Our results support UGIs with high molecular weight HA as a treatment for the early stages of hip OA. HA is a major component of synovial fluid and provides lubrication and shock absorption in joints. In OA there has been shown to be a decrease in HA molecular weight and concentration in the synovial fluid as the disease progresses, leading to a reduction in viscoelastic properties [
28]. Hence, there is potential for symptomatic improvement by supplementing HA with better viscoelastic properties. Hyaluronateis composed of alternating N-acetyl-d-glucosamine and d-glucuronic acid residues attached by β(1–4) and β(1–3) bonds with molecular mass ranging from 6500 to 10,900 kDa [
18]. Its rheological characteristics involved in the main function of synovial fluid to serve as a lubricant, scavenger for free radicals, and for the regulation of cellular activities such as binding of proteins [
19]. During the progression of OA, the endogenous HA in the joint is depolymerized from being of a high molecular weight (6500–10,900 kDa) into a lower molecular weight (2700–4500 kDa), which consequently diminishes the mechanical and viscoelastic properties of the synovial fluid in the affected joint [
28,
29]. Thus, exogenous HA injections have been clinically used to mitigate the macerated functions of the depolymerized endogenous HA of OA patients [
28], although the exogenous HA does not restore and replace the full properties and activities of the depolymerized endogenous HA of the synovial fluid but it may induce satisfactory pain relief via several mechanisms [
28]. These mechanisms include synthesis of proteoglycan and/or glycosaminoglycan, anti-inflammatory effect, and viscoelasticity maintenance [
28]. Nevertheless, there is a clear heterogeneity in the therapeutic trajectory for OA patients following HA injections. As some studies reporting an overall beneficial effect while others report that there is only a small benefit [
21]. One of the potential reasons for the variable effect of HA treatments on OA patients is levels of hyaluronidases in a patient’s synovial fluid. Hyaluronidases are a family of enzymes that degrade hyaluronic acid through cleaving the β(1–4) linkages of HA, fracturing the large molecule into smaller pieces before degrading it [
30]. HA is being administrated into OA patients via two main ways either oral administration or local injection [
31,
32]. Several preparations of injectable HA used for clinical use include Synvisc® and Synvisc-One® (Genzyme); Gel-One® (Zimmer); Hyalubrix® (Fidia); Supartz FX™(Bioventus); Orthovisc® (Anika); Euflexxa®, previously named Nuflexxa (Savient); Monovisc® (Anika Therapeu-tics); and Gel-Syn™ (Institut Biochimique SA) [
33]. Each product differs in many characteristics, including source (animal versus bacterial biofermentation using modified organisms), mean molecular weight ranging from (500 to 6000 kDa), distribution of molecular weight, molecular structure (linear, cross-linked or both), method of crosslinking, concentration (0.8–30 mg/mL), volume of injection (0.5–6.0 mL), and posology [
34], although animal source of HA (rooster combs) was considered as a traditional source for many years. The alternative ways of producing hyaluronic acid currently in use, such as bio-fermentation using genetically modified organisms, are the result of many investigations that have been carried out over time. This modified bacterial source is currently used as the main source as it’s associated with lower costs and less side effects [
35,
36]. For oral HA treatment, the body absorbs the high molecular weight polymer as a decomposed 2–6 membered polysaccharide [
37]. One proposed mechanism of action shows that ingested HA binds to toll-like receptor-4 and promotes the expressions of interleukin-10 and cytokine signaling, which both lead to anti-inflammation of arthritis [
38]. A systematic review of 13 reports on oral HA clinical trials, Oe et al. found that patients that were on a highly pure HA regiment reported a beneficial effect on knee pain compared to placebo[
31]. In terms of safety, it has been shown that on a 12 month study of 30 patients taking an oral HA regiment, no statistically significant negative side effects were seen [
39]. As stated before, locally injected HA is distinct in many different characteristics. The most fundamental change is the molecular weight of the HA in the injection, and it was shown that there is no significant difference in the long term outcome regardless the preparation [
40]. Unlike oral treatment, the complete HA molecule is introduced to the synovial fluid of the affected joint, providing a variety of different mechanisms for symptom relief [
41] .These include enhancing the synthesis of extracellular matrix proteins, altering inflammatory mediators in order to shift away from degradation, reducing the motility of lymphocytes, and maintaining cartilage thickness, area and surface smoothness [
32]. However, it must be stated that these are not the only proposed mechanism of actions for locally injected HA and further research trials needs to be performed in order to fully investigate the physiological effects of the treatment. Based on the study of 76 trials by Bellamy et al. locally injected HA treatment is an effective treatment for OA based on its effects of patients pain, function and patient global assessment [
