Abstract-Little or no practical decision-making data are available to the foot-care provider regarding the selection of orthotic materials used in therapeutic footwear. A device for simulating in-shoe forefoot conditions for the testing of orthosis materials is described. Materials are tested for their effectiveness by evaluating and comparing stress-strain and dynamic compression fatigue characteristics. The device, called the Cyclical Compression Tester (CCT), has been optimized for size, simplicity of construction, and cost. Application of the device ranges from the clinician deciding the useful life of single- and multidensity orthosis materials to the researcher characterizing materials for finite-element analysis modeling. This real-time CCT device and custom user interface combine to make an evaluation tool useful for testing how the pressure distribution of in-shoe materials changes over time in therapeutic footwear for those with peripheral neuropathy at risk for foot injury. Key words: Cyclical Compression Tester, diabetic footwear, finite element analysis, materials testing, orthosis, orthotic, peripheral neuropathy, pressure distribution, rehabilitation, therapeutic footwear.

Abbreviations: ASTM = American Society for Testing and Materials, CCT = Cyclical Compression Tester, DC = direct current, DCF = dynamic compression fatigue, LP = linear potentiometer, MP = Microcel Puff, P = Poron, P1 = Plastazote 1, P2 = Plastazote 2, PLC = programmable logic controller, S-S = stress-strain, UDSP = user- defined set point.

INTRODUCTION

Sensory neuropathy and abnormal levels of mechanical stress are considered the primary causes of plantar foot ulceration and are common factors in the pathway to lower-limb amputation [1-3]. Localized prominent areas resulting from skeletal deformities are most affected by mechanical stress. Therapeutic footwear is thought to be a frontline defense modality for reducing harmful stress and thus preventing the occurrence or recurrence of neuropathic plantar ulceration [4-8]. Equally important is the foot/shoe interface, namely the orthosis, which has been shown to effectively reduce forefoot pressures when compared with a shoe without an orthosis [9- 13].

Currently, little or no practical decision-making data are available to the foot-care provider regarding the selection of orthotic materials used in therapeutic footwear. Manufacturer and other commercial information only provide static data, such as apparent density, durometer, and compression set. The ideal dynamic properties to test for when determining in-shoe material characteristics are mechanical stress (a perpendicular force applied over area), strain (the change of thickness to the original thickness ratio), and fatigue (the material breakdown after repeated cyclical stress). From these properties, most applicable material characteristics can be derived, such as hardness, durability, compressibility, and resilience. For determination of these material properties, a benchtop device would provide essential data for an objective material-selection pathway that is still very much a subjective determination.

Previous laboratory bench studies have examined various materials under rapid impact loading, primarily simulating heel-strike during running [14-18]. The diabetic forefoot, where ulceration occurs most often [1,19- 20], is better modeled quasistatistically or at very low rates of loading [21]. The diabetic forefoot is modeled mathematically, suggesting no impact during the foot-flat to toe- off phase of gait. Dynamic compression testing using a determined number of cycles, loading rate, and pressures equivalent to the conditions found in the footwear forefoot better characterizes the useful service life of materials [22].

Several investigators have examined forefoot loading conditions by using custom bench devices capable of measuring mechanical stress and strain. Using a modified procedure from the American Society for Testing and Materials (ASTM) designated ASTM D573-67, Campbell et al. compared the compression properties of a set of materials, 2.85 cm standardized diameter, by using a normalized log linear method after a series of 250,000 cycles, at a rate of 1 Hz and pressure of 294 kPa [23]. Brodsky et al. compared commonly used orthosis materials under cyclical loading of 5,000 cycles per day for 2 days with the original compression characteristics of the materials [24]. While the previous authors used noninstrumented analog devices, Sanders et al. and Petre et al. used digitally controlled devices capable of quantitatively monitoring stress-strain (S-S) mechanical behavior for each cycle over the period tested [25-26]. No previous studies, however, have reported fatigue, which is called dynamic compression fatigue (DCF) in this article. Additionally, no previous studies have demonstrated materials testing using a compact, portable benchtop device that is digitally controlled, programmable, repeatable, and cost-effective.

In this technical note, a device for simulating in-shoe forefoot conditions for the testing of orthosis materials is described. The simulation will test materials for their effectiveness by evaluating and comparing the S-S and DCF characteristics. The device has been optimized for size, simplicity of construction, and cost. The device is called the Cyclical Compression Tester (CCT).

METHODS

A single CCT device measures 15.2 cm wide x 34.3 cm long x 15.2 cm high and weighs approximately 4 kg (Figure 1). The unit has a 12.7 x 27.9 cm footprint and is designed to sit on a desktop within 1 m of a computer with Microsoft Windows XP (Redmond, Washington) operating system and the custom user interface (Lab- VIEW, National Instruments Corp; Austin, Texas).

All structural components of the CCT are made from nominal size aluminum and steel items for lower cost and ease of assembly. The extent of machine shop work for assembly is limited to cutting, drilling, and tapping followed by the attachment of the ball screw actuator (Industrial Devices Corp; Rockford, Illinois), load cell (Transducer Techniques; Temecula, California), linear potentiometer (LP) (UniMeasure; Corvallis, Oregon), and rubber footings. The baseplate is 15.2 x 30.5 x 1.3 cmthick aluminum reinforced on the underside with two 2.5 x 27.9 x 1.3 cm steel channels. The aluminum tailpiece, where the load cell and rear compression plate are anchored, is 3.8 x 2.5 x 5.1 cm and the two 1.6 cm-thick compression plates are parted from 5.7 cm-diameter aluminum round stock.

The electronic control system interfaces between the computer and the CCT (Figure 2). A strain gauge input module (Dataforth; Tucson, Arizona) amplifies the signal from the load cell in-line with the actuator, which is fed to a programmable logic controller (PLC) (International Parallel Machines, Inc; New Bedford, Massachusetts) monitoring minimum and maximum force predetermined by the user. When minimum or maximum force is sensed, the PLC sends a proportional analog direct current (DC) voltage to the servo motor controller (Advanced Motion Controls; Camarillo, California) for direction and speed of the actuator, thus producing the cyclical motion. The PLC enables the system to respond to the cyclical events with predictable results rather than depending on the laptop computer's operating system, which cannot guarantee immediate response because of its multitasking nature. The principal components of the CCT device are listed in the Table.

Data acquisition is accomplished by monitoring the in-line force and position of the actuator head. These two variables occupy analog voltage input channels in the data acquisition module (National Instruments Corp) in which real-time control and feedback are obtained through the customized user interface (Figure 3). The user determines the frequency with which data are recorded between cycles. Before testing, the data are analyzed and graphically interpreted with Microsoft Excel spreadsheet software.

The control system components have four analog input channels, facilitating the simultaneous use of two CCT devices.