Fat grafting is a well-established method for facial contouring, including cosmetic temporal augmentation. Various approaches have been documented in the literature, focusing on precise fat placement to enhance outcomes. Traditional lipoinjection, a widely accepted technique, involves harvesting fat from donor sites like the abdomen or thighs, processing it, and injecting it into the desired areas. This method allows for effective correction of volume loss in the temporal region and has been extensively used due to its reliability and adaptability.

For instance, Hu et al. utilized traditional lipoinjection for temporal augmentation, where fat was harvested from donor sites, decanted, and injected into the temporal area to correct volume loss (2). Similarly, Chiu et al. described a technique of autologous fat grafting to the temple and forehead, where fat is harvested, processed through centrifugation, and then carefully injected into the target areas (14). Both studies emphasize the effectiveness of traditional fat grafting methods in restoring facial fullness, particularly in the temporal region, highlighting their safety and efficacy in clinical practice.

Some studies have explored more specialized fat grafting methods. For instance, microautologous Fat Transfer-Gun (MAFT-Gun) Using microautologous fat transfer (MAFT). In two articles, the use of MAFT for cosmetic temporal augmentation is described (12, 15). Additionally, Lee et al. used a special device to apply the microautologous fat graft in the temporal fossa, called “MAFT Gun.” The MAFT-Gun can inject specific aliquots of fat precisely into each layer (12). The “3M3l” Technique Huang et al. described the targeted fat graft technique by following the principle of “3M3l”, i.e., Multitunnel, Multilayer, Multipoint, Low-pressure suction, Low Speed, and Low volume (13).

Further variations of fat grafting included the use of Stromal Vascular Fraction (SVF)- or Platelet-Rich Plasma (PRP)-assisted cell therapy. Stromal Vascular Fraction (SVF) and Platelet Rich Plasma (PRP) Assisted Cell Therapy Further variations of pure fat grafting are SVF- or PRP-assisted cell therapy for temporal augmentation (14, 18). In the included articles of this systematic review, Li et al. used SVF-assisted fat transfer for cosmetic temporal augmentation (18). After the isolation of SVF from the adipocytes, it is injected into the temporal area combined with the fat graft (18). Chiu et al. combined autologous fat grafting with PRP to restore temporal volume loss (14).

The average amount of injected pure fat volume per temple was 9.56 ml (range 6–16.2) on the right side and 9.5 ml (range 5.9–16.2) on the left side (Table 1). The amount of injected volume combined with SVF was 17.5 ml per temple. In only three studies, the evaluation of fat graft retention was part of the follow-up. Huang et al. detected a volume retention rate of 65.7% (13). Hu et al. noted a fat graft survival rate of 33% on the right and 32% on the left side (2). Li et al. described a higher fat graft survival rate by using SVF-assisted fat transfer (64.8% per temple) compared to using a pure fat graft only (46.4% per temple) (18). Most surgeons in the reviewed articles decided the amount of injected volume by their experience and according to the patient's wish (2, 15). Some surgeons subsequently added 30% more fat graft since fat-atrophy after the procedure was already assumed (2, 15, 18).

