ABSTRACT

Introduction

Osseointegrated implants for direct skeletal attachment of transtibial prosthesis carry risks that are yet to be fully resolved, such as early loosening, mechanical failure of percutaneous and medullar parts of implant, periprosthetic issues, and infections. Underloading could lead to early loosening and infection. Overloading might compromise the bone–implant interface. Therefore, Goldilocks loading regimen applied by transtibial bone-anchored prostheses is critical for safe and efficient development of osseointegration around the implant during rehabilitation and beyond. We hypothesized that Goldilocks loading could be achieved when ambulating with a so-called anthropomorphic prosthetic ankle showing moment–angle relationship similar to a sound ankle.

Materials and Methods

Quantitative characteristics of the moment–angle curve of the sound ankle during dorsiflexion phase of a free-pace walking were extracted for 4 able-bodied participants (experiment 1). A slope of the moment–angle curve (stiffness) was calculated twice: for the first half and for the second half of the moment–angle curve. The difference of stiffnesses (those at the second half minus at the first half) was called the index of anthropomorphicity (IA). By definition, positive IA is associated with concave shape of the moment–angle curve, and the negative IA is associated with convex shape. In experiment 2, the same recordings and calculations were performed for 3 participants fitted with transtibial osseointegrated fixation during walking with their usual feet and the Free-Flow Foot (Ohio Willow Wood). The Free-Flow Foot was selected for its anthropomorphicity demonstrated in the previous studies with amputees using traditional socket attachment.

Results

The IA was 5.88 ± 0.93 for the able-bodied participants, indicating that the stiffness during the first part of the dorsiflexion phase was substantially fewer than during the second parts, as the calf muscles resisted to angulation in ankle substantially less than during the second part of dorsiflexion phase. For amputees fitted with Free-Flow Foot, IA was 2.68 ± 1.09 and −2.97 ± 2.37 for the same amputees fitted with their usual feet.

Conclusions

Indexes of anthropomorphicity, while of different magnitude, were positive in control able-bodied group and in the amputee group wearing Free-Flow Foot, which was qualitatively associated with concave shape of their moment–angle curves. The 3 usual feet worn by the participants were classified as nonanthropomorphic as their individual moment–angle curves were convex and the corresponding IAs were negative. Furthermore, this study showed that a foot with anthropomorphic characteristics tends to decrease maximal loads at the bone–implant interface as compared to the nonanthropomorphic feet and possibly may minimize the risks to compromise the integrity of this interface.

INTRODUCTION

Over 2.2 million U.S. patients are projected to have limb loss by 2020, with the number rising to 3.6 million by 2050.1 In military medicine, advances in combat casualty care have increased survival rates of battlefield injuries, including high-energy open tibial fractures.2 Through 2015, over 1,600 service members have had a major limb amputation as the result of combat wounds sustained in Iraq and Afghanistan.3 Over 20% of these members have lost more than 1 limb.4

Compensatory Synergy in Prosthetic Gait

Every step on a leg prosthesis requires the amputee to create a bending moment in the prosthetic ankle in order to overcome its resistance to flexion-dorsiflexion. To achieve it, the amputee generates by the stump a pair of equal and opposite forces applied to the socket (Fig. 1 (a) and (b)).5 When a prosthetic ankle is excessively resistive to angulation, a larger bending moment is required from the stump, and a correspondingly greater loading is applied back to it from the socket. This is illustrated with the convex moment–angle graph in Fig. 1, indicating that the initiation of the angulation requires greater moment than its progression.

FIGURE 1.

Upper panel—example of lean and bulbous transtibial residuum, where compressive and frictional forces from the socket contribute to immune dysregulation.12 Lower panel—pair of forces F, −F (a) and F1, −F1 (b), which the amputee has to generate by the stump to dorsiflex the prosthetic ankle with convex (1) or concave (2) shape of its moment of resistance to dorsiflexion.5

FIGURE 1.

