Stem cell-based cartilage regeneration: Biological strategies, engineering innovations, and clinical translation

Abstract

Articular cartilage damage caused by trauma or degenerative diseases such as osteoarthritis remains a major therapeutic challenge due to the tissue’s limited regenerative capacity. Traditional surgical interventions-including microfracture, autologous chondrocyte implantation, and osteochondral grafting-often result in the formation of biomechanically inferior fibrocartilage and fail to restore long-term joint function. In contrast, stem cell-based strategies have emerged as a promising approach for regenerating hyaline-like cartilage by combining the biological potential of mesenchymal stem cells and induced pluripotent stem cells with advances in tissue engineering. This review synthesizes the current understanding of cartilage structure and repair limitations, evaluates the regenerative potential of various stem cell sources, and highlights engineering innovations such as bioactive scaffolds, controlled growth factor delivery, and three-dimensional bioprinting. We also examine key preclinical studies and early-phase clinical trials demonstrating the safety and efficacy of stem cell-based therapies. Finally, we explore future directions, including gene editing, exosome-based therapeutics, and personalized regenerative strategies, that may address remaining translational barriers. Collectively, stem cell-centered approaches offer a transformative avenue toward durable, functional cartilage repair and hold strong potential for clinical application.

Key Words: Cartilage regeneration; Mesenchymal stem cells; Induced pluripotent stem cells; Tissue engineering; Three-dimensional bioprinting; Osteoarthritis; Gene editing

Core Tip: This review highlights the emerging role of stem cell-based strategies in articular cartilage regeneration. By integrating the biological potential of mesenchymal stem cells and induced pluripotent stem cells with engineering advances such as bioactive scaffolds, growth factor delivery, and three-dimensional bioprinting, these approaches aim to overcome the limitations of traditional therapies. We also discuss cutting-edge developments including gene editing and exosome-based therapeutics, offering new insights into personalized and durable cartilage repair solutions.

INTRODUCTION

Articular cartilage is a highly specialized connective tissue that covers the ends of long bones, providing a smooth, load-bearing surface essential for joint movement. Its unique extracellular matrix (ECM), composed primarily of type II collagen and proteoglycans, endows it with remarkable tensile strength and compressive resistance[1,2]. However, the intrinsic capacity for repair is extremely limited due to the tissue’s avascular, aneural, and low-cell-density nature[3]. Consequently, even minor cartilage injuries can progress to degenerative joint conditions such as osteoarthritis (OA), leading to chronic pain, functional impairment, and reduced quality of life[4,5].

OA is now recognized as a whole-joint disease, involving not only cartilage but also the subchondral bone, menisci, synovium, and infrapatellar fat pad. These structures interact dynamically through mechanical, biochemical, and inflammatory signaling pathways, which further complicate disease progression and therapeutic intervention[6]. In clinical practice, a variety of interventions are employed to manage cartilage lesions, including autologous chondrocyte implantation (ACI), a procedure in which a patient’s own cartilage cells are harvested, expanded, and re-implanted into the defect site. Conventional treatments-including microfracture surgery, ACI, and osteochondral grafting-are widely used to manage cartilage defects. While these interventions may offer temporary symptom relief or structural coverage, they rarely result in the restoration of native hyaline cartilage with appropriate biomechanical properties[7-10]. Moreover, issues such as fibrocartilage formation, donor site morbidity, and limited long-term durability highlight the pressing need for more effective regenerative solutions[11,12]. In this context, stem cell-based therapies have garnered substantial interest for their potential to regenerate articular cartilage and restore joint function. One of the primary challenges in treating cartilage defects lies in the limited proliferative capacity of native chondrocytes and the inability of adult cartilage to mount an effective repair response[13,14]. Traditional interventions, such as microfracture or ACI, often result in the formation of fibrocartilage rather than true hyaline cartilage, leading to inferior mechanical properties and limited durability. Furthermore, these methods are invasive, involve multiple procedures, and are often not suitable for large or complex lesions[15-17].

Stem cells offer several key advantages over these conventional approaches. Mesenchymal stem cells (MSCs), particularly those derived from bone marrow, adipose tissue, and synovium, possess robust chondrogenic potential and are capable of modulating the local immune microenvironment through the secretion of anti-inflammatory cytokines and extracellular vesicles[18]. Clinical studies have demonstrated that intra-articular injection of autologous MSCs can lead to symptomatic improvement and increased cartilage thickness on magnetic resonance imaging (MRI) in patients with early-stage OA, often with fewer complications than ACI[19]. For instance, a trial by Lee et al[20] showed that a single injection of adipose-derived MSCs resulted in improved pain scores and cartilage volume over 12 months, with no serious adverse effects.

