Bridging the gap in ligament healing: A systematic review of secretome-derived therapies

INTRODUCTION

Anterior cruciate ligament (ACL) injuries are among the most prevalent knee injuries, particularly affecting athletes and physically active individuals. In the United States, approximately 150,000 ACL injuries are reported annually.^[1] In Asia, the incidence of ACL injuries has been rising, especially among children and adolescents, due to increased participation in sports. However, comprehensive epidemiological data specific to Asian countries remain limited. Although precise incidence rates are scarce, research indicates a growing focus on ACL-related studies within Southeast Asia.^[2] A bibliometric analysis revealed a steady increase in ACL-related publications from Southeast Asian countries between 2010 and 2019, reflecting a heightened awareness and interest in ACL injuries within the region.^[3]

In Indonesia, the exact annual incidence of ACL injuries has not been well documented. However, given the country’s population of approximately 270 million, estimates suggest that up to one million individuals may sustain ACL ruptures each year. The prevalence of sports-related injuries, including ACL tears, is notably high due to the increasing engagement in both competitive and recreational sports.^[4] Furthermore, a significant proportion of ACL reconstructions (ACLRs) in Indonesia is performed on younger patients who require expedited and complete recovery to resume daily activities or professional athletic careers.^[5] Despite the increasing demand for ACL treatment, access to advanced biological therapies remains limited, and existing rehabilitation programs are often prolonged due to suboptimal graft healing, highlighting the need for improved treatment strategies. ACLR using autografts, such as hamstring tendon and bone–patellar tendon–bone grafts, remains the gold standard for surgical management. However, despite surgical advancements, challenges such as prolonged graft remodeling, inadequate integration, and donor site morbidity persist, potentially affecting long-term functional outcomes.^[6]

Notwithstanding progress in ACL restoration methodologies, issues such as graft integration, mechanical integrity, and prolonged functional recovery remain prevalent. Regenerative medicine strategies, such as biologic augmentation, have garnered interest as prospective methods to improve transplant healing. Regenerative medicine techniques have incorporated secretome-based therapies as an innovative method to improve graft healing and integration.^[7] The secretome denotes the assemblage of bioactive molecules released by mesenchymal stem cells (MSCs), encompassing cytokines, growth factors (e.g., vascular endothelial growth factor [VEGF], transforming growth factor-beta [TGF-β], insulin-like growth factor-1 [IGF-1]), and extracellular vesicles (EVs), which exhibit considerable regenerative capacity in diverse musculoskeletal contexts. The advantageous effects of the secretome encompass increased cellular proliferation, regulation of inflammation, extracellular matrix remodeling, and facilitation of angiogenesis, all of which are essential for graft integration and ligamentization in ACLR.^[8]

Pre-clinical investigations indicate that secretome administration can expedite graft healing by optimizing collagen organization, augmenting vascularization, and enhancing biomechanical characteristics. Nonetheless, clinical translation is constrained by diversity in study designs, discrepancies in secretome preparation, and uneven documentation of clinical outcomes. This systematic review aims to critically analyze and synthesize existing evidence about the effects of secretome therapy in ACL autograft restoration, in light of the increasing interest in biologic augmentation for this procedure.^[9] We will evaluate its effects on histological healing, biomechanical strength, imaging-based graft integration, and clinical functional results to deliver a thorough synthesis of its potential advantages.

MATERIALS AND METHODS

Eligibility criteria

Studies will be chosen according to the subsequent inclusion criteria: (1) Utilization of MSC-derived secretome (including EVs, exosomes, or conditioned media) in ACL autografts and (2) conventional ACLR devoid of secretome or supplemented with placebo treatment. Randomized controlled trials (RCTs), prospective and retrospective cohort studies, and animal studies assessed the secretome in ACL autograft restoration. Exclusion criteria for this review: (1) Research involving allografts or synthetic grafts, (2) editorials and narrative reviews, (3) studies without quantitative result data, and (4) non-English articles (unless accompanied by a high-quality translation) see Figure 1. The primary outcome of this study comprised histological results: Collagen arrangement, cellular growth, and angiogenesis; biomechanical outcomes: Ultimate tensile strength and graft stiffness; clinical functional outcomes: International Knee Documentation Committee (IKDC) score, Lysholm score, Tegner activity scale, KT-1000/2000 arthrometer values; and imaging results: Scores for graft integration based on magnetic resonance imaging (MRI). The Population, Intervention, Comparison, Outcome (PICO) structure is illustrated in Table 1.^[10-15]

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Figure 1:

Preferred reporting items for Systematic Reviews and Meta-analyses flow chart. *: Database recorded, **: Data screened.

