Tendon graft revitalization using adult anterior cruciate ligament (ACL)-derived CD34+ cell sheets for ACL reconstruction

Cells Remain Viable When Collected With an In-Line-Suction Tissue Collector From Byproducts of Anterior Cruciate Ligament Reconstruction Surgery

Purpose

To investigate the viability of cells collected with an in-line-suction autologous tissue collector from the tissue byproducts of arthroscopic anterior cruciate ligament (ACL) reconstruction, to characterize cells from different tissue types, and to identify mesenchymal stem cells.

Methods

Patients aged 14 to 50 years with ACL injuries requiring arthroscopic reconstruction surgery were offered enrollment and screened for participation. In total, 12 patients were enrolled in the descriptive laboratory study. Arthroscopic byproduct tissue was collected with an in-line-suction autologous tissue collector from 4 intraoperative collection sites for each patient: ACL stump, ACL fat pad, notchplasty debris, and tunnel drilling debris. All tissue samples were digested using collagenase, and the derived cellular populations were analyzed in vitro, characterizing cellular viability, proliferative potential, qualitative multipotent differentiation capacity, and cell-surface marker presence.

Results

An equivalent mass of arthroscopic byproduct tissue was taken from each of the 4 intraoperative collection sites (1.12-1.61 g, P = .433), which all showed an average viability of at least 99.95% and high average total nucleated cells (≥1.37 × 107 cells/mL). No significant differences in collected mass (P = .433), cellular viability (P = .880), or total nucleated cells (P = .692) were observed between the 4 byproduct tissues. The byproduct tissues did exhibit significant differences in monocyte (P = .037) and red blood cell (P = .038) concentrations, specifically with greater values present in the ACL stump tissue. Cells from all byproduct tissues adhered to plastic cell culture flasks. Significant differences were found between colony-forming unit fibroblast counts of the 4 byproduct tissues when plated at 106 (P = .003) and 103 (P = .016) cells as the initial seeding density. There was a significant relationship found between both the starting concentration (χ2 = 32.7, P < .001) and the byproduct tissue type (χ2 = 30.4, P < .001) to the presence of ≥80% confluency status at 10 days. Cells obtained from all 4 byproduct tissues qualitatively showed positive tri-lineage (adipocyte, osteoblast, chondroblast) differentiation potential compared with negative controls under standardized in vitro differentiation conditions. Cells derived from all 4 byproduct tissues expressed cell-surface antigens CD105+, CD73+, CD90+, CD45-, CD14-, and CD19- (>75%), and did not express CD45 (<10%). There were no statistically significant differences in cell-surface antigens between the four byproduct tissues.

Conclusions

This descriptive laboratory study demonstrated that cells derived from arthroscopic byproduct tissues of ACL reconstruction remain viable when collected with an in-line-suction autologous tissue collector and these cells meet the ISCT criteria to qualify as mesenchymal stem cells.

Clinical Relevance

It is known that viable mesenchymal stem cells reside in byproduct tissue of anterior cruciate ligament reconstruction surgery (ACLR). Practical methods to harvest these cells at the point of care require further development. This study validates the use of an in-line-suction autologous tissue collector for the harvest of viable mesenchymal stem cells after ACLR.

Cell Sheet Technology: An Emerging Approach for Tendon and Ligament Tissue Engineering

Tendon and ligament injuries account for a substantial proportion of disorders in the musculoskeletal system. While non-operative and operative treatment strategies have advanced, the restoration of native tendon and ligament structures after injury is still challenging due to its innate limited regenerative ability. Cell sheet technology is an innovative tool for tissue fabrication and cell transplantation in regenerative medicine. In this review, we first summarize different harvesting procedures and advantages of cell sheet technology, which preserves intact cell-to-cell connections and extracellular matrix. We then describe the recent progress of cell sheet technology from preclinical studies, focusing on the application of stem cell-derived sheets in treating tendon and ligament injuries, as well as highlighting its effects on mitigating inflammation and promoting tendon/graft-bone interface healing. Finally, we discuss several prerequisites for future clinical translation including the selection of appropriate cell source, optimization of preparation process, establishment of suitable animal model, and the fabrication of vascularized complex tissue. We believe this review could potentially provoke new ideas and drive the development of more functional biomimetic tissues using cell sheet technology to meet the needs of clinical patients.

Synovium-Derived Mesenchymal Stem Cell-Based Scaffold-Free Fibrocartilage Engineering for Bone-Tendon Interface Healing in an Anterior Cruciate Ligament Reconstruction Model

BACKGROUND

Current tendon and ligament reconstruction surgeries rely on scar tissue healing which differs from native bone-to-tendon interface (BTI) tissue. We aimed to engineer Synovium-derived mesenchymal stem cells (Sy-MSCs) based scaffold-free fibrocartilage constructs and investigate in vivo bone-tendon interface (BTI) healing efficacy in a rat anterior cruciate ligament (ACL) reconstruction model.

METHODS

Sy-MSCs were isolated from knee joint of rats. Scaffold-free sy-MSC constructs were fabricated and cultured in differentiation media including  TGF-β-only, CTGF-only, and TGF-β + CTGF. Collagenase treatment on tendon grafts was optimized to improve cell-to-graft integration. The effects of fibrocartilage differentiation and collagenase treatment on BTI integration was assessed by conducting histological staining, cell adhesion assay, and tensile testing. Finally, histological and biomechanical analyses were used to evaluate in vivo efficacy of fibrocartilage construct in a rat ACL reconstruction model.

RESULTS

Fibrocartilage-like features were observed with in the scaffold-free sy-MSC constructs when applying TGF-β and CTGF concurrently. Fifteen minutes collagenase treatment increased cellular attachment 1.9-fold compared to the Control group without affecting tensile strength. The failure stress was highest in the Col + D + group (22.494 ± 13.74 Kpa) compared to other groups at integration analysis in vitro. The ACL Recon + FC group exhibited a significant 88% increase in estimated stiffness (p = 0.0102) compared to the ACL Recon group at the 4-week postoperative period.

CONCLUSION

Scaffold-free, fibrocartilage engineering together with tendon collagenase treatment enhanced fibrocartilaginous BTI healing in ACL reconstruction.

Advances focusing on the application of decellularization methods in tendon-bone healing

BACKGROUND

The tendon or ligament is attached to the bone by a triphasic but continuous area of heterogeneous tissue called the tendon-bone interface (TBI). The rapid and functional regeneration of TBI is challenging owing to its complex composition and difficulty in self-healing. The development of new technologies, such as decellularization, has shown promise in the regeneration of TBI. Several ex vivo and in vivo studies have shown that decellularized grafts and decellularized biomaterial scaffolds achieved better efficacy in enhancing TBI healing. However further information on the type of review that is available is needed.

AIM OF THE REVIEW

In this review, we discuss the current application of decellularization biomaterials in promoting TBI healing and the possible mechanisms involved. With this work, we would like to reveal how tissues or biomaterials that have been decellularized can improve tendon-bone healing and to provide a theoretical basis for future related studies.

KEY SCIENTIFIC CONCEPTS OF THE REVIEW

Decellularization is an emerging technology that utilizes various chemical, enzymatic and/or physical strategies to remove cellular components from tissues while retaining the structure and composition of the extracellular matrix (ECM). After decellularization, the cellular components of the tissue that cause an immune response are removed, while various biologically active biofactors are retained. This review further explores how tissues or biomaterials that have been decellularized improve TBI healing.

Do the Differences in the Epiligament of the Proximal and Distal Parts of the Anterior Cruciate Ligament Explain Their Different Healing Capacities? Quantitative and Immunohistochemical Analysis of CD34 and α-SMA Expression in Relation to the Epiligament Theory

The aim of this study was to assess the epiligament theory by determining the normal epiligament morphology of the proximal and distal parts of the anterior cruciate ligament in humans and analyzing the differences between them and the midportion of the ligament in terms of cell numbers and expression of CD34 and α-SMA. Samples were obtained from the anterior cruciate ligaments of 12 fresh knee joints. Monoclonal antibodies against CD34 and α-SMA were used for immunohistochemistry. Photomicrographs were analyzed using ImageJ software, version 1.53f. The cell density was higher in the epiligament than in the ligament connective tissue. Cell counts were higher in the proximal and distal thirds than in the midsubstance of the epiligament. CD34 was expressed similarly in the proximal and distal thirds, although it seemed slightly more pronounced in the distal third. α-SMA expression was more robust in the proximal than the distal part. The results revealed that CD34 and α-SMA are expressed in the human epiligament. The differences between the numbers of cells in the proximal and distal parts of the epiligament and the expression of CD34 and α-SMA enhance epiligament theory. Future investigations into improving the quality of ligament healing should not overlook the epiligament theory.

Augmentation of ACL Autograft Reconstruction With an Amnion Collagen Matrix Wrap and Bone Marrow Aspirate Concentrate: A Pilot Randomized Controlled Trial With 2-Year Follow-up

Background

It is theorized that the lack of a synovial lining after anterior cruciate ligament (ACL) injury and ACL reconstruction (ACLR) contributes to slow ligamentization and possible graft failure. Whether graft maturation and incorporation can be improved with the use of a scaffold requires investigation.

