Mesenchymal stem cell (MSC) treatment is a new regenerative therapy for treating tendon disorders. Preclinical studies have reported that MSC therapy may increase the number of tenocytes and regenerate the injured tendon tissue [1-4]. While several studies with animals support the treatment of tendon disorders using MSCs, little is known about the efficacy and safety of MSCs to treat these conditions in humans. Although a few clinical reports suggested the therapeutic potentials of MSCs in tendon disorders, they are mostly case reports or case series.

A systematic review of MSC therapy on tendon disorder [5] analyzed three case series [6-8] and one matched non-randomized trial [9]. The authors concluded that MSC treatment is not yet suitable for clinical practice because the included studies are at high risk of bias. However, the result should be reconsidered, as three [6,7,9] of the four studies included in this review were not performed with isolated MSCs but with bone marrow aspirates or stromal vascular fractions cells. Moreover, this study was not conducted with a meta-analysis methodology, which combines the results from multiple studies. Furthermore, two current clinical studies [10,11] that used isolated MSCs on tendon disorder were not included in the review.

Although an increasing number of research studies on stem cell treatments have been published, no meta-analyses have been conducted on this topic to date. Furthermore, concerns regarding the possible adverse events of MSC treatments that were raised by physicians or scientists reluctant to the therapy [12] should be thoroughly reviewed. Thus, we performed an updated meta-analysis of the prospective clinical studies to evaluate the efficacy and safety of MSC therapies in patients with tendon disorders.

The meta-analysis was conducted in accordance with the updated guidelines of the Preferred Reporting Items for Systematic review and Meta-Analysis Protocols (PRISMA-P) [13]. Searches on PubMed-MEDLINE, EMBASE, and the Cochrane Library were performed in March 2021 by using the following key terms and syntax: (Tendinopathy OR Tendon OR Tendon disorder OR Tendon injuries OR Tendinosis OR Tendinitis OR Tennis elbow OR Elbow Tendinopathy OR Lateral epicondylitis OR Lateral epicondylosis OR Golfer’s elbow OR Rotator cuff OR Rotator Cuff Injuries OR De Quervain disease OR Jumper’s knee OR Achilles tendon) AND (Stem cells OR Mesenchymal stem cells OR Progenitor cells OR Mother cells OR Multipotent OR Pluripotent OR Totipotent) AND Clinical studies [14,15]. An overview of the search strategy is presented in Supplement A. We included all prospective clinical studies that investigated the effects of MSC administration on tendon disorders. We imposed no language restriction. We also searched for unpublished and gray literature using the following databases and trial registries: World Health Organization Clinical Trial Register, EU clinical trials register, ClinicalTrials.gov, and OpenGrey.

Identified records were saved to the EndNote software (X7.2; Thomson Reuters). Two independent reviewers (WSC and SYL) screened all the titles and abstracts to identify relevant investigations. The inclusion criteria were as follows: (1) articles reporting a prospective clinical study with at least a 6-month follow-up that (2) described the effect of MSC therapy in patients with any tendon disorder. Although no limitations were set for the types of MSCs, that is, cell origin, either autologous or allogeneic, we excluded studies that did not use isolated MSCs such as bone marrow aspirates or stromal vascular fractions cells. Reviews, basic science articles, comments, letters, and protocols were excluded. When updates of earlier studies were available, we used only the most recent ones.

The primary outcome of interest was defined as pain on motion related to tendon disorders. All types of pain measurements such as visual analog scale or numerical rating scale were included. The secondary outcomes analyzed in this study were as follows: (1) joint function scores such as the Constance score, University of California Los Angeles (UCLA) score, modified Mayo Elbow Performance Index, or Shoulder Pain and Disability Index; (2) radiological parameters to measure tendon defects using magnetic resonance imaging (MRI) or ultrasonography; and (3) arthroscopic findings to measure tendon defects with a calibrated arthroscopic probe. For every eligible study, the following data were extracted and entered into a spreadsheet by the two reviewers (WSC and SYL): first author’s family name, year of publication, study design, types of tendon disorder, origin of the MSCs, number of patients, MSC injection methods, cell dose, follow-up duration, safety assessment, and efficacy measurement. We assessed publication bias using the Begg funnel plot [16] and Egger test [17].

Effect sizes were computed as standardized mean difference (SMD) measures [18], representing the magnitude of the pretest-posttest difference for each outcome. SMD was calculated separately for all the available control and treatment groups for each study. Heterogeneity between comparable studies was tested with the chisquare (χ²) and I² tests. p-values >0.1 and I² values <50% were considered statistically significant. As no significant heterogeneity was observed among the four studies (p=0.658 and I²=0.0%), we used a fixed-effects metaanalysis to quantify the pooled effect size of the studies included. In each analysis by outcome, the following parameters were also analyzed using the fixed-effects model: pain (p=0.093 and I²=47.0%), functional scores (p=0.313 and I²=15.3%), radiological parameters (p=0.406 and I²=0.0%), and arthroscopic findings (p=0.588 and I²=0.0%). In addition, we performed a meta-regression analysis to assess the relationship between the MSC dose and the pooled effect size in each cell dose. All analyses were performed using the Comprehensive Meta-Analysis version 3.3 software (Biostat, Englewood, NJ, USA). This study was exempted from the Institutional Review Board review, as no human subjects were involved.

