Application of Amniotic Membrane Allograft in the Treatment of Foot and Ankle Pathologies: A Review of the Basic Science and Clinical Evidence
1Department of Orthopedics and Rehabilitation, Tufts Medical Center, Boston, Massachusetts, United States
2Foot & Ankle Research and Innovation Lab (FARIL), Department of Orthopedic Surgery, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts, United States
3Foot and Ankle Service, Department of Orthopedic Surgery, Massachusetts General Hospital, Boston, Massachusetts, United States
4The BIOS Orthopedic Institute, Sacramento, CA, United States
5Department of Orthopedics and Rehabilitation, Tufts Medical Center, Boston, Massachusetts, United States; Boston Sports & Biologics, Wellesley, Massachusetts, United States
Corresponding Author: Walter I Sussman, Department of Orthopedics and Rehabilitation, Tufts Medical Center, Boston, Massachusetts, United States; Boston Sports & Biologics, Wellesley, Massachusetts, United States, Phone: (781) 591-7855, e-mail: email@example.com
Received on: 28 August 2022; Accepted on: 26 October 2022; Published on: 31 December 2022
Background: In recent years, an increasing number of studies have investigated the use of AM for various orthopedic indications. AM allografts’ anti-inflammatory and anti-scarring properties make them a promising agent used to augment the healing of pathological soft tissue.
Purpose: The purpose of this study is to review the published studies on the use of amniotic membrane (AM)-derived allografts in human subjects for various indications in the foot and ankle.
Results: Studies suggest that when it is used for the treatment of plantar fasciitis, the efficacy of AM-derived products is at least equivocal to corticosteroid injections in the short term and superior to placebo injection in the long term. Fewer studies have investigated the application of AM-derived products in Achilles tendinitis, ankle and foot osteoarthritis (OA), or peripheral nerve pathology.
Conclusion: Additional high-quality supportive data is needed to support the efficacy and safety of AM-derived products in the foot and ankle. Furthermore, clinicians should be aware of the regulatory status of any AM-derived product before its use in clinical practice.
How to cite this article: Nakagawa H, Ashkani-Esfahani S, Waryasz GR, et al. Application of Amniotic Membrane Allograft in the Treatment of Foot and Ankle Pathologies: A Review of the Basic Science and Clinical Evidence. J Foot Ankle Surg (Asia-Pacific) 2023;10(1):13-19.
Source of support: Nil
Conflict of interest: None
Keywords: Amniotic graft, Amniotic membrane, Amniotic mesenchymal stem cells, Achilles tendonitis, Nerve wrap, Osteoarthritis, Plantar fasciitis.
The clinical application of AM tissue dates back over a century ago.1 Since its first use in the early 1900s, AM tissue allografts have been used mainly in the field of wound care and ophthalmology.1-5 Recently, the therapeutic applications of AM tissue allografts have expanded to include a variety of orthopedic indications. This paper provides a review of the structure and function of the placenta and an overview of published studies on the use of AM-derived allografts in human subjects for the treatment of plantar fasciitis, Achilles tendonitis, and OA, as well as its application in peripheral nerve pathology. We will also discuss efficacy, safety, and regulatory concerns.