Purpose

To evaluate the safety and efficacy of wrapping an ACL autograft with an amnion collagen matrix and injecting bone marrow aspirate concentrate (BMAC), quantify the cellular content of the BMAC samples, and assess 2-year postoperative patient-reported outcomes.

Study Design

Randomized controlled trial; Level of evidence, 2.

Methods

A total of 40 patients aged 18 to 35 years who were scheduled to undergo ACLR were enrolled in a prospective single-blinded randomized controlled trial with 2 arms based on graft type: bone-patellar tendon-bone (BTB; n = 20) or hamstring (HS; n = 20). Participants in each arm were randomized into a control group who underwent standard ACLR or an intervention group who had their grafts wrapped with an amnion collagen matrix during graft preparation, after which BMAC was injected under the wrap layers after implantation. Postoperative magnetic resonance imaging (MRI) mapping/processing yielded mean T2* relaxation time and graft volume values at 3, 6, 9, and 12 months. Participants completed the Single Assessment Numeric Evaluation Score, Knee injury and Osteoarthritis Outcome Score, and pain visual analog scale. Statistical linear mixed-effects models were used to quantify the effects over time and the differences between the control and intervention groups. Adverse events were also recorded.

Results

No significant differences were found at any time point between the intervention and control groups for BTB T2* (95% CI, -1.89 to 0.63; P = .31), BTB graft volume (95% CI, -606 to 876.1; P = .71), HS T2* (95% CI, -2.17 to 0.39; P = .162), or HS graft volume (95% CI, -11,141.1 to 351.5; P = .28). No significant differences were observed between the intervention and control groups of either graft type on any patient-reported outcome measure. No adverse events were reported after a 2-year follow-up.

Conclusion

In this pilot study, wrapping a graft with an amnion collagen matrix and injecting BMAC appeared safe. MRI T2* values and graft volume of the augmented ACL graft were not significantly different from that of controls, suggesting that the intervention did not result in improved graft maturation.

Registration

NCT03294759 (ClinicalTrials.gov identifier).

Preclinical tendon and ligament models: Beyond the 3Rs (replacement, reduction, and refinement) to 5W1H (why, who, what, where, when, how)

Several tendon and ligament animal models were presented at the 2022 Orthopaedic Research Society Tendon Section Conference held at the University of Pennsylvania, May 5 to 7, 2022. A key objective of the breakout sessions at this meeting was to develop guidelines for the field, including for preclinical tendon and ligament animal models. This review summarizes the perspectives of experts for eight surgical small and large animal models of rotator cuff tear, flexor tendon transection, anterior cruciate ligament tear, and Achilles tendon injury using the framework: "Why, Who, What, Where, When, and How" (5W1H). A notable conclusion is that the perfect tendon model does not exist; there is no single gold standard animal model that represents the totality of tendon and ligament disease. Each model has advantages and disadvantages and should be carefully considered in light of the specific research question. There are also circumstances when an animal model is not the best approach. The wide variety of tendon and ligament pathologies necessitates choices between small and large animal models, different anatomic sites, and a range of factors associated with each model during the planning phase. Attendees agreed on some guiding principles including: providing clear justification for the model selected, providing animal model details at publication, encouraging sharing of protocols and expertise, improving training of research personnel, and considering greater collaboration with veterinarians. A clear path for translating from animal models to clinical practice was also considered as a critical next step for accelerating progress in the tendon and ligament field.

Biologics, Stem Cells, Growth Factors, Platelet-Rich Plasma, Hemarthrosis, and Scaffolds May Enhance Anterior Cruciate Ligament Surgical Treatment

Biologics including mesenchymal stem cells (MSCs), growth factors, and platelet-rich plasma may enhance anterior cruciate ligament (ACL) reconstruction and even ACL primary repair. In addition, hemarthrosis after acute ACL injury represents a source of biologic factors. MSCs can differentiate into both fibroblasts and osteoblasts, potentially providing a transition between the ligament or graft and bone. MSCs also produce cytokines and growth factors necessary for cartilage, bone, ligament, and tendon regeneration. MSC sources including bone marrow, synovium, adipose tissue, ACL-remnant, patellar tendon, and umbilical cord. Also, scaffolds may represent a tool for ACL tissue engineering. A scaffold should be porous, which allows cell growth and flow of nutrients and waste, should be biocompatible, and might have mechanical properties that match the native ACL. Scaffolds have the potential to deliver bioactive molecules or stem cells. Synthetic and biologically derived scaffolds are widely available. ACL reconstruction with improved outcome, ACL repair, and ACL tissue engineering are promising goals. LEVEL OF EVIDENCE: Level V, expert opinion.

BMP-2/TGF-β1 gene insertion into ligament-derived stem cells sheet promotes tendon-bone healing in a mouse

Bone morphogenetic protein-2 (BMP-2) and transforming growth factor-β1 (TGF-β1) reportedly induce the osteogenic and tenogenic differentiation of anterior cruciate ligament (ACL)-derived stem cells (LDSCs), respectively. However, few studies have investigated the effect of BMP-2/TGF-β1 on the differentiation of LDSC. We developed a BMP-2/TGF-β1 gene insertion into an LDSC cell sheet that promotes tendon-bone healing in a mouse ACL reconstruction (ACLR) model. CD34⁺ LDSCs were isolated from human ACL stump tissues, virally transduced to express BMP-2 or TGF-β1, and then embedded within cell sheets. All mice underwent ACLR using an autograft wrapped with a cell sheet and were randomly divided into three groups: BMP-2-, TGF-β1-, and BMP-2/TGF-β1-transduced. At 4 and 8 weeks, tendon-bone healing was evaluated by micro-CT, biomechanical test, and histological analysis. BMP-2 and TGF-β1 promoted the osteogenic and tenogenic differentiation of LDSC in vitro. BMP-2/TGF-β1-transduced LDSC sheet application contributed to early improvement in mean failure load and graft stiffness, accelerated maturation of the tendon-bone junction, and inhibited bone tunnel widening. Furthermore, reduced M1 macrophage infiltration and a higher M2 macrophage percentage were observed in the BMP-2/TGF-β1-transduced LDSC group. This work demonstrated that BMP-2 and TGF-β1 promoted CD34⁺ LDSCs osteogenic and tenogenic differentiation in vitro and in vivo, which accelerated the tendon-bone healing after ACLR using autografts wrapped with cell sheets in a mouse model.

Bioactive Decellularized Tendon-Derived Stem Cell Sheet for Promoting Graft Healing After Anterior Cruciate Ligament Reconstruction

BACKGROUND

Stem cell sheets provide a scaffold-free option for the promotion of graft healing after anterior cruciate ligament reconstruction (ACLR). However, cell viability, stability, and potential uncontrolled actions create challenges for clinical translation. The decellularization of cell sheets may overcome these problems as studies have shown that the natural extracellular matrix of stem cells is bioactive and can promote tissue repair.

HYPOTHESIS

The decellularized tendon-derived stem cell (dTDSC) sheet can promote graft healing after ACLR.

STUDY DESIGN

Controlled laboratory study.

METHODS

An optimized decellularization protocol was developed to decellularize the TDSC sheets. A total of 64 Sprague-Dawley rats underwent ACLR with or without the dTDSC sheet wrapping the tendon graft (n = 32/group). At 2 and 6 weeks after surgery, graft healing was assessed by micro-computed tomography, histology, and biomechanical testing. The accumulation of iNOS⁺ and CD206⁺ cells and the expression of metalloproteinase 1 (MMP-1), MMP-13, and tissue inhibitor of metalloprotease 1 (TIMP-1) were assessed by immunohistochemistry.

RESULTS

The decellularization was successful, with the removal of 98.4% nucleic acid while preserving the collagenous proteins and bioactive factors. The expression of bone morphogenetic protein 2 (BMP-2) and VEGF in the dTDSC sheet was comparable with the TDSC sheet (P > .05). Micro-computed tomography showed significantly more tunnel bone formation in the dTDSC sheet group. The dTDSC sheet group demonstrated better graft osteointegration and higher integrity of graft midsubstance with significantly higher ultimate failure load (16.58 ± 7.24 vs 8.93 ± 2.45 N; P = .002) and stiffness (11.97 ± 5.21 vs 6.73 ± 2.20 N/mm; P = .027). Significantly fewer iNOS⁺ cells but more CD206⁺ cells, as well as lower MMP-1 and MMP-13 but higher TIMP-1 expression, were detected at the tendon-bone interface and graft midsubstance in the dTDSC sheet group.

CONCLUSION

An optimized decellularization protocol for producing bioactive dTDSC sheets was developed. Wrapping tendon graft with a dTDSC sheet promoted graft healing after ACLR, likely via enhancing bone formation and angiogenesis by BMP-2 and VEGF, modulating macrophage polarization and MMP/TIMP expression, and physically protecting the tendon graft.

CLINICAL RELEVANCE

dTDSC sheets alleviate the quality control and safety concerns of cell transplantation and can be used as a cell-free alternative for the promotion of graft healing in ACLR.

A comparative study of the epiligament of the medial collateral and anterior cruciate ligaments in the human knee: Immunohistochemical analysis of CD 34, α-smooth muscle actin and vascular endothelial growth factor in relation to epiligament theory

BACKGROUND

This study evaluated and compared the expression of VEGF, CD34, and α-SMA in the anterior cruciate ligaments and medial collateral ligaments in healthy human knees in order to enrich the epiligament theory regarding ligament healing after injury.

