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

Abstract

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.

Keywords: Biomaterial; Growth factor; Stem cell; Tendon/ligament-to-bone interface; Tissue engineering.

Graphical abstract

Fig. 1

The schematic of scaffolds, growth factors and stem cells as the biomimetic components for tendon/ligament-to-bone interface regeneration. ECM, extracellular matrix; PRP, platelet-rich plasma.

Fig. 2

The histological staining and schematic of tendon/ligament-to-bone insertion. (A) The H&E staining of rabbit supraspinatus tendon (a) and anterior cruciate ligament (c) insertion sites. The Masson staining of rabbit supraspinatus tendon (b) and anterior cruciate ligament (d) insertion sites. Reproduced with permission from Ref. [8]. Copyright 2017, Springer Nature. (B) The tidemark stained with H&E. Reproduced with permission from Ref. [32]. Copyright 2002, Elsevier Science Inc. (C) The schematic of tendon/ligament-to-bone insertion. The fibrocartilaginous enthesis is composed of four distinct zones: tendon/ligament, non-mineralized fibrocartilage, mineralized fibrocartilage, and bone. RCT, rotator cuff tendon; ACL, anterior cruciate ligament; NFC, non-mineralized fibrocartilage; MFC, mineralized fibrocartilage; UF, uncalcified fibrocartilage; CF, calcified fibrocartilage; T, tidemark; ECM, extracellular matrix.

Fig. 3

The biphasic/multiphasic scaffolds for tendon/ligament-to-bone healing. (A) The schematic of the application of bipolar nanofibrous membrane. Reproduced with permission from Ref. [70]. Copyright 2017, Acta Materialia Inc. (B) The co-electrospun dual nano-scaffolds and the fixation technique. Reproduced with permission from Ref. [71]. Copyright 2016, The Royal Society of Chemistry. (C) The schematic of manufacturing triphasic silk graft used for restoration of osseointegration in the rabbit anterior cruciate ligament-defect model. Reproduced with permission from Ref. [72]. Copyright 2016, Elsevier Ltd. (D) Macroscopic images of a collagen-based four-layer scaffold and the cross-sectional images of the tendon layer, uncalcified fibrocartilage layer, calcified fibrocartilage layer, and bone layer. Reproduced with permission from Ref. [73]. Copyright 2014, Wiley Periodicals, Inc.

Fig. 4

The gradient mineral scaffolds for tendon/ligament-to-bone healing. (A) The schematic of the procedure for generating a graded coating of calcium phosphate on a nonwoven mat of electrospun nanofibers. Reproduced with permission from Ref. [75]. Copyright 2009, American Chemical Society. (B) The diagram of electrospinning apparatus depicting offset spinnerets and the fluorescent images of nHAP-PCL fibers (green) (a), transition region (b) and PEUUR2000 fibers (red) (c). Reproduced with permission from Ref. [77]. Copyright 2011, Acta Materialia Inc. (C) The 3D scaffolds produced by crochet using PCL/gelatin and PCL/gelatin/HAP microfibers. Micro‐CT scans of different sections of the scaffold and HAP particles content. Reproduced with permission from Ref. [80]. Copyright 2019, WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. (D) The schematic of the fabrication of a hierarchically structured scaffold for tendon-to-bone repair. Reproduced with permission from Ref. [81]. Copyright 2018, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. PCL, polycaprolactone; HAP, hydroxyapatite; nHAP, nanohydroxyapatite; PLGA, poly (lactic-co-glycolic acid).

Fig. 5

The topographical cues incorporated into the scaffold. (A) Three spinnerets were used to form two transition regions (a). The photograph and scanning electron microscopy micrographs of electrospun mesh comprising 4 regions: random PLGA, transition, aligned PCL, and random PCL (b). Reproduced with permission from Ref. [88]. Copyright 2014, Wiley Periodicals, Inc. (B) The preparation of random-aligned-random tendon extracellular matrix composite scaffold. The microstructure of the cross section of the random ends (a). Surface morphology of the transitional region (b) of the aligned (c) and random (d) portions. Reproduced with permission from Ref. [91]. Copyright 2017, Elsevier Ltd. (C) The schematic of developed gelatin-based hydrogels. The F-actin filaments (a), osteogenic differentiation-related marker OPN (b) and tendon tissue-related marker TNC (c) expression evaluation in each of the different phase. Reproduced with permission from Ref. [92]. Copyright 2019, American Chemical Society. PLGA, poly (lactide-co-glycolide); PCL, polycaprolactone.

Fig. 6

The region-specific growth factors incorporated into the scaffold. (A) Design of 3D-printed scaffolds with spatiotemporal delivery of CTGF, TGFβ3 and BMP2 (a) and implantation site (b). The 3D-printed, growth factors embedded three-layered PCL scaffolds were prepared as sheets (c) with high flexibility to fit to anatomical contour of humeral heads (d). Reproduced with permission from Ref. [140]. Copyright 2019, IOP Publishing Ltd. (B) The schematic of the formation of PCL/Pluronic F127 membrane with reverse gradients of PDGF-BB and BMP-2 (a). The immunohistochemical images for tenomodulin (red) and bone sialoprotein (green) of ASCs cultured on the membrane with reverse PDGF-BB and BMP-2 concentration gradients (b). Reproduced with permission from Ref. [142]. Copyright 2013, Acta Materialia Inc. (C) The polydopamine gradient on substrate by spatially controlling oxygen availability could be used as template for graded immobilization of PDGF (a). The immunofluorescence images for scleraxis and tenomodulin of ASCs on PDGF gradient aligned nanofiber (b). Reproduced with permission from Ref. [143]. Copyright 2018, Elsevier Ltd. (D) The generation of a concentration gradient of osteogenic induction medium to promote the cultured stem cells in the scaffold differentiation into different cell types to mimic the tendon-to-bone interface. Reproduced with permission from Ref. [144]. Copyright 2020, The Royal Society of Chemistry. (E) The schematic of growth factor layout (a). The immunofluorescence images for aggrecan (green) and decorin (red), collagen I (green) and collagen II (red), and osteopontin (green) and tenomodulin (red) (b). Reproduced with permission from Ref. [145]. Copyright 2020, IOP Publishing Ltd. PCL, polycaprolactone; ASCs, adipose-derived stem cells.