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25/ medical sciences
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УДК 617.3-089
Reshma Neena Ramachandran
Master degree 4rd course student HSEEU "Bukovinian State Medical University " Dudko Oleksii Gennadiyovich Ph.D., Associate Professor Traumatology and Orthopaedics Department, HSEEU "Bukovinian State Medical University" Shayko-Shaykovskiy Oleksandr Gennadiyovich PhD, D.Tech.Sci, Professor of General Physics Department Yuriy Fedkovych Chernivtsi National University, DOI: 10.24412/2520-2480-2020-2880-21-23 PROPERTIES OF MATERIALS USED IN ORTHOPAEDIC FIXATION
Abstract.
As the number of injuries of the locomotor system is continuously increasing, the research in the field of orthopaedic devices can improve the outcome of treatment of such the most common and severe injuries as bone fractures. In the paper the authors describe main parameters of materials and orthopaedics devices that are used for their manufacturing. The main requirements are highlighted, as high biocompatibility, low toxicity, high mechanical properties that are essentially important for proper fixation offracture fragments. As new demands for materials arise, such properties as biodegradation and self-resolving abilities are preferable, so different types of these materials were analysed in the paper.
Keywords: bone fractures, orthopaedic devices, materials, biomechanics.
Actuality. Orthopaedics and traumatology is a medical field which studies how to treat fractures, as well as to correct the malformations of the musculoskeletal system caused by accidents, life activities and sports. Recently there is an increase in the number of cases of osteoporosis related to the malfunctioning of endocrine glands, low physical activity, etc. One of the best ways to treat these problems is surgical management by using implants. We can use internal and external fracture fixation. The implants used for internal fracture fixation are: k-wires, pins, screws, plates, intramedullary nails. Each implant has its own merits and demerits, as well as indications for its use.
Aim or research: To analyse the properties of materials used for internal fracture fixation.
Mechanical properties for bone and implants. Bone is an anisotropic material. When we choose an implant - it should be non-toxic to the body, non-corroding, strong, chemically non-reactive, able to resist deformations due to wear and loads, osseointegration and biocompatible. Then it may be considered to be biomaterial. The main laws that should be respected for proper fracture fixation with implants are Wolff's law and Hooke's law.
Wolffs law by German surgeon Julius Wolff states that bones will adapt according to the demands placed on them [1]. That means that the load applied to the bone effects significantly on its mechanical properties. When bone fractures are fixed with implants, the early movements in joints are possible, that helps fracture healing and increase mechanical properties of bone.
Hooke's law by English scientist states that stress is directly proportional to the strain.
Types of Orthopaedic implants.
K-wires are metallic wires usually made up of stainless steel [2]. They are highly biocompatible and are of different diameters, standard tips and special designs. Standard tips include trocar, round, flat, lancet,
triangle and square. Special designs include partial thread, full thread, trocar knurled lengthwise, special eyelet and drill tip. Due to their small size K-wires don't have high mechanical properties, but they can be used for preliminary fracture fixation, followed by plate fixation. If k-wires are used alone additional external support with plaster bandage should be used after surgery.
Screws were used in orthopaedic surgery for the first time in 1850 by French surgeons Cucel and Rigaud. Carl Hansmann used nickel plated screws along with a removable steel plate in fixation. Screws consist of the head, the core and the thread. Some of the important features of screws include cannulation, thread depth, pitch, and single-vs double-lead threads [3]. During screw tightening, stress induced by the screw on the bone increases and osteoblastic activity also increases. Wolff's law has been applied in this case. Hence due to the differences of mechanical parameter of bone tissue and metal, screws become less biocompatible and lead to implants' failure [4].
Screws also can be made of the polymeric materials, but not all polymers can be used for this purpose, so many experimental studies should be performed to prove the compatibility of every polymer that is going to be used for implantation. Such studies were performed for polymer Polyamide-12, that showed good results in clinical application for fracture fixation [5], and have proved its high biocompatibility properties in long-term outcome [6].
Other biodegradable polymers that were well studied and used for fracture fixation are polyglycolic acid and polylactic acid [7, 8]. They have a good property to degrade after the fracture heals, so further removal of implant with another surgery should not be performed.
Plates are usually used for the internal fixation of various fractures occurring in human body especially for forearm fractures. Plates are of different design, broad or narrow, straight or curved with different types
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of holes for screws insertion. Plates and screws are often used together [9]. They usually can provide stable fixation. Plates are made up of stainless steel or titanium [10]. They are usually used for the treatment of major fractures, but require large skin incisions, that disrupt the blood supply and cause skin irritations. Plates hold the fracture fragments together and later after fracture healing in several months or years they are usually removed. The new generation of plates with locking head screws provide much better fixation. These plates are locked with screws and reduce movements of fracture fragments, as well as are less harmful for underlying bone.
Biomaterials for implant production.
