Intra-articular injection of umbilical cord–derived mesenchymal stem cells is safe and effective for moderate to severe knee osteoarthritis with synovitis: a double‑blinded and randomized controlled trial

Abstract

Objectives

To evaluate the safety and clinical efficacy of intra-articular injection of human umbilical cord–derived mesenchymal stem cells (UC-MSCs) for knee osteoarthritis (KOA).

Methods

This was a randomized, double-blind, placebo-control trial consisting of 3 arms including placebo, hyaluronic acid (HA), and UC-MSCs. 69 individuals were screened, with 55 participants subsequently randomized. The primary endpoint was the safety after injection. The secondary endpoints were improvements in visual analog scale (VAS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), Knee Society Clinical Rating System (KSS), EuroQol five-dimensional questionnaire (EQ-5D), and magnetic resonance imaging (MRI) examinations at 6-month after injection.

Results

Adverse events (AEs) did not demonstrate significant differences among the groups. No serious treatment-related AEs were observed. Compared with placebo and HA, injection of UC-MSCs resulted in a greater improvement in VAS and EQ-5D; however, the differences were not statistically significant. Patients in the UC-MSCs group exhibited significant improvement in synovitis at the 6-month follow-up as compared with the placebo and HA groups (P = 0.003). At 6 months, UC-MSCs injection showed significantly better improvements in VAS (P = 0.011), WOMAC (P = 0.014), and EQ-5D scores (P = 0.049) in patients with KOA incorporating synovitis, particularly those experiencing moderate to severe pain. MRI indicated no significant difference in change of joint structures among the groups, except for the synovium.

Conclusions

Treatment with UC-MSCs was shown to be a viable therapeutic option for KOA combined with synovitis, showing clinical improvement at the end of follow-up, especially in those with moderate to severe pain. Further phase III clinical trials would be required to confirm the efficacy.

Trial registration

The trial was registered at chictr.org on October 13, 2020, with the registration number ChiCTR2000039017.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-025-09440-y.

Introduction

Knee osteoarthritis (KOA) is a common joint disorder that affects around 650 million individuals worldwide aged over 40 years in 2020 and its occurrence is rapidly increasing due to an aging society [1]. As KOA is a chronic degenerative condition, it ultimately leads to persistent knee pain, deformity, and economic burden for patients. However, despite decades of research, no disease-modifying treatments are available for KOA. Hence, patients with refractory symptoms and advanced stages of OA eventually resort to joint replacement surgery. However, a prior study revealed that despite improvements in functional outcomes following knee arthroplasty, the rate of sports participation among patients actually declined [2]. In this regard, the development of effective and feasible disease-modifying treatments for KOA is recognized as a medical priority.

Recently, mesenchymal stromal cells (MSCs)-based therapies have been considered as an effective disease-modifying treatment for KOA in numerous preclinical [3] and clinical studies [4, 5], given its promising effects. Initially, it was believed that transplanting MSCs into cartilage defects could grow in situ and differentiate for repairing damaged tissue. However, recent research has found that the paracrine effects of MSCs may also play a crucial role in therapeutic efficacy, including the secretion of cytokines and exosomes to achieve anti-inflammatory, immune-modulatory, and chondroprotective effects [6]. In animal models of osteoarthritis, MSCs transplantation has shown the potential to prevent, halt, or even reverse cartilage degradation [7]. Moreover, MSCs obtained from different sources have been demonstrated to effectively suppress inflammatory responses and alleviate cartilage damage in these models, promoting the regeneration and repair of injured cartilage [8, 9].

Umbilical cord–derived MSCs (UC-MSCs) is an increasingly potent cell source suitable for allogeneic MSC-based therapy because of its high proliferation capacity, hypo-immunogenic, non-tumorigenic, noninvasively harvesting method, and relatively minor ethical issue [10]. Whereas adipose-derived MSCs (ADMSCs) and bone marrow-derived MSCs (BMMSCs) have been extensively researched for treating KOA [11, 12], UC-MSCs are a relatively novel avenue in this field. Recently, a systematic review and meta-analysis compared the efficacy of MSCs from different sources for the treatment of KOA. The researchers concluded that, considering factors such as pain relief, promotion of joint function recovery, and minimization of patient trauma, UC-MSCs should be prioritized, followed by ADMSCs, with BMMSCs being the least preferred option [13]. Our previous preclinical investigations have unveiled a novel role of UC-MSCs in preserving articular cartilage anabolism, especially superficial zone chondrocytes, and alleviating synovitis during OA progression [14]. Recently, a systematic review has demonstrated significant pain reduction and functional enhancement following intra-articular UC-MSCs injections for KOA patients, with apparent safety profiles [15]. However, the majority of these studies included in this review were case series [16–18], highlighting the necessity for additional high-quality randomized trials to better ascertain the efficacy of UC-MSCs in KOA treatment.

