Current Concepts and Clinical Applications in Cartilage Tissue Engineering

Orthobiologics

The field of TE has advanced in parallel with the field of orthobiologics, with both disciplines critically influencing each other. Due to their ease, biological therapies are common in clinical practice and thus provided the ability to better understand the role of cells and signaling agents within a joint. ⁹ Platelet-rich plasma (PRP) was one of the earliest biological therapies utilized in orthopedics and continues to be popular today. It was first employed in reconstructive treatment in 1975 and first reported in the context of bone regeneration in 1999.^10,11 Since the breakthrough understanding of PRP’s anti-inflammatory properties, the field of orthobiologics and TE has advanced significantly, with improvements in technological variety and intervention efficacy.^12,13 Particularly, it has been discovered that varying the composition ratios of leukocytes, platelets, growth factors, and other mediators offers different advantages depending on the application, as described by Everts et al. (2020). ¹² Furthermore, Baghersad et al. (2024) described PRP’s role as a bioactivator of engineered cartilage scaffolds, as PRP and scaffolds work synergistically to promote a regenerative environment. ¹⁴

Cartilage TE has focused on strategies that combine biological, cellular, and material science approaches to aid and optimize native tissue regeneration. The lessons learned during this endeavor have expanded the use of PRP and mesenchymal cell-based therapies in clinical orthopedic practice with promising results. A recent systematic review and meta-analysis aimed at assessing the synergy of PRP, mesenchymal stem cells (MSCs), and hyaluronic acid (HA) found improvement in Lysholm, Western Ontario and McMaster Universities Arthritis Index, and Knee Society scores, indicating a potential benefit in effective cartilage regeneration in osteoarthritic knees when these agents are combined. ¹⁵

Cell sources

Cell-based biological therapy is another pillar within cartilage and reconstructive interventions and uses a variety of cell sources. These include mesenchymal stem cells (MSCs), embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), and autologous chondrocytes.^16–18 Currently, MSCs and autologous chondrocytes are the most prevalent in the field of cartilage TE. ESCs are not commonly explored due to the potential for disease, tumorigenicity, and ethical concerns, whereas iPSCs are not popular yet due to the complexity of preparation. However, iPSCs are viewed by some as the most promising cell source for cartilage applications, especially after the discovery of the chondroprogenitor cell, derived from iPSCs that holds promise as a new cell source with unique characteristics.^17,18 Specifically, chondroprogenitor cells have a rich differentiation potential toward osteocytes, chondrocytes, as well as adipocytes. These cells appear to have the advantage of forming hyaline-like cartilage without hypertrophy and are mechanoresponsive and initiate chondrogenesis without increase in type X collagen or alkaline phosphatase.^19,20

MSCs can differentiate into chondrocyte lineages, thereby mediating mitochondrial function, regulating cytokines, and balancing the synthesis of the extracellular matrix (ECM). ²¹ The two most common MSCs in use for cartilage TE are bone-derived and adipose-derived stem cells. Briefly, bone-derived MSCs have a high capacity for chondrocyte differentiation and respond well to transforming growth factor-beta (TGF-β); however, these cells are most prone to endochondral bone formation that limits their mechanical functionality. ²² To counter these limitations, adipose-derived MSCs have been studied and boast advantages such as ease of harvest and lower risk of hypertrophy.^23,24 However, both bone-derived and adipose-derived MSCs have the potential to differentiate into multiple cell types, which can lead to a mixture of cartilaginous and fibrocartilaginous tissue, with disadvantages such as poor mechanical properties and hypertrophy. This is a significant disadvantage when attempting to build pure hyaline cartilage because this heterogenous composition may eventually fail due to its inadequate structural integrity.^8,25 Therefore, it is thought that MSCs may be more effective in a paracrine environment-supporting role rather than in direct cartilage production.

