Fat Necrosis After Autologous Fat Transfer (AFT) to Breast: Comparison of Low-Speed Centrifugation with Sedimentation

Are We Speaking the Same Language? A Systematic Review on the Use of Consistent Language in Reporting Fat Necrosis in Autologous Fat Grafting of the Breast

BACKGROUND

Autologous fat grafting is a widely adopted approach to optimize outcomes in breast reconstruction and augmentation. Although fat necrosis is a well-known consequence of autologous fat grafting, it remains inconsistently defined in the literature. In late 2014, the Food and Drug Administration released a draft guidance to restrict future autologous fat grafting-a statement that was permissively modified in late 2017. In the context of evolving guidelines and autologous fat grafting outcome data, the language and descriptions of fat necrosis are inconsistent in the literature.

METHODS

Five databases were queried for studies reporting fat necrosis following autologous fat grafting for breast reconstruction or augmentation from inception to August 11, 2022. Studies were temporally stratified according to released FDA guidelines: pre-2015, 2015-2017, and 2018-2022.

RESULTS

Sixty-one articles met inclusion criteria. Prior to 2015, 6 of 21 studies (28.6%) offered clear definitions of fat necrosis. In contrast, the 2015-2017 period demonstrated an absence of clear fat necrosis definitions (0/13 studies, p = 0.03). Though the 2018-2022 period exhibited a rise in annual publications compared with the pre-2015 period (5.4 vs. 1.9, respectively, p = 0.04), this was not matched by a rise in clear fat necrosis reporting (14.8% studies, p = 0.45). Across all periods, only 16.4% of articles offered clear definitions, which exhibited wide heterogeneity.

CONCLUSION

Despite the increasing popularity of autologous fat grafting, fat necrosis remains inconsistently defined and described, especially in the context of changing FDA guidelines. This limits the reliable interpretation and application of the current literature reporting fat necrosis outcomes.

LEVEL OF EVIDENCE IV

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 .

Comparative Analysis of a New Automatic System and Four Existing Techniques for Autologous Fat Grafting

Background

Autologous fat grafting is increasingly used worldwide and is a very attractive technique in many ways. However, treatment duration and postinjection tissue resorption remain problematic elements, which are largely related to the preparation method used. Moreover, few scientific studies objectively compare different fat preparation methods. This study analyzes the efficiency and quality of lipoaspirates prepared with a new filtration/centrifugation system (Adipure) in comparison with several existing techniques.

Methods

Patient lipoaspirates were processed by five different techniques: decantation, centrifugation, Macrofill, Puregraft, and Adipure. Adipose tissue was evaluated in vitro for tissue resorption and oil formation, as well as in vivo after subcutaneous injections in immunodeficient mice. Adipose grafts were collected after 1 month, weighed, and analyzed by histology with a detailed scoring method.

Results

Decanting gives inferior results to all other techniques, in terms of amount of tissue and oil in vitro, or graft weight and histological analysis in vivo. Methods using classical Coleman centrifugation (1200g), or a modified one (400g) associated with washes (Macrofill) produce very similar results, both in vitro and in vivo. Techniques using filtration systems (Puregraft and Adipure) produce less oil overall and have a higher grafting efficiency. The best results regarding grafting efficiency and oil quantity are found with the Adipure device.

Conclusions

A combination of filtration and very low-speed centrifugation potentiates the advantages of these techniques, in terms of graft efficiency. The adipose tissue purification being done in a few minutes, in an automatic way, undoubtedly provides a strong advantage for the use of this new system.

Optimizing Prepectoral Implant Placement and Concomitant Fat Grafting After Tissue Expansion

BACKGROUND

Prepectoral implant-based breast reconstruction is often supplemented by autologous fat grafting to optimize aesthetic outcomes. This usually entails several rounds of modest fat transfer to minimize risk of necrosis; however, the limits of fat grafting at expander exchange are not known.

METHODS

A single-institution retrospective review from July 2016 to February 2022 was performed of all patients who underwent (1) mastectomy, (2) prepectoral tissue expander placement, (3) expander exchange for implant, and (4) at least one round of autologous fat transfer. Student t test and χ2 test were used.

