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Introduction

Osteoarthritis (OA) affects approximately 32.5 million adults in the United States alone [1]. Serving as the most common form of arthritis, OA commonly affects individuals between the ages of 55 and 64 [1]. OA can lead to severe pain, psychological disorders, restrictions on activities, and decreased quality of life. Management of the disease currently involves pharmacological or surgical treatments [2]. Stem cell therapy offers a promising alternative due to the cells’ ability to differentiate and generate new chondrocytes. Additionally, scaffolds provide the necessary structure for cellular adhesion, nutrient diffusion, and tissue development. While these treatments can offer some relief to patients, further investigation into innovative cartilage regenerative techniques is warranted. 

Articular Cartilage

Articular cartilage is a type of fibrocartilage commonly found in synovial joints [3]. This cartilage contains both type I and type II collagen, with type I collagen providing additional strength and durability [3]. Collagen plays a crucial role in promoting elasticity and structural integrity within the joint. Additionally, articular cartilage has a high water content, allowing it to function as both a shock absorber and joint lubricant, while also maintaining an avascular environment that depends on diffusion for nutrient exchange and waste removal [3,4]. A lack of direct blood supply contributes to the cartilage’s limited self-healing capacity, making it susceptible to degeneration and injury. With time, mechanical stress, aging, and trauma can result in cartilage deterioration, leading to conditions such as OA [3].

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Cartilage is produced and maintained by chondrocytes, which play a vital role in preserving joint integrity and health through the extracellular matrix (ECM) [3,5]. The ECM contains essential components such as water, collagen, proteoglycans, and non-collagenous proteins, which protect against injury and serve as mechanical signal transducers within cells [4,6]. The ECM is mainly constructed from water, promoting extensive frictional characteristics [4]. 

Over time, cartilage undergoes remodeling due to degradation. Aging leads to diminished chondrocyte function, resulting in reduced cartilage maintenance and repair [6]. Significant cartilage degeneration, pain, and dysfunction are primary contributors to OA. Cartilage damage most commonly results from injury, overuse, or degeneration [3]. Young patients are more prone to sports-related injuries, whereas older patients are more commonly affected by wear and tear and age-related degeneration [3].

Defining Osteoarthritis 

Osteoarthritis presents in numerous fashions—pathological, radiographic, and clinical [7]. Radiographic OA is the ‘standard’ for defining the disease. The Kellgren-Lawrence (K/L) system is used to grade radiographic OA [7]. This classification evaluates OA on a scale from 0 to 4, with Grade ≥2 suggesting the presence of an osteophyte. Further advanced grades are based on features such as joint space narrowing, sclerosis, cyst formation, and deformity [7, Table 1]. Additionally, more sensitive imaging, such as magnetic resonance imaging (MRI), can help identify structural variations in joints [7]. Current research suggests further investigation into the effectiveness of MRI in identifying potential disease-modifying interventions faster than standard radiography. Meanwhile, not all individuals with radiographic OA experience clinical symptoms. 

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Table 1: Kellgren-Lawrence (K/L) grading system for radiographic OA

Householder et al. recognize knee OA as a deficiency of articular cartilage at the medial and lateral condyles of the femur, tibial plateau, and patellofemoral facet [8]. Symptoms present as chronic pain, joint inflammation, and restricted mobility [8]. The exacerbated pain then contributes to decreased joint function and, eventually, leads to physical disability. 

Risk Factors

Radiographic OA

Risk factors for radiographic osteoarthritis are multifactorial [7]. Both systemic and local causes contribute to disease development. Systemic factors include, but are not limited to, age, sex, ethnicity, nutrition, and bone density. Meanwhile, local causes are recognized as obesity, a history of trauma, and mechanical factors like muscle weakness [7].

Systemic Causes

Age

Age is the most prominent contributing risk factor for all joints [7]. In patients over the age of 60, radiographic knee OA occurs in over 30% of patients, although not all may note pain symptoms [7]. With age, cartilage degenerates and chondrocyte function declines. Along with mechanical stress, chronic inflammation can alter the subchondral bone metabolism, contributing to progressive OA signs and symptoms [7].