The MSC paracrine function is best illustrated and applied through coculture, which is defined by the culture of multiple, distinct cell types, either directly or indirectly, within the same culture environment. ²⁶ In the case of direct coculture, MSCs and chondrocytes share physical contact, whereas for indirect coculture only the environment is shared. Both methods have a common goal: to drive cartilage tissue formation and maintain potency of stem cells during their expansion. ²⁶ Direct coculture has advantages of robust cartilage formation due to direct cell–cell interactions; however, the risk of hypertrophy is higher. To lower the hypertrophy risk, indirect coculture takes advantage of MSCs paracrine abilities to provide a more controlled, albeit slower, differentiation. Coculture techniques are used in the production of injectable hydrogels, and the potential use of coculture in combination with current surgical interventions is an active and promising area of research.^27–30

Autologous chondrocytes have been considered an irreplaceable cell source for use in cartilage injury interventions. ³¹ Unfortunately, these chondrocytes are known to dedifferentiate in vitro, which results in fibrocartilage during cartilage repair and regeneration in vivo. ³² Additionally, the ability for autologous chondrocytes to proliferate is negatively affected by patient age, which can limit the success of this cell source in older patients.^33,34 Autologous chondrocytes are currently used within surgical procedures to repair focal cartilage lesions, which will be the focus of the following section. ³¹ Overall, the use of an autologous cell source is advantageous because there is no concern for disease or immune rejection; however, obtaining autologous chondrocytes results in significant donor site morbidity.^35,36

An additional challenge within chondrocyte culture is adequate cell proliferation. Once chondrocytes have been obtained, they must be proliferated in vitro to reach the number of cells needed to generate an implant. As described by Nordberg et al., as cell therapies continue to advance, researchers must know how to set up and maintain a Master Cell Bank, used for long-term supply of cells, and a Working Cell Bank, used for experimentation, testing, or production. ⁷ A major cell therapy breakthrough in cartilage TE is the method of conservative chondrogenic passaging and aggregate rejuvenation that allows preservation of chondrogenic phenotype to P11 and significantly enhanced glycosaminoglycans content and type II collagen. ³⁷

Scaffold and scaffold-less approaches

In native tissues, the ECM provides structural support in an anchorage-dependent manner.^38,39 Synthetic scaffolds, typically made of polymeric biomaterials, aim to emulate the ECM, thus providing structural support and promoting subsequent tissue development. An ideal cartilage scaffold is one which (1) closely resembles the physiological structure of interest, (2) is biocompatible without immune rejection, (3) contains an appropriate pore size to allow nutrient diffusion, and (4) is reproduceable for widespread use. ⁴⁰ Since native cartilage has the ECM and lacks an actual scaffold, the presence of the scaffold has to be temporary and mimic architecture and organization of ECM.

Recent advancements in cartilage TE have focused on improving scaffold structures, developing scaffold-free constructs, and enhancing cell stimulation. ⁷ Efforts within scaffold technology are primarily focused on maximizing three-dimensional (3D) printing capabilities. Techniques such as electrospinning, 3D bioprinting, and 3D weaving have allowed researchers to pursue complex scaffold structures that mimic the internal organization of cartilage tissue.^7,41–46

Electrospinning allows engineers to produce flexible, dense fibrous membranes in a multilayer configuration.^27,44 This technique has been utilized for the production of a 3D nanofiber scaffold, which demonstrated cartilage-like character in rabbit models. ⁴⁷ 3D bioprinting has led to customizable implants to the exact specifications of cartilage defects, and in the future, this technology may be used to create on-demand patient-specific TE cartilage. Microextrusion is the most promising type of 3D bioprinting due to its ability to deposit bioinks with high cell density, and engineers have found success in the repair of swine cartilage defects using MSCs within a 3D bioprinted gelatin/hydroxyapatite scaffold.^48,49 Lastly, 3D weaving is a manufacturing technique used to create unique, thick, and anisometric scaffolds. This technique has been used in a study that combined 3D-woven polycaprolactone fibers, MSCs, and viable bone to assess the resulting tissue synthesis. ⁵⁰ Scaffold-based approaches excel in applications where load bearing is necessitated (due to the exogenous material), which is one of the reasons it holds promise for cartilage. However, there are significant limitations including poor remodeling and integration in vivo, as well as toxicity concerns. ⁵¹

