Instruments and materials

Question 1

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).

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Question 2

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.

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Question 3

Yield point is the point at which plastic deformation begins to occur; breaking point is the point at which the material fractures.

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Question 4

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

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Question 6

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.

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Question 7

All steels, including stainless steel, are alloys composed of primarily iron with small amounts of carbon and other elements.

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Question 9

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”.

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Question 10

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.

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Question 11

Stainless steel is further subdivided based on its crystal structure. 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.

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Question 12

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.

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Question 12

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).

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Question 13

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.

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Question 14

The most common scalpel and blade system used in dermatologic surgery is the Bard–Parker. Numerous handle options are available, each of which can connect with a variety of blades.

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Question 15

Scissors have the largest breadth of options for the dermatologic surgeon: general operating, dissecting, suture, and bandage scissors. They should generally be held with the thumb and ring finger on the rings with the index finger on the fulcrum. Three forces are at work while operating scissors: closing, shear, and torque forces.

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Question 16

There are four main grips to holding the needle driver: thumb/ring finger, thenar, palmed, and pencil. The needle should usually be placed perpendicular to the driver jaws, being careful not to damage the tip or swage.

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Question 17

Certain practices should be taken to care for stainless steel instruments. Blood and other visible debris should be removed as soon as possible and have the instrument soak in a commercial instrument soaking solution. All of the following should be avoided, as they can cause pitting and/or staining: placing dissimilar metals together, chlorine, phosphate, acidic/alkaline detergents. After visible debris has been removed, ultrasonic cleaning can be an effective means of dislodging small particles.

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Question 18

Once cleaned and rinsed, the instrument must be allowed to thoroughly dry. Locking instruments, such as needle drivers, should be kept unlocked to avoid weakening their mechanisms. A thorough inspection of instruments should be performed (e.g., cutting edges of scissors, jaws of needle holders, skin hook tips). Scissor sharpness can be evaluated by cutting through tissue or surgical glove. Prior to autoclaving, instruments with metal-to-metal contact can be treated with specialized lubricants.

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Question 19

Sterilization of surgical instruments can take one of four forms: dry, steam, chemical (formaldehyde/alcohol), and gas (ethylene oxide). Steam is the most practical of these options, with the main drawback being the humidity’s potential for dulling.

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Question 20

Halogens (chlorine, fluorine, iodine) easily pass through the passive layer and can damage instruments

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Question 21

Bleach solutions are not be used to clean surgical stainless steel.

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Question 22

The suture is wound inside an inner folder, often with an outer folder, and finally a non-sterile external wrapping layer. For most sutures, the inner folder is dry, but for gut sutures, an alcohol mixture is used to bathe the suture.

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Question 23

Today, gamma radiation and ethylene oxide are used for sutures. Gamma irradiation is effective and has the advantage of being able to sterilize sealed packages, but it can degrade some sutures such as synthetic absorbables.

Answer 23

T Natural gut sutures and nylon can be safely sterilized with gamma irradiation and are often sterilized as such in currently manufactured sutures. Most others are treated with ethylene oxide gas, requiring sealing of the outer packaging after treatment.

Question 24

Newer martensitic stainless steels, such as S42000, S42020, and S45500, provide several of the properties essential to a needle: sharpness, resistance to bending and resistance to fracture. S45500 contains nickel at a level of 7.5–9.5% and titanium at 1.5%, whereas S42000 and S42020 lack both. S45500 has the same modulus as earlier steels, but benefits from a 60% higher yield strength and 25% higher maximal tensile strength, which translates into improved resistance to plastic deformation (bending) and improved resistance to breakage, respectively.

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Question 25

Newer proprietary high nickel needle alloys with increased amounts (3–4%) of molybdenum have also been developed, such as Surgalloy and Ethalloy. On formal testing, Surgalloy needles showed 32% higher yield strength and were able to withstand almost twice the number of deformations prior to breaking as the S45500 stainless steel needles.

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Question 26

Suture needles are composed of three segments: the swage (which connects strand to needle), body, and point. The size of the needle is determined by a number of factors

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Question 27

Needle length is Distance along arc from tip to end of swage

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Question 28

The body of the needle connects the swage to the point and can have one of several cross-sectional shapes: triangular, round, rectangular, trapezoidal. The rectangular body was created to help minimize needle bending and twisting.

