A Review on Materials for Orthotic Devices: Processing and Characterization

A Review on Materials for Orthotic Devices: Processing and Characterization

Hridayjit Kalita (BIT Mesra, India) and Divya Zindani (National Institute of Technology Silchar, India)
DOI: 10.4018/978-1-5225-8235-9.ch009
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The recent use of fiber-reinforced composites as bio-material in fabrication of orthoses have accelerated the research in finding the most compatible combinations of bio-polymeric composites. The strength, flexibility, biological compatibility, durability, and fatigue strength can be easily altered considering different combinations of polymers and their reinforced structures. In this chapter, a review of the presently used polymeric composites as orthotic materials, their properties, and their applications based on their availability for common people, for a variety of specialized functions, and for the best quality and compatibility factor were discussed. The common orthotic devices mainly used on patients to support weak joints or limbs, materials used, and processing techniques were discussed. Characterization tests for polyethylene composites performed for material properties like ultimate tensile and bending strength, elastic modulus, thermal degradation properties, and surface resistance were discussed, which validates the usability of polyethylene in orthotic fabrication.
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Fibre- reinforced composites have gained popularity and is widely used today for its ease of suitably altering the properties to be suited to any body parts in certain ranges of compatibility in stiffness, tensile strength, fatigue strength, surface interaction between body and orthotic, durability and functional needs (sporting purpose, general purpose). Polymer composites can be selected to operate under certain conditions which can be classified based on their surface compatibility and their mechanical compatibility [Ramakrishna S et al., 2001]. Surface compatibility factors include all body/prosthetic interface interactions such as allergic to the material, biological, chemical and physical suitability of the orthotic material to the muscles and bones in contact with it. Mechanical compatibility factors include strength, stiffness, elasticity, impulsive load on the body part by the orthotic [Ramakrishna S et al., 2001; Wintermantel E et al., 2001].

The stress being subjected to the human bones regularly is approximately 4MPa while the tendons and ligaments take about 80MPa or above. The motion of the joints and fingers in body can be calculated in terms of cycles/per day to be 10^6 [Ramakrishna S et al., 2001]. The advancement of polymer technology have facilitated its modification to mimic the mechanical behaviour of a human bone and synchronizing with the functions being transmitted from the body in contact. Metal plates and screws like stainless steel were commonly used as materials for orthotic and prosthetic implants. But metal plates possess few drawbacks. Metal plates are stiffer and stronger than human bone which on using with it makes the bone the weaker member and non resisting to fatigue shocks or surface wear. The surface of the interface between soft bone material and the metal greatly influences the compatibility factor between the two materials. Polymer composites can be a better replacement while considering these compatibility issues. The stiffness and other properties in the orthotic material can be altered and maintained to adjust itself to mimic the bone material property so that necessary rigidity, shocks and load could be provided to the bone joint. The polymer composite holds the ability to get processed in a wide variety of properties and the range of properties for the compatibility with the bone material is easily achieved.

One disadvantage is that, though polymer material (fibreless) can be synchronized well with the bone in the domain of comfort and softness, the stiffness and strength are still very less to hold the necessary weight of the body or any peak load. So the property of the preferred best material lies somewhere in between both the polymer and the metal. Fibre reinforced polymer holds the solution to the above mentioned problem, which is basically a combination of a polymeric matrix reinforced with metallic or non-metallic fibres. The necessary stiffness, hardness and wear resistance is provided by the fibres while the matrix provides the softness, surface finish and cushioning effect to the body [Kumari et al., 2018]. The strength of the orthotic material made of fibre reinforced polymer mainly depends on the relative arrangement of both the constituents or the alignment of the fibres apart from their individual properties [Ramakrishna S et al., 2001, Sen N., 2004]. Replacing polymer composites in place of metal prosthetics or orthotics also enabled visual imaging of injuries or healing process imaging behind the metal plates and screws [Scholz et al., 2011]. In this paper, section 1 is dedicated to classification of different orthotic materials and the significance of using a fibre reinforced thermosetting polymer composite (carbon-epoxy) over a fibre reinforced thermoplastic (glass fibre-nylon) in achieving the best quality orthotics. Section 2 deals in material processing and fabrication techniques involved in production of orthotic devices. Section 3 gives an account of the significance of Polyethylene (PE) as orthotic material and its property characterization.

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