Hydrogel scaffolds have been widely investigated in cartilage TE due to their high (∼90%) water content, porous structure, and biocompatibility. ²⁷ Natural (e.g., HA) or synthetic (e.g., polyethylene glycol) materials are used in combination with growth factors and stem cells or chondrocytes to form injectable in situ forming implants. ⁵² Injectable hydrogels have gained attention as a potential replacement for implantation surgery due to their ability to fill irregular defects and noninvasive mode of delivery. ²⁷ Recently, injectable hydrogels have been used toward the development of a favorable joint environment via immunoregulation that can enhance cartilage repair and potentially delay osteoarthritic changes. ⁵³

Noninjectable hydrogels have been the mainstay of cartilage hydrogel technology due to their superior mechanical properties. Recent advancements in noninjectable hydrogel technology include the engineering of an Native denatured crosslinking (FL)₈ protein hydrogel using chain entanglement to create a stiff, tough, yet highly compressible hydrogel for use in cartilage applications. ⁵⁴ Additionally, for any noninjectable hydrogel scaffold to perform clinically, it must withstand the ∼10-kilopascal of hydraulic pressure in arthroscopic irrigation. The inferior mechanical stability and, thus, limited durability is one of the problems of hydrogels as this is a balance between functionality and toughness.^55,56 Hua et al. report the engineering of a hybrid photocrosslinkable hydrogel that can withstand these forces as demonstrated in vivo with swine models. ⁵⁷ Formation of self-assembled peptide fibers clad with metal ions could form a hydrogel of high fracture stress (∼4.1 MPa), toughness (25.3 kJ/m²), and high fatigue threshold (∼424 J/m²), thus highlighting the importance of tailoring hydrogel network structures at the molecular level to improve mechanical properties. ⁵⁸ Hydrogel technology will continue to advance and become more widely used in clinical settings due to their biocompatibility and adaptability to many different clinical scenarios.

Scaffold-free approaches have been pursued to counter the limitations of poor remodeling, integration, and toxicity concerns seen with scaffold-based approaches. Preliminary studies have shown greater biocompatibility, mechanical responsiveness, and capacity for integration and maturation. ⁵¹ Scaffold-free technology does not require cell seeding within an exogenous material (as in scaffold-based) but rather uses pellet and aggregate culture to create a self-assembling environment.^59,60 Notable recent advances in scaffold-free technology include the creation of larger, anatomically shaped implants, and the use of varying cell sources including adult dermal stem cells and osteochondritis dissecans fragments.^61–64 Overall, the benefits to scaffold-free constructs include a reduced time required for tissue construction and the ability to create complicated tissue and organ architecture. ⁶⁵

Cell stimulation

Cell stimulation is a prominent area of research within cartilage TE and can be divided into three broad categories: biomechanical, biochemical, and genetic alterations. ⁷ Biomechanical stimulation involves applying mechanical loading, hydrostatic pressure, vibration, or shear stress to stimulate the production of ECM and therefore enhance cartilage formation and maturation. ⁶⁶ Further exploration of the role of signaling molecules in chondrogenic differentiation identified multiple ways that biochemical and biomechanical stimuli can be applied in engineered tissue toward improvement of the functional properties of tissue engineered neocartilage.^67–70 A recent experiment by Kowsari-Esfahan et al. demonstrated that 10% strain (compared with 0%, 5%, 15%, and 20%), using a microfluidic device, optimized chondrogenesis in encapsulated adipose-derived stem cells in an alginate hydrogel. ⁷¹

