## 4 Structure and Construction of Compression Orthopaedic Products
The compressive properties of knitted fabrics are influenced by various factors: (a) raw material, i.e., ground yarn type and linear density and elastomeric yarn type; and (b) manufacturing process, i.e., the characteristics of the knitting machine and specific parameters of production process. Compression products are worn repeatedly many times; accordingly, the properties of resilience are equally important as elastic properties [66]. The recovery power of the product is used to evaluate its response to the body's movement immediately after the impact. The higher elastomeric yarn tension results in a higher recovery power in the knitting area, which results in shorter loops formation (in comparison with the length of loops without elastomeric yarn) [90; 91]. Deformation properties of elastic knits widely differ from knitted fabrics without elastomeric yarns in the structure. The higher amount of elastomeric yarn in the knitted fabric composition leads to a greater tensile force and, therefore, a human limb is affected by the higher compression [19; 25; 31; 66; 96; 96; 102; 103; 104; 105]. It is important to notice that a different geometry of knitted structure generates different mechanical properties that are strongly related to the fabric structure, yarn properties and direction in which fabric is used [74; 86; 106]. Many studies have been published on the deformability of knitted fabrics [1; 97; 12; 107; 12]. Compression of the support depends on the support area, shape and knitting characteristics, such as knitting pattern, loop density, etc. The ways in which the textile material deforms under applied stresses play an important role in its processing and end-use. Alternating compression in the length of the product can be achieved by changing the knitting density, knitting pattern and/or pre-tension of a laid-in elastomeric yarn. Knitted orthopaedic supports are often designed with additional details for different purposes. Orthopaedic supports often have added silicone or other parts for functional application and may also comprise other components, such as straps, and fasteners, including a disengage-able two-part fastener system for engaging the support with a body [9]. All the rigid elements inserted into the support can change the elasticity of the entire product and, consequently, its compression. In the area of low extensions, there is a strong linear dependence between the relative area of the rigid element and the compression generated by the knitted orthopaedic support--compression linearly increases by increasing the area of the rigid element [86]. It is not always possible to obtain the desired features of the support only with the knit, so additional parts may be inserted into the knitted product. Such parts can be one-sided or double-sided plates with special hinges and straps, foam rubber, silicone parts, metal plates, hook and loop strips (Velcro), and neoprene parts. For example, special hinges and straps may be inserted into the knee support to strengthen the sideway support from both sides of the knee and to control the degree of joint movement. These devices also are beneficial for reducing the compression forces of the joints caused by weight and the \"valgus-varus\" (pulling, pushing forces) load caused by contact activities. Hinges are usually made of metal alloys (titanium or aluminium) or non-metallic composite materials with a specific strength, also it must be as light as possible and ensure the comfort of the patient. The extra straps inserted into the support facilitate the attaching and removing processes of the support and provide individual compression. In this case, the patient can regulate the compression level [5]. Moreover, the structure of the support can also be modified: an opening for the kneecap can be made, and one or more additional straps and special reinforcements inserted [5; 50; 51]. Special reinforcements may be round-shaped, C-shaped, J-shaped or H-shaped [5; 25].