Introduction

Over the last two decades, techniques of fat transplantation have significantly improved from simple free transfers of intact adipose tissue to free composite fat-cell transplantation techniques [1]. There are various indications of fat grafting, ranging from breast and craniofacial reconstruction to improvement of joint mobility after surgery for ankylosis [2].

The application of autologous fat grafts (AFG) for soft tissue augmentation has increased in popularity, creating an inexpensive and readily available product for lipo-grafting [3]. Krastev et al. (2018) conducted a systematic review that demonstrated the effectiveness of AFGs in treating soft-tissue abnormalities with minimal complications [4]. Conversely, other studies mention that there is a lack of reliable scientific evidence on the long-term viability of the transferred fat [3]. Traditional techniques of AFG for lipo-sculpting have recorded poor rates of volume retention ranging from 25 to 80%, which often requires additional procedures to optimise cosmetic outcomes [5,6,7,8].

Stem cells have been able to augment tissue regeneration within a host matrix by differentiating along specific cell lineages [9]. This principle has been utilised in lipofilling when reconstructing defects through the enrichment of fat grafts with stromal vascular fraction [10]. Stem cell enriched fat grafts (SCEFG) have been routinely adapted to enrich harvested fat with adipose-derived stromal cells which can increase graft take and volume retention by differentiation into mature adipocytes [11, 12]. Surgeons have been able to correct deformities in the breast, face as well as other anatomical regions utilising this method [13,14,15,16,17].

Although randomised controlled trials (RCTs) and non-randomised studies have shown promising results [17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32], no study in the literature currently analyses the outcomes of all comparative studies. This is the first systematic review and meta-analysis focusing on all comparative studies assessing SCEFG versus routine AFG for soft tissue reconstruction.

Methods

This systematic review and meta-analysis were conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) statement standards [33], but it was not registered at the International Prospective Register of Systematic Reviews.

Eligibility Criteria

All comparative studies including randomised as well as non-randomised controlled trials and observational studies comparing SCEFG versus routine AFG for reconstructive purposes were included. SCEFG was the intervention of interest and AFGs was the comparator. All patients were included irrespective of age, gender, co-morbidity status or anatomical region reconstructed as long as they belonged to either a study or control group. Case reports and cohort studies where no comparison was conducted were excluded from the review process.

Outcome Measures

The primary outcome measures included volume retention, fat necrosis, cancer recurrence, redness and swelling, infection, and cysts. Secondary outcomes included patient satisfaction post-surgery, scar assessment, operation time and number of fat grafting sessions.

Literature Search Strategy

Two authors independently searched the electronic databases of MEDLINE, EMBASE, CINAHL and the Cochrane Central Register of Controlled Trials (CENTRAL). The last search was conducted on 1st August 2022. The search strategy was adapted according to the thesaurus headings, search operators and limits in each of the above databases. The search terms for our intervention of interest consisted of “stem cell enriched”, “stromal vascular fraction enriched”, “ADSC”, “ASCs”, “ADRCs”, “progenitor enriched”, “call assisted lipotransfer”. Search terminologies used for the control group consisted of “fat graft”, “fat harvest”, “fat transplantation”, “autologous fat harvest”, “lipofilling”, “fat transfer”, “lipograft”, All terms were combined with adjuncts of “and” as well as “or”. To extend the screening for eligible articles, the bibliographic lists were also reviewed for the relevant studies. No search restriction was placed on language as a limiting factor.

Selection of Studies

The titles and abstracts of articles retrieved from the literature were independently assessed by two authors. Articles that met the eligibility criteria were selected after their full texts were reviewed. A consultation was obtained from an independent third author for any discrepancies in study selection.

Data Extraction and Management

A Microsoft Excel data extraction spreadsheet was amalgamated that abided with Cochrane’s data collection form for intervention reviews. A pilot test was conducted with the spreadsheet extracting data from random articles and adapting it as needed. Three authors independently extracted and recorded data.