Upper panel—example of lean and bulbous transtibial residuum, where compressive and frictional forces from the socket contribute to immune dysregulation.12 Lower panel—pair of forces F, −F (a) and F1, −F1 (b), which the amputee has to generate by the stump to dorsiflex the prosthetic ankle with convex (1) or concave (2) shape of its moment of resistance to dorsiflexion.5

Conversely, if the ankle moment–angle graph is concave (Figs. 1, 2), allowing for almost free initial dorsiflexion, the force couple F1, −F1 (Fig. 1 (b)) and corresponding loads on the stump are lower than F, −F (Fig. 1 (a)).5

FIGURE 2.

Potential relationship between Goldilocks zone and anthropomorphicity of prosthetic feet characterized by stiffness curve with concave shape for anthropomorphic prosthetic ankles (A) and convex stiffness curve nonanthropomorphic ankles (B).18

FIGURE 2.

Potential relationship between Goldilocks zone and anthropomorphicity of prosthetic feet characterized by stiffness curve with concave shape for anthropomorphic prosthetic ankles (A) and convex stiffness curve nonanthropomorphic ankles (B).18

To decrease excessive loads from the socket walls, the wearer intuitively avoids bending the rigid ankle unit, which synergistically decreases the knee stance-flexion angle in the involved leg even though the anatomical knee joint is intact, resulting in increased gait asymmetry with the uninvolved leg.6 Another compensatory locomotor strategies include asymmetric loading of the limbs, altering kinematic parameters, and developing abnormal trunk motion.7–9 Relying heavily on their intact limb during ambulation, trans-tibial amputees have an increased risk of developing osteoarthritis in the knee of their intact limb.10–12

Amputee patients who have endured battle trauma have a disproportionately high prevalence of skin problems on their stumps compared with civilian patients.13,14 These issues are experienced by 73.9% of patients who became amputees because of mine explosion and by 80.3% of patients who became amputees because of gunshot.11 The skin in the socket of a prosthesis must withstand the compressive and frictional forces for which it is poorly adapted (Fig. 1, upper panel).10–12,15,16

Some of these shortcomings could be alleviated by direct skeletal attachment where the socket is replaced by an osseointegrated fixation including a percutaneous part (abutment) that enables attachment of bone-anchored prosthesis. However, the surgical implantations of osseointegrated fixation carry risks that are yet to be fully resolved. For example, consistent excessive bending of the abutment because of nonregulated ankle resistance to angulation may compromise the bone–implant interface, leading to early loosening, mechanical failure of percutaneous and medullar parts of implant.17 Underloading could also lead to early loosening and infection.

Therefore, Goldilocks loading regimen applied by transtibial bone-anchored prostheses is critical for safe and efficient development of osseointegration around the fixation during rehabilitation and beyond (Fig. 2).

The purpose of the study was to verify a hypothesis that Goldilocks loading can be achieved with a so-called anthropomorphic prosthetic ankle, being defined as showing moment–angle relationship that resembles those generated by a sound ankle.5,6 To quantitatively compare the moment–ankle graphs, we developed a numerical parameter called index of anthropomorphicity (IA). We developed a technique for its calculation using 2 consecutive slopes of each moment–ankle graph (ankle stiffnesses).18

Specifically, the difference of these stiffnesses (those at the second half minus at the first half) has been called the IA. By definition, positive IA is associated with concave shape of the moment–angle curve, and the negative IA is associated with the convex shape.

A guidance for designing prosthetic feet suggests to mimic the stiffness of the anatomical ankle (anthropomorphic) to minimize the damaging moment on the residuum or the osseointegrated fixation as compared to nonanthropomorphic prostheses (Fig. 2).7,19

Unfortunately, it is not seen in the majority of the current prostheses lead to overloading of amputees’ residuum, with serious, which negatives consequences.

We believe that properly attuned stiffness of an anthropomorphic prosthetic ankle could significantly mitigate the risks of commonly experienced adverse events, particularly loosening, periprosthetic issues, and, possibly, infections.

In the long term, we plan to demonstrate experimentally an urgent need to develop regulations for the prosthetic feet performance and suggest the IA as the first numerical criterion of safe body–prosthesis interface.

