Powered Hip Exoskeleton Reduces Residual Hip Effort Without Affecting Kinematics and Balance in Individuals With Above-Knee Amputations During Walking.

OBJECTIVE: A unilateral, lightweight powered hip exoskeleton has been shown to improve walking economy in individuals with above-knee amputations. However, the mechanism responsible for this improvement is unknown. In this study we assess the biomechanics of individuals with above-knee amputations walking with and without a unilateral, lightweight powered hip exoskeleton. We hypothesize that assisting the residual limb will reduce the net residual hip energy. METHODS: Eight individuals with above-knee amputations walked on a treadmill at 1 m/s with and without a unilateral powered hip exoskeleton. Flexion/extension assistance was provided to the residual hip. Motion capture and inverse dynamic analysis were performed to assess gait kinematics, kinetics, center of mass, and center of pressure. RESULTS: The net energy at the residual hip decreased from 0.05±0.04 J/kg without the exoskeleton to -0.01±0.05 J/kg with the exoskeleton (p = 0.026). The cumulative positive energy of the residual hip decreased on average by 18.2% with 95% confidence intervals (CI) (0.20 J/kg, 0.24 J/kg) and (0.16 J/kg, 0.20 J/kg) without and with the exoskeleton, respectively. During stance, the hip extension torque of the residual limb decreased on average by 37.5%, 95% CI (0.28 Nm/kg, 0.36 Nm/kg), (0.17 Nm/kg, 0.23 Nm/kg) without and with the exoskeleton, respectively. CONCLUSION: Powered hip exoskeleton assistance significantly reduced the net residual hip energy, with concentric energy being the main contributor to this change. We believe that the reduction in residual hip extension torque during early stance is the main contributor to this reduction. SIGNIFICANCE: This analysis shows that by assisting the residual hip, the exoskeleton significantly decreased the net hip energy produced by the residual limb, which may explain the improvements in walking economy previously observed.

Assistive Powered Hip Exoskeleton Improves Self-Selected Walking Speed in One Individual with Hemiparesis: A Case Study.

Most individuals suffering a stroke have permanent weakness on one side of the body (hemiparesis) that reduces their ability to ambulate. Autonomous powered exoskeletons have been proposed as a possible solution to this problem. Studies with healthy subjects show that assistive powered exoskeletons have the potential to improve gait, for example, by reducing the metabolic cost of walking. However, only a handful of studies have been conducted with individuals with hemiparesis. Thus, the ability of autonomous exoskeletons to improve gait in this population remains largely unknown. In this study, we assess self-selected walking speed with and without an autonomous powered hip exoskeleton in one individual with hemiparesis walking on level ground. Results show that the proposed exoskeleton improves self-selected walking speed by ~30%. The biomechanical analysis suggest that the increased walking speed is the result of the powered hip exoskeleton enabling the subject to take longer strides on the hemiparetic side. This case study provides important information to inform future exoskeleton development and clinical study design.

Powered hip exoskeleton improves walking economy in individuals with above-knee amputation.

Above-knee amputation severely reduces the mobility and quality of life of millions of individuals. Walking with available leg prostheses is highly inefficient, and poor walking economy is a major problem limiting mobility. Here we show that an autonomous powered hip exoskeleton assisting the residual limb significantly improves metabolic walking economy by 15.6 ± 2.9% (mean ± s.e.m.; two-tailed paired t-test, P = 0.002) in six individuals with above-knee amputation walking on a treadmill. The observed metabolic cost improvement is equivalent to removing a 12-kg backpack from a nonamputee individual. All participants were able to walk overground with the exoskeleton, including starting and stopping, without notable changes in gait balance or stability. This study shows that assistance of the user's residual limb with a powered hip exoskeleton is a viable solution for improving amputee walking economy. By significantly reducing the metabolic cost of walking, the proposed hip exoskeleton may have a considerable positive impact on mobility, improving the quality of life of individuals with above-knee amputations.

Self-Aligning Mechanism Improves Comfort and Performance With a Powered Knee Exoskeleton.

Misalignments between powered exoskeleton joints and the user's anatomical joints are inevitable due to difficulty locating the anatomical joint axis, non-constant location of the anatomical joint axis, and soft-tissue deformations. Self-aligning mechanisms have been proposed to prevent spurious forces and torques on the user's limb due to misalignments. Several exoskeletons have been developed with self-aligning mechanisms based on theoretical models. However, there is no experimental evidence demonstrating the efficacy of self-aligning mechanisms in lower-limb exoskeletons. Here we show that a lightweight and compact self-aligning mechanism improves the user's comfort and performance while using a powered knee exoskeleton. Experiments were conducted with 14 able-bodied subjects with the self-aligning mechanism locked and unlocked. Our results demonstrate up to 15.3% increased comfort and 38% improved performance when the self-aligning mechanism was unlocked. Not surprisingly, the spurious forces and torques were reduced by up to 97% when the self-aligning mechanism was unlocked. This study demonstrates the efficacy of self-aligning mechanisms in improving comfort and performance for sit-to-stand and position tracking tasks with a powered knee exoskeleton.

