Most instruments are fabricated from stainless steel which contains a minimum of 12% chromium for satisfactory corrosion resistance. The selection of a stainless steel is based on several factors, such as design and function. Stainless steel used for orthopedic implants or instruments are classified according to composition and microstructure. The most versatile compositions are the straight chromium alloys with a martensitic microstructure and the chromium-nickel alloys with an austenitic microstructure. Other types of stainless steel known as ferritic or precipitation hard enable alloys are less common, but they may occasionally be used for specialized instruments.
Martensitic stainless steel may contain 12%-18% chromium and have a medium to high carbon content. They are highly magnetic and are capable of being heat-treated to a very high hardness. Specialized heat- treating procedures have been established to provide a good balance between hardness and fracture properties. High hardness provides good wear resistance, and cutting edges retain their sharpness. These alloys may be used for chisels, bone- cutting forceps, pliers, taps, drill bits, medullary reamer heads, etc.
Austenitic stainless steels typically contain 16%- 18% chromium plus 8%- 10% nickel and have a low to medium carbon content. The alloys may be hardened by cold working but cannot be hardened by heat treatment. They are generally nonmagnetic, but some compositions may be slightly magnetic, depending on specific metallurgical conditions. The austenitic stainless steels have a lower hardness but better corrosion resistance than the martensitic stainless steels. Noncutting instruments, such as drill guides or aiming devices, and components, such as hollow sleeves, springs, and actuators, are typical examples.
Stainless steel instruments are typically given a bead blasting treatment as part of the surface finishing operations. Tiny beads of aluminium or silica powder are blasted onto the instrument surface to create a highly textured finish. Bead blasting is followed by controlled electropolishing, to remove any contamination and chemically restore corrosion resistance. Bead blasting plus electropolishing provide a uniform corrosion resistance matte surface with the reduced light reflectivity desirable in the operating room.
Various types of plastics are used for certain instrument parts. They can be divided into two groups, thermosetting plastics and thermoplastics.
Thermosetting plastics are pressure- setting plastics (e.g. Duroplastic) that do not deform at normal sterilizing temperatures, but that can be damaged or strained by prolonged exposure to strong disinfectants. The handles of screwdrivers, hammers, periosteal elevators, etc. are manufactured from this type of phenol resin.
Thermoplastics are injection- molded plastics that lend to soften during sterilization, depending on the material and the temperatures used. Polyamide 66, for example, used for the medullary tube and the spiral hose softens at 170℃ (approximately 338⁰F). Further thermoplastics are ethylene- propylene (for the outer tube of the double hose) and acetyl resin (for protective caps and tube plugs for the external fixator).
Carbon fiber- reinforced epoxy is used for small and large external fixation rods. This composite material provides good axial frame stiffness, significant weight reduction, and radiolucency when compared to stainless steel. The rods are resistant to distortion, delamination, and thermal damage during steam autoclaving.
Some instrument handles are composed of a linen- impregnated phenolic composite material known as Tufnell or LE grade phenolic. The handles usually appear light tan in color when they are new and tend to become slightly darker in appearance after repeated handling and multiple steam- autoclave cycles. This is an aging phenomenon and does not influence the performance of the instrument. Hot-air sterilization is not recommended. For more information about orthopedic implants manufacturing and supplying process, you may visit our website www.siiora.com.