MATERIALS AND METHODS

Ethics Review Statement

Gait study with able-bodied subjects was approved by Sinai IRB, New England Sinai Hospital, 150 York St., Stoughton, MA 02072, USA. Gait study with amputee subjects was approved by Queensland University of Technology IRB, 2 George St, Brisbane, 4000 QLD, Australia.

Study Design

The observational case series involves 2 experiments focusing on able-bodied and individuals with transtibial amputation. The sampling frequency for the recording on the 3D kinematic and kinetic data for experiment 1 was 200 Hz and that on the 2D kinematic data for experiment 2 was 60 Hz. In both experiments, participants were encouraged to take as much rest as needed between walking trials.

Experiment 1: Control Arm With Able-bodied Participants

The stiffness characteristics of sound ankle were extracted for 4 able-bodied participants (2 males, 2 females, 25 ± 2.5 years, 1.71 ± 0.12 m, 68 ± 1.72 kg). Participants were recruited by New England Sinai Hospital between January and April 2014.

Able-bodied participants walked 3 trials at self-selected speed in the Gait lab equipped with 6 cameras (Vicon Motion Analysis System, Oxford, UK) and 2 Kistler force plates (Kistler Instrument Corp., Novi, MI, USA). Dorsiflexion angle data were extracted from the 3D motion capture with the video and force plate sampling frequency of 200 Hz and using the Modified Helen Hayes full body reflective 9-mm marker set. Bending moment data were calculated using inverse dynamics.

Experiment 2: Intervention Arm With Participants With Transtibial Amputation

The experiment 2 was primarily designed to measure directly the load applied on the residuum of individuals with transtibial osseointegrated implant during daily activities. One of the objectives of this experiment was to analysis kinetic data applied during daily activities performed in a nonexperimental environment. This reduced opportunities to record complementary kinematic data using a typical 3D motion capture. Alternatively, each trial was video recorded to assist a retrospective visual inspection, when needed. These footages were sufficient to determine the angle of dorsiflexion but not the speed of walking. Alternatively, we provided the cadence and other spatiotemporal characteristics.

The stiffness characteristics of the prosthetic feet was extracted for 3 participants with transtibial amputation fitted with press-fit osseointegrated fixation (2 males, 1 female, 58 ± 17 years, 1.76 ± 0.17 m, 83.46 ± 24.84 kg). Participants were recruited by prosthetists in Brisbane, Australia, between May and July 2017.

First, participants 1, 2, and 3 walked 5 trials with their own prostheses including RUSH foot (RUSH), Trias 1C30 foot, and Triton Vertical shock 1C6 foot (both by Otto Bock), respectively. Then, participants walked with the Free-Flow Foot (Ohio Willow Wood).18 The Free-Flow Foot was selected because of its stiffness curve’s concavity, which is also observed in the ankle in the norm (Fig. 3).20,21

FIGURE 3.

Example of long axes of the leg (LGL) and foot (LGF) used to determine ankle angle of dorsiflexion of the instrumented transtibial bone-anchored prosthesis attached to residuum (A) and percutaneous part of osseointegration fixation (B) including connector (C), transducer (D), pylon (E), and multi-axial rolling Free-Flow Foot (Ohio Willow Wood) (F) with anthropomorphic moments of dorsiflexion, inversion/eversion, and axial rotation featuring screw for adjustment of initial stiffness (1), tibial surface of rolling contact (2), cushion (3), and base talar surface of rolling (4).19

FIGURE 3.

Example of long axes of the leg (LGL) and foot (LGF) used to determine ankle angle of dorsiflexion of the instrumented transtibial bone-anchored prosthesis attached to residuum (A) and percutaneous part of osseointegration fixation (B) including connector (C), transducer (D), pylon (E), and multi-axial rolling Free-Flow Foot (Ohio Willow Wood) (F) with anthropomorphic moments of dorsiflexion, inversion/eversion, and axial rotation featuring screw for adjustment of initial stiffness (1), tibial surface of rolling contact (2), cushion (3), and base talar surface of rolling (4).19

