle.

BACKGROUND: Arthrodesis of the first metatarsophalangeal joint is indicated for severe osteoarthritis or as a revision of failed treatment for hallux valgus. The literature suggests that an optimum fused dorsiflexion angle is between 20 degrees and 25 degrees from the axis of the first metatarsal. The purpose of this study was to investigate the relationship between dorsiflexion angle and plantar pressure in the postoperative gait. We assumed that there is a fused dorsiflexion angle at which pressures are minimized under the hallux and the first metatarsal head. METHODS: Six cadaver foot specimens underwent incremental changes in simulated fused metatarsophalangeal joint dorsiflexion angle followed by dynamic gait simulation. A robotic gait simulator performed at 50% of body weight and one-fifteenth of physiologic velocity. In vitro tibial kinematics and tendon forces were based on normative in vivo gait and electromyographic data and were manually tuned to match the in vitro ground reaction force and tendon force behavior. Regression lines were calculated for peak pressure and pressure-time integral under the hallux and the metatarsal head by dorsiflexion angle. RESULTS: Peak pressure and pressure-time integral under the hallux were negatively correlated with dorsiflexion angle (p < 0.004), while peak pressure and pressure-time integral under the metatarsal head were positively correlated with dorsiflexion angle (p < 0.004). The intersection of the regression lines that represented the angle at which peak pressure and pressure-time integral were minimized was 24.7 degrees for peak pressure and 21.3 degrees for pressure-time integral. CONCLUSIONS: Our findings support the hypothesis that an angle-pressure relationship exists following arthrodesis of the first metatarsophalangeal joint and that it is inversely related for the hallux and the metatarsal head. Our results encompass the suggested range of 20 degrees to 25 degrees.

Gait simulation via a 6-DOF parallel robot with iterative learning control.

We have developed a robotic gait simulator (RGS) by leveraging a 6-degree of freedom parallel robot, with the goal of overcoming three significant challenges of gait simulation, including: 1) operating at near physiologically correct velocities; 2) inputting full scale ground reaction forces; and 3) simulating motion in all three planes (sagittal, coronal and transverse). The robot will eventually be employed with cadaveric specimens, but as a means of exploring the capability of the system, we have first used it with a prosthetic foot. Gait data were recorded from one transtibial amputee using a motion analysis system and force plate. Using the same prosthetic foot as the subject, the RGS accurately reproduced the recorded kinematics and kinetics and the appropriate vertical ground reaction force was realized with a proportional iterative learning controller. After six gait iterations the controller reduced the root mean square (RMS) error between the simulated and in situ; vertical ground reaction force to 35 N during a 1.5 s simulation of the stance phase of gait with a prosthetic foot. This paper addresses the design, methodology and validation of the novel RGS.