More recently, induced pluripotent stem cells (iPSCs) have emerged as a promising cell source that combines pluripotency with the possibility of autologous derivation, thereby reducing the risk of immune rejection. iPSC-derived chondrocytes have been shown to produce cartilage-like matrix in preclinical models and to integrate well into host cartilage[21,22]. Notably, the use of iPSCs enables the generation of a virtually unlimited supply of patient-specific chondrogenic cells, overcoming many of the scalability issues associated with primary chondrocytes or MSCs.

When combined with advances in biomaterials, controlled growth factor delivery systems, and three-dimensional (3D) bioprinting technologies, stem cell-based approaches enable precise spatial and temporal control over tissue regeneration. These integrated platforms not only support cell survival and differentiation but also allow for the fabrication of anatomically tailored cartilage constructs with zonal organization, which is difficult to achieve using traditional surgical methods[23,24]. Together, these innovations position stem cell therapy as a transformative strategy with the potential to restore durable, functional cartilage and significantly improve long-term outcomes for patients with joint disease.

This review provides a comprehensive synthesis of the current landscape of stem cell-based cartilage regeneration. We begin by discussing cartilage structure and the challenges of repair, followed by an in-depth evaluation of stem cell types, differentiation strategies, tissue engineering platforms, and preclinical and clinical evidence. Finally, we explore future perspectives, including gene editing, exosome-based therapy, and personalized regenerative medicine, highlighting the innovations that may bring durable, functional cartilage repair closer to clinical reality.

ARTICULAR CARTILAGE: STRUCTURE, FUNCTION, AND CHALLENGES

Structure and function

Articular cartilage is composed of a dense ECM and a sparse population of chondrocytes. The ECM primarily consists of type II collagen and proteoglycans, particularly aggrecan, which provide tensile strength and compressive resistance. Cartilage is organized into distinct zones: The superficial zone, containing flattened chondrocytes aligned parallel to the surface; the middle (transitional) zone, composed of rounded chondrocytes and a more random collagen matrix; the deep zone, characterized by columnar chondrocytes aligned perpendicular to the surface; and the calcified zone, which interfaces with subchondral bone[2]. Chondrocytes, the only cell type in cartilage, are often found within chondrons - functional units consisting of a chondrocyte surrounded by a pericellular matrix. Chondrons play a key role in mechanotransduction and matrix homeostasis. This zonal organization is essential for distributing mechanical loads and maintaining joint function. The absence of blood vessels, lymphatics, and nerves in cartilage contributes to its limited regenerative capability and poor response to injury[25].

Cartilage lesions can arise from a variety of causes, including acute trauma (e.g., sports injuries, joint dislocations), repetitive mechanical stress, and degenerative joint diseases such as OA and rheumatoid arthritis. Other etiologies include developmental disorders (e.g., osteochondritis dissecans) and metabolic conditions (e.g., crystal arthropathies)[26]. These insults disrupt the cartilage matrix and chondrocyte homeostasis, leading to progressive degeneration, inflammation, and pain. In OA, these zonal structures become disrupted. Chondrocyte density declines, matrix composition changes, and chondron organization is lost. These alterations lead to decreased stiffness, impaired load distribution, and progressive degeneration. Biomechanically, OA cartilage exhibits reduced elastic modulus and increased permeability, weakening its ability to withstand repetitive loading (Figure 1).

Figure 1 Structural differences between normal and degenerated articular cartilage. A and B: Schematic representation of healthy (A) and osteoarthritis-affected (B) articular cartilage. Normal cartilage is composed of four zones-superficial, middle, deep, and calcified-organized above the subchondral bone and enriched in collagen type II and aggrecan. In osteoarthritis, progressive matrix degradation, fissures, loss of chondrocytes, and elevated expression of matrix metalloproteinases and aggrecanases (a disintegrin and metalloproteinase with thrombospondin motifs) result in biomechanical failure and tissue degeneration. MMPs: Matrix metalloproteinases; ADAMTs: A disintegrin and metalloproteinase with thrombospondin motifs.

Clinically, cartilage damage manifests as joint stiffness, swelling, crepitus, and reduced range of motion[6]. In advanced cases, patients experience chronic pain, joint deformity, and disability. Imaging modalities such as MRI, particularly T2 mapping and delayed gadolinium-enhanced MRI of cartilage, are critical for evaluating cartilage integrity and monitoring therapeutic outcomes[27]. Arthroscopy remains the gold standard for direct visualization and grading of cartilage lesions[28].

