Fat, Stem Cells, and Platelet-Rich Plasma

The ideal filler for aesthetic surgery is inexpensive and easy to obtain, natural in appearance and texture, immunologically compatible, and long lasting without risk of infection. By most metrics, autologous fat grafts meet these criteria perfectly. Although facial fat grafting is now a commonly accepted surgical procedure, there has been a wave of activity applying stem cells and platelet-rich plasma (PRP) therapies to aesthetic practice. This article addresses technical considerations in the use of autologous fat transfer for facial rejuvenation, and also explores the current evidence for these stem cell and PRP therapies in aesthetic practice.

Key points

  • Advances in technique led to the development of Coleman fat grafting, which has been well studied and standardized, and is a reliable technique for minimally invasive facial rejuvenation.

  • However, over the past 10 years, advances in regenerative medicine have opened the door to new strategies designed to enhance graft retention and regenerative potency.

  • Adipose-derived stem cells (ASCs) and platelet-rich plasma (PRP) are among the most promising current options, but research in the field is still in its infancy.

  • Even with the promise that stem cell–based therapies might hold for the future of the field, the benefits and risks have not yet been adequately studied for widespread adoption beyond clinical trials.

  • More importantly, physicians, researchers, and national societies/organizations need to work to inform the public about which practices are evidence based and to encourage support of additional research.

Background

The ideal filler for aesthetic surgery is inexpensive and easy to obtain, natural in appearance and texture, immunologically compatible, and long lasting without risk of infection. By most metrics, autologous fat grafts meet these criteria perfectly. Since the first reported case more than 120 years ago, fat grafting has had a waxing and waning role in plastic surgery practice, with resorption being a primary clinical problem. Advances in harvesting and injection technique have made facial aesthetic applications more reliable and very effective. Accordingly, facial fat grafting, alone or in combination with open procedures, is widely practiced. Although facial fat grafting is a commonly accepted surgical procedure, there has been a wave of activity in stem cell and platelet-rich plasma (PRP) therapies in aesthetic practice. This article addresses technical considerations in the use of autologous fat transfer for facial rejuvenation, and also explores the current evidence for stem cell and PRP therapies in aesthetic practice.

Background

The ideal filler for aesthetic surgery is inexpensive and easy to obtain, natural in appearance and texture, immunologically compatible, and long lasting without risk of infection. By most metrics, autologous fat grafts meet these criteria perfectly. Since the first reported case more than 120 years ago, fat grafting has had a waxing and waning role in plastic surgery practice, with resorption being a primary clinical problem. Advances in harvesting and injection technique have made facial aesthetic applications more reliable and very effective. Accordingly, facial fat grafting, alone or in combination with open procedures, is widely practiced. Although facial fat grafting is a commonly accepted surgical procedure, there has been a wave of activity in stem cell and platelet-rich plasma (PRP) therapies in aesthetic practice. This article addresses technical considerations in the use of autologous fat transfer for facial rejuvenation, and also explores the current evidence for stem cell and PRP therapies in aesthetic practice.

History of fat grafting

Neuber was the first to describe autologous fat transfer in 1893 when he transferred fat from the arm to a depressed facial scar. Even then, he reported that the primary limitation of the technique was the inability to transplant anything “[larger than an almond]” because of resorption. Two years later, Czerny published the first description of fat transfer to the breast, using a large lipoma from the lower back to fill a lumpectomy defect. In 1909, Hollander described a technique involving fat injection through a cannula to correct contour deformities. The problem of fat reabsorption was well known, so he combined his patients’ fat with a special preparation of male ram fat in attempt to thicken and preserve the graft. Although patients experienced a painful rash when they rejected the xenograft, he reported generally favorable long-term aesthetic results.

The first reports of applying fat transfer specifically to aesthetic surgery also came in 1909 when Lexer described the use of fat to solve several facial aesthetic challenges. Numerous surgeons followed suit, reporting the use of fat for a wide variety of facial defects. In 1926, Miller published a detailed review of his development and use of cannulas for fat injection, but the technique still failed to achieve widespread adoption. The following decades saw a decline in fat grafting, highlighted by Peer’s historic observation that half of fat graft volume was lost in the first year with mostly fibrous tissue remaining. As a result of his experiments, Peer concluded that revascularization was necessary for fat graft survival and that a larger number of small aliquots fared better than a large single graft because of enhanced surface area. Even with these groundbreaking observations, the first major efforts to apply his findings did not come until the 1980s.