Therefore, we performed a clinical trial through a randomized, double-blinded, placebo-controlled study to evaluate the safety and efficacy of UC-MSCs in the treatment of KOA. The study hypothesis was that individuals receiving an intra-articular injection of UC-MSCs would demonstrate safe and significantly superior pain relief and functional improvement compared to both the placebo group and the HA group.

Materials and methods

Study design

The present study was performed between January 2021 and Jun 2022 in Wuhan Union Hospital. The protocol was registered at chictr.org (ChiCTR2000039017) and was approved by the Institutional Review Board in accordance with the principles of the Declaration of Helsinki prior to patient recruitment (Approval No. 2019SC02). It was designed as a randomized, double-blinded, placebo-controlled study to assess the safety and efficacy of intra-articular injection of UC-MSCs in patients with KOA.

Patient

The target population included individuals aged 40 to 75 years who were willing to participate in this study and meeting the following inclusion criteria: unilateral KOA (Kellgren-Lawrence grade 1–3) per the criteria of the Chinese College of Rheumatology, with disease course of more than 6 months and ineffective physical basic treatment. Patients were excluded if they had other serious bone and joint diseases; severe axial deviation defined by valgus/varus (10°), meniscus III ° injury, cruciate ligament or collateral ligament injury, arthroscopy or intra-articular steroids or hyaluronic acid in the past 1 years, local or systemic infection, ipsilateral hip or ankle pain, hypersensitivity to any component used in the study, chronic treatment with immunosuppressive or anticoagulant drugs, previous malignancy, or body mass index (BMI) ≥ 30 kg/m². Those who were judged by the investigator to have the following conditions that might interfere with the clinical trial and interpretation of the results were also excluded, such as excessive tension, sensitivity, or cognitive impairment. All enrolled patients provided written informed consent.

UC-MSCs isolation and culture

The use of human umbilical cord tissue in this study was supported by the Institutional Review Board of Wuhan University People’s Hospital (Approval No. WDRY2019-G001), and informed consent was obtained from the donor. UC-MSCs were isolated, expanded, and identified using the methods described in our previous work to meet the characteristics of MSCs [19, 20]. The more detailed information on the characterization of the UC-MSCs was showed in Fig. 1. In a sterile petri dish, the umbilical cord was sliced into approximately 1.0 mm small pieces. The tissue fragments were then rinsed with PBS and centrifuged at 1900 r/min for 6 min. The umbilical cord fragments were cultured in serum-free medium (NC0103, Yocon, China) supplemented at 37 °C in a 5% carbon dioxide incubator. The primary cells were harvested upon reaching 90% confluency. The confluent culture was digested with 0.25% trypsin-EDTA (25200-072, Sigma, USA). When UC-MSCs reached 90% confluency, passage was performed. The fifth generation of UC-MSCs was used in this clinical experiment. UC-MSCs are manufactured in a clean environment following the requirements of current Good Manufacturing Practices (GMP). Cells were cultured until the number reached the clinical grade with an average number of 2 × 10⁷ cells per dose injected per patient. Prior to injection, UC-MSCs were tested for mycoplasma, endotoxin, microbial contamination, etc. The UC-MSCs used for injection were washed and suspended in 2.5 ml of 0.9% saline solution supplemented with 1% human serum albumin.

Fig. 1.

Identification of UC-MSCs. A UC-MSCs. B The potency of adipogenesis, osteogenesis, and chondrogenesis of P5 UC-MSCs were confirmed by Oil Red O, Alizarin Red, and Alcian Blue, respectively. C Specific surface markers of of P5 UC-MSCs were examined by flow cytometry. The UC-MSCs associated with markers were positive for CD90, CD105, CD73, and were negative for CD34, CD19, CD45 and HLA-DR

Intervention

The eligible participants were randomly assigned to one of three groups, receiving intra-articular knee injections of placebo, HA, or UC-MSCs at a 1:1:1 ratio, according to a computer-generated randomization schedule. Intra-articular injections were administered only once at baseline by a specialized orthopedic surgeon physician who was not involved in the subsequent assessment of the participants, and the procedure was performed under ultrasound guidance. Both the patient and the physician performing the assessment were blinded to the treatment assignment, ensuring double-blinding of the study. The UC-MSCs injections contained 2 × 10⁷ cells in 2.5 mL of saline with 1% human serum albumin, the HA injections contained 2.5mL of sodium hyaluronate (ARTZ Dispo, 25 mg/2.5mL) and the placebo injections contained an equal volume of stem cell resuspension without cells, i.e., 2.5 mL of saline with 1% human serum albumin. Participants were enrolled and followed for 24 weeks with 4 distinct study visits scheduled at 2, 6, 12, and 24 weeks after injection. These follow-up visits were conducted by another physician who was not involved in the trial procedures and remained blinded to the treatment assignment.

Outcomes

The primary outcome was the safety of UC-MSCs treatment, as assessed by physical examination, laboratory tests, electrocardiography, and adverse events (AEs). The AEs were recorded at each follow-up and described in terms of symptoms, incidence, severity, and relatedness according to the World Health Organization-Uppsala Monitoring Centre causality assessment system. The severity of AEs was classified into 5 levels according to National Cancer Institute-Common Terminology Criteria for Adverse Events (NCI-CTCAE).

The secondary outcome of the trial was efficacy, based on the validated clinical outcome scales and radiologic examination. Clinical evaluation included a 10-point Visual Analog Scale (VAS) for knee pain, Western Ontario and McMaster Universities Arthritis Index (WOMAC) and Knee Society Clinical Rating System (KSS) for knee function, and a five-level EuroQol five-dimensional questionnaire (EQ-5D-5 L) [21] for quality of life. WOMAC consists of 3 sub-scores, pain, with 5 items; stiffness, with 2 items; and functionality, with 17 items. EQ-5D-5 L ranges from − 0.391 (worst state) to 1 (best state), with 5 dimensions: mobility, self-care, usual activities, pain/discomfort, and anxiety/depression. Radiologic outcomes were measured with magnetic resonance imaging (MRI) according to the MRI Osteoarthritis Knee Score (MOAKS), which is a validated semi-quantitative tool to describe OA progression over time. The features assessed in this study were articular cartilage, synovitis, and bone marrow lesions (BMLs). All data were collected at intervals of baseline, 2, 6, 12, and 24 weeks follow-up by an orthopedic physician with no knowledge of the treatment group.

Sample size

This study represents a pilot randomized controlled trial (RCT) to evaluate treatment protocols for safety and efficacy prior to conducting a larger RCT and the sample size determination was based on the WOMAC scale. To date there is inadequate published data on the effects of UC-MSCs therapy in KOA, the estimated effective rate of UC-MSCs was 60%, according to a previously published study on MSCs in the treatment of KOA [22]. Using the sample size calculation method for comparing two proportions, and assuming a significance level of α = 0.05 and a test power of 1-β = 0.8, we calculated the required sample size based on the hypothesis testing formula, with a treatment difference δ = 0.45 between the UC-MSCs group and the HA group, and a 1:1 ratio in sample sizes. To account for potential losses to follow-up, a loss rate of 20% was assumed. Therefore, the estimated sample size was 18 participants per group, for a total of 54 participants.

Statistical analysis

The prespecified analyses were performed on the intent-to-treat (ITT). ITT refers to analyzing patients based on their assigned randomization groups, regardless of the treatment they actually received. The sample description included the frequency and percentages for categorical variables and mean with standard deviations (SD) for continuous variables. Demographics and baseline characteristics were analyzed by using one-way ANOVA, chi-square test, or Fisher´s exact test as appropriate to ensure comparability between treatments. One-way ordinal CMH Chi-square tests or Fisher’s exact test were used to compare differences in ordinal categorical variables among treatment groups at baseline and during follow-up. The Wilcoxon signed-rank test was used for within-group comparisons of changes from baseline for quantitative variables, and the Kruskal-Wallis test was used to test significant differences between treatments. The significance level was determined by a P value of < 0.05, and all statistical analyses were carried out using IBM SPSS Statistics version 21.0 (IBM Corp., Armonk, NY).

