Tension-compression loading with chemical stimulation results in additive increases to functional properties of anatomic meniscal constructs

Biomechanical evaluation of suture-holding properties of native and tissue-engineered articular cartilage

The purpose of this study was to determine suture-holding properties of tissue-engineered neocartilage relative to native articular cartilage. To this end, suture pull-out strength was quantified for native articular cartilage and for neocartilages possessing various mechanical properties. Suture-holding properties were examined in vitro and in vivo. Neocartilage from bovine chondrocytes was engineered using two sets of exogenous stimuli, resulting in neotissue of different biochemical compositions. Compressive and tensile properties and glycosaminoglycan, collagen, and pyridinoline cross-link contents were assayed (study 1). Suture pull-out strength was compared between neocartilage constructs, and bovine and leporine native cartilage. Uniaxial pull-out test until failure was performed after passing 6-0 Vicryl through each tissue (study 2). Subsequently, neocartilage was implanted into a rabbit model to examine short-term suture-holding ability in vivo (study 3). Neocartilage glycosaminoglycan and collagen content per wet weight reached 4.55 ± 1.62% and 4.21 ± 0.77%, respectively. Tensile properties for neocartilage constructs reached 2.6 ± 0.77% MPa for Young's modulus and 1.39 ± 0.63 MPa for ultimate tensile strength. Neocartilage reached ~ 33% of suture pull-out strength of native articular cartilage. Neocartilage cross-link content reached 50% of native values, and suture pull-out strength correlated positively with cross-link content (R² = 0.74). Neocartilage sutured into rabbit osteochondral defects was successfully maintained for 3 weeks. This study shows that pyridinoline cross-links in neocartilage may be vital in controlling suture pull-out strength. Neocartilage produced in vitro with one-third of native tissue pull-out strength appears sufficient for construct suturing and retention in vivo.

Comparison of meniscal fibrochondrocyte and synoviocyte bioscaffolds toward meniscal tissue engineering in the dog

Tissue engineering is a promising field of study toward curing the meniscal deficient stifle; however the ideal cell type for this task is not known. We describe here the extraction of synoviocytes and meniscal fibrochondrocytes from arthroscopic debris from six dogs, which were cultured as tensioned bioscaffolds to synthesize meniscal-like fibrocartilage sheets. Despite the diseased status of the original tissues, synoviocytes and meniscal fibrochondrocytes had high viability at the time of removal from the joint. Glycosaminoglycan and collagen content of bioscaffolds did not differ. Meniscal fibrochondrocyte bioscaffolds contained more type II collagen, but collagen deposition was disorganized, with only 30-40% of cells viable. The collagen of synoviocyte bioscaffolds was organized into sheets and bands and 80-90% of cells were viable. Autologous, diseased meniscal fibrochondrocytes and synoviocytes are plausible cell sources for future meniscal tissue engineering research, however cell viability of meniscal fibrochondrocytes in the tensioned bioscaffolds was low.

Biomechanics and mechanobiology in functional tissue engineering

The field of tissue engineering continues to expand and mature, and several products are now in clinical use, with numerous other preclinical and clinical studies underway. However, specific challenges still remain in the repair or regeneration of tissues that serve a predominantly biomechanical function. Furthermore, it is now clear that mechanobiological interactions between cells and scaffolds can critically influence cell behavior, even in tissues and organs that do not serve an overt biomechanical role. Over the past decade, the field of "functional tissue engineering" has grown as a subfield of tissue engineering to address the challenges and questions on the role of biomechanics and mechanobiology in tissue engineering. Originally posed as a set of principles and guidelines for engineering of load-bearing tissues, functional tissue engineering has grown to encompass several related areas that have proven to have important implications for tissue repair and regeneration. These topics include measurement and modeling of the in vivo biomechanical environment; quantitative analysis of the mechanical properties of native tissues, scaffolds, and repair tissues; development of rationale criteria for the design and assessment of engineered tissues; investigation of the effects biomechanical factors on native and repair tissues, in vivo and in vitro; and development and application of computational models of tissue growth and remodeling. Here we further expand this paradigm and provide examples of the numerous advances in the field over the past decade. Consideration of these principles in the design process will hopefully improve the safety, efficacy, and overall success of engineered tissue replacements.