To a similar end, biochemical stimulation relies on TGF-β, bone morphogenetic protein, fibroblast growth factor, insulin-like growth factor 1, and platelet-derived growth factor to maximize the growth and repair of cartilage. ⁷² The incorporation of select factors into scaffolds to optimize outcomes is a particularly active area of research. Bordbar et al. investigated the controlled release of TGF-β1 in an ECM-derived hydrogel and determined that applied mechanical stimuli and prolonged delivery of TGF-β1 upregulated chondrogenic genes and promoted cartilage tissue formation. ⁷³ Lastly, genetic alterations have been explored for the optimization of cartilage growth and regeneration, and more comprehensive details may be found in other reviews.^7,74,75 Briefly, recent advancements in this area have focused on advanced gene editing using CRISPR/Cas9 as well as optimizing gene delivery systems.^76,77

Microfracture

Often the first-line therapy for small to medium focal defects, this procedure harnesses the body’s natural healing response. A surgeon will arthroscopically create small holes in the subchondral bone beneath the cartilage defect and rely on marrow elements that form a “superclot,” with stromal cells that promote cartilage regeneration. ⁷⁸ This technique is generally indicated for patients with small (<2–4 cm²), full-thickness lesions without significant bone loss. ⁷⁹ Advantages of microfracture are its minimally invasive approach, low cost, and low technical demand. ⁸⁰ The disadvantages include the formation of fibrocartilage instead of hyaline cartilage, which has been demonstrated to be structurally inferior to the viscoelastic hyaline cartilage, limited durability of the fibrocartilage, and weight bearing restrictions to allow the cartilage to form and heal.^81–83 Survival rates have been reported as 88.8%, 67.9%, and 45.6% at 5, 10, and 12 years, respectively. ⁸⁴

A systematic review including 28 studies and 3122 patients undergoing microfracture for knee cartilage injury reported improved knee function during the first 24 months but conflicting evidence of durability afterward. ²⁵ This procedure is further limited as it is associated with worse medium-term outcomes in patients with elevated body mass index above 25 kg/m², age above 35, and for defects located in the patellofemoral compartment.^85–87 Finally, microfracture compromises the outcome of subsequent cartilage repair techniques as it violates the subchondral plate.^88–90 Microdrilling, a technique using instruments with smaller diameter, has been shown to yield better clinical results and causes less compression and destruction of the subchondral bone than the classical microfracture awl.^91,92 Due to its ease and low cost, microfracture remains one of the most commonly performed cartilage surgical interventions. However, the risk of compromising the effectiveness of future cartilage procedures has raised reasonable concerns and redefined clinical indications. Future relevant research is seeking to further improve its application.

Osteochondral autograft transplantation

This procedure is primarily used to treat small- to medium-sized well-contained cartilage lesions. It involves harvesting both surface cartilage and underlying subchondral bone from a nonweight bearing area of the patient’s own joint, such as the superomedial and superolateral portions of the femoral trochlea or the femoral notch area. The surgeon then prepares the harvested cartilage to fit the size and shape of the defect and performs the transplantation. ⁹³ This technique has advantages including the use of an autologous graft and immediate repair without risk of immune rejection; however, the obvious disadvantage is the morbidity at the donor site and imperfections in the alignment of the donor tissue within the defect. Another consideration is the potential chondrocyte death due to the mechanical impaction to press-fit the implant into the defect. A systematic review including 610 patients undergoing osteochondral autograft transplantation (OAT) with a mean 10-year follow-up reported a failure rate of 28% and reoperation rate of 19%. ⁹⁴ Outcomes have been shown to vary with increased failure rates in women, patients older than 40 years, and defect sizes >3 cm² (Solheim 2013). ⁹⁵ In a 10-year randomized control trial, the OAT procedure was found to have a significantly higher failure rate than autologous chondrocyte implantation (ACI) (55% vs 17%). ⁹⁶

