Ligament tissue engineering and its potential role in anterior cruciate ligament reconstruction

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.

Mechanical Actuation Systems for the Phenotype Commitment of Stem Cell-Based Tendon and Ligament Tissue Substitutes

High tensile forces transmitted by tendons and ligaments make them susceptible to tearing or complete rupture. The present standard reparative technique is the surgical implantation of auto- or allografts, which often undergo failure.Currently, different cell types and biomaterials are used to design tissue engineered substitutes. Mechanical stimulation driven by dedicated devices can precondition these constructs to a remarkable degree, mimicking the local in vivo environment. A large number of dynamic culture instruments have been developed and many appealing results collected. Of the cells that have been used, tendon stem cells are the most promising for a reliable stretch-induced tenogenesis, but their reduced availability represents a serious limitation to upscaled production. Biomaterials used for scaffold fabrication include both biological molecules and synthetic polymers, the latter being improved by nanotechnologies which reproduce the architecture of native tendons. In addition to cell type and scaffold material, other variables which must be defined in mechanostimulation protocols are the amplitude, frequency, duration and direction of the applied strain. The ideal conditions seem to be those producing intermittent tension rather than continuous loading. In any case, all physical parameters must be adapted to the specific response of the cells used and the tensile properties of the scaffold. Tendon/ligament grafts in animals usually have the advantage of mechanical preconditioning, especially when uniaxial cyclic forces are applied to cells engineered into natural or decellularized scaffolds. However, due to the scarcity of in vivo research, standard protocols still need to be defined for clinical applications.

Anterior Cruciate Ligament: Structure, Injuries and Regenerative Treatments

Anterior cruciate ligament (ACL) is one of the most vulnerable ligaments of the knee. ACL impairment results in episodic instability, chondral and meniscal injury and early osteoarthritis. The poor self-healing capacity of ACL makes surgical treatment inevitable. Current ACL reconstructions include a substitution of torn ACL via biological grafts such as autograft, allograft. This review provides an insight of ACL structure, orientation and properties followed by comparing the performance of various constructs that have been used for ACL replacement. New approaches, undertaken to induce ACL regeneration and fabricate biomimetic scaffolds, are also discussed.

Decellularized Versus Fresh-Frozen Allografts in Anterior Cruciate Ligament Reconstruction: An In Vitro Study in a Rabbit Model

BACKGROUND

The common fresh-frozen allografts that are used for anterior cruciate ligament (ACL) reconstructions behave slower during the remodeling process and produce weaker tendon-bone integrations than do autografts. Decellularization of allogenic tendons results in a clean and porous collagen scaffold with low antigenicity and high compatibility, which may be more suitable for ACL reconstructions.

HYPOTHESIS

Allograft decellularization will result in a tissue structure with suitable mechanical characteristics for ACL reconstruction, thereby promoting graft remodeling and enhancing tendon-bone healing.

STUDY DESIGN

Controlled laboratory study.

METHODS

Decellularized allograft tissues were prepared with a pH-modified decellularization process and evaluated for their biocompatibility and biomechanical character in vitro. Eighty New Zealand White rabbits were divided into 2 groups, with 40 in each group, to receive ACL reconstruction with either fresh-frozen (common) allografts or decellularized allografts on both knees. At 2, 4, 8, and 12 weeks postoperatively, the rabbits were euthanized for biomechanical testing, micro-computed tomography analysis, and histologic analysis.

RESULTS

The pH-modified decellularized allograft tissues kept excellent biocompatibility and biomechanical character during the in vitro study. Biomechanical testing indicated that the decellularized allograft had significantly higher ultimate load (P = .02) and stiffness (P = .01) levels than the common allograft at 12 weeks, and there was no significant difference between the 2 groups at any other time point. The micro-CT evaluation determined significantly higher bone mineral density (P < .01) in the decellularized allograft group than that in the common allograft group at 12 weeks, but no difference between the 2 groups was observed at any other time point. Regarding bone volume/total volume, there was no difference between the 2 groups at any time point. Fibroblast ingrowths, vascular formation, and connective tissue formation in the tendon-bone interface were better in the decellularized group within 8 weeks. New bone formation was more common in the decellularized allograft group. The collagen birefringence was restored more quickly in the decellularized allograft group than in the common allograft group at all time points.

CONCLUSION

The use of pH-modified decellularized allografts compared with the common allografts resulted in better cellularity, vascularity, collagen matrix remolding, new bone formation around the graft, enhanced tendon-bone healing, and higher ultimate failure load and stiffness of the graft after ACL reconstruction in the rabbit model.

CLINICAL RELEVANCE

The pH-modified decellularized allograft may be a better graft option than the common fresh-frozen allograft for knee ligament reconstructions.