A kinematic and kinetic dataset of 18 above-knee amputees walking at various speeds.

Motion capture is necessary to quantify gait deviations in individuals with lower-limb amputations. However, access to the patient population and the necessary equipment is limited. Here we present the first open biomechanics dataset for 18 individuals with unilateral above-knee amputations walking at different speeds. Based on their ability to comfortably walk at 0.8 m/s, subjects were divided into two groups, namely K2 and K3. The K2 group walked at [0.4, 0.5, 0.6, 0.7, 0.8] m/s; the K3 group walked at [0.6, 0.8, 1.0, 1.2, 1.4] m/s. Full-body biomechanics was collected using a 10-camera motion capture system and a fully instrumented treadmill. The presented open dataset will enable (i) clinicians to understand the biomechanical demand required to walk with a knee and ankle prosthesis at various speeds, (ii) researchers in biomechanics to gain new insights into the gait deviations of individuals with above-knee amputations, and (iii) engineers to improve prosthesis design and function.

ExoProsthetics: Assisting Above-Knee Amputees with a Lightweight Powered Hip Exoskeleton.

In this paper, we explore the effect of residual hip assistance in one above-knee amputee subject using a novel lightweight powered hip exoskeleton. Differently from a powered prosthesis, a powered hip exoskeleton adds mass proximally. Thus, we expect that the negative effect of the added mass will be lower for a powered hip exoskeleton than a powered ankle and knee prosthesis. Consequently, residual-hip assistance may more easily lead to a net reduction of metabolic effort. To preliminarily assess this hypothesis, we measured the physiological cost index (PCI) while an above-knee subject walked with and without a powered hip exoskeleton. Experimental results show 20.5% reduction of PCI when walking with the powered hip exoskeleton.

A Dual-Pivot Pattern Simulating Native Knee Kinematics Optimizes Functional Outcomes After Total Knee Arthroplasty.

BACKGROUND: Few studies on kinematics correlate patterns to functional outcomes after total knee arthroplasty (TKA). The purpose of this study was to determine if lateral pivot motion in early flexion and medial pivot in high flexion, simulating native knee kinematics, produces superior clinical outcomes. A second objective was to determine if specific kinematic patterns produce superior outcomes. METHODS: One hundred twenty consecutive TKAs were performed using sensor trials to record intraoperative knee kinematics. Lateral and medial pivot pattern designations were based on the center of rotation within 3 flexion zones: 0°-45° (early), 45°-90°, and 90° to full flexion (late). Knee Society Scores, pain scores, and patient satisfaction were analyzed in relation to kinematic patterns. RESULTS: Knee Society function scores were higher in TKAs with early lateral pivot/late medial pivot intraoperative kinematics compared to all other kinematic patterns (P = .018), and there was a greater decrease in the proportion who reported that their knee never feels normal (P = .011). Early lateral/late medial pivot had greater function scores at 1-year (P < .001) and improvement from preoperative baseline (P = .008) compared to those with the least ideal pattern. All patients with the most ideal pattern compared to none of the least ideal pattern reported they were very satisfied (P = .003). CONCLUSION: Patients with an intraoperative early lateral pivot pattern followed by medial pivot motion in later flexion, reported higher functional outcome scores along with higher overall patient satisfaction. Replicating the dual-pivot kinematic pattern observed in native knees may improve function and satisfaction after TKA.

Do Medial Pivot Kinematics Correlate With Patient-Reported Outcomes After Total Knee Arthroplasty?