Osteochondral allograft transplantation

This procedure is similar to the above OAT procedure, yet osteochondral allograft transplantation (OCA) utilizes cartilage donation from a deceased donor. The indications for OCA are larger defects, which cannot be reasonably replaced using the patient’s own tissue. Advantages of the OCA procedure are lack of donor site morbidity and the ability to restore larger defects. Disadvantages include graft failure, imperfections in fit, and immune rejection. Additionally, high costs and the risk of OCA disease transmission are additional problems. ³⁵ Gracitelli et al. evaluated 46 knees who underwent OCA and found a failure rate of 11% and reoperation rate of 24%. ⁹⁷

It is important to recognize that survival rate can be variable and worsens overtime. A meta-analysis reported an average 5-year survival rate of 86.7% (range 64.1–100.0%), while the 10-year survival rate was 78.7% (range 39.0–93.0%). ⁹⁸ The variability in survival of OCA can be explained partially by the recognition of risk factors for failure after OCA, such as bipolar or kissing chondral defects, male sex, older age, and greater body mass index. ⁹⁹ Survival rate is 72.8% and 67.5% at 15 and 20 years, respectively. ⁹⁸ The failure in one out of the three patients after 20 years appears relatively low, but it can be a challenging situation, especially when OCA is used in the management of chondral defects in adolescents and young adults.

Autologous chondrocyte implantation

Results from this technique were first described in 1994, and it involves an operation to harvest a modicum of cartilage from the patient (usually notch area or nonweight bearing area of the lateral femoral condyle), subsequent culture of the chondrocytes, and a second operation to suture a periosteal flap around the damaged cartilage and inject the cultured cells. ¹⁰⁰ This technique has advantages of producing hyaline-like cartilage, use for defects >2 cm², and that it uses autologous tissue. Additionally, there is less trauma than osteochondral autografts, and the procedure is easier to perform. ³⁵ However, disadvantages include the two-stage surgery, high cost, and technical demands on the surgeon. In addition, the periosteal flap can lead to hypertrophy, scarring, and inflammation, which necessitates a third operation.^101,102 A review of ACI procedures in 771 patients reported a failure and reoperation rate of 18% and 37%, respectively, 11.4 years after the surgical intervention. ¹⁰³

Matrix-associated autologous chondrocyte implantation

As an evolution and improvement to the ACI procedure, matrix-associated autologous chondrocyte implantation (MACI) has become increasingly popular for the treatment of focal cartilage lesions. This procedure was FDA approved in 2016. The key difference is that for MACI, the chondrocytes are preseeded onto a scaffold and then glued in place, as opposed to being injected beneath the sutured periosteal flap. The advantages of MACI, as compared with ACI, are a lower risk of hypertrophy, ability for a single-stage operation, and a more durable repair. However, there remain disadvantages: high cost and the lack of long-term data. Wang et al. performed a systematic review including 168 patients who underwent MACI for knee chondral defects with a minimum 10-year follow-up. ¹⁰⁴ They report significant and durable long-term improvements across multiple patient reported outcomes measures, in addition to satisfactory defect filling on magnetic resonance imaging (MRI). The progression to total knee arthroplasty was 7.4% at 10–17 years of follow-up. ¹⁰⁴

In summary, current clinical solutions for cartilage defects offer improvement in clinical outcomes and have a survival rate that is variable depending on patient and chondral defect characteristics. Microfracture is an easy and low-cost technique, but unfortunately offers only a short-term solution with high failure rates after the first 5 years. OAT appears to be also a low-cost technique, but it appears to have a failure rate close to 30% at 10 years and high associated morbidity. OCA offers high survival rate that can be up to 67% at 20 years, but the high cost and imperfection in alignment of the cartilage surface are major considerations.