Results

Patient characteristics

69 patients were screened, of which 14 were excluded after screening, leaving 55 participants who were randomized into three groups (placebo group: 18, HA group: 19, UC-MSCs group: 18). One participant in the HA group voluntarily withdrew before intervention, resulting in 18 participants per group. During follow-up, 2 participants in the placebo group were lost to follow-up and 1 discontinued intervention (final n = 15); 1 in the HA group was lost to follow-up and 1 discontinued intervention (final n = 16); and 1 in the UC-MSCs group was lost to follow-up (final n = 17, Fig. 2). Patients in each group had similar demographic characteristics and clinical status (Table 1).

Table 1.

Baseline patient characteristics

Characteristics	Placebo group	HA group	UC-MSCs group	P value
No. of patients	18	18	18
Age, mean (SD)	55.89 (9.82)	54.44 (6.26)	55.22 (8.24)	0.869
Sex, n (%)				0.574
Male	8 (44.44)	7 (38.89)	5 (27.78)
Female	10 (55.56)	11 (61.11)	13 (72.22)
BMI, mean (SD)	24.10 (1.51)	24.72 (2.15)	23.29 (1.70)	0.071
Kellgren-Lawrence grade, n (%)
Grade 1	9 (50)	9 (50)	8 (44.4)	0.929
Grade 2	6 (33.3)	9 (50)	8 (44.4)	0.589
Grade 3	3 (16.7)	0 (0)	2 (11.1)	0.349
WOMAC, mean (SD)	59.7 (44.0)	48.2 (24.5)	45.1 (25.4)	0.372
VAS, mean (SD)	4.0 (1.9)	3.7 (1.2)	3.5 (1.5)	0.636
EQ-5D, mean (SD)	0.760 (0.22)	0.753 (0.15)	0.754 (0.12)	0.990
KSS, mean (SD)	91.2 (4.9)	88.1 (6.2)	89.9 (5.6)	0.243
Concomitant diseases				0.594
Yes	8 (44.4)	11 (61.1)	10 (55.6)
No	10 (55.6)	7 (38.9)	8 (44.4)

Data are presented as n (%) or mean ± SD

Abbreviations: BMI, body mass index; HA, hyaluronic acid; UC-MSCs, umbilical cord-derived mesenchymal stromal cells; WOMAC, Western Ontario and McMaster Universities Arthritis Index; VAS, visual analog scale; KSS: knee society clinical rating system

Fig. 2.

Patient flow diagram illustrating the number of subjects screened, randomized, allocated to treatment groups, and followed throughout the study period

Safety outcomes

No serious AEs, permanent disability, deaths, septic arthritis, or neoplasia cases were reported in this study. AEs were observed in 3 (18.8%) patients in the HA group and 5 (29.4%) patients in the UC-MSCs group. The most common AE related to intra-articular injection was pain without reaching statistical difference between groups. Injection-related pains were transient and responsive to rest or oral acetaminophen, and those completely disappeared within 2 days. One patient in the UC-MSCs group had a fever about 6 h after injection with a maximum body temperature of 37.8℃, and the body temperature returned to normal the next morning without taking antipyretics. All AEs were mild or moderate. No patients were discontinued from the trial due to AEs (Table 2).

Table 2.

Summary of adverse events

Adverse Events	Placebo group	HA group	UC-MSCs group	P value
Patients with AEs, n (%)a	0	3 (18.8)	5 (29.4)	0.091
Injection-related AE, n (%)	0	2 (12.5)	5 (29.4)	0.055
Pain	0	2 (12.5)	4 (23.5)	0.166
Fever	0	0	1 (5.9)	>0.05
Bleeding	0	0	0
Septic lesions	0	0	0
Allergic reaction	0	0	0
Patients with SAEsb	0	0	0
Injection-related SAEs	0	0	0
AEs by grade, n (%)c
Grade 1	0	3 (18.8)	4 (23.5)	0.165
Grade 2	0	0	1 (5.9)	>0.05
Grade 3	0	0	0
Grade 4	0	0	0
Grade 5	0	0	0

^aAEs were defined as undesired medical incidents in participants that do not necessarily have a cause-and-effect relationship with the treatment

^bSAEs were defined as undesired medical incidents that cause hospitalization, life-threatening, disability, congenital abnormality, or death

^cNCI-CTCAE scale

Abbreviations: AEs, advance events; SAEs, serious adverse events; HA, hyaluronic acid; UC-MSCs, umbilical cord-derived mesenchymal stromal cells