RESULTS

A total of 82 breasts underwent a single round of fat grafting during implant placement (group 1); 75 breasts underwent fat grafting that occurred in multiple rounds and/or in delay to implant placement (group 2). Group 1 received more fat at the time of implant placement (100 mL; interquartile range, 55-140 mL; P < 0.001) and underwent fewer planned operative procedures compared with group 2 (1.0 vs 2.2, P < 0.001). Total fat volume in group 2 did not significantly exceed that of group 1 until after 3 rounds of fat transfer (128.5 mL; interquartile range, 90-130 mL; P < 0.01). There was no difference in the rate of fat necrosis between groups after the first round (15.9% vs 9.3%, P = 0.2) and final round (15.9% vs 12.0%, P = 0.5) of fat grafting. Complication rates were similar between groups (3.7% vs 8.0%, P = 0.2).

CONCLUSIONS

A 2-stage approach of prepectoral tissue expander placement with single round of larger volume fat transfer at expander exchange reduces overall number of operative procedures without increased risks.

“Achieving Optimal Clinical Outcomes in Autologous Fat Grafting: A Systematic Review of Processing Techniques”

BACKGROUND

Autologous fat grafting (AFG) is a versatile technique in reconstructive and cosmetic surgery. Graft processing is a key source of variability resulting in unreliable clinical outcomes, with no consensus on the optimal methodology. This systematic review identifies the evidence base supporting different processing paradigms.

METHODS

A systematic literature search was conducted using the PubMed, Scopus and The Cochrane Foundation databases. Studies comparing AFG processing methods and reporting long-term patient outcomes were identified.

RESULTS

Twenty-four studies (2413 patients) were identified. Processing techniques evaluated included centrifugation, decantation, washing, filtration, gauze rolling, as well as commercial devices and adipose-derived stem/stromal cell (ASC) enrichment methods. Objective volumetric and subjective patient-reported outcomes were discussed. There was a variable reporting of complications and volume retention rates. Complications were infrequent; palpable cysts (0-20%), surgical-site infections (0-8%) and fat necrosis (0-58.4%) were the most reported. No significant differences in long-term volume retention between techniques were found in AFG in the breast. In head and neck patients, greater volume retention was documented in ASC enrichment (64.8-95%) and commercial devices (41.2%) compared to centrifugation (31.8-76%).

CONCLUSIONS

Graft processing through washing and filtration, including when incorporated into commercial devices, results in superior long-term outcomes compared to centrifugation and decantation methods. ASC enrichment methods and commercial devices seem to have superior long-term volume retention in facial fat grafting.

The Importance of Protecting the Structure and Viability of Adipose Tissue for Fat Grafting

BACKGROUND

Fat grafting is widely used for soft-tissue augmentation; however, the related clinical outcome remains variable and technique-dependent. The mechanisms underlying fat graft survival are not fully understood, particularly regarding the contributions of different cell types, such as functional adipocytes. This study evaluated the importance of adipose tissue structure and viability in fat grafting and, to some extent, revealed the effect of adipocytes in fat grafting.

METHODS

Human lipoaspirate was harvested using suction-assisted liposuction and processed using three separate methods: cotton-pad filtration, soft centrifugation (400 g for 1 minute), and Coleman centrifugation (1200 g for 3 minutes). Then all samples were subjected to second cotton-pad concentration. Adipose tissue structure and viability, the numbers of adipose-derived stem cells, and their proliferation and multilineage differentiation abilities were compared in vitro. The volume retention rate and fat graft quality were evaluated in vivo.

RESULTS

Cell structure destruction and viability decline were more evident in the Coleman centrifugation group compared to the cotton-pad filtration group and the soft centrifugation group. However, no intergroup differences were observed in the numbers, proliferation, or multilineage differentiation abilities of adipose-derived stem cells. After transplantation, the volume retention rates were similar in the three groups. However, greater structural and functional damage was associated with poorer graft quality, including decreased levels of graft viability, vessel density, and vascular endothelial growth factor secretion and increased levels of vacuoles, necrotic areas, fibrosis, and inflammation.

CONCLUSIONS

Protecting adipose tissue structure and viability is crucial for improving fat grafting outcomes.