In-office needle arthroscopy

Recently, a new technique, in-office needle arthroscopy (IONA), has made it possible to perform diagnostic arthroscopy in the office, without general anesthesia.^105,106 Bi et al. first described the use of needle arthroscopy for cartilage repair of chondral or osteochondral lesions using BioCartilage, a cartilage allograft ECM originally designed for use in conjunction with microfracture. ¹⁰⁷ This procedure allows for a cartilage biopsy for ACI to be performed in the office, making the MACI into a single-stage procedure. ¹⁰⁸ IONA can also be used for postoperative evaluation of osteochondral allograft transplant, allowing for accurate visualization of healing progress and can give more accurate recommendations for progression to the next phase of recovery. ¹⁰⁹

Agili-C

Approved by the FDA in 2022, this device is a cell-free implant for use in OA and osteochondral defects ranging from 1 to 7 cm². ¹¹⁰ It is composed of interconnected natural inorganic calcium carbonate, making the implant porous, biocompatible, and resorbable. Altshuler et al. reported the results of a multicenter randomized controlled trial comparing Agili-C to the control of microfracture/debridement for knee chondral and osteochondral defects. The study included 251 participants from 26 medical centers with a follow-up of 2 years. The Agili-C group showed statistically superior outcomes in Knee injury and Osteoarthritis Outcome Score subscores, International Knee Documentation Committee Subjective Knee Form scores, and defect fill by MRI. In the first 2 years only, the failure rate was 7.2% versus 21.4% for the implant and control groups, respectively. ¹¹¹

Chondro-Gide

This product received FDA Breakthrough Device designation in 2021 and is an acellular bilayer collagen I/III porcine membrane. It functions as a scaffold and is used in conjunction with ACI operations to regenerate cartilage. Steinwachs et al. performed a systematic review and meta-analysis of ACI using the Chondro-Gide product in knee chondral and osteochondral defects and included 12 studies with 375 patients. ¹¹² There was clinically significant improvement in pain Visual Pain Scale, Lysholm, and IKDC scores from baseline. IKDC scores at 5 years ranged between 72.2 and 90.9, and longer follow-up data are still pending.

Hyalofast

This product is a biodegradable, resorbable, nonwoven scaffold of proprietary hyaluronic matrix, which has been approved by the FDA in 2023 for use with MSCs from bone marrow aspirate. ¹¹³ There is an ongoing randomized controlled study (Hyalofast Phase III 15–01, NCT02659215), which aims to establish the superiority of Hyalofast compared with the control of microfracture. The study completion is estimated to be June 30, 2026. Kacprazak et al. have reported results of Hyalofast within 49 professional athletes who underwent microfracture and Hyalofast scaffold placement for knee cartilage injuries with a mean follow-up time of 19.75 months. ¹¹⁴ They report significant improvements with pain and quality of life compared with baseline.

Despite these advancements and integration into clinical practice, challenges still remain in both the regulatory logistics and actual implementation of these technologies. Premarket challenges arise in the conception, development, and clinical trials of the device or biological as it must progress through all stages of preclinical and clinical studies. Industry companies or academic institutions must secure stable funding throughout a somewhat uncertain period of research and development in order to see clinical impacts. Funding institutions must also navigate regulations from government entities. ¹¹⁵ The particular details regarding regulatory pathway progress can be found in Nordberg et al. ¹¹⁶

In addition, there must be reasonable reimbursement and adoption of new technologies; without reasonable reimbursement, these scientific achievements may seem fruitless for those who fund these endeavors and may halt clinical integration. ¹¹⁶ When a product does reach clinics, it faces an additional physician adoption barrier. Dodson et al. conducted interviews with experts regarding novel cell-based therapies and found that physicians were hesitant to adopt new therapies even if clinical trials reported superior results to existing therapies. ¹¹⁷ This may be associated to adaptation of a new procedure that sometimes is technically challenging, the high cost associated, and the lack of long-term outcomes for these new technologies initially. Additional skepticism for new technologies comes from the fact that microfracture is still used as the gold standard for cartilage lesions in these trials, while other treatments have been proven more effective in clinical practice.

