Dimensional accuracy of ankle-foot orthoses constructed by rapid customization and manufacturing framework

INTRODUCTION According to the Americans with Disabilities report, in 2005 approximately 27 million people over the age of 15 had a walking-related disability [1]. Ankle joint musculature is extremely important during walking and is thought to be the primary muscle group that supports upright stance and produces forward propulsion [2-3]. Individuals with muscular weakness about the ankle, an impairment often caused by upper motor neuron disorders and lower-limb injuries, are frequently prescribed ankle-foot orthoses (AFOs), which brace the ankle during gait and aim to improve gait function [4]. Passive-dynamic AFOs (PD-AFOs) constitute a class of ankle braces that rely on material properties and physical features to establish functional characteristics such as bending or rotational stiffness and the storage and return of mechanical energy [5]. PD-AFOs are traditionally composed of foot plate, strut, and cuff components, which may be fabricated using continuous material [5-7] or connected as components in various manners [8-9]. Despite the great potential for biomechanical assessment and treatment using this classification of orthoses, currently prescribed PDAFOs are often generic, having standardized size and shape (fit) and bending or rotational stiffness (functional) characteristics [10-11]. When customization of functional characteristics is sought, PD-AFOs are typically manually fabricated by orthotists--which may introduce undesired manufacturing variability in PD-AFO quality or effectiveness, depending on an orthotists' skill and experience [12]--and require substantial time and expertise to ultimately manufacture orthoses having functional characteristics that match the unique gait dynamics of each patient [6-7,13]. Furthermore, the annual cost of these devices is substantial. In 2007, the prescription of orthotics accounted for US$458,000,000 of Medicare expenditures alone [14]. Therefore, the capability to rapidly design and manufacture customized orthoses with precisely controlled characteristics would help transform the PD-AFO customization and fabrication process from a craft-based industry into a modern clinical specialty [15]. Fit customization is an important design factor for obtaining optimal function from a PD-AFO. The size and shape characteristics, which describe the fit of a PDAFO, can be customized through a variety of methods. Traditionally, an orthotist casts a patient's shank and foot to create a negative mold. A positive mold is generated from the negative mold, and then the PD-AFO is manually fabricated around this positive mold similar to methods for fabricating a foot orthosis [16]. While manual manufacturing methods can sufficiently generate a PDAFO with customized size, augmented shape, and functional characteristics, manual manufacturing methods can contribute to undesirable variability in quality of manufactured components. Recent efforts have worked to utilize computer-aided design (CAD) models and associated parameterization tools to customize orthoses. Darling and Sun designed a parameterized orthosis model that consisted of two rigid components, one for each the foot and shank, which were attached by a single-degree-of-freedom hinge [17]. Parameterization of this model was based on two anatomically relevant coordinate systems, one for each of the components. Patient-specific imaging data were fit to the parameterized model to scale the orthosis. While this CAD model was parameterized for size and ankle angle, the orthosis design lacked the organic shape characteristics and parameterization of orthosis functional characteristics. In this article, we define parameterization as the process of identifying the parameters necessary for the complete specification as well as manipulation of the PDAFO CAD model. Using parameters to specify and manipulate a PD-AFO CAD model has profound implications for the objective and precise customization of biomechanically designed orthoses that match patient needs with orthosis function [15]. …