Limited regenerative capacity

Articular cartilage lacks intrinsic repair mechanisms due to its avascular and aneural nature. Chondrocytes are terminally differentiated and exhibit minimal proliferative activity, limiting their ability to respond to injury. Additionally, the cartilage environment is characterized by low oxygen tension and a paucity of progenitor cells, further impeding spontaneous regeneration. Inflammatory mediators such as interleukin-1 beta and tumor necrosis factor-alpha exacerbate tissue damage by upregulating catabolic enzymes like matrix metalloproteinases (MMPs) and aggrecanases, which degrade the ECM and compromise biomechanical integrity[29-31].

Limitations of conventional therapies

Conventional treatment strategies for cartilage damage include non-operative approaches (e.g., physiotherapy, non-steroidal anti-inflammatory drugs, corticosteroid injections), as well as surgical techniques like microfracture, ACI, and osteochondral autograft or allograft transplantation[32,33]. Microfracture is a marrow-stimulation technique that involves creating perforations in the subchondral bone to recruit mesenchymal progenitors. However, it often leads to the formation of fibrocartilage, which lacks the biomechanical resilience of native hyaline cartilage and deteriorates over time[34]. ACI involves harvesting autologous chondrocytes from a non-load-bearing region, expanding them in vitro, and reimplanting them into the defect site. While this method can generate hyaline-like tissue, it is technically demanding, requires two surgeries, and poses risks such as chondrocyte dedifferentiation and periosteal hypertrophy[35]. Osteochondral grafting transfers cartilage and underlying bone from either the patient (autograft) or a donor (allograft) to the defect site. Although this technique can restore structural continuity and load-bearing function, it is limited by donor tissue availability, graft mismatch, and potential immunogenic responses in the case of allografts[36]. The persistent shortcomings of these therapies emphasize the need for regenerative approaches that can restore durable, hyaline-like cartilage capable of withstanding long-term joint loading (Figure 1).

STEM CELL SOURCES FOR CARTILAGE REGENERATION

The application of stem cells in cartilage repair has revolutionized the field of regenerative orthopedics. Unlike traditional surgical interventions that aim to alleviate symptoms or restore partial function, stem cells offer the potential to regenerate tissue that closely mimics native hyaline cartilage. Their ability to differentiate into chondrocytes, secrete trophic factors, and modulate immune responses makes them ideal candidates for treating cartilage defects.

MSCs

Among all stem cell types, MSCs are the most extensively investigated for cartilage regeneration. MSCs can be isolated from multiple adult tissues, including bone marrow, adipose tissue, and synovial membrane. They exhibit a robust capacity for chondrogenic differentiation when cultured under specific conditions [e.g., transforming growth factor β (TGF-β), bone morphogenetic protein 2 (BMP-2) induction], and also possess immunomodulatory properties beneficial for controlling inflammation in the joint microenvironment[37-39].

Bone marrow-derived MSCs (BMSCs) have demonstrated promising results in preclinical and clinical studies. Wakitani et al[40] first reported successful implantation of BMSCs into cartilage defects in human knees, with improvements in both histology and clinical outcomes. More recently, a randomized controlled trial by Vega et al[41] in 2015 showed that intra-articular injection of autologous BMSCs improved pain and cartilage quality compared to hyaluronic acid (HA) in patients with knee OA.

Adipose-derived stem cells (ADSCs) are easily accessible and abundant, commonly harvested from abdominal fat, though increasing attention has been given to infrapatellar fat pad-derived MSCs[42]. They have been used in both scaffold-free systems and combined with hydrogels or scaffolds. Lee et al[20] in 2019 reported single injection of adipose tissue-derived MSCs led to a significant improvement of the Western Ontario and McMaster Universities Osteoarthritis index (WOMAC) score at 6 months. An intra-articular injection of autologous adipose tissue-derived MSCs provided satisfactory functional improvement and pain relief for patients with knee OA in the outpatient setting, without causing adverse events at 6 months’ follow-up[20]. A 2019 clinical trial by Hong et al[43] demonstrated that intra-articular injection of ADSCs significantly improved WOMAC scores and MRI-assessed cartilage thickness over 12 months in patients with early knee OA. However, ADSCs may be more prone to hypertrophic differentiation, necessitating optimized induction protocols to stabilize the chondrogenic phenotype.