CLINICAL RELEVANCE STATEMENT

The protection of the structure and viability of adipose tissue should be ensured throughout the whole process of fat grafting to reduce complications and improve graft quality.

Oil Cyst Formation after Lower Blepharoplasty with Fat Grafts

BACKGROUND

Fat grafting is increasingly used as an adjuvant surgery to blepharoplasty to refill the volume loss of an aged face and promote cellular regeneration. Complications, such as hematoma, infection, seroma, and palpable mass, may occur. We collected the patients that underwent lower blepharoplasty combined with fat graft to evaluate the incidence of oil cyst formation in the lower eyelid and to identify risk factors.

MATERIAL AND METHODS

A retrospective review was performed of all patients who underwent lower or total blepharoplasty combined with fat graft at the authors' institution between January 2018 and June 2020. Complication rates were observed, and associations between preoperative variables and outcomes were assessed.

RESULTS

A total of 119 patients were included in the series (all bilateral, 238 eyelids). The average patient age was 54.88 ± 11.94 years, and the average grafted fat was 1.88 ± 1.0 mL. On a per-eyelid basis for all patients, the complication rate of oil cyst formation was 6.72% (16 of 238 eyelids). The occurrence of oil cyst formation was associated with hypertension (P = 0.012; adjusted odds ratio, 9.27; 95% confidence interval, 1.62-52.99) and diabetes mellitus (P = 0.005; adjusted odds ratio, 14.02; 95% confidence interval, 2.22-88.45), but not associated with anticoagulants use (P = 0.931), age (P = 0.784), sex (P = 0.317), or fat volume (P = 0.215). The mean interval between the fat graft procedure and oil cyst noted was 236.5 ± 118.9 days.

CONCLUSIONS

Oil cyst in lower eyelid can be defined as a palpable, firm, and persistent subcutaneous cystic lesion found postoperatively in any size during physical examination. The complication rate of oil cyst formation occurring after lower blepharoplasty with autologous fat grafting is 6.72%. Hypertension and diabetes mellitus maybe are risk factors of oil cyst formation. Steroid injection, needle capsulotomy, liposuction, and excision are safe and effective treatments. Reduce surgical trauma by diminishing anterior lamina trauma and capsulopalpebral fascia repair might decrease the complication rate of oil cyst formation.Transconjunctival lower blepharoplasty with fat graft or 2-stage surgery may be a choice to prevent oil cyst formation.

Efficacy and Safety of External Volume Expansion (EVE) on Fat Grafting: A Systematic Review and Single-Arm Meta-Analysis

BACKGROUND

The autologous fat grafting is commonly used for reconstructive or aesthetic purposes. However, due to the huge variation in methods, its retention rate varies a lot. External volume expansion (EVE) has been used to treat recipient sites of fat grafting. Concerns have been raised regarding its efficacy and safety.

METHODS

We have searched PubMed, EMBASE, and the Cochrane Library for studies on EVE-assisted fat grafting published from 2000 to 2020. A meta-analysis was conducted to pool the retention rate. The incidence of complications was assessed for reconstructive or aesthetic purposes.

RESULTS

The 11 included studies involved 1152 patients with operations on 1794 breasts. Four studies were included in the meta-analysis. The pooled retention rate was 65% [95%CI 49, 79]. Eight studies reported the complications. The total complication incidence was 34%, which is 35% for the aesthetic group and 33% for the reconstructive group. The complication rate was not obviously different between the two groups.

CONCLUSIONS

The study shows that the EVE-assisted fat grafting has better retention rate than traditional fat grafting. However, the data showed that the complication rate was much higher in the EVE-assisted group.

[Diagnosis and management of fat necrosis after autologous fat transplantation of breast]

Objective

To review the diagnosis and management of fat necrosis after autologous fat transplantation of breast.

Methods

Based on the latest related literature, the pathology, clinical and radiographic examinations, influence factors, as well as the management of fat necrosis after autologous fat transplantation for breast augmentation and reconstruction were summarized.