Sonographically Guided Anterior Cruciate Ligament Injection: Technique and Validation

OBJECTIVE

To describe and validate a practical technique for sonographically guided anterior cruciate ligament (ACL) injections.

DESIGN

Prospective, cadaveric laboratory investigation.

SETTING

Procedural skills laboratory in a tertiary medical center.

SUBJECTS

Ten unembalmed, cadaveric mid-thigh-knee-ankle foot specimens (5 left knees and 5 right knees; 5 male and 5 female) from 10 donors aged 76 to 93 years (mean 85.6 years) with body mass indices of 17.6 to 42.2 kg/m(2) (mean 28.8 kg/m(2)).

METHODS

A single, experienced operator used a 22-gauge, 63.5-mm stainless steel needle and a 12-3-MHz linear transducer to inject 1.5 mL of diluted colored latex into the ACLs of 10 unembalmed cadaveric specimens via an in-plane, caudad-to-cephalad approach, long axis to the ACL. At a minimum of 24 hours postinjection, specimens were dissected, and the presence and distribution of latex within the ACL assessed by a study co-investigator.

MAIN OUTCOME

Presence and distribution of latex within the ACL.

RESULTS

All 10 injections accurately delivered latex into the proximal (femoral), midsubstance, and distal (tibial) portions of the ACL. No specimens exhibited evidence of needle injury or latex infiltration with respect to the menisci, hyaline cartilage, or posterior cruciate ligament.

CONCLUSIONS

Sonographically guided intra-ligamentous ACL injections are technically feasible and can be performed with a high degree of accuracy. Sonographically guided ACL injections could be considered for research and clinical purposes to directly deliver injectable agents into the healing ACL postinjury or postreconstruction.

PLA-poloxamer/poloxamine copolymers for ligament tissue engineering: sound macromolecular design for degradable scaffolds and MSC differentiation

The treatment of anterior cruciate ligament (ACL) failures remains a current clinical challenge. The present study aims at providing suitable degradable scaffolds for ligament tissue engineering. First, we focus on the design and the evaluation of poly(lactide)/poloxamer or poly(lactide)/poloxamine multiblock copolymers selected and developed to have suitable degradation and mechanical properties to match ACL repair. In the second part, it is shown that the copolymers can be processed in the form of microfibers and scaffolds consisting of a combination of twisted/braided fibers to further modulate the mechanical properties and prepare scaffold prototypes suitable for ligament application. Finally, after assessment of their cytocompatibility, the polymer scaffolds are associated with mesenchymal stem cells (MSCs). MSC differentiation toward a ligament fibroblast phenotype is promoted by a dual stimulation including an inductive culture medium and cyclic mechanical loads. RT-qPCR analyses confirm the potential of our scaffolds and MSCs for ACL regeneration with upregulation of some differentiation markers including Scleraxis, Tenascin-C and Tenomodulin.

Anterior cruciate ligament regeneration using mesenchymal stem cells and collagen type I scaffold in a rabbit model

PURPOSE

The objective of this study was to determine whether using mesenchymal stem cells (MSC) seeded in a collagen type I scaffold would be sufficient to regenerate the torn anterior cruciate ligament (ACL).

METHODS

Anterior cruciate ligament transection was performed on both knees in 10 New Zealand rabbits and then repaired with as follows: suture alone (suture-treated group, n = 6), suture associated with collagen type I scaffold (collagen type I scaffold-treated group, n = 8) or suture associated with autologous MSC seeded on collagen type I scaffold (MSC/collagen type I scaffold-treated group, n = 6). At 12-week post-intervention, the animals were killed and the ACLs were characterised macroscopically and histologically. Data of the 3 groups were against normal ACL (normal group, n = 10).

RESULTS

Macroscopic observation found that in MSC/collagen type I scaffold group, 33% of specimens showed a complete ACL regeneration, with a tissue similar to the normal ACL. Regeneration was not observed in the group treated with suture alone or associated with collagen type I scaffold without cells. In the latter, only a reparative attempt at the ends was observed. Histological analysis of the regenerated ACL showed a tissue with organised collagen and peripheric vessels.

CONCLUSIONS

These results provide evidence that the use of MSC seeded in a collagen type I scaffold in the treatment of ACL injuries is associated with an enhancement of ligament regeneration. This MSC-based technique is a potentially attractive tool for improving the treatment of ACL ruptures.

Histologic, biomechanical, and biological evaluation of fan-folded iliotibial band allografts for anterior cruciate ligament reconstruction

PURPOSE

The purpose of this study was to thoroughly characterize the fan-folded iliotibial band (FITB) allograft and compare it with anterior tibialis tendons (ATs) and native anterior cruciate ligaments (ACLs) to determine whether it measures up to those tissues.

