Regeneration of the anterior cruciate ligament: Current strategies in tissue engineering

Intermittent Cyclic Stretch of Engineered Ligaments Drives Hierarchical Collagen Fiber Maturation in a Dose- and Organizational-Dependent Manner

Hierarchical collagen fibers are the primary source of strength in tendons and ligaments; however, these fibers largely do not regenerate after injury or with repair, resulting in limited treatment options. We previously developed a static culture system that guides ACL fibroblasts to produce native-sized fibers and early fascicles by 6 weeks. These constructs are promising ligament replacements, but further maturation is needed. Mechanical cues are critical for development in vivo and in engineered tissues; however, the effect on larger fiber and fascicle formation is largely unknown. Our objective was to investigate whether intermittent cyclic stretch, mimicking rapid muscle activity, drives further maturation in our system to create stronger engineered replacements and to explore whether cyclic loading has differential effects on cells at different degrees of collagen organization to better inform engineered tissue maturation protocols. Constructs were loaded with an established intermittent cyclic loading regime at 5 or 10% strain for up to 6 weeks and compared to static controls. Cyclic loading drove cells to increase hierarchical collagen organization, collagen crimp, and tissue tensile properties, ultimately producing constructs that matched or exceeded immature ACL properties. Further, the effect of loading on cells varied depending on degree of organization. Specifically, 10% load drove early improvements in tensile properties and composition, while 5% load was more beneficial later in culture, suggesting a shift in mechanotransduction. This study provides new insight into how cyclic loading affects cell-driven hierarchical fiber formation and maturation, which will help to develop better rehabilitation protocols and engineer stronger replacements. STATEMENT OF SIGNIFICANCE: Collagen fibers are the primary source of strength and function in tendons and ligaments throughout the body. These fibers have limited regenerate after injury, with repair, and in engineered replacements, reducing treatment options. Cyclic load has been shown to improve fibril level alignment, but its effect at the larger fiber and fascicle length-scale is largely unknown. Here, we demonstrate intermittent cyclic loading increases cell-driven hierarchical fiber formation and tissue mechanics, producing engineered replacements with similar organization and mechanics as immature ACLs. This study provides new insight into how cyclic loading affects cell-driven fiber maturation. A better understanding of how mechanical cues regulate fiber formation will help to develop better engineered replacements and rehabilitation protocols to drive repair after injury.

Assembly of Interfacial Polyelectrolyte Complexation Fibers with Mineralization Gradient for Physiologically-Inspired Ligament Regeneration

Current synthetic grafts for ligament rupture repair often fail to integrate well with the surrounding biological tissue, leading to complications such as graft wear, fatigue, and subsequent re-rupture. To address this medical challenge, this study aims at advancing the development of a biological ligament through the integration of physiologically-inspired principles and tissue engineering strategies. In this study, interfacial polyelectrolyte complexation (IPC) spinning technique, along with a custom-designed collection system, to fabricate a hierarchical scaffold mimicking native ligament structure, is utilized. To emulate the bone-ligament interface and alleviate stress concentration, a hydroxyapatite (HAp) mineral gradient is strategically introduced near both ends of the scaffold to enhance interface integration and diminish the risk of avulsion rupture. Biomimetic viscoelasticity is successfully displayed to provide similar mechanical support to native ligamentous tissue under physiological conditions. By introducing the connective tissue growth factor (CTGF) and conducting mesenchymal stem cells transplantation, the regenerative potential of the synthetic ligament is significantly amplified. This pioneering study offers a multifaceted solution combining biomimetic materials, regenerative therapies, and advanced techniques to potentially transform ligament rupture treatment.

Intermittent Cyclic Stretch of Engineered Ligaments Drives Hierarchical Collagen Fiber Maturation in a Dose- and Organizational-Dependent Manner

Hierarchical collagen fibers are the primary source of strength in tendons and ligaments, however these fibers do not regenerate after injury or with repair, resulting in limited treatment options. We previously developed a culture system that guides ACL fibroblasts to produce native-sized fibers and fascicles by 6 weeks. These constructs are promising ligament replacements, but further maturation is needed. Mechanical cues are critical for development in vivo and in engineered tissues; however, the effect on larger fiber and fascicle formation is largely unknown. Our objective was to investigate whether intermittent cyclic stretch, mimicking rapid muscle activity, drives further maturation in our system to create stronger engineered replacements and to explore whether cyclic loading has differential effects on cells at different degrees of collagen organization to better inform engineered tissue maturation protocols. Constructs were loaded with an established intermittent cyclic loading regime at 5 or 10% strain for up to 6 weeks and compared to static controls. Cyclic loading drove cells to increase hierarchical collagen organization, collagen crimp, and tissue mechanics, ultimately producing constructs that matched or exceeded immature ACL properties. Further, the effect of loading on cells varied depending on degree of organization. Specifically, 10% load drove early improvements in mechanics and composition, while 5% load was more beneficial later in culture, suggesting a cellular threshold response and a shift in mechanotransduction. This study provides new insight into how cyclic loading affects cell-driven hierarchical fiber formation and maturation, which will help to develop better rehabilitation protocols and engineer stronger replacements.

Enthesis repair - State of play

The fibrocartilaginous enthesis is a highly specialised tissue interface that ensures a smooth mechanical transfer between tendon or ligament and bone through a fibrocartilage area. This tissue is prone to injury and often does not heal, even after surgical intervention. Enthesis augmentation approaches are challenging due to the complexity of the tissue that is characterised by the coexistence of a range of cellular and extracellular components, architectural features and mechanical properties within only hundreds of micrometres. Herein, we discuss enthesis repair and regeneration strategies, with particular focus on elegant interfacial and functionalised scaffold-based designs.

Scaffold-induced compression enhances ligamentization potential of decellularized tendon graft reseeded with ACL-derived cells

Anterior cruciate ligament (ACL) reconstruction is often performed using a tendon graft. However, the predominant synthesis of fibrotic scar tissue (type III collagen) occurs during the healing process of the tendon graft, resulting in a significantly lower mechanical strength than that of normal ACL tissue. In this study, ACL-derived cells were reseeded to the tendon graft, and scaffold-induced compression was applied to test whether the compressive force results in superior cell survival and integration. Given nanofiber polycaprolactone (PCL) scaffold-induced compression, ACL-derived cells reseeded to a tendon graft demonstrated superior cell survival and integration and resulted in higher gene expression levels of type I collagen compared to non-compressed cell-allograft composites in vitro. Translocation of Yes-associated protein (YAP) into the nucleus was correlated with higher expression of type I collagen in the compression group. These data support the hypothesis of a potential role of mechanotransduction in the ligamentization process.

Temporal application of lysyl oxidase during hierarchical collagen fiber formation differentially effects tissue mechanics

The primary source of strength in menisci, tendons, and ligaments are hierarchical collagen fibers; however, these fibers are not regenerated after injury nor in engineered replacements, resulting in limited repair options. Collagen strength is reliant on fiber alignment, density, diameter, and crosslinking. Recently, we developed a culture system which guides cells in high-density collagen gels to develop native-like hierarchically organized collagen fibers, which match native fiber alignment and diameters by 6 weeks. However, tensile moduli plateau at 1MPa, suggesting crosslinking may be lacking. Collagen crosslinking is regulated by lysyl oxidase (LOX) which forms immature crosslinks that condense into mature trivalent crosslinks. Trivalent crosslinks are thought to be the primarily source of strength in fibers, but it's not well understood how they form. The objective of this study was to evaluate the effect of exogenous LOX in our culture system at different stages of hierarchical fiber formation to produce stronger replacements and to better understand factors affecting crosslink maturation. We found treatment with LOX isoform LOXL2 did not restrict hierarchical fiber formation, with constructs still forming aligned collagen fibrils by 2 weeks, larger fibers by 4 weeks, and early fascicles by 6 weeks. However, LOXL2 treatment did significantly increase mature pyridinium (PYD) crosslink accumulation and tissue mechanics, with timing of LOXL2 supplementation during fiber formation having a significant effect. Overall, we found one week of LOXL2 supplementation at 4 weeks produced constructs with native-like fiber organization, increased PYD accumulation, and increased mechanics, ultimately matching the tensile modulus of immature bovine menisci. STATEMENT OF SIGNIFICANCE: Collagen fibers are the primary source of strength and function in connective tissues throughout the body, however it remains a challenge to develop these fibers in engineered replacements, greatly reducing treatment options. Here we demonstrate lysyl oxidase like 2 (LOXL2) can be used to significantly improve the mechanics of tissue engineered constructs, but timing of application is important and will most likely depend on degree of collagen organization or maturation. Currently there is limited understanding of how collagen crosslinking is regulated, and this system is a promising platform to further investigate cellular regulation of LOX crosslinking. Understanding the mechanism that regulates LOX production and activity is needed to ultimately regenerate functional repair or replacements for connective tissues throughout the body.

Development of three-layer collagen scaffolds to spatially direct tissue-specific cell differentiation for enthesis repair

Enthesis repair remains a challenging clinical indication. Herein, a three-layer scaffold composed of a tendon-like layer of collagen type I, a fibrocartilage-like layer of collagen type II and a bone-like layer of collagen type I and hydroxyapatite, was designed to recapitulate the matrix composition of the enthesis. To aid tenogenic and fibrochondrogenic differentiation, bioactive molecules were loaded in the tendon-like layer or the fibrocartilage-like layer and their effect was assessed in in vitro setting using human bone marrow derived mesenchymal stromal cells and in an ex vivo model. Seeded human bone marrow mesenchymal stromal cells infiltrated and homogeneously spread throughout the scaffold. As a response to the composition of the scaffold, cells differentiated in a localised manner towards the osteogenic lineage and, in combination with differentiation medium, towards the fibrocartilage lineage. Whilst functionalisation of the tendon-like layer did not improve tenogenic cell commitment within the time frame of this work, relevant fibrochondrogenic markers were detected in the fibrocartilage-like layer when scaffolds were functionalised with bone morphogenetic protein 2 or non-functionalised at all, in vitro and ex vivo, respectively. Altogether, our data advocate the use of compartmentalised scaffolds for the repair and regeneration of interfacial tissues, such as enthesis.

Recent advances in tendon tissue engineering strategy

Tendon injuries often result in significant pain and disability and impose severe clinical and financial burdens on our society. Despite considerable achievements in the field of regenerative medicine in the past several decades, effective treatments remain a challenge due to the limited natural healing capacity of tendons caused by poor cell density and vascularization. The development of tissue engineering has provided more promising results in regenerating tendon-like tissues with compositional, structural and functional characteristics comparable to those of native tendon tissues. Tissue engineering is the discipline of regenerative medicine that aims to restore the physiological functions of tissues by using a combination of cells and materials, as well as suitable biochemical and physicochemical factors. In this review, following a discussion of tendon structure, injury and healing, we aim to elucidate the current strategies (biomaterials, scaffold fabrication techniques, cells, biological adjuncts, mechanical loading and bioreactors, and the role of macrophage polarization in tendon regeneration), challenges and future directions in the field of tendon tissue engineering.

Advanced Gene Therapy Strategies for the Repair of ACL Injuries

The anterior cruciate ligament (ACL), the principal ligament for stabilization of the knee, is highly predisposed to injury in the human population. As a result of its poor intrinsic healing capacities, surgical intervention is generally necessary to repair ACL lesions, yet the outcomes are never fully satisfactory in terms of long-lasting, complete, and safe repair. Gene therapy, based on the transfer of therapeutic genetic sequences via a gene vector, is a potent tool to durably and adeptly enhance the processes of ACL repair and has been reported for its workability in various experimental models relevant to ACL injuries in vitro, in situ, and in vivo. As critical hurdles to the effective and safe translation of gene therapy for clinical applications still remain, including physiological barriers and host immune responses, biomaterial-guided gene therapy inspired by drug delivery systems has been further developed to protect and improve the classical procedures of gene transfer in the future treatment of ACL injuries in patients, as critically presented here.

3D-Braided Poly-ε-Caprolactone-Based Scaffolds for Ligament Tissue Engineering

The anterior cruciate ligament (ACL) is the most commonly injured intra-articular ligament of the knee. Due to its limited intrinsical healing potential and vascularization, injuries of the ACL do not heal satisfactorily, and surgical intervention is usually required. The limitations of existing reconstructive grafts and autologous transplants have prompted interest in tissue-engineered solutions. A tissue engineering scaffold for ACL reconstruction must be able to mimic the mechanical properties of the native ligament, provide sufficient porosity to promote cell growth of the neoligament tissue, and be biodegradable. This study investigates long-term biodegradable poly-ε-caprolactone (PCL)-based scaffolds for ACL replacement using the 3D hexagonal braiding technique. The scaffolds were characterized mechanically as well as morphologically. All scaffolds, regardless of their braid geometry, achieved the maximum tensile load of the native ACL. The diameter of all scaffolds was lower than that of the native ligament, making the scaffolds implantable with established surgical methods. The 3D hexagonal braiding technique offers a high degree of geometrical freedom and, thus, the possibility to develop novel scaffold architectures. Based on the findings of this study, the 3D-braided PCL-based scaffolds studied were found to be a promising construct for tissue engineering of the anterior cruciate ligament.

Electrospun aligned poly(ε-caprolactone) nanofiber yarns guiding 3D organization of tendon stem/progenitor cells in tenogenic differentiation and tendon repair

Hierarchical anisotropy structure directing 3D cellular orientation plays a crucial role in designing tendon tissue engineering scaffolds. Despite recent development of fabrication technologies for controlling cellular organization and design of scaffolds that mimic the anisotropic structure of native tendon tissue, improvement of tenogenic differentiation remains challenging. Herein, we present 3D aligned poly (ε-caprolactone) nanofiber yarns (NFYs) of varying diameter, fabricated using a dry-wet electrospinning approach, that integrate with nano- and micro-scale structure to mimic the hierarchical structure of collagen fascicles and fibers in native tendon tissue. These aligned NFYs exhibited good in vitro biocompatibility, and their ability to induce 3D cellular alignment and elongation of tendon stem/progenitor cells was demonstrated. Significantly, the aligned NFYs with a diameter of 50 μm were able to promote the tenogenic differentiation of tendon stem/progenitor cells due to the integration of aligned nanofibrous structure and suitable yarn diameter. Rat tendon repair results further showed that bundled NFYs encouraged tendon repair in vivo by inducing neo-collagen organization and orientation. These data suggest that electrospun bundled NFYs formed by aligned nanofibers can mimic the aligned hierarchical structure of native tendon tissue, highlighting their potential as a biomimetic multi-scale scaffold for tendon tissue regeneration.

Functional Ultra-High Molecular Weight Polyethylene Composites for Ligament Reconstructions and Their Targeted Applications in the Restoration of the Anterior Cruciate Ligament

The selection of biomaterials as biomedical implants is a significant challenge. Ultra-high molecular weight polyethylene (UHMWPE) and composites of such kind have been extensively used in medical implants, notably in the bearings of the hip, knee, and other joint prostheses, owing to its biocompatibility and high wear resistance. For the Anterior Cruciate Ligament (ACL) graft, synthetic UHMWPE is an ideal candidate due to its biocompatibility and extremely high tensile strength. However, significant problems are observed in UHMWPE based implants, such as wear debris and oxidative degradation. To resolve the issue of wear and to enhance the life of UHMWPE as an implant, in recent years, this field has witnessed numerous innovative methodologies such as biofunctionalization or high temperature melting of UHMWPE to enhance its toughness and strength. The surface functionalization/modification/treatment of UHMWPE is very challenging as it requires optimizing many variables, such as surface tension and wettability, active functional groups on the surface, irradiation, and protein immobilization to successfully improve the mechanical properties of UHMWPE and reduce or eliminate the wear or osteolysis of the UHMWPE implant. Despite these difficulties, several surface roughening, functionalization, and irradiation processing technologies have been developed and applied in the recent past. The basic research and direct industrial applications of such material improvement technology are very significant, as evidenced by the significant number of published papers and patents. However, the available literature on research methodology and techniques related to material property enhancement and protection from wear of UHMWPE is disseminated, and there is a lack of a comprehensive source for the research community to access information on the subject matter. Here we provide an overview of recent developments and core challenges in the surface modification/functionalization/irradiation of UHMWPE and apply these findings to the case study of UHMWPE for ACL repair.