Clinical outcomes

VAS, WOMAC, KSS, and EQ-5D clinical scores were utilized to provide a comprehensive understanding of how patients perceived their progression at 6 months, but no significant differences were observed among the three groups after injection compared with baseline (Fig. 3). Interestingly, we screened patients with KOA combined with synovitis based on baseline MRI and found that only UC-MSCs group had significant improvements in synovitis from baseline at 24 weeks follow-up, in contrast to both the placebo and HA group (P = 0.325, P = 0.309, P = 0.003, for placebo, HA and UC-MSCs groups, respectively) (Table 3). Representative MRI images in the UC-MSCs group are shown in Fig. 4. At baseline, the synovium was markedly thickened and oedematous, and a large amount of synovial fluid was present, whereas the manifestation of synovitis gradually improved after injection of UC-MSCs.

Fig. 3.

Efficacy outcomes. A-D Comparison with baseline in each group. A VAS score. B KSS score. C WOMAC score. D EQ-5D score

Table 3.

Changes in synovitis in MRI after injection

	Placebo group	HA group	UC-MSCs group	P value
Baseline				0.643
I	8 (72.7)	6 (60)	7 (53.8)
II	3 (27.3)	4 (40)	4 (30.8)
III	0 (0)	0 ()	2 (15.4)
24 weeks				0.140
None	2 (18.2)	3 (30)	8 (61.5)
I	8 (72.7)	5 (50)	3 (23.1)
II	1 (9.1)	2 (20)	2 (15.4)
III	0 (0)	0 (0)	0 (0)
P value	0.325	0.309	0.003

Data are presented as n (%)

Abbreviations: HA, hyaluronic acid; UC-MSCs, umbilical cord-derived mesenchymal stromal cells

Fig. 4.

Representative knee MRI images from a patient in UC-MSCs group at baseline, 2, 6, 12, and 24 weeks. A, A′ Baseline sagittal and axial views showing marked joint effusion and severe synovitis. B–D, B′–D′ Sagittal and axial views at 2, 6, 12, and 24 weeks after UC-MSCs injection, demonstrating gradual resolution of joint effusion and synovial inflammation

Patients with synovitis in the placebo group showed no significant changes in VAS, KSS, WOMAC, and EQ-5D scores. In the HA group, the VAS and WOMAC scores increased transiently from baseline at 2 weeks and decreased gradually. The UC-MSCs group showed a larger reduction in VAS scores compared with the control group and HA group, but this difference was not statistically significant. Of note, the UC-MSCs group showed significant improvement in WOMAC score at final follow-up (P = 0.036). Moreover, a significant improvement in EQ-5D score was observed at 12 Scanzello CR, Goldring SR. The role of synovitis in osteoarthritis pathogenesis. Bone 2012;51(2):249-57. https://doi.org/10.1016/j.bone.2012.02.012 (Fig. 5).

Fig. 5.

Efficacy outcomes in patients with KOA and synovitis. A-D Comparison with baseline in each group. A VAS score. B KSS score. C WOMAC score. D EQ-5D score

Considering the interference of pain degree on treatment effect, we further screened individuals with VAS ≥ 4 at baseline from patients with KOA combined with synovitis for analysis. No statistically significant changes in VAS, WOMAC, KSS, and EQ-5D scores of the placebo group and HA group were detected from baseline to the last follow-up period. In the UC-MSCs group, the VAS for knee pain significantly decreased at 6 and 24 weeks compared with baseline. Furthermore, although patients in the UC-MSCs group did not perceive some improvement in WOMAC score initially, this perception was not sustained after long-term follow-up. Similarly, a statistically significant improvement in EQ-5D value was observed in patients receiving UC-MSCs at 24 weeks follow-up (Fig. 6).

Fig. 6.

Efficacy outcomes in KOA patients with synovitis and VAS ≥ 4. A-D Comparison with baseline in each group. A VAS score. B KSS score. C WOMAC score. D EQ-5D score

Radiological outcomes

There were no signs of new cyst formation, neoplasms (benign or malignant), heterotopic ossification of the bone, cartilage, vasculature, or synovium. Table 4 indicates the baseline prevalence and change of articular cartilage and BMLs in Global MOAKS at 6 months follow-up. The size of the cartilage defect and BMLs in MRI at 6 months were not significantly changed among groups.

Table 4.