BACKGROUND: Many total knee arthroplasty (TKA) implants are designed to facilitate a medial pivot kinematic pattern. The purpose of this study was to determine whether intraoperative medial pivot kinematic patterns are associated with improved patient outcomes. METHODS: A retrospective review of consecutive primary TKAs was performed. Sensor-embedded tibial trials determined kinematic patterns intraoperatively. The center of rotation (COR) was identified from 0° to 90° and from 0° to terminal flexion, and designated medial-pivot or non-medial pivot based on accepted criteria. Patient-reported outcomes were measured preoperatively and at minimum one-year follow-up. RESULTS: The analysis cohort consisted of 141 TKAs. Mean age and median BMI were 63.7 years and 33.8 kg/m2, respectively. Forty-percent of TKAs demonstrated a medial pivot kinematic pattern intraoperatively. A medial pivot pattern was more common with posterior cruciate-retaining (CR) and posterior cruciate-substituting/anterior lipped (CS) implants when compared to posterior stabilized (PS) TKAs (P ≤.0150). Regardless of bearing type, minimum one-year Knee Society scores and UCLA activity level did not significantly differ based on medial vs non-medial pivot patterns (P ≥.292). For patients with posterior cruciate-sacrificing implants, there were trends for greater median improvement in Knee Society objective (46 vs 31.5 points, P =.057) and satisfaction (23 vs 14 points, P =.067) scores in medial pivot knees. CONCLUSION: A medial pivot pattern may not significantly govern clinical success after TKA based on intraoperative kinematics and modern outcome measures. Further research is warranted to determine if a particular kinematic pattern promotes optimal clinical outcomes.

Leaving Residual Varus Alignment After Total Knee Arthroplasty Does Not Improve Patient Outcomes.

BACKGROUND: Recent popularity of kinematic alignment and constitutional varus has caused some surgeons to leave varus limbs in residual varus after total knee arthroplasty (TKA). This study assessed whether if patients left in residual varus have improved outcomes compared with those fully corrected to neutral alignment. METHODS: A retrospective review of 361 consecutive primary TKAs was performed. Anatomic tibiofemoral alignment was measured and knees were categorized as neutral, varus, or valgus. Modern Knee Society scores and University of California Los Angeles Activity Level scores were collected at minimum 1-year follow-up. RESULTS: After exclusions for confounds and loss to follow-up, 262 knees were available for analysis, 67% (176) of which were preoperatively varus. Sixty-six percent of varus knees were corrected to neutral, 25.6% were left in residual varus, and 8.5% were corrected to valgus. Median Knee Society objective scores at latest follow-up were greater in knees corrected to neutral (97), followed by knees corrected to varus (95), and valgus (93; P = .025), but post hoc comparisons between pairs of medians were not significant. There was no difference between groups in any other outcome measure (P ≥ .245) or the amount of improvement from baseline (P ≥ .423). Sixty percent of native varus patients corrected to neutral, 64% of those corrected to varus, and 40% of those corrected to valgus reported that their knee felt normal (P = .193). CONCLUSION: Findings fail to support the notion that leaving varus knees in residual varus will improve outcomes and pain. Caution is advised when leaving limbs in residual varus after TKA.

Safe Selection of Outpatient Joint Arthroplasty Patients With Medical Risk Stratification: the "Outpatient Arthroplasty Risk Assessment Score".

BACKGROUND: Current patient selection criteria and medical risk stratification methods for outpatient primary total joint arthroplasty (TJA) surgery are unproven. This study assessed the predictive ability of a medically based risk assessment score in selecting patients for outpatient and short stay surgery. METHODS: A retrospective review of 1120 consecutive primary TJAs in an early discharge program was performed. An Outpatient Arthroplasty Risk Assessment ("OARA") score was developed by a high-volume arthroplasty surgeon and perioperative internal medicine specialist to stratify patients as "low-moderate risk (≤59)" and "not appropriate" (≥60) for early discharge. OARA, American Society of Anesthesiologists Physical Status Classification System (ASA-PS), and Charlson comorbidity index (CCI) scores were analyzed with respect to length of stay. RESULTS: The positive predictive value of the OARA score was 81.6% for the same or the next day discharge, compared with that of 56.4% for ASA-PS (P < .001) and 70.3% for CCI (P = .002) scores. Patients with OARA scores ≤59 were 2.0 (95% confidence interval [CI], 1.4-2.8) times more likely to be discharged early than those with scores ≥60 (P < .001), while a low ASA-PS score was 1.7 (95% CI, 1.2-2.3) times more likely to be discharged early (P = .001). CCI did not predict early discharge (P ≥ .301). With deliberate patient education and expectations for outpatient discharge, the odds of early discharge predicted by the OARA score, but not the ASA-PS score, increased to 2.7 (95% CI, 1.7-4.2). CONCLUSION: The OARA score for primary TJA has more precise predictive ability than the ASA-PS and CCI scores for the same or next day discharge and is enhanced with a robust patient education program to establish appropriate expectations for early discharge. Early results suggest that the OARA score can successfully facilitate appropriate patient selection for outpatient TJA, although consideration of clinical program maturity before adoption of the score is advised.