Melt-Spun, Cross-Section Modified Polycaprolactone Fibers for Use in Tendon and Ligament Tissue Engineering

Tissue Engineering is considered a promising route to address existing deficits of autografts and permanent synthetic prostheses for tendons and ligaments. However, the requirements placed on the scaffold material are manifold and include mechanical, biological and degradation-related aspects. In addition, scalable processes and FDA-approved materials should be applied to ensure the transfer into clinical practice. To accommodate these aspects, this work focuses on the high-scale fabrication of high-strength and highly oriented polycaprolactone (PCL) fibers with adjustable cross-sectional geometry and degradation kinetics applying melt spinning technology. Four different fiber cross-sections were investigated to account for potential functionalization and cell growth guidance. Mechanical properties and crystallinity were studied for a 24-week exposure to phosphate-buffered saline (PBS) at 37 °C. PCL fibers were further processed into scaffolds using multistage circular braiding with three different hierarchical structures. One structure was selected based on its morphology and scaled up in thickness to match the requirements for a human anterior cruciate ligament (ACL) replacement. Applying a broad range of draw ratios (up to DR9.25), high-strength PCL fibers with excellent tensile strength (up to 69 cN/tex) could be readily fabricated. The strength retention after 24 weeks in PBS at 37 °C was 83–93%. The following braiding procedure did not affect the scaffolds’ mechanical properties as long as the number of filaments and the braiding angle remained constant. Up-scaled PCL scaffolds resisted loads of up to 4353.88 ± 37.30 N, whilst matching the stiffness of the human ACL (111–396 N/mm). In conclusion, this work demonstrates the fabrication of highly oriented PCL fibers with excellent mechanical properties. The created fibers represent a promising building block that can be further processed into versatile textile implants for tissue engineering and regenerative medicine.

Development and assessment of biologically compatible anterior cruciate ligament using braided ultra-high molecular weight polyethylene

In this study, a ultra-high molecular weight polyethylene (UHMWPE) braided structure was surface modified with low temperature plasma and was coated with cationized gelatin and hyaluronic acid to improve its biocompatibility for the reconstruction of an anterior cruciate ligament (ACL). The ligament was studied for its various mechanical properties. Surface modifications were studied through FESEM. Biological compatibility of the ligament was assessed in accordance to ISO 10993 standard. Tensile strength of the UHMWPE reconstructed ligament ranges between 2628 and 5937 N; maximum tensile strength was attained in 1600 denier 2/2 pattern of triple braided structure along with higher strain at failure of 36.1%. In 1600 denier 2/2 pattern of triple braid structure, the linear stiffness was found to be high at 375 N/mm. Among the developed materials, four braided structures namely as 800 denier 2/2 pattern of double braids and triple braids, 1600 denier 1/1 pattern of double braid and 2/2 pattern of triple braid were found to be mechanically suitable. Specifically, the 1600 denier 2/2 pattern of triple braid having higher mechanical properties was selected for coating. The results of in-vitro cytotoxicity and genotoxicity confirmed the extract of ACL to non-toxic and non-mutant. Furthermore, in-vivo analysis of the extract and the coated ACL graft proved the ligament to be non-irritant, non-sensitizer and also found to promote new tissue formation around the graft. Based on the results, the CG and HA coated ACL graft were concluded to be biocompatible and having considerable potential as an alternate for autograft/allograft.

Second Generation of Tissue-Engineered Ligament Substitutes for Torn ACL Replacement: Adaptations for Clinical Applications

The anterior cruciate ligament (ACL) of the knee joint is one of the strongest ligaments of the body and is often the target of traumatic injuries. Unfortunately, its healing potential is limited, and the surgical options for its replacement are frequently associated with clinical issues. A bioengineered ACL (bACL) was developed using a collagen matrix, seeded with autologous cells and successfully grafted and integrated into goat knee joints. We hypothesize that, in order to reduce the cost and simplify the model, an acellular bACL can be used as a substitute for a torn ACL, and bone plugs can be replaced by endobuttons to fix the bACL in situ. First, acellular bACLs were successfully grafted in the goat model with 18% recovery of ultimate tensile strength 6 months after implantation (94 N/mm² vs. 520). Second, a bACL with endobuttons was produced and tested in an exvivo bovine knee model. The natural collagen scaffold of the bACL contributes to supporting host cell migration, growth and differentiation in situ post-implantation. Bone plugs were replaced by endobuttons to design a second generation of bACLs that offer more versatility as biocompatible grafts for torn ACL replacement in humans. A robust collagen bACL will allow solving therapeutic issues currently encountered by orthopedic surgeons such as donor-site morbidity, graft failure and post-traumatic osteoarthritis.

Current and future of anterior cruciate ligament reconstruction techniques

In recent years, anterior cruciate ligament (ACL) reconstruction has generally yielded favorable outcomes. However, ACL reconstruction has not provided satisfactory results in terms of the rate of returning to sports and prevention of osteoarthritis (OA) progression. In this paper, we outline current techniques for ACL reconstruction such as graft materials, double-bundle or single-bundle reconstruction, femoral tunnel drilling, all-inside technique, graft fixation, preservation of remnant, anterolateral ligament reconstruction, ACL repair, revision surgery, treatment for ACL injury with OA and problems, and discuss expected future trends. To enable many more orthopedic surgeons to achieve excellent ACL reconstruction outcomes with less invasive surgery, further studies aimed at improving surgical techniques are warranted. Further development of biological augmentation and robotic surgery technologies for ACL reconstruction is also required.

Early development of a polycaprolactone electrospun augment for anterior cruciate ligament reconstruction

Despite the clinical success of Anterior Cruciate Ligament reconstruction (ACLR) in some patients, unsatisfactory clinical outcomes secondary to graft failure are seen, indicating the need to develop new regeneration strategies. The use of degradable and bioactive textiles has the potential to improve the biological repair of soft tissue. Electrospun (ES) filaments are particularly promising as they have the ability to mimic the structure of natural tissues and influence endogenous cell behaviour. In this study, we produced continuous polycaprolactone (PCL) ES filaments using a previously described electrospinning collection method. These filaments were stretched, twisted, and assembled into woven structures. The morphological, tensile, and biological properties of the woven fabric were then assessed. Scanning electron microscopy (SEM) images highlighted the aligned and ACL-like microfibre structure found in the stretched filaments. The tensile properties indicated that the ES fabric reached suitable strengths for a use as an ACLR augmentation device. Human ACL-derived cell cultured on the fabric showed approximately a 3-fold increase in cell number over 2 weeks and this was equivalent to a collagen coated synthetic suture commonly used in ACLR. Cells generally adopted a more elongated cell morphology on the ES fabric compared to the control suture, aligning themselves in the direction of the microfibres. A NRU assay confirmed that the ES fabric was non-cytotoxic according to regulatory standards. Overall, this study supports the development of ES textiles as augmentation devices for ACLR.

Biomimetic Biphasic Electrospun Scaffold for Anterior Cruciate Ligament Tissue Engineering

BACKGROUND

Replacing damaged anterior cruciate ligaments (ACLs) with tissue-engineered artificial ligaments is challenging because ligament scaffolds must have a multiregional structure that can guide stem cell differentiation. Here, we designed a biphasic scaffold and evaluated its effect on human marrow mesenchymal stem cells (MSCs) under dynamic culture conditions as well as rat ACL reconstruction model in vivo.