METHODS

We compared the histologic structure, tensile strength to failure, creep, and stress-relaxation properties of FITBs with those of ATs and ACLs. In vitro cytotoxicity and biocompatibility of FITBs were also compared with ATs.

RESULTS

No structural difference was observed between the tissues studied. FITB ultimate tensile strength (3,459 ± 939 N) was not significantly different (P > .9999) from ultimate tensile strength of ATs (3,357 ± 111 N) and was significantly greater (P = .0005) than that of ACLs (886 ± 254 N). No significant difference (P > .9999) was observed in the increase in length resulting from creep testing between FITBs (9.5 ± 3.0 mm) and ATs (9.7 ± 4.0 mm). During stress-relaxation testing, FITBs reached 181 ± 46 N, which was not significantly different (P > .9999) from ATs (166 ± 40 N). Finally, we showed that cytotoxicity of FITBs and ATs was negligible. In vitro biocompatibility of FITBs and ATs was very good, whereas FITBs had a higher propensity to favor the attachment and infiltration of cells that proliferated for at least 4 weeks on their contact.

CONCLUSIONS

We found that FITBs, ACLs, and ATs shared a similar structure made of aligned collagen fibers. No significant difference was observed between FITB and AT ultimate tensile strength, creep, and stress-relaxation viscoelastic properties. Ultimate tensile strength to failure of ACLs was lower than that of FITBs and ATs, whereas ACLs were superior to both FITBs and ATs during creep and stress-relaxation testing. FITBs and ATs showed low cytotoxicity and excellent biocompatibility in vitro, with a somewhat higher propensity of FITBs to favor cell attachment and infiltration over time.

CLINICAL RELEVANCE

This study suggests that FITBs have the potential to perform as well as ATs for ACL reconstruction.

Growth factors and stem cells for the management of anterior cruciate ligament tears

The anterior cruciate ligament (ACL) is fundamental for the knee joint stability. ACL tears are frequent, especially during sport activities, occurring mainly in young and active patients. Nowadays, the gold standard for the management of ACL tears remains the surgical reconstruction with autografts or allografts. New strategies are being developed to resolve the problems of ligament grafting and promote a physiological healing process of ligamentous tissue without requiring surgical reconstruction. Moreover, these strategies can be applicable in association surgical reconstruction and may be useful to promote and accelerate the healing process. The use of growth factors and stem cells seems to offer a new and fascinating solution for the management of ACL tears. The injection of stem cell and/or growth factors in the site of ligamentous injury can potentially enhance the repair process of the physiological tissue. These procedures are still at their infancy, and more in vivo and in vitro studies are required to clarify the molecular pathways and effectiveness of growth factors and stem cells therapy for the management of ACL tears. This review aims to summarize the current knowledge in the field of growth factors and stem cells for the management of ACL tears.

Effect on ligament marker expression by direct-contact co-culture of mesenchymal stem cells and anterior cruciate ligament cells

Ligament and tendon repair is an important topic in orthopedic tissue engineering; however, the cell source for tissue regeneration has been a controversial issue. Until now, scientists have been split between the use of primary ligament fibroblasts or marrow-derived mesenchymal stem cells (MSCs). The objective of this study was to show that a co-culture of anterior cruciate ligament (ACL) cells and MSCs has a beneficial effect on ligament regeneration that is not observed when utilizing either cell source independently. Autologous ACL cells (ACLcs) and MSCs were isolated from Yorkshire pigs, expanded in vitro, and cultured in multiwell plates in varying %ACLcs/%MSCs ratios (100/0, 75/25, 50/50, 25/75, and 0/100) for 2 and 4 weeks. Quantitative mRNA expression analysis and immunofluorescent staining for ligament markers Collagen type I (Collagen-I), Collagen type III (Collagen-III), and Tenascin-C were performed. We show that Collagen-I and Tenascin-C expression is significantly enhanced over time in 50/50 co-cultures of ACLcs and MSCs (p≤0.03), but not in other groups. In addition, Collagen-III expression was significantly greater in MSC-only cultures (p≤0.03), but the Collagen-I-to-Collagen-III ratio in 50% co-culture was closest to native ligament levels. Finally, Tenascin-C expression at 4 weeks was significantly higher (p≤0.02) in ACLcs and 50% co-culture groups compared to all others. Immunofluorescent staining results support our mRNA expression data. Overall, 50/50 co-cultures had the highest Collagen-I and Tenascin-C expression, and the highest Collagen-I-to-Collagen-III ratio. Thus, we conclude that using a 50% co-culture of ACLcs and MSCs, instead of either cell population alone, may better maintain or even enhance ligament marker expression and improve healing.