Hyaluronic acid (HA) is a high viscosity polysaccharide naturally produced by B cells in the synovial membrane. It may also be manufactured from animal sources (typically avian) or bacteria in laboratory (biological) fermentation. Biochemically, it is part of the glycosaminoglycan group[10], acting as a salt under physiological conditions. It is also called sodium hyaluronate, or hyaluronan[11]. Intra-articular hyaluronan preparations have been shown to relieve osteoarthritis-derived pain more than conventional treatments such as physical therapy, exercise, nonsteroidal anti-inflammatory drugs (NSAIDs), intra-articular corticosteroids or even arthroscopic lavage[12].

The physicochemical properties of HA are determined by molecular mass and spatial conformation. It can also be classified as high, medium or low molecular weight. Normal (non-osteoarthritic) autologous HA in joints weighs around 5 to 7 million Daltons, whereas osteoarthritic joint HA weighs an average of 1 million Daltons. High molecular weight HA molecules intertwine to form a high viscosity solution, which serves both as a lubricant and shock absorber, and has biomolecular properties compatible with and favorable to cell growth[13].

HA is the major non-protein hydrodynamic component of synovial fluid. This substance, biological or synthetic, forms a layer around the cells where it interacts with pro-inflammatory mediators and binds to cellular receptors, modulating cell proliferation and migration, as well as gene expression[14]. It is a collagen stimulator, capable of promoting tissue recovery and maintaining cellular integrity[15]. These properties make HA useful for healing or even regeneration of tendon and chondral tissues in some cases.

Intra-articular (IA) injection of HA is considered a local treatment with no reports of systemic adverse events observed after the administration of other types of IA injections, such as corticosteroids, or even with oral administration of NSAIDs[16]. This therapy represents an alternative treatment, especially for patients with comorbidities.

This alternative approach has shown beneficial effects in vitro. It is suggested that the extracellular matrix (ECM) has an influence on cell metabolism, especially on osteoarthritic subchondral bone osteoblasts. HA can reverse its abnormal synthetic activity[17].

An IA injection of HA has been shown to reduce chondrocyte apoptosis whilst increasing its proliferation[18]. This therapy has been shown to promote chondroprotection due to the HA binding to CD44 receptors. This event inhibits the expression of IL-1β, which leads to a decrease in the biosynthesis of MMP-1, 2, 3, 9, and 13[19-21]. This process avoids catabolic enzyme activity within the joint cartilage[22]. It is our experience, in human patient treatment, that it is important to use medium or high molecular weight HA, similar to the molecular weight that is normally produced in the body. Also, it is important to use HA derived from biological synthesis, to avoid undesired side effects. Another advantage of HA is the possibility of its combination with other products, biological or not. For example, the combination of calcitonin[23], sorbitol[24], platelet-rich plasma (PRP)[25,17] and/or bone marrow aspirate concentrate (BMAC)[26,27] have been postulated. Clinical trials reporting the use of HA for hip disorders are listed in Table 1.

PRP is an autologous non-immunogenic therapy, produced through the concentration of platelets in a small plasma volume (typically 3-6 times above baseline)[28]. PRP therapy has advantages such as rapid preparation and technical simplicity; it is also a point-of-care procedure, and can be carried out in-office due to its minimal invasiveness, permitting intra-articular, intra-tendinous or even intra-osseous injection. As a result of its autologous origin, it exhibits a unique safety profile, lacking many drug-related side effects or interactions[29]. In osteoarthritis (OA), PRP can interfere in the catabolic and inflammatory cascade, in order to promote anabolic responses. The bioactive molecules in PRP can act in inflammatory modulation, ECM synthesis and vessel remodeling in order to improve the natural healing of the tissue[30,31]. One of the greatest problems regarding PRP is the lack of standardization. Important variables such as the number of platelets, buffy-coat content, exogenous activation and number of centrifugations are lacking in many publications, resulting in heterogeneous results, which therefore cannot be compared. In order to minimize this problem, some PRP classifications have been proposed. Recently, Lana et al[32] published a thoughtful classification incorporating all these variables. Besides the aforementioned variables, light activation, image guidance method and red blood cell content were included. All these parameters are important for a standardization protocol.