METHODS

We designed a novel dual-phase electrospinning strategy wherein the scaffolds comprised randomly arranged phases at the two ends and an aligned phase in the middle. The morphological, mechanical properties and scaffold degradation were investigated. MSCs proliferation, adhesion, morphology and fibroblast markers were evaluated under dynamic culturing. This scaffold were tested if they could induce ligament formation using a rodent model in vivo.

RESULTS

Compared with other materials, poly(D,L-lactide-co-glycolide)/poly(ε-caprolactone) (PLGA/PCL) with mass ratio of 1:5 showed appropriate mechanical properties and biodegradability that matched ACLs. After 28 days of dynamic culturing, MSCs were fusiform oriented in the aligned phase and randomly arranged in a paving-stone-like morphology in the random phase. The increased expression of fibroblastic markers demonstrated that only the alignment of nanofibers worked with mechanical stimulation to promote effective fibroblast differentiation. This scaffold was a dense collagenous structure, and there was minimal difference in collagen direction in the orientation phase.

CONCLUSION

Dual-phase electrospun scaffolds had mechanical properties and degradability similar to those of ACLs. They promoted differences in the morphology of MSCs and induced fibroblast differentiation under dynamic culture conditions. Animal experiments showed that ligamentous tissue regenerated well and supported joint stability.

Maintenance of Ligament Homeostasis of Spheroid-Colonized Embroidered and Functionalized Scaffolds after 3D Stretch

Anterior cruciate ligament (ACL) ruptures are usually treated with autograft implantation to prevent knee instability. Tissue engineered ACL reconstruction is becoming promising to circumvent autograft limitations. The aim was to evaluate the influence of cyclic stretch on lapine (L) ACL fibroblasts on embroidered scaffolds with respect to adhesion, DNA and sulphated glycosaminoglycan (sGAG) contents, gene expression of ligament-associated extracellular matrix genes, such as type I collagen, decorin, tenascin C, tenomodulin, gap junctional connexin 43 and the transcription factor Mohawk. Control scaffolds and those functionalized by gas phase fluorination and cross-linked collagen foam were either pre-cultured with a suspension or with spheroids of LACL cells before being subjected to cyclic stretch (4%, 0.11 Hz, 3 days). Stretch increased significantly the scaffold area colonized with cells but impaired sGAGs and decorin gene expression (functionalized scaffolds seeded with cell suspension). Stretching increased tenascin C, connexin 43 and Mohawk but decreased decorin gene expression (control scaffolds seeded with cell suspension). Pre-cultivation of functionalized scaffolds with spheroids might be the more suitable method for maintaining ligamentogenesis in 3D scaffolds compared to using a cell suspension due to a significantly higher sGAG content in response to stretching and type I collagen gene expression in functionalized scaffolds.

Tissue-Specific Decellularized Extracellular Matrix Bioinks for Musculoskeletal Tissue Regeneration and Modeling Using 3D Bioprinting Technology

The musculoskeletal system is a vital body system that protects internal organs, supports locomotion, and maintains homeostatic function. Unfortunately, musculoskeletal disorders are the leading cause of disability worldwide. Although implant surgeries using autografts, allografts, and xenografts have been conducted, several adverse effects, including donor site morbidity and immunoreaction, exist. To overcome these limitations, various biomedical engineering approaches have been proposed based on an understanding of the complexity of human musculoskeletal tissue. In this review, the leading edge of musculoskeletal tissue engineering using 3D bioprinting technology and musculoskeletal tissue-derived decellularized extracellular matrix bioink is described. In particular, studies on in vivo regeneration and in vitro modeling of musculoskeletal tissue have been focused on. Lastly, the current breakthroughs, limitations, and future perspectives are described.

Rupture, reconstruction, and rehabilitation: A multi-disciplinary review of mechanisms for central nervous system adaptations following anterior cruciate ligament injury

BACKGROUND

Despite surgical reconstruction and extensive rehabilitation, persistent quadriceps inhibition, gait asymmetry, and functional impairment remain prevalent in patients after anterior cruciate ligament (ACL) injury. A combination of reports have suggested underlying central nervous system adaptations in those after injury govern long-term neuromuscular impairments. The classic assumption has been to attribute neurophysiologic deficits to components of injury, but other factors across the continuum of care (e.g. surgery, perioperative analgesia, and rehabilitative strategies) have been largely overlooked.

OBJECTIVE

This review provides a multidisciplinary perspective to 1) provide a narrative review of studies reporting neuroplasticity following ACL injury in order to inform clinicians of the current state of literature and 2) provide a mechanistic framework of neurophysiologic deficits with potential clinical implications across all phases of injury and recovery (injury, surgery, and rehabilitation) RESULTS: Studies using a variety of neurophysiologic modalities have demonstrated peripheral and central nervous system adaptations in those with prior ACL injury. Longitudinal investigations suggest neurophysiologic changes at spinal-reflexive and corticospinal pathways follow a unique timecourse across injury, surgery, and rehabilitation.

CONCLUSION

Clinicians should consider the unique injury, surgery, anesthesia, and rehabilitation on central nervous system adaptations. Therapeutic strategies across the continuum of care may be beneficial to mitigate maladaptive neuroplasticity in those after ACL injury.

Principles for establishment of the stem cell bank and its applications on management of sports injuries

BACKGROUND

The stem cells of the stem cell banks have prominent problems for insufficient sources, easy contamination, unstable biological characteristics after serial subcultivations, and high cost.

METHODS

After collecting the construction processes of the existing stem cell banks and suggestions from authoritative experts in the past 10 years, 230 reference principles were obtained, and finally, the principles of "5C" for the establishment of modern standardized stem cell banks were summarized, and their related applications on the management of sports injuries were reviewed as well.

RESULTS

The basic principles of "5C" for the establishment of modern standardized stem cell banks include (1) principle of informed consent, (2) confidentiality principle, (3) conformity principle, (4) contamination-free principle, and (5) commonweal principle. The applications of stem cells on repairs, reconstructions, and regenerations of sports injuries were also reviewed, especially in tissue-engineered cartilage, tissue-engineered meniscus, and tissue-engineered ligament.

CONCLUSIONS

The proposal of the basic principles of "5C" is conducive to relevant stem cell researchers and clinical medical experts to build modern stem cell banks in a more standardized and efficient manner while avoiding some major mistakes or problems that may occur in the future. On this basis, stem cells from stem cell banks would be increasingly used in the management of sports injuries. More importantly, these days, getting stem cell samples are difficult in a short time, and such banks with proper legal consent may help the scientific community.

Acceleration of ligamentization and osseointegration processes after anterior cruciate ligament reconstruction with autologous tissue-engineered polyethylene terephthalate graft

Background

Despite the advantages of excellent mechanical properties for rapid return to sports and early rehabilitation after anterior cruciate ligament (ACL) reconstruction with polyethylene terephthalate (PET) artificial ligament, the graft failure rate during long-term follow-up is relatively high due to poor graft-host incorporation. The purpose of the present study was to investigate the effect of autologous tissue-engineered PET (ATE-PET) grafts on osseointegration and ligamentization after ACL reconstruction.

Methods

Forty-eight New Zealand white rabbits were randomly divided into PET group (n=24) and ATE-PET group (n=24). In the ATE-PET group, the rabbits initially underwent subcutaneous implantation of the PET ligament. Two weeks later, unilateral ipsilateral ACL reconstruction was performed using an ATE-PET graft. In the PET group, the rabbits underwent ACL reconstruction using PET grafts as controls. Macroscopic observation, micro-computed tomography, histological and immunofluorescent staining, and biomechanical tests were conducted to evaluate the effects at 4 and 12 weeks postoperatively.