Sex

Further, there is a greater incidence and severity of disease among women compared to men. There is a known increase in OA around menopause in women, which highlights some theories that hormones play a role in the development of OA [7]. For instance, a study investigating patients on hormone replacement therapy displayed a 15% decrease in the need for a total hip or knee arthroplasty compared to women not taking supplemental hormones [9]. 

Ethnicity

Nelson et al. studied OA prevalence across racial groups, finding comparable hip OA rates between African American (AA) and white women (23% vs. 22%) but slightly higher rates in AA men than white men (21% vs. 17%) [10]. Radiographic OA findings, such as joint space narrowing and osteophytes, are also more common in AAs [7]. Additionally, anatomical differences in femoral and acetabular structures among racial groups warrant further investigation [7].

Similarly, AAs exhibit higher rates of radiographic and symptomatic knee OA than non-Hispanic whites, leading to greater pain and severity scores [11]. Callahan et al. reported that AAs are 50–65% more likely to develop knee OA than whites [12]. Interestingly, Chinese women have a 45% higher prevalence of knee OA than white women, though no significant difference is observed between Chinese and white men [12].

Genetics

Studies have investigated the potential genetic component of OA. Both twin and familial studies have determined heritable components of OA, estimating a 50-65% greater genetic relationship to hand and hip OA [7]. Congenital disorders such as Legg-Calve-Perthes disease, Slipped Capital Femoral Epiphysis, or congenital subluxation have a known relationship with hip OA later in life [7].

Diet

Vitamin D deficiency is one of OA's most notable nutritional risk factors [7]. Depleted levels of vitamin D can lead to fragile bones. McAlindon et al. examined participants in the Framingham Heart Study to evaluate whether serum vitamin D levels and dietary intake could predict the incidence and progression of knee OA [13]. The study determined that the lowest (<27 ng/mL) and middle (27-33 ng/mL) tertiles of serum 25-hydroxyvitamin D have a three-fold increased risk of knee OA compared to subjects in the highest tertile (>33 ng/mL) [13]. Regarding radiographic OA, Lane et al. noted that decreased 25-vitamin D is possibly related to changes found in hip OA imaging, such as joint space narrowing [14]. Nonetheless, there is limited data on the effects of vitamin deficiencies on OA, suggesting further research needs to be conducted to investigate the effects of vitamin D and additional vitamins on OA. 

Local Causes

Obesity

Patients with high BMIs have a significant risk of developing OA, especially knee OA. Women who had lost about ten pounds in the Framingham Knee Osteoarthritis Study had approximately a 50% reduced risk of symptomatic knee OA development [15]. The researchers also indicated a clear connection between weight loss and decreased risk of radiographic knee OA [15]. On the other hand, when exploring hip OA risks, patient weight and disease development have no clear relationship [7]. 

Injury

Knee injury is another prominent risk factor for OA. Common examples of injury include meniscal tears, ACL injuries, and trans-articular fractures [7]. 

Mechanical Factors

The relationship between muscle strength and OA is an extremely complicated topic of study and is not fully understood. Data suggests that muscle weakness and atrophy are connected to knee OA [7]. Slemenda et al. concluded that patients with asymptomatic radiographic knee OA without muscle atrophy, but with quadriceps weakness, had an increased risk of developing symptomatic knee OA [16]. 

Discussion

Current Guidelines for Osteoarthritis Management 

Although there is a significant disparity in OA prevalence, an effective treatment for the disease remains lacking [17]. As the disease is influenced by multiple factors, its treatment approach must also be multifaceted. The American College of Rheumatology (ACR) and the Arthritis Foundation collaborated to develop the 2019 guidelines for managing OA of the hand, knee, and hip [2, Table 1]. When approaching OA treatment, there are two key options: surgical or non-surgical [17]. Regarding the non-surgical approaches, there are strong recommendations for exercise and weight loss in overweight or obese patients with hip or knee OA [17]. Additional knee-specific recommendations include topical NSAIDs, intra-articular injections, and tibiofemoral bracing [8,17]. NSAIDs play a crucial role in reducing inflammation and managing OA symptoms [3]. The first-line pharmacologic agent for managing OA is Tylenol [17]. NSAIDs and COX-2 inhibitors fall under second-line treatment [17]. Topical medications can be used in conjunction with oral treatments to offer more relief. Not to mention, injections, especially those that contain hyaluronic acid (HA), can restore the joint’s lubrication process [17]. It is important to note that the medical management component of OA, specifically injections, does not delay the progression of the disease [8]. Thus, the focus of treatment has shifted to the biomechanisms of cartilage deterioration. 