Baseline prevalence and change of articular cartilage and BMLs at 6 months follow-up

	Placebo group	HA group	UC-MSCs group	P value
Articular cartilage, n (%)
Baseline	15	16	17
-Progression	3 (20)	3 (18.8)	2 (11.8)	0.605
-No change	10 (66.7)	11 (68.8)	13 (76.5)	0.511
-Improvement	2 (13.3)	2 (12.5)	2 (11.8)	0.245
BMLs, n (%)
Baseline	12	14	14
-Progression	3 (25)	3 (21.4)	2 (14.3)	0.628
-No change	6 (50)	8 (57.1)	8 (57.1)	0.264
-Improvement	3 (25)	3 (21.4)	4 (28.6)	0.315

Data are presented as n (%)

Abbreviations: HA, hyaluronic acid; UC-MSCs, umbilical cord-derived mesenchymal stromal cells; BMLs: bone marrow lesions

Discussion

The current study aimed to evaluate the safety and therapeutic efficacy of intra-articular injection of UC-MSCs in KOA by assessing AEs, clinical outcome scales, and MRI. Clinical outcomes showed that UC-MSCs therapy was safe and well-tolerated. It could relieve pain and enhance function in patients with KOA combined with synovitis, particularly those experiencing moderate to severe pain.

KOA is a comprehensive joint disorder, characterized by a multifaceted and diverse progress involving mechanical, inflammatory, and metabolic factors, which could result in the deterioration of articular cartilage, synovitis, subchondral bone sclerosis, osteophyte formation, as well as muscle and ligament injuries. Recent progress in osteoarthritis research has provided a new insight into its pathophysiology, highlighting the significant roles of inflammation, growth factors, and cytokine-mediated signaling pathways in its progression [23]. In this regard, the inhibition of the inflammatory process in the knee joint has garnered attention. Recently, intra-articular injection of MSCs has been proposed as an innovative therapy for KOA by inflammation suppression besides cartilage regeneration. Due to their high proliferation rate, strong immunomodulatory properties, and chondrogenic capabilities, MSCs have emerged as a promising therapeutic option for KOA. Among the various sources of MSC-based therapies, UC-MSCs are widely used in clinical application, as they exhibit superior proliferative potential and reduced immunogenicity compared to MSCs derived from other sources like the bone marrow and adipose tissue [24]. Furthermore, since umbilical cords are often considered medical waste and discarded, there is a plentiful supply of these cells, thereby mitigating medical risks for donors and alleviating ethical concerns [25]. Several preclinical studies [14, 26] and clinical trials [16–18, 27–29] regarding the intra-articular UC-MSCs injection in KOA have demonstrated therapeutic effect, improved joint function, and ameliorated radiographic signs, whereas most of these studies have been conducted as pilot trials with a single-arm design [16–18, 28, 29]. Therefore, we designed a randomized, double-blind, controlled clinical trial to evaluate the safety and efficacy of intra-articular injection of UC-MSCs.

Our study revealed that treatment with UC-MSCs was safe and well-tolerated, with a safety characteristic comparable to that of the placebo and HA groups. No significant AEs or undesirable effects were observed during 6-month follow-up period. AEs in our study were primarily local pain, which was notably observed in patients randomized to the HA and UC-MSCs groups, with incidence rates of 12.5% and 29.4%, respectively. This injection-related pain was resolved with rest or analgesics within 2 days. The incidence of these complications was consistent with findings from another study utilizing UC-MSCs [27]. In another study using intra-articular injection of allogeneic BMMSCs (40 million cells suspended in 8 ml of Ringer-Lactate) in KOA reported post-injection pain in 60% and 53% of patients in both the control and treatment groups [22], which is notably higher than in our research. Hence, we propose that the characteristics of UC-MSCs, including their hypoimmunogenicity and immunomodulatory properties, likely contributed to the reduced frequency of complications. It is important to note that, in addition to the source of MSCs, various factors such as higher dosage, injection volume, cell activity, and repeated injections may influence the occurrence of complications in MSCs therapy for KOA. Recently, Chen et al. conducted a study using different doses of allogeneic ADMSCs. The high-dose group exhibited a larger percentage of general disorders, including pain and swelling at the injection site of the joint, compared to the HA group and low-dose group [30]. This may be attributed to a higher proportion of cell death associated with increased MSCs dosage, which could trigger inflammation and consequently cause pain. Joswing and colleagues found that repeated intra-articular injections of allogeneic MSCs in KOA therapy resulted in increased adverse clinical reactions after a second injection, four weeks later [31]. However, another study demonstrated that repeated intra-articular injection of UC-MSCs in patients did not result in increased frequency or severity of AEs [27]. This may be related to the more pronounced humoral immune response elicited by ADMSCs [32], as well as potentially being influenced by the short intervals between repeated injections.