The main disadvantage is the difficult prediction of resorption and survival rates. Fat graft retention was inconsistently reported and assessed. Huang et al. reported a retention rate of 65.7%, while Hu et al. observed a survival rate of 33% on the right and 32% on the left side (13). Li et al. found that SVF-assisted fat transfer led to a higher survival rate (64.8%) compared to pure fat grafting alone (46.4%) (18). One plausible reason for the low survival rate of fat grafts is the lack of vascularization which results in necrosis and ultimately in a loss of the graft (12). Therefore, different methods have been established to improve fat graft retention. Generally, clinical data on volume retention after temporal augmentation by AFG is limited. A large number of studies depicted that the temporal area is one of the facial subunits with the highest fat resorption rate and the lowest patient satisfaction rate (8). An explanation for the low-fat graft survival rate might be the continuous movement of the temporalis muscle and the relatively avascular nature of the upper and lower temporal compartments (12). Unfortunately, the methods of measuring fat survival rate in the temporal area are seldomly standardized. Indeed, all included articles used pre-and postoperative photographic documentation for objective assessment but only two articles used further objective assessment like a three-dimensional (3D) laser scanning system or computed tomography (CT) to evaluate fat graft survival rate in the temporal area (1, 18). One article used molded plasticine for objective assessment (2). We acknowledge the great importance of photos as an essential and useful tool for a pre-and postoperative documentary but especially for cosmetic purposes, photographs can be misleading due to for example variability of film color, patient's/photographer's position, flash intensity, and facial expression. Therefore, it is of great importance to define objective assessment criteria, which allow reproducible and comparable results. The fact that different and not standardized measurement scales have been used makes the result of the high patient satisfaction not representative. Most articles in our study evaluated fat grafting success in the temporal area just by patient's and/or surgeon's and/or layperson's subjective assessment using various scales like the Likert scale, Hollowness Severity Rating Scale (HSRS), or Visual Analogue scale (VAS) and hereby try to equate postoperative satisfaction results with fat survival rate (1, 12, 14, 16). The effect of fat grafting has been evaluated by nonvalidated questionnaires for many years and objective assessments have relied on pre-and postoperative photographs (14, 16, 17). Even though a large number of objective assessment methods exist, which measure facial fat atrophy, none of them have been used to evaluate systematically fat volume loss in the temporal region. With regards to cosmetic temporal augmentation by AFG, only Li et al. used CT for evaluating fat survival rate in the temporal area (18). Another option for measuring the success of temporal augmentation by using AFG is a three-dimensional scanning system like used by Huang et al. (12). Three-dimensional surface imaging systems are suitable for quantifying fat graft survival rates (24, 25). Magnet resonance imaging (MRI) is also a possible method to measure facial volumetric changes (10). Del Vecchio et al. used MRI to measure volume retention after AFG of the breast and by that they could standardize injection techniques (26).

After systematically reviewing the included articles, we found out that there is also a great lack of investigating the long-term survival rates of fat grafts to correct temporal hollowness. Only one of our reviewed articles considered volume stability after temporal augmentation at a long-term follow-up of 8 years (17). The others (n = 7/8) evaluated volume retention only for a short period (6–12 months) and in one study, they followed the patients up for around 24 months.

Very promising options to improve survival rates of autologous fat grafts are from the fields of cell-assisted lipotransfer (27, 28). Our results show that two studies addressed this problem by adding platelet-rich plasma (PRP) or stromal vascular fraction (SVF, Stromal Vascular Fraction) to the transplanted fat graft (14, 18).

While PRP has been investigated as mentioned above, Platelet-Rich Fibrin (PRF) is considered a next-generation platelet concentrate. PRF differs from PRP by its fibrin matrix, which leads to a slower release of growth factors and promotes better tissue repair and angiogenesis. PRF holds potential to further enhance graft retention rates and could offer a more sustained improvement in fat graft survival, though its application in temporal augmentation has yet to be fully explored in the literature. Future studies comparing the efficacy of PRP vs. PRF in cosmetic procedures, particularly in temporal fat grafting, are warranted.

The Stromal Vascular Fraction presents a component of fat tissues which typically includes different cell types such as ASCs, endothelial cells, and other supporting cells with the idea to enhance and improve fat retention and survival (29). Adipose stem cells and activated platelets release several biochemical factors, for example, VEGF (vascular endothelial growth factor) that improve angiogenesis (30). A key limitation of this study is that while SVF-augmented fat grafting shows promise in enhancing fat graft survival, it is not currently approved for clinical use under FDA regulations due to its classification as a drug. This classification requires extensive and costly approval processes, significantly restricting its practical application in routine clinical settings.

In a controlled prospective trial Li et al. demonstrated the benefits of SVF to autologous fat grafts for restoration purposes of temporal hollowness (18). They examined two study groups; 6 out of 12 patients received temporal augmentation with fat graft only and 13 out of 26 individuals received an SVF-assisted fat graft transfer (18). The fat graft survival rate was postoperatively measured by computed tomography (CT) and/or by photographic documentation (18). Overall, fat graft survival was higher with SVF (64.8 ± 10.2%) than fat grafting alone (46.6% ± 9.3%) (18). Yao et al. also analyzed the use of SVF-assisted lipo-injection and the use of fat graft injection only for facial augmentation (27). Compared to the patient group that received lipo-injection only, the patient satisfaction in the SVF-group was significantly higher and the rate of a second surgery to achieve satisfactory results was significantly lower in the SVF-group (27). In the SVF-group 77.3% were satisfied (54.5%) or very satisfied (22.8%) with their results (27). On the contrary, in the patients’ group who underwent lipoinjection only 53.8% were satisfied (48.7%) or very satisfied (5.1%) (27). In contrast to that, the combined use of AFG and PRP seems to make no significant difference in long-term volume retention (31). Fontdevila et al. performed a randomized controlled trial, in which the use of AFG only and AFG with PRP in the therapy of facial human immunodeficiency virus (HIV) atrophy has been investigated (31). The fat volume increase was measured by CT (31). A difference in facial volume gain between both groups could not be confirmed, so the use of PRP was not associated with a better clinical outcome (31). Chiu et al. examined in a retrospective study n = 175 patients who underwent temporal and forehead augmentation by using AFG combined with PRP (14). They describe a high patient satisfaction rate of 90% (with 14% very much improved, 60% much improved, and 16% improved) (14). Unfortunately, it is not a controlled study, and the graft survival rate was not objectively measured (14). Therefore it is important, that cell-assisted lipo-transfer for temporal augmentation should gain further attention in the future and its clinical efficacy should be examined by objective measurement methods since current data mostly use subjective assessment criteria (e.g., Likert scale), which tend to have very high risks of patients’ and clinicians’ bias.