Synovium-derived stem cells (SMSCs) are increasingly recognized for their superior chondrogenic potential and proximity to articular cartilage. Shirasawa et al[44] demonstrated that SMSCs showed higher expression of cartilage matrix genes and less tendency toward hypertrophy compared to BMSCs in vitro. SMSCs may be particularly promising for intra-articular applications due to their native residence in the joint environment. Although the synovium is inflamed in OA, SMSCs have been shown to retain their regenerative capacity and maintain high expression of chondrogenic markers even under inflammatory conditions[45].

iPSCs

iPSCs offer the advantage of pluripotency without the ethical concerns associated with embryonic stem cells. They can be derived from a patient’s own somatic cells, reducing the risk of immune rejection. iPSC-derived chondrocytes have been shown to form cartilage-like tissues with matrix characteristics resembling native cartilage[46]. For example, Yamashita et al[47] in 2015 developed a stepwise differentiation protocol yielding hyaline cartilage constructs from iPSCs that integrated well into cartilage defects in rats. Other emerging sources such as nasal chondrocytes have gained attention for their remarkable proliferative capacity and phenotypic stability. Autologous nasal chondrocytes have been successfully used in phase I clinical studies, showing good integration and functional repair in knee cartilage defects[48]. These cells are easily accessible and possess intrinsic chondrogenic properties, making them promising candidates for future regenerative strategies. Nevertheless, concerns regarding genomic instability and tumorigenicity remain major hurdles for clinical translation. Advances in non-integrating reprogramming methods, such as episomal vectors and mRNA-based systems, are helping to address these issues.

Other emerging sources

Other tissue-specific progenitor cells, including periosteum-derived cells, muscle-derived stem cells, and dental pulp stem cells, have also shown chondrogenic potential in vitro and in animal models[49,50]. Articular cartilage contains a subpopulation of chondrocyte progenitors, particularly within the superficial zone and near the tidemark, that exhibit stem-like characteristics. These cells express markers such as Notch1, CD44, CD105, and proteoglycan 4, show colony-forming ability, and can differentiate into chondrocytes under specific conditions. They play a role in tissue homeostasis and may serve as an endogenous repair source. Recent studies have suggested the therapeutic potential of harnessing or stimulating these cells for in situ regeneration, which may reduce the need for cell transplantation[13]. These cells may offer niche-specific advantages or secrete unique paracrine factors that support cartilage regeneration, though their clinical application remains exploratory. The therapeutic rationale for using stem cells in cartilage repair lies not only in their differentiation potential but also in their ability to secrete a broad spectrum of cytokines, growth factors, and extracellular vesicle[13,51]. These secreted factors contribute to creating a pro-regenerative niche by reducing inflammation, promoting angiogenesis in subchondral bone, and recruiting endogenous progenitor cells. In summary, stem cells-particularly MSCs and iPSCs-represent a transformative approach to cartilage regeneration. Their multipotency, immunomodulation, and interaction with biomaterials make them foundational elements in modern cartilage tissue engineering. Ongoing studies continue to refine cell sources, improve safety, and enhance functional outcomes to facilitate their widespread clinical translation (Table 1).

Table 1 Summary of stem cell therapies for cartilage regeneration.

Stem cell type	Advantages	Challenges	Clinical stage	Ref.
Bone marrow-derived MSCs	Well-studied; strong chondrogenic potential	Painful harvest; limited yield in elderly	Phase I/II trials	Wakitani et al[40], 2004
Adipose-derived MSCs	High yield; minimally invasive harvest	Variable quality; less matrix production than BMSCs	Phase I/II trials	Lee et al[20], 2019
Infrapatellar fat pad MSCs	Joint-derived; OA-primed immunomodulatory phenotype	Limited standardization; less explored	Early clinical research	Koh and Choi[101], 2012
Synovium-derived MSCs	High chondrogenic capacity; joint niche origin	Harvest from inflamed OA synovium; invasive	Preclinical to early clinical	Jeyaraman et al[102], 2022
Induced pluripotent stem cells	Unlimited expansion; autologous potential	Tumorigenicity risk; ethical and regulatory hurdles	Preclinical	Yamashita et al[47], 2015
Nasal chondrocytes	Phenotypic stability; easy to harvest	Non-joint origin; limited studies	Phase I	Mumme et al[48], 2016

MSCs: Mesenchymal stem cells; BMSCs: Bone marrow-derived mesenchymal stem cells; OA: Osteoarthritis.