Challenged by the poor results of contemporary techniques, Ellenbogen developed a method of mincing fat into 4- to 6-mm “pearls” before placing them directly into the recipient bed. Using this method, he was able to achieve success with acne pitting, facial wrinkles, scars, and chin augmentation. Even though Ellenbogen initially reported good results using his technique, he later retracted his previous claims and concluded that the fat did not have long-term survival with his mincing technique. Around the same time, Illouz and Fournier described using unprocessed liposuction for fat transplantation. However, resorption was even worse than with traditional fat transfer, creating a necessity for substantial overcorrection and limiting adoption. This led Chajchir and Benzaquen to develop several handling strategies, including removing oil and blood before reinjection. However, it was not until Coleman developed a meticulous, standardized means of processing and injecting lipoaspirate, termed lipostructure, that fat grafting became widely used in aesthetic surgery.

Adipose stem cells

Adipose Stem Cells History and Biology

In 2001, Zuk and colleagues described a population of multipotent mesenchymal stem cells present in lipoaspirate that were similar to bone marrow mesenchymal stem cells. These adipose-derived stem cells (ASCs) have since become one of the most widely used stem cell populations in regenerative medicine because of ease of harvest and diverse differentiation capacity. ASCs have the capacity to differentiate to mesoderm derivatives, including fat, cartilage, muscle, and bone. Furthermore, they can be induced to form endodermal and ectodermal tissue types, including skin epithelium, neural cells, hepatocytes, and pancreatic islets. In addition to their differentiation capacity, ASCs are also involved with paracrine signaling for angiogenesis, immunomodulation, and tissue regeneration.

Isolating Adipose Stem Cells

ASCs reside in the perivascular compartments of adipose stroma. As a result, stromal dissociation is necessary to liberate enough cells for practical use. This is most often achieved by enzymatic digestion with collagenase followed by lysis of red blood cells. The resulting stromal vascular faction (SVF) contains a diverse progenitor cell population, which includes ASCs, pericytes, endothelial progenitor cells, and transit amplifying cells. To isolate ASCs, the SVF is plated on tissue culture plastic where ASCs adhere preferentially. In addition to collagenase digestion, several nonenzymatic approaches, including washing and mechanical separation, are also described for SVF isolation but are less widely used. Numerous point-of-care devices have been developed to harvest the SVF, but they generally use similar enzymatic or mechanical digestion strategies.

Clinical Use of Adipose Stem Cells: Cell-Assisted Lipotransfer

ASC or SVF-enriched fat grafts were first reported by Moseley and colleagues in 2006 based on unpublished work by Llull in 2005. The technique was more formally described by Matsumoto and colleagues shortly thereafter and termed “cell-assisted lipotransfer” (CAL). Numerous preclinical animal studies have since demonstrated improvements in fat graft volume with CAL, leading to the initiation of human trials.

To date, two randomized controlled trials have been conducted in humans. Koh and colleagues assessed ASC-enriched microfat grafts in 10 patients with Parry-Romberg hemifacial atrophy and found that volume retention improved substantially in the group receiving CAL (47% resorption in control subjects vs 21% in CAL). Similarly, in a triple-blind RCT of 10 patients, Kolle and colleagues found improved retention in the arm receiving ASC-enriched fat grafts compared with standard fat grafting into the contralateral control arm.

Five human cohort studies have also been published examining SVF-enriched fat grafts in the face and breast. Four of five showed improvement with CAL. One study comparing SVF-enriched fat with fat obtained by water-jet-assisted liposuction in 20 women undergoing breast augmentation found no difference between groups. However, the use of water-jet-assisted liposuction in the control group limits the generalizability of these findings.

Taken together, these studies make a strong case that CAL enhances fat graft retention. However, longer term studies with safety data are necessary before general clinical adoption is recommended, particularly in patients undergoing oncologic reconstruction. Elevated risk of malignant transformation or recurrence has not been evident in existing clinical studies of fat grafting, but long-term studies remain limited, particularly with regard to cell-enriched fat grafts in humans.