The individual cadence, duration of GC and support phases ranged from 40 ± 1 strides/min to 52 ± 1 strides/min, 1.158 ± 0.02 s to 1.488 ± 0.034 s, and 61 ± 1%GC to 68 ± 2%GC with usual foot as well as 45 ± 1 strides/min to 51 ± 1 strides/min, 1.175 ± 0.016 s to 1.46 ± 0.029 s, and 61 ± 1%GC to 64 ± 2%GC with Free-Flow Foot, respectively.19

No marker set was used for experiment 2. Dorsiflexion angle data was collected using a digital camera (Canon, IXUS, USA) recording the 2D sagittal view on the prosthetic side. We clicked frame-by-frame on body landmarks (Center knee joint, Centre ankle joint, toe) using Kinovea angle tool to determine the ankle angle between the long axis of the leg and the foot.22 Bending moment was recorded at the sampling rate of 200 Hz using iPecsLab (RTC, USA) portable kinetic system including a tri-axial transducer embedded between the percutaneous part and ankle unit (Fig. 3).23,24

Characterization of Ankle Stiffness

In both experiments, the ankle stiffness was characterized following the 12-step process we have recently published.18 This automated data-based criterion process eliminated all subjective detections of key variables characterizing stiffness, and, therefore eliminated interrater variability in data extraction.

First, the shape of the stiffness curve was identified based on the isolation of the flat foot phase, the recognition of the curvature and the identification of the point of inflection of the stiffness curve. This point of inflection was the maximum point above or below the reference line between the lowest and highest point of the stiffness curve for convex and concave curve, respectively. Then, the stiffness variables were extracted. The overall ankle stiffness (K0) was to the slope of the line reference line. K1 and K2 were the slopes of the regression lines of stiffness curve before and after the point of inflection. The IA was calculated using IA = K2−K1.

RESULTS

Mean dorsiflexion angle, bending moment and regression lines for K0, K1, K2, and IA of moment–angle curves exerted by sound ankle of able-bodied and prosthetic ankle of participants with osseointegrated prostheses fitted with Free-Flow Foot and their usual feet are presented in Fig. 4.

FIGURE 4.

Angle of dorsiflexion and bending moment and regression lines for K0 and IA = K2−K1 of moment–angle curves for able-bodied (A) and participants with osseointegrated prostheses with Free-Flow (B) and their own feet (C). Upper graphs: averaged moment–angle diagrams in able-bodied and amputee subjects wearing either their own prosthetic feet or the Free-Flow foot. Lower table: values of K1, K2, and IA = K2−K1. HC, heel contact; TC, toe contact; HO, heel off; TO, toe off.

FIGURE 4.

Angle of dorsiflexion and bending moment and regression lines for K0 and IA = K2−K1 of moment–angle curves for able-bodied (A) and participants with osseointegrated prostheses with Free-Flow (B) and their own feet (C). Upper graphs: averaged moment–angle diagrams in able-bodied and amputee subjects wearing either their own prosthetic feet or the Free-Flow foot. Lower table: values of K1, K2, and IA = K2−K1. HC, heel contact; TC, toe contact; HO, heel off; TO, toe off.

DISCUSSION

Outcomes

  • Bending moments applied to the implant progressed by 73.27 ± 29.62 Nm for usual feet and by 54.99 ± 13.37 Nm for Free-Flow Foot. The averaged maximal bending moments generated by Free-Flow Foot decreased by 25% corresponding to 18 Nm that is well and truly larger than load measurements errors.18,25

  • Ankle stiffnesses in control group of the able-bodied participants were all positive with an IA = 5.88 ± 0.93.

  • Ankle stiffnesses the Free-Flow Foot demonstrated concave shape and were positive: IA = 2.68 ± 1.09 confirming the Free-Flow Foot’s classification as an anthropomorphic foot.

  • Correspondingly, the 3 usual feet wore by the subjects were classified as nonanthropomorphic as their individual ankle stiffnesses demonstrated convex shape and were negative: IA = −2.97 ± 2.37.