3D bioprinting

The ability to use 3D bioprinting in clinical practice holds tremendous promise. The strength of this technology is the ability to produce patient-personalized constructs in preoperative planning to create an osteochondral implant perfectly complementary to the patient’s cartilage defect based on data obtained via computer tomography and MRI scans. ¹¹⁸ One primary challenge within TE cartilage is the tendency of chondrocytes to homogenously distribute throughout a biomaterial scaffold. Native chondrocytes are arranged in clusters or zones, and an even distribution of TE chondrocytes negatively impacts their integration into native tissue. Bioprinting can serve as a remedy, allowing for the precise delivery of cells and other bioactive substances (including growth factors) in an additive layer-by-layer approach that can recreate the zonal organization of the tissue. ¹¹⁹ Currently, the available techniques include primarily inkjet bioprinting, extrusion-based bioprinting, and less commonly vat polymerization and laser assister bioprinting. ¹¹⁸

Promising advances have been made as synthetic biomaterial inks are combined with cell-laden hydrogel bioinks to create hybrid structures with improved biomechanical and biological properties. Shim et al. developed a biomimetic multiphasic scaffold using multimaterial extrusion bioprinting of human turbinate-derived MSC (htMSC)-laden atelocollagen and HA bioinks, which included growth factors to induce the desired differentiation into osteocytes and chondrocytes of the htMSCs. This construct was successfully implanted into rabbit joints, where they observed neocartilage formation and osteochondral integration with no immune reaction over 8 weeks. ¹²⁰ More recently, Sun et al. addressed the challenge of the hierarchical nature of osteochondral tissue by developing a gradient-structured MSC-laden constructs that provides cells with zone-specific biochemical cues. This provided the material mechanical properties and chondrocyte differentiation in the superficial zone and smaller micropores in the deep zone. This was successfully transplanted into a rabbit osteochondral defect model, yielding great repairing effects. ¹²¹ Furthermore, artificial intelligence has been leveraged to aid in the bioprinting process including bioink selection and printing process. ⁷²

The major challenge that remains is the development of engineered grafts with inherent tissue integration capability, especially at the cartilage–cartilage interface, which would eliminate the current risk for implant failure when using donor tissues and allow for a single procedure.^40,122 Additionally, many of the available constructs have not been tested in large animal disease models, thus the translational ability of bioprinted tissues for clinical application has not been established.

Genetics and personalized care

The use of CRISPR/Cas9 gene editing would allow physicians to directly edit the genes responsible for ECM production and chondrocyte proliferation. ⁷⁶ Gene editing technology is currently being used in countless domains of research and will continue to become more relevant clinically in the coming years. Particularly, an in vivo study using CRISPR-Cas9 successfully induced the activation of the DANCR gene responsible for adipose to chondrocyte differentiation in a rat model. ¹²³ We expect these gene editing techniques to work in a synergistic manner with the other cell stimulation methods discussed previously and eventually transform outcomes for patients.

In addition, recent evolution in genetics demonstrated that the response or side effects to different pharmacologic agents can be different among patients based on their genetic profile.^124,125 As a result, the ability to personalize the response to various treatments and identify the potential risks for poor integration or in vivo degeneration may allow the engineering of products that have certain characteristics to allow successful treatment based on patient characteristics. This may impact dosing for biological treatments in order to maximize outcomes.