Platelet-Rich Plasma

In 1998, Marx and colleagues provided one of the first descriptions of the regenerative use of PRP as a strategy for enhancing bone grafts in oral surgery. PRP is obtained by centrifugation of whole blood to obtain a five to seven times concentration of autologous platelets suspended in a small volume of plasma. Before injection, platelets are activated using calcium chloride and bovine thrombin to trigger degranulation of α granules. Activated platelets then release a variety of growth factors meant to initiate wound healing, including platelet-derived growth factor, transforming growth factor-β1 and 2, vascular endothelial growth factor, thrombospondin, and epidermal growth factor. The equipment required for isolation of PRP is already available in most hospitals via hemotherapy, clinical pathology, or the blood bank facilities. Additionally, there are several point-of-care devices designed to make the isolation process more convenient. However, many of these devices are unable to meet the minimum standard for PRP of 10 platelets/μL.

In vitro studies suggest that PRP enhances ASC proliferative and secretory functions, whereas in vivo data have shown improved fat graft retention. PRP and ASCs act synergistically when combined in rodents, with better retention combined than when either are used independently.

The addition of PRP to human fat grafts has also been promising, with multiple reports of improved volume retention. However, findings remain far from consensus, as evidenced by a recent trial by Rigotti and colleagues, which reported no benefit when adding PRP to ASC-enriched fat grafts for facial rejuvenation. In fact, PRP resulted in increased inflammation and undesirable vascular changes.

In addition to its use as a fat graft supplement, PRP has also been used independently for a variety of facial aesthetic procedures, from reducing edema and ecchymosis post facelift to treating facial rhytides. PRP may also have applications for hair regrowth. A double-blind placebo-controlled randomized controlled trial found that PRP was able to significantly improve hair regrowth in alopecia areata better than either placebo or steroid cream. Similarly, several studies have seen enhanced hair growth in androgenic alopecia.

Unfortunately, determining efficacy for PRP treatment is not without challenges. First and foremost, the literature remains difficult to interpret due to lack of standardized dosing or isolation technique. Additionally, most human trials have been case series and have lacked objective outcomes or adequate controls. Large randomized controlled trials with quantifiable outcome measures are necessary to define the appropriate role for PRP in the plastic surgeon’s armamentarium.

Emerging strategies

Although less relevant for facial fat grafting, Khouri and DelVeccio have popularized the concept of preconditioning the recipient bed, presenting promising results using the BRAVA suction device (BRAVA, LLC, Miami, FL, USA) to enhance fat graft take in the breast. Additionally, several options are available to assist with larger volume harvests, including power-assisted, water-assisted, ultrasound-assisted, and laser-assisted liposuction.

Compared with harvesting by hand using a 10-mL syringe, suction-assisted machine harvest does not seem to influence graft viability. Similarly, no differences have been seen in animal studies of ultrasound-assisted harvest. No significant human or animal trials exist for mechanical or laser-assisted liposuction, but in vitro studies have revealed a lower stem cell yield and poorer ASC proliferation. Preliminary in vitro and rodent studies with water-jet-assisted liposuction suggest that it improves ASC content, results in more viable adipocytes, and increases volume retention of grafts. Harvest cannula size has also been investigated to reduce shear stress and improve graft viability. Larger, 5- to 6-mm harvesting cannulas seem to improve adipocyte viability in cell culture and in animal models. Unfortunately, the lack of head-to-head human clinical trials limits the usefulness of these data in current clinical practice.

Similar to harvest, there are numerous techniques used to prepare aspirated fat for grafting, including washing, decanting, centrifugation, filtration, and rolling on Telfa gauze. Of these techniques, only Telfa rolling has any evidence of improved retention over centrifugation, based on studies conducted in the nude mouse xenograft model. However, the lack of human clinical trials also limits the generalizability of these findings.

Harvesting and processing techniques often focus on ensuring adipocyte survival. Yet, mounting evidence suggests that ASCs are more important for fat graft success, and in some cases, the rejuvenating effect of the fat graft on overlying skin may be more important than pure volume replacement. This is evidenced by the emergence of microfat grafting and sharp-needle intradermal fat grafting, which aim to improve rhytides and scars through small-volume fat injections.