STRUCTURE AND FUNCTION OF THE PLACENTA
The placenta is a discoid fetal organ that develops during pregnancy and provides an interface between the mother and fetus. The chorionic plate represents the fetal side of the placenta and is covered by the AM. It is the innermost layer of the placenta and amniotic sac (Fig. 1). The AM is composed of two distinct layers: (1) the amnion that lines the inner surface of the amniotic sac facing the fetus; and (2) the chorion, that is, the layer in contact with the maternal tissue (decidua and myometrium) (Fig. 2). The amnion layer is composed of a simple squamous epithelium with an underlying basement membrane and a band of loose connective tissues. The connective tissues of the amnion and chorion are distinct, with the amnion layer only weakly attached to the underlying chorion (Fig. 3).6,7
The umbilical cord ensures an exchange of blood between the mother and fetus, providing oxygenated blood to the fetus and removing by-products to the maternal circulation. The umbilical cord consists of the umbilical vessels (one vein and two arteries) and a bulk of mucous connective tissues known as Wharton’s jelly.8 Umbilical cord blood is the blood that remains in the placenta and umbilical cord following childbirth and is rich in hematopoietic stem cells, similar to those found in the bone marrow.9 The amniotic fluid (AF) envelopes the developing fetus within the amniotic sac and provides nutritional and immunomodulatory support to the growing fetus in the utero.7
PROCUREMENT AND PROCESSING OF THE AM AND PLACENTA
The placenta, umbilical cord, and umbilical cord blood have all emerged as an area of interest in cell-based therapy and regenerative medicine. The subject of placental-derived-tissue grafts is broad and several different formulations are available. Stromal cells isolated from each of the layers of the placenta differ, but most of the studies evaluating AM in foot and ankle pathology use the amnion layer alone, or a combination of the amnion and chorion layers.7,10,11
Amniotic membrane (AM)-derived products are harvested from donated placenta tissues with full consent from the mother.1,7,12,13 Cesarean delivery is preferred to eliminate the risk of contamination from normal flora during vaginal delivery.13 The collected placental membrane grafts are then aseptically processed, separated into individual membrane components, and then preserved.7,13,14
In the past decade, harvesting, processing, and preservation methods of AM grafts have dramatically improved, reducing the risk of disease transmission that once prevented their widespread use.1,15,16 Potential donors undergo strict screening protocols for hepatitis B and C, syphilis, cytomegalovirus, human immunodeficiency virus infection, and tuberculosis through regulation by the Federal Drug Administration (FDA) and the American Association of Tissue Banks.17,18
Methods of preservation have also led to the commercialization of placenta-derived products. Preservation methods include dehydration, cryopreservation, and lyophilization.1,19,20 The AM allografts used clinically for the nonoperative management of plantar fascia and Achilles pathology are mostly pulverized to particulate matter (i.e., micronized), cryopreserved, or lyophilized and are used as an injectable amniotic allograft.1,13 AM can also be processed to form sheets of tissues that can be used intraoperatively as tendons, ligaments, or nerve wraps.13,21 These sheets can also be deployed intratendinously during interventional procedures.
Methods of sterilization and preservation of placental tissues render themselves safe off-the-shelf products but can damage components of the placental tissues. Dehydration, cryopreservation, and lyophilization have been shown to preserve the structures of the basement membrane, which seems to allow AM products to serve as a scaffold and allow cell adhesion.22,23 Although some cryopreservation and lyopreservation processing methods may result in products containing viable cells, most commercially available products do not contain viable cells.1,23-29
BASIC SCIENCE OF AMNIOTIC GRAFTS: IN VITRO EFFECTS AND ANIMAL MODELS
Amniotic membranes (AMs) are a rich source of stem cells, including amniotic epithelial cells (AECs) and amniotic mesenchymal stromal cells (AMSCs). AECs have markers consistent with epithelial cells and stem cells and have the ability to differentiate into all of the mature cell lineages, including mesoderm, endoderm, and ectoderm.30 AMSCs are derived from the embryonic mesoderm and also have the ability to differentiate into all three germ layers.31 While AMSCs are known to be present in AF in vivo, the majority of commercial products have shown no living AMSCs after commercial processing (i.e., decellularization).24,25 Therefore, AM grafts should not be considered a mesenchymal “stem cell” product, as the cellular component of most commercial products is no longer viable after commercial processing.