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Question 28

Most needles are connected to the suture strand with a laser-drilled swage. Prior to this technology, mechanical drilling required the proximal end of the needle to be at least 0.36 mm. Obviously, this was too large a needle for smaller 5-0 and 6-0 suture strands, in which a flanged end was traditionally used. The channel in the flange created tissue drag and undue trauma. With laser drilling, smaller sutures can be attached to a correspondingly smaller needle. The laser also allows a shorter channel to be created, thereby allowing the surgeon to grasp closer to the swaged end without damaging it.

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Question 29

Radius of curvature is Center of radius of circle created by needle arc to any point along the needle

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Question 30

Chord length is Straight-line distance from tip to end of swage

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Question 31

Arc is Fraction of complete circle (3/8 most common)

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Question 32

The cross-section of the point can be round or triangular. Triangular points can be further divided into traditional cutting (the apex of the triangle faces the inside of the arc) and reverse cutting (the apex of the triangle faces outward).

Answer 32

T The disadvantage of the traditional cutting style is that the sharp apex is cutting the tissue on the inside of the hole created by the needle, which could lead to a tear of the hole into the narrow space between the puncture hole and wound edge. The reverse cutting needle leaves a flat “wall” of tissue on the inside edge of the needle hole, making it less susceptible to tearing through the tissue

Question 33

Triangular points usually arise from triangular bodies. The exception is taper-cut needles, which allow a transition from a round/oval body to triangular point. Another important consideration for the needle point is the taper ratio. This is the ratio of the length of the tapered portion to its diameter. A longer taper ratio will minimize resistance to needle penetration into tissue.

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Question 34

Covidien labels cutting needles as “Conventional Cutting” and reverse cutting needles as “Cutting”. Reading the label is facilitated by knowing that cutting needles have an arrowhead pointing up on the label and reverse cutting needles have an arrowhead pointing down

Answer 34

T Most applications in dermatology call for a reverse cutting needle

Question 35

Non-absorbable sutures are, in theory, resistant to biodegradation, whereas absorbable sutures degrade in 2–3 months.

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Question 36

Sutures of larger diameter have higher tensile strengths. The knot is the weakest portion of any suture and can be separately tested using a tensio­meter and sutures tied with various knot configurations.

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Question 37

Polybutester (Novafil) exhibits a biphasic load–extension curve whereas nylon and polypropylene have a linear elastic regionThis biphasic elastic region in Novafil allows the suture to elongate up to 25% at very low tensile loads

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Question 38

# . Of the common sutures used in dermatologic surgery, polybutester (Novafil) and Monocryl (followed closely by PDS II) have the least bending stiffness and best handling

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Question 39

Larger diameter sutures will have more stiffness than smaller sutures. It is worth noting that tensile stiffness is a bit different from memory, which is the ability of a suture to recall its prior configuration. Memory is a consequence of bending stiffness.

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Question 40

Capillarity of a suture is its ability to transport fluid through a fixed length of suture in a given time period of which there are numerous testing protocols.

Answer 40

T The suture's ability to absorb fluid may be an equally important indicator of its ability to also absorb bacteria, since absorbed fluid may be contaminated.

Question 41

Braided sutures have higher capillarity than monofilament sutures.

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Question 42

Synthetic sutures absorb very little fluid, whereas catgut absorbs the most

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Question 43

While capillarity and fluid absorption help predict propensity of a suture to potentiate infection, this property can be more directly measured by implanting sutures contaminated with known quantities of bacteria in vivo and then assessing bacterial growth after several days. Monofilaments such as Caprosyn and Biosyn show significantly lower bacterial growth than braided counterparts chromic gut and Vicryl, respectively. Of note, lack of absorbable suture of any kind provides the least bacterial growth after 4 days, underscoring the logic of avoiding deep sutures in grossly contaminated traumatic wounds.

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Question 44

Tissue drag involves measuring the force required to overcome resistance as a suture is pulled through a given material. A monofilament such as Monocryl requires less force to be pulled through tissue than a gut suture. Lower drag would be expected to reduce damage to the tissue while pulling the strand.

