Fat grafting in humans: An assessment of the resorption dynamics of fat grafts and evaluation of methods to measure fat graft retention in the breast
Author: Mikkel Herly, MD
Supervisors: Professor Christian von Buchwald, MD, DMSc, Peter Viktor Vester-Glowinski, MD, PhD, Professor Emeritus Krzysztof T. Drzewiecki, MD, DMSc
Date, Institution and University: 2020, Department of Plastic Surgery and Burns Treatments, Rigshospitalet
Fat grafting is an increasingly popular technique for the correction of soft tissue defects and now has an important role in plastic surgery. The procedure consists of harvesting fat from one part of the body where fat is abundant and transplanting that fat to another part of the body where more soft tissue volume is needed. The fat is harvested as a block of tissue and placed via an incision or it is harvested as lipoaspirate and injected in a fan like pattern in multiple channels and planes with a blunt cannula. Fat grafting is used for both reconstructive and cosmetic purposes. The advantages of fat grafting are a low risk of donor site morbidity, the absence of foreign material, and a natural feel. The major drawback of the fat grafting technique is that some of the fat cells are lost to resorption, which result in a loss of volume. For this reason, multiple sessions of fat grafting are often necessary to achieve the desired result, which poses a challenge when planning the surgery.
The resorption of the fat graft is thought to be due to a lack of oxygenation and nutritional support of the transplanted fat cells.4–6 It is expected that approximately half of the transplanted fat is resorbed after surgery.7 Therefore, many researchers are eager to design interventions that may improve the volume retention of fat grafts. One of the most promising techniques is to add the patient’s own adipose-derived stem/stromal cells (ASCs) to the fat graft. A study performed by Kølle et al.8 from our group, which was published in 2013, showed that fat grafts enriched with expanded adipose-derived stem cells can significantly increase the volume retention of bolus injected fat grafts. This discovery led to the planning of two additional RCT’s that would be used to translate the first proof-of-concept study to clinical practice. The first study would investigate ASC-enriched fat grafting for breast augmentation (EudraCT 2014-000510-59) and the other ASC-enriched fat grafting for breast reconstruction (EudraCT 2016-005186-31). The first of these follow-up-studies has been conducted with Peter Glowinski as the principal investigator and I had the opportunity to be a part of the trial group from the beginning as a medical student. subsequently, I got the opportunity to be the principal investigator of the final trial that would investigate ASC-enriched fat grafting for breast reconstruction in women who had undergone a mastectomy.
However, there were still many aspects of non-enriched fat grafting that had yet to be studied, to ensure the highest possible quality of the RCTs. We learned that there was a lack of studies investigating the length and extent of the normal resorption process. Furthermore, there was a need for validated and reproducible techniques for measuring the fat graft retention rate in the breast, which could be used to compare results between research groups.
In this PhD thesis, I present four studies that investigate the resorption dynamics of fat grafts and methods for measuring the volume retention of fat grafts. The four studies share a common foundation; they all investigate fat graft retention. The four studies investigate fat graft retention at different levels: two studies are concerned with methods for measuring fat graft volume retention in the breast, one study is concerned with the basic resorption dynamics of excised fat grafts over time and finally, one study is concerned with the very clinically relevant question of how many fat grafting treatment sessions are needed to complete a breast reconstruction. Furthermore, all four studies are important supporting studies for a randomized clinical trial (RCT) that I plan to conduct as the principal investigator. The RCT is summarized in study 5.
The first four studies presented in this thesis build upon individual research questions that are addressed in manuscripts and summaries of this thesis. However, the studies also have roles to play in relation to the planning of the randomized clinical trial (study 5) that will be addressed exclusively in this thesis. The research questions and their relation to the trial protocol are described below:
Question 1: What is the time to volumetric steady state of an excised fat graft, and what is the retention rate at steady state? The answer to these questions would help us choose an appropriate follow-up time to measure fat graft volume retention in the randomized clinical trial (study 5).
In study 1, I present a retrospective analysis of patients who have undergone surgery to remove a vestibular schwannoma. The patients were reconstructed with an excised fat graft and followed by CT and MRI for many years after the surgery to identify relapse of the schwannoma. These images were used to measure fat graft volume retention over time and to identify the time to steady-state of the fat graft’s volume retention in a statistical model. We used the analysis as a guide for the length of the clinical follow-up in the RCT of ASC-enriched fat grafting for breast reconstruction after mastectomy.
Question 2: How many treatment sessions are needed to complete a breast reconstruction after a skin-sparing mastectomy? This question is important for the RCT (study 5) in which we sought to investigate whether a potentially increased fat graft volume retention from the ASC-enrichment could decrease the number of fat grafting treatment sessions needed to complete a breast reconstruction.