Results

The ATE-PET graft was highly pre-vascularized with myofibroblast aggregation after two weeks of subcutaneous implantation. With regard to the intraosseous part of the graft, the ATE-PET group had significantly higher bone mineral density and bone volume/total volume ratio at 12 weeks. Histologically, the width of the interface between the graft and bone was smaller. Regarding the intra-articular part, thicker tissue coverage with a glossy appearance was observed in the ATE-PET group at 12 weeks on macroscopic observation. Histological staining also showed more collagen fibers grew in the grafts with fewer inflammatory reactions of the ATE-PET group at both 4 and 12 weeks. Immunofluorescently, both α-SMA-positive vessels and α-SMA-positive myofibroblasts were found to be significantly greater around the graft in the ATE-PET group at 4 weeks and markedly declined at 12 weeks. Moreover, the ATE-PET group presented significantly greater failure load and stiffness than the PET group at 12 weeks (53.7±5.4 vs. 42.5±4.5 N, P<0.01; 12.9±3.0 vs. 9.8±1.3 N/mm, P=0.04).

Conclusions

The ATE-PET artificial ligament with pre-vascularization and myofibroblast aggregation could effectively accelerate intra-articular graft ligamentization and intraosseous graft osseointegration, thus enhancing the biomechanical properties after ACL reconstruction in a rabbit model.

Physeal-Sparing Anterior Cruciate Ligament Reconstruction for Skeletally Immature Patients: All-Epiphyseal Technique Using Quadricep Tendon Autograft

The anterior cruciate ligament (ACL) is a major stabilizing structure of the knee and one of the most common injured structures. The true incidence of ACL injury in children and adolescents is unknown, but recent studies suggest increased ACL injury rates, especially in the sports-participating population. The mechanism of injury, clinical examination, and diagnosis of ACL injury in children is the same as in adults. The main concerns in the management of pediatric ACL injuries are the open physes and the eventual long-term consequences of the ACL deficient knee. The ideal treatment strategy of pediatric ACL injuries is still controversial, because there is still no universal consensus for techniques, graft choices, and postoperative rehabilitation. We present a case of a 12-year-old male patient who underwent ACL reconstruction using an all-inside, physeal-sparing technique with a quadriceps tendon autograft and discuss the current treatment strategies.

Fibrous Systems as Potential Solutions for Tendon and Ligament Repair, Healing, and Regeneration

Tendon and ligament injuries caused by trauma and degenerative diseases are frequent and affect diverse groups of the population. Such injuries reduce musculoskeletal performance, limit joint mobility, and lower people's comfort. Currently, various treatment strategies and surgical procedures are used to heal, repair, and restore the native tissue function. However, these strategies are inadequate and, in some cases, fail to re-establish the lost functionality. Tissue engineering and regenerative medicine approaches aim to overcome these disadvantages by stimulating the regeneration and formation of neotissues. Design and fabrication of artificial scaffolds with tailored mechanical properties are crucial for restoring the mechanical function of tendons. In this review, the tendon and ligament structure, their physiology, and performance are presented. On the other hand, the requirements are focused for the development of an effective reconstruction device. The most common fiber-based scaffolding systems are also described for tendon and ligament tissue regeneration like strand fibers, woven, knitted, braided, and braid-twisted fibrous structures, as well as electrospun and wet-spun constructs, discussing critically the advantages and limitations of their utilization. Finally, the potential of multilayered systems as the most effective candidates for tendon and ligaments tissue engineering is pointed out.

Driving Hierarchical Collagen Fiber Formation for Functional Tendon, Ligament, and Meniscus Replacement

Hierarchical collagen fibers are the primary source of strength in musculoskeletal tendons, ligaments, and menisci. It has remained a challenge to develop these large fibers in engineered replacements or in vivo after injury. The objective of this study was to investigate the ability of restrained cell-seeded high density collagen gels to drive hierarchical fiber formation for multiple musculoskeletal tissues. We found boundary conditions applied to high density collagen gels were capable of driving tenocytes, ligament fibroblasts, and meniscal fibrochondrocytes to develop native-sized hierarchical collagen fibers 20-40 μm in diameter. The fibers organize similar to bovine juvenile collagen with native fibril banding patterns and hierarchical fiber bundles 50-350 μm in diameter by 6 weeks. Mirroring fiber organization, tensile properties of restrained samples improved significantly with time, reaching ~1 MPa. Additionally, tendon, ligament, and meniscal cells produced significantly different sized fibers, different degrees of crimp, and different GAG concentrations, which corresponded with respective juvenile tissue. To our knowledge, these are some of the largest, most organized fibers produced to date in vitro. Further, cells produced tissue specific hierarchical fibers, suggesting this system is a promising tool to better understand cellular regulation of fiber formation to better stimulate it in vivo after injury.

Fibrin-Based Biomaterial Systems to Enhance Anterior Cruciate Ligament Healing

Anterior cruciate ligament (ACL) tears are a common and potentially career-ending injury, particularly for athletes and soldiers. Partial and complete ruptures of this ligament cause instability in the knee, and the ACL does not have the capacity for healing due, in part, to its position within the highly thrombolytic synovial fluid environment of the knee joint. Traditional methods of ACL reconstruction, such as graft replacement with attached bone anchors for bone integration, restore stability, but do not prevent the development of post-traumatic osteoarthritis. To enhance therapeutic treatment options, novel fibrin-based technologies and repair techniques have been recently explored and show promise for improved patient outcomes. Through modification of existing surgical methods, such as the use of fibrin glues incorporating growth factors and cells and the implementation of scaffolds containing platelet-rich plasma, platelet-rich fibrin, and other blood derivatives, surgeons are attempting to overcome the shortcomings of traditional treatments. This mini-review will detail current efforts using fibrin-based treatments and discuss opportunities to further enhance ACL healing.

Artificial ligament made from silk protein/Laponite hybrid fibers

With developments in tissue engineering, artificial ligaments are expected to be future materials for anterior cruciate ligament (ACL) reconstruction. However, poor healing of the intraosseous part after ACL reconstruction significantly hinders their applications in this field. In this study, a bioactive clay Laponite (LAP) was introduced into the regenerated silk fibroin (RSF) spinning dope to produce functional RSF/LAP hybrid fibers by wet-spinning. These RSF/LAP hybrid fibers were then woven into artificial ligament for ACL reconstruction. The structure and mechanical properties of RSF/LAP hybrid fibers were extensively studied by different means. Results confirmed the presence of LAP in RSF fibers and revealed that the addition of LAP slightly deteriorated the comprehensive mechanical properties of RSF fibers. However, they were still much tougher (with higher breaking energy) than those of degummed natural silkworm silk that was earlier used for making artificial ligament. The artificial ligament woven from RSF/LAP hybrid fibers showed better cytocompatibility and osteogenic differentiation with mouse pre-osteoblasts in vitro than those made from degummed natural silkworm silks and pure RSF fibers. Furthermore, in vivo study in a rat ACL reconstruction model demonstrated that the presence of LAP in the artificial ligament could significantly enhance the graft osseointegration process and also improve the corresponding biomechanical properties of the artificial ligament. Based upon these results, the RSF/LAP hybrid fibers, which can be mass produced by wet-spinning process, are believed to have a great potential for use as artificial ligament materials for ACL reconstruction. STATEMENT OF SIGNIFICANCE: In this study, we successfully introduced Laponite (LAP), a kind of clay that has the function of osteogenic induction, into regenerated silk fibroin (RSF) fibers, which was prepared by a mature wet-spinning method developed in our lab. We believe that through artificial spinning, additional functional components can be added into RSF fibers, which one can hardly achieve with natural silks. We showed that the artificial ligament woven from RSF/LAP hybrid fibers had better cytocompatibility and osteogenic differentiation for mouse pre-osteoblasts in vitro, and significantly enhanced the graft osseointegration process and improved the corresponding biomechanical properties in a rat ACL reconstruction model in vivo, compared to those artificial ligaments made from degummed natural silkworm silks and pure RSF fibers.