CHONDROGENIC DIFFERENTIATION AND ENGINEERING STRATEGIES

Signaling pathways in chondrogenesis

Successful chondrogenesis is governed by a tightly regulated network of signaling pathways and transcription factors. Among these, the TGF-β superfamily-particularly TGF-β1, TGF-β3, and BMP-2-is widely used to induce chondrogenic differentiation of MSCs and iPSCs[52,53]. For example, the combination of TGF-β3 and dexamethasone could significantly enhance cartilage-specific ECM production in BMSC pellets[54]. Similarly, BMP-2 has been shown to upregulate sex determining region Y-box 9 (SOX9) expression, a master regulator of chondrogenesis, and promote matrix synthesis[55]. The SOX trio-SOX5, SOX6, and SOX9-is critical for maintaining the chondrogenic lineage and suppressing hypertrophic differentiation. Overexpression of SOX9 in MSCs not only promotes collagen type II expression but also reduces the expression of hypertrophy-related markers such as collagen type X alpha 1 and MMP13[56,57]. Conversely, dysregulation of Wnt/β-catenin signaling can promote hypertrophy and endochondral ossification[58]. Studies have shown that inhibition of canonical Wnt signaling using small molecules like inhibitor of Wnt productions-2 or Dickkopf-1 can stabilize the chondrogenic phenotype and reduce calcification[59]. These findings underscore the need for fine-tuned pathway modulation in engineered chondrogenesis.

Scaffold and biomaterial design

The selection and design of scaffolds are central to successful cartilage tissue engineering. Natural materials such as type I/II collagen, gelatin, HA, and chitosan are favored for their biocompatibility and ECM-mimicking properties[60]. For example, collagen-based scaffolds seeded with BMSCs have been used to generate cartilage-like tissue in both small and large animal models[61]. HA-based hydrogels, such as those used in the Hyalograft^® C system, have been tested in clinical trials with promising long-term outcomes, with study reported that patients implanted with Hyalograft^® showed durable hyaline-like cartilage repair and sustained pain relief[62,63].

Synthetic polymers like poly-lactic-co-glycolic acid (PLGA) and polycaprolactone provide tunable mechanical strength and controlled degradation rates. When combined with natural polymers in composite scaffolds, they offer improved structural integrity without compromising cell viability[64]. For instance, Zhang et al[65] developed a PLGA-gelatin composite scaffold that enhanced cell attachment and supported sustained chondrogenesis of ADSCs in a rabbit model. Innovative modifications-such as incorporating micro- and nanofibers, adjusting porosity, or integrating functional groups (e.g., RGD peptides)-can further enhance cell adhesion, nutrient diffusion, and mechanical mimicry of native cartilage[66].

Growth factor delivery systems

Controlled and localized delivery of growth factors is essential to direct stem cell differentiation and maintain the stability of the newly formed cartilage. Encapsulation of TGF-β3 or BMP-2 into microspheres or nanocarriers can provide sustained release while minimizing systemic exposure[67]. For example, Park et al[68] reported that PLGA microspheres loaded with TGF-β3 could sustain chondrogenesis of BMSCs in vitro and in vivo for over 4 weeks.

Layer-by-layer assembly and core-shell nanoparticles have also been used to sequentially release multiple factors. For instance, a dual-delivery system releasing TGF-β3 followed by insulin-like growth factor-1 mimicked developmental chondrogenesis and led to improved ECM production and tissue mechanics in engineered constructs[69]. Injectable hydrogels that gel in situ, such as Gelatin methacrylate (GelMA) or polyethylene glycol-based systems, can deliver both stem cells and growth factors simultaneously, allowing for minimally invasive therapies with spatial control over cell-fate decisions.

3D bioprinting and advanced fabrication

3D bioprinting technologies enable the construction of patient-specific, anatomically accurate cartilage constructs. Using extrusion-based systems, stem cells can be encapsulated in bioinks-composed of materials like alginate, GelMA, and gellan gum-for precise deposition layer by layer[23]. For instance, Daly et al[70] successfully printed zonal cartilage constructs using MSC-laden GelMA bioinks with region-specific mechanical and biochemical cues, mimicking native articular cartilage structure. Another study bioprinted a cartilage model using ADSCs embedded in nanocellulose-alginate hydrogels, demonstrating long-term viability and enhanced glycosaminoglycan and collagen II deposition[71]. Emerging printing methods, such as digital light processing and stereolithography, allow for high-resolution fabrication and tunable stiffness gradients across constructs[72]. Volumetric bioprinting, for example, enables the rapid fabrication of complex, centimeter-scale living tissue constructs within seconds, maintaining high cell viability. Additionally, the integration of artificial intelligence (AI) into 3D bioprinting processes is enhancing design optimization, bioink formulation, and real-time process control, thereby improving the precision and efficiency of tissue fabrication[73]. AI-driven optimization of bioink composition, printing parameters, and construct architecture is gaining traction, enabling more efficient and reproducible outcomes in cartilage bioprinting[74]. These advancements are paving the way for more effective and personalized regenerative therapies. Furthermore, bioprinting techniques can integrate vascular channels or osteochondral layers into the construct, facilitating integration with subchondral bone and improving long-term functionality.