Data Synthesis

The authors aimed to perform a meta-analysis for outcomes reported by at least three studies. The odds ratio (OR) was reserved for dichotomous variables whereas the mean difference (MD) was aimed to assess continuous variables for the study and control groups.

Review Manager 5.3 and Microsoft excel was used for data analysis and the fixed and random effects models were used. Reported outcomes were given in forest plots at 95% confidence intervals (CIs).

Heterogeneity was assessed using the Cochran Q test (χ2) and we aimed to quantify inconsistency by calculating I2 which was interpreted per the following: 0% to 25% (low heterogeneity); 25% to 75% (moderate heterogeneity); and 75% to 100% (considerable heterogeneity). The authors also intended to develop funnel plots and asses their symmetry to review publication bias, however, only if a minimum of 10 studies were available.

Sensitivity and Sub-Group Analyses

To identify areas of heterogeneity and assess the robustness of results, the authors aimed to perform additional analyses for outcomes that were reported by at least four studies.

Methodological Quality and Risk Of Bias Assessment

Two authors independently assessed the methodological quality as well as the risk of bias for articles matching the inclusion criteria. Cochrane’s tool for evaluating the risk of bias for randomised trials was used. Domains assessed included selection bias, performance bias, detection bias, attrition bias, reporting bias, and other sources. It classifies studies into low, unclear, and high risk of bias. In addition, Newcastle-Ottawa Scale [34] was used for the assessment of bias of non-randomised studies in terms of three domains: selection, comparability and exposure. It uses a star scoring system with a maximum total score of nine stars for each study.

Results

Literature Search Results

A literature search reported 248 articles in total which were reviewed by two independent authors to filter out duplicates, abstracts, review articles, studies without the intervention of interest as well as those without comparative control groups and reports involving non-human subjects. Sixteen studies were selected which met the eligibility criteria of which included 5 randomized control trials and 11 non-randomised case-control studies.

Description of Studies

Table 1 summarizes the baseline characteristic of the included studies. The studies were homogenous in the study population and interventions of interest comparing SCEFG and AFG in reconstructive surgery. The current meta-analysis included a total of 16 studies with a total sample size of 686 grafts. Eight studies focused on facial reconstruction, six studies on breast reconstruction and two studies on limb reconstruction (Fig. 1).

Table 1 Baseline characteristics of the included studies
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Fig. 1
figure 1

Prisma Flow Diagram. the PRISMA diagram details the search and selection processes applied during the overview. prisma, preferred reporting items for systematic reviews and meta-analyses

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Primary Outcomes

Volume Retention

In Fig. 2, mean volume retention was reported in 13 studies enrolling 555 subjects. There was a statistically significant difference seen in the mean difference analyses showing a higher mean percentage volume retention for the SCEFG group than the AFG group (Standardised MD = 2.76, CI = 1.56 to 3.95, P < 0.000001). A considerable level of heterogeneity was found amongst the studies (I2 = 95%, P < 0.00001).

Fig. 2
figure 2

Forest plot for mean difference of cal versus aft - Volume retention. Quantitative analysis showing a statistically higher mean percentage volume retention in stem cell enhanced lipo-transfer compared with routine aft

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Moreover, Yoshimura et al. revealed that the volume of fat retention showed a minimal change in the SCEFG group whereas it continued to be reduced in the AFG group. In addition, Malik et al. reported that there’s a more significant increase in the mean fat area on MRI scans pre-operatively and 6 months post-operatively in the SCEFG (17.8800 to 26.8280; P = 0.009) than the control group (24.0720 to 28.7620; P = 0.158).

Fat Necrosis

In Fig. 3, the rate of fat necrosis was reported in three studies enrolling 194 subjects. There was a statistically insignificant difference seen in the odds ratio analyses showing a lower rate of fat necrosis for the SCEFG group than the routine group (OR = 2.23, CI = 0.56 to 8.97, P = 0.26). A moderate level of heterogeneity was found amongst the studies (I2 = 31%, P = 0.24). Furthermore, Kølle et al reported a lower mean area of necrosis in the SCEFG group than the control group, with values of 4.6% and 16.1% respectively. Gentile et al. (2015), however, reported no complications in any patient in both groups.