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Table 1: Physical and medical approaches to managing OA [2]. 

Dark blue represents strongly recommended techniques. Light blue reflects conditionally recommended strategies.

Conditional recommendations for OA include yoga, cognitive behavioral therapy (CBT), and self-management strategies [2]. Physical therapy has been shown to improve joint strength and flexibility, while balance training has demonstrated promising outcomes for knee and hip OA [3]. Additionally, orthoses have been particularly effective in managing hand OA [2]. 

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The current standard of surgical interventions for osteochondral defects includes marrow stimulation techniques, autologous chondrocyte implantation, and osteochondral grafting [3,18]. These current treatment options often result in the formation of both type I & II collagen-forming fibrocartilage, whose strength and resilience are lower compared to articular cartilage. Furthermore, they are only applicable for smaller defects, and total joint replacement remains the definitive treatment of OA with larger cartilage defects [18]. Several types of cells, such as chondrocytes or pluripotent cell types that can differentiate into chondrocytes, such as mesenchymal, embryonic, bone marrow, or adipose-derived stem cells, can be potentially used to stimulate cartilage regeneration and maintenance when applied alongside the proper scaffold, such as collagen and proteoglycans [3,18].

Techniques Based On Cell Type

Chondrocytes

The technique of autologous chondrocyte implantation (ACI) was employed initially for cell-based therapy to repair cartilage. In this technique, healthy chondrocytes are harvested from a non-weight-bearing portion of the joint in the initial procedure and then cultured in vitro for 14 to 21 days [19]. The cultured chondrocytes were then injected into the defect, which was covered with a periosteal flap, harvested from the proximal tibia, in a suspension fluid containing collagen [19]. Fibrin glue is then used to seal the affected area. Promising results were seen for this technique, with both pain and swelling reduced and knee locking eliminated [19]. However, ACI comes with the risk of morbidity at the donor site as both the collagen and periosteum are harvested and later implanted, contributing to two separate surgeries [4,18].

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The collagen-covered ACI (CACI) is the improved second generation of ACI, which reduced the risk of periosteal compromise by using collagen to cover the defective site [18]. Further advancements led to the third generation of ACI, which uses biomaterial scaffolds as the base on which harvested chondrocytes are attached and later inserted at the lesion site [18]. This new technique was termed matrix-assisted ACI (MACI) [18]. The use of MACI prevents the chondrocytes from being damaged during the injection phase, as the chondrocytes are maintained on the scaffold rather than suspended in collagen as seen in ACI and CACI [18].

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A limitation of ACI is that the ex vivo growth of the chondrocytes may lead to loss of differentiation or loss of phenotype due to age-related decline in harvested chondrocytes, which renders them impotent for cartilage regeneration [20]. Although fetal and neonatal chondrocytes can develop significantly faster and with better collagen type II and proteoglycan structure, their limited availability has enabled a deeper investigation into other sources [18].

Pluripotent Stem Cells (PSCs)

Pluripotent stem cells (PSCs) are found to have the ability to differentiate into chondrocytes like embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) [18]. Even though ESCs are a phenomenal option for tissue regeneration, their unlimited potential to self-renew and differentiate into numerous tissue types raises ethical concerns as they are harvested from embryos in the blastocyst stage [18]. A superior alternative would be iPSCs, which can be generated from various cell types, including fibroblasts, cord blood cells, peripheral blood monocytes, and even a patient's skin or blood [18,21]. This provides a more ethical, less invasive, and reasonable option for PSCs to be harvested and used for cartilage regeneration. Further studies need to be conducted to evaluate the prevention of teratoma formation, as these undifferentiated cells can grow uncontrollably and are at risk for tumor formation [18]. 