Co-culture systems and mechanical stimulation

Co-culturing stem cells with primary chondrocytes or synovial fibroblasts enhances chondrogenic differentiation through paracrine signaling[75]. For example, co-culture of MSCs with articular chondrocytes at optimized ratios has been shown to significantly increase SOX9 and collagen II expression while reducing hypertrophy markers. This strategy allows MSCs to benefit from chondrocyte-derived cues without the need for exogenous growth factors[76]. However, the feasibility of applying co-culture systems clinically remains limited due to challenges in cell sourcing, regulatory concerns, and scalability.

Mechanical stimulation-mimicking native joint loading-is a potent enhancer of ECM production and cartilage maturation. Studies using bioreactors to apply dynamic compression or hydrostatic pressure have shown enhanced matrix organization and biomechanical properties of engineered cartilage. For instance, Thorpe et al[77] demonstrated that cyclic hydrostatic pressure improved collagen and glycosaminoglycan deposition in MSC-seeded scaffolds compared to static culture. In vivo, mechanical stimulation is reflected in the clinical use of continuous passive motion devices, which apply controlled joint movement after cartilage repair surgery. Continuous passive motion has been shown to improve cartilage quality and reduce fibrous tissue formation in animal models and some clinical settings[78]. The integration of mechanical stimulation with biochemical cues in bioreactors represents a cutting-edge approach to achieving functional cartilage tissues for clinical translation (Figure 2).

Figure 2 Stem cell-based therapeutic strategies for cartilage repair. A simplified schematic representing the application of stem cells - including mesenchymal stem cells and induced pluripotent stem cells - in combination with bioengineering tools such as biomaterial scaffolds, growth factors, and three-dimensional bioprinting. These integrated approaches collectively drive chondrogenesis and facilitate functional cartilage regeneration. MSCs: Mesenchymal stem cells; iPSCs: Induced pluripotent stem cells; 3D: Three-dimensional.

PRECLINICAL AND CLINICAL APPLICATIONS

Preclinical validation

Animal studies are indispensable for evaluating the safety, efficacy, and integration capacity of stem cell-based cartilage regeneration strategies. Various models have been developed, including small animals (mice, rats, rabbits) for rapid screening, and large animals (goats, pigs, sheep, horses) for translational relevance[79,80]. For example, Atluri et al[81] transplanted autologous BMSCs embedded in a collagen matrix into rabbit femoral defects, resulting in hyaline-like cartilage formation with histological improvement over 12 weeks. In a goat model, Wang et al[82] used a scaffold-free MSC construct that demonstrated superior integration and mechanical strength compared to microfracture controls. Large animal models provide physiological relevance. In a porcine model, Lee et al[83] delivered iPSC-derived chondroprogenitors in a layered GelMA scaffold and observed ECM-rich cartilage with good biomechanical properties after 6 months. Each model has distinct advantages and limitations in terms of cartilage thickness, joint load, immune compatibility, and cost (Table 2). Preclinical assessments typically use histological staining (Safranin O, Alcian blue), immunohistochemistry (collagen II, aggrecan), and biomechanical testing (indentation modulus) to assess regeneration. Imaging modalities like micro-computed tomography (CT), MRI, and contrast-enhanced ultrasound help track scaffold degradation, neotissue formation, and inflammation.

Table 2 Animal models for preclinical cartilage regeneration.

Species	Cartilage thickness	Advantages	Limitations
Mouse	20-30 μm	Low cost, genetic models available	Extremely thin cartilage, not load-bearing
Rat	100-150 μm	Inexpensive, widely used, easy handling	Thin cartilage, less similar to humans
Rabbit	200-300 μm	Moderate size, suitable for defect modeling	Rapid healing not reflective of humans
Goat	1000-2000 μm	Load-bearing joints, similar cartilage size	Higher cost, ethical concerns
Pig	900-1300 μm	Joint size, cartilage thickness similar to humans	Expensive, difficult handling
Sheep	700-1200 μm	Weight-bearing joints, suitable for long-term studies	Expensive, anatomical differences

Clinical trials and outcomes

Clinical trials investigating stem cell-based therapies for cartilage repair are steadily increasing. A pioneering study by Centeno et al[84] reported significant improvement in pain and joint function following autologous BMSC injection in patients with knee OA. In a double-blind randomised controlled trial, Chen et al[85] showed that a single intra-articular injection of high-dose ADSCs led to improved cartilage volume and reduced pain on MRI at 6 to 12 months follow-up. Combination therapies are also gaining traction. In a phase I/II trial, Park et al[86] treated patients with a composite scaffold loaded with allogeneic BMSCs (Cartistem^®), which resulted in the regeneration of hyaline-like cartilage and improved ICRS scores over 7 years of follow-up. Notably, long-term safety was confirmed with no tumor formation or immune rejection. Emerging trials are also exploring iPSC-based therapies. A recent first-in-human study in Japan tested iPSC-derived chondrocyte sheets for knee cartilage lesions, demonstrating initial safety and tissue integration over a 1-year period[87]. However, scalability and cost remain barriers to broader adoption.

Outcomes are commonly assessed using clinical indices (WOMAC, Knee injury and Osteoarthritis Outcome Score, visual analog scale), MRI grading (e.g., Magnetic Resonance Observation of Cartilage Repair Tissue score), and sometimes second-look arthroscopy with biopsy. While short-term improvements are encouraging, few studies exceed 5 years of follow-up, emphasizing the need for long-term surveillance. While numerous clinical trials report outcomes with stem cell-based therapies, a few have shown limited or no significant improvement. For instance, a randomized controlled trial comparing ACI with debridement reported no significant difference in functional scores after 24 months follow-up. Other trials have observed modest effects of MSC injections in patients with late-stage OA, likely due to hostile inflammatory microenvironments and advanced matrix damage. These findings underscore the importance of careful patient selection and optimizing delivery timing for stem cell-based treatments.

Translational barriers

Despite compelling evidence, several hurdles limit widespread clinical adoption of stem cell-based cartilage regeneration. First, cell variability remains a major concern. Donor age, comorbidities, and harvest site affect MSC proliferation and differentiation. For instance, aged MSCs show senescence-associated secretory phenotypes, which impair regenerative potential[88,89]. Standardized good manufacturing practice-compliant protocols are essential for reproducible cell quality. Second, phenotypic instability, particularly hypertrophic differentiation, reduces the biomechanical durability of regenerated tissue. Strategies such as stepwise differentiation, hypoxic preconditioning, and small-molecule inhibitors (e.g., parathyroid hormone-related protein) are under investigation to improve stability[90]. Third, immunogenicity and safety concerns, especially for allogeneic MSCs and iPSC-derived products, require rigorous preclinical validation. While MSCs are considered immune-privileged, repeated administration may induce immune memory. iPSCs carry the theoretical risk of teratoma formation, necessitating high-purity differentiation and exhaustive screening[91]. Lastly, regulatory complexity and cost remain obstacles. The classification of stem cell products as advanced therapy medicinal products under European Medicines Agency and Food and Drug Administration frameworks demands extensive documentation, which slows commercialization. Additionally, lack of insurance reimbursement for stem cell interventions hampers patient access[92,93]. Addressing these challenges will require not only scientific innovation but also regulatory reform, industrial partnerships, and the development of scalable, cost-effective manufacturing systems.

FUTURE PERSPECTIVES

Gene editing and molecular engineering

The integration of gene-editing technologies into stem cell-based cartilage regeneration offers a powerful strategy to enhance efficacy and reduce risks. CRISPR/Cas9, in particular, allows for precise manipulation of genes involved in chondrogenesis, hypertrophy, and immune response. For example, targeted knockdown of collagen type X alpha 1 and MMP13 using CRISPR has been shown to suppress hypertrophic differentiation in MSCs, preserving the hyaline cartilage phenotype during long-term culture[94,95]. Similarly, overexpression of SOX9 via lentiviral or CRISPRa systems has been reported to enhance ECM deposition and stabilize chondrogenic commitment. Recent advancements in mRNA-based therapeutics have shown potential in OA treatment. For instance, intra-articular delivery of lipid nanoparticle-encapsulated recombinant human fibroblast growth factor 18 mRNA has been demonstrated to promote cartilage regeneration and mitigate OA progression in preclinical models[96]. This approach enhances mRNA expression within the joint cavity, leading to improved ECM synthesis and subchondral bone remodeling. These findings suggest that mRNA-based therapies could offer a promising avenue for OA management.

Recent advances also focus on modifying stem cells to secrete therapeutic factors or express inducible anti-inflammatory molecules. Engineered MSCs to overexpress interleukin-1 receptor antagonist, significantly reducing cartilage damage and inflammation in a rat OA model[97]. These examples highlight how molecular reprogramming can both boost regenerative potential and protect against joint degradation. Moreover, engineered exosomes derived from gene-modified MSCs are gaining traction as cell-free alternatives. Exosomes loaded with miR-140, a microRNA known to regulate cartilage homeostasis, have been shown to promote cartilage repair and inhibit catabolism in preclinical OA models[98].

Personalized regenerative strategies

The future of cartilage regeneration lies in personalized, data-driven approaches that account for patient-specific variables such as age, comorbidities, biomechanics, and genetic background. Advances in 3D imaging (MRI, CT) allow for the generation of anatomically tailored bioprinted scaffolds that match cartilage defect geometry with sub-millimeter precision. A study by Di Bella et al[99] successfully used CT-guided bioprinting to create personalized cartilage implants that were surgically placed in large animal defects with excellent integration and mechanical function.

In addition, multi-omics technologies-such as single-cell RNA-sequencing and proteomics-are beginning to reveal inter-individual variability in stem cell differentiation and immune profiles[100]. These data can guide the selection of optimal cell types, culture conditions, and scaffold materials. For instance, researcher can use transcriptomic analysis to predict which MSC subpopulations would perform best under chondrogenic stimuli. AI and machine learning are also being deployed to analysis large clinical and molecular datasets. Algorithms are being trained to predict therapeutic outcomes based on patient characteristics and to optimize scaffold composition, mechanical cues, and drug combinations in silico before clinical application.

Interdisciplinary collaboration and clinical translation

A major determinant of success in stem cell-based cartilage therapy will be the formation of integrated, multidisciplinary teams. Collaboration among orthopedic surgeons, biomaterials engineers, stem cell biologists, and regulatory specialists is essential to bridge the gap between bench and bedside. Initiatives such as the European Project BIORIMA and the United States-based Regenerative Medicine Innovation Project are establishing standardized safety and efficacy frameworks, including biocompatibility, sterility, and potency assays, for stem cell and scaffold-based products. Furthermore, international registries are being developed to track long-term outcomes and adverse events, supporting evidence-based regulatory decision-making.

Industry-academia partnerships are critical for good manufacturing practice-scale manufacturing of stem cells and biomaterials. Bioreactor platforms for automated, closed-system expansion of MSCs are now commercially available and will likely play a pivotal role in scaling up production. Parallel progress is needed in storage, transportation, and on-site delivery systems, such as cryopreserved cell sheets or injectable hydrogels. Finally, policy advocacy and patient education are necessary to foster public trust and ensure equitable access. As reimbursement structures evolve, real-world cost-effectiveness data will be vital to justify the clinical adoption of next-generation therapies.

CONCLUSION

Over the course of this review, we have discussed the biological characteristics of articular cartilage, the mechanisms underlying its limited regenerative capacity, and the clinical limitations of conventional repair techniques. Stem cells-particularly MSCs and iPSCs-have emerged as promising candidates for cartilage regeneration due to their multipotency, immunomodulatory functions, and ability to respond to bioengineering cues. We reviewed current strategies to enhance chondrogenic differentiation through biochemical signals, scaffold design, and mechanical stimulation, as well as emerging technologies such as 3D bioprinting. Preclinical animal studies and early-phase clinical trials have provided growing evidence that stem cell-based therapies can promote hyaline-like cartilage regeneration, improve joint function, and alleviate pain in patients with focal cartilage defects or early-stage OA.

Despite these advances, stem cell-based cartilage regeneration still faces significant challenges before achieving routine clinical translation. The risk of hypertrophic differentiation, variability in donor cell quality, and incomplete integration of engineered tissue into native cartilage remain pressing concerns. Furthermore, current therapies lack long-term outcome data and are often limited by scalability, cost, and regulatory complexity. Looking forward, emerging tools-such as CRISPR/Cas9-mediated gene editing, patient-specific 3D printing, multi-omics profiling, and machine learning-guided therapy design-are poised to reshape the landscape of personalized cartilage regeneration. In parallel, innovations in cell-free therapies, such as exosome-based delivery and smart biomaterials, may offer less invasive yet effective alternatives. To fully realize the clinical potential of stem cell-based cartilage repair, future research must emphasize long-term efficacy, standardized protocols, and cross-disciplinary collaboration to bridge the remaining gaps between laboratory discoveries and real-world clinical solutions.