Given the advantage small injection cannulas provide with regard to precision, Tonnard and colleagues described a technique to further reduce particle size, allowing injection of fat through a 27-gauge needle. Termed “nanofat,” this filler provides limited volume to the patient but can be placed intradermally with a high degree of precision, permitting the treatment of rhytides. When the composition of the graft was examined, it was noted that there were virtually no surviving adipocytes. The cellular composition was more similar to SVF isolates (with the addition of fragmented adipocytes), and rather than seeing an immediate postoperative change, improvements with nanofat were generally delayed by 4 to 12 weeks.

Many advertised stem cell therapies have nonvalidated claims

As with other frontier fields, the marketing of unvalidated claims remains a substantial problem in aesthetic surgery. Direct-to-consumer advertising can lead to therapeutic misconceptions and unrealistic expectations. A salient example is the deceptive marketing of so-called stem cell facelifts and stem cell breast augmentations. Many of these procedures are advertised by “noncore” practitioners, practitioners without the formal aesthetic training necessary to ensure safe patient care. Moreover, many of these groups use isolation devices and techniques that have not been approved by the Food and Drug Administration. Others perform standard fat grafting, but add “stem cell” to the name of the procedure for marketing purposes. In addition to financial and safety concerns, these spurious marketing claims can serve to undermine the credibility of legitimate stem cell clinical trials with the public.

In 2011, a joint task force of the American Society for Aesthetic Plastic Surgery and the American Society of Plastic Surgeons convened to address deceptive advertising practices. After screening more than 9000 articles, they concluded that the marketing and promotion of stem cell procedures in aesthetic surgery was not adequately supported by the existing evidence. Until more evidence is available, stem cell therapies should be conducted only within clinical studies subject to institutional review board approval and in compliance with Food and Drug Administration regulations.

Similarly, according to a survey of members of the American Society for Aesthetic Plastic Surgery and American Society of Plastic Surgeons published in 2014 by Nayar and colleagues, most plastic surgeons indicated that application of stem cells into clinical practice is not yet adequately justified by current knowledge. In addition, most reported that the risks and benefits of many stem cell–based therapies had not been adequately studied to be able to provide informed consent to patients.

Patient evaluation, indications, and contraindications for aesthetic fat grafting

The Aging Face

The applications of fat grafting are diverse and improvements are well documented. In the context of aesthetics, patients most commonly seek fat grafting to improve aging-related changes to the face, including reduced fullness, skin thinning, loss of elasticity, and sun damage. Because of fat’s ability to restore volume and improve skin quality, it is ideal for facial rejuvenation. In many cases, fat grafting alone can achieve dramatic effects, but it becomes a particularly powerful technique when used in conjunction with excision of excess skin. Skin quality continues to improve over long-term follow-up.

Aesthetic Recontouring in Younger Patients

Younger patients may seek fat grafting because of dissatisfaction with facial proportions. Fat grafting has the capacity to achieve aesthetic balance while maintaining a natural appearance, particularly with such features as the jaw line, cheeks, or chin. The minimally invasive nature of fat grafting is ideal compared with alternatives, such as bone grafts, temporary injectables, or solid implants.

Reconstruction of Congenital or Trauma-Related Defects

Fat grafting has also become an important tool for reconstruction of congenital and traumatic facial deformities and asymmetries. In congenital disorders, such as hemifacial microsomia or Treacher Collins syndrome, fat grafting helps to improve facial proportions and restore volumetric symmetry. Larger volumes and touch-up procedures may be necessary to deal with more complex baseline bone and soft tissue irregularities, but fat grafting can achieve excellent long-term results. Because fat is supple and can be placed differentially throughout recipient bed, it can restore complex contour irregularities better than other strategies. When applied to sites of prior significant trauma, fat grafts also improve scar appearance and can fill fairly substantial defects. Fat grafting also provides an excellent option for patients seeking revision of iatrogenic deformities.

Drug-Related Lipodystrophy

The success of antiretroviral therapies and protease inhibitors for the treatment of human immunodeficiency virus has led to an increase in drug-related facial lipodystrophies, marked by buccal and temporal hollowing. Fat grafting can provide an excellent long-term solution for these patients as well.

Improvement in Scar Appearance and Skin Quality

The regenerative properties of fat, likely caused by the presence of ASCs, make fat grafts useful for more than just volume replacement. This is exemplified by the work of Rigotti and colleagues with radiation injury. Fat grafting essentially cured radiation-related tissue damage in nonhealing radiated wounds. In addition, there have been numerous reports of improved scar quality after fat grafting. In addition to restoring volume, fat grafting rejuvenated the overlying skin, as evidenced by improvements in texture, contour, and color.