Fig. 3
figure 3

Forest plot for odds ratio of stem cell enriched fat versus routine autologous fat transfer – fat necrosis. Quantitative analysis showing a statistically insignificant lower rate of fat necrosis in stem cell enhanced lipo-transfer compared with routine autologous fat transfer

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Cancer Recurrence

In Fig. 4, the rate of cancer recurrence was reported in five studies enrolling 363 subjects. There was a statistically insignificant difference seen in the odds ratio analyses showing a higher rate of cancer recurrence for the SCEFG group than the AFG group (OR = 1.39, CI = 0.43 to 4.43, P = 0.58). A low level of heterogeneity was found amongst the studies (I2 = 0%, P = 0.84).

Fig. 4
figure 4

Forest plot for odds ratio of stem cell enriched fat versus routine autologous fat transfer – cancer recurrence. Quantitative analysis showing a statistically insignificant higher rate of cancer recurrence in stem cell enhanced lipo-transfer compared with routine autologous fat transfer

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Redness and Swelling

In Fig. 5, the rate of redness and swelling was reported in three studies enrolling 90 subjects. There was a statistically significant difference seen in the odds ratio analyses showing a higher rate of redness and swelling for the SCEFG group than the AFG group (OR = 441, CI = 7.98 to 24372.70, P = 0.003). Additionally, Chang et al. reported improvement in colour at the site in both groups. Li et al., Tanikawa et al. and Bashir et al. all reported swelling in both groups in the early postoperative period that was self-limiting with no further complications. Bashir et al. also reported bruising in 11 (69%) patients in SCEFG enriched group, whereby bruising occurred after 36 sessions and resolved in 2–3 weeks. Similarly, Sasaki described transient swelling experienced by all patients lasting for 2–3 weeks until full recovery with no difference between both groups, whereby there were no recorded incidences of hematomas or tissue loss.

Fig. 5
figure 5

Forest plot for odds ratio of stem cell enriched fat versus routine autologous fat transfer – redness and swelling. Quantitative analysis showing a statistically significant higher rate of redness and swelling in stem cell enhanced lipo-transfer compared with routine autologous fat transfer

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Infection

In Fig. 6, the rate of infection was reported in six studies enrolling 425 subjects. There was no significant difference seen in the odds ratio analysis between the SCEFG group and the AFG group (OR = 0.36, CI = 0.05 to 2.49, P = 0.30). Heterogeneity was revealed to be low between the studies (I2= 0%, P=0.89). Moreover, Kølle et al. and Tanikawa et al. did not report any adverse events or surgical complications respectively.

Fig. 6
figure 6

Forest plot for odds ratio of stem cell enriched fat versus routine autologous fat transfer – infection. Quantitative analysis showing a statistically insignificant lower rate of infection in stem cell enhanced lipo-transfer compared with routine autologous fat transfer

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Cysts

In Fig. 7, the rate of cysts was reported in three studies enrolling 203 subjects. There was no statistically significant difference seen in the odds ratio analyses showing a similar rate of cysts for the SCEFG group and the routine group (OR = 0.88, CI = 0.11 to 7.18, P = 0.91). A low level of heterogeneity was found amongst the studies (I2 = 0%, P = 0.91). In addition, Gentile et al. (2015) highlighted oily cysts detected by ultrasound in 45.83% of study group at 12 months after the last lipofilling.

Fig. 7
figure 7

Forest plot for odds ratio of stem cell enriched fat versus routine autologous fat transfer – cysts. Quantitative analysis showing a statistically insignificant similar rate of cysts in stem cell enhanced lipo-transfer compared with routine autologous fat transfer

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Patient Satisfaction Post-Surgery

Sterodimas et al. highlighted more satisfaction in the SCEFG group than the non-enriched group at 6 months; however, no difference was noted at 18 months. Visual analog scale was used by Koh et al. for the assessment of patient satisfaction, highlighting a higher score for the intervention group (4.5) than the control group (3.1). Similarly, Bashir et al. did not only report a significantly higher patient satisfaction score (mean score: 2.52 ± 0.521 [ADSC enriched group] vs. 4.25 ± 0.68 [routine AFT group]) in the intervention group but also a higher physician satisfaction score (mean score: 3.69 ± 0.79 [ADSC enriched group] vs. 2.14 ± 0.36 [routine AFT group]).