Improving intra-articular environment

The immune response plays a direct role in regulating the process of cartilage repair. ⁵³ During cartilage injury, the immune system secretes various proinflammatory, anti-inflammatory, and chondrogenic cytokines to facilitate cartilage repair. However, an imbalance between the secreted cytokines may end up causing more damage to joint cartilage tissue than repair as in OA or driving the repair into less favorable results as in replacing hyaline cartilage into fibrocartilage. One of the key mediators of the immune response is macrophages, which play a role in both the proinflammatory and anti-inflammatory state during cartilage repair. ¹²⁶

In a study done by Donahue et al., they were able to create a coculture model to study the effects of neocartilage scaffolds with different mechanical properties on the polarization of macrophages to either a pro- or anti-inflammatory state. ¹²⁶ They were able to conclude that scaffold stiffness and biofactors play a role in modulating the macrophages response. The development of this coculture model would eventually open doors to further studies to understand cartilage mechano-immunology.

Recent advancements in the field of immunology have allowed the use of anticlonal antibodies to immune-modulate inflammatory response of arthritis. ¹²⁷ Specifically, the use of targeted monoclonal antibodies in RA has significantly delayed the degenerative process of arthritis and, thus, improved the quality of life of numerous patients.^128,129 Parallel treatment focusing on modulation of the inflammatory process at the same time as TE cartilage implantation could allow to successfully overcome the current challenges of cartilage TE implantation.

Biological total knee joint replacement

In the mind of everyone involved in TE, the potential of biological total knee joint reconstruction that could supplement and eliminate the need for traditional knee replacement is closer than ever before. Current knee replacement surgeries, while effective, face challenges such as limited longevity, potential for complications, and issues with material compatibility. ¹³⁰ The ability to engineer tissues that can replace injured or damaged tissue has several challenges, but it has the major advantage that can be customized to meet the needs of the patient and the joint. The first step is to engineer tissues that can act as transplants and can successfully replace the damaged tissue.

Advances in the field such as 3D printing of bone, formation of large hyaline-like cartilage tissue via aggressive chondrocyte expansion, proliferation while maintaining the chondrogenic profile, and successful integration of bone and cartilage may allow for biological resurfacing of the joint instead of the use of metal implants (Fig. 3).^131–133 Osteochondral implants are among these advances, and advances in cartilage TE include cartilage with robust mechanical properties, allowing the implants to survive the joint environment. ⁷ A shape-specific cartilage-engineered osteochondral implant that could resurface the whole joint and be used as a biological implant—instead of a metal one—could transform the field of orthopedics and joint replacement. ¹³⁴

OPEN IN VIEWER

FIG. 3.

Timeline illustrating notable developments specifically related to cartilage TE within the past 30 years. TE, tissue engineering.

An example of successful osteochondral integration was the development of biphasic osteochondral constructs by Ding et al (2013). ¹³⁵ In this study, tissue-engineered goat femoral heads grown after implanting them subcutaneously into nude mice resulted in adequate mechanical and morphological properties. In 2010, Lee et al. achieved a breakthrough by creating a cell-free anatomical humeral head replacement in a rabbit model that functions through cell homing. ¹³⁶ Using 3D printing, they fabricated an anatomical humeral head with an intramedullary stem made out of poly-ε-caprolactone hydroxyapatite, which was infused with TGF-β3-adsorbed collagen hydrogel. After surgical implantation into live rabbits, the articular surface fully regenerated within 4 months, forming vascularized bone and likely hyaline cartilage through cell homing of endogenous stem cells. The TGF-β3-mediated cartilage had viscoelastic properties similar to native tissue, allowing for normal weightbearing and locomotion.

Osteochondral implants have been designed to emulate the osteochondral unit as a whole, promoting integration of engineered tissue with host tissue. ¹³⁷ Current work is evaluating engineered cartilage implants’ ability to survive inflammatory environments through the use of immunomodulatory biomaterials and synthetic biology. ⁷ Achieving this goal could significantly enhance patients’ quality of life while lowering healthcare costs, as these biological implants would have native tissue properties and can be manufactured at low cost. With that said, quality control methods for tissue-engineered products currently rely on expensive, time-consuming testing. ¹³⁸ Continued research and development in TE are crucial to bringing the vision of biological joint resurfacing, which could transform the future of joint replacement, to fruition.