Despite a lack of viable cells in most commercial products, the clinical benefits of AM-derived products seem to be maintained, suggesting that the effects are likely mediated by matrix proteins and growth factors within the AM-derived tissue.24,25 AM products are more accurately classified as a metabolically active biologic scaffold, and clinical studies in chronic wound management have demonstrated that placental products that contain viable cells result in similar wound closure rates as devitalized products.32,33
The mechanism of action of AM products likely varies depending on the underlying pathology. AM allografts are believed to aid in tissue repair by providing key growth factors to guide the connective tissues through the different phases of tissue repair. The release of vascular endothelial growth factors promotes angiogenesis, basic fibroblast growth factors promote early tendon healing, and transforming growth factor β stimulates the production of extracellular matrix.23,34
In vitro, AM has been shown to release anti-inflammatory cytokines, such as interleukin-10 (IL-10). These cytokines downregulate the inflammatory phase of healing, which is important in guiding tissue recovery and reducing scar tissue formation. In peripheral neuropathy, local inflammation triggers a cascade of processes that results in perineural fibrosis, scar formation, and adhesion.35 Anti-inflammatory cytokines contained in AM, such as IL-1 receptor antagonist and IL-10, can also downregulate inflammation to attenuate scar formation around the nerve.14,35 Anti-inflammatory cytokines contained in AM can also downregulate inflammation to attenuate cartilage degeneration.14,35 Synovial fluid in OA contains a high concentration of inflammatory cytokines, such as IL-1 and tumor necrotic factor-α. These cytokines cause an increased production of matrix metalloproteinase (MMP) that contributes to cartilage degradation. AM releases tissue inhibitors of metalloproteinase that reduce MMP activity and inhibits further collagen degradation.23
Animal studies have varied in the type of animal models used, pathology and indications, placental cell types, tissue preparation and preservation, doses, and administration routes. It is challenging to perform a comparative analysis, and a review of these preclinical animal studies is beyond the scope of this publication. In one review of 35 animal studies using AM-derived grafts, all reviewed publications demonstrated some degree of efficacy when treated with orthopedic sports medicine indications and when compared with controls.36
APPLICATIONS IN FOOT AND ANKLE ORTHOPEDICS
The clinical literature of the use on AM-derived allografts in orthopedics and sports medicine is limited. Human studies using AM allografts have been published for the treatment of plantar fasciitis, Achilles tendonitis, and knee OA. While AM has been used clinically for the treatment of OA in the foot, ankle, and nerve wrapping, to the authors’ knowledge, there are no publications describing clinical outcomes for these indications.
Current treatment for plantar fasciitis is typically conservative and can include rest, physical therapy, orthoses, nonsteroidal anti-inflammatory drugs (NSAIDs), and corticosteroid injections.37,38 In 90% of the cases, symptoms are self-limiting and are resolved within 12 months with conservative treatments.39 Operative treatment is indicated in recalcitrant cases, although there is no consensus on the preferred operative technique since no procedure has demonstrated superior results.40
Several randomized controlled trials have shown a significant improvement in function and pain with AM graft injections for the treatment of chronic plantar fasciitis. Zelen et al. performed a randomized single-center clinical trial consisting of 45 patients, who were randomly divided into a control group (normal saline) and two treatment groups that received different volumes of acellular human AF. Both treatment groups had a significant improvement in pain and function as assessed by the American Orthopaedic Foot and Ankle Society (AOFAS) Hindfoot scores and Short Form 36 Standard Health Survey from their respective baseline levels at all-time points when compared to the control group, and the improvement was maintained at the 8-week follow-up.41 There are several concerns with this study. First, this was a small study with a relatively short follow-up and was funded by the AM manufacturer MiMedex. After the injections, the subjects were managed with orthosis, night splints, and tramadol, which could have potentially affected the outcomes.
Cazzell et al. conducted a prospective randomized multicenter clinical trial consisting of 145 patients, comparing a control group (normal saline injections) to a group that received micronized dehydrated human amnion/chorion membrane (mdHACM) injections. The patients were followed at various points over a year. The micronized amniotic injection group had significant improvement in pain and function as measured by the foot function index-revised (FFI-R) scores.37 This study is the largest randomized controlled study to date with an adequate follow-up and showed statistically significant improvements in pain and function. However, similar concerns exist for this study, including the fact that all patients were instructed to off-load their heels and to use night splints or orthotics. This study was also funded by MiMedex.