In study 2 we conducted a systematic review of the literature on breast reconstructions performed with fat grafting as the only treatment modality. The studies of this review were in turn used to estimate the number of fat grafting sessions needed to complete a breast reconstruction following a skin-sparing mastectomy, a modified radical mastectomy and breast conserving surgery. These estimates were used in the design of the RCT of ASC-enriched fat grafting for breast reconstruction, which had “number of treatment sessions needed to complete the reconstruction” as a secondary endpoint.
Question 3: What is the most accurate and objective method for measuring fat graft volume retention in the breast? The RCT (study 5) relies on a measurement of fat graft volume retention in the breast as the primary endpoint, and therefore it is necessary to ensure that the method for measuring this endpoint is accurate and precise.
Study 3 is a validation study of the technique that we believe to be the most commonly used method for measuring fat graft volume retention in the breast based on MRI scans. To the best of our knowledge, this technique has not been previously validated for its ability to measure changes in breast volume, although this is prerequisite for measuring fat graft volume retention. Study 4 is a presentation of a new method for measuring fat graft volume retention in the breast for use in future studies. The method was validated according to the same principles used in study 3 and it will be used to measure fat graft volume retention in the RCT, which is described in study 5.
In study 5, the trial protocol for the RCT (EudraCT 2016-005186-31) is summarized. The knowledge acquired from the first four PhD studies were used to design the trial.
Study 1 was used to model the resorption process of excised fat grafts placed in the mastoid cavity. The model estimated that it takes approximately 2.2 years for the fat grafts to reach volumetric steady state. This tells us that the resorption process has the potential to go on for a very long time, although we do not yet know whether the resorption process of injected fat grafts in the breast is equally long. However, the results of this study led us to change the follow-up period of the primary endpoint in the randomized clinical trial (study 5) to two years, and we generally plan for a longer follow-up periods in future studies of fat grafting.
Study 2 provided an estimate of the number of treatments typically needed to reconstruct a breast after a mastectomy. Our analysis relied on meta-analytical methods of the literature and estimated that approximately three treatment sessions are necessary to completely reconstruct a breast that has not been treated with radiotherapy after a skin-sparing mastectomy. This estimate was used in the planning of the randomized clinical trial (study 5) by giving us an estimate of the background population. The study found that significantly more fat grafting treatment sessions were necessary to reconstruct an irradiated breast than a non-irradiated breast. Surprisingly, we also found that the number of treatment sessions to completely reconstruct the breast was not dependent on whether the patient had been treated with a modified radical mastectomy or a skin-sparing mastectomy.
Study 3 and 4 comprise our efforts to find an accurate and precise method for measuring the volume retention of fat grafts in the breast based on MRI scans. We found that the most commonly used method for measuring fat graft retention in the breast overestimated increases in breast volume. We also presented our new method for measuring fat graft volume retention in the breast and showed that the method was able to measure changes in breast volume with high accuracy and reproducibility across observers. The new method which we call Lipovol will be used in future studies in our group and we plan to use it in the randomized controlled trial of breast reconstruction with ASC-enriched fat grafting that I describe in study 5. Furthermore, we hope that other researchers will find the method useful in their studies. The method will be freely available to other researchers upon request.
Preliminary results from the randomized controlled trial of ASC-enriched fat grafting for breast augmentation (EudraCT 2014-000510-59)
The second RCT of our group to investigate ASC-enriched fat grafting in women undergoing a breast augmentation has been finalized. The results have been presented9, and the scientific manuscript is in the final stages before being submitted to a peer-reviewed journal. Unfortunately, the trial did not show any effect of the ASC-enrichment. The breasts treated with ASC-enriched fat grafting did not show a significantly higher fat graft volume retention than the breasts treated with normal fat grafting (54.0% vs. 55.9%). These results have important implications for my plans to initiate the next randomized controlled trial of ASC-enriched fat grafting for breast reconstruction. This recent development has prompted me to outline a research plan to investigate the differences between the first trial that was published in 20138, which showed a significant effect; and the second trial that did not. These studies will investigate in detail how to isolate and expand ASCs and ensure that the cells maintain the properties that make them able to improve fat graft volume retention.
This PhD thesis consists of five studies. The thesis is generally concerned with the resorption dynamics of fat grafts and methods to measure fat graft volume retention in the breast. The final study of the PhD thesis is a protocol for a randomized controlled trial that will investigate ASC-enriched fat grafting for breast reconstruction. The first four studies all played a role in the design of the trial protocol.
The preliminary results of the second RCT investigating ASC-enriched fat grafting for breast augmentations recently showed no effect of the ASC-enrichment. This outcome has prompted us to conduct a series of studies that will investigate the discrepancy between the first study that found a significant effect of the ASC-enrichment in the upper arms of healthy individuals and the second RCT that failed to reproduce this effect in the breast. The research plan will be used to optimize the isolation and expansion of ASCs which hopefully, will enable us to improve breast reconstructions with fat grafting so that we can offer a better treatment to our patients in the future.