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Nyland J, Sirignano MN, Richards J, Krupp RJ. Regenerative Anterior Cruciate Ligament Healing in Youth and Adolescent Athletes: The Emerging Age of Recovery Science. J Funct Morphol Kinesiol 2024; 9:80. [PMID: 38804446 PMCID: PMC11130880 DOI: 10.3390/jfmk9020080] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/03/2024] [Revised: 04/15/2024] [Accepted: 04/19/2024] [Indexed: 05/29/2024] Open
Abstract
Anterior cruciate ligament (ACL) injuries mainly arise from non-contact mechanisms during sport performance, with most injuries occurring among youth or adolescent-age athletes, particularly females. The growing popularity of elite-level sport training has increased the total volume, intensity and frequency of exercise and competition loading to levels that may exceed natural healing capacity. Growing evidence suggests that the prevailing mechanism that leads to non-contact ACL injury from sudden mechanical fatigue failure may be accumulated microtrauma. Given the consequences of primary ACL injury on the future health and quality of life of youth and adolescent athletes, the objective of this review is to identify key "recovery science" factors that can help prevent these injuries. Recovery science is any aspect of sports training (type, volume, intensity, frequency), nutrition, and sleep/rest or other therapeutic modalities that may prevent the accumulated microtrauma that precedes non-contact ACL injury from sudden mechanical fatigue failure. This review discusses ACL injury epidemiology, current surgical efficacy, the native ACL vascular network, regional ACL histological complexities such as the entheses and crimp patterns, extracellular matrix remodeling, the concept of causal histogenesis, exercise dosage and ligament metabolism, central nervous system reorganization post-ACL rupture, homeostasis regulation, nutrition, sleep and the autonomic nervous system. Based on this information, now may be a good time to re-think primary ACL injury prevention strategies with greater use of modified sport training, improved active recovery that includes well-planned nutrition, and healthy sleep patterns. The scientific rationale behind the efficacy of regenerative orthobiologics and concomitant therapies for primary ACL injury prevention in youth and adolescent athletes are also discussed.
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Affiliation(s)
- John Nyland
- Norton Orthopedic Institute, 9880 Angie’s Way, Suite 250, Louisville, KY 40241, USA (J.R.); (R.J.K.)
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Multiparity and Aging Impact Chondrogenic and Osteogenic Potential at Symphyseal Enthesis: New Insights into Interpubic Joint Remodeling. Int J Mol Sci 2023; 24:ijms24054573. [PMID: 36902004 PMCID: PMC10003663 DOI: 10.3390/ijms24054573] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2022] [Revised: 02/21/2023] [Accepted: 02/21/2023] [Indexed: 03/03/2023] Open
Abstract
Pregnancy and childbirth cause adaptations to the birth canal to allow for delivery and fast recovery. To accommodate delivery through the birth canal, the pubic symphysis undergoes changes that lead to the interpubic ligament (IpL) and enthesis formation in primiparous mice. However, successive deliveries influence joint recovery. We aimed to understand tissue morphology and chondrogenic and osteogenic potential at symphyseal enthesis during pregnancy and postpartum in primiparous and multiparous senescent female mice. Morphological and molecular differences were found at the symphyseal enthesis among the study groups. Despite the apparent incapacity to restore cartilage in multiparous senescent animals, the symphyseal enthesis cells are active. However, these cells have reduced expression of chondrogenic and osteogenic markers and are immersed in densely packed collagen fibers contiguous to the persistent IpL. These findings may indicate alterations of key molecules in the progenitor cell population maintenance of the chondrocytic and osteogenic lineages at the symphyseal enthesis in multiparous senescent animals, possibly compromising the mouse joint histoarchitecture recovery. This sheds light on the distention of the birth canal and the pelvic floor that may play a role in pubic symphysis diastasis (PSD) and pelvic organ prolapse (POP), both in orthopedic and urogynecological practice in women.
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Edwards JH, Jones GL, Herbert A, Fisher J, Ingham E. Integration and functional performance of a decellularised porcine superflexor tendon graft in an ovine model of anterior cruciate ligament reconstruction. Biomaterials 2021; 279:121204. [PMID: 34736146 PMCID: PMC8683753 DOI: 10.1016/j.biomaterials.2021.121204] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2020] [Revised: 10/10/2021] [Accepted: 10/20/2021] [Indexed: 12/30/2022]
Abstract
The objective was to evaluate the performance of decellularised porcine superflexor tendon (pSFT) as an anterior cruciate ligament (ACL) reconstruction device. The ACL of adult sheep was reconstructed with decellularised pSFT or ovine allograft SFT and animals sacrificed at 4, 12 and 26 weeks (n = 4 per group) for biological evaluation and 26 weeks (n = 6) for biomechanical evaluation of the grafts. Both grafts showed good in vivo performance with no major differences at macroscopic evaluation post euthanasia. Histopathology revealed an inflammatory reaction to both grafts at 4 weeks, which reduced by 26 weeks. There was advanced cellular ingrowth from 12 weeks, ligamentisation of intra-articular grafts, ossification and formation of Sharpey's fibers at the graft/bone junctions. Immunohistochemistry showed that at 4 and 12 weeks, the host response was dominated by CD163+ M2 macrophages and a cell infiltrate comprising α-SMA + myofibroblasts, CD34+ and CD271+ progenitor cells. At 26 weeks the biomechanical properties of decellularised pSFT and oSFT grafts were comparable, with all grafts failing in the intra-articular region. This study provides new insight into constructive remodelling of tendons used for ACL replacement and evidence of integration and functional performance of a decellularised xenogeneic tendon with potential as an alternative for ACL reconstruction.
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Affiliation(s)
- Jennifer Helen Edwards
- Institute of Medical and Biological Engineering, School of Biomedical Sciences, University of Leeds, Leeds, LS2 9JT, UK; Institute of Medical and Biological Engineering, School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UK.
| | - Gemma Louise Jones
- Institute of Medical and Biological Engineering, School of Biomedical Sciences, University of Leeds, Leeds, LS2 9JT, UK.
| | - Anthony Herbert
- Institute of Medical and Biological Engineering, School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UK.
| | - John Fisher
- Institute of Medical and Biological Engineering, School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UK.
| | - Eileen Ingham
- Institute of Medical and Biological Engineering, School of Biomedical Sciences, University of Leeds, Leeds, LS2 9JT, UK.
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Gögele C, Hoffmann C, Konrad J, Merkel R, Schwarz S, Tohidnezhad M, Hoffmann B, Schulze-Tanzil GG. Cyclically stretched ACL fibroblasts emigrating from spheroids adapt their cytoskeleton and ligament-related expression profile. Cell Tissue Res 2021; 384:675-690. [PMID: 33835257 PMCID: PMC8211585 DOI: 10.1007/s00441-021-03416-9] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2020] [Accepted: 01/13/2021] [Indexed: 01/09/2023]
Abstract
Mechanical stress of ligaments varies; hence, ligament fibroblasts must adapt their expression profile to novel mechanomilieus to ensure tissue resilience. Activation of the mechanoreceptors leads to a specific signal transduction, the so-called mechanotransduction. However, with regard to their natural three-dimensional (3D) microenvironment cell reaction to mechanical stimuli during emigrating from a 3D spheroid culture is still unclear. This study aims to provide a deeper understanding of the reaction profile of anterior cruciate ligament (ACL)-derived fibroblasts exposed to cyclic uniaxial strain in two-dimensional (2D) monolayer culture and during emigration from 3D spheroids with respect to cell survival, cell and cytoskeletal orientation, distribution, and expression profile. Monolayers and spheroids were cultured in crosslinked polydimethyl siloxane (PDMS) elastomeric chambers and uniaxially stretched (14% at 0.3 Hz) for 48 h. Cell vitality, their distribution, nuclear shape, stress fiber orientation, focal adhesions, proliferation, expression of ECM components such as sulfated glycosaminoglycans, collagen type I, decorin, tenascin C and cell-cell communication-related gap junctional connexin (CXN) 43, tendon-related markers Mohawk and tenomodulin (myodulin) were analyzed. In contrast to unstretched cells, stretched fibroblasts showed elongation of stress fibers, cell and cytoskeletal alignment perpendicular to strain direction, less rounded cell nuclei, increased numbers of focal adhesions, proliferation, amplified CXN43, and main ECM component expression in both cultures. The applied cyclic stretch protocol evoked an anabolic response and enhanced tendon-related marker expression in ACL-derived fibroblasts emigrating from 3D spheroids and seems also promising to support in future tissue formation in ACL scaffolds seeded in vitro with spheroids.
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Affiliation(s)
- Clemens Gögele
- Institute of Anatomy and Cell Biology, Paracelsus Medical University, Prof.-Ernst-Nathan Str. 1, 90419 Nuremberg and Salzburg, Nuremberg, Germany
- Department of Biosciences, Paris Lodron University Salzburg, Hellbrunnerstr. 34, 5020 Salzburg, Austria
| | - Christina Hoffmann
- Institute of Biological Information Processing: IBI-2, Forschungszentrum Jülich, 52425 Jülich, Germany
| | - Jens Konrad
- Institute of Biological Information Processing: IBI-2, Forschungszentrum Jülich, 52425 Jülich, Germany
| | - Rudolf Merkel
- Institute of Biological Information Processing: IBI-2, Forschungszentrum Jülich, 52425 Jülich, Germany
| | - Silke Schwarz
- Institute of Anatomy and Cell Biology, Paracelsus Medical University, Prof.-Ernst-Nathan Str. 1, 90419 Nuremberg and Salzburg, Nuremberg, Germany
| | - Mersedeh Tohidnezhad
- Department of Anatomy and Cell Biology, RWTH Aachen University, Wendlingweg 2, 52074 Aachen, Germany
| | - Bernd Hoffmann
- Institute of Biological Information Processing: IBI-2, Forschungszentrum Jülich, 52425 Jülich, Germany
| | - Gundula Gesine Schulze-Tanzil
- Institute of Anatomy and Cell Biology, Paracelsus Medical University, Prof.-Ernst-Nathan Str. 1, 90419 Nuremberg and Salzburg, Nuremberg, Germany
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Takahashi T, Kimura M, Higuchi H, Suzuki K, Yamada Y, Takeshita K. A Histological Comparison Between Anterior Cruciate Ligament Remnant Tissue, Anatomically Reconstructed Graft, and Non-Anatomically Reconstructed Graft. Cureus 2021; 13:e13016. [PMID: 33542886 PMCID: PMC7847778 DOI: 10.7759/cureus.13016] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
Introduction: To our knowledge, no studies have investigated the histological comparison between primary injured anterior cruciate ligament (ACL), initially anatomically reconstructed grafts and non-anatomically reconstructed grafts at the time of revision ACL reconstruction. The purpose of this study was to histologically clarify the differences between ACL remnant tissue, reconstructed graft after anatomic double-bundle ACL reconstruction, and reconstructed graft after non-anatomic single-bundle ACL reconstruction. Methods: This histological study included five patients after anatomic double-bundle ACL reconstruction, three patients after non-anatomic single-bundle ACL reconstruction performed who injured their operated knees again, and five patients who injured their ACL for the first time and agreed to participate. All of the grafts and ACL remnant tissue were harvested, stained with hematoxylin and eosin, S-100, and alpha smooth muscle actin and evaluated using light microscopy. Results: There was no area of necrosis in the reconstructed graft after an anatomic double-bundle ACL reconstruction. However, there were obvious areas of necrosis in the reconstructed graft after non-anatomic single-bundle ACL reconstruction. Additionally, the collagen fibers were more longitudinally oriented, and most cells were spindle shaped like those in ACL remnant tissue after an anatomic double-bundle ACL reconstruction in contrast with the finding of the grafts after non-anatomic single-bundle ACL reconstruction. Conclusion: Initially reconstructed graft after an anatomic double-bundle ACL reconstruction may be beneficial if preserved at the time of the revision surgery.
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Affiliation(s)
| | | | - Hiroshi Higuchi
- Orthopaedic Surgery, Asakura Sports Rehabilitation Clinic, Maebashi, JPN
| | - Kosuke Suzuki
- Orthopaedic Surgery, Zenshukai Hospital, Maebashi, JPN
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Ristaniemi A, Regmi D, Mondal D, Torniainen J, Tanska P, Stenroth L, Finnilä MAJ, Töyräs J, Korhonen RK. Structure, composition and fibril-reinforced poroviscoelastic properties of bovine knee ligaments and patellar tendon. J R Soc Interface 2021; 18:20200737. [PMID: 33499766 DOI: 10.1098/rsif.2020.0737] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022] Open
Abstract
Tissue-level stress-relaxation of ligaments and tendons in the toe region is characterized by fast and long-term relaxations and an increase in relaxation magnitude with strain. Characterizing the compositional and structural origins of these phenomena helps in the understanding of mechanisms of ligament and tendon function and adaptation in health and disease. A three-step tensile stress-relaxation test was conducted on dumbbell-shaped pieces of bovine knee ligaments and patellar tendon (PT) (n = 10 knees). Their mechanical behaviour was characterized by a fibril-reinforced poroviscoelastic material model, able to describe characteristic times and magnitudes of fast and long-term relaxations. The crimp angle and length of tissues were measured with polarized light microscopy, while biochemical contents were determined by colorimetric biochemical methods. The long-term relaxation time was longer in the anterior cruciate ligament (ACL) and PT compared with collateral ligaments (p < 0.05). High hydroxyproline content predicted greater magnitude and shorter time of both fast and long-term relaxation. High uronic acid content predicted longer time of long-term relaxation, whereas high crimp angle predicted higher magnitude of long-term relaxation. ACL and PT are better long-term stabilizers than collateral ligaments. The long-term relaxation behaviour is affected or implied by proteoglycans and crimp angle, possibly relating to slow structural reorganization of the tissue.
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Affiliation(s)
- Aapo Ristaniemi
- Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
| | - Dristi Regmi
- Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
| | - Diponkor Mondal
- Research Unit of Medical Imaging, Physics and Technology, University of Oulu, Oulu, Finland
| | - Jari Torniainen
- Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.,Diagnostic Imaging Center, Kuopio University Hospital, Kuopio, Finland
| | - Petri Tanska
- Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
| | - Lauri Stenroth
- Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
| | - Mikko A J Finnilä
- Research Unit of Medical Imaging, Physics and Technology, University of Oulu, Oulu, Finland
| | - Juha Töyräs
- Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.,Diagnostic Imaging Center, Kuopio University Hospital, Kuopio, Finland.,School of Information Technology and Electrical Engineering, The University of Queensland, Brisbane, Australia
| | - Rami K Korhonen
- Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
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7
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Mayr HO, Stoehr A, Herberger KT, Haasters F, Bernstein A, Schmal H, Prall WC. Histomorphological Alterations of Human Anterior Cruciate Ligament Grafts During Mid-Term and Long-Term Remodeling. Orthop Surg 2020; 13:314-320. [PMID: 33295125 PMCID: PMC7862156 DOI: 10.1111/os.12835] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/30/2020] [Revised: 08/29/2020] [Accepted: 09/28/2020] [Indexed: 11/30/2022] Open
Abstract
Objective The aim of the present paper is to analyze mid‐term and long‐term alterations of human anterior cruciate ligament (ACL) grafts during the remodeling process with special regards to cellularity, α‐smooth muscle protein (αSMP) expression, and crimp length in comparison to the native ACL. Methods A total of 34 patients were included (23 male and 11 female). Biopsies of 13 semitendinosus tendon and 14 patellar tendon autografts were obtained during surgical revision secondary to an ACL reconstruction. According to the interval between the index procedure and sample collection, the patients were divided into four groups: 4–12 months, 13–60 months, 61–108 months, and >108 months. Seven samples of native ruptured ACL tissue obtained during surgical intervention served as control. All biopsies were taken from the intraligamentous part of the ACL or the graft. Histomorphological and immunohistochemical analyses were conducted after samples were stained using hematoxylin–eosin, Giemsa, and αSMP enzyme‐labeled antibodies. The total cell density, the numbers of fibroblasts and fibrocytes, the fibroblast/fibrocyte ratio, the number of αSMP+ cell nuclei, and the percentage of αSMP+ cells per fibroblast as well as the crimp lengths were determined using light microscopy. Results In the early phase of remodeling, the grafts featured extensively high total cell counts (1021.2 ± 327.8, P = 0.001), with high numbers of fibroblasts (841.4 ± 245.2, P = 0.002), fibrocytes (174.5 ± 113.0, P = 0.04), and αSMP+ cells (78.3 ± 95.0, P = 0.02) compared to controls (390.1 ± 141.7, 304.5 ± 160.8, 65.6 ± 31.4 and 2.3 ± 2.6, respectively). Thereafter, the numbers of all cell entities decreased. After more than 108 months, the percentage of αSMP+ cells per fibroblast reached physiological values (ratio 1.3 ± 1.0, P = 0.41; control 0.8 ± 0.8), while the total cell count (834.3 ± 183.7, P = 0.001) as well as the numbers of fibroblasts (663.5 ± 192.6, P = 0.006) and fibrocytes (134.1 ± 73.0, P = 0.049) remained significantly high. The fibroblast/fibrocyte ratio showed no significant alterations over the course of time compared to the controls. The collagen crimp lengths were elongated by tendency in the early phase (28.8 ± 12.9 mm, P = 0.15; control 20.7 ± 2.2 mm) and significantly shortened over time, with the lowest values in the long term (14.8 ± 2.0 mm, P = 0.001). The comparison of biopsies from semitendinosus tendon and patellar tendon autografts revealed no significant differences for any of the histomorphological parameters investigated. Conclusion This study reveals distinctive mid‐term and long‐term immunomorphological alterations during human ACL graft remodeling. These data clearly indicate that the remodeling is a process that continues for 9 years or more. Furthermore, it seems to be a process of adaptation rather than full restoration. Even in the long run, several biological properties of the native ACL are not completely reestablished.
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Affiliation(s)
- Hermann O Mayr
- FIFA Medical Centre of Excellence, Division of Knee, Hip and Shoulder Surgery, Schoen Clinic Munich Harlaching, Academic Teaching Hospital of the Paracelsus Medical University, Salzburg, Austria.,Department of Orthopaedics and Trauma Surgery, Freiburg University Hospital, Albert Ludwig University of Freiburg, Freiburg, Germany
| | | | - Katrin T Herberger
- Department of Orthopaedics and Trauma Surgery, Freiburg University Hospital, Albert Ludwig University of Freiburg, Freiburg, Germany
| | - Florian Haasters
- FIFA Medical Centre of Excellence, Division of Knee, Hip and Shoulder Surgery, Schoen Clinic Munich Harlaching, Academic Teaching Hospital of the Paracelsus Medical University, Salzburg, Austria.,Department of General, Trauma and Reconstructive Surgery, Munich University Hospital, Ludwig Maximilian University, Munich, Germany
| | - Anke Bernstein
- Department of Orthopaedics and Trauma Surgery, Freiburg University Hospital, Albert Ludwig University of Freiburg, Freiburg, Germany
| | - Hagen Schmal
- Department of Orthopaedics and Trauma Surgery, Freiburg University Hospital, Albert Ludwig University of Freiburg, Freiburg, Germany.,Department of Orthopaedic Surgery, Odense University Hospital, Odense, Denmark
| | - Wolf C Prall
- FIFA Medical Centre of Excellence, Division of Knee, Hip and Shoulder Surgery, Schoen Clinic Munich Harlaching, Academic Teaching Hospital of the Paracelsus Medical University, Salzburg, Austria.,Department of General, Trauma and Reconstructive Surgery, Munich University Hospital, Ludwig Maximilian University, Munich, Germany
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8
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van Vijven M, van Groningen B, Kimenai JN, van der Steen MC, van Doeselaar M, Janssen RPA, Ito K, Foolen J. Identifying potential patient-specific predictors for anterior cruciate ligament reconstruction outcome - a diagnostic in vitro tissue remodeling platform. J Exp Orthop 2020; 7:48. [PMID: 32623555 PMCID: PMC7335379 DOI: 10.1186/s40634-020-00266-2] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/01/2020] [Accepted: 06/24/2020] [Indexed: 02/06/2023] Open
Abstract
Purpose Upon anterior cruciate ligament (ACL) rupture, reconstruction is often required, with the hamstring tendon autograft as most widely used treatment. Post-operative autograft remodeling enhances graft rupture risk, which occurs in up to 10% of the patient population, increasing up to 30% of patients aged under 20 years. Therefore, this research aimed to identify potential biological predictors for graft rupture, derived from patient-specific tissue remodeling-related cell properties in an in vitro micro-tissue platform. Methods Hamstring tendon-derived cells were obtained from remnant autograft tissue after ACL reconstructions (36 patients, aged 12–55 years), and seeded in collagen I gels on a micro-tissue platform. Micro-tissue compaction over time – induced by altering the boundary constraints – was monitored. Pro-collagen I expression was assessed using ELISA, and protein expression of tenomodulin and α-smooth muscle actin were measured using Western blot. Expression and activity of matrix metalloproteinase 2 were determined using gelatin zymography. Results Only micro-tissues corresponding to younger patients occasionally released themselves from the constraining posts. Pro-collagen I expression was significantly higher in younger patients. Differences in α-smooth muscle actin and tenomodulin expression between patients were found, but these were age-independent. Active matrix metalloproteinase 2 expression was slightly more abundant in younger patients. Conclusions The presented micro-tissue platform exposed patient-specific remodeling-related differences between tendon-derived cells, with the micro-tissues that released from constraining posts and pro-collagen I expression best reflecting the clinical age-dependency of graft rupture. These properties can be the starting point in the quest for potential predictors for identifying individual patients at risk for graft rupture.
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Affiliation(s)
- Marc van Vijven
- Orthopaedic Biomechanics, Department of Biomedical Engineering, Eindhoven University of Technology, Building 15, Groene Loper, Gemini-Zuid 4.12, PO Box 513, 5600MB, Eindhoven, The Netherlands. .,Institute for Complex Molecular Systems, Eindhoven University of Technology, Eindhoven, the Netherlands.
| | - Bart van Groningen
- Department of Orthopaedic Surgery, Máxima MC, Eindhoven, the Netherlands
| | - Joyce N Kimenai
- Orthopaedic Biomechanics, Department of Biomedical Engineering, Eindhoven University of Technology, Building 15, Groene Loper, Gemini-Zuid 4.12, PO Box 513, 5600MB, Eindhoven, The Netherlands
| | - Maria C van der Steen
- Department of Orthopaedic Surgery, Máxima MC, Eindhoven, the Netherlands.,Department of Orthopaedic Surgery, Catharina Hospital Eindhoven, Eindhoven, the Netherlands
| | - Marina van Doeselaar
- Orthopaedic Biomechanics, Department of Biomedical Engineering, Eindhoven University of Technology, Building 15, Groene Loper, Gemini-Zuid 4.12, PO Box 513, 5600MB, Eindhoven, The Netherlands
| | - Rob P A Janssen
- Orthopaedic Biomechanics, Department of Biomedical Engineering, Eindhoven University of Technology, Building 15, Groene Loper, Gemini-Zuid 4.12, PO Box 513, 5600MB, Eindhoven, The Netherlands.,Department of Orthopaedic Surgery, Máxima MC, Eindhoven, the Netherlands.,Fontys University of Applied Sciences, Eindhoven, the Netherlands
| | - Keita Ito
- Orthopaedic Biomechanics, Department of Biomedical Engineering, Eindhoven University of Technology, Building 15, Groene Loper, Gemini-Zuid 4.12, PO Box 513, 5600MB, Eindhoven, The Netherlands.,Institute for Complex Molecular Systems, Eindhoven University of Technology, Eindhoven, the Netherlands
| | - Jasper Foolen
- Orthopaedic Biomechanics, Department of Biomedical Engineering, Eindhoven University of Technology, Building 15, Groene Loper, Gemini-Zuid 4.12, PO Box 513, 5600MB, Eindhoven, The Netherlands.,Institute for Complex Molecular Systems, Eindhoven University of Technology, Eindhoven, the Netherlands
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9
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Zuskov A, Freedman BR, Gordon JA, Sarver JJ, Buckley MR, Soslowsky LJ. Tendon Biomechanics and Crimp Properties Following Fatigue Loading Are Influenced by Tendon Type and Age in Mice. J Orthop Res 2020; 38:36-42. [PMID: 31286548 PMCID: PMC6917867 DOI: 10.1002/jor.24407] [Citation(s) in RCA: 19] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/31/2019] [Accepted: 06/25/2019] [Indexed: 02/04/2023]
Abstract
In tendon, type-I collagen assembles together into fibrils, fibers, and fascicles that exhibit a wavy or crimped pattern that uncrimps with applied tensile loading. This structural property has been observed across multiple tendons throughout aging and may play an important role in tendon viscoelasticity, response to fatigue loading, healing, and development. Previous work has shown that crimp is permanently altered with the application of fatigue loading. This opens the possibility of evaluating tendon crimp as a clinical surrogate of tissue damage. The purpose of this study was to determine how fatigue loading in tendon affects crimp and mechanical properties throughout aging and between tendon types. Mouse patellar tendons (PT) and flexor digitorum longus (FDL) tendons were fatigue loaded while an integrated plane polariscope simultaneously assessed crimp properties at P150 and P570 days of age to model mature and aged tendon phenotypes (N = 10-11/group). Tendon type, fatigue loading, and aging were found to differentially affect tendon mechanical and crimp properties. FDL tendons had higher modulus and hysteresis, whereas the PT showed more laxity and toe region strain throughout aging. Crimp frequency was consistently higher in FDL compared with PT throughout fatigue loading, whereas the crimp amplitude was cycle dependent. This differential response based on tendon type and age further suggests that the FDL and the PT respond differently to fatigue loading and that this response is age-dependent. Together, our findings suggest that the mechanical and structural effects of fatigue loading are specific to tendon type and age in mice. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:36-42, 2020.
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Affiliation(s)
- Andrey Zuskov
- McKay Orthopaedic Research Laboratory, University of Pennsylvania, Philadelphia, PA, USA
- Department of Orthopaedic Surgery, Wake Forest School of Medicine, Winston-Salem, NC, USA
| | - Benjamin R. Freedman
- McKay Orthopaedic Research Laboratory, University of Pennsylvania, Philadelphia, PA, USA
- John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA
- Wyss Institute for Biologically Inspired Engineering, Harvard University, Boston, MA
| | - Joshua A. Gordon
- McKay Orthopaedic Research Laboratory, University of Pennsylvania, Philadelphia, PA, USA
| | - Joseph J. Sarver
- Department of Biomedical Engineering, Drexel University, Philadelphia, PA, USA
| | - Mark R. Buckley
- Department of Biomedical Engineering, University of Rochester, Rochester, NY, USA
| | - Louis J. Soslowsky
- McKay Orthopaedic Research Laboratory, University of Pennsylvania, Philadelphia, PA, USA
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10
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Migrating Myofibroblastic Iliotibial Band-Derived Fibroblasts Represent a Promising Cell Source for Ligament Reconstruction. Int J Mol Sci 2019; 20:ijms20081972. [PMID: 31013670 PMCID: PMC6514966 DOI: 10.3390/ijms20081972] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/04/2019] [Revised: 04/13/2019] [Accepted: 04/16/2019] [Indexed: 12/24/2022] Open
Abstract
The iliotibial band (ITB) is a suitable scaffold for anterior cruciate ligament (ACL) reconstruction, providing a sufficient mechanical resistance to loading. Hence, ITB-derived fibroblasts attract interest for ligament tissue engineering but have so far not been characterized. This present study aimed at characterizing ITB fibroblasts before, during, and after emigration from cadaveric ITB explants to decipher the emigration behavior and to utilize their migratory capacity for seeding biomaterials. ITB and, for comparison, ACL tissues were assessed for the content of alpha smooth muscle actin (αSMA) expressing fibroblasts and degeneration. The cell survival and αSMA expression were monitored in explants used for cell isolation, monolayer, self-assembled ITB spheroids, and spheroids seeded in polyglycolic acid (PGA) scaffolds. The protein expression profile of targets typically expressed by ligamentocytes (collagen types I-III, elastin, lubricin, decorin, aggrecan, fibronectin, tenascin C, CD44, β1-integrins, vimentin, F-actin, αSMA, and vascular endothelial growth factor A [VEGFA]) was compared between ITB and ACL fibroblasts. A donor- and age-dependent differing percentage of αSMA positive cells could be detected, which was similar in ITB and ACL tissues despite the grade of degeneration being significantly higher in the ACL due to harvesting them from OA knees. ITB fibroblasts survived for several months in an explant culture, continuously forming monolayers with VEGFA and an increased αSMA expression. They shared their expression profile with ACL fibroblasts. αSMA decreased during the monolayer to spheroid/scaffold transition. Using self-assembled spheroids, the migratory capacity of reversible myofibroblastic ITB cells can be utilized for colonizing biomaterials for ACL tissue engineering and to support ligament healing.
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Bell R, Gendron NR, Anderson M, Flatow EL, Andarawis-Puri N. A potential new role for myofibroblasts in remodeling of sub-rupture fatigue tendon injuries by exercise. Sci Rep 2018; 8:8933. [PMID: 29895865 PMCID: PMC5997675 DOI: 10.1038/s41598-018-27196-5] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2017] [Accepted: 05/30/2018] [Indexed: 12/14/2022] Open
Abstract
Tendons are ineffective at repairing sub-rupture fatigue injuries. Accordingly, we evaluated whether an exercise protocol that we have previously found to decrease structural damage kinks in fatigue damaged tendons, leads to improvement in mechanical properties. We hypothesized that exercise that promotes repair of fatigue damage will decrease apoptosis and increase the population of myofibroblasts. Rat patellar tendons underwent in vivo fatigue loading for 500 or 7200 cycles. Animals resumed cage activity for 2-weeks, then either remained cage active or began treadmill running until sacrifice at 4- or 10-weeks post-fatigue loading. Exercise following fatigue damage increased the stiffness back towards naïve levels, decreased apoptosis and increased the population of myofibroblasts. Next, proteins associated with inhibition of apoptosis (Collagen VI) or activation of myofibroblast (pSmad 2/3, fibrillin, integrin subunits αV and α5) were evaluated. Data suggests that collagen VI may not be integral to inhibition of apoptosis in this context. Exercise increased pSmad 2/3 and fibrillin in the insertion region for the 7200-cycles group. In addition, exercise decreased integrin αV and increased integrin α5 in fatigue damaged tendons. Data suggests that a decrease in apoptosis and an increase in population of myofibroblasts may be integral to remodeling of fatigue damaged tendons.
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Affiliation(s)
- Rebecca Bell
- Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY, USA
| | - N Remi Gendron
- Leni and Peter W. May Department of Orthopaedics, Icahn School of Medicine at Mount Sinai, New York, NY, USA
| | - Matthew Anderson
- Leni and Peter W. May Department of Orthopaedics, Icahn School of Medicine at Mount Sinai, New York, NY, USA
| | - Evan L Flatow
- Leni and Peter W. May Department of Orthopaedics, Icahn School of Medicine at Mount Sinai, New York, NY, USA
| | - Nelly Andarawis-Puri
- Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY, USA. .,Nancy E. and Peter C. Meinig School of Biomedical Engineering, Cornell University, Ithaca, NY, USA. .,Hospital for Special Surgery, New York, NY, USA.
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12
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Spiesz EM, Thorpe CT, Thurner PJ, Screen HRC. Structure and collagen crimp patterns of functionally distinct equine tendons, revealed by quantitative polarised light microscopy (qPLM). Acta Biomater 2018; 70:281-292. [PMID: 29409868 PMCID: PMC5894809 DOI: 10.1016/j.actbio.2018.01.034] [Citation(s) in RCA: 26] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/08/2017] [Revised: 01/19/2018] [Accepted: 01/24/2018] [Indexed: 01/15/2023]
Abstract
Structure-function relationships in tendons are directly influenced by the arrangement of collagen fibres. However, the details of such arrangements in functionally distinct tendons remain obscure. This study demonstrates the use of quantitative polarised light microscopy (qPLM) to identify structural differences in two major tendon compartments at the mesoscale: fascicles and interfascicular matrix (IFM). It contrasts functionally distinct positional and energy storing tendons, and considers changes with age. Of particular note, the technique facilitates the analysis of crimp parameters, in which cutting direction artefact can be accounted for and eliminated, enabling the first detailed analysis of crimp parameters across functionally distinct tendons. IFM shows lower birefringence (0.0013 ± 0.0001 [−]), as compared to fascicles (0.0044 ± 0.0005 [−]), indicating that the volume fraction of fibres must be substantially lower in the IFM. Interestingly, no evidence of distinct fibre directional dispersions between equine energy storing superficial digital flexor tendons (SDFTs) and positional common digital extensor tendons (CDETs) were noted, suggesting either more subtle structural differences between tendon types or changes focused in the non-collagenous components. By contrast, collagen crimp characteristics are strongly tendon type specific, indicating crimp specialisation is crucial in the respective mechanical function. SDFTs showed much finer crimp (21.1 ± 5.5 µm) than positional CDETs (135.4 ± 20.1 µm). Further, tendon crimp was finer in injured tendon, as compared to its healthy equivalents. Crimp angle differed strongly between tendon types as well, with average of 6.5 ± 1.4° in SDFTs and 13.1 ± 2.0° in CDETs, highlighting a substantially tighter crimp in the SDFT, likely contributing to its effective recoil capacity. Statement of Significance This is the first study to quantify birefringence in fascicles and interfascicular matrix of functionally distinct energy storing and positional tendons. It adopts a novel method – quantitative polarised light microscopy (qPLM) to measure collagen crimp angle, avoiding artefacts related to the direction of histological sectioning, and provides the first direct comparison of crimp characteristics of functionally distinct tendons of various ages. A comparison of matched picrosirius red stained and unstained tendons sections identified non-homogenous staining effects, and leads us to recommend that only unstained sections are analysed in the quantitative manner. qPLM is successfully used to assess birefringence in soft tissue sections, offering a promising tool for investigating the structural arrangements of fibres in (soft) tissues and other composite materials.
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Affiliation(s)
- Ewa M Spiesz
- School of Engineering and Materials Science, Queen Mary University of London, Mile End Rd, London E1 4NS, United Kingdom; Department of Bionanoscience, Kavli Institute of Nanoscience, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands.
| | - Chavaunne T Thorpe
- School of Engineering and Materials Science, Queen Mary University of London, Mile End Rd, London E1 4NS, United Kingdom; Comparative Biomedical Sciences, Royal Veterinary College, Royal College Street, London NW1 0TU, United Kingdom.
| | - Philipp J Thurner
- Institute of Lightweight Design and Structural Biomechanics, TU Wien, Getreidemarkt 9, A-1060 Vienna, Austria.
| | - Hazel R C Screen
- School of Engineering and Materials Science, Queen Mary University of London, Mile End Rd, London E1 4NS, United Kingdom.
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Hirukawa M, Katayama S, Sato T, Yamada M, Kageyama S, Unno H, Suzuki Y, Miura Y, Shiratsuchi E, Hasegawa M, Miyamoto K, Horiuchi T. Development of Tissue-Engineered Ligaments: Elastin Promotes Regeneration of the Rabbit Medial Collateral Ligament. Artif Organs 2017; 42:E102-E113. [PMID: 29266271 DOI: 10.1111/aor.13066] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2017] [Revised: 09/05/2017] [Accepted: 09/12/2017] [Indexed: 12/14/2022]
Abstract
When ligaments are injured, reconstructive surgery is sometimes required to restore function. Methods of reconstructive surgery include transplantation of an artificial ligament and autotransplantation of a tendon. However, these methods have limitations related to the strength of the bone-ligament insertion and biocompatibility of the transplanted tissue after surgery. Therefore, it is necessary to develop new reconstruction methods and pursue the development of artificial ligaments. Elastin is a major component of elastic fibers and ligaments. However, the role of elastin in ligament regeneration has not been described. Here, we developed a rabbit model of a medial collateral ligament (MCL) rupture and treated animal knees with exogenous elastin [100 µg/(0.5 mL·week)] for 6 or 12 weeks. Elastin treatment increased gene expression and protein content of collagen and elastin (gene expression, 6-fold and 42-fold, respectively; protein content, 1.6-fold and 1.9-fold, respectively), and also increased the elastic modulus of MCL increased with elastin treatment (2-fold) compared with the controls. Our data suggest that elastin is involved in the regeneration of damaged ligaments.
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Affiliation(s)
- Masaki Hirukawa
- Department of Chemistry for Materials, Mie University, Graduate School of Engineering, Tsu, Mie, Japan
| | - Shingo Katayama
- Department of Chemistry for Materials, Mie University, Graduate School of Engineering, Tsu, Mie, Japan
| | - Tatsuya Sato
- Department of Chemistry for Materials, Mie University, Graduate School of Engineering, Tsu, Mie, Japan
| | - Masayoshi Yamada
- Department of Chemistry for Materials, Mie University, Graduate School of Engineering, Tsu, Mie, Japan
| | - Satoshi Kageyama
- Department of Chemistry for Materials, Mie University, Graduate School of Engineering, Tsu, Mie, Japan
| | - Hironori Unno
- Department of Orthopedic Surgery, Mie University, Graduate School of Medicine, Tsu, Mie, Japan
| | - Yoshiaki Suzuki
- Department of Orthopedic Surgery, Mie University, Graduate School of Medicine, Tsu, Mie, Japan
| | - Yoshihiro Miura
- Department of Orthopedic Surgery, Mie University, Graduate School of Medicine, Tsu, Mie, Japan
| | | | - Masahiro Hasegawa
- Department of Orthopedic Surgery, Mie University, Graduate School of Medicine, Tsu, Mie, Japan
| | - Keiichi Miyamoto
- Department of Chemistry for Materials, Mie University, Graduate School of Engineering, Tsu, Mie, Japan
| | - Takashi Horiuchi
- Department of Chemistry for Materials, Mie University, Graduate School of Engineering, Tsu, Mie, Japan
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14
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Lavagnino M, Brooks AE, Oslapas AN, Gardner KL, Arnoczky SP. Crimp length decreases in lax tendons due to cytoskeletal tension, but is restored with tensional homeostasis. J Orthop Res 2017; 35:573-579. [PMID: 27878991 DOI: 10.1002/jor.23489] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/17/2016] [Accepted: 11/10/2016] [Indexed: 02/04/2023]
Abstract
Collagen crimp morphology is thought to contribute to the material behavior of tendons and may reflect the local mechanobiological environment of tendon cells. Following loss of collagen tension in tendons, tenocytes initiate a contraction response that shortens tendon length which, in turn, may alter crimp patterns. We hypothesized that changes in the crimp pattern of tendons are the result of cell-based contractions which are governed by relative tautness/laxity of the collagen matrix. To determine the relationship between crimp pattern and tensional homeostasis, rat tail tendon fascicles (RTTfs) were either allowed to freely contract or placed in clamps with 10% laxity for 7 days. The freely contracting RTTfs showed a significant decrease in percent crimp length on both day 5 (3.66%) and day 7 (7.70%). This decrease in crimp length significantly correlated with the decrease in freely contracting RTTf length. Clamped RTTfs demonstrated a significant decrease in percent crimp length on day 5 (1.7%), but no significant difference in percent crimp length on day 7 (0.57%). The results demonstrate that the tendon crimp pattern appears to be under cellular control and is a reflection of the local mechanobiological environment of the extracellular matrix. The ability of tenocytes to actively alter the crimp pattern of collagen fibers also suggests that tenocytes can influence the viscoelastic properties of tendon. Understanding the interactions between tenocytes and their extracellular matrix may lead to further insight into the role tendon cells play in maintaining tendon heath and homeostasis. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:573-579, 2017.
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Affiliation(s)
- Michael Lavagnino
- Laboratory for Comparative Orthopaedic Research, College of Veterinary Medicine, Michigan State University, 784 Wilson Road, East Lansing, Michigan, 48824
| | - Andrew E Brooks
- Laboratory for Comparative Orthopaedic Research, College of Veterinary Medicine, Michigan State University, 784 Wilson Road, East Lansing, Michigan, 48824
| | - Anna N Oslapas
- Laboratory for Comparative Orthopaedic Research, College of Veterinary Medicine, Michigan State University, 784 Wilson Road, East Lansing, Michigan, 48824
| | - Keri L Gardner
- Laboratory for Comparative Orthopaedic Research, College of Veterinary Medicine, Michigan State University, 784 Wilson Road, East Lansing, Michigan, 48824
| | - Steven P Arnoczky
- Laboratory for Comparative Orthopaedic Research, College of Veterinary Medicine, Michigan State University, 784 Wilson Road, East Lansing, Michigan, 48824
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YAMAKAWA S, DEBSKI RE, FUJIE H. Strain distribution in the anterior cruciate ligament in response to anterior drawer force to the knee. ACTA ACUST UNITED AC 2017. [DOI: 10.1299/jbse.16-00582] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Affiliation(s)
| | - Richard E. DEBSKI
- Departments of Bioengineering and Orthopaedic Surgery University of Pittsburgh
| | - Hiromichi FUJIE
- Graduate School of System Design Tokyo Metropolitan University
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16
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Raspanti M, Reguzzoni M, Protasoni M, Congiu T. Mineralization-related modifications in the calcifying tendons of turkey (Meleagris gallopavo). Micron 2015; 71:46-50. [DOI: 10.1016/j.micron.2015.01.003] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2014] [Revised: 01/20/2015] [Accepted: 01/22/2015] [Indexed: 10/24/2022]
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17
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Legerlotz K, Dorn J, Richter J, Rausch M, Leupin O. Age-dependent regulation of tendon crimp structure, cell length and gap width with strain. Acta Biomater 2014; 10:4447-55. [PMID: 24907659 DOI: 10.1016/j.actbio.2014.05.029] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/20/2014] [Revised: 05/19/2014] [Accepted: 05/23/2014] [Indexed: 12/13/2022]
Abstract
The black-and-white patterning of tendon fascicles when visualized by light microscopy, also known as crimp, is a well-known feature of fiber-forming collagens. However, not much is known about its development, function and response to strain. The objective of this study is to investigate the interaction of tenocyte and crimp morphology as well as their changes with increasing age and acute strain. In contrast to previous studies, which used indirect measures, such as polarized light, to investigate the crimp structure, this study visualizes internal crimp structure in three dimensions without freezing, sectioning, staining or fixing the tissue, via two-photon imaging of green fluorescent protein expressing cells within mouse tail tendon fascicles. This technique further allows straining of the live tissue while visualizing changes in crimp morphology and cell shape with increasing specimen length. Combining this novel microscopy technique with computational image and data analysis revealed a complex relationship between tenocytes and the extracellular matrix that evolves with increasing age. While the reduction of crimping with strain was observed as expected, most of the crimps were gone at 0-1% strain already. Even relatively low strains of 3% led to pronounced changes in the crimp structure after relaxation, particularly in the young animals, which could not be seen with bright-field imaging. Cell length and gap width increased with strain. However, while the cells were able to return to their original length even after high strains of 6%, the gaps between the cells widened, which may imply modified cell-cell communication after overstretching.
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"Ligamentization" in hamstring tendon grafts after anterior cruciate ligament reconstruction: a systematic review of the literature and a glimpse into the future. Arthroscopy 2013; 29:1712-21. [PMID: 23859954 DOI: 10.1016/j.arthro.2013.05.009] [Citation(s) in RCA: 100] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/18/2013] [Revised: 04/26/2013] [Accepted: 05/06/2013] [Indexed: 02/02/2023]
Abstract
PURPOSE The aims of this study were to provide a systematic review of the literature on "ligamentization" in hamstring tendon (HT) grafts after anterior cruciate ligament (ACL) reconstruction and analyze the postoperative remodeling process in human patients and animal models. METHODS We performed a search in the Medline, PubMed, Embase and The Cochrane Library databases, followed by a manual search of reference lists to identify relevant articles. Only studies that investigated the ligamentization of hamstring grafts by histologic examination and comprehensively reported on the remodeling process were deemed eligible for review. RESULTS A total of 4 studies were determined appropriate for systematic review: 2 of the included studies investigated human ACL grafts and 2 were performed in animal models. The studies included the examination of 79 human hamstring graft biopsy specimens and grafts of 27 skeletally mature sheep. To verify the remodeling process, authors reported on various aspects of cellularity, vascularity, and collagen organization. None of the included studies reached a level of evidence higher than 3. CONCLUSIONS A postoperative ligamentization process can be found in HT grafts after ACL reconstruction and shows a typical progression through 3 distinguishable remodeling phases in humans and in animal models, whereas the progression and intensity of remodeling is distinctly increased in animals. CLINICAL RELEVANCE Because postoperative remodeling influences biomechanical properties of hamstring grafts, a better understanding of this process and its timing could lead to substantial improvements in postoperative care strategies and indirectly to the optimization of surgical techniques.
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