Medial stabilizing technique preserves anatomical joint line and increases range of motion compared with the gap-balancing technique in navigated total knee arthroplasty

BACKGROUND

Medial compartment stability is important in total knee arthroplasty. The medial stabilizing technique (MST) has been proposed to achieve medial stability without excessive medial soft tissue release in total knee arthroplasty. Herein, we compare the MST and the gap-balancing technique (GBT) in navigated total knee arthroplasty.

METHODS

We retrospectively analyzed 70 patients with varus knee osteoarthritis who underwent primary total knee arthroplasty using the navigation system. They were divided into MST (n = 39) and GBT (n = 31) groups. We assessed intraoperative navigation data, radiographic data, and insert thickness. Preoperative and postoperative joint line changes were measured. We also assessed range of motion and clinical instability before and after total knee arthroplasty. These parameters were statistically compared between the groups.

RESULTS

Compared with the GBT group, medial extension gaps were significantly smaller in the MST group (P = 0.008). The gap difference between medial and lateral extension was significantly greater in the MST group (P = 0.018). Other navigation data showed no significant differences. Insert thickness and joint line changes were significantly lower in the MST group (P = 0.001, P = 0.018, respectively). Postoperative range of motion was significantly greater in the MST group (P = 0.032). There was no objective or subjective knee instability in either group.

CONCLUSION

The MST could avoid knee joint line changes and might increase postoperative range of motion. Although the MST permitted a discrepancy between medial and lateral gaps, no patients felt knee instability. The MST might improve the results of total knee arthroplasty.

Biodegradable polymer nanocomposites for ligament/tendon tissue engineering

Ligaments and tendons are fibrous tissues with poor vascularity and limited regeneration capacity. Currently, a ligament/tendon injury often require a surgical procedure using auto- or allografts that present some limitations. These inadequacies combined with the significant economic and health impact have prompted the development of tissue engineering approaches. Several natural and synthetic biodegradable polymers as well as composites, blends and hybrids based on such materials have been used to produce tendon and ligament scaffolds. Given the complex structure of native tissues, the production of fiber-based scaffolds has been the preferred option for tendon/ligament tissue engineering. Electrospinning and several textile methods such as twisting, braiding and knitting have been used to produce these scaffolds. This review focuses on the developments achieved in the preparation of tendon/ligament scaffolds based on different biodegradable polymers. Several examples are overviewed and their processing methodologies, as well as their biological and mechanical performances, are discussed.

Evolutionary approaches in protein engineering towards biomaterial construction

The tailoring of proteins for specific applications by evolutionary methods is a highly active area of research. Rational design and directed evolution are the two main strategies to reengineer proteins or create chimeric structures. Rational engineering is often limited by insufficient knowledge about proteins' structure-function relationships; directed evolution overcomes this restriction but poses challenges in the screening of candidates. A combination of these protein engineering approaches will allow us to create protein variants with a wide range of desired properties. Herein, we focus on the application of these approaches towards the generation of protein biomaterials that are known for biodegradability, biocompatibility and biofunctionality, from combinations of natural, synthetic, or engineered proteins and protein domains. Potential applications depend on the enhancement of biofunctional, mechanical, or other desired properties. Examples include scaffolds for tissue engineering, thermostable enzymes for industrial biocatalysis, and other therapeutic applications.

Characterization of the structure of rabbit anterior cruciate ligament and its stem/progenitor cells

BACKGROUND

It is known that anterior cruciate ligament (ACL) of the knee joint is prone to injuries with poor healing potential. The healing capacity of a tissue-like ACL is dependent on its structural components and the properties of the stem cells (SCs). Therefore, this study aimed to characterize the structure of ACL tissue and the properties of the SCs derived from the tissue components.

METHODS

The tissue structure of rabbit ACL was determined using a scanning electron microscope, hematoxylin and eosin, and immunohistochemical staining. The biological properties of SCs derived from the structural components of ACL were studied by colony formation, cell proliferation assay, SC marker expression and collagen exhibition, and multidifferentiation potential.

RESULTS

The two distinct components of ACL are classified as sheath and core, which possess differential properties in terms of collagen type, organization, and presence of blood vessels. The sheath tissue contains vascular SCs and the core tissue contains ligamentous SCs, respectively. The two types of SCs differ in clonogenicity, proliferation, and multidifferentiation potential.

CONCLUSION

This study shows that ACL consists of sheath and core tissues, which contain sheath and core SCs with distinctive biological properties. These findings highlight the need for use of both sheath and core SCs to promote the repair of the complex structure of injured ACL.

Osteointegration of a Novel Silk Fiber-Based ACL Scaffold by Formation of a Ligament-Bone Interface

BACKGROUND

Given the unsatisfactory results and reported drawbacks of anterior cruciate ligament (ACL) reconstruction, such as donor site morbidity and the limited choice of grafts in revision surgery, new regenerative approaches based on tissue-engineering strategies are currently under investigation.

PURPOSES

To determine (1) if a novel silk fiber-based ACL scaffold is able to initiate osteointegration in the femoral and tibial bone tunnels under in vivo conditions and (2) if the osteointegration process will be improved by intraoperatively seeding the scaffolds with the autologous stromal vascular fraction, an adipose-derived, stem cell-rich isolate from knee fat pads.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 33 sheep underwent ACL resection and were then randomly assigned to 2 experimental groups: ACL reconstruction with a scaffold alone and ACL reconstruction with a cell-seeded scaffold. Half of the sheep in each group were randomly chosen and euthanized 6 months after surgery and the other half at 12 months. To analyze the integration of the silk-based scaffold in the femoral and tibial bone tunnels, hard tissue histology and micro-computed tomography measurements were performed.

RESULTS

Hard tissue histological workup showed that in all treatment groups, with or without the application of the autologous stromal vascular fraction, an interzone of collagen fibers had formed between bone and silk-based graft. This collagen-fiber continuity partly consisted of Sharpey fibers, comparable with tendon-bone healing known for autografts and allografts. Insertion sites were more broad based at 6 months and more concentrated on the slightly protruding, bony knoblike structures at 12 months. Histologically, no differences between the treatment groups were detectable. Analysis of micro-computed tomography measurements revealed a significantly higher tissue density for the cell-seeded scaffold group as compared with the scaffold-alone group in the tibial but not femoral bone tunnel after 12 months of implantation.

CONCLUSION

The novel silk fiber-based scaffold for ACL regeneration demonstrated integration into the bone tunnels via the formation of a fibrous interzone similar to allografts and autografts. Histologically, additional cell seeding did not enhance osteointegration. No significant differences between 6 and 12 months could be detected. After 12 months, there was still a considerable amount of silk present, and a longer observation period is necessary to see if a true ligament-bone enthesis will be formed.

CLINICAL RELEVANCE

ACL regeneration with a silk fiber-based scaffold with and without additional cell seeding may provide an alternative treatment option to current techniques of surgical reconstruction.

Synthetic polymer scaffolds for soft tissue engineering

Tissue engineering (TE) and regenerative medicine are progressively developed areas due to many novel tissue replacements and implementation strategies. Increasing knowledge involving the fabrication of biomaterials with advanced physicochemical and biological characteristics, successful isolation and preparation of stem cells, incorporation of growth and differentiation factors, and biomimetic environments gives us a unique opportunity to develop various types of scaffolds for TE. The current strategies for soft tissue reconstitution or regeneration highlight the importance of novel regenerative therapies in cases of significant soft tissue loss and in cases of congenital defects, disease, trauma and ageing. Various types of biomaterials and scaffolds have been tested for soft tissue regeneration. The synthetic types of materials have gained great attention due to high versatility, tunability and easy functionalization for better biocompatibility. This article reviews the current materials that are usually the most used for the fabrication of scaffolds for soft TE; in addition, the types of scaffolds together with examples of their applications for the regenerative purposes of soft tissue, as well as their major physicochemical characteristics regarding the increased applicability of these materials in medicine, are reviewed.

