Instruments and Materials

Mechanical Properties of Materials

In discussing materials, the concept of stress and strain ( Fig. 4.1 ) will repeatedly arise. A stress is a pressure, tension, or shear applied to a material. Tensile stress is defined as a force per unit area and is measured in Newtons per square meter (N/m ² ), Pascals (Pa) (1 N/m ² = 1 Pa), or in pounds per square inch (psi). Strain is the resulting deformation in the object and is measured as the fraction of the deformation to the original length for a tensile strain (%). When a material is minimally stressed, it is often able to return to its original form if the stress is removed in a timely fashion. This is the elastic portion of the curve. If stress is applied beyond a certain threshold, permanent deformation will occur in the so-called plastic region of the curve. Yield point is the point at which plastic deformation begins to occur; breaking point is the point at which the material fractures. Each point has a corresponding strength and elongation at which it occurs. Maximal tensile strength is the highest point of the tensile strength curve and is often higher than the breaking point strength. The slope of the elastic portion of the curve is called the modulus of elasti­city . This reflects the material's resistance to elastic deformation or tensile stiffness . The modulus of stainless steel used in suture needles, for example, is approximately 200 GPa. The moduli of elasticity of suture materials are in the range of 0.5–3 GPa.

A classic dichotomy contrasts a ductile material, such as stainless steel, versus a brittle material, such as glass ( Fig. 4.2 ). Once glass reaches its elastic limit, it will break instead of undergoing plastic deformations. Stainless steel, on the other hand, is ductile and able to withstand a great degree of elongation beyond its yield point.

Instrument Material Selection

The materials used to fashion surgical instruments have largely been a reflection of the popular materials of that age. Instruments from the late Roman era (1st–4th century ad ), were primarily made using copper alloys. From the seventeenth to the nineteenth century, steel grew in prominence along with a smaller proportion of nickel-plated instruments. After the introduction of stainless steel in the early twentieth century, practitioners and instrument manufacturers rapidly adopted it. By 1938, 60% of surgical instruments were made of stainless steel and by 1963, that proportion had reached over 90%.

All steels, including stainless steel, are alloys composed of primarily iron with small amounts of carbon and other elements. Chromium imparts corrosion resistance in stainless steel and must be present at a content of at least 10.5% to receive the designation “stainless steel”. Increasing the proportion of chromium in steel will impart a parabolic decline in its propensity to corrode due to the formation of a small “passive” layer of chromium oxide (Cr ₂ O ₃ ) on the surface. While it is only a few nanometers thick, the passive layer prevents the iron within the steel from interacting with oxygen and salts to corrode. Damage to the passive layer of Cr ₂ O ₃ can also immediately “self heal” in that free chromium atoms on the surface of the alloy will become oxidized to “heal” the layer.

Stainless steel is further subdivided based on its crystal structure. The iron atoms within the steel can assume various configurations (outlined in Table 4.1 ). Ferrite has the same body-centered cubic structure as found in pure iron in which atoms are found at the corners of a cube and one in the center. If heated past 900°C, ferritic steel can assume an austenite structure with a face-centered crystal structure. If then slowly cooled back to room temperature, its face-centered structure returns to the body-centered cubic ferritic form. Addition of nickel to the alloy will allow stabilization of the austenitic structure at room temperature. Hence, all austenitic steels contain nickel ( Table 4.1 ). If ferritic steel is heated above 900°C and then cooled quickly (a process called quenching ), it can form a structure in which the cube is stretched in one direction (body-centered tetragonal) and is saturated with carbon: martensite . In its “as-quenched” form, martensitic steel is very brittle, making it difficult to form. Further heat treatments ( tempering ) improve ductility and are a mainstay in the production of martensitic steels. Martensitic steels are the most used in dermatologic surgery instruments.

Table 4.1

Types of stainless steel used in surgical instruments

	Crystal structure	Magnetic properties	Naming (AISI)	Composition
Ferritic	Body-centered cubic	Magnetic Simplest manufacturing	400 series	Iron Chromium (10.5–30%) Molybdenum (0–5%) Carbon (up to 0.08%)
Austenitic	Face-centered cubic	Non-magnetic Most ductile	200 series (Cr-Ni-Mn) 300 series (Cr-Ni)	Iron Chromium (16–26%) Nickel (3.5–26%) Manganese (up to 11% in 200-series austenitic) Molybdenum (0–7%) Carbon (up to 0.1%)
Martensitic	Body-centered tetragonal	Magnetic Most complex manufacturing Highest tensile strength Majority of surgical instruments	400 series	Iron Chromium (11.5–20%) Nickel (0–3%) Molybdenum (0–4%) Carbon (up to 1.5%)

(Diagrams for crystal structures are from Figure 3.3 of Tisza M. Physical Metallurgy for Engineers ).

Duplex and precipitation hardening steels, the other two major categories of stainless steel, are not found in most standards for surgical stainless steel and will not be discussed further. The properties of each type of stainless steel are summarized in Table 4.1 .

Finally, there are a variety of engineering and standards bodies throughout the world, each with its own nomenclature for materials, including stainless steel. The American Iron and Steel Institute (AISI) uses a 3-digit code, often with a modifier (e.g., 316L). The United Numbering System (UNS) uses the AISI code, then adds two extra digits and the “S” prefix for stainless steel (e.g., S45500).

Small surface irregularities can contribute to difficulty in cleaning and maintaining hygiene in instruments. Roughness can be formally evaluated in stainless steel by measuring the peaks and valleys in micrometers (µm) and averaging the results to the roughness average (Ra). A smaller Ra (e.g., <0.5 µm) will provide a smoother surface and make cleaning easier. Electropolishing uses an electrochemical process to allow manufacturers to obtain these small roughness values and smooth finishes.

Surgical Instruments