Hanselman et al. compared the efficacy of acellular human AF injections to that of corticosteroid injections and there was no significant difference in pain or function between the groups at the 6, 12, or 18-week follow-up.42 This study suggested that the efficacy of AM for the treatment of plantar fasciitis is at least comparable to that of corticosteroid injection, in terms of providing pain relief in the short and intermediate terms. However, unlike corticosteroid injections, AM may stimulate a reparative process that can provide long-term benefits with continued improvement seen at the long-term follow-up. This study was a low-powered pilot study (only 23 patients) with a short follow-up period. Patients were instructed to perform stretching exercises for the plantar fascia and calf at least five times a day, which may have had impactful results. This study was also funded by an AM manufacturer Amniox Medical, Inc.
Although there is promising evidence for using AM for plantar fasciitis, further high-quality studies are needed to establish its clinical utility. Overall, three randomized trials exist, but only one of these studies was adequately powered and reported long-term follow-up data. These studies suggest AM-derived products are either superior to placebo or noninferior over corticosteroid injections, but the results need to be interpreted carefully as various postintervention treatments were also used following the injections. Furthermore, all these studies were funded by manufacturers of AM.
Conservative treatments for Achilles tendonitis include physical therapy, rest, stretch exercise, NSAIDs, and steroid injections.43,44 Until recently, the use of AM graft for tendon repair was limited to in vivo studies.
In the past few years, several human clinical trials have been published, mostly as case series. In a series of patients (n = 44) experiencing recalcitrant Achilles tendinosis, Werber applied 1.0 mL of AM allograft along the tendon under ultrasound guidance. There was a significant reduction in pain scores starting in 4th week after the treatment, and most patients reported only mild pain by the 12thweek.43
Lullove published a series of 10 patients with various chronic tendon pathologies treated with human placental tissue. The composition of the placental tissue allograft was not reported in this publication or on the manufacturer’s website. All 10 patients had complete resolution of pain 5 weeks after the treatment. The study included two patients with Achilles tendonitis.
In a similar study, Gellhorn and Han conducted a case series of 40 patients with various chronic tendinosis or arthropathy who received mdHACM injections. This study included two patients with Achilles tendonitis. The cohort showed a significant reduction in average pain, starting 1 month after the procedure, with a significant improvement in functional levels. No serious adverse events occurred during the study.45
Finally, Spector et al. published a case series of 32 patients with Achilles tendinopathy treated with mdHACM. The study only reported short-term follow-ups. They found that while 97% of the patients reported severe or moderate pain prior to the intervention, 66% of treated patients reported complete symptom resolution, and the remaining reported partial improvement at the 45-day follow-up.46
The existing data seems to suggest that AM-derived products may be effective for chronic Achilles tendinopathy, but further high-quality studies are needed. The existing studies lack power and provide no long-term follow-up. No randomized controlled trials exist to date.
Conventional management for OA includes weight loss, low impact aerobic exercise, physical therapy, NSAIDs, and steroid or hyaluronic acid injection.47 Surgical intervention is performed when these conventional managements are unsuccessful.47 Most of the studies using AM for the treatment of OA are for the knee and used micronized lyophilized, cryopreserved, or dehydrated AM or AF.45,48-52 Overall, these studies suggest that there is promising evidence for using AM to treat knee OA. Complications were limited to localized pain and swelling in a very small number of subjects. The one randomized trial by Gomoll et al. for knee OA has adequate power with an adequate follow-up period. This study suggested that AM may be superior to hyaluronic acid injection. It is important to note that most of these studies were funded by manufacturers of AM, and no studies have been published on AM allograft injections for ankle or foot OA.