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Question 45

The only natural absorbable suture commonly used, and discussed here, are gut sutures. These are primarily degraded by proteases released by phagocytic cells recruited during the inflammatory phase of wound healing.

Answer 45

T As substantiation for the need for this cellular response, experimentally treating an incision of the synovium in sheep with corticosteroids will significantly delay breakdown of chromic gut sutures, likely due to the absence of inflammation and subsequent phagocytic cells.

Question 46

Synthetic non-absorbable sutures are polymers composed of a variety of cyclic lactone monomers. The nature of the monomers allows for unique properties for each synthetic absorbable.

Answer 46

T For example, addition of glycolide imparts stiffness and susceptibility to hydrolysis. This leads to the shorter absorption time of Vicryl compared with polydioxane and the new generation of high lactide sutures

Question 47

Degradation of synthetic absorbable sutures is usually effected by hydrolysis rather than proteolysis as for catgut.

Answer 47

T Treating the same sheep synovium with corticosteroids as detailed above will have minimal impact on breakdown of synthetic absorbable sutures. Of note, the often quoted “absorption times” for sutures are very variable and situation-dependent, since absorption of suture strands can be affected by many factors

Question 48

synovial fluid and gamma irriadiation accelerates the degradation of sutures

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Question 49

Breaking strength retention (BSR) curves can be created by measuring the percentage of original tensile strength in the suture at various time points. The BSR profiles were created using a composite of the available studies of in-vivo suture degradation. Of interest, the size of the absorbable suture has a minimal effect on the BSR curve. While larger suture has a higher tensile strength than smaller suture, each loses relative strength at remarkably similar rates, facilitating comparisons.

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Question 50

Biosyn has the highest initial tensile strength, followed by Vicryl.

Answer 50

T Next are Monocryl, Caprosyn and PDS, with similar initial tensile strengths. Variants of gut suture are the weakest absorbable sutures. In spite of having a similar BSR profile to Vicryl, Biosyn's initial strength is adequately high to enable it to have higher tensile strength at 2 weeks than Vicryl had at the time of surgery. In similar fashion, despite its rapid BSR decline, Monocryl maintains excellent tensile strength for the first week of healing due to its high initial tensile strength.

Question 51

Of current absorbables, catgut has the highest tissue reactivity, followed by Vicryl, PDS and Maxon, followed by Biosyn and finally Caprosyn and Monocryl with the least amount of reactivity. Silk has the most reactivity of the non-absorbables.

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Question 52

Wound strength is estimated at 10% of normal at 2 weeks; 50% of normal at between 4–5 weeks, and plateaus at 80% of normal

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Question 53

Silk is produced by Bombyx mori

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Question 54

Polypropylene has the lowest reactivity of non-absorbables

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Question 56

Novafil has the lowest stiffness of non-absorbable sutures

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Question 57

PDS has longest absorption times

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Question 58

Glycomer 631 17 Biosyn– Covidien Highest absorbable suture initial tensile strength

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Question 59

Plain gut is derived from serosal layer of cow or submucosal layer of sheep intestines

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Question 60

Traditional staples are usually manufactured from austenitic stainless steel (e.g., 316L) and are placed percutan­eously. They have the advantage of high tensile strength, but require removal and can result in epithelialization of the puncture sites (“train-tracks”). Absorbable dermal staples (INSORB) are manufactured using a polymer of L-lactide (70%) and glycolide (30%). They retain 40% of original strength at 2 weeks and complete mass absorption at 3–4 months.

Answer 60

T Staples are used for closure of longer wounds or scalps where underlying tension has already been relieved by deep sutures if necessary. Absorbable dermal staples (INSORB) have been extensively used in closure of C-sections and meta-analysis of six studies showed that metal staples allowed for faster closures, but INSORB-closed wounds had lower complications (infection and dehiscence). 33 If an absorbable suture is going to be used in a wound under tension, subcutaneous absorbable sutures can be placed to relieve excess tension prior to stapling.

Question 60

Vicryl Rapide time to complete loss of mass is 6 weeks

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Question 61

Understanding the properties of stainless steel encourages the surgeon to understand that it is not stain-“proof”. Use of bleach and abrasives can damage the thin passive layer and result in pitting and corrosion. Staff must be diligent to get the instrument clean and not damage the passive layer in the process.

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