Gentile et al. (2012) reported satisfaction from all the patients in both groups in terms of the resulting softness, texture and contour. In addition, Gentile et al. (2015) reported satisfaction with the same characteristics from all the patients in the study group, although there was no report of the satisfaction of the control group. Similarly, Sasaki did not compare the satisfaction of both groups; however, 90–95% of patients were satisfied.

No significant differences between the intervention and control groups in patient satisfaction with the results of fat grafting were reported by Tissiani et al. and Gentile et al. (2019), with P values of 0.52 and 0.603 in turn.

Scar Assessment

Clinical assessment of scars by Malik et al. was performed using the observer scar assessment scale (POSAS) score preoperatively and at 1 and 6 months postoperatively, whereby both groups showed similar improvement in scar characteristics. Li et al. reported that no scars were present in patients during follow‐up.

Operative Time

Both Peltoniemi et al. and Tanikawa et al. reported that the operation time was longer in the stem-cell enriched group than the control group. Peltoniemi et al. reported a prolongation of 2–2.5 hours in the former group depending on the volume of enrichment used. Tanikawa et al. highlighted a shorter mean surgical time for the control group than the intervention group, with 80 minutes and additional 45 minutes in turn.

Number of fat grafting sessions

Five studies reported the number of fat grafting sessions that took place. Three studies (Chang et al 2013, Gentille et al 2015 and Koh et al 2012) used the same number of fat injections for both the SCEFG and AFG groups. The other two studies (Bashir et al 2019 and Sterodimas et al 2011) had an average of 1 session for the SCEFG group and 2 sessions for the AFG group.

Subgroup Analysis – Volume Retention

In Fig. 8, subgroup analysis was done showcasing mean difference in volume retention across 13 different studies with a total number of 555 participants. Overall, there is a significant difference seen in the mean difference analyses favouring the intervention group whereby volume retention is higher (MD:2.76, CI: 1.56–3.95, P<0.00001) compared to the AFG group. Additionally, there is no significant difference seen between the subgroups (P=0.73).

Fig. 8
figure 8

Forest plot depicting subgroup analysis of volume retention for stem-cell enriched fat grafts vs routine autologous fat grafts- Quantitative analysis showing significant differences seen for participants undergoing breast and face grafts and no differences between the subgroups

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Methodological Quality and Risk of Bias Assessment

The Cochrane Collaboration’s Tool was used to summarise the risk of bias in randomised studies (Table 2). The Newcastle-Ottawa Scale [16] was used to assess the quality of the three non-randomised studies, which offers a star system for analysis (Table 3). Although the comparability was low in most studies, selection and exposure were of higher quality. Overall, all studies were of good quality except for Sasaki et al which was of fair quality based on the AHRQ standards [16].

Table 2 Assessment of risk of bias of the randomised trials using the cochrane collaboration’s tool
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Table 3 Newcastle-Ottawa Scale (NOS) to assess the quality of non-randomised studies
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Discussion

SCEFG produced a superior effect when compared with AFG in terms of mean volume retention shown by the results of the analyses. There was a significant (P < 0.0001) improvement in the Intervention group compared with the control groups in the combined analysis(Fig. 2) and the subgroup assessment showing no differences between them (Fig. 8). Conversely, there was a significantly (P = 0.003) increased rate of redness and swelling in the experimental group (Fig. 5). However, there were no differences observed in the analyses of fat necrosis (P = 0.26) and cancer recurrence (P = 0.58) in Fig.s 3 and 4. Similarly, the rates of infection (P = 0.48) and cysts (P = 7.18) showed no significant differences as demonstrated in Figs. 6, 7. This shows that SCEFG improves mean volume retention whilst not increasing the overall risk of complications. Regarding the between-study heterogeneity, it was low to moderate for cancer recurrence (I2 = 0%), cysts (I2 = 0%) and fat necrosis (I2 = 31%); however, it was considerably high for volume retention (I2 = 95%). The heterogeneity assessment was not applicable for the redness and swelling.