Regarding the efficacy of treatment evaluated by VAS, WOMAC, KSS, and EQ-5D, there were no significant differences observed among the three groups. Intra-articular injection of UC-MSCs led to greater improvements in VAS and EQ-5D compared with placebo and HA, but the differences were not statistically significant. This is presumably attributed to the small sample size and heterogeneity of response, and potentially the mild baseline involvement as indicated by the KL-grade and VAS scores. In our study, intra-articular injection of HA did not significantly improve symptoms of KOA, which is consistent with the AAOS 2013 guidelines that strongly recommend against the use of HA due to its minimal or no clinical benefit [33].

As an independent risk factor for OA, synovitis closely correlates with OA pain and leads to the secretion of various inflammatory cytokines, exacerbating cartilage degeneration and OA progression [34]. MSCs possess the capability to modulate inflammation [35], and our previous preclinical research illustrated that UC-MSCs attenuated OA progression by inhibiting synovitis [14]. Therefore, in this study, we further identified patients with both OA and synovitis through baseline MRI screening. Our findings revealed that compared to the placebo and HA groups, patients receiving UC-MSCs therapy exhibited significant improvement in synovitis at the 6-month follow-up. There was no significant difference in VAS score changes between the UC-MSCs group and the placebo or HA groups. Strikingly, the UC-MSCs group showed a significant reduction of WOMAC scores at 6 months. Additionally, patients using UC-MSCs were superior to those patients in the placebo and HA groups in terms of improvement of quality of life as was shown by EQ-5D after months 3 and 6 from baseline. Although previous research has demonstrated the anti-inflammatory properties of MSCs [36], to our knowledge, this is the first randomized, double-blinded study to definitively prove that intra-articular injection of UC-MSCs can ameliorate synovitis through MRI imaging.

Recently, some studies have suggested that MSCs treatment is more effective in patients with severe KOA, particularly those with K-L grades 3 [37]. Given that the majority of patients screened in this study had K-L grades below 3, we specifically selected individuals experiencing moderate to severe pain at baseline, identified by VAS ≥ 4, from among those with KOA combined with synovitis. This is because moderate to severe pain can largely reflect the degree of inflammation and deterioration of the knee joint, thereby providing a more accurate representation of the genuine therapeutic efficacy of UC-MSCs. Our results revealed markedly superior improvements in VAS and WOMAC scores in the UC-MSCs group at 6 months follow-up. The EQ-5D score in the UC-MSCs group showed a progressive improvement up to months post-infection. These findings align with a previous study that demonstrated significant clinical improvement following intra-articular injection of ADMSCs, with results improving from 6 months after injection [38]. Further, the outcomes of VAS and WOMAC in other studies significantly increased 3 months post-injection and remained steady for up to 12 months [30, 36]. This is presumably due to the paracrine ability of MSCs through the secretion of growth factors and immunomodulatory cytokines [39]. In addition, a recent trial indicated that intra-articular injection of ADMSCs yielded safe and effective clinical improvement in VAS and WOMAC scores, with sustained safety observed for up to 5 years [40]. These findings may further validate the evidence regarding the clinical effectiveness of MSCs, indicating their potential as a disease-modifying therapy for individuals with KOA.

Our MRI assessments at the 6-month follow-up did not indicate significant structural alterations in cartilage status after intra-articular injection of UC-MSCs. A previous study noted that ADMSCs played a role in facilitating cartilage repair, as evidenced by arthroscopy and histological findings [41]. However, demonstrating a notable therapeutic advancement in cartilage status through radiographic evaluations in clinical trials remains challenging. Recently, a meta-analysis of RCTs presented contentious findings regarding cartilage alterations on MRI evaluations, with insufficient evidence supporting cartilage regeneration after intra-articular injection of MSCs in short-term follow-up [42]. Nevertheless, preclinical animal models have consistently demonstrated the disease-modifying effects of MSCs for KOA through immunohistochemical and histologic evaluations [43]. Our previous preclinical study also confirmed that UC-MSCs could alter the biochemical environment of OA towards regenerative and anti-inflammatory conditions via paracrine effects [44]. In the current study, no cartilage regeneration or structural alteration were observed at 6 months after injection, which may be attributed to the small sample size, the variability and insensitivity of radiographic measures, and the influence of natural disease progression. A recent clinical trial showed significant improvement in cartilage alterations at 3 years after UC-MSCs implanted in the cartilage defect, as determined by serial MRI evaluations [29]. Thus, structural enhancement of cartilage necessitates a more extended follow-up to fully capture the regenerative and chondroprotective effects of UC-MSCs.