PRP was used alongside hip arthroscopy surgery for a variety of pathologies[33]. The technique consists of the administration of 4.5 mL of PRP into the repaired hip joint capsule in the peripheral compartment through the arthroscopic cannula, as well as 10 mL of PRP in the surrounding soft tissues as suggested by Marc Philippon and Robert Laprade’s group. Growth factors present in PRP in addition to the aforementioned healing potential also aid in postoperative hemostasis[33]. Regarding the use of leukocytes, it has been reported that leukocyte-rich PRP is effective in the initiation of a proinflammatory healing response to stimulate vascular tissue regrowth[34]. However, for the treatment of hypovascular tissues, it was reported that it may induce profibrotic tissue formation, which reduces the quality of the tissue itself[33]. PRP can also be used in combination with different orthobiologics, such as HA and bone marrow preparations[23-27]. Also, even if PRP does not lead to cartilage regeneration, it may still offer symptomatic and functional benefits via modulation of inflammation and direct analgesia[29]. The description of clinical trials for hip disorders is summarized in Table 1. PRP has also been compared to surgery, regarding gluteus medium tendonitis in a recent meta-analysis[35] and has been deemed effective and safer than endoscopic surgical procedures. In addition, in another recent meta-analysis, PRP was compared to HA and steroid injections and was considered to achieve the highest rank for pain relief at 6 mo in patients suffering from OA[36].

In our clinical practice, after a standardization protocol for PRP therapy, we opted for a handmade PRP preparation, with double centrifugation, enriched with buffy-coat (i.e., leukocyte-rich PRP) using the modified protocol of Amable et al[37] and Huber et al[38]. With this technique, we obtained a 3-5-fold increase in platelet concentration and a 2-4 fold increase in white blood cells from baseline (therefore leukocyte rich).

Bone marrow has been studied in the context of regeneration for many years. It encompasses erythroblasts, neutrophils, eosinophils, basophils, mononuclear cells [monocytes and mesenchymal stem cells (MSCs)], lymphocytes, megakaryocytes, and plasma. The erythroblasts, eosinophils and neutrophils are present in various stages of differentiation[39]. Especially relevant are the bone marrow-derived mesenchymal stem cells (BM-MSCs), hematopoietic stem cells (HSCs) and other progenitors. The use of BM-MSCs has demonstrated benefits in the regeneration process. The HSCs are thought to be the true drivers for enhancing cartilage and bone regeneration, with an important role in the direct conversion to stromal MSCs and orchestration of the bone formation[40] as described by Omsted-Davis in 2003, who pointed out that a side population of marrow stem cells could regenerate the hematopoietic compartment of lethally irradiated mice when transplanted and could also differentiate to osteoblasts through a mesenchymal intermediate[41]. This effect was also demonstrated clinically in the study by Marx et al[42] in 2014 where they devised a study with 40 adults undergoing craniomandibular reconstruction. Those patients attaining higher CD34+ counts registered a 100% regeneration of implantable bone, as opposed to 40% of this effect, when CD34+ counts were much lower[42]. Pettine also demonstrated improved results in a study on intradiscal injection of bone marrow concentrate (BMC) with a three-year follow-up, and found that HSC (CD34+) also played an immunomodulatory role similar to MSCs and reported that patients who received greater concentrations of progenitor cells [colony forming unit-fibroblast, (CFU-F), and CD34+ lineage] experienced faster and greater pain reduction. The authors also claimed the following: ”This is the first study to link a clinical improvement to CFU-F and CD34+ cell concentrations in BMC[43]”.

MSCs were first discovered in the bone marrow, presenting an important paracrine effect. MSCs thus exhibit a secretory function, comprising anti-inflammatory, pro-angiogenic, immunomodulatory, anti-apoptotic, anti-fibrotic and wound healing properties, including a proliferative role[44]. MSCs represent 0.01% to 0.001% of total bone marrow nucleated cells. Due to this low number, there is a trend for cellular expansion in order to obtain higher cell counts for clinical applications[45]. However, the use of non-cultured cells presents some advantages that go beyond cost and good manufacturing practice and regulation. One of the major advantages is that the direct extraction allows the maintenance of its regenerative capacity and avoids senescence[44]. Expansion techniques risk cells with variable differentiation capacity, increased senescence markers and tumor degeneration liability. Also, these cells must be cultured for two or three weeks – a slow and expensive process that in clinical practice demands a two-staged tissue implantation surgery. One important issue is that the use of pure, cultured MSCs do not contain HSCs, cytokines and growth factors as concentrated bone marrow does[46]. The outcomes of concentrated vs cultured cells should be assessed more carefully, taking into account the costs, time expenditure and clinical practice suitability[46].