In addition to the aforementioned outcomes, the results of the current study reported several secondary outcomes that proved SCEFG to have similar effects to the AFG. In the long-term, there were no significant differences noted between both groups in terms of patient satisfaction post-operatively and scar formation. However, operative time was reported to be longer in the SCEFG group.

SCEFG have been developed to overcome the unreliability of the final volume retention of AFG as well as to reduce complications [35]. The current study findings emphasise that SCEFG is superior to AFG. This is supported by the several proposed mechanisms whereby stem cell enrichment increases adipose survival [12]. Initially stem cells differentiate into adipocytes and endothelial cells assist in adipose tissue regeneration and angiogenesis, respectively [12]. Subsequently, stem cell growth factor release helps to protect surrounding tissues from ischaemia and hypoxia [12, 36]. Finally, they survive as original adipose-derived stem cells [12]. Other studies within the literature have reported similar findings to the results of the current review with Lu et al. concluding that there is an enhancement in the survival and quality of transplanted fat tissues with adipose derived stem cells transduced with vascular endothelial growth factor (VEGF) [37]. A further study by Zhu et al. showed that the supplementation of adipose-derived regenerative cells does not only increase adipose graft retention by 2-fold at both 6 and 9 months post-operatively but also enhances the quality of the grafts with higher capillary density, demonstrating the effect of promoting neovascularisation [38].

Several studies question the safety of stem cell application in oncological cases. Stem cells are suggested by several studies to enhance breast tumour growth [39,40,41,42]. Similarly, other studies highlight an increased risk of tumour recurrence with their application [43, 44]. The quantitative assessment conducted by the current meta-analysis however shows no significant difference in the cancer recurrence rate between both stem cell enriched and standard fat transfer cohorts.

While the outcomes of this review and the numerous benefits of SCEFG offer promising results for future practise, there are several limitations that must be considered adapting them in routine clinical practice. ADSC isolation is challenging, with most included studies in this review employing SVF instead [45]. The paucity of published clinical research, a lack of standard methodology, and economic impedance all pose a hinderance to the use of ADSC- or SVF-based cell therapy in clinical activity. Furthermore, despite substantial increases in volume retention when comparing SCEFG for breast and face, none of the included studies for breast reconstruction were prospective or randomised thus reducing the quality of evidence.

Taking into consideration the results from the most reliable evidence and the physiological advantages of stem cells [12, 36], the enhancement of stem cells in AFG should be a routine practice for the management of patients undergoing reconstructive operations. However, in cases of cancer, it is important to be cautious with stem cell enrichment before the establishment of cancer remission [42].

A summary of the best available evidence was obtained by applying a systematic approach to assess the risk of bias of relevant studies [17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32]. The 16 studies were standardised in terms of their design and included patients undergoing reconstructive surgeries. The included studies were homogenous in both the intervention of interest used and the comparative routine grafting, allowing non-biased comparison. However, the current meta-analysis should be reviewed in terms of inherit limitations. 16 studies were included enrolling a total sample size of 686 subjects, only 5 of which were RCTS which could be inadequate to draw definitive conclusions as there is a higher selection bias. Further randomised clinical trials are required to fully assess the efficacy and safety of SCEFG in reconstructive cases.

Conclusions

The findings of this meta-analysis involving 16 studies comparing stem cell-enriched fat grafts and routine fat grafts suggest that stem cell enrichment improves certain outcomes in patients undergoing reconstruction surgery including the mean volume retention and patient satisfaction and surgical complications, except for redness and swelling. SCEFG remain a promising alternative to AFG bearing in mind issues with cost and lack of standardised protocols. The authors highlight the requirement of further randomised clinical studies to enhance the evidence base for the efficiency of SCEFGs.