While informative, the present RCT has some limitations. First, this study was conducted as a preliminary pilot trial with only 55 randomized participants. The relatively small sample size inevitably limits the statistical power to detect potential differences between groups and may increase the risk of type II error, underscoring the importance of future studies with larger sample sizes. Second, the 6-month follow-up period may be too short to draw definitive conclusions regarding the safety and efficacy of the intra-articular injection of UC-MSCs. Longer follow-up is particularly important in KOA studies, as unforeseen outcomes related to clinical efficacy, adverse events, and structural changes may emerge over time. Therefore, future investigations with extended follow-up durations of at least 12 months are warranted to provide more robust long-term evidence. Third, this study primarily compares UC-MSCs with HA, without evaluating additional treatment options such as corticosteroid injections or physical therapy. Future research should focus on incorporating these comparators to better reflect the diverse clinical management strategies for KOA and to offer a more comprehensive understanding of the potential advantages of UC-MSC therapy. Finally, some patients did not complete the follow-up; nevertheless, the loss rates were only 11.1%, 5.3%, and 5.6% in the placebo, HA, and UC-MSCs groups, respectively, which is a normal and acceptable occurrence in RCT studies. Additionally, missing data were addressed through valid statistical methods for imputation. Future studies with larger sample sizes and longer follow-up periods will be required.

Conclusions

In this trial, the safety and therapeutic efficacy of UC-MSCs therapy for treating KOA patients were reported. UC-MSCs treatment was found to be safe, well-tolerated, and presented a comparable safety profile to both the placebo and HA treatments. The intra-articular injection of UC-MSCs provided significant pain alleviation and functional improvements in KOA patients combined with synovitis, especially those with moderate to severe pain. However, the relatively small sample size and the short 6-month follow-up period remain important limitations. Larger multicenter trials with longer-term follow-up are needed to confirm and extend these results. Additionally, as a novel therapeutic approach, MSCs therapy warrants future studies incorporating cost-effectiveness analyses to clarify its economic value compared with standard care.

Abbreviations

KOA

Knee osteoarthritis

UC-MSCs

Umbilical cord-derived mesenchymal stromal cells

ADMSCs

Adipose-derived mesenchymal stromal cells

BMMSCs

Bone marrow-derived mesenchymal stromal cells

HA

Hyaluronic acid

BMI

Body mass index

GMP

Good manufacturing practices

AEs

Adverse events

NCI-CTCAE

National Cancer Institute-Common Terminology Criteria for Adverse Events

VAS

Visual analog scale

WOMAC

Western Ontario and McMaster Universities Arthritis Index

KSS

Knee Society Clinical Rating System

MOAKS

MRI Osteoarthritis Knee Score

BMLs

Bone marrow lesions

Authors’ contributions

H.T. and D. W. made the study design. Y.S. and W.T. drafted the manuscript. J.J., D.X., Q.L., and C.X. collected and analyzed the data. E.X., W.R., Q.H. W.X, and S.Z. revised and supervised the manuscript. All authors have read and approved the manuscript.

Funding

This work was supported by the Innovation and Entrepreneurship Leading Team Project in Zengcheng District, Guangzhou (202001002); Key Research and Development Program of Science and Technology of Hubei Province (2021BCA134, 2024BCB107); Key Research Project of Wuhan Donghu New Technology Development Zone (2022KJB118); the Department of Science and Technology of Hubei Province (2023BCB089); and the National Natural Science Foundation of China (82372465).

Data availability

Data will be made available on request.

Declarations

Ethical approval and consent to participate

All procedures performed in studies involving human participants were in accordance with the Helsinki Declaration of 1975, an revised in 2008. Informed consent was obtained from all patients. This trial was approved by the ethics committee of Wuhan Union Hospital (2019SC02).

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.