Results

Fat necrosis after breast autologous fat transplantation is histologically manifested as hyaline degeneration, fibrosis, and calcification. The diagnosis of fat necrosis includes clinical examination, imaging examination (ultrasound, mammography, and MRI), and biopsy. The occurrence of fat necrosis is closely related to patient's own reason and fat transplantation technology. Optimizing the process of fat acquisition, purification, and transplantation can reduce the occurrence of fat necrosis. Intervention or not after fat necrosis depends on the nature of the nodules. According to the nature of the the nodules, various methods such as simple aspiration, vibration amplification of sound energy at resonance liposuction, or direct excision can be selected.

Conclusion

Fat necrosis after autologous fat transplantation of breast are difficult to control. How to process fat to minimize the injury and maximize the activity of grafted fat needs further researches.

Autologous Fat Transplantation for Aesthetic Breast Augmentation: A Systematic Review and Meta-Analysis

BACKGROUND

Autologous fat transplantation has already become a part of clinical practice for aesthetic breast augmentation even though evidence regarding its efficacy is still lacking.

OBJECTIVES

The authors sought to determine the current worldwide status and efficacy, techniques, and oncologic safety on this subject.

METHODS

PubMed, EMBASE, and Cochrane Library databases were searched to identify all relevant studies.

RESULTS

Eighty-four articles published between 1987 and April 2020, consisting of 6468 patients, were included, and 64 studies consisting of 5162 unique patients were included in the meta-analysis. Most studies had a low level of evidence (levels 2b-5); In this meta-analysis, there were 17 prospective cohort studies, 4 retrospective cohort studies, 6 case-control studies, and 38 case series. The publications were from 21 countries. Indications for autologous fat transplantation were aesthetic augmentation (93.2%) and congenital malformation (6.8%). Among the 5162 patients, 2 cases (0.04%) of cancer were reported. The meta-analysis revealed very high overall patient and surgeon satisfaction rates of 93% and 87%, respectively. Overall, only 1.56 sessions were needed to achieve the desired result. Long-term survival was calculated to be approximately 60% to 70% at 1-year follow-up. Only 8% of procedures resulted in clinical complications, and 5% of patients required biopsy because of abnormal clinical or radiological findings.

CONCLUSIONS

Autologous fat transplantation seems to be a major tool in aesthetic breast augmentation. Preoperative patient selection is essential but under-reported. Future research should focus on evaluating the technical and patient factors influencing the rate of fat survival and its oncological safety.

LEVEL OF EVIDENCE: 4

Fat Processing Techniques: A Narrative Review

OBJECTIVE

This study reviewed the literature regarding different fat processing techniques, in order to update the information for healthcare personnel and provide the latest evidence in selecting purification methods.

METHODS

PubMed (MEDLINE), EMBASE, the Cochrane Library, and the Cochrane Central Register of Controlled Trials were comprehensively searched to identify studies that compared different fat purification methods for animal and human studies published through July 2020. Papers were screened using inclusion and exclusion criteria, and relative data were collected for review.

RESULTS

A total of 3292 studies were identified, of which 30 were included for review. The findings of existing clinical studies showed that the filtration and washing filtration methods performed better in the volume retention rate of adipose tissue. In terms of postoperative complications (fat necrosis, nodules, cysts, etc.), the incidence of complications of centrifugation is generally higher than that of other purification methods, while Telfa gauze and washing filtration system show better safety. More comparative studies are needed to draw conclusions about clinical efficacy and satisfaction. The existing basic science studies generally believe that centrifugation has no advantage in the integrity and metabolic activity of adipose tissue. However, there is no definite conclusion about the volume retention rate of grafts in animal experiments.

CONCLUSION

In recent years, studies on the cost-effectiveness of various purification methods have emerged, and the efficiency advantages of commercial systems have also been gradually reflected. In the future, the purification efficiency will be improved based on ensuring clinical efficacy, which will be translated into cost savings.

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 .

Evaluation of a new adipose tissue processing method for breast and buttock fat grafting procedures

Background

There is still some debate about the best adipose processing method for autologous fat transfer procedures. This article reports a series of 66 fat grafting procedures using a novel fat processing technique that involves washing, “micronization,” and decantation.

Methods

We retrospectively reviewed the files of all patients operated on over a 2-year period by the two investigators. They underwent breast or buttock surgery that employed the novel fat processing technique for fat transfer. Complications and patient satisfaction were recorded, and fat resorption was qualitatively assessed based on clinical examination and photographs.