METHODS

Samples from the mid-substance of the anterior cruciate ligament and the medial collateral ligament of 12 fresh knee joints were used. Monoclonal antibodies against CD34, α-SMA, and VEGF were used for immunohistochemical analysis. Photomicrographs were analyzed using the ImageJ software.

RESULTS

The epiligament of the anterior cruciate ligament showed slightly higher expression of CD34, α-SMA, and VEGF than the epiligament of the medial collateral ligament. Overall, among the tested markers, α-SMA expression was most pronounced in anterior cruciate ligament epiligament images and CD34 dominated in medial collateral ligament epiligament images. The intensity of DAB staining for CD34, α-SMA, and VEGF was higher in vascular areas of the epiligament than in epiligament connective tissue.

CONCLUSIONS

The results illustrate that CD34, α-SMA, and VEGF are expressed in the human epiligament. The differences between the epiligament of the investigated ligaments and the fact that CD34, α-SMA, and VEGF, which are known to have a definite role in ligament healing, are predominantly expressed in the main vascular part of the ligament-epiligament complex enlarge the existing epiligament theory. Future investigations regarding better ligament healing should not overlook the epiligament tissue.

Comparison of ligamentization potential between anterior cruciate ligament-derived cells and adipose-derived mesenchymal stem cells reseeded to acellularized tendon allograft

AIMS

To test the hypothesis that reseeded anterior cruciate ligament (ACL)-derived cells have a better ability to survive and integrate into tendon extracellular matrix (ECM) and accelerate the ligamentization process, compared to adipose-derived mesenchymal stem cells (ADMSCs).

METHODS

Acellularized tibialis allograft tendons were used. Tendons were randomly reseeded with ACL-derived cells or ADMSCs. ACL-derived cells were harvested and isolated from remnants of ruptured ACLs during reconstruction surgery and cultured at passage three. Cell suspensions (200 µl) containing 2 × 106 ACL-derived cells or ADMSCs were prepared for the purpose of reseeding. At days 1, 3, and 7 post-reseeding, graft composites were assessed for repopulation with histological and immunohistochemical analysis. Matrix protein contents and gene expression levels were analyzed.

RESULTS

In the graft reseeded with ACL-derived cells, a large number of elongated cells that integrated into the matrix were evident at day 3 and day 7. However, in the graft reseeded with ADMSCs, only a small number of elongated cells were found integrated into the matrix. Immunofluorescence for Ki-67 and type I collagen confirmed the pronounced production of type I collagen by Ki-67-positive ACL-derived cells integrated into the ECM. A messenger RNA (mRNA) expression assay demonstrated significantly higher gene expression levels of types I (p = 0.013) and III (p = 0.050) collagen in the composites reseeded with ACL-derived cells than ADMSCs.

CONCLUSION

ACL-derived cells, when reseeded to acellularized tendon graft, demonstrated earlier better survival and integration in the tendon ECM and resulted in higher gene expression levels of collagen, which may be essential to the normal ligamentization process compared to ADMSCs.Cite this article: Bone Joint Res 2022;11(11):777-786.

Exosomes derived from magnetically actuated bone mesenchymal stem cells promote tendon-bone healing through the miR-21-5p/SMAD7 pathway

Graft healing after anterior cruciate ligament reconstruction (ACLR) involves slow biological processes, and various types of biological modulations have been explored to promote tendon-to-bone integration. Exosomes have been extensively studied as a promising new cell-free strategy for tissue regeneration, but few studies have reported their potential in tendon-to-bone healing. In this study, a novel type of exosome derived from magnetically actuated (iron oxide nanoparticles (IONPs) combined with a magnetic field) bone mesenchymal stem cells (BMSCs) (IONP-Exos) was developed, and the primary purpose of this study was to determine whether IONP-Exos exert more significant effects on tendon-to-bone healing than normal BMSC-derived exosomes (BMSC-Exos). Here, we isolated and characterized the two types of exosomes, conducted in vitro experiments to measure their effects on fibroblasts (NIH3T3), and performed in vivo experiments to compare the effects on tendon-to-bone integration. Moreover, functional exploration of exosomal miRNAs was further performed by utilizing a series of gain- and loss-of-function experiments. Experimental results showed that both BMSC-Exos and IONP-Exos could be shuttled intercellularly into NIH3T3 fibroblasts and enhanced fibroblast activity, including proliferation, migration, and fibrogenesis. In vivo, we found that IONP-Exos significantly prevented peri-tunnel bone loss, promoted more osseous ingrowth into the tendon graft, increased fibrocartilage formation at the tendon-bone tunnel interface, and induced a higher maximum load to failure than BMSC-Exos. Furthermore, overexpression of miR-21-5p remarkably enhanced fibrogenesis in vitro, and SMAD7 was shown to be involved in the promotive effect of IONP-Exos on tendon-to-bone healing. Our findings may provide new insights into the regulatory roles of IONPs in IONP-Exos communication via stimulating exosomal miR-21-5p secretion and the SMAD7 signaling pathway in the fibrogenic process of tendon-to-bone integration. This work could provide a new strategy to promote tendon-to-bone healing for tissue engineering in the future.

Cellular Augmentation of Anterior Cruciate Ligament Surgery Is Not Currently Evidence Based: A Systematic Review of Clinical Studies

PURPOSE

To perform a systematic review of clinical outcome studies exploring cellular augmentation of anterior cruciate ligament (ACL) surgery, including stem cell techniques.

METHODS

A systematic search was performed according to the Preferred Reporting Items of Systematic Reviews and Meta-analyses (PRISMA) guidelines using the Cochrane, PubMed, MEDLINE, SPORTDiscus, and CINAHL (Cumulative Index to Nursing and Allied Health Literature) databases from 2000 to 2019. The inclusion criteria were clinical studies that reported on ACL surgery augmented with stem cells or cellular therapy and patient-reported outcome measures or graft healing. Risk of bias was assessed using the Cochrane risk-of-bias tool for randomized clinical trials, and nonrandomized trials were assessed using the Methodological Items for Non-randomized Studies (MINORS) tool. Methodologic assessment was performed according to the Modified Coleman Methodology Score.

RESULTS

Four studies were found: 2 randomized clinical trials, 1 cohort study with a matched historical control group, and 1 case series. The mean Modified Coleman Methodology Score in these studies was 59, and there was a low risk of bias in 1 study. One study reported outcomes of augmented ACL repair, and 3 studies reported the results of augmented ACL reconstruction. Cellular therapies varied and included concentrated bone marrow aspirate, collagenase/centrifuge processed adipose, and marrow stimulation combined with platelet-rich plasma, as well as cells cultured from allograft bone marrow aspirate. The concentrated bone marrow aspirate and adipose tissue study results did not support their use. The marrow stimulation technique combined with repair led to promising clinical results. The use of allograft cultured cells improved patient-reported outcomes and postoperative radiographic findings.

CONCLUSIONS

Augmentation of ACL surgery with cellular therapy is not supported by clinical evidence at this time.

LEVEL OF EVIDENCE

Level IV.

In situ regeneration of bone-to-tendon structures: Comparisons between costal-cartilage derived stem cells and BMSCs in the rat model

Bone-tendon interface (BTI), also called enthesis, is composed of the bone, fibrocartilage, and tendon/ligament with gradual structural characteristics. The unique gradient structure is particularly important for mechanical stress transfer between bone and soft tissues. However, BTI injuries result in fibrous scar repairs and high incidences of re-rupture, which is attributed to the lack of local stem cells with tenogenic and osteogenic potentials. In the rat model, we identified unique stem cells from costal cartilage (CDSCs) with a high in situ regeneration potential of BTI structures. Compared to bone-marrow mesenchymal stem cells (BMSCs), CDSCs exhibit higher self-renewal capacities, better adaptability to low-oxygen and low-nutrient post-transplantation environments, as well as strong bi-potent differentiation abilities of osteogenesis and tenogenesis. After transplantation, CDSCs can survive, proliferate, and in situ gradually regenerate BTI structures. Therefore, CDSCs have a great potential for tissue engineering regeneration in BTI injuries, and have future clinical application prospects. STATEMENT OF SIGNIFICANCE: Tissue engineering is a promising technique for bone-to-tendon interface (BTI) regeneration after injury, but it is still a long way from clinical application. One of the major reasons is the lack of suitable seed cells. This study found an ideal source of seed cells derived from costal cartilages (CDSCs). Compared to the traditional seed cell BMSCs, CDSCs have higher proliferation ability, strong chondrogenic and tenogenic differentiation potential, and better adaptability to low-oxygen and low nutrient conditions. CDSCs were able to survive, proliferate, and regenerate BTI structures in situ, in contrast to BMSCs. CDSCs transplantation showed strong BTI structures regeneration potential both histologically and biomechanically, making it a suitable seed cell for the tissue engineering regeneration of BTI.

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.

Tackling the Challenges of Graft Healing After Anterior Cruciate Ligament Reconstruction-Thinking From the Endpoint

Anterior cruciate ligament (ACL) tear is common in sports and accidents, and accounts for over 50% of all knee injuries. ACL reconstruction (ACLR) is commonly indicated to restore the knee stability, prevent anterior–posterior translation, and reduce the risk of developing post-traumatic osteoarthritis. However, the outcome of biological graft healing is not satisfactory with graft failure after ACLR. Tendon graft-to-bone tunnel healing and graft mid-substance remodeling are two key challenges of biological graft healing after ACLR. Mounting evidence supports excessive inflammation due to ACL injury and ACLR, and tendon graft-to-bone tunnel motion negatively influences these two key processes. To tackle the problem of biological graft healing, we believe that an inductive approach should be adopted, starting from the endpoint that we expected after ACLR, even though the results may not be achievable at present, followed by developing clinically practical strategies to achieve this ultimate goal. We believe that mineralization of tunnel graft and ligamentization of graft mid-substance to restore the ultrastructure and anatomy of the original ACL are the ultimate targets of ACLR. Hence, strategies that are osteoinductive, angiogenic, or anti-inflammatory should drive graft healing toward the targets. This paper reviews pre-clinical and clinical literature supporting this claim and the role of inflammation in negatively influencing graft healing. The practical considerations when developing a biological therapy to promote ACLR for future clinical translation are also discussed.

Anterior Cruciate Ligament Reconstruction: Is Biological Augmentation Beneficial?

Surgical reconstruction in anterior cruciate ligament (ACL) ruptures has proven to be a highly effective technique that usually provides satisfactory results. However, despite the majority of patients recovering their function after this procedure, ACL reconstruction (ACLR) is still imperfect. To improve these results, various biological augmentation (BA) techniques have been employed mostly in animal models. They include: (1) growth factors (bone morphogenetic protein, epidermal growth factor, granulocyte colony-stimulating factor, basic fibroblast growth factor, transforming growth factor-β, hepatocyte growth factor, vascular endothelial growth factor, and platelet concentrates such as platelet-rich plasma, fibrin clot, and autologous conditioned serum), (2) mesenchymal stem cells, (3) autologous tissue, (4) various pharmaceuticals (matrix metalloproteinase-inhibitor alpha-2-macroglobulin bisphosphonates), (5) biophysical/environmental methods (hyperbaric oxygen, low-intensity pulsed ultrasound, extracorporeal shockwave therapy), (6) biomaterials (fixation methods, biological coatings, biosynthetic bone substitutes, osteoconductive materials), and (7) gene therapy. All of them have shown good results in experimental studies; however, the clinical studies on BA published so far are highly heterogeneous and have a low degree of evidence. The most widely used technique to date is platelet-rich plasma. My position is that orthopedic surgeons must be very cautious when considering using PRP or other BA methods in ACLR.

Nanog/NFATc1/Osterix signaling pathway-mediated promotion of bone formation at the tendon-bone interface after ACL reconstruction with De-BMSCs transplantation

Background

Bone formation plays an important role in early tendon–bone healing after anterior cruciate ligament reconstruction (ACLR). Dedifferentiated osteogenic bone marrow mesenchymal stem cells (De-BMSCs) have enhanced osteogenic potential. This study aimed to investigate the effect of De-BMSCs transplantation on the promotion of bone formation at the tendon–bone interface after ACLR and to further explore the molecular mechanism of the enhanced osteogenic potential of De-BMSCs.

Methods

BMSCs from the femurs and tibias of New Zealand white rabbits were subjected to osteogenic induction and then cultured in medium without osteogenic factors; the obtained cell population was termed De-BMSCs. De-BMSCs were induced to undergo osteo-, chondro- and adipo-differentiation in vitro to examine the characteristics of primitive stem cells. An ACLR model with a semitendinosus tendon was established in rabbits, and the animals were divided into a control group, BMSCs group, and De-BMSCs group. At 12 weeks after surgery, the rabbits in each group were sacrificed to evaluate tendon–bone healing by histologic staining, micro-computed tomography (micro-CT) examination, and biomechanical testing. During osteogenic differentiation of De-BMSCs, an siRNA targeting nuclear factor of activated T-cells 1 (NFATc1) was used to verify the molecular mechanism of the enhanced osteogenic potential of De-BMSCs.

Results

De-BMSCs exhibited some properties similar to BMSCs, including multiple differentiation potential and cell surface markers. Bone formation at the tendon–bone interface in the De-BMSCs group was significantly increased, and biomechanical strength was significantly improved. During the osteogenic differentiation of De-BMSCs, the expression of Nanog and NFATc1 was synergistically increased, which promoted the interaction of NFATc1 and Osterix, resulting in increased expression of osteoblast marker genes such as COL1A, OCN, and OPN.

Conclusions

De-BMSCs transplantation could promote bone formation at the tendon–bone interface after ACLR and improve the biomechanical strength of the reconstruction. The Nanog/NFATc1/Osterix signaling pathway mediated the enhanced osteogenic differentiation efficiency of De-BMSCs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02643-9.

Adipose-Derived Stem Cell Sheets Improve Early Biomechanical Graft Strength in Rabbits After Anterior Cruciate Ligament Reconstruction

BACKGROUND

Although various reconstruction techniques are available for anterior cruciate ligament (ACL) injuries, a long recovery time is required before patients return to sports activities, as the reconstructed ACL requires time to regain strength. To date, several studies have reported use of mesenchymal stem cells in orthopaedic surgery; however, no studies have used adipose-derived stem cell (ADSC) sheets in ACL reconstruction (ACLR).

HYPOTHESIS

ADSC sheet transplantation can improve biomechanical strength of the autograft used in ACLR.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 68 healthy Japanese white rabbits underwent unilateral ACLR with a semitendinosus tendon autograft after random enrollment into a control group (no sheet; n = 34) and a sheet group (ADSC sheet; n = 34). At 2, 4, 8, 16, and 24 weeks after surgery, rabbits in each group were sacrificed to evaluate tendon-bone healing using histological staining, micro-computed tomography, and biomechanical testing. At 24 weeks, scanning transmission electron microscopy of the graft midsubstance was performed.

RESULTS

The ultimate failure load for the control and sheet groups, respectively, was as follows: 17.2 ± 5.5 versus 37.3 ± 10.3 (P = .01) at 2 weeks, 28.6 ± 1.9 versus 47.4 ± 10.4 (P = .003) at 4 weeks, 53.0 ± 14.3 versus 48.1 ± 9.3 (P = .59) at 8 weeks, 66.2 ± 9.3 versus 95.2 ± 43.1 (P = .24) at 16 weeks, and 66.7 ± 27.3 versus 85.3 ± 29.5 (P = .39) at 24 weeks. The histological score was also significantly higher in the sheet group compared with the control group at early stages up to 8 weeks. On micro-computed tomography, relative to the control group, the bone tunnel area was significantly narrower in the sheet group at 4 weeks, and the bone volume/tissue volume of the tendon-bone interface was significantly greater at 24 weeks. Scanning transmission electron microscopy at 24 weeks indicated that the mean collagen fiber diameter in the midsubstance was significantly greater, as was the occupation ratio of collagen fibers per field of view, in the sheet group.

CONCLUSION

ADSC sheets improved biomechanical strength, prevented bone tunnel enlargement, and promoted tendon-bone interface healing and graft midsubstance healing in an in vivo rabbit model.

CLINICAL RELEVANCE

ADSC sheets may be useful for early tendon-bone healing and graft maturation in ACLR.

Biological modulations to facilitate graft healing in anterior cruciate ligament reconstruction (ACLR), when and where to apply? A systematic review

Background

When and where to apply the biological modulations is effective to promote healing in the anterior cruciate ligament (ACL) reconstruction remains unclear.

Purpose

To perform a systematic review of preclinical animal studies on biological modulation in anterior cruciate ligament reconstruction (ACLR) concerning the time and site of delivery.

Study design

Systematic review of controlled laboratory studies.

Methods

PubMed, Ovid, and Scopus were searched until December 2020 using a combination of keywords and their synonym to retrieve all animal studies about biological modulation in ACLR. Studies that assessed mechanical strength after ACLR and compared with negative control were included. The methodological quality of animal studies was evaluated.

Results

33 studies were included in this review and the majority reported mechanical strength improvement. 79 ​% of studies applied the biological modulations intra-operatively with different delivery systems used. For 21 ​% of post-operative delivery studies, intermittent delivery was tried. 21 of the included studies directly applied the biological modulations in the bone tunnels, 5 studies applied intra-articularly while 7 studies applied both in the bone tunnels and intra-articular part. Biological modulations applied intra-operatively and those applied in both parts showed better mechanical strength increase. A shift of the failure mode of pull-out from the bone tunnel in the early healing phase, to mid-substance rupture in the later phase was observed in most studies.

Conclusion

The improvement of the mechanical strength depends on how the biological modulations (delivery phase, delivery site, delivery form) are applied. The intra-operative delivery showed an overall higher mechanical strength increase and bone tunnel only delivery or intra-articular and bone tunnel both delivery are preferred than intra-articular only delivery. In addition, intra-articular and bone tunnel both delivery can have better mechanical strength increase for a long follow-up time. Thus, intra-operative application with a carrier to control release rate in both parts should be recommended. Further studies are needed to achieve a better healing outcome and more attention should be given to the intra-articular remodeling of the graft along with the tendon bone healing to increase the final mechanical strength.

The Translational potential of this article

Here, a systematic review of preclinical evidence of the time, site and the method the biological modulations being applied for ACLR to improve the graft healing would be performed. After reviewing the available studies, a choice of when and where to apply the biological modulations can achieve better mechanical strength after ACLR can be obtained. It provides evidence for both researchers and clinicians to decide when and where to apply the biological modulations can achieve their best effectiveness for ACLR before implementing. Promoting graft healing with targeted time and targeted site may reduce the risk of graft failure, safeguard return to sport.

Application of Stem Cell Therapy for ACL Graft Regeneration

Graft regeneration after anterior cruciate ligament (ACL) reconstruction surgery is a complex three-stage process, which usually takes a long duration and often results in fibrous scar tissue formation that exerts a detrimental impact on the patients' prognosis. Hence, as a regeneration technique, stem cell transplantation has attracted increasing attention. Several different stem cell types have been utilized in animal experiments, and almost all of these have shown good capacity in improving tendon-bone regeneration. Various differentiation inducers have been widely applied together with stem cells to enhance specific lineage differentiation, such as recombinant gene transfection, growth factors, and biomaterials. Among the various different types of stem cells, bone marrow-derived mesenchymal stem cells (BMSCs) have been investigated the most, while ligament stem progenitor cells (LDSCs) have demonstrated the best potential in generating tendon/ligament lineage cells. In the clinic, 4 relevant completed trials have been reported, but only one trial with BMSCs showed improved outcomes, while 5 relevant trials are still in progress. This review describes the process of ACL graft regeneration after implantation and summarizes the current application of stem cells from bench to bedside, as well as discusses future perspectives in this field.

Stem cell therapies in tendon-bone healing

Tendon-bone insertion injuries such as rotator cuff and anterior cruciate ligament injuries are currently highly common and severe. The key method of treating this kind of injury is the reconstruction operation. The success of this reconstructive process depends on the ability of the graft to incorporate into the bone. Recently, there has been substantial discussion about how to enhance the integration of tendon and bone through biological methods. Stem cells like bone marrow mesenchymal stem cells (MSCs), tendon stem/progenitor cells, synovium-derived MSCs, adipose-derived stem cells, or periosteum-derived periosteal stem cells can self-regenerate and potentially differentiate into different cell types, which have been widely used in tissue repair and regeneration. Thus, we concentrate in this review on the current circumstances of tendon-bone healing using stem cell therapy.

Graft healing after anterior cruciate ligament reconstruction (ACLR)

Anterior cruciate ligament reconstruction (ACLR) is a commonly performed procedure in Orthopaedic sports medicine. With advances in surgical techniques providing better positioning and fixation of the graft, subsequent graft failure to certain extent should be accounted by poor graft healing. Although different biological modulations for enhancement of graft healing have been tried in different clinical and animal studies, complete graft incorporation into bone tunnels and the “ligamentization” of the intra-articular part have not been fully achieved yet. Based on the understanding of graft healing process and its failure mechanism, the purpose of this review is to combine both the known basic science & clinical evidence, to provide a much clearer picture of the obstacle encountered in graft healing, so as to facilitate researchers on subsequent work on the enhancement of ACL graft healing.

Known data on applied regenerative medicine in tendon healing

Tendons and ligaments are important structures in the musculoskeletal system. Ligaments connect various bones and provide stability in complex movements of joints in the knee. Tendon is made of dense connective tissue and transmits the force of contraction from muscle to bone. They are injured due to direct trauma in sports or roadside accidents. Tendon healing after repair is often poor due to the formation of fibro vascular scar tissues with low mechanical property. Regenerative techniques such as PRP (platelet-rich plasma), stem cells, scaffolds, gene therapy, cell sheets, and scaffolds help augment repair and regenerate tissue in this context. Therefore, it is of interest to document known data (repair process, tissue regeneration, mechanical strength, and clinical outcome) on applied regenerative medicine in tendon healing.

Cruciate Ligament Cell Sheets Can Be Rapidly Produced on Thermoresponsive poly(glycidyl ether) Coating and Successfully Used for Colonization of Embroidered Scaffolds

Anterior cruciate ligament (ACL) cell sheets combined with biomechanically competent scaffolds might facilitate ACL tissue engineering. Since thermoresponsive polymers allow a rapid enzyme-free detachment of cell sheets, we evaluated the applicability of a thermoresponsive poly(glycidyl ether) (PGE) coating for cruciate ligamentocyte sheet formation and its influence on ligamentocyte phenotype during sheet-mediated colonization of embroidered scaffolds. Ligamentocytes were seeded on surfaces either coated with PGE or without coating. Detached ligamentocyte sheets were cultured separately or wrapped around an embroidered scaffold made of polylactide acid (PLA) and poly(lactic-co-ε-caprolactone) (P(LA-CL)) threads functionalized by gas-phase fluorination and with collagen foam. Ligamentocyte viability, protein and gene expression were determined in sheets detached from surfaces with or without PGE coating, scaffolds seeded with sheets from PGE-coated plates and the respective monolayers. Stable and vital ligamentocyte sheets could be produced within 24 h with both surfaces, but more rapidly with PGE coating. PGE did not affect ligamentocyte phenotype. Scaffolds could be colonized with sheets associated with high cell survival, stable gene expression of ligament-related type I collagen, decorin, tenascin C and Mohawk after 14 d and extracellular matrix (ECM) deposition. PGE coating facilitates ligamentocyte sheet formation, and sheets colonizing the scaffolds displayed a ligament-related phenotype.

Biomimetic strategies for tendon/ligament-to-bone interface regeneration

Tendon/ligament-to-bone healing poses a formidable clinical challenge due to the complex structure, composition, cell population and mechanics of the interface. With rapid advances in tissue engineering, a variety of strategies including advanced biomaterials, bioactive growth factors and multiple stem cell lineages have been developed to facilitate the healing of this tissue interface. Given the important role of structure-function relationship, the review begins with a brief description of enthesis structure and composition. Next, the biomimetic biomaterials including decellularized extracellular matrix scaffolds and synthetic-/natural-origin scaffolds are critically examined. Then, the key roles of the combination, concentration and location of various growth factors in biomimetic application are emphasized. After that, the various stem cell sources and culture systems are described. At last, we discuss unmet needs and existing challenges in the ideal strategies for tendon/ligament-to-bone regeneration and highlight emerging strategies in the field.

A Cell-free Biodegradable Synthetic Artificial Ligament for the Reconstruction of Anterior Cruciate Ligament in a Rat Model

Traditional Anterior Cruciate Ligament (ACL) reconstruction is commonly performed using an allograft or autograft and possesses limitations such as donor site morbidity, decreased range of motion, and potential infection. However, a biodegradable synthetic graft could greatly assist in the prevention of such restrictions after ACL reconstruction. In this study, artificial grafts were generated using "wet" and "dry" electrospinning processes with a biodegradable elastomer, poly (ester urethane) urea (PEUU), and were evaluated in vitro and in vivo in a rat model. Four groups were established: (1) Wet PEUU artificial ligament, (2) Dry PEUU artificial ligament, (3) Dry polycaprolactone artificial ligament (PCL), and (4) autologous flexor digitorum longus tendon graft. Eight weeks after surgery, the in vivo tensile strength of wet PEUU ligaments had significantly increased compared to the other synthetic ligaments. These results aligned with increased infiltration of host cells and decreased inflammation within the wet PEUU grafts. In contrast, very little cellular infiltration was observed in PCL and dry PEUU grafts. Micro-computed tomography analysis performed at 4 and 8 weeks postoperatively revealed significantly smaller bone tunnels in the tendon autograft and wet PEUU groups. The Wet PEUU grafts served as an adequate functioning material and allowed for the creation of tissues that closely resembled the ACL.

Biomaterials developed for facilitating healing outcome after anterior cruciate ligament reconstruction: Efficacy, surgical protocols, and assessments using preclinical animal models

Anterior cruciate ligament (ACL) reconstruction is the recommended treatment for ACL tear in the American Academy of Orthopaedic Surgeons (AAOS) guideline. However, not a small number of cases failed because of the tunnel bone resorption, unsatisfactory bone-tendon integration, and graft degeneration. The biomaterials developed and designed for improving ACL reconstruction have been investigated for decades. According to the Food and Drug Administration (FDA) and the International Organization for Standardization (ISO) regulations, animal studies should be performed to prove the safety and bioeffect of materials before clinical trials. In this review, we first evaluated available biomaterials that can enhance the healing outcome after ACL reconstruction in animals and then discussed the animal models and assessments for testing applied materials. Furthermore, we identified the relevance and knowledge gaps between animal experimental studies and clinical expectations. Critical analyses and suggestions for future research were also provided to design the animal study connecting basic research and requirements for future clinical translation.

Does Manual Drilling Improve the Healing of Bone-Hamstring Tendon Grafts in Anterior Cruciate Ligament Reconstruction? A Histological and Biomechanical Study in a Rabbit Model

Background:

Heat necrosis due to motorized drilling during anterior cruciate ligament (ACL) reconstruction could be a factor in delayed healing at the bone–tendon graft interface.

Hypothesis:

The process of osteointegration could be enhanced using manual drilling. It reduces the invasiveness of mechanical-thermal stress normally caused by the traditional motorized drill bit.

Study Design:

Controlled laboratory study.

Methods:

ACL reconstruction using semitendinosus tendon autografts was performed in 28 skeletally mature female New Zealand white rabbits, which were randomly divided into 3 groups. In group A (n = 12), the tunnels were drilled using a motorized device; in group B (n = 12), the tunnels were drilled using a manual drill bit; and group C (n = 4) served as a control with sham surgical procedures. The healing process in the tunnels was assessed histologically at 2, 4, 8, and 12 weeks and graded according to the Tendon–Bone Tunnel Healing (TBTH) scoring system. In addition, another 25 rabbits were used for biomechanical testing. The structural properties of the femur–ACL graft–tibia complex, from animals sacrificed at 8 weeks postoperatively, were determined using uniaxial tests. Stiffness (N/mm) and ultimate load to failure (N) were determined from the resulting load-elongation curves.

Results:

The time course investigation showed that manual drilling (group B) had a higher TBTH score and improved mechanical behavior, reflecting better organized collagen fiber continuity at the bone–fibrous tissue interface, better integration between the graft and bone, and early mineralized chondrocyte-like tissue formation at all the time points analyzed with a maximum difference at 4 weeks (TBTH score: 5.4 [group A] vs 12.3 [group B]; P < .001). Stiffness (23.1 ± 8.2 vs 17.8 ± 6.3 N/mm, respectively) and ultimate load to failure (91.8 ± 60.4 vs 55.0 ± 18.0 N, respectively) were significantly enhanced in the specimens treated with manual drilling compared with motorized drilling (P < .05 for both).

Conclusion:

The use of manual drilling during ACL reconstruction resulted in better tendon-to-bone healing during the crucial early weeks. Manual drilling was able to improve the biological and mechanical properties of bone–hamstring tendon graft healing and was able to restore postoperative graft function more quickly. Tunnel drilling results in bone loss and deficient tendon-bone healing, and heat necrosis after tunnel enlargement may cause mechanical stress, contributing to a delay in healing. Manual drilling preserved the bone stock inside the tunnel, reduced heat necrosis, and offered a better microenvironment for faster healing at the interface.

Clinical Relevance:

Based on study results, manual drilling could be used successfully in human ACL reconstruction, but further clinical studies are needed. A clinical alternative, called the original “all-inside” technique, has been developed for ACL reconstruction. In this technique, the femoral and tibial tunnels are manually drilled only halfway through the bone for graft fixation, reducing bone loss. Data from this study suggest that hamstring tendon–to–bone healing can be improved using a manual drilling technique to form femoral and tibial tunnels.

American Society for Bone and Mineral Research-Orthopaedic Research Society Joint Task Force Report on Cell-Based Therapies - Secondary Publication

Cell-based therapies, defined here as the delivery of cells in vivo to treat disease, have recently gained increasing public attention as a potentially promising approach to restore structure and function to musculoskeletal tissues. Although cell-based therapy has the potential to improve the treatment of disorders of the musculoskeletal system, there is also the possibility of misuse and misrepresentation of the efficacy of such treatments. The medical literature contains anecdotal reports and research studies, along with web-based marketing and patient testimonials supporting cell-based therapy. Both the American Society for Bone and Mineral Research (ASBMR) and the Orthopaedic Research Society (ORS) are committed to ensuring that the potential of cell-based therapies is realized through rigorous, reproducible, and clinically meaningful scientific discovery. The two organizations convened a multidisciplinary and international Task Force composed of physicians, surgeons, and scientists who are recognized experts in the development and use of cell-based therapies. The Task Force was charged with defining the state-of-the art in cell-based therapies and identifying the gaps in knowledge and methodologies that should guide the research agenda. The efforts of this Task Force are designed to provide researchers and clinicians with a better understanding of the current state of the science and research needed to advance the study and use of cell-based therapies for skeletal tissues. The design and implementation of rigorous, thorough protocols will be critical to leveraging these innovative treatments and optimizing clinical and functional patient outcomes. In addition to providing specific recommendations and ethical considerations for preclinical and clinical investigations, this report concludes with an outline to address knowledge gaps in how to determine the cell autonomous and nonautonomous effects of a donor population used for bone regeneration. © 2020 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:485-502, 2020.

Anterior cruciate ligament remnant cells have different potentials for cell differentiation based on their location

Histological and cytological observations of the human anterior cruciate ligament (ACL) had been described, but the differentiation potency based on their location is still unknown. To determine and compare proliferation and differentiation potential of cells derived from distal and middle thirds of the ACL remnant, ACL remnant was initially marked at the distal third (within 10  mm from the tibial insertion) and middle third (between 10-20  mm from the tibial insertion) and then dissected. Both the middle and distal third regions of ACL remnant were analyzed using CD34+ cell counting. Cell proliferation rate did not differ in both middle and distal third regions of ACL remnant, but they showed different characteristics in cell differentiation depending on their location. The distal third region of the ACL remnant had a tendency for chondrogenic differentiation with higher expression of CD34+ cells. On the other hand, the middle third region of ACL remnant had a strong tendency for osteogenic and ligamentous differentiation. Characteristics of the ACL remnant tissues should be considered when performing remnant-preserving or harvesting ACL remnants for tissue engineering.

Arthroscopic Primary Repair of the Anterior Cruciate Ligament With Single-Bundle Graft Augmentation

Recently, there has been a resurgence of interest in ligament preservation of the anterior cruciate ligament (ACL) that has the advantage of preserving the native tissue and maintaining proprioceptive function. Studies reporting outcomes of remnant-preserving ACL surgery have shown encouraging results with a higher potential for early healing and better functional outcomes compared with remnant-resecting surgery. Over the past decade, several surgical techniques for remnant preservation of the ACL have been proposed. In this technical note, the technique of primary ACL repair with graft augmentation is described. The goal of this technique is to preserve and tension the native tissue, thereby restoring the anatomy as much as possible while avoiding cyclops lesions, whereas the additional graft provides strength to the repaired ligament.

Tendon and Ligament Healing and Current Approaches to Tendon and Ligament Regeneration

Ligament and tendon injuries are common problems in orthopedics. There is a need for treatments that can expedite nonoperative healing or improve the efficacy of surgical repair or reconstruction of ligaments and tendons. Successful biologically-based attempts at repair and reconstruction would require a thorough understanding of normal tendon and ligament healing. The inflammatory, proliferative, and remodeling phases, and the cells involved in tendon and ligament healing will be reviewed. Then, current research efforts focusing on biologically-based treatments of ligament and tendon injuries will be summarized, with a focus on stem cells endogenous to tendons and ligaments. Statement of clinical significance: This paper details mechanisms of ligament and tendon healing, as well as attempts to apply stem cells to ligament and tendon healing. Understanding of these topics could lead to more efficacious therapies to treat ligament and tendon injuries. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:7-12, 2020.

American Society for Bone and Mineral Research-Orthopaedic Research Society Joint Task Force Report on Cell-Based Therapies

Cell-based therapies, defined here as the delivery of cells in vivo to treat disease, have recently gained increasing public attention as a potentially promising approach to restore structure and function to musculoskeletal tissues. Although cell-based therapy has the potential to improve the treatment of disorders of the musculoskeletal system, there is also the possibility of misuse and misrepresentation of the efficacy of such treatments. The medical literature contains anecdotal reports and research studies, along with web-based marketing and patient testimonials supporting cell-based therapy. Both the American Society for Bone and Mineral Research (ASBMR) and the Orthopaedic Research Society (ORS) are committed to ensuring that the potential of cell-based therapies is realized through rigorous, reproducible, and clinically meaningful scientific discovery. The two organizations convened a multidisciplinary and international Task Force composed of physicians, surgeons, and scientists who are recognized experts in the development and use of cell-based therapies. The Task Force was charged with defining the state-of-the art in cell-based therapies and identifying the gaps in knowledge and methodologies that should guide the research agenda. The efforts of this Task Force are designed to provide researchers and clinicians with a better understanding of the current state of the science and research needed to advance the study and use of cell-based therapies for skeletal tissues. The design and implementation of rigorous, thorough protocols will be critical to leveraging these innovative treatments and optimizing clinical and functional patient outcomes. In addition to providing specific recommendations and ethical considerations for preclinical and clinical investigations, this report concludes with an outline to address knowledge gaps in how to determine the cell autonomous and nonautonomous effects of a donor population used for bone regeneration. © 2019 American Society for Bone and Mineral Research.

Knitted Silk-Collagen Scaffold Incorporated with Ligament Stem/Progenitor Cells Sheet for Anterior Cruciate Ligament Reconstruction and Osteoarthritis Prevention

Current surgical management of anterior cruciate ligament (ACL) rupture still remains an intractable challenge in ACL regeneration due to the weak self-healing capability of ACL. Inadequate cell numbers and vascularization within the articular cavity contribute mainly to the poor prognosis. This time, we fabricated a new tissue engineering scaffold by adding ligament stem/progenitor cell (LSPC) sheets to our previous knitted silk-collagen sponge scaffold, which overcame these limitations by providing sufficient numbers of seed cells and a natural extracellular matrix to facilitate regeneration. LSPCs display excellent proliferation and multilineage differentiation capacity. Upon ectopic implantation, the knitted silk-collagen sponge scaffold incorporated with an LSPC sheet exhibited less immune cells but more fibroblast-like cells, deposited ECM and neovascularization, and better tissue ingrowth. In a rabbit model, we excised the ACL and performed a reconstructive surgery with our scaffold. Increased expression of ligament-specific genes and better collagen fibril formation could be observed after orthotopic transplantation. After 6 months, the LSPC sheet group showed better results on ligament regeneration and ligament-bone healing. Furthermore, no obvious cartilage and meniscus degeneration were observed at 6 months postoperation. In conclusion, these results indicated that the new tissue engineering scaffold can promote ACL regeneration and slow down the progression of osteoarthritis, thus suggesting its high clinical potential as an ideal graft in ACL reconstruction.

In vitro behavior of tendon stem/progenitor cells on bioactive electrospun nanofiber membranes for tendon-bone tissue engineering applications

Purpose

In order to accelerate the tendon-bone healing processes and achieve the efficient osteointegration between the tendon graft and bone tunnel, we aim to design bioactive electrospun nanofiber membranes combined with tendon stem/progenitor cells (TSPCs) to promote osteogenic regeneration of the tendon and bone interface.

Methods

In this study, nanofiber membranes of polycaprolactone (PCL), PCL/collagen I (COL-1) hybrid nanofiber membranes, poly(dopamine) (PDA)-coated PCL nanofiber membranes and PDA-coated PCL/COL-1 hybrid nanofiber membranes were successfully fabricated by electrospinning. The biochemical characteristics and nanofibrous morphology of the membranes, as well as the characterization of rat TSPCs, were defined in vitro. After co-culture with different types of electrospun nanofiber membranes in vitro, cell proliferation, viability, adhesion and osteogenic differentiation of TSPCs were evaluated at different time points.

Results

Among all the membranes, the performance of the PCL/COL-1 (volume ratio: 2:1 v/v) group was superior in terms of its ability to support the adhesion, proliferation, and osteogenic differentiation of TSPCs. No benefit was found in this study to include PDA coating on cell adhesion, proliferation and osteogenic differentiation of TSPCs.

Conclusion

The PCL/COL-1 hybrid electrospun nanofiber membranes are biocompatible, biomimetic, easily fabricated, and are capable of supporting cell adhesion, proliferation, and osteogenic differentiation of TSPCs. These bioactive electrospun nanofiber membranes may act as a suitable functional biomimetic scaffold in tendon-bone tissue engineering applications to enhance tendon-bone healing abilities.

Stem cells, blood vessels, and angiogenesis as major determinants for musculoskeletal tissue repair

This manuscript summarizes 20 years of research from my laboratories at the University of Pittsburgh and more recently, at the University of Texas Health Science Center at Houston and the Steadman Philippon Research Institute in Vail, Colorado. The discovery of muscle-derived stem cells (MDSCs) did not arise from a deliberate search to find a novel population of muscle cells with high regenerative potential, but instead was conceived in response to setbacks encountered while working in muscle cell transplantation for Duchenne muscular dystrophy (DMD). DMD is a devastating inherited X-linked muscle disease characterized by progressive muscle weakness due to lack of dystrophin expression in muscle fiber sarcolemma.¹ Although the transplantation of normal myoblasts into dystrophin-deficient muscle can restore dystrophin, this approach has been hindered by limited survival (less than 1%) of the injected cells.¹ The fact that 99% of the cells were not surviving implantation was seen as a major weakness with this technology by most. My research team decided to investigate which cells represent the 1% of the cells surviving post-implantation. We have subsequently confirmed that the few cells which exhibit high survival post-implantation also display stem cell characteristics, and were termed "muscle-derived stem cells" or MDSCs. Herein, I will describe the origin of these MDSCs, the mechanisms of MDSC action during tissue repair, and finally the development of therapeutic strategies to improve regeneration and repair of musculoskeletal tissues. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1212-1220, 2019.

Cellular Complexity at the Interface: Challenges in Enthesis Tissue Engineering

The complex heterogeneous cellular environment found in tendon-to-bone interface makes this structure a challenge for interface tissue engineering. Orthopedic surgeons still face some problems associated with the formation of fibrotic tissue or re-tear occurring after surgical re-attachment of tendons to the bony insertion or the application of grafts. Unfortunately, an understanding of the cellular component of enthesis lags far behind of other well-known musculoskeletal interfaces, which blocks the development of new treatment options for the healing and regeneration of this multifaceted junction. In this chapter, the main characteristics of tendon and bone cell populations are introduced, followed by a brief description of the interfacial cellular niche, highlighting molecular mechanisms governing tendon-to-bone attachment and mineralization. Finally, we describe and critically assess some challenges faced concerning the use of cell-based strategies in tendon-to-bone healing and regeneration.

Biological augmentation of graft healing in anterior cruciate ligament reconstruction: a systematic review

Aims

The success of anterior cruciate ligament reconstruction (ACLR) depends on osseointegration at the graft-tunnel interface and intra-articular ligamentization. Our aim was to conduct a systematic review of clinical and preclinical studies that evaluated biological augmentation of graft healing in ACLR.

Materials and Methods

In all, 1879 studies were identified across three databases. Following assessment against strict criteria, 112 studies were included (20 clinical studies; 92 animal studies).

Results

Seven categories of biological interventions were identified: growth factors, biomaterials, stem cells, gene therapy, autologous tissue, biophysical/environmental, and pharmaceuticals. The methodological quality of animal studies was moderate in 97%, but only 10% used clinically relevant outcome measures. The most interventions in clinical trials target the graft-tunnel interface and are applied intraoperatively. Platelet-rich plasma is the most studied intervention, but the clinical outcomes are mixed, and the methodological quality of studies was suboptimal. Other biological therapies investigated in clinical trials include: remnant-augmented ACLR; bone substitutes; calcium phosphate-hybridized grafts; extracorporeal shockwave therapy; and adult autologus non-cultivated stem cells.

Conclusion

There is extensive preclinical research supporting the use of biological therapies to augment ACLR. Further clinical studies that meet the minimum standards of reporting are required to determine whether emerging biological strategies will provide tangible benefits in patients undergoing ACLR. Cite this article: Bone Joint J 2018;100-B:271-84.

Boosting tendon repair: interplay of cells, growth factors and scaffold-free and gel-based carriers

Background

Tendons are dense connective tissues and critical components for the integrity and function of the musculoskeletal system. Tendons connect bone to muscle and transmit forces on which locomotion entirely depends. Due to trauma, overuse and age-related degeneration, many people suffer from acute or chronic tendon injuries. Owing to their hypovascularity and hypocellularity, tendinopathies remain a substantial challenge for both clinicians and researchers. Surgical treatment includes suture or transplantation of autograft, allograft or xenograft, and these serve as the most common technique for rescuing tendon injuries. However, the therapeutic efficacies are limited by drawbacks including inevitable donor site morbidity, poor graft integration, adhesion formations and high rates of recurrent tearing. This review summarizes the literature of the past 10 y concerning scaffold-free and gel-based approaches for treating tendon injuries, with emphasis on specific advantages of such modes of application, as well as the obtained results regarding in vitro and in vivo tenogenesis.

Results

The search was focused on publications released after 2006 and 83 articles have been analysed. The main results are summarizing and discussing the clear advantages of scaffold-free and hydrogels carriers that can be functionalized with cells alone or in combination with growth factors.

Conclusion

The improved understanding of tissue resident adult stem cells has made a significant progress in recent years as well as strategies to steer their fate toward tendon lineage, with the help of growth factors, have been identified. The field of tendon tissue engineering is exploring diverse models spanning from hard scaffolds to gel-based and scaffold-free approaches seeking easier cell delivery and integration in the site of injury. Still, the field needs to consider a multifactorial approach that is based on the combination and fine-tuning of chemical and biomechanical stimuli. Taken together, tendon tissue engineering has now excellent foundations and enters the period of precision and translation to models with clinical relevance on which better treatment options of tendon injuries can be shaped up.

Current Evidence of Adult Stem Cells to Enhance Anterior Cruciate Ligament Treatment: A Systematic Review of Animal Trials

PURPOSE

To systematically review the available preclinical evidence of adult stem cells as a biological augmentation in the treatment of animal anterior cruciate ligament (ACL) injury.

STUDY DESIGN

Systematic review.

METHODS

PubMed (MEDLINE) and Embase were searched for the eligible studies. The inclusion criteria were controlled animal trials of adult stem cells used in ACL treatment (repair or reconstruction). Studies of natural ACL healing without intervention, in vitro studies, ex vivo studies, and studies without controls were excluded. Evidence level, methodologic quality, and risk of bias of each included study were identified using previously established tools.

RESULTS

Thirteen animal studies were included. Six of 7 studies using bone marrow-derived mesenchymal stem (stromal) cells (BMSCs) reported a positive enhancement in histology, biomechanics, and biochemistry within 12 weeks postoperatively. Four studies using ACL-derived vascular stem cells showed a promoting effect in histology, biomechanics, and imaging within 8 weeks postoperatively. Two studies focusing on animal tendon-derived stem cells (TDSCs) and human umbilical cord blood-derived mesenchymal stem cells (hUCB-MSCs) reported promotable effects for the early healing in a small animal ACL model.

CONCLUSIONS

BMSCs, ACL-derived vascular stem cells, TDSCs, and hUCB-MSCs were shown to enhance the healing of ACL injury during the early phase in small animal models.

CLINICAL RELEVANCE

Results of clinical trials using adult stem cells in ACL treatment are conflicting, and a systematic review of the current best preclinical evidence is crucial to guide further application.

Appropriate Tensile Mode and Timing of Applying Tension to Promote Tendon Gel Regeneration

"Tendon gel" secreted from a parent tendon is regenerated for tendon repair by applying tension. However, the details of the tensile stimulus have not been clarified. This study aimed to evaluate an appropriate tensile stimulus mode and the optimal timing of applying tension to promote tendon gel regeneration. Tendon gel was prepared using a film model method in mice and was preserved in vivo for 3, 5, and 10 days. Unlike tendon gel on day 3 or day 5, a fibrous structure developed in the tendon gel on day 10 when tension was applied. Infrared spectroscopy revealed that characteristic peaks appearing for the tendon gel on days 3 and 5 disappeared on day 10. Disappearance of the peaks indicated maturity of the tendon gel, and it showed the optimal timing for tension application to the tendon gel. The effect of tensile load on tendon gel preserved for 10 days was investigated using a tensile test, a creep test, or a cycle test. In the tensile test, tendon gel was elongated into a thin cord of collagen fibers with an increase in stress, and the maximum diameter of the collagen fiber was approximately 50 times larger than that in the normal Achilles tendon of mice. The results suggest that the diameter of the oriented collagen fiber is controllable by adjusting the applied load and the time in mature tendon gel.

Single-Bundle Augmentation for a Partial Tear of the Anterior Cruciate Ligament

Given the prevalence of anterior cruciate ligament (ACL) tears in young athletes, it is essential to elucidate and illustrate surgical interventions that return the most favorable outcomes. Although most ACL injuries are full-thickness tears, occasionally either the anteromedial (AM) or posterolateral (PL) bundle is torn in isolation, allowing consideration of an ACL reconstruction or augmentation. Because the ACL-deficient knee has been shown to exhibit less proprioceptive feedback, the preservation of the intact bundle may offer an inherent component of stability. After arthroscopy has confirmed a partial ACL tear, the decision to augment the intact bundle can be made. The technique is adapted from principles of the double-bundle reconstruction such that the graft follows the path of either the native AM bundle or the native PL bundle, depending on the location of the partial tear. We present our surgical technique for ACL AM bundle augmentation with PL bundle reconstruction using a semitendinosus tendon autograft.

Viable Stem Cells Are in the Injury Effusion Fluid and Arthroscopic Byproducts From Knee Cruciate Ligament Surgery: An In Vivo Analysis

PURPOSE

To examine the number of viable stem cells contained in the postinjury effusion fluid and the waste byproducts of arthroscopic cruciate ligament surgery.

METHODS

This study included patients older than 18 years of age with acute (<5 weeks old) cruciate ligament injuries requiring arthroscopic surgery. The postinjury effusion fluid (effusion fluid), fat pad and cruciate ligament stump debridement tissue (byproduct tissue), and arthroscopic fluid collected during fat pad and/or stump debridement (byproduct fluid) were collected at the time of surgery from 30 individuals. Specimens were analyzed, investigating cell viability, nucleated cell counts, cell concentrations, colony-forming unit assays, and flow cytometry. Samples from the first 20 individuals were collected in small specimen containers, and samples from the last 10 individuals were collected in larger specimen containers.

RESULTS

Cells of the injury effusion exhibited the greatest viability (86.4 ± 1.31%) when compared with the small volume harvest byproduct tissue (50.2 ± 2.5%, P = .0001), small volume harvest byproduct fluid (48.8 ± 1.88%, P = .0001), large volume harvest byproduct tissue (70.1 ± 5.6%, P = .0001), and large volume harvest byproduct fluid (60.3 ± 3.41%, P = .0001). The culture analysis of fibroblast colony-forming units found on average 1916 ± 281 progenitor cells in the effusion fluid, 2488 ± 778 progenitor cells in the byproduct tissue, and 2357 ± 339 progenitor cells in the byproduct fluid. Flow cytometry confirmed the presence of immature cells and the presence of cells with markers typically expressed by known stem cell populations.

CONCLUSIONS

Viable stem cells are mobilized to the postinjury effusion at the time of cruciate ligament injury and can be found in the byproduct waste of cruciate ligament surgery.

CLINICAL RELEVANCE

The methodology around effusion fluid and byproduct tissue capture during cruciate ligament surgery should be investigated further. Cell amounts available from these tissues with current technologies are not sufficient for immediate evidence-based clinical application.

Anterior Cruciate Ligament-Derived Stem Cells Transduced With BMP2 Accelerate Graft-Bone Integration After ACL Reconstruction

BACKGROUND

Strong graft-bone integration is a prerequisite for successful graft remodeling after reconstruction of the anterior cruciate ligament (ACL) using soft tissue grafts. Novel strategies to accelerate soft tissue graft-bone integration are needed to reduce the need for bone-tendon-bone graft harvest, reduce patient convalescence, facilitate rehabilitation, and reduce total recovery time after ACL reconstruction.

HYPOTHESIS

The application of ACL-derived stem cells with enhanced expression of bone morphogenetic protein 2 (BMP2) onto soft tissue grafts in the form of cell sheets will both accelerate and improve the quality of graft-bone integration after ACL reconstruction in a rat model.

STUDY DESIGN

Controlled laboratory study.

METHODS

ACL-derived CD34+ cells were isolated from remnant human ACL tissues, virally transduced to express BMP2, and embedded within cell sheets. In a rat model of ACL injury, bilateral single-bundle ACL reconstructions were performed, in which cell sheets were wrapped around tendon autografts before reconstruction. Four groups containing a total of 48 rats (96 knees) were established (n = 12 rats; 24 knees per group): CD34+BMP2 (100%), CD34+BMP2 (25%), CD34+ (untransduced), and a control group containing no cells. Six rats from each group were euthanized 2 and 4 weeks after surgery, and each graft was harvested for immunohistochemical and histological analyses. The remaining 6 rats in each group were euthanized at 4 and 8 weeks to evaluate in situ tensile load to failure in each femur-graft-tibia complex.

RESULTS

In vitro, BMP2 transduction promoted the osteogenic differentiation of ACL-derived CD34+ cells while retaining their intrinsic multipotent capabilities. Osteoblast densities were greatest in the BMP2 (100%) and BMP2 (25%) groups. Bone tunnels in the CD34+BMP2 (100%) and CD34+BMP2 (25%) groups had the smallest cross-sectional areas according to micro-computed tomography analyses. Graft-bone integration occurred most rapidly in the CD34+BMP2 (25%) group. Tensile load to failure was significantly greater in the groups containing stem cells at 4 and 8 weeks after surgery. Tensile strength was greatest in the CD34+BMP2 (100%) group at 4 weeks, and in the CD34+BMP2 (25%) group at 8 weeks.

CONCLUSION

ACL-derived CD34+ cells transduced with BMP2 accelerated graft-bone integration after ACL reconstruction using soft tissue autografts in a rat model, as evidenced by improved histological appearance and graft-bone interface biology along with tensile load to failure at each time point up to 8 weeks after surgery.

CLINICAL RELEVANCE

A primary disadvantage of using soft tissue grafts for ACL reconstruction is the prolonged time required for bony ingrowth, which delays the initiation of midsubstance graft remodeling. The lack of consistent correlation between the appearance of a "healed" ACL on postoperative magnetic resonance imaging and readiness to return to sport results in athletes being released to sport before the graft is ready to handle high-intensity loading. Therefore, it is desirable to identify strategies that accelerate graft-bone integration, which would reduce the time to biologic fixation, improve the reliability of biologic fixation, allow for accelerated rehabilitation, and potentially reduce the incidence of early graft pullout and late midsubstance failure.

Rotator cuff repair using cell sheets derived from human rotator cuff in a rat model

To achieve biological regeneration of tendon-bone junctions, cell sheets of human rotator-cuff derived cells were used in a rat rotator cuff injury model. Human rotator-cuff derived cells were isolated, and cell sheets were made using temperature-responsive culture plates. Infraspinatus tendons in immunodeficient rats were resected bilaterally at the enthesis. In right shoulders, infraspinatus tendons were repaired by the transosseous method and covered with the cell sheet (sheet group), whereas the left infraspinatus tendons were repaired in the same way without the cell sheet (control group). Histological examinations (safranin-O and fast green staining, isolectin B4, type II collagen, and human-specific CD31) and mRNA expression (vascular endothelial growth factor; VEGF, type II collagen; Col2, and tenomodulin; TeM) were analyzed 4 weeks after surgery. Biomechanical tests were performed at 8 weeks. In the sheet group, proteoglycan at the enthesis with more type II collagen and isolectin B4 positive cells were seen compared with in the control group. Human specific CD31-positive cells were detected only in the sheet group. VEGF and Col2 gene expressions were higher and TeM gene expression was lower in the sheet group than in the control group. In mechanical testing, the sheet group showed a significantly higher ultimate failure load than the control group at 8 weeks. Our results indicated that the rotator-cuff derived cell sheet could promote cartilage regeneration and angiogenesis at the enthesis, with superior mechanical strength compared with the control. Treatment for rotator cuff injury using cell sheets could be a promising strategy for enthesis of tendon tissue engineering. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:289-296, 2017.