Contraindications

Fat grafting is generally well tolerated and has few contraindications. Small-volume fat grafting can be performed using only local anesthesia, whereas larger defects typically require general anesthesia. As such, patients who are too frail or high risk to undergo general anesthesia are not candidates for fat grafting.

Typically, even very thin patients have sufficient fat for most aesthetic applications, particularly in the face or hand. However, in some extreme cases, patients may have too little body fat to permit adequate harvest. These patients are encouraged to gain weight for the procedure, but they should be cautioned that postoperative weight loss results in loss of graft volume.

Finally, patients with excessive loose skin do not achieve satisfactory results with fat grafting alone. However, excellent results are achieved if skin excision is combined with fat grafting for residual volume deficits. For example, fat grafting can optimize youthful contour and skin quality after excess skin is removed in a traditional facelift.

Techniques

Overview

The Coleman method of lipostructure involves gentle harvest of fat to preserve delicate architecture, centrifugation to remove nonviable components, and injection of fat into small aliquots to maximize access to blood supply. Adherence to these principles limits resorption and facilitates predictable results. Scientific investigation of this technique has shown that shown that the Coleman method yields fat with high survival and normal cellular enzymatic profile.

Donor Site Selection and Preparation

The ideal donor site is patient-dependent and based on patient preference and availability of fat at the donor site. Although fat from different donor depots may be slightly different, no conclusive differences have been demonstrated with regard to graft take. The posterior hip, back, love handles, and lateral thighs do have the advantage of being more forgiving, whereas the abdomen and medial thighs are more prone to wrinkling and deformity. Small incisions are hidden in scars, stretch marks, creases, or hair-bearing areas whenever possible. Using these incisions, a blunt cannula is used to infiltrate tumescent fluid. For smaller cases, performed under local anesthesia only, this fluid consists of 0.5% lidocaine with 1:200,000 epinephrine. For larger cases using general anesthesia, 0.2% lidocaine with 1:400,000 epinephrine is used instead. Ideally, the volume infiltrated should equal the volume of fat the surgeon intends to remove.

Fat Harvest

Fat harvest is carried out using a two-hole Coleman harvesting cannula and 10-mL syringes. Syringes are drawn up slowly during suctioning (only a few millimeters) to avoid rupturing adipocytes with excess negative pressure. Harvesting cannula size is also optimized to allow collection of intact fat parcels while ensuring that fat is small enough to pass through a 17-gauge injection cannula.

Processing

After 9 to 10 mL has been aspirated, the syringe is disconnected from the cannula and replaced with another 10-mL syringe until adequate volume has been obtained. Harvest incisions are then closed using interrupted nylon suture. Syringes are fitted with Luer-lok caps, and sterile centrifuge sleeves are used to prevent contamination of syringe tubes during spinning. Samples are then centrifuged at 1286 × g for 3 minutes to facilitate the separation of nonviable components. Early syringes obtained by this method are likely to contain more of the aqueous component, consisting primarily of tumescent solution. As liposuction continues in a given area, increasingly more blood is captured by the syringe. Fortunately centrifugation allows the removal of these undesirable components.

After centrifugation, excess oil is decanted and wicked away using neuropaddies or Telfa pads. The Luer-Lok plug is removed to drain the aqueous fraction from the syringe. Fat is then aliquoted to 1-mL or 3-mL syringes depending on the size of the recipient site. Fat grafting to the face or hands should be done using 1-mL syringes; 3-mL syringes can be used for convenience in larger volume sites, such as the breast.

Placement

As with harvesting sites, incision sites for placement should be hidden whenever possible. Planned sites are anesthetized with 0.5% lidocaine with 1:200,000 epinephrine, and a small stab incision is made just large enough to fit injection cannulas through. In the face, a small amount of the 0.5% lidocaine/1:200,000 epinephrine solution can also be infiltrated into the recipient bed to induce vasoconstriction before fat placement. In addition to reducing postoperative bleeding, this technique reduces the likelihood of accidental fat embolization from intravascular injection.

When placing fat it is imperative to maximize surface area contact with surrounding tissue to ensure close proximity of grafted fat with recipient blood supply. Larger globules of fat undergo central necrosis, volume loss, and may result in oil cysts. Graft to recipient volume ratio is also an important consideration because grafts tend to coalesce at higher ratios despite best efforts at dispersion.