The anatomical complexity of the temporal region poses unique challenges for autologous fat grafting, which can result in suboptimal outcomes such as fat necrosis, irregularities, low retention rates or lead to complications. Huang et al.'s study on temporal fat compartments provides a detailed understanding of these structures, revealing four distinct compartments (Lateral Temporal-Cheek Fat Compartment, Lateral Orbital Fat Compartment, Upper temporal Compartment, Lower temporal compartment) (1). Their findings highlight the importance of precise fat placement using multi-plane injection techniques to ensure safe and effective outcomes in temporal augmentation. One the relevant findings ist the zone of caution (anterior half of the LTC) which includes the temporal branch of the facial nerve, the sentinel vein and middle temporal vessel perforators that can be injured (32).

The studies included in this review reported only a few minor complications like bruising and swelling which revolved spontaneously but overall, no major complications have been reported by using AFG for cosmetic temporal augmentation. However, prior studies in the field have reported on major complications such as blindness, cerebral fat embolism, or pulmonary embolism after accidental intravascular (venous/arterial) fat injection (3, 5, 33, 34). While the procedure may have a low rate of minor complications, it is not without significant risks that must be carefully considered and communicated to patients. In this context, the zone of caution, an area believed to be associated with the highest complication rate, is the anterior half of the lower temporal compartment that contains these crucial neurovascular structures (1). Injection in this region heightens the risk of severe complications. Technically, several precautions are vital to reduce these risks. Avoiding deep injections near the superficial temporal artery and using blunt cannulas, which reduce the likelihood of vascular injury, are essential steps. Slow, low-pressure injections with constant cannula motion help prevent fat from entering the bloodstream as high pressure can lead to retrograde arterial injection. One can aspirate to assure that the cannula is not intravascular. Additionally, understanding and recognizing the early signs of complications, such as visual disturbances, enables rapid intervention, potentially averting serious outcomes like blindness.

Other complications can be related to the technique of fat injection. For example, high pressure injection and over injection of fat volume in one location can limit diffusion of oxygen and nutrition in the early phase and impair neovascularization (35). This is because grafts rely on the recipient site blood supply and plasmatic imbibition in the early phases. Consequently, cells can necrose with reduced volume retention and possibly oil cyst formation which may lead to contour irregularities. In addition to optimizing harvest, processing and injection, one must consider the potential to prepare the recipient site for fat grafting.

To further improve graft take and consistency of temporal fat grafting several strategies could be systematically explored. Graft site preparation is intended to enhance fat graft take (e.g., via external volume expansion, alloplastic material implantation, addition of cell-proliferative factors, microneedling) (36). External volume expansion techniques of the recipient site have been explored which applies negative pressure over the recipient site hoping to stretch and expand it over time (37). One commonly discussed method for achieving this is through external expansion techniques, such as the use of external vacuum-assisted devices (e.g., the Brava system for breasts) before or after fat injection. These devices create a vacuum environment that stretches the recipient tissue, thereby increasing blood flow and tissue oxygenation. This can enhance the integration of the injected fat by facilitating early revascularization—a key determinant for the survival of fat grafts. Tissue expansion is thought to create a more favorable environment for the grafted fat by increasing the available space, improving local perfusion, and reducing mechanical compression that might otherwise impede graft survival. Other adjuncts such as hyperbaric oxygen (HBO) therapy after temporal fat grafting could be explored to enhance graft take. In preclinical studies, the positive effect of HBO on grafted adipose tissue was demonstrated (38). Further systematic studies on the impact of recipient site preparation and other adjuncts to improve graft take in temporal fat grafting could provide valuable insights.