42]. In terms of safety, it has also been shown that there is no statistically significant negative side effects in patients receiving injection treatment [43]. Studies have shown that both local injections and oral supplementation of HA can combat OA symptoms, especially with those with early osteoarthritis [44]. Interestingly, Panuccio et al. showed that if these two types of treatments are combined, the oral supplementation of HA can extend the benefits of the injection treatments [45]. Thus, patients would not have to visit hospitals and receive the sometimes uncomfortable injections as often [45]. Further randomized clinical trials are required to be designed in order to determine the exact outcomes of combined treatment. Many studies on OA of the hip indicated that the intra-articular use of HA products may be a relevant option in the management of patients suffering from hip OA with persistent pain, who do not respond to conventional analgesic or pharmacological treatment alternatives [
29,
30]. However, the evidence suggesting improvements with these injections is weak [
31] and controversial results can be found in literature on the clinical benefit of HA injections [
28,
32,
33]. Indeed, Holen et al. in a systematic review observed a benefit of intra-articular HA inside joints including hip but concluded that the benefit was not better than placebo [
33]. The effectiveness of the procedure can be influenced by several factors. Therefore, it is important to carefully select the patients who will be offered this treatment to increase the success rate. In 2017, Eymard et al., [
30] on behalf of the Osteoarthritis Group of the French Society of Rheumatology and the French Research Group in Interventional Rheumatology, published their results from a multi-center, open-label, prospective, trial. They evaluated on the subset of hip OA patients that benefited the most by a single intraarticular injection of a cross-linked HA combined with mannitol on day 90. Patients with moderate pain, moderate disability, moderate joint space narrowing, superomedial and axial femoral head migration, with femoral-acetabular impingement or coxa profunda displayed better results. They reported a reduction of pain in 50% of their patients at day 90. Benefits in similar subset of patients have been reported by other authors, as well. Pogliacomi et al. [
34] in their study published in 2018 reported the efficacy of intraarticular single dose of high-weight HA in patients under a follow-up of 12 months. They concluded that patients with a moderate grade of OA are the ones that benefit the most from the said injection in according also with our study.
In 15.3% of the patients, symptoms persisted despite the injections and THA was needed. Retrospective analysis of THA rates in a large series on hip OA with HA injection suggested that injections are useful to postpone arthroplasty [
38]. However, HA does not seem effective in severe hip OA. In advanced stages, the treatment could be considered only in patients who refuse a THA. These patients must be informed that the clinical benefit could be poor. Moreover, in the event of a lack of clinical benefit from injections, THA must be delayed for a minimum of 3 months due to the reported increased risk of periprosthetic joint infection in the preceding period, as different authors suggested [
39,
40].More generally, the effectiveness of this procedure is still debated for effective and durable benefits. To clarify some aspects of viscosupplementation treatment, a review of the literature confirmed that IA HA is an effective treatment for mild to moderate osteoarthritis but it is not an alternative to surgery in advanced cartilage degeneration [
41]. Another important aspect is the technique that you can use to perform injection. Giordano et al. concluded that use of an automated delivery system for US-guided intra-articular hip injections did not show superior efficiency or patient comfort over traditional ultrasound guided syringe injections [
42]. In our daily practice, we do not use these systems both because the US guidance alone is deemed sufficient for the success of the procedure and because there is more margin of maneuver of the probe and the needle independent of each other.
This study has some limitations. The absence of a control group did not allow us to compare the efficacy of UGIs with HA with other products, including steroids, platelet-rich plasma and other orthobiologics. Although we monitored which patients needed to undergo THA, the follow-up was not long enough to draw conclusions on the medium- and long-term efficacy of this treatment or whether repeated treatment over time would have the same efficacy. We did not include instrumental investigations to monitor the radiographic progression of the arthrosis or an MRI in the younger patients to assess the effect on the chondropathy. Therefore, conclusions are limited to the clinical side only.