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Table 1: PICO structure.

Population	Patients undergoing ACLR using autografts (e.g., hamstring tendon, bone-patellar tendon-bone) with or without adjunctive biological treatment.
Intervention	Application of secretome-based therapy (derived from mesenchymal stem cells or other regenerative sources) to enhance autograft healing and integration in ACLR.
Comparison	•Standard ACL autograft reconstruction without secretome therapy (conventional treatment). •Other biological augmentation methods (e.g., platelet-rich plasma, growth factors, or stem cells).
Outcome	•Graft integration (magnetic resonance imaging, histological analysis). •Biomechanical strength of the graft. •Functional recovery (International Knee Documentation Committee, Lysholm, Tegner scores).

ACLR: Anterior cruciate ligament reconstruction

RESULTS

The initial database search yielded 678 studies, of which 6 studies met the inclusion criteria after screening’ see Table 2. The included studies analyzed the effects of secretome therapy on ACL autografts in pre-clinical and clinical settings. Our studies report that few studies used bone marrow-derived MSC secretome, while others utilized adipose-derived secretome. The follow-up periods varied from 12 weeks to more than 24 months.

The secretome denotes the entirety of bioactive chemicals released by cells into their extracellular milieu. This includes proteins, cytokines, chemokines, growth factors, EVs (including exosomes), and other signaling molecules.^[6] The secretome plays a crucial role in intercellular communication and is involved in various biological processes, including tissue repair, immune modulation, angiogenesis, and inflammation regulation.^[16] In regenerative medicine, the secretome is predominantly examined concerning MSCs because of its potential therapeutic benefits without requiring direct cell transplantation. Conditioned medium (CM) is the culture medium harvested from cells following a specified incubation duration, encompassing the secretome and its bioactive constituents.^[17] CM is regarded as a cell-free treatment strategy that utilizes the advantageous properties of the secretome while mitigating issues related to live cell therapies, including immunological rejection and tumorigenicity. The composition of the conditioned media is contingent upon the cell source, culture circumstances, and external stimuli, all of which might affect its regeneration capacity.^[18]

In the realm of ACLR, secretome-based therapies are being investigated as a supplementary approach to enhance graft integration and healing.^[4] Research indicates that the application of secretome can improve collagen remodeling, increase fibroblast proliferation, and facilitate angiogenesis, resulting in enhanced biomechanical qualities of the graft. Growth factors, including TGF-β, VEGF, and IGF-1, found in the secretome, facilitate extracellular matrix formation, tissue remodeling, and enhanced vascularization.^[19] Moreover, EVs in the secretome have demonstrated the ability to enhance cellular signaling pathways that promote ligamentization, a process wherein the graft develops into a ligament-like structure. Secretome-based therapy in orthopedic applications, especially in ACL restoration, signifies a viable approach for improving surgical outcomes.^[5] Nonetheless, additional study, encompassing extensive clinical trials, is essential to ascertain the ideal protocols for secretome formation, delivery mechanisms, and long-term safety and efficacy. With the progression of regenerative medicine, secretome-based therapies may offer a novel and efficacious alternative to conventional ACL graft augmentation methods.^[20]

DISCUSSION

This systematic review indicates that secretome-treated autografts demonstrate notable enhancements in histological results, biomechanical characteristics, imaging-based recovery, and functional outcomes relative to untreated controls.^[8] These findings highlight the promise of secretome as a novel therapeutic approach to improve autograft integration in ACL rehabilitation.^[21] Grafts treated with secretome exhibited superior collagen organization, heightened fibroblast proliferation, and greater vascularization compared to controls. The enhanced collagen alignment and density indicate that the secretome promotes extracellular matrix remodeling, essential for graft strength and durability.^[22] The growth factors and cytokines in the secretome presumably promote fibroblast proliferation and migration, hence expediting tissue regeneration. Moreover, heightened vascularization, evidenced by increased capillary density and upregulation of VEGF, augments oxygen and nutrition supply, hence promoting enhanced graft survival and integration.^[18]