Nerve Entrapment or Injury
Several in vivo studies have demonstrated AM’s anti-inflammatory and antifibrotic properties when used for peripheral nerve pathology.53-55 Autologous nerve grafts are a standard surgical technique for partial or complete loss of nerve function.54 Postoperative perineural scar formation and undesired adhesion can result in painful dysesthesias with progressive functional loss of the nerve.53 AM allografts’ anti-inflammatory and anti-scarring properties make AM allografts a promising candidate for nerve wrapping, but studies in human subjects remain limited. While the application of AM nerve allografts has not been reported in the foot and ankle literature, this application has been studied in the upper extremity. Gaspar et al. treated entrapped radial nerve sensory branch (n = 2) and cubital tunnel syndrome (n = 8) using revision neurolysis with AM nerve wrapping and demonstrated improvements in subjective pain and functional scores (Table 1).35,56
|Pathology||Study type||Allograft type||Duration||Summary|
|Zelen et al.41||Plantar fasciitis||Prospective, randomized (N = 45)||mdHACM a(AmnioFix Injectable, MiMedx)||8 weeks||AOFAS and pain scores improved with the treatment within 1 week of treatment compared to saline injections. No adverse events related to the treatments were observed.|
|Hanselman et al.42||Plantar fasciitis||Prospective, randomized (N = 23)||mdHACM (Clarix FLO, Medcore Biologix)||12 weeks||No difference between cryopreserved AM and corticosteroid injections. No adverse events related to the treatments were observed.|
|Werber43||Plantar fasciitis and Achilles tendonitis||Case series (N = 44)||Cryopreserved human AM-AF (PalinGen SportFLOW, Amnio Technology)||12 weeks||Pain scores improved 4 weeks after the treatment with AM-AF. No adverse events related to the treatments were observed.|
|Lullove65||Various tendinopathies||Retrospective case series (N = 10)||Human placental tissue b(PX50, Human Regenerative Technologies LLC)||6 weeks||By the 5th week, all 10 patients had resolved their pain.|
|Gellhorn and Han45||Various tendon and joint pathologies||Case series (N = 40)||mdHACM a(AmnioFix Injectable, MiMedx)||12 weeks||Pain scores improved 4 weeks after the treatment. No serious adverse events related to the treatments were observed.|
|Cazzell et al.37||Plantar fasciitis||Prospective, randomized (N = 145)||mdHACM a(AmnioFix Injectable, MiMedx)||12 weeks||FFI-R and pain scores improved 3 months after the treatment compared to placebo. No adverse events related to the treatments were observed.|
|Spector et al.46||Achilles tendonitis||Retrospective case series (N = 32)||mdHACM a(AmnioFix Injectable, MiMedx)||4 weeks||All patients reported either complete symptom resolution or partial symptom improvement after the treatment. No adverse events related to the treatments were observed.|
|Chin and LaViolette66||Achilles tendonitis||Retrospective case series (N = 10)||mdHACM (Clarix FLO, Medcore Biologix)||12 months||Pain scores improved 4 weeks after the treatment. No adverse events related to the treatments were observed.|
aNo longer available in the market; bFormulation not clear in the publication or on the manufacturer’s website
APPLICATIONS OUTSIDE OF ORTHOPEDIC PRACTICE
Based on the number of clinical trials and publications, orthopedics is the field in which AM is the second most used. AM tissue has been most used in the field of dermatology and wound care.57 AM allograft, when applied to wound tissue, has been found to promote collagen secretion and angiogenesis.58,59 Other fields that use AM include ophthalmology, urology, neurology, dentistry, otolaryngology, and hematology/oncology.57
SAFETY OF AM ALLOGRAFTS
Concerns over the safety of AM-derived products include a lack of standardization in both the content and administration of these products.60 The content of commercially available AM grafts has not been well studied, and equivalency cannot be assumed among the different commercial manufacturers.25 The level of growth factors has been shown to be lower in commercially available AM-derived products than in unprocessed AF.25 AM products represent varying mixtures of different tissue components, and there appears to be no unified approach to preparation or storage.12 In addition, some of the products used in many of the clinical studies (i.e., AmnioFix Injectables) are no longer commercially available, and thus, clinicians need to be cautious about making generalized conclusions from the published studies.
There are limited human studies assessing the safety of AM-derived products for musculoskeletal indications. AM tissue is characterized by its low immunogenicity, which theoretically improves its safety profile. During gestation, AM tissues arise from the inner cell mass and should theoretically be recognized as foreign tissue in the maternal environment. However, the maternal immune system demonstrates tolerance for AM tissue.5 The immunomodulatory properties of AM appear to be due to the lack of major histocompatible antigens (human leukocyte antigens A, B, C, or DR) on the surface epithelial cells.61,62 Current literature suggests that acute rejection does not occur even when used for severely immunocompromised patients.43 In another study, when a monolayer of human AEC was transplanted into healthy volunteers, there was no evidence of acute rejections.63
Despite the privileged immune status of AM-derived cells and existing literature, there is some concern that commercially available products may retain either viable or fractured cells, which may result in an immune cascade of unknown consequence.27,60 Further high-quality supportive data on safety are necessary to address these concerns.