The primary database search yielded 1,135 records. After duplicates were removed, the titles and abstracts of 897 articles were initially screened, of which 25 were selected for full-text review. The full-text articles were read, and four articles were considered relevant by qualitative analysis [8,10,11,19]. The studies selected for final inclusion or exclusion are shown in Fig. 1, and the characteristics of the included studies are summarized in Table 1. In terms of quantitative analysis, these four studies (published from 2015 to 2019) fulfilled our inclusion criteria.

Three papers [8,10,19] were open-label prospective studies, while one [11] was a double-blind randomized controlled trial. The studies identified for meta-analysis included 52 participants. In two studies [8,19], adipose tissue-derived MSCs were used, and in the other two [10,11], bone marrow-derived MSCs were administered. The number of cells used in each study ranged from 10⁶ to a maximum of 10⁸. Regarding tendon disorder types, most of the studies conducted were on rotator cuff tears, but one study [8] was on lateral epicondylitis. The followup duration ranged from 6 to 12 months.

The MSC therapies showed a significant pooled effect size (overall Hedges’ g pooled SMD=1.868; 95% confidence interval [CI], 1.274–2.462; p<0.001) (Fig. 2). The pain parameters, functional scores, radiological parameters (MRI or ultrasonography findings), and arthroscopic findings all improved with MSC treatment (Fig. 3). In the meta-regression analysis, a significant cell dosedependent response in pain relief (Q=9.06, p=0.029) was observed (Fig. 4). While three studies reported mild adverse events after MSC injection, these were not severe and were relieved spontaneously (Table 2). Publication bias was not evident, as shown by the symmetrical Begg’s funnel plot (Supplement B), and the p-value for bias was 0.625 (Egger test; all four trials).

Potential evidence has shown that MSC injection improves pain, joint functional, radiological, and arthroscopic parameters in patients with tendon disorders. Although all the included studies had a small sample size, the results clearly presented MSC dose-dependent responses regarding pain relief. To the best of our knowledge, this is the first clinical meta-analysis describing the pooled effects of MSC therapies in patients with tendon disorders.

Tendon injuries are a common health problem, which are defined as painful conditions occurring around tendons that limit the function of the affected tendons [20]. Tendons are susceptible to repeated use or degenerative condition. Injuries in those structures are rarely regenerated but repaired by scar tissue and fibrosis. The healed tissue presents inferior tensile strength and is prone to further injuries. Preclinical studies support that MSCs have a regenerative potential, as they can differentiate into targeted tissues and replace injured resident cells [1]. Therefore, MSC administration has been regarded as a possible curative treatment option for tendon degeneration.

Implanted stem cells survive in tendon defects, differentiate into the tenogenic cell lineage, and secrete their own extracellular matrix to promote tendon healing [4]. Mazzocca et al. [21] showed that bone marrow-derived stem cells differentiated into tendon-like cells. Lee et al. [4] also reported that transplanted human adipose tissue-derived stem cells survived for at least 4 weeks in the rat tendon injury model and released human-specific collagen type I and tenascin C (TnC). TnC expression is known to increase rapidly during the early period of recovery after tendon injuries and thereby used as a marker of tenogenic differentiation [22].

In this meta-analysis, three of the four included studies examined radiological data (MRI or ultrasonography) or arthroscopic findings after MSC injections. These tests could confirm that the injected cells not only relieved pain and improved functions but also regenerated the damaged tissue. Noteworthy, Jo et al. [19] conducted the second-look arthroscopic examination at 6 months after MSC injection and MRI follow-up. They reported that regenerated tendon tissues were identified in all the subjects regardless of the location and size of the tear. The defect volumes were decreased in the patients who received mid (5.0×10⁷ cells) and high doses (1.0×10⁸ cells). Although this is a macroscopic observation, it may be considered as strong supporting evidence for the regeneration effect of MSCs.

The benefits of MSCs in the treatment of tendon disorders are not confined to their differentiation potential alone. Another important biological mechanism that supports the use of MSC therapy is that MSCs release diverse cytokines, chemokines, and growth factors [1]. Several studies found that these secreted factors may stimulate their proliferation, allowing the promotion of tissue regeneration. The benefits of MSC-conditioned media proven by in vitro studies also encourage the paracrine effects of MSCs. Kinnaird et al. [23] found that the growth of endothelial cells and smooth muscle cells may be promoted by the use of medium conditioned with MSCs. This phenomenon might be partly explained by the presence of VEGF (vascular endothelial growth factor) and bFGF (basic fibroblast growth factor), which appeared in high levels in a MSC-conditioned medium [24]. They can recruit macrophages and endothelial cells into the injured site, allowing enhancement of the healing process.