Even though their potential for regenerative medicine and disease modeling has no ceiling, bioengineered grafts based on iPSCs are limited to only scientific and laboratory-scale productions [21]. Another limitation of iPSCs is the use of viral vectors for their generation, such as retro- or lentiviruses, which pose the possibility of genetic instability by integrating into the host cell genome or creating interference for host cell functions [21]. Several iPSC protocols have been studied in the past for chondrogenesis, but none have yet been approved for clinical trials. Thus, further studies are still required to further analyze controls and create a safe and stringent protocol for iPSC differentiation [21]. 

Mesenchymal Stem Cells (MSC)

MSCs, derived from adult tissue, carry a high proliferative capacity and capability to be multipotent, differentiating into adipocytes, chondrocytes, osteoblasts, and myocytes [18]. Another added benefit of using MSCs for chondrocyte regeneration is that they carry anti-inflammatory and regenerative characteristics, which help promote self-healing, while also producing collagen and proteoglycans, aiding in the growth and maintenance of cartilage [3]. The use of MSC and HA injected into the injured intra-articular site of porcine models showed cartilage regeneration [22]. However, it is important to acknowledge that although these cells display early clinical success, tissue repair commonly fails due to their negligible mechanical components [4].

The use of bone marrow stem cells (BMSCs), most commonly MSCs, has been associated with improved defect filling and higher collagen type II content following implantation in multiple animal studies. However, no clinical studies have yet been conducted to see if similar results can be replicated in humans [18]. 

Adipose-derived stem cells (ADSCs) are an alternative to BMSCs and are more plentiful and accessible, but do carry slightly lower chondrogenic potential [18]. Synovial-derived stem cells (SDSCs) have been associated with superior chondrogenic capacity when compared with other MSCs, such as skeletal muscle, adipose, bone marrow, and periosteum. Nonetheless, SDSCs are seen to withhold some fibroblastic capacity even after implantation, which deems them unreliable as a substitute for cartilage repair and regeneration [18]. 

The use of MSCs in the treatment of cartilage repair and regeneration has been promising, especially given their ability to differentiate into chondrocytes while also harboring anti-inflammatory mechanisms to promote healing and decrease rejection rates. Nevertheless, more thorough research is still required to understand the mechanism of signaling and optimal conditions for survival, proliferation, and MSC differentiation in vivo, while also needing strict protocols for administration methods and doses before they can be used in a broader clinical setting [3]. The use of autologous MSCs along with new tissue engineering techniques may hold the keys to becoming the panacea for cartilage repair and regeneration.

Scaffolds

The goal of employing scaffolds is to provide a temporary structure that establishes a three-dimensional design on which seeded cells can adhere, while also providing mechanical support that aids in cartilage development over time [18]. The scaffold material must possess specific characteristics: (i) controllable biodegradation and pliability without producing cytotoxic materials, (ii) the ability to allow nutrients and metabolite diffusion across the scaffold and regulate cell activity; (iii) support for cell proliferation, survival, differentiation and extracellular matrix production; (iv) the capacity to anchor into the cartilage defect site and integrate into the surrounding tissue; (v) provide mechanical stability and support for the new tissue; (vi) and appropriate regulation of surface chemistry to promote overall growth [18,21]. 

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Scaffolds used for tissue engineering are broadly grouped into either natural or synthetic matrix materials [21]. Natural materials can be further categorized as protein or carbohydrate-based polymers, while composite polymers serve as a combination of natural and synthetic materials [18]. Several clinical studies have shown positive results from the use of natural or synthetic scaffolds for cartilage repair and regeneration, with most candidates claiming reduced pain and injury site healing compared to their counterparts, who received the traditional treatment of microfracture [18]. 