In 2017, the United States FDA published a framework for regulating regenerative medicine products under the authority of the Public Health Service Act (PHSA).64 These regulations govern human cells, tissues, and cellular and tissue-based products (HCT/Ps), defined as “articles containing or consisting of human cells or tissues that are intended for implantation, transplantation, infusion, or transfer into a human recipient.”64 HCT/P can be allografts (grafts taken from a donor, i.e., placenta-derived products) or autologous grafts taken from the patient’s own body (i.e., autologous whole blood, platelet-rich plasma, bone marrow concentrate, and adipose-derived grafts).
The regulations set forth in the Code of Federal Regulations, Title 21, Part 1271 regulate whether the HCT/P product is classified and regulated as a biologic drug (Section 351 of the PHSA) or whether the product can be used without premarket approval from the FDA (Section 361 of the PHSA).64 Products that are deemed a “361 HCT/P” must satisfy the following four criteria set forth by the FDA and can avoid the costly and time-consuming process of an FDA review and licensure. These criteria include: (1) minimal manipulation; (2) homologous use; (3) not combined with drugs or devices; (4) and not reliant on cell metabolic activity as a primary function.65 If the product does not meet all these criteria, the product is deemed a “351 HCT/P,” and these products constitute a “biologic product” that requires FDA review and licensure under the PHSA.66
Currently, there are no 351 HCT/P placenta-based biological drugs that have been approved by the FDA in the United States. AM-derived products that are flowable or micronized formulations are considered beyond minimal manipulation and, as of 31st May 2021, require FDA review under Section 351. These 351 HCT/Ps are now required to file for an Investigational New Drug (IND) application and conduct phase 1–2 clinical trials. At this time, no flowable amniotic product, umbilical cord blood, Wharton jelly, or exosomes should be administered to a patient that is not enrolled in an IND study with the physician acting as a listed investigator on the study. AM-derived allograft products that are minimally manipulated and used for homologous use (i.e., “repair, reconstruction, replacement, or supplementation of a recipient’s cells or tissues via implantation, transplantation, infusion, or transfer into a human recipient”) are regulated as 361 HCT/P products. These products include sheet amnion, sheet chorion, and sheet amnion/chorion products.
The regulatory status of individual AM-derived products may change as companies pursue approval for various biologic products through the Center for Biologics Evaluation and Research. Clinicians in the United States should be aware of the regulatory environment and the status of any AM-derived product before using the product in clinical practice. Furthermore, as AM remains an investigational drug, it is difficult to comment on its cost implications at this time. The direct cost of AM allografts is typically not covered by insurance.
Amniotic membrane (AM)-derived products have shown promise in the field of orthopedics and sports medicine, although the literature remains limited. The existing studies suggest that the efficacy of AM-derived products is at least equivocal to corticosteroid injections in the short-term and demonstrated superiority to placebo injections for plantar fasciitis in long-term follow-up (i.e., 1 year). Fewer studies have explored the application of AM-derived products in Achilles tendinitis, ankle and foot OA, or peripheral nerve pathology. Additional high-quality supportive data is needed to support the efficacy and safety of AM-derived products in foot and ankle pathology. Despite promising preclinical data and clinical interest in AM-derived products, regulatory concerns in the United States may limit the clinical application of AM-derived products. Currently, no commercially available AM products have been approved by the FDA as a “biologic drug,” and clinicians should be aware of the regulatory status of any AM-derived product before use in clinical practice.
Hirotaka Nakagawa https://orcid.org/0000-0002-7491-4986
Soheil Ashkani-Esfahani https://orcid.org/0000-0003-2299-6278
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