Implants used in orthopaedics can be metallic or non-metallic. Metallic implants are usually made of special alloys: Stainless steel (316L according to American standards), Titanium alloys, Cobalt chrome alloys [11]. Non metal implants include ceramics and bioac-tive glasses, polymers (bone cement, polyethylene).
Biomaterials for implants are classified into first, second and third generations which include bioinert material, bioactive and biodegradable materials, materials designed to stimulate specific responses at the molecular level respectively [12].
Stainless steels are mainly used for making implants. Not all types of stainless steels can be used for making implants. The special stainless steel according to USA standards that is used for this purpose is 316 L stainless steel. It has less carbon compared to 316 stainless steel and it is more resistant to corrosion. Elastic modulus is 165 GPa [11]. Mechanical properties of stainless steel are given in Table 1 [9].
Titanium alloys are mostly used for implants manufacturing because of its light weight strength and ability to withstand extremes of temperature. Titanium alloys contain Titanium (89%), Aluminium (6%), Vanadium (4%). Modulus of elasticity or Young's modulus of titanium is 116 GPa, which is lower than for stainless steel and hence is suitable for movement using less stress. Young's modulus of beta type alloy is the lowest. They are non-corrosive [13]. Good osseointe-gration is an important feature of titanium alloys. For best prognosis the implants should have properties similar to the human compact bones [14]. The possibility to perform an MRI scan is another advantage of titanium implants, as for many stainless steel implants it is prohibited.
Table 1
Mechanical Properties of Type 316L Stainless Steel
Mechanical Properties Type 316L stainless steel
Yield Point, MPa 332
Tensile strength, MPa 673
Modulus of Elasticity, MPa 165
Strength at break, MPa 586
Elongation at break, mm 35.5
Titanium forms titanium dioxide (TiO2) on its surface. TiO2 is anodized. Oxide layers undergo dissolution and form pores. Electric field assisted dissolution leads to deepening of pores. Incorporation of adjacent small pores into big pores and thus nanotube array is formed. Titanium nanotubes enhance the activity of osteoblast cells and thus osteointegration occurs [15].
Cobalt-chromium alloys are mainly used in knee replacement surgeries and hip joint surgeries. It was first discovered by Elwood Haynes. These alloys are highly non-corrosive, malleable, biocompatible, resistant to scratch, wear and tear. Sometimes these alloys are used along with other metals like molybdenum, nickel, tungsten. Since nickel is toxic, the usage of alloys without nickel is preferable. Some alloys implants exhibit antibacterial properties. Cobalt-chromium alloys on ultra high molecular weight polyethylene (UHMWPE) are used to reduce abrasion.
One of the important disadvantages of these implants is that they release chromium ions into surrounding tissues and blood especially into erythrocytes. This leads to delayed hypersensitivity reactions (cell cytotoxicity, type IV hypersensitivity) like Aseptic lymphocytic vasculitis associated lesions, necrosis, reduction of CD8 lymphocyte levels, chromosomal and DNA mutations. Therefore serum analysis of the patient must be done periodically after implantation of stainless steel
devices in cases when there is a continuous friction between device components [16]. These are cases when metal on metal total hip prosthesis are implanted. So friction between stainless steel components can significantly increase the amount of harmful ions in surrounding tissues up to the dangerous level and requires periodic expensive investigations.
Biomechanical properties of orthopaedic implants.
Tensile and compression strength is a capacity of material to resist the load before the object will start to decrease or increase the size. It is measured in MPa for materials, objects and their parts. For bone tissue the compression strength is 7 MPa and tensile strength is up to 20 MPa, and shear strength is about 15 MPa.
Shear strength is a capacity of material to resist the load which is perpendicular to the axis of the object or material sample.
Flexural strength is a capacity of a material sample to resist the load before its bending starts.
Yield strength is a maximum load that the material can resist before its irreversible deformation begins.
Toughness of a material is its ability to resist the load without its damage, as the fracture or irreversible deformation. For this the materials require curtain level of strength and ductility.
Elasticity is the ability of material to resist the load with reversible deformation.
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Poisson's ratio shows how the material responds to the axial load with a decreasing or increasing of its volume.
Elasticity describes the deformation of material in a response to axial load with the return to its previous size and form.
Young's modulus (E) is responsible for the stiffness of the material, and shows the strain in the material under load. It may be expressed as the relation of load applied along the axis of the object (c) on square to the strain that appears in the object and causes its deformity (e).
So there are some important mechanical parameters that are responsible for material behaviour under load and should be considered when a fixation device is manufactured and applied. Complex forces are affecting the bone and fixation device in the most of the cases. This is a combination of compression, extension, bending in various planes, as well as torsion. The effect of load is usually calculated for each force separately and due to this it is hard to predict exactly the implant behaviour.
Conclusion. So as many devices now are used in orthopaedics for implantation, the various materials are used. For the most common devices for fracture fixation, as plates, screws and intramedullary nails such material, as stainless steel is gradually replaced by titanium and its alloys. But for future progress devices with biodegradable properties are developing, that will allow to decrease treatment expenses for bone fractures and improve outcome.
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