Interpretation

The study demonstrated that it is possible to differentiate angle of dorsiflexion and bending moment profile using IA between feet with different design characteristics. The study showed that a foot with anthropomorphic characteristics tends to decrease maximal loads. This could possibly minimize the risks and increase the longevity of the bone–implant interface while reducing rate of revisions without modifying existing technology of osseointegration. This study also revealed that a prosthetic foot with anthropomorphic characteristics could generate a stiffness profile that is comparable with an able-bodied ankle. This might suggest that an anthropomorphic foot, like to Free-Flow, could generate loading characteristics attuned with expected bone loading and therefore could potential promote osseointegration. Such foot could then increase safety of the treatment provided. However, this hypothesis remained to be validated depending if the stiffness applied by anthropomorphic foot is, indeed, within the most favorable Goldilocks zone.

Limitations

This study presented the typical limitations of a retrospective preliminary study comparing kinematic and kinetic data collected with 2 small and unrelated series.

As we reported Frossard et al. (2019),18 the interpretation of differences in stiffness outcomes between prostheses was limited mainly because of unknown effects of confounders (e.g., individual length of residuum, distal position of transducer, short acclimation with Free-Flow Foot, foot size, and footwear).18 Unknown are also differences in spatiotemporal gait characteristics (e.g., speed of walking, walking base, step, and stride length), kinematics (e.g., trunk bending, knee flexion, and hip range of movement) and kinetics (e.g., knees and hips joint power) information.26

Furthermore, the comparison on stiffness between series of able-bodied and individuals with transtibial amputation was limited by the discrepancies in measurements of ankle angle of dorsiflexion (e.g., automated 3D Vicon motion capture vs. manual 2D Kinovea movement analysis) and bending moment (e.g., inverse dynamics using fixed force plates vs. direct measurement using wearable load cell).

Generalization

Generalization of the outcomes must be considered carefully giving the intrinsic shortcoming of a case-series study. Nonetheless, this study provided initial benchmark stiffness profile involving 42% of existing population with transtibial amputation fitted with press-fit fixation worldwide.

More generalizable is the methodology presented here to evidence ankle stiffness profile.18 This information should be considered and, possibly, educate the design of subsequent studies focusing on characterization of transtibial BAP. For example, the range of differences angle of dorsiflexion and bending moment might informed the sample size required to reach sufficient statistical power.21,27

Altogether, this study provided initial evidence of worthiness of the IA that could possibly be integrated in ISO 22523, ISO 22675, ISO 10328, and the WHO Standards for Prosthetics and Orthotics, as the first quantitative standard of the prosthesis’ performance.28–31

Future Studies

Future longitudinal studies could compare ankle stiffness with various prosthetic constructs (e.g., components, alignment) for a larger cohort of individuals fitted with a transtibial prosthesis.32,33 This could provide a better understanding of intra- and intervariability inherent to design of components and daily activities.34,35

Subsequent cross-sectional studies could establish a link between ankle stiffness outcomes and additional 3D biomechanical (e.g., dynamics, kinematics, joint work, and power), physiological (e.g., EMG of residuum muscles, metabolic energy consumption, development of osseointegration, and skin damages), and participants’ experience (e.g., PEQm, TAPES-M) information.

CONCLUSIONS

This work was an initial effort toward providing benchmark of stiffness data exerted by feet with and without anthropomorphic designs. Altogether, this study should be considered as a stepping stone for manufacturers of components, prosthetic care providers, and decision-makers developing international standards and guidelines toward better evidence-based prescription of safe prosthetic components to growing population of individuals with transtibial amputation fitted with osseointegrated fixation worldwide.

FUNDING

This study was supported in part by DOD Orthotics and Prosthetics Outcomes Research Program (W81XWH-16-1-0475) and National Institute of Arthritis and Musculoskeletal and Skin Diseases, NIH (AR43290).

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Author notes

Poster presentation at the 2019 Military Health System Research Symposium, Kissimmee, FL, MHSRS-19-00186.

The views expressed in this article are those of the author and do not necessarily represent the official position or policy of the U.S. Government, the Department of Defense, or the National Institutes of Health.

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