Results

Data were retrieved from a total of 66 procedures performed on 43 patients: 17 breast augmentations, 8 buttock augmentations, and 41 breast reconstructions. Five patients also underwent concomitant facial fat grafting. The complications were one case of transient liponecrotic cyst in the breast, one case of transient areolar sensitivity reduction, and two cases of infection. The infections were reported in patients from the breast reconstruction cohort who had a history of radiotherapy. There was no complication associated with facial fat grafting. Patient satisfaction was achieved in 82.4 to 92.5% of cases depending on the study cohort. Fat resorption was graded as minimal to moderate in 95% of all cases.

Conclusions

This retrospective study supports the safety and effectiveness of this novel fat processing method in breast and buttock surgeries. Further controlled studies are needed to compare its performance with that of other techniques.

Level of evidence: Level III, therapeutic study

Concentrated nanofat: a modified fat extraction promotes hair growth in mice via the stem cells and extracellular matrix components interaction

Background

Fat graft transplantation seems a promising cell therapy for hair loss. However, impurities in lipoaspirate weaken the treatment effect. Here, we developed the lipoaspirate extraction method then investigate the effect and mechanism on hair growth-promoting in a mouse model.

Methods

Fat graft was prepared into concentrated nanofat (CNF), decellularized CNF (DCNF), and adipose-derived stem cells (ADSCs). They were injected subcutaneously in the back of depilated mice to test the hair promoting effect. Conditioned media (CM) from the adipose extracts were applied to dermal papilla cells (DPCs) to evaluate the cell viability and the anagen related signal.

Results

CNF and a high dose of ADSCs promoted hair growth and induced telogen-to-anagen transition in depilated mice. DCNF and a low dose of ADSCs did not show such effect; however, hair growth was promoted when they were used in combination. In vitro study showed the CNF-CM treated DPCs exhibited increased proliferation, migration, cell cycle progression, and elevated Wnt/β-catenin pathway protein levels compared with the other treatment groups.

Conclusions

CNF has a better effect than ADSCs in hair promotion via activating the DPCs and anagen induction. In this nature complex of stem cells (SCs) and extracellular matrix (ECM), ECM serves a significant supplementary role and amplifies the power of ADSCs. These results supply a theoretical basis on the clinical application of CNF to treat hair loss.

Total breast reconstruction using large-volume condensed and viable fat grafting after mastectomy

Autologous fat grafting (AFG) has become a third alternative for breast reconstruction after mastectomy. However, total breast reconstruction using AFG remains a great challenge. We performed breast reconstruction using large-volume condensed and viable fat grafting in 30 postmastectomy patients (irradiated, n = 9; nonirradiated, n = 21). The grafts were purified and condensed by the washing and cotton gauze rolling technique. The surgical maneuver is designed to preserve the grafts viability. Three-dimensional expansion was applied where strong adhesions were present. Seven patients wore the Brava device for the expansion of the recipient site. The mean (SD) volume of fat grafted in each procedure was 230.5 (57.8) mL. The average number of sessions was 3.3 (0.7). The irradiated patients required more sessions than the nonirradiated patients (p=.017). The mean follow-up period was 12.8 (4.3) months. One patient (3%) developed postoperative cellulitis. Cysts and palpable nodules occurred in eight patients (27%) and one patient (3%), respectively. Severe dermatitis and skin pigmentation change occurred in one patient wearing the Brava device. The esthetic scores significantly improved after the treatments (p<.001). Approximately 90% of the patients were satisfied. No local recurrence or remote metastasis was documented during the follow-up period. Our study showed the success of total breast reconstruction using AFG. As a third alternative for breast reconstruction, total breast reconstruction using large volume condensed and viable fat grafting is an effective and safe approach for postmastectomy patients.

Comparison of Telfa Rolling and a Closed Washing System for Autologous Fat Processing Techniques in Postmastectomy Breast Reconstruction

BACKGROUND

The purpose of this study was to compare the commonly used fat grafting techniques-Telfa rolling and a closed washing system-in breast reconstruction patients.

METHODS

Consecutive patients undergoing fat grafting were retrospectively reviewed and grouped by technique. Patients with less than 180 days of follow-up were excluded. Demographics, operative details, and complications were compared using univariate analysis with significance set at p < 0.05.