Feasibility of a Three-Dimensional Porous Uncalcined and Unsintered Hydroxyapatite/poly-d/l-lactide Composite as a Regenerative Biomaterial in Maxillofacial Surgery

This study evaluated the feasibility of a novel three-dimensional (3D) porous composite of uncalcined and unsintered hydroxyapatite (u-HA) and poly-d/l-lactide (PDLLA) (3D-HA/PDLLA) for the bony regenerative biomaterial in maxillofacial surgery, focusing on cellular activities and osteoconductivity properties in vitro and in vivo. In the in vitro study, we assessed the proliferation and ingrowth of preosteoblastic cells (MC3T3-E1 cells) in 3D-HA/PDLLA biomaterials using 3D cell culture, and the results indicated enhanced bioactive proliferation. After osteogenic differentiation of those cells on 3D-HA/PDLLA, the osteogenesis marker genes runt-related transcription factor-2 (Runx2), and Sp7 (Osterix) were upregulated. For the in vivo study, we evaluated the utility of 3D-HA/PDLLA biomaterials compared to the conventional bone substitute of beta-tricalcium phosphate (β-TCP) in rats with critical mandibular bony defects. The implantation of 3D-HA/PDLLA biomaterials resulted in enhanced bone regeneration, by inducing high osteoconductivity as well as higher β-TCP levels. Our study thus showed that the novel composite, 3D-HA/PDLLA, is an excellent bioactive/bioresorbable biomaterial for use as a cellular scaffold, both in vitro and in vivo, and has utility in bone regenerative therapy, such as for patients with irregular maxillofacial bone defects.

Intrasubstance Anterior Cruciate Ligament Injuries in the Pediatric Population

Pediatric intrasubstance anterior cruciate ligament (ACL) tears have a significant epidemiologic impact as their numbers continue to grow globally. This review focuses on true pediatric intrasubstance ACL tears, which occur >400,000 times annually. Modifiable and non-modifiable risk factors include intercondylar notch width, ACL size, gender, landing mechanisms, and hormonal variations. The proposed mechanisms of injury include anterior tibial shear and dynamic valgus collapse. ACL tears can be associated with soft tissue and chondral defects. History and physical examination are the most important parts of evaluation, including the Lachman test, which is considered the most accurate physical examination maneuver. Imaging studies should begin with AP and lateral radiographs, but magnetic resonance imaging is very useful in confirming the diagnosis and preoperative planning. ACL injury prevention programs targeting high risk populations have been proven to reduce the risk of injury, but lack uniformity across programs. Pediatric ACL injuries were conventionally treated nonoperatively, but recent data suggest that early operative intervention produces best long term outcomes pertaining to knee stability, meniscal tear risk, and return to previous level of play. Current techniques in ACL reconstruction, including more vertically oriented tunnels and physeal sparing techniques, have been described to reduce the risk of physeal arrest and limb angulation or deformity. Data consistently show that autograft is superior to allograft regarding failure rate. Mean durations of postoperative therapy and return to sport were 7 ± 3 and 10 ± 3 months, respectively. These patients have good functional outcomes compared to the general population yet are at increased risk of additional ACL injury. Attempts at primary ACL repair using biological scaffolds are under investigation.

Modulating microfibrillar alignment and growth factor stimulation to regulate mesenchymal stem cell differentiation

The ideal tissue engineering (TE) strategy for ligament regeneration should recapitulate the bone - calcified cartilage - fibrocartilage - soft tissue interface. Aligned electrospun-fibers have been shown to guide the deposition of a highly organized extracellular matrix (ECM) necessary for ligament TE. However, recapitulating the different tissues observed in the bone-ligament interface using such constructs remains a challenge. This study aimed to explore how fiber alignment and growth factor stimulation interact to regulate the chondrogenic and ligamentous differentiation of mesenchymal stem cells (MSCs). To this end aligned and randomly-aligned electrospun microfibrillar scaffolds were seeded with bone marrow derived MSCs and stimulated with transforming growth factor β3 (TGFβ3) or connective tissue growth factor (CTGF), either individually or sequentially. Without growth factor stimulation, MSCs on aligned-microfibers showed higher levels of tenomodulin (TNMD) and aggrecan gene expression compared to MSCs on randomly-oriented fibers. MSCs on aligned-microfibers stimulated with TGFβ3 formed cellular aggregates and underwent robust chondrogenesis, evidenced by increased type II collagen expression and sulphated glycosaminoglycans (sGAG) synthesis compared to MSCs on randomly-oriented scaffolds. Bone morphogenetic protein 2 (BMP2) and type I collagen gene expression were higher on randomly-oriented scaffolds stimulated with TGFβ3, suggesting this substrate was more supportive of an endochondral phenotype. In the presence of CTGF, MSCs underwent ligamentous differentiation, with increased TNMD expression on aligned compared to randomly aligned scaffolds. Upon sequential growth factor stimulation, MSCs expressed types I and II collagen and deposited higher overall levels of collagen compared to scaffolds stimulated with either growth factor in isolation. These findings demonstrate that modulating the alignment of microfibrillar scaffolds can be used to promote either an endochondral, chondrogenic, fibrochondrogenic or ligamentous MSC phenotype upon presentation of appropriate biochemical cues.

STATEMENT OF SIGNIFICANCE

Polymeric electrospun fibers can be tuned to match the fibrillar size and anisotropy of collagen fibers in ligaments, and can be mechanically competent. Therefore, their use is attractive when attempting to tissue engineer the bone-ligament interface. A central challenge in this field is recapitulating the cellular phenotypes observed across the bone-ligament interface. Here we demonstrated that it is possible to direct MSCs seeded onto aligned electrospun fibres towards either a ligamentogenic, chondrogenic or fibrochondrogenic phenotype upon presentation of appropriate biochemical cues. This opens the possibility of using aligned microfibrillar scaffolds that are spatially functionalized with specific growth factors to direct MSC differentiation for engineering the bone-ligament interface.

Ectopic tissue engineered ligament with silk collagen scaffold for ACL regeneration: A preliminary study

Anterior cruciate ligament (ACL) reconstruction remains a formidable clinical challenge because of the lack of vascularization and adequate cell numbers in the joint cavity. In this study, we developed a novel strategy to mimic the early stage of repair in vivo, which recapitulated extra-articular inflammatory response to facilitate the early ingrowth of blood vessels and cells. A vascularized ectopic tissue engineered ligament (ETEL) with silk collagen scaffold was developed and then transferred to reconstruct the ACL in rabbits without interruption of perfusion. At 2weeks after ACL reconstruction, more well-perfused cells and vessels were found in the regenerated ACL with ETEL, which decreased dramatically at the 4 and 12week time points with collagen deposition and maturation. ACL treated with ETEL exhibited more mature ligament structure and enhanced ligament-bone healing post-reconstructive surgery at 4 and 12weeks, as compared with the control group. In addition, the ETEL group was demonstrated to have higher modulus and stiffness than the control group significantly at 12weeks post-reconstructive surgery. In conclusion, our results demonstrated that the ETEL can provide sufficient vascularity and cellularity during the early stages of healing, and subsequently promote ACL regeneration and ligament-bone healing, suggesting its clinic use as a promising therapeutic modality.

STATEMENT OF SIGNIFICANCE

Early inflammatory cell infiltration, tissue and vessels ingrowth were significantly higher in the extra-articular implanted scaffolds than theses in the joint cavity. By mimicking the early stages of wound repair, which provided extra-articular inflammatory stimulation to facilitate the early ingrowth of blood vessels and cells, a vascularized ectopic tissue engineered ligament (ETEL) with silk collagen scaffold was constructed by subcutaneous implantation for 2weeks. The fully vascularized TE ligament was then transferred to rebuild ACL without blood perfusion interruption, and was demonstrated to exhibit improved ACL regeneration, bone tunnel healing and mechanical properties.