Engineering functional anisotropy in fibrocartilage neotissues

The knee meniscus, intervertebral disc, and temporomandibular joint (TMJ) disc all possess complex geometric shapes and anisotropic matrix organization. While these characteristics are imperative for proper tissue function, they are seldom recapitulated following injury or disease. Thus, this study's objective was to engineer fibrocartilages that capture both gross and molecular structural features of native tissues. Self-assembled TMJ discs were selected as the model system, as the disc exhibits a unique biconcave shape and functional anisotropy. To drive anisotropy, 50:50 co-cultures of meniscus cells and articular chondrocytes were grown in biconcave, TMJ-shaped molds and treated with two exogenous stimuli: biomechanical (BM) stimulation via passive axial compression and bioactive agent (BA) stimulation via chondroitinase-ABC and transforming growth factor-β1. BM + BA synergistically increased Col/WW, Young's modulus, and ultimate tensile strength 5.8-fold, 14.7-fold, and 13.8-fold that of controls, respectively; it also promoted collagen fibril alignment akin to native tissue. Finite element analysis found BM stimulation to create direction-dependent strains within the neotissue, suggesting shape plays an essential role toward driving in vitro anisotropic neotissue development. Methods used in this study offer insight on the ability to achieve physiologic anisotropy in biomaterials through the strategic application of spatial, biomechanical, and biochemical cues.

Correlation of meniscal T2* with multiphoton microscopy, and change of articular cartilage T2 in an ovine model of meniscal repair

OBJECTIVE

To correlate meniscal T2* relaxation times using ultra-short echo time (UTE) magnetic resonance imaging (MRI) with quantitative microscopic methods, and to determine the effect of meniscal repair on post-operative cartilage T2 values.

DESIGN

A medial meniscal tear was created and repaired in the anterior horn of one limb of 28 crossbred mature ewes. MR scans for morphological evaluation, meniscal T2* values, and cartilage T2 values were acquired at 0, 4 and 8 months post-operatively for the Tear and Non-Op limb. Samples of menisci from both limbs were analyzed using multiphoton microscopy (MPM) analysis and biomechanical testing.

RESULTS

Significantly prolonged meniscal T2* values were found in repaired limbs than in control limbs, P < 0.0001. No regional differences of T2* were detected for either the repaired or control limbs in the anterior horn. Repaired limbs had prolonged cartilage T2 values, primarily anteriorly, and tended to have lower biomechanical force to failure at 8 months than Non-Op limbs. MPM autofluorescence and second harmonic generation data correlated with T2* values at 8 months (ρ = -0.48, P = 0.06).

CONCLUSIONS

T2* mapping is sensitive to detecting temporal and zonal differences of meniscal structure and composition. Meniscal MPM and cartilage T2 values indicate changes in tissue integrity in the presence of meniscal repair.

High density type I collagen gels for tissue engineering of whole menisci

This study investigates the potential of high density type I collagen gels as an injectable scaffold for tissue engineering of whole menisci, and compares these results with previous strategies using alginate as an injectable scaffold. Bovine meniscal fibrochondrocytes were mixed with collagen and injected into micro-computed tomography-based molds to create 10 and 20mgml(-1) menisci that were cultured for up to 4weeks and compared with cultured alginate menisci. Contraction, histological, confocal microscopy, biochemical and mechanical analysis were performed to determine tissue development. After 4weeks culture, collagen menisci had preserved their shape and significantly improved their biochemical and mechanical properties. Both 10 and 20mgml(-1) menisci maintained their DNA content while significantly improving the glycosaminoglycan and collagen content, at values significantly higher than the alginate controls. Collagen menisci matched the alginate control in terms of the equilibrium modulus, and developed a 3- to 6-fold higher tensile modulus than alginate by 4weeks. Further fibrochondrocytes were able to reorganize the collagen gels into a more fibrous appearance similar to native menisci.

Self-organization and the self-assembling process in tissue engineering

In recent years, the tissue engineering paradigm has shifted to include a new and growing subfield of scaffoldless techniques that generate self-organizing and self-assembling tissues. This review aims to cogently describe this relatively new research area, with special focus on applications toward clinical use and research models. Particular emphasis is placed on providing clear definitions of self-organization and the self-assembling process, as delineated from other scaffoldless techniques in tissue engineering and regenerative medicine. Significantly, during formation, self-organizing and self-assembling tissues display biological processes similar to those that occur in vivo. These processes help lead to the recapitulation of native tissue morphological structure and organization. Notably, functional properties of these engineered tissues, some of which are already in clinical trials, also approach native tissue values. This review endeavors to provide a cohesive summary of work in this field and to highlight the potential of self-organization and the self-assembling process for providing cogent solutions to currently intractable problems in tissue engineering.

The effect of IGF-I on anatomically shaped tissue-engineered menisci

This study investigates the effect of insulin-like growth factor (IGF)-I on the development of anatomically-shaped alginate menisci seeded with meniscal fibrochondrocytes. To accomplish this, bovine meniscal fibrochondrocytes were seeded into 2% w/v alginate, crosslinked with calcium sulfate, and injected into anatomical molds derived from microcomputed tomography scans. The meniscal constructs were then cultured for up to 4 weeks with or without 100 ng/mL IGF-I supplemented in the media. Histological, immunohistological, biochemical, and mechanical analyses were performed to characterize tissue development, accumulation and localization of extracellular matrix, and mechanical properties. After 4 weeks of culture, IGF-I treatment significantly improved mechanical and biochemical properties, while maintaining DNA content, with a 26-fold increase in glycosaminoglycan (GAG) content and 10-fold increase in collagen content compared to 0-week controls, and a 3-fold increase in the equilibrium modulus at 2 weeks compared to controls. IGF-I-treated menisci had ∼60% of the GAG content of native tissue and the compressive equilibrium modulus matched native properties by 2 weeks of culture. Further, IGF-I-treated menisci developed a distinct surface layer similar to native tissue with elongated cells and collagen fibers aligned parallel to the surface, the presence of types I and II collagen, and accumulation of lubricin. This study demonstrates that IGF-I treatment can greatly increase the mechanical and biochemical properties of engineered tissues and aid in the development of a distinct surface zone similar to the superficial zone of native menisci.

Chondrogenically tuned expansion enhances the cartilaginous matrix-forming capabilities of primary, adult, leporine chondrocytes

When expanded through passage, chondrocytes lose their ability to produce high-quality cartilaginous matrix. This study attempts to improve the properties of constructs formed with expanded chondrocytes through alterations in the expansion protocol and the ratio of primary to expanded chondrocytes used to form cartilage constructs. A chondrogenically tuned expansion protocol provided similar monolayer growth rates as those obtained using serum-containing medium and enhanced cartilaginous properties of resultant constructs. Various ratios of primary to chondrogenically expanded chondrocytes were then self-assembled to form neocartilage. Biochemical analysis showed that constructs formed with only expanded cells had twice the GAG per wet weight and collagen II/collagen I ratio compared to constructs formed with primary chondrocytes. Biomechanically, compressive properties of constructs formed with only passaged cells matched the instantaneous modulus and exceeded the relaxation modulus of constructs formed with only primary cells. These counterintuitive results show that, by applying proper expansion and three-dimensional culture techniques, the cartilage-forming potential of adult chondrocytes expanded through passage can be enhanced over that of primary cells.