Previous studies showed variables that could influence the number of MSCs, such as anatomic harvest site, aspirated volume, age and sex of the patient. Traditionally, MSCs could be predictably obtained from the iliac crest[47]. It is also regarded that the aspiration of larger volumes decreases the concentration of osteoblast progenitors due to the dilution of bone marrow with peripheral blood[48]. Regarding the anatomic site of harvesting, Pierini et al[49] published a study comparing bone marrow collection from both the anterior and posterior iliac crest of twenty-two patients. The yield of colony-forming connective-tissue progenitors was 1.6 times greater in the posterior compared to the anterior iliac crest. In relation to the volume aspirated, Hernigou et al[50] published a study evaluating the aspiration of bone marrow from iliac crest using 10 mL and 50 mL syringes, and compared MSCs and progenitor cell content. It was verified that the aspirate performed with a 10 mL syringe had a greater concentration of MSCs and 300% more progenitor cells in comparison with the 50 mL syringe aspiration.

In order to increase the relative number of MSCs per volume of injectate for therapeutical purposes, the aspirate of bone marrow can be concentrated, thus the term BMAC.

BMAC is the concentration of whole bone marrow aspirate that can be manufactured in different ways in order to concentrate nucleated cells. Common techniques used to achieve this concentration are the use of Ficoll density gradients, closed centrifugation systems and in-house preparations[51]. It is believed that increased cell concentration would also enhance the amount of MSCs, which could then provide local microenvironment improvement in order to achieve healing and regeneration[52,53]. Potentially, these MSCs are able to provide a direct cell source for repair. In addition, the nucleated cells may have a significant paracrine effect, releasing and delivering a myriad of cytokines and growth factors to orchestrate tissue repair processes[54,55]. In comparison to PRP, there is a significant variation in the final products achieved. Fortier et al[56] evaluated the constituents of PRP and BMAC, showing a reduction in platelet content and an increase in white blood cell content in BMAC. The differences between these products could represent a different mechanism of action[56]. Ziegler also performed a comparison between these products and concluded that BMAC had a significantly higher IL-1 receptor antagonist and therefore offers a more relevant source of anti-inflammatory therapy for OA[57].

The majority of studies have focused on BMAC for osteonecrosis of the femoral head (ONFH). Hernigou et al[58] described a technique for injecting BMAC as an adjunct to core decompression (CD). The beneficial effect is attributed not only to disruption of the necrotic zone, but also to the introduction of osteogenic progenitor cells into the femoral head. This study evaluated 189 decompressed hips using BMAC harvested from the iliac crest. The results showed that patients in early stages (Steinberg I-II) had excellent outcomes at 5 years, with only 6.2% of the hips requiring total hip arthroplasty. On the other hand, patients in advanced stages (Steinberg III-IV) had poor outcomes, where 60% required total hip arthroplasty (THA). In addition, it was reported that superior outcomes were verified in patients that presented increased numbers of progenitor cells, suggesting a relationship between the clinical outcomes and their count[58]. Similar results were obtained in the study by Einhorn et al[59], aimed at patients with symptomatic ONFH stages I-II. The researchers observed minimization of pain, preservation of joint function, and obviation or at least delayed need for THA within 2 years of follow-up. In this study 62%-75% of CD-BMAC recipients successfully avoided the need for additional interventions, including THA.

The use of BMAC has been suggested to restore joint harmony and minimize further chondral deterioration. One advantage of this orthobiologic is that MSCs are ideal for the chondrolabral junction, since these cells are able to differentiate into both fibrocartilage and hyaline-like tissue products[60]. Clinical trials registered in the literature demonstrate the efficacy and safety of BMAC in treating arthritic cartilage symptoms and focal chondral lesions[61,62]. Additionally, significant cartilage growth, for instance, was also demonstrated with the administration of BMAC in association with other treatment modalities such as microfracture and biologic scaffolds[63]. The use of BMAC for femoroacetabular impingement and OA of the hip is scarce. In a case report of a professional soccer player, after two arthroscopies, the patient presented remitting hip pain 3 mo after the second surgery. He was treated with 3 PRP and 2 BMAC injections and was able to return to full activities[64]. Mardones et al[45] published a technique for the treatment of chondral hip lesions using BMAC and PRP clot. In this technique, hip arthroscopy was performed and microfracture was carried out for full thickness chondral lesions. Bone marrow was harvested and processed, as well as PRP, which was exogenously activated in order to obtain a clot. To finalize the arthroscopic procedure, the fibrin clot was placed over the microfracture site using a cannula. BMAC was inserted under the PRP clot. This technique was used in 13 patients. As a result, the authors reported improvement in the symptoms for a mean of 8 mo of follow-up[45]. Table 1 shows clinical results of the use of bone marrow derivatives for hip disorders.

The results of BMAC in bone, cartilage and tendon injuries are encouraging. We believe that cell characterization is necessary for outcome optimization, and larger randomized controlled trials are highly warranted in order to further support these results. On the other hand, the use of BMAC especially in the early stages of hip diseases resulted in positive results for pain and function, and could represent a promising contemporary treatment strategy.