Numerous investigations indicated enhanced biomechanical performance in secretome-treated grafts, exhibiting greater tensile strength and stiffness relative to controls. The findings indicate that the secretome improves the mechanical integration of the graft, potentially diminishing the likelihood of graft failure.^[7] The enhanced mechanical qualities may result from expedited extracellular matrix deposition and collagen maturation, both of which augment graft durability under physiological stress. MRI analyses indicated that secretome-treated grafts displayed increased signal intensity, implying improved ligamentization.^[9] Enhanced MRI results suggest expedited and thorough graft remodeling, consistent with histological and biomechanical observations. The results corroborate the concept that the secretome expedites the transformation from an immature graft to a more ligamentous structure, hence enhancing long-term functionality.^[22]

Clinical investigations indicated that patients receiving secretome-enhanced grafts exhibited superior functional results, as assessed by the IKDC and Lysholm scores.^[16] These enhancements indicate improved joint stability, less discomfort, and overall knee functionality, which are essential for patient rehabilitation and resumption of physical activity.^[23] Furthermore, secretome therapy may diminish post-operative problems, such as stiffness and graft failure, therefore enhancing long-term patient satisfaction.^[17] The elevated functional ratings indicate that patients might undergo expedited recovery periods and enhanced trust in joint stability.^[24] These effects are especially pertinent for athletes or individuals with substantial physical demands, since a swifter return to full activity can greatly influence career longevity and overall quality of life. Subsequent research ought to concentrate on longitudinal functional evaluations, integrating objective biomechanical assessments and patient-reported outcomes to further substantiate the advantages of secretome therapy in ACL restoration.^[25]

The findings of this analysis could be highly relevant to the Indonesian community, particularly given the high frequency of ACL injuries among athletes and physically active adults.^[18] Sports such as football, badminton, and martial arts are prevalent in Indonesia, making ACL injuries a frequent worry. In addition, labor-intensive occupations popular in Indonesia could benefit from faster recovery times given by secretome treatment.^[26] However, various variables must be examined before clinical implementation in Indonesia. Regulatory approval from Badan Pengawas Obat dan Makanan is necessary, and subsequent clinical trials on Indonesian patients will assist in establishing its efficacy and safety in the local community.^[19] Another major consideration is cost-effectiveness, as modern biologic medicines can be expensive, and accessibility may be limited to specialist medical institutes. Future research should focus on understanding whether secretome-based treatments are possible and effective in Indonesia’s diversified healthcare system, particularly in balancing cost with improved patient outcomes.^[27]

The encouraging results from this investigation indicate that the administration of secretome in ACL restoration may enhance healing efficacy and patient outcomes. Nevertheless, additional study is required to standardize secretome preparation, dose, and administration techniques.^[28] Existing discrepancies in secretome formulations and administration techniques may influence therapy effectiveness, underscoring the necessity for thorough optimization and regulatory endorsements before extensive clinical use. Extended clinical investigations are necessary to evaluate the persistence of these effects and their influence on graft life.^[29] Furthermore, comparative analyses of secretome therapy and traditional biologic enhancement techniques, including platelet-rich plasma and MSCs, may elucidate the most efficacious regenerative approach for ACL restoration.

A crucial factor is the cost-effectiveness of secretome treatment. Preliminary evidence indicates possible clinical advantages; nevertheless, economic assessments must determine if these enhancements warrant the supplementary expenses in surgical and rehabilitation contexts.^[30] If validated, secretome therapy has the potential to transform graft healing methodologies, shorten recovery durations, decrease problems, and enhance overall patient quality of life. Subsequent studies ought to examine personalized methodologies in secretome therapy, considering patient-specific variables such as age, activity level, and genetic susceptibility to enhance treatment efficacy. Advancements in bioengineering approaches may improve secretome efficacy through controlled-release formulations and scaffold-based delivery systems for prolonged therapeutic benefits.

CONCLUSION

Secretome-treated grafts exhibit significant advantages in histological organization, biomechanical properties, imaging-based healing, and functional outcomes. These findings support the potential role of the secretome as a regenerative adjunct in ACLR. While early evidence is promising, further clinical trials are essential to validate these benefits and optimize implementation in orthopedic surgery.