RESULTS

Between January of 2013 and September of 2017, 186 women underwent a total of 319 fat grafting procedures. There was no difference in demographics, number of procedures performed, volume of fat grafted, and number of days after reconstruction that fat grafting was performed between groups (p > 0.05). Telfa rolling patients had longer operative times for second fat grafting procedures (implant exchange often completed prior) [100.0 minutes (range, 60.0 to 150.0 minutes) versus 79.0 minutes (range, 64.0 to 94.0 minutes); p = 0.03]. Telfa rolling breasts had more palpable masses requiring imaging (26.0 percent versus 14.4 percent; p = 0.01) and an increased incidence of fat necrosis (20.6 percent versus 8.0 percent; p < 0.01). The closed washing system was found to be an independent predictor of decreased rates of imaging-confirmed fat necrosis (OR, 0.29; p = 0.048). There was no difference in fat necrosis excision or cancer recurrence between the groups.

CONCLUSION

The closed washing system was independently associated with decreased rates of imaging-confirmed fat necrosis compared to Telfa rolling without an increase in other complications.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, III.

The Effect of Thymosin beta4 on the Survival of Autologous Fat Grafting: A Preliminary Study

BACKGROUND

Autologous fat grafting is a common procedure to improve tissue deficiencies. However, the survival rate of fat grafting is unpredictable. Thymosin beta 4 (Tß4), a multifunctional peptide containing 43 amino acids, is effective in angiogenesis, inhibiting apoptosis and inflammation.

OBJECTIVES

The authors initially investigated the potential effect of Tß4 in fat grafting.

METHODS

Adipose tissue premixed exogenous Tß4 were transplanted into rabbit ears. Rabbits were randomly assigned to 3 groups: group A, 5 μg/mL Tß4; group B, 10 μg/mL Tß4; and group C, phosphate-buffered saline buffer as a blank control. The fat grafts were subjected to magnetic resonance imaging at 2, 4, and 12 weeks in vivo. Each harvested graft was analyzed at 3 time points after transplantation.

RESULTS

The fat grafts in the Tß4-treated groups showed better volume and weight retention, greater adipose tissue integrity, adipocyte viability, and angiogenesis. The results of dynamic contrast-enhanced magnetic resonance imaging also showed that the experimental groups increased microcirculation perfusion of the grafts.

CONCLUSIONS

The study proved that Tß4 could improve adipose tissue survival and neovascularization. It may be useful for fat grafting as a potential protective reagent.

The Vacuum-Assisted Breast Biopsy System is an Effective Treatment Strategy for Breast Lumps After Augmentation with Autologous Fat Grafting

BACKGROUND

With the extensive application of autologous fat grafting (AFG) to the breasts, postoperative complications such as breast lumps attract high attention. Breast lumps greatly reduce patient satisfaction and bring mental stress. However, there are few detailed reports about minimally invasive treatment strategies for breast lumps after AFG. Our study aimed to investigate the effectiveness of the vacuum-assisted breast biopsy (VABB) system for patients with lumps after AFG.

MATERIALS AND METHODS

We retrospectively reviewed 37 patients with breast lumps between April 2015 and January 2019. The characteristics of patients and breast lumps were analyzed. Breast lumps were classified into four types, including cystic, solid, complex and calcification. The vacuum-assisted breast biopsy (Mammotome and Encor) was performed for the patients with lumps after AFG. The efficacy, safety, complications and patient satisfactions were recorded during postoperative follow-up periods.

RESULTS

Under the guidance of ultrasound, the breast lumps could be thoroughly and accurately excised by the vacuum-assisted biopsy system. No patient experienced breast infections or major complications requiring treatment. Hematoma was observed in only 2 patients and gradually resolved without any special management. With a median follow-up of 29 months, no recurrence was observed. Furthermore, there were no statistical differences in duration of the procedures and complications between the two VABB systems. All the patients recovered well and were satisfied with the cosmetic outcome.

CONCLUSION

The vacuum-assisted breast biopsy system can be used as an effective and minimally invasive approach for the surgical management of lumps after AFG.

LEVEL OF EVIDENCE IV

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 .