Mechanical Loading Improves Tendon-Bone Healing in a Rabbit Anterior Cruciate Ligament Reconstruction Model by Promoting Proliferation and Matrix Formation of Mesenchymal Stem Cells and Tendon Cells

Background/Aims: This study investigated the effect of mechanical stress on tendon-bone healing in a rabbit anterior cruciate ligament (ACL) reconstruction model as well as cell proliferation and matrix formation in co-culture of bone-marrow mesenchymal stem cells (BMSCs) and tendon cells (TCs). Methods: The effect of continuous passive motion (CPM) therapy on tendon-bone healing in a rabbit ACL reconstruction model was evaluated by histological analysis, biomechanical testing and gene expressions at the tendon-bone interface. Furthermore, the effect of mechanical stretch on cell proliferation and matrix synthesis in BMSC/TC co-culture was also examined. Results: Postoperative CPM therapy significantly enhanced tendon-bone healing, as evidenced by increased amount of fibrocartilage, elevated ultimate load to failure levels, and up-regulated gene expressions of Collagen I, alkaline phosphatase, osteopontin, Tenascin C and tenomodulin at the tendon-bone junction. In addition, BMSC/TC co-culture treated with mechanical stretch showed a higher rate of cell proliferation and enhanced expressions of Collagen I, Collagen III, alkaline phosphatase, osteopontin, Tenascin C and tenomodulin than that of controls. Conclusion: These results demonstrated that proliferation and differentiation of local precursor cells could be enhanced by mechanical stimulation, which results in enhanced regenerative potential of BMSCs and TCs in tendon-bone healing.

Collagen and Elastin Biomaterials for the Fabrication of Engineered Living Tissues

Collagen and elastin represent the two most predominant proteins in the body and are responsible for modulating important biological and mechanical properties. Thus, the focus of this review is the use of collagen and elastin as biomaterials for the fabrication of living tissues. Considering the importance of both biomaterials, we first propose the notion that many tissues in the human body represent a reinforced composite of collagen and elastin. In the rest of the review, collagen and elastin biosynthesis and biophysics, as well as molecular sources and biomaterial fabrication methodologies, including casting, fiber spinning, and bioprinting, are discussed. Finally, we summarize the current attempts to fabricate a subset of living tissues and, based on biochemical and biomechanical considerations, suggest that future tissue-engineering efforts consider direct incorporation of collagen and elastin biomaterials.

Hamstring graft tibial insertion preservation versus detachment in anterior cruciate ligament reconstruction: a prospective randomized comparative study

BACKGROUND

Anterior cruciate ligament (ACL) reconstruction with hamstring graft (HG) is a commonly performed procedure. Despite the type of reconstruction chosen, the detached HG undergoes a remodeling process known as ligamentization. In order to shorten the ligamentization process, the maintenance of HG tibial insertion, aimed to spare the tendons vascular supply, has been postulated. The aim of this paper is to report the results of a prospective randomized study comparing clinical and MRI results between two different ACL reconstructive procedures with and without HG tibial insertion preservation.

METHODS

Forty patients (mean age 27.5 ± 9.5 years) were enrolled and randomly divided into two groups. The study group underwent an ACL reconstruction using a distally inserted HG, while the control group underwent a technique encompassing HG tibial detachment. Subjective and objective IKDC score was administered preoperatively and at 3-, 6-, 12- and 24-month follow-up. Graft morphology was assessed through MRI evaluation performed at 6-month follow-up.

RESULTS

Clinical results were excellent in both groups. Regarding MRI results, a better intra-articular graft morphology was observed in the study group (Tau = 0.313, p = 0.024). No differences in graft integration were noticed.

CONCLUSION

The main finding of this preliminary study is that preservation of the hamstring tibial insertion seems to enhance graft ligamentization with improved morphology of the intra-articular portion of the graft compared to a detachment of the hamstring tendons from the tibial side. Further well-designed studies with higher number of patients as well as more serial MRI evaluations are required to validate these preliminary findings.

A Novel Silk Fiber-Based Scaffold for Regeneration of the Anterior Cruciate Ligament: Histological Results From a Study in Sheep

BACKGROUND

Because of ongoing problems with anterior cruciate ligament (ACL) reconstruction, new approaches in the treatment of ACL injuries, particularly strategies based on tissue engineering, have gained increasing research interest. To allow for ACL regeneration, a structured scaffold that provides a mechanical basis, has cells from different sources, and comprises mechanical as well as biological factors is needed. Biological materials, biodegradable polymers, and composite materials are being used and tested as scaffolds. The optimal scaffold for ACL regeneration should be biocompatible and biodegradable to allow tissue ingrowth but also needs to have the right mechanical properties to provide immediate mechanical stability.

HYPOTHESES

The study hypotheses were that (1) a novel degradable silk fiber-based scaffold with mechanical properties similar to the native ACL will be able to initiate ligament regeneration after ACL resection and reconstruction under in vivo conditions and (2) additional cell seeding of the scaffold with autologous stromal vascular fraction-containing adipose-derived stem cells will increase regenerative activity.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 33 mountain sheep underwent ACL resection and randomization to 2 experimental groups: (1) ACL reconstruction with a scaffold alone and (2) ACL reconstruction with a cell-seeded scaffold. Histological evaluation of the intra-articular portion of the reconstructed/regenerated ligament was performed after 6 and 12 months.

RESULTS

After 6 months, connective tissue surrounded the silk scaffold with ingrowth in some areas. The cell-seeded scaffolds had a significant lower silk content compared with the unseeded scaffolds and demonstrated a higher content of newly formed tissue. After 12 months, the density of the silk fibers decreased significantly, and the ingrowth of newly formed tissue increased in both groups. No differences between the 2 groups regarding silk fiber degradation and regenerated tissue were detected at 12 months.

CONCLUSION

The novel silk fiber-based scaffold was able to stimulate ACL regeneration under in vivo conditions. Additional cell seeding led to increased tissue regeneration and decreased silk fiber content at 6 months, whereas these differences were not present at 12 months.

CLINICAL RELEVANCE

ACL regeneration using a silk fiber-based scaffold with and without additional cell seeding may provide a new treatment option after joint injuries.

Protein selective adsorption properties of a polyethylene terephtalate artificial ligament grafted with poly(sodium styrene sulfonate) (polyNaSS): correlation with physicochemical parameters of proteins

Immediately after surgical placement of biomaterials, a first step consists in the adsorption of proteins from the biological environment on the artificial surfaces. Because the composition of the adsorbed protein layer modulates the cell response to the implanted material, researchers in the biomaterials field have focused on coating proteins or peptides onto surfaces to improve cell response and therefore the long-term compatibility of the implant. However, some materials used in tissue engineering, mainly synthetic polymers, are too hydrophobic to allow the optimal adsorption of proteins and have to be first submitted to physical or chemical treatments. In our laboratory, we have demonstrated that grafting of poly(sodium styrene sulfonate) (polyNaSS) onto biomaterials can strongly modulate the protein adsorption and the cellular response compared to unmodified surfaces. In this study, we used a liquid chromatography strategy coupled to proteomics to evaluate the adsorptive properties of a polyethylene terephtalate (PET) artificial ligament grafted with polyNaSS, and to identify and analyse proteins adsorbed on PET fibers. Results obtained with platelet rich plasma (PRP) proteins demonstrated that grafting significantly increases the protein adsorption of the PET and also selectively modulates the adsorption of proteins on PET fibers. Finally, regarding physicochemical parameters calculated from the amino acid sequence of identified proteins, we found that the aliphatic index is highly correlated with the selective adsorption of proteins onto the polyNaSS/PET surface. Therefore, the proteomic approach complemented with physicochemical property evaluation could provide a powerful tool for the elaboration of new biomaterials based on protein layer deposition.