Northwestern University Feinberg School of Medicine

Northwestern University Prosthetics-Orthotics Center

Investigation of Ankle Axis Misalignment in Ankle Foot Orthoses (AFOs) Using a Three-Dimensional Model

Principal Investigator: Stefania Fatone, PhD, BPO(Hons)

Co-Investigators: Kerice-Ahmun Tucker, BS; William Brett Johnson, PhD

Funded by: National Institute on Disability and Rehabilitation Research (NIDRR)

Status: Completed

Figure 1: The program consists of a Command Window and a Plot Window.Figure 2: Displacement map of misalignments in two planes.

Figure 1: The program consists of two main windows: a Command Window where the simulation is setup and run, and a Plot Window shown above where the results of the simulation are displayed.Figure 2: Displacement map of misalignments in two planes at 20 degrees of dorsiflexion. More information on this project can be found in the 2013 spring issue of Capabilities, 21(2):1-2.

Purpose

Accurate alignment of anatomical and mechanical joint axes is one of the major biomechanical principles pertaining to articulated orthoses, yet knowledge of the potential effects of axis misalignment is limited. Congruency between anatomical and mechanical joint axes is considered important as misalignment results in undesirable forces (both shear and compressive) and moments, generated as the joints move through their ranges of motion. Inappropriate alignment of joint axes has consequences not only for the soft tissue at the interface, but also the integrity of the joints (mechanical and anatomical).

Joint axis misalignment consists of two components: linear (anterior-posterior and proximal-distal) and angular (transverse and coronal plane) misalignments. While the consequences of linear misalignments (i.e., 2-dimensional [2D] misalignments) at the ankle joint of an ankle foot orthosis (AFO) were modeled in a previous RERC-funded project, there has been more limited consideration for combined angular and linear misalignments at the ankle (i.e., 3-dimensional [3D] misalignments).

Current understanding of the effects of ankle axis misalignments in ankle foot orthoses is limited to linear misalignments. Since there is natural torsion of the tibia, rotational alignment of the ankle joints and the consequences of ankle axis misalignment in 3D needs to be considered.

Specific Objectives

Our objectives for this project include: (1) developing a 3D model to explore the effects of ankle axis misalignment; (2) using the model to systematically analyze the various combinations of ankle axis misalignments possible in an AFO and their effect on motion of the device relative to the limb and compressive displacements at the orthosis-user interface (which relate to pressure on the limb); and (3) compiling the completed program into an executable file that can be uploaded to the Web as an educational, interactive tool for use by orthotics students.

Progress to Date

A linear spring model was developed to represent the leg and AFO (Figure 1). The program can calculate and display all compressive displacements and corresponding pressure magnitudes for combinations of misalignments (Figure 2). The completed program, "AFO3D", has been compiled into an executable file, AFO 3D Installer (Windows 7 compatible) that can be downloaded from the project site, Simulation of Ankle Foot Orthotic Joint Misalignment.

Publications

Fatone S, Johnson WB, Kwak S (2016) Using a Three-Dimensional Model of the Ankle-Foot Orthosis to Explore the Effects of Combinations of Axis Misalignments. Prosthetics Orthotics International, 40(2)247-252. DOI: 10.1177/0309364614556839. http://poi.sagepub.com/content/early/2014/11/11/0309364614556839

Fatone S, Johnson WB, Tucker K (2016) A 3D Model to Assess the Effect of Ankle Joint Axis Misalignments in Ankle-Foot Orthoses. Prosthetics Orthotics International, 40(2):240-246. DOI: 10.1177/0309364614543551. ISPO has designated this publication an open access article. It is available at: http://poi.sagepub.com/content/40/2/240.full 

Presentations

Tucker K, Fatone S. (2013) Development of a three-dimensional simulation tool to investigate the effects of ankle foot orthotic joint misalignments. Midwest Chapter of the American Academy of Orthotists and Prosthetists, May 30-June 1, Lake Geneva, Wisconsin.

Fatone S, Tucker K. (2013) Development of a three-dimensional simulation tool to investigate the effects of ankle foot orthosis joint misalignments. Capabilities, 21(2):1-2.

Fatone S, Tucker K, Johnson WB, Kwak S. (2014) A 3D simulation tool to investigate the effects of ankle foot orthosis joint misalignments. 40th American Academy of Orthotists and Prosthetists Annual Meeting & Scientific Symposium, February 26-March 1, Chicago, Illinois.

Kwak S, Johnson WB, Fatone S. (2014) The Consequences of Ankle Foot Orthosis Joint Misalignment Using a 3D Model. National Conference on Undergraduate Research, April 3-5, Lexington, Kentucky.