The ability of the MSCs to produce a wide range of immunomodulatory factors has also attracted great attention [25]. Both in vitro and in vivo studies have elicited that MSCs can downregulate the excessive response of numerous immune cells such as T cells, B cells, dendritic cells, macrophages, and natural killer cells. MSCs can also induce regulatory T cells and thereby expand and maintain a long-lasting immune-modulating activity, which is similar to the role of catalysts. Considering that the inflammation-derived tissue damage is one of the key processes in most tendon disorders, immunomodulation induced by injected MSCs can also play an important role in promoting treating tendon diseases, in addition to their differentiation potential and paracrine effects [26].

Several concerns remain regarding the use of MSCs as a treatment option for tendon disorders. Potential long-term adverse events from the stem cell treatment have been poorly reported in several clinical studies. In the studies included in this meta-analysis, most of the reported adverse events were not related to the treatment (Table 2). The treatment-related side effects were mild joint effusion and regional swelling after allogeneic stem cell injection [8] or engrafted patch-related foreign body reaction [11]. The joint swelling spontaneously subsided, while the patch-related adverse event needed additional surgery. Considering the prognosis of the reported adverse events, these side effects might have come from the localized inflammatory response related to the treatment procedure itself, or immunological response against allogeneic cells, but are less likely to have arisen from the MSC itself.

The safety issues related to the use of MSCs have already been sufficiently assessed in clinical trials in the field of internal medicine, in which MSCs are injected systemically. The POSEIDON trial [27] was designed to investigate the safety and efficacy of autologous and allogeneic MSC therapies for ischemic cardiomyopathy. The study reported that after trans-endocardial stem cell injection, the treated group showed improvement in structural and functional outcomes, while no serious adverse events, including immunologic reactions occurred. Indeed, the long-term adverse events from and possible teratogenicity of the stem cell treatment should be thoroughly considered. One animal study reported undesired cartilage formation after the injection of human MSCs in 81 rat tendon injury models [28]. While no histological evidence of tumor formation was found in the study, concerns for possible teratogenicity still remain.

Although numerous challenges still need to be overcome and analyzed, MSC therapy can be a promising treatment option for tendon disorders. Approximately 17% of patients with tendon disorders are known to have no effects after undergoing conservative treatment for >1 year [29]. In some patients, the rate of retear is fairly high, even after surgical repair for tendon injuries [19]. Thus, the limitations of the current therapies suggest a need for more fundamental regenerative treatments, and MSCs might offer a regenerating opportunity for the tendon by yielding a more robust repaired tissue [30]. For MSC injections to be established in tendon disorders, the aforementioned long-term safety issues should be better verified. Furthermore, well-designed clinical trials should be performed to support the evidence.

This meta-analysis has several limitations. First, we included a limited number of studies in our meta-analysis. Moreover, only one randomized controlled study was available. As MSCs have been applied for the treatment of tendon disorder for only a short time, the number of studies that fulfilled our criteria was limited. If a sufficient number of studies had been analyzed, more solid evidence could have been obtained. However, it is meaningful to combine the data through a meta-analysis because related studies are inadequate. Second, the outcome variables showed heterogeneity among the included trials. Three studies [10,11,19] used the Constant score for functional assessment, while one study used the modified Mayo Elbow Performance Index [8], and two studies additionally used the UCLA score [10] and Shoulder Pain and Disability Index [19], respectively. Although we used the combined pooled effect sizes to deal with this issue, the effect sizes should be cautiously interpreted from the clinical point of view. Furthermore, the heterogeneities of the MSC origin and target tissue were also limitations of this analysis. Two studies were performed with the administration of bone marrow-derived MSCs [10,11], while the other two studies used adipose tissue-derived MSCs [8,19]. The specific tendon disorders presented in the studies were also different, namely three studies aimed at treating the rotator cuff disease and one, lateral epicondylitis. And two studies added MSCs injection therapy to surgical treatment [10,11], and the other two studies confirmed the effect of MSCs injection therapy alone [8,19]. However, to assure that the mechanisms and efficacy of MSC therapies in tendon disorder are clear and evident, whether these treatments are suitable for not just a single specific tendinopathy but also for other tendon disorders, which may involve various musculoskeletal structure, must be evaluated.

In conclusion, our meta-analysis revealed that MSC therapy may improve pain, function, and radiological and arthroscopic parameters in patients with tendon disorders. Owing to the limited sample size of this meta-analysis and considering the increasing MSC applications, large-scale randomized controlled trials are strongly needed to confirm the long-term functional improvement and adverse effects of MSC therapies in tendon disorders.