Microcannulas are liposuction cannulas with an inside diameter (ID) less than or equal to 2.2 mm, which is the ID of a 12-gauge hypodermic needle.
Microcannulas are constructed from standard, fully tempered, stainless-steel hypodermic needle tubing. In contrast, larger cannulas are made of stainless-steel tubing with a wall thickness that is significantly greater than that of microcannulas.
The relatively thin wall of a microcannula significantly limits the shape and size of the apertures in the cannula; this limits the design of the microcannula tip. A tip design appropriate for a cannula with thick-walled stainless-steel tubing may not be safe or practical with hypodermic needle tubing. The thin wall of hypodermic needle tubing imposes structural limitations on the type of aperture patterns than can be used and still maintain sufficient structural integrity.
The qualitative differences between microcannulas and macrocannulas are determined in part by the fibrous stroma within fat. The tenacious fibrous septa of fat compartments and fat pearls resist penetration by larger cannulas but are easily penetrated by microcannulas.
Microcannulas remove small volumes of fat with each stroke, so they can be used to remove superficial layers of fat with less risk of creating irregularities in the skin. Microcannulas require more time to complete a case but, paradoxically, remove greater amounts of fat. Whereas larger cannulas tend to be directed deeply to avoid creating visible furrows or depressions on the skin surface, microcannulas can be used more superficially and thus can remove more fat.
Microcannulas permit the removal of superficial fat layers without disrupting or disconnecting the many fibrous attachments that connect the skin to the muscle below. After tumescent liposuction removes the heavy weight of fat pulling downward on the skin, these fibrous attachments contract, returning the skin to its normal position.
Cannular diameter is directly correlated with the degree of discomfort associated with liposuction totally by local anesthesia in an awake patient. If tumescent infiltration has been correctly accomplished, tumescent liposuction should be painless even with larger cannulas. If the surgeon encounters a localized area of incomplete anesthesia, a larger cannula will cause even more burning discomfort, whereas a smaller cannula will cause much less pain with continued liposuction.
The use of microcannulas decreases the probability that an awake patient will experience discomfort during tumescent liposuction. This important fact has gone unnoticed by many surgeons with inadequate training in tumescent liposuction. These surgeons may continue to use large cannulas and often cannot accomplish liposuction totally by local anesthesia.
The use of microcannulas was not practical or safe before the advent of tumescent vasoconstriction. The extensive use of microcannulas is only feasible with the profound hemostasis provided by the tumescent technique.
Before the tumescent technique, blood loss was a limiting factor in determining how much liposuction could be done safely. As discussed next, microcannulas cause more surgical bleeding than large-diameter cannulas. Without good hemostasis, liposuction surgeons preferred to use larger cannulas. The profound hemostasis of the tumescent technique almost completely eliminates liposuction blood loss and permits the use of microcannulas.
Bleeding Analysis. The following explanation reveals the relation between the diameter of a liposuction cannula and the degree of liposuction-induced intraoperative surgical bleeding.
Theoretically the shape of the wound made by a liposuction cannula within fat is approximately a cylindrical tunnel, with a circular cross section corresponding to the cannula’s ID. A cylinder with a circular cross section of radius R and length L has a surface area A = 2πRL and volume V = πR2L. For such a cylindrical tunnel the relation of A to V is given by the following equation:
A = 2V/R
In other words, for a fixed volume, the surface area of a cylinder is inversely proportional to its radius. Thus, as the cannula diameter increases, the surface area of the wound decreases (Figure 27-1).
For example, a patient has equal volumes of fat removed by liposuction from each outer thigh, with a small-diameter cannula used on one thigh and a larger cannula on the opposite thigh. For this fixed volume of fat, the small-diameter cannula will have the largest surface area associated with its multiple cylindrical wounds. The larger the surface area of a cylindrical wound within fat, the greater is the number of transected capillaries, and thus the greater the amount of bleeding. Larger cannulas produce a wound with a smaller surface area and fewer transected capillaries.
Before the tumescent technique, large cannulas minimized the bleeding associated with liposuction. Thus the earlier and bloodier liposuction techniques required larger cannulas to minimize bleeding.
Based on this analysis, a 2-mm cannula theoretically causes four times the bleeding as an 8-mm cannula. Because bleeding associated with tumescent liposuction is virtually zero, no significantly increased bleeding occurs with tumescent liposuction using microcannulas.
This analysis assumes thorough tumescent infiltration, which is necessary for optimal hemostasis. Incomplete tumescent infiltration will result in proportionately more bleeding.
Designs and terminology
The Finesse microcannula is designed to minimize the risk of injury to the deep surface of the dermis when liposuction is done near the skin. Early cannula designs had one or more oblong apertures or oval slots arranged linearly along the distal end of the cannula. The Finesse microcannula has two in-line oblong slots along one side and a spatula-like tip. This design was motivated by its simplicity and ease of fabrication (Figure 27-2, A).
To avoid injury to the superficial vascular plexus on the skin’s undersurface when doing superficial liposuction, the apertures of a Finesse microcannula should always be directed away from the dermis. This is achieved by having a “thumb rest” depression machined into the cannula hub. The tube is attached to the hub with the thumb rest facing 180 degrees away from the oblong apertures. This arrangement allows the surgeon to control the direction of the microcannula apertures.
When the cannula is in subcutaneous fat, the cannula is held so that the thumb rest on the hub faces away from the skin. This ensures that the apertures are continuously oriented away from the dermal undersurface.
The Capistrano microcannula is designed to maximize liposuction cannula efficiency (volume of fat liposuctioned per in-and-out stroke). Because the apertures are arranged around the entire microcannula circumference, some are always directed toward the skin. To prevent dermal injury, Capistrano microcannulas should never approach within one cannula diameter of the dermal undersurface.
The Capistrano microcannula has multiple round apertures arranged in a helical pattern along the distal end of the cannula (Figure 27-2, B). It has a single slot located distally to facilitate cleaning and the removal of debris (Figure 27-3).
Because of its size and delicacy, the 16-gauge Capistrano microcannula has only seven round apertures and the one distal slot. Standard 14-gauge and 12-gauge cannulas have 12 round apertures and one distal slot.
Special Capistrano-style cannulas have been designed specifically for reduction mammoplasty by microcannular tumescent liposuction. This design permits efficient liposuction in the fibrous fat and glandular tissue within the female breast. These microcannulas have smaller holes but also many more holes than a Capistrano microcannula. The small holes minimize the risk of vascular injury.
In the early years of liposuction the terminology for specifying the diameter of urinary catheters and abortion cannulas was also used to specify the diameter of the liposuction cannulas. Thus the diameter of a liposuction cannula was specified as being either 8 mm or 24 French. Subsequently, it became common practice to refer to the diameter of cannulas only in terms of approximate millimeter size.
In the engineering of hypodermic needles, where the diameters of progressive sizes vary by fractions of a millimeter, it is traditional to specify tube diameter in terms of wire gauges rather than millimeters. The terminology that describes microtubes in terms of gauge has been applied to microcannulas to simplify communication between manufacturers of hypodermic needle tubing, manufacturers of Luer connectors, machinists, and surgeons.
The gauge sizes of the hypodermic needle tube are associated with standard ID, outside diameter (OD), nominal wall thickness, and dimensional tolerances. Because all physicians and surgeons are familiar with the standard needle gauge designations for hypodermic sizes, microcannular size is designated by the corresponding hypodermic needle gauge (Table 27-1).
Table 27-2 lists available microcannulas according to gauge and length specifications.
As with any surgical technique or instrument, the use of microcannulas for liposuction assumes the surgeon has had specific training with regard to the risks, benefits, and indications.
20 Gauge. The 20-gauge Capistrano microcannula is ultrasmall and specifically designed for delicate surgery on the lower face, including the nasolabial cheeks and submental area. It is helpful in removing small, incremental volumes of fat. This extremely fragile instrument will bend easily if it is used roughly or forced through excessively fibrous tissue. Microcannulas such as the 20-gauge Capistrano should never be used near the eyelids.
18 Gauge. The 18-gauge Capistrano microcannula is extrasmall and intended for fine, incremental surgery of the submental area, cheeks, and jowls. When passed through subcutaneous fat of these areas, an 18-gauge microcannula encounters less resistance than microcannulas with larger diameters. An 18-gauge microcannula can be used within the subcutaneous fat of the medial submental area, with or without suction, to define a deep surgical plane immediately above the platysma muscle.
The 20-gauge and 18-gauge microcannulas can be used to create a precise network of fine tunnels through which the larger 16- and 14-gauge microcannulas can subsequently be passed with minimal resistance and improved accuracy.
For the most precise control when using 20-, 18-, and 16-gauge microcannulas, many surgeons prefer to attach the microcannula directly to Fine-Touch Aspiration Tubing (modified intravenous tubing) instead of using a microcannula handle and the heavier, standard aspiration tubing. This lightweight tubing allows a delicate three-finger grip of the cannula hub, using the thumb, index, and middle finger to control and maneuver the microcannula.
16 Gauge. The 16-gauge microcannula has an ID of 1.2 mm and OD of 1.6 mm.
A 16-gauge microcannula is useful for liposuction of excessively fibrous fat. Some areas are naturally more fibrous, such as the periumbilical area, back, and breasts; other areas are excessively fibrous because of previous liposuction. In such areas a 16-gauge cannula can penetrate the fibrous septa with minimal force and create multiple fenestrations through the fibrous partitions. After a 16-gauge cannula has prepared an area of fibrous fat, a larger microcannula can be passed through the fenestrations, accomplishing liposuction with minimal resistance, minimal discomfort, and maximal effectiveness.
The 16-gauge microcannula is especially useful for areas that are sensitive or painful during liposuction, such as the medial knees.
These microcannulas allow extremely fine control of the volume of fat that is removed. Thus a 16-gauge cannula is indicated for areas where only minimal fat needs to be removed (e.g., face, neck) or where a small liponot requires minimal liposuction for a smoother result.
14 Gauge. The 14-gauge microcannula (ID 1.6 mm, OD 2.1 mm) is remarkably efficient. It is the most versatile and most frequently used microcannula. The 14-gauge cannula is particularly useful for liposuction of the arms and thighs. These areas often are treated only with 16-gauge and 14-gauge cannulas.
A 14-gauge microcannula fits through a 1.5-mm adit (punch excision) in the skin. Because of the elastic nature of skin, a 1.5-mm punch excision creates a small hole that expands to approximately 2.0 to 2.1 mm in diameter and readily accommodates a 14-gauge microcannula. Thus, in addition to encouraging postoperative drainage, a 1.5-mm adit also provides easy access for both 16-gauge and 14-gauge microcannulas and eventually heals with an imperceptible scar (see later discussion).
As always, to avoid postinflammatory hyperpigmentation, the surgeon must be cautious not to traumatize the cannula access site. Trauma to the dermal-epidermal junction can occur either by allowing the cannula to rub the skin at the site where it passes through the dermis or by allowing the cannula hub to pound the skin repeatedly at the entrance site.
12 Gauge. The 12-gauge microcannula (ID 2.15 mm, OD 2.75 mm) is used regularly but not necessarily with every case. A 12-gauge microcannula is more efficient than the smaller 16-gauge and 14-gauge cannulas and thus removes fat more rapidly.
Removing fat more rapidly is an advantage as long as the surgeon has the skill and experience to know when and where such an efficient cannula can be used. Inappropriate use of an especially efficient cannula can remove fat too fast and increase the risk of inadvertently creating lipotrops or irregularities within a treated area.
For liposuction of such areas as the abdomen, female hips, and male flanks, the 12-gauge Capistrano microcannula is often employed only after use of a 14-gauge microcannula. For the inner thighs the 12-gauge Finesse microcannula is often the last cannula used, treating the area of maximum depth of medial thigh fat while minimizing the risk of injury to the overlying thin dermis.
10 Gauge. The largest cannula that I ever use (perhaps once a year) is a 10-gauge Finesse cannula (ID 2.7 mm). Ten-gauge Capistrano microcannulas are not recommended because they can remove fat so rapidly that a large lipotrop can be created without the surgeon’s awareness. If a 10-gauge Capistrano cannula is used too superficially, it may cause full-thickness dermal necrosis as a result of obliterating the vascular plexus in the apical fat layer along the skin’s undersurface.
16-Gauge and 14-Gauge Brest Microcannulas. The female breast microcannulas, specialized versions of the Capistrano microcannula, are designed to minimize the risk of trauma to blood vessels and sensory nerves. The distal round holes are more numerous and extend proximally from the cannula tip for a greater distance. The 16-gauge Brest microcannula is used in the initial stages of female breast surgery and typically accounts for 10% to 20% of the supranatant aspirate. The 14-gauge, 15-cm-long Brest microcannula usually accounts for the remainder of the total aspirate.
Tumescent liposuction for female breast reduction is capable of reducing the volume of the breast by as much as 50% without visible scars, without significant mammographic changes postoperatively, and without the prolonged postoperative recovery and delayed return to normal activities typically associated with the traditional excisional breast reduction surgery.
A 1-mm adit is sufficient to accommodate the passage of a 16-gauge or 14-gauge Brest microcannula through the skin of the female breast. Although 12-gauge Brest microcannulas are available, they are not intended for breast liposuction; some surgeons use them for liposuction of the abdomen, hips, and lateral thighs.
An adit is a technical engineering term that describes an extra opening or collateral tunnel by which a mine is entered or drained. An adit used in tumescent liposuction is a small circular hole made by a tiny, skin biopsy punch. Adits facilitate and promote the open drainage of residual blood-tinged anesthetic solution associated with tumescent liposuction.
Round 1.0-mm, 1.5-mm, and 2-mm adits, created by disposable skin biopsy punches, allow better drainage than simple incisions. The edges of a linear slit or microincision made by a scalpel blade may close and heal prematurely, entrapping blood-tinged anesthetic solution in the subcutaneous space.
The round hole of a 1.5-mm adit can easily accommodate a 16-gauge or 14-gauge microcannula with little or no epidermal friction as the microcannula is pushed and pulled through the skin. A 12-gauge microcannula usually requires a 2-mm adit, although a 1.5-mm adit may accommodate a 12-gauge cannula in elastic skin such as the inner thigh.
Small, 2-mm to 3-mm linear incisions can be used for tumescent liposuction of the face and neck, but I prefer to use 1.0-mm adits. Elsewhere, 1.5-mm and 2-mm adits are used almost exclusively for microcannular access into the subcutaneous fat.
Microcannulas are necessary for optimal results with tumescent liposuction. They have many advantages over larger cannulas.
Less Pain. Microcannulas are less painful than larger cannulas. This is an important advantage when doing liposuction totally by local anesthesia.
Better Accuracy. Microcannulas are more accurate and reduce the risk of liposuction-induced dermal complications. On changing the direction of a cannula, a larger cannula is more likely to follow a path of least resistance and be inadvertently advanced along an existing tunnel. A smaller cannula is more easily advanced through fibrous tissue and thus is less likely to deviate from its intended direction. A microcannula reduces the risk of liposuction along an unintended path.
Greater Finesse. By removing smaller volumes of fat per stroke, microcannulas provide greater assurance against inadvertently removing too much fat. Proper technique using microcannulas minimizes the risk of skin irregularities after liposuction of areas with less margin for error, such as jowls, cheeks, and nasolabial fat pad; medial and anterior thighs; and buttocks.
Superficial Liposuction. The superior accuracy in directing the cannula and the more delicate control of fat removal permit more uniform, smoother results. This enables more superficial removal of fat and minimizes risks of irregularities. Superficial liposuction is feasible only with tumescent vasoconstriction and small cannulas.
Excessively superficial liposuction that injures the vascular plexus within the apical fat adjacent to the dermis can cause dermal necrosis.
More Complete Removal. An increased confidence in achieving uniformly smoother results permits more liposuction and removal of larger amounts of fat from any area. Thus more fat can be confidently removed using microcannulas.
The surgeon must always be careful not to remove too much fat. Aesthetic considerations require that the liposuction produce results that look and feel normal; removing all the subcutaneous fat from an area usually does not give natural-appearing results.
Easier Penetration. Microcannulas can penetrate fibrous fatty tissue with minimal force, thus permitting liposuction of areas that are nearly impossible to treat adequately with larger cannulas. Examples of very fibrous areas include the glandular tissue of male or female breasts, the dorsolateral fat just below the bra straps on women, and areas previously treated by liposuction.
Adits and Microincisions. Adits and microincisions permit microcannular access into the subcutaneous fat. An adit is a small, round opening in the skin created by 1.0-mm, 1.5-mm, or 2.0-mm skin biopsy punches. A microincision is an incision so small, usually only 2 to 4 mm long, that it does not require sutures for optimal postoperative healing.
Provided that the cannula does not traumatize the epidermis surrounding an incision, scars from adits and microincisions are rarely perceptible. This permits the use of more incisions, allowing greater accessibility to all subcutaneous fat compartments. Because adits are patulous round holes, unlike linear incisions, they tend to minimize the traumatic friction on the epidermis where the microcannula penetrates the skin. They also promote drainage of fluids after tumescent liposuction.
No Sutures. Adits and microincisions for microcannulas do not require sutures. Although not using sutures to close a surgical wound might seem antithetical to accepted practice, with microincisions the proposition is plausible. Assume that adjacent sutures in a routine surgical closure of a 100-mm incision are usually spaced at least 5 mm apart. Therefore, because the length of a microincision is less than or equal to the distance between adjacent sutures in a larger wound, there is no a priori need to use sutures to close a microincision.
By eliminating the foreign body inflammation and cross-hatch scarring associated with sutures, nonsutured incisions for microcannulas heal better and faster than suture incisions.
Accelerated Healing. Multiple incisions without sutures accelerate drainage, which reduces bruising, swelling, and soreness. Wounds created within fat by microcannulas are narrow tunnels that heal more rapidly than larger tunnels. The greater net surface area of the walls of these “microtunnels” promotes more rapid absorption of residual fluid within the subcutaneous space. The net effect is greatly accelerated healing.
Greater Time Efficiency. By eliminating the need for sutures, the use of microcannulas saves the surgical time required to place the sutures. Furthermore, no sutures means no postoperative visit for suture removal.
Nonsutured incisions and appropriate compression optimize drainage of blood-tinged anesthetic solution, which in turn hastens postoperative recovery and return to normal activities. Fewer postoperative problems and less patient anxiety dramatically decrease follow-up visits. The first postoperative visit can be postponed for 4 to 6 weeks.
Microcannulas do not remove as much fat per stroke as do larger cannulas. Thus treating any specific area by liposuction can be completed more rapidly with a large cannula. Larger cannulas are less accurate, however, and more likely to cause skin irregularities that require secondary “touch-up” procedures. Such procedures must be considered when assessing the “total time” required for liposuction. Microcannulas minimize the need for touch-up sessions.
Less Muscle Strength Required. The small crosssectional area of a microcannula and the flat, semiblunt microcannula tip minimize the resistance encountered when advancing a cannula through the fibrous septa in subcutaneous fat. With the microcannula, no surgeon should be at a disadvantage because of lack of muscle strength.
Similarly, the decreased resistance and decreased force requirement permit more accurate control of the speed and direction of the microcannula tip. This minimizes the risk of an inadvertent penetration of other nontargeted tissues, such as muscle or the peritoneal cavity. To my knowledge, the peritoneal cavity has never been penetrated by a microcannula.
Less Elbow Trauma. The force required to push a cannula through fibrous fatty tissue is minimized by using microcannulas. This is a great advantage in terms of protecting the surgeon’s elbow from the chronic stress and trauma of performing thousands of liposuction surgeries.
Excessive Efficiency. The danger of excessive efficiency is an ever-present concern when using Capistrano microcannulas. The surgeon may not be aware of these cannulas’ tendency to remove more fat than is usually removed with other cannulas. Therefore the surgeon may unwittingly remove more fat than intended and produce a disfiguring lipotrop.
Novice liposuction surgeons should use Capistrano microcannulas with great caution. I am aware of two cases in which experienced liposuction surgeons faced litigation because of inattentive or overaggressive use of Capistrano microcannulas on the medial thighs.
Fragility. Delicate instruments cannot be used as a pry bar to lift and shift tissue. Using microcannulas requires a straight, in-and-out, pistonlike stroke, similar to the stroke of a billiard cue stick. Microcannulas may bend when used clumsily. Repeated flexion of the thin metal tube will result in metal fatigue and cracking at the point where the tube enters the hub.
Microcannulas are susceptible to being crimped or bent with lateral stress. Flexion can occur when an excessive forceful thrust is stopped by exceptionally dense fibrous tissue.
More Incisions. Microcannulas require more incisions than was the practice in the past with larger cannulas. Although the incisions are so small that scars are rarely visible, the surgeon must take extra care not to cause prolonged dyschromia by unnecessarily injuring the epidermis.
Dyschromia. Postoperative dyschromia of the incision site is typically the result of repeated traumatic pounding or friction to the surrounding epidermis. Carefully avoiding injury to the dermal-epidermal junction ensures that scars are virtually invisible. Liposuction with microcannulas should be done with a light touch and finesse.
Postinflammatory hyperpigmentation is a particular risk in individuals who have inherited darkly pigmented skin. The skin of the upper abdomen and the back is especially susceptible to postinflammatory hyperpigmentation or hypopigmentation; the lateral thorax and extremities are less predisposed.
Inspissated Fat in Apertures. If a small fragment of fat remains in a microcannula aperture and is allowed to dry out, the aperture will become plugged, requiring special cleaning. After using a microcannula or when changing from one microcannula to another, care must be taken to prevent residual fat in the small apertures from becoming desiccated and obstructing the small holes of the cannula tip. When the fat is still moist, the holes can be cleaned out by simply wiping the distal portion of the microcannula with sterile gauze while the vacuum pump is aspirating.
Proper Technique and Function
Microcannulas are smaller and more fragile than the rugged, thick-walled steel tubes of large liposuction cannulas. When used correctly, microcannulas are durable, reliable surgical instruments that should last for years. Being more delicate, however, they can be broken more easily if used roughly.
Microcannulas must be advanced with a firm but gentle, pistonlike motion along a linear axis. Repeatedly using a microcannula aggressively to lift or push the skin and fatty tissue will place repetitive structural stress on the tubing.
Finesse Versus Force
A small-diameter stainless-steel tube does not have the strength to endure an excessive force vector applied in a direction that is perpendicular to the tube’s long axis. Excessive force may cause the microcannula to bend beyond its limit of flexibility and become permanently damaged. Using a microcannula with inappropriate, aggressive surgical technique will cause the microcannula to fracture at the point of maximum flexion. This point of maximum stress is where the tube enters the cannula hub.
The smaller the diameter, the more flexible is the microcannula. Also, for any given diameter, the longer the microcannula, the greater is its inherent flexibility. Greater flexibility means a microcannula can be more easily bent. Greater care is required to avoid applying excessive force when advancing or pushing more flexible microcannulas through adipose tissue.
Microcannulas must be used with care and finesse. When the surgeon encounters an area of resistance or fibrosis, forceful application of muscle strength is contraindicated. The microcannula should be advanced somewhat tentatively, probing for a path of less resistance. With a gentle technique the cannula can be finessed through fibrous tissue without exerting undue force. Too much force will cause the cannula to buckle and break.
Microcannulas are designed to be effective and efficient. They are not heavy-duty industrial-grade tools, but rather delicate surgical instruments. Microcannulas permit smoother and more accurate results, but they require greater skill and more gentleness than larger, more rugged cannulas.
Local Versus Systemic Anesthesia
Microcannulas penetrate the fibrous septa of adipose tissue with minimal resistance. They are less likely to cause painful traction on distant tissues and are most appropriate for liposuction totally by local anesthesia.
Since they require less force to be pushed through adipose tissue, microcannulas permit more surgical finesse and greater surgical precision. They also cause less repetitive trauma to a surgeon’s wrist, elbow, and shoulder.
In contrast, large traditional cannulas rely on muscle strength and usually require systemic anesthesia. A large cannula must be rammed through the fibrous tissue partitions of subcutaneous adipose tissue with such force that most awake patients would find it intolerable.
The force required to push a large, blunt cannula through adipose tissue exerts traction on fibrous tissues and transfers the force to distant tissues. Because large cannulas can cause pain beyond the locally traumatized tissue, they are frequently incompatible with local anesthesia.
Fat is more fragile than most other tissues and is more easily penetrated by a blunt probe. The density and strength of fibrous attachments within fat decrease with decreasing size of the structures in the hierarchy of adipose tissue anatomy (see Chapter 25).
Adipose tissue within a fat pearl generally contains relatively little fibrous tissue and has a jellylike or custard pudding consistency. The tissue within a fat pearl is easily deformed and easily plucked from its weak attachments to the surrounding, denser fibrous stroma. Microcannulas require relatively little force to aspirate the gelatinous mass of fat lobules found inside a fat pearl.
In contrast, large cannulas require considerable force to detach groups of fat pearls from the more fibrous stroma within a fat section. Whereas large liposuction cannulas ingest multiple intact fat pearls and then literally rip large chunks of fat from fibrous attachments, microcannulas remove the fat from within individual fatty pearls.
The desirable qualities in microcannular design include the following:
- Safety, with minimal risk of injury to nonlipocytic tissues
- Minimal pain during liposuction in a fully awake and alert patient
- Optimal efficiency
- Quality construction that produces durable, easily manufactured cannulas.
The following relationships apply to any cannular design:
- The smaller the cannula OD, the lower the incidence of pain
- The smaller the OD, the less the pain caused by pushing the cannula through subcutaneous fat.
- The smaller the cannula apertures, the less the pain caused by suctioning adipocytes off fibrous septa within adipose tissue
At least two configurations satisfactorily meet these requirements: Finesse and Capistrano. The original microcannular design, now known as the Finesse microcannula, has two distal, in-line oblong openings. For years the Finesse microcannula was the only microcannula designed specifically for tumescent liposuction. Capistrano-style microcannulas, including Brest cannulas, are a more recent design and are more efficient.
Cautious Use. The surgeon must be extra careful when first using the Capistrano microcannula to avoid inadvertent excessive liposuction. The Capistrano microcannula is so efficient than an inattentive surgeon can unintentionally remove more fat than desired.
I recommend that novice surgeons approach tumescent liposuction with caution by first using the Finesse-type microcannula. The Capistrano microcannula is approximately 30% to 50% more efficient than the Finesse microcannula.
The Physics of Liporaspiration
This section addresses the following questions:
- How do liposuction cannulas work?
- What are the factors that determine the efficiency of a microcannula?
- Are microcannulas (ID 1.2 to 2.2 mm) qualitatively different from macrocannulas (ID 4 to 8 mm)?
The answers are based on clinical observations rather than published scientific studies.
Hydrodynamics is the branch of physics that studies the forces acting on or exerted by liquids. The mechanical dynamics of a liposuction cannula designed only to suction adipose tissue differ from other systems. The mechanical principles of liposuction cannulas are hydrodynamically distinct from surgical cannulas designed to aspirate liquids such as blood or other body fluids.
A cannula that aspirates blood works on the simple hydrokinetics of pressure gradients. Liposuction involves ripping and tearing of semisolid tissue with the assistance of a pressure gradient. Thus, from the perspective of pure physics, the word liposuction is a misnomer. A lexicologic purist would prefer the term liporaspiration (lipo, fat; rasp, to scrape off or away).
If there is no movement of the cannula relative to the enveloping fat, no fat is removed. In other words, after a cannula is inserted into fat and a vacuum is established within the cannular lumen, no aspiration of fat occurs when the cannula is motionless. If fat were a fluid, an intense vacuum would result in fat being aspirated through a motionless cannula located within subcutaneous adipose tissue. However, adipose tissue consists of fat cells intimately embedded within a fibrous stroma.
Fat is not a fluid, and the physics of liposuction is quite different from the physics of sucking water through a small tubular cannula. Only when the liposuction cannula is in motion, moving to and fro, does fat flow down the tube.
A liposuction cannula works by sucking a discrete morsel of fat into one of its apertures. It then maintains a firm hold on the morsel until it can be ripped away from its attachment by the force of the cannula being pushed and pulled through adipose tissue. Fat enters a cannula through perforations, or apertures, which are located along the cannula’s distal 2 to 4 cm.
Because the contents of individual fat pearls are minimally fibrous, the vacuum within a microcannula can pull morsels of puddinglike lobular fat through the cannula apertures. Initially, each fat morsel remains attached by fibrous stroma to the fat outside the cannula lumen. A fat morsel cannot actually be aspirated into the collecting canister until it is sheared off its fibrous attachments by the cannula’s pistonlike motion (Figure 27-4).
The true efficiency (e) of a microcannula design is a theoretic number, defined as the mean volume of fat suctioned per one complete stroke (stroke count, stroke cycle) of the cannula. A stroke of the cannula consists of the inward thrust and outward pull on a cannula. For any particular cannula, a statistical estimate (ê) of the true efficiency of the cannula can be defined as the average
ê = V/n
where V is the total volume of supranatant fat aspirated with n strokes of the cannula, and n is an arbitrarily specified number. In actual practice the choice of n depends on several factors, including the gauge of the cannula. Clearly, V increases as n increases.
Testing. From a statistical point of view, n should be large enough to permit the detection of a difference in efficiency between two cannulas when a significant difference truly exists. The number of strokes should be small enough to permit the testing of more than one cannula per area of the body.
Sputum Traps. In practice, if one is using a 60-ml sputum trap to collect and measure the volume of fat per n strokes, n should be chosen to permit all the supranatant fat and infranatant anesthetic to be collected in just one 60-ml traps (Figure 27-5).
Two sputum traps are placed in sequence between the collection canister and the microcannula suction tubing. Aspirated fat is trapped in the first sputum trap, where its volume can be measured. The second sputum trap collects any fat that inadvertently “overflows” or splashes out of the first trap. The number n is chosen such that the volume of aspirate will not exceed the volume of the sputum trap for any cannula under consideration.
Volume of Fat. The efficiency of a microcannula is defined as the average volume of fat removed by one complete in-and-out stroke of the microcannula. The volume of fat removed per stroke is a function of the following:
- Microcannula tip design (size, number, and placement of apertures)
- Microcannula gauge (ID)
- Microcannula length
- Velocity of cannula motion through the fat
When comparing the efficiency of various tip designs, the microcannulas should have the same inside radius and length. They also should be thrust through the fat at approximately the same velocity.
For example, the efficiency of a 14-gauge cannula 15 cm in length may be estimated as the volume of fat aspirated per 50 in-and-out strokes within fat that has not been previously suctioned. For a 12-gauge cannula, efficiency might be defined as the volume of suctioned fat per 25 strokes (Figure 27-6). The efficiency of a 14-gauge 10-hole Capistrano microcannula, for example, is estimated to be 28 ml/100 strokes, or 0.28 ml/stroke.
Variables. It is important to avoid confounding variables when comparing the efficiency of two or more different cannulas. Each individual trial comparison should use either the same or a contralateral symmetric area of fat to avoid bias caused by different degrees of fibrousness. Similarly, each comparison test should be done by one surgeon. Because this testing procedure cannot be reliably accomplished in a blinded, unbiased fashion, the surgeon can be a source of measurement bias in this estimation process.
For any given cannula, the actual volumes of fat per n strokes will vary as a function of the degree of fibrousness and the surgeon’s technique. For any given area of fat, however, the most efficient cannula will tend to harvest the most fat per n strokes.
When two cannulas are compared twice, first using abdominal fat and then using inner thigh fat, both tests should give the same qualitative (nonparametric) or rank-ordered results in terms of which cannula is the most efficient. Similarly, with any surgeon, despite the idiosyncratic nature of surgical technique, the most efficient cannula will aspirate the most fat per number of strokes.
Because of statistical variation, determining which of two cannulas is the most efficient requires repeated testing, then comparison of the mean efficiency measurements using standard statistical techniques.
Factors in Microcannula Efficiency
Along with cannula ID and length, the factors that determine liposuction cannula efficiency include the following (Figure 27-7):
- Total aperture area
- Total linear length of distal and proximal edges of apertures
- Velocity of aperture translocation (speed of the cannula’s in-and-out motion)
Aperture Area. The total aperture area is the sum of the cross-sectional areas of all the apertures through which fat is aspirated. If one compares cannulas that have only one round aperture, the efficiency of a cannula is an increasing function of total aperture area. A tiny round aperture will aspirate less fat per stroke than a slightly larger round aperture. The diameter of a round aperture is limited by the cannula’s ID.
If the cross-sectional area of an aperture is greater than cross-sectional area of the cannula itself, π[(ID)/2]2, the aperture must assume an elongated or oblong shape. If one compares cannulas with a single oblong aperture, the cannula efficiency seems to approach a maximum when the length of the oblong slot is approximately four times its width.
Aperture Edges. The number of distal and proximal edges of the apertures also affects cannula efficiency. The distal (proximal) edge of a round aperture is arbitrarily defined as the portion of the distal (proximal) aperture margin that subtends a 90-degree angle from the center of the aperture. After tiny morsels of fat have been sucked into a microcannula aperture, the distal and proximal edges of the aperture act as foci where the pieces of fat are torn off by the cannula’s reciprocating in-and-out motion.
The greater the total length of all proximal and distal aperture edges available to tear off tiny morsels of fat, the greater is the cannula’s likely efficiency.
If cannula C1 has one round hole with an aperture area of A, and if cannula C2 has two round holes, each with an aperture area of ½A, the cannulas have the same total aperture area. Although each cannula has the same total aperture area, C2, with two holes, has a greater total aperture circumference and thus a longer total length of proximal and distal aperture edges. Therefore C2 is more efficient.
Nevertheless, a point of diminishing returns occurs where more and smaller apertures become less efficient.
Aperture Translocation. There is an optimum velocity of aperture translocation, that is, the linear or angular velocity of the cannula. As mentioned earlier, if no movement of the apertures occurs relative to the enveloping fat, no fat is aspirated. Again, if a cannula is inserted into fat, and a vacuum is established within the lumen, no fat will be aspirated until the cannula is in motion.
On the other hand, there is probably an upper limit on how rapidly the cannula should be advanced and retracted. The rate of cannular oscillations is limited by concerns about accuracy, the surgeon’s endurance, and the patient’s safety.
Manufacture and Fabrication
Microcannulas are manufactured from standard stainless-steel hypodermic needle tubing that is permanently attached to a stainless-steel hub. At the end of the hub is a helical female Luer connector for attaching the cannula hub to either a cannula handle or a standard Luer syringe.
The microcannula tube is permanently attached to its hub by a process known as press fitting. This process involves placing one metal part into an excessively small hole in a second metal part.
Press fitting relies on a basic physical property of solid metals. When a metal is chilled, it shrinks; when heated, a metal expands. Thus chilling the cannula tube causes it to shrink. Simultaneously, heating causes the hub to expand in all its dimensions; in particular, the hole in the hub expands. The combined effect of shrinking the tube and expanding the hub allows the relatively large cannula tube to be slipped into the relatively small hole in the hub.
After the cannula has been inserted, the temperatures of the two metal parts equalize. The cannula tube expands while the hole in the hub contracts, causing an exceedingly tight fit.
Special welding techniques can also achieve a strong, durable attachment between a stainless-steel microcannula tube and hub.
|BOX 27-1 Care of Microcannulas|
|1. Soak in germicide-detergent. Immediately after surgery, cannulas, handles, and other instruments are placed in a bactericidal, fungicidal, and virucidal detergent solution (e.g., Control 3*). This is done to prevent desiccation of blood, body fluids, and tissue on the instruments; to minimize presence of pathogens on the instruments; and to reduce risk of exposing staff members to pathogens during initial phases of cleaning.|
|2. Rinse in tap water. All instruments are rinsed thoroughly with warm tap water immediately after removal from germicidal soak.|
|3. Flush microcannulas. A solution of Kleenzyme (1 oz = 30 ml of Kleenzyme in 4 L of tap water) is freshly prepared in a plastic tub or bucket.† With cannula apertures submerged in solution, repeatedly flush this solution through cannula to remove any loose tissue debris, using a 30-ml syringe attached to cannula’s Luer connector.|
|4. Scour with cleaning brush. With cannula tip submerged in Kleenzyme solution, push a special nylon brush on twisted stainless-steel wire back and forth through cannulas to dislodge any adherent tissue debris. For each cannula, use brush with largest diameter that will fit inside. Carefully examine cannula holes for adherent tissue.‡|
|5. Flush microcannulas again. Repeat step 3, flushing microcannulas with Kleenzyme.|
|6. Apply instrument polish. Scrub instruments with a toothbrush after applying polish, then rinse them under tap water.|
|7. Use ultrasonic cleaner. First, soak instruments in a fresh solution of Kleenzyme and cleaner for ultrasonic instruments for 10 minutes. Next, submerge instruments (e.g., microcannulas, handles) in the ultrasonic cleaner solution and sonicate for 10 to 15 minutes. Then rinse instruments under tap water.|
|8. Soak instruments in distilled water. Soak for at least 1 minute to remove minerals, prevent oxidation or discoloration, and moisten lumen of cannulas. Moistened lumen ensures optimum sterilization on autoclaving.|
|9. Wrap and sterilize. All instruments can then be immediately packaged, pouched, and autoclaved.|
|*Maril Products, Tustin, Calif.†Kleenzyme (enzymatic cleaner) and Hinge-Free (instrument lubricant and rust inhibitor), Merck.‡Surgical cleaning brushes for 18-, 16-, 14-, and 12-gauge microcannulas.§Sklar instrument polish, Sklar, Westchester, Pa.|
|TABLE 27-1 Manufacturer’s Specifications for Hypodermic Needle Gauge and Dimensional Tolerance*|
|Gauge||Outside Diameter||Outside Diameter Tolerance||Nominal Wall||Wall Tolerance||Approximate Inside Diameter|
|20||0.0355 (0.9 mm)||0.0005||0.00625||0.0005||0.023 (0.58 mm)|
|18||0.050 (1.27 mm)||0.0005||0.0085||0.0005||0.033 (0.84 mm)|
|16||0.065 (1.6 mm)||0.0005||0.009||0.0005||0.047 (1.2 mm)|
|14||0.083 (2.1 mm)||0.001||0.01||0.001||0.063 (1.6 mm)|
|12||0.109 (2.75 mm)||0.001||0.012||0.001||0.085 (2. 15 mm)|
|10||0.134 (3.4 mm)||0.001||0.014||0.001||0.106 (2.7 mm)|
|*Numbers are fractions of an inch.|
|TABLE 27-2 Standard Sizes of Microcannulas|
|Gauge||Length: cm (inches)|
|16||5 (2), 8 (3)|
|14||10 (4), 15 (6)|
|12||10 (4), 15 (6)|
|18||5 (2), 8 (3)|
|16||5 (2), 8 (3), 12 (4.75), 15 (6)|
|14||15 (6), 23 (9)|
|12||15 (6), 23 (9)|
|Breast (Capistrano Design)|
|16 (24 holes)||12 (4.75)|
|16 (24 holes)||15 (6)|
|14 (30 holes)||15 (6)|
|12 (30 holes)||15 (6)|
|TABLE 27-3 Author’s Preferred Microcannulas|
|Gauge||Length: cm (inches)|
|16||12 (4.75), 15 (6)|
|14||15 (6), 23 (9)|
|12||15 (6), 23 (9)|
|Female Breast Liposuction|