Fat should be placed gently, in small parcels while withdrawing the blunt injection cannula. Grafts are placed at a variety of depths to promote greater dispersion, and specific locations are targeted when certain outcomes are desired. For example, injecting fat just below the dermis is more likely to enhance skin quality, decrease pore size, reduce wrinkles, and improve scarring. However, care must be taken with these techniques because superficial injections are the most likely to cause visible contour irregularity, particularly in such areas as the eyelids where the skin is very thin. Alternatively, fat is targeted to just above the periosteum to simulate the appearance of bony structure. However, as with other locations, deposits must be built up slowly with fat distributed into tiny parcels.

Molding to displace previously injected fat should be avoided except in instances of noticeable irregularity after injection. If applied to larger aliquots or more densely placed fat, molding promotes fat pooling, resulting in necrosis and long-term contour irregularities. When grafting is complete, incisions are closed with single interrupted nylon sutures.

Postoperative Care

Postoperative care depends on the body regions involved. The core principle is to pad recipient sites and provide compression to donor sites. Tegaderm is effective for most areas of the face, but care must be taken with eyelid dressings to ensure that the lower eyelids are not pulled down. Areas that received liposuction are dressed with 0.5-inch Reston foam (3M, St. Paul, MN, USA), followed by Tegaderm or Microfoam tape (3M, St. Paul, MN, USA) to compress the foam. Cool therapy also used for the first 72 hours to limit inflammation, but care should be taken to avoid direct icing or overcooling as low temperatures may precipitate apoptosis of the newly transplanted fat. Recipient sites in the hand are dressed with Microfoam tape, whereas donor sites on the legs or torso are dressed with compression garments or an abdominal binder.

Dressings remain in place until the patient returns to the office on postoperative Day 3 or 4. At this time, dressings and any sutures in the face are removed. Sutures at donor sites or on the hands remain in place until Day 5 to 7. Deep massage of recipient sites should be avoided during the first month, but very light touch to promote lymphatic drainage is advantageous.

Complications and Follow-up

Common complications are generally mild and self-resolving. Contour irregularity is the primary aesthetic concern, but this can usually be avoided by vigilance during harvest and placement with careful adherence to proper technique. Bruising, pigmentation, and small bumps may be present in donor or recipient sites but typically resolve within 2 to 3 weeks. Rarely, pigmentation lasting for several months has occurred in thin-skinned areas, such as the eyelids. Infection is generally rare, but it can severely compromise volume retention and aesthetic outcome when it occurs. Consequently, strict sterile technique is critical when processing and reinjecting fat graft. Furthermore, grafting near nonsterile areas, such as the lips or nose, should be conducted only after grafting to sterile areas is complete.

As with other facial soft tissue fillers, embolization remains a rare yet devastating complication, although this is most often associated with the use of sharp cannulas. The preconditions for fat embolism have been well established in the orthopedic literature and include: well vascularized tissue, fragmentation of the parenchyma, and a local increase in tissue pressure. The overwhelming majority of reported cases of embolism following facial fat grafting have occurred during injection into glabellar lines, the dorsal nose, and the nasolabial fold where cannulation of the supratrochlear, dorsal nasal, or angular artery results in retrograde transfer of fat into the ophthalmic or middle cerebral arteries. As such, caution must be taken when injecting in these areas. Blunt cannulas that are introduced slowly and gently, should not be able to cannulate these arteries or their tributaries. However, in areas that have acute tissue injury or undermining, high local tissue pressure from overfilling can force fat graft into damaged arterioles and result in embolization. Fortunately, elevations in tissue pressure and opportunities for embolization are minimized when small volumes of fat are injected with each pass.

Even with careful technique, revision procedures may be necessary to address resorption or overcorrection. When patients gain or lose a substantial amount of weight postoperatively, the graft changes size accordingly, potentially necessitating debulking or additional grafting. In regions with thin or delicate skin, it is difficult to remove excess fat once placed. Therefore, it is better to err on the side of undercorrection and plan to stage filling over multiple procedures.

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Nov 21, 2017 | Posted by in Dental Materials | Comments Off on Fat, Stem Cells, and Platelet-Rich Plasma
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