Microscale strain concentrations in tissue-engineered osteochondral implants are dictated by local compositional thresholds and architecture.

Tissue-engineered osteochondral implants manufactured from condensed mesenchymal stem cell bodies have shown promise for treating focal cartilage defects. Notably, such manufacturing techniques have shown to successfully recapture the bulk mechanical properties of native cartilage. However, the relationships among the architectural features, local composition, and micromechanical environment within tissue-engineered cartilage from cell-based aggregates remain unclear. Understanding such relationships is crucial for identifying critical parameters that can predict in vivo performance. Therefore, this study investigated the relationship among architectural features, composition, and micromechanical behavior of tissue-engineered osteochondral implants. We utilized fast-confocal microscopy combined with a strain mapping technique to analyze the micromechanical behavior under quasi-static loading, as well as Fourier Transform Infrared Spectroscopy to analyze the local compositions. More specifically, we investigated the architectural features and compositional distributions generated from tissue maturation, along with macro- and micro-level strain distributions. Our results showed that under compression, cell-based aggregates underwent deformation followed by body movement, generating high local strain around the boundaries, where local aggrecan concentration was low and local collagen concentration was high. By analyzing the micromechanics and composition at the single aggregate length scale, we identified a strong threshold relationship between local strain and compositions. Namely at the aggrecan concentration below 0.015 arbitrary unit (A.U.) and the collagen concentration above 0.15 A.U., the constructs experienced greater than threefold increase in compressive strain. Overall, this study suggests that local compositional features are the primary driver of the local mechanical environment in tissue-engineered cartilage constructs, providing insight into potential quality control parameters for manufacturing tissue-engineered constructs.

Finite element modeling to predict the influence of anatomic variation and implant placement on performance of biological intervertebral disc implants.

Background: Tissue-engineered intervertebral disc (TE-IVD) constructs are an attractive therapy for treating degenerative disc disease and have previously been investigated in vivo in both large and small animal models. The mechanical environment of the spine is notably challenging, in part due to its complex anatomy, and implants may require additional mechanical support to avoid failure in the early stages of implantation. As such, the design of suitable support implants requires rigorous validation. Methods: We created a FE model to simulate the behavior of the IVD cages under compression specific to the anatomy of the porcine cervical spine, validated the FE model using an animal model, and predicted the effects of implant location and vertebral angle of the motion segment on implant behavior. Specifically, we tested anatomical positioning of the superior vertebra and placement of the implant. We analyzed corresponding stress and strain distributions. Results: Results demonstrated that the anatomical geometry of the porcine cervical spine led to concentrated stress and strain on the posterior side of the cage. This stress concentration was associated with the location of failure of the cages reported in vivo, despite superior mechanical properties of the implant. Furthermore, placement of the cage was found to have profound effects on migration, while the angle of the superior vertebra affected stress concentration of the cage. Conclusions: This model can be utilized both to inform surgical procedures and provide insight on future cage designs and can be adopted to models without the use of in vivo animal models.

Instabilities induced by mechanical loading determine the viability of chondrocytes grown on porous scaffolds.

Tissue-engineered cartilage constructs have shown promise to treat focal cartilage defects in multiple clinical studies. Notably, products in clinical use or in late-stage clinical trials often utilize porous collagen scaffolds to provide mechanical support and attachment sites for chondrocytes. Under loading, both the local mechanical responses of collagen scaffolds and the corresponding cellular outcomes are poorly understood, despite their wide use. