Gluteal augmentation procedures are becoming more prevalent in the field of plastic surgery. Specifically, buttock augmentation with fat grafting has seen a dramatic increase in the United States in recent years. In 2016, there were 18,489 buttock augmentation with fat grafting procedures performed, which is a 26 percent increase from the year prior and nearly double the amount performed in 2011.1 There are various reports in the literature on techniques that include fat grafting in either the subcutaneous plane, the intramuscular plane, or a combination of both.2–27 As demand for this procedure has increased, a number of fatalities that are a direct result of gluteal fat transplantation have surfaced.28–30 In a recent report by Mofid et al., the Aesthetic Surgery Education and Research Foundation formed a task force to evaluate the incidence of fatal and nonfatal pulmonary fat embolisms associated with gluteal fat grafting and to provide safety recommendations.30 The authors reported a cases-per-career mortality rate of one in 6214 cases and a case rate of fatal and nonfatal fat pulmonary emboli of one in 1931 with gluteal fat grafting, with an estimated annual mortality rate of one in 3448 cases. These fatalities have been attributed to fat embolism, which is thought to be a result of either direct injection into a large gluteal vein or injury to a gluteal vein with subsequent inflow of fat lobules from the surrounding tissues. These venous injuries were associated with deep gluteal muscle fat grafting. However, there are opposing views that report no fatalities with intramuscular injections.31 Therefore, it is of utmost importance to learn from these cases and the paucity of literature on this topic to further improve the safety of this procedure. In this article, we review the key anatomy and delineate what we feel are the most important concepts for the purpose of improving patient safety during buttock augmentation with fat grafting.
Thorough history and physical examination are essential before performing any aesthetic procedure, including gluteal augmentation. Important issues to discuss with patients include history of hematologic disorders and use of anticoagulants or herbal remedies that increase bleeding risks.22 , 32 Also, patients with a personal or family history of deep venous thrombosis or pulmonary embolism need to be identified and need to undergo appropriate hematologic evaluation. However, these key elements are important to identify and investigate before performing buttock augmentation with fat grafting. Examination of lower extremities and posterior thighs for the presence of large varicosities is important, as these may potentially increase the risk of venous injury. Furthermore, asking patients about a history of sciatic nerve symptoms, both motor and sensory, is also important, as these should be considered a relative contraindication to the procedure. First, the sciatic nerve is at risk of injury during the procedure and a preoperative baseline needs to be determined.3 , 33 Furthermore, it has been suggested that patients with sciatic nerve symptoms may have varicose veins in the region of the sciatic nerve distribution causing the symptoms, and this can also place the patient at greater risk for venous injury29 (Table 1).
Understanding the anatomy is this region is critical to ensure a safe approach to gluteal fat transplantation. There are two main groups of muscles in the gluteal region, which are commonly grouped as superficial and deep (Fig. 1). The superficial muscles are the larger muscles and include gluteus maximus, gluteus minimus, gluteus medius, and laterally the tensor fasciae latae. The deep muscles are smaller and include piriformis, obturator internus, gemellus superior, gemellus inferior, and quadratus femoris.
The vascular supply to this region originates from the superior and inferior gluteal arteries (Fig. 2). The superior gluteal artery leaves the pelvic cavity with the superior gluteal nerve through the greater sciatic foramen above the piriformis muscle and divides into a superficial branch and a deep branch. The superficial branch passes onto the deep surface of the gluteus maximus muscle and the deep branch passes between the gluteus medius and minimus muscles. The inferior gluteal artery exits the pelvic cavity with the inferior gluteal nerve through the greater sciatic foramen inferior to the piriformis muscle and descends through the gluteal region into the posterior thigh. The superior and inferior gluteal veins follow the respective gluteal arteries into the pelvis, where they join the pelvic plexus of veins. Peripherally, the veins anastomose with superficial gluteal veins, which ultimately drain anteriorly into the femoral vein.
The sciatic nerve enters the gluteal region through the greater sciatic foramen inferior to the piriformis muscle (Fig. 3). It is found in the plane between the superficial and deep group of gluteal region muscles, crossing the posterior surfaces of first the obturator internus and associated gemellus muscles and then the quadratus femoris muscle. It lies just deep to the gluteus maximus at the midpoint between the ischial tuberosity and the greater trochanter.
In addition to preoperative markings for identification of areas of liposuction and lipofilling, it is important to identify the danger triangles on each buttock. The apex of the triangles is the posterior superior iliac spines. The inferior lateral point is located at the greater trochanter, and the inferior medial point is at the ischial tuberosity (Fig. 4). The area with in these triangles identifies the regions of the major gluteal vessels and the sciatic nerve. Of note, one of our senior authors safely performs this procedure with a slightly smaller area of safety that is directly over the course of superior gluteal vessels and the sciatic nerve.34 However, to optimize the safety of gluteal fat transplantation, we recommend using this larger safety triangle, where the fat injections must be placed superficial to the muscle.
Donor-site selections should coincide with sites that best require liposuction because of excess adipose tissue. It has been previously demonstrated that there is no significant difference in adipose cell viability when the donor sites include the abdomen, flank, thighs, or knees, especially when performing larger volume (>100 cc) fat augmentation.35 , 36 In addition, studies have also demonstrated that donor site did not have any effect on the volume of fat graft take or duration of graft survival.37 , 38 Standard wetting solution liposuction techniques using larger bore cannulas and low-pressure suction minimize blood loss and improve adipocyte viability.39 Various fat processing techniques have been described; however, there is no significant peer-reviewed evidence to show that one method is superior to others.36 We recommend using a closed system in which the lipoaspirate is not exposed to the environment to minimize possible open-air contamination. Simultaneous separation and tumescence of fat, first described by Khouri and Del Vecchio, is the recommended technique of preparing the donor site before liposuction.34 , 40 Fat is harvested manually by means of multihole cannulas (Del Vecchio Cannula; Wells Johnson, Inc., Tucson, Ariz.), or by power-assisted means (Microaire, Inc., Charlottesville, Va.; Möller Medical, Fulda, Germany). Fat is collected in-line using large sterile canisters (Medela, Inc., McHenry, Ill.). Once the fat separates from the blood and crystalloid, the unwanted blood and crystalloid is decanted off and the donor fat is then transferred through a peristaltic pump system (Byron EZ Pump; Mentor Worldwide, Santa Barbara, Calif.) to injection cannulas, either manual or power-assisted. We do not routinely centrifuge the fat when used in these large-volume cases, nor do we add antibiotics to the donor fat. Although reports have described the addition of antibiotic solution to the lipoaspirate to decrease the risk of infection, no significant data support this currently.2 , 17 , 21
Following positioning for lipoaspiration, the patient is placed in a prone position with hips flexed in a “jackknife” position (Fig. 5). While placing patients in this position, attention must be directed at also bending the knees to avoid lower extremity venous stasis, which increases the risk of deep venous thrombosis. This allows for greater control and safety with use of the injection cannulas. Use of this position also allows for directing of the injection cannula into more of a subcutaneous and superficial muscular plane, if so desired by the surgeon. In addition, combining this positioning with the use of an angled cannula improves the surgeon’s ability to remain in superficial planes.
There are four key principles to remaining safe during the gluteal fat grafting portion of the procedure, as follows: using a larger bore blunt cannula (≥4 mm), using a continual motion, remaining primarily in the subcutaneous (our preference) or superficial muscle throughout and only in the subcutaneous plane in the triangle of danger, and avoiding filling to excessively high recipient-site pressures (Table 2).
The use of a large-bore (4- to 5-mm) blunt cannula can minimize the risk of venous injury and can potentially make penetration into the deeper musculature more difficult. It is our preference to use an exploded-tip basket cannula, which further increases the size of the tip of the cannula. The large-bore cannulas are more rigid, providing better control and precise anatomical placement compared with more flexible smaller gauge cannulas.41 Flexibility misguidance is a phenomenon that may be seen when using thin-gauge injection cannulas and syringe for fat grafting.34 When the thin cannula is advanced through the tissues, there is flexing of the cannula-syringe interface, which leads to deeper penetration despite the, misleadingly, straight appearance of the cannula, indicating to the surgeon that they are superficial when they are not. All that is required is one misguided stroke to injure a deep vein and potentially cause a fat embolism. Furthermore, large-bore cannulas have been demonstrated to improve adipocyte viability.39
Continual motion of the cannula while injecting the fat graft is critical during this procedure. First, continual motion of the cannula can prevent direct and continuous injection into a vessel. Also, with continual motion, the graft is dispersed throughout the tissue planes, allowing for greater likelihood of adequate lobular diffusion and improved graft take. The continual motion of using a power-assisted liposuction device in an off-label manner to insert grafted fat may further improve dispersion and evenness of graft placement. Making the theoretical assumption that if the cannula is in constant motion, it can never be in one location long enough to cause injury or bolus into an injured vein, one may further consider a power-assisted liposuction–derived vibrational injection of fat, expansion vibration lipofilling, to be a potentially safer mechanism of graft insertion, compared with manual axial grafting.34 When using syringe injection, there are three neurologic functions that are imperative to proper fat grafting. First, there is the axial to-and-fro motion that is required to create the channels. This is served by the large muscle groups of the elbow and shoulder region. Second, to initiate fat flow with the syringe, the thenar muscles and the hand intrinsics are necessary to propel fat with a syringe. Third, proprioception is mandatory to know where the tip of the cannula is at all times. Fatigue-induced decrease in proprioception is a well-documented phenomenon that may lead to decreased understanding of cannula position with traditional injection techniques.42–44 With expansion vibration lipofilling, fat flow is initiated with an infusion pump and there is less active axial motion as the cannula drives through the tissue, similar to a self-propelled lawnmower. Lastly, the only neurologic function that is mandatory with expansion vibration lipofilling is the proprioception. The theoretical reduction in fatigue and the increase in proprioception with expansion vibration lipofilling may eventually prove it to be a safer technique for avoiding deep structures and flexibility misguidance.
Of these key principles, we believe knowing the exact planar position of the cannula tip 100 percent of the time, with every stroke of the cannula, is the most important variable in staying safe. The most dangerous part of this procedure is inadvertent injection into the deep medial gluteal region. Remaining in a subcutaneous plane within the bounds of the triangle of danger should decrease the risk of inadvertent vascular injury or direct vessel injection (Fig. 6). In addition, it will help avoid injury to the sciatic nerve. Outside of the triangle of danger, the fat grafting should remain in more superficial planes, as large vessels lie just deep to the gluteus maximus. The nondominant hand should be used to continually assess the exact location of the tip of the injection cannula.
The total volume of fat grafting has not been directly correlated to deaths, and numerous authors have reported good outcomes with low- to high-volume gluteal fat transplantation. However, given the proposed mechanism of fat embolization, we believe that care should be taken to not overfill the recipient site with excessive graft pressure during this procedure.3 , 27–29 , 31 , 45 The optimal fill volumes for a given patient are impossible to delineate before surgery. This is because of the iterative nature of the procedure, dependent on achieving desired convexities, hip-to-waist ratios, and overall buttock shape, while assessing recipient-site tissue compliance and respecting individual tissue characteristics. Although it is difficult to objectively quantify the maximal fill volume, we equate the feel of tissues similar to that achieved during tumescent infiltration. In addition, when a decrease in resistance during lipofilling is detected, maximal fill has likely been reached. This occurs when the recipient site is overloaded with donor fat to the point where there is minimal intervening connective tissue. Also, when fat is seen continuously exiting the port incision during grafting, this is a clear sign of extremely high recipient-site pressure and should be avoided. The risk with excessive pressures in the setting of a possible venous injury can lead to pressure gradients that favor fat entrance into the lower pressure vein lumen. It is also important to note that this is a theoretical risk of excessive pressure, but we believe the inciting event is deep venous injury. Stated differently, there can be high subcutaneous pressure with the insertion of fat, but there can be no gradient unless there is large-caliber venous injury. Overfilling to excessive recipient-site pressures theoretically increases the risk of fat embolism in the setting of venous injury. Overfilling and increased pressure also reduce oxygen diffusion capabilities, which decreases the amount of graft survival.
Patients should be placed in compressive garments during the postoperative period to provide compression on the liposuction donor sites.2 Standard postoperative deep venous thrombosis prophylaxis should be given, depending on the patient’s Caprini score, and early ambulation is recommended.46 Although there is no consensus in the literature, keeping pressure off of the grafted site is recommended for a minimum of 2 weeks postoperatively.17 , 20 Patients can return to normal activities within 1 week of the procedure.
Buttock fat transplantation is increasing in demand and should be performed by highly trained plastic surgeons because of the mortality rate associated with the procedure. Proper patient selection, favorable instrumentation, patient positioning, proper technique (i.e., continual motion and knowledge of cannula location at all times), and knowledge of anatomy (i.e., superficial planes of injection and avoidance of excessively high recipient-site pressure) are critical to improving the safety of this technique. Proper management of the above principles should allow a reduction in mortality from this procedure, which would safely allow its continued offering by properly trained and educated plastic surgeons, in the setting of increasingly high demand.
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