Stem Cell Enriched Fat Grafts versus Autologous Fat Grafts in Reconstructive Surgery: Systematic Review and Meta-Analysis

Objective To compare the outcomes of stem cell-enrichment fat grafting (SCEFG) versus autologous fat grafting (AFG) for reconstructive purposes. Methods A systematic review and meta-analysis was performed as per the preferred reporting items for systematic reviews and meta-analyses. Guidelines and a search of electronic information was conducted to identify all Randomised Controlled Trials (RCTs), case-control studies and cohort studies comparing the outcomes of SCEFG versus AFG. Volume retention, fat necrosis, cancer recurrence, redness and swelling, infection, and cysts were primary outcome measures. Secondary outcome measures included patient satisfaction post-surgery, scar assessment, operation time and number of fat grafting sessions. Fixed and random effects modelling were used for the analysis. Results 16 studies enrolling 686 subjects were selected. Significant differences between the SCEFG and AFG groups were seen in mean volume retention (standardised mean difference = 3.00, P < 0.0001) and the incidence of redness and swelling (Odds Ratio [OR] = 441, P = 0.003). No significant difference between the two groups in terms of fat necrosis (OR = 2.23, P = 0.26), cancer recurrence (OR = 1.39, P = 0.58), infection (OR = 0.30, P = 0.48) and cysts (OR = 0.88, P = 0.91). For secondary outcomes, both cohorts had similar results in patient satisfaction, scar assessment and number of fat grafting sessions. Operation time was longer for the intervention group. Conclusions SCEFG offers better outcomes when compared to AFG for reconstructive surgery as it improves the mean volume retention and does not worsen patient satisfaction and surgical complications except for self-limiting redness and swelling. Further clinical trials are recommended to support this argument and validate the use of SCEFG in clinical practice. Level of Evidence III This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266.


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 liposculpting 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].

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 (CEN-TRAL).The last search was conducted on 1 st 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 (v 2 ) and we aimed to quantify inconsistency by calculating I 2 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.

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).

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 (I 2 = 95%, P \ 0.00001).
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 preoperatively 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 (I 2 = 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.

Cancer Recurrence
In Fig. 4, the rate of cancer recurrence was reported in five studies enrolling 363 subjects.There was a statistically

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

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 (I 2 = 0%, P=0.89).Moreover, Kølle et al. and Tanikawa et al. did not report any adverse events or surgical complications respectively.

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 (I 2 = 0%, P = 0.91).In addition,

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).

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].

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 (I 2 = 0%), cysts (I 2 = 0%) and fat necrosis (I 2 = 31%); however, it was considerably high for volume retention (I 2 = 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 postoperatively 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 metaanalysis 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.

Fig. 1 Fig. 2 Fig. 3
Fig. 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

Fig. 4
Fig. 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

Fig. 6
Fig. 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

Fig. 8
Fig.8Forest 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

Table 1
Baseline characteristics of the included studies

Table 3
Newcastle-Ottawa Scale (NOS) to assess the quality of nonrandomised studies