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Techniques Based on Cellular Signaling

Cellular signaling using cytokines and growth factors is crucial in promoting the proliferation and growth of tissues and cartilage [21]. Chondrogenesis requires these endogenous molecules to signal in a coordinated autocrine and paracrine fashion to stimulate proper repair and regeneration [21]. The most common and studied cytokines involved in cartilage regeneration include the transforming growth factor (TGF) superfamily; platelet-derived growth factors (PDGF), insulin-like growth factor 1 (IGF-1), and fibroblast growth factors (FGF) [21]. 

Challenges of Generating Cartilage 

One of the most challenging factors is due to the avascular nature of articular cartilage [17]. The lack of vascularization hinders progenitor cell penetration, thus limiting repair processes. Further, the restoration of osteochondral damage that includes subchondral bone is instituted by undifferentiated MSCs from bone marrow. The new tissue generated is comprised of fibrocartilage, which is an insufficient replacement for hyaline articular cartilage [17]. When considering the osteochondral grafting methods, injured osteochondral regions are replaced with autologous (mosaicplasty) or allogeneic (allograft transplantation) tissue [17]. There are concerns with donor site morbidity and graft failure during mosaicplasty, in addition to potential disease transmission and limited cell viability with allograft methods [17].

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Huey et al. note that the greatest challenge in cartilage engineering is ensuring that the tissue can effectively perform one of its primary functions: bearing weight [4]. Currently, no biomedically generated materials on the market can fully replicate the compressive, tensile, and frictional properties of healthy cartilage on a large scale. Additionally, existing techniques fail to mimic the essential structure-function relationship of natural cartilage. When cartilage is loaded, the interstitial fluid within the joint becomes pressurized due to steric and electrostatic interactions, which hinder water movement and bear the majority of the load [4]. The remaining weight is then supported by the ECM. The complex ECM structure and tensile strength properties have been challenging to generate in a laboratory environment. However, advancements have been made with remodeling components, TGF-beta, as well as mechanical stimuli to improve the biomechanically engineered tensile forces [4]. A focus should be directed at augmenting collagen arrangement, maturation, and cross-linking [4]. 

The field of regenerative medicine has been continuously studying the various methods for cartilage regeneration to combat OA progression. Various approaches used currently to tackle cartilage repair include cell-based therapies, biologics, and physical therapy [3]. Tissue engineering is the future of cartilage regeneration, using biology and material science principles to promote tissue regeneration and repair [21]. This allows for potential biological and synthetic cartilage constructs to be implanted in a single step to promote durable tissue repair [18]. The advantages of tissue engineering lie in its ability to provide personalized treatment strategies while also promoting tissue repair and regeneration with minimal rejection and donor dependence [21]. The use of scaffold materials as the architectural support required for the cultured cells--harvested and expanded ex vivo for transplantation and stimulation of self-healing--is the core basis of this technology [21]. 

Conclusion

While medical and surgical methods have been the longstanding management for OA, cartilage regeneration provides promising results for future treatment of the disease. However, further research is still needed to fill the knowledge gaps before scaffolds and stem cells can be effectively combined for the successful creation of type II cartilage in previously injured osteoarthritic joints. The field of tissue engineering opens a gateway for durable tissue repair, incorporating stem cells and various scaffolds. 

References

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20. Adkisson, H. D., 4th, Martin, J. A., Amendola, R. L., Milliman, C., Mauch, K. A., Katwal, A. B., Seyedin, M., Amendola, A., Streeter, P. R., & Buckwalter, J. A. (2010). The potential of human allogeneic juvenile chondrocytes for restoration of articular cartilage. The American journal of sports medicine, 38(7), 1324–1333. https://doi.org/10.1177/0363546510361950

21. Chen, M., Jiang, Z., Zou, X., You, X., Cai, Z., & Huang, J. (2024). Advancements in tissue engineering for articular cartilage regeneration. Heliyon, 10(3), e25400. https://doi.org/10.1016/j.heliyon.2024.e25400

22. Lee, K. B., Hui, J. H., Song, I. C., Ardany, L., & Lee, E. H. (2007). Injectable mesenchymal stem cell therapy for large cartilage defects--a porcine model. Stem cells (Dayton, Ohio), 25(11), 2964–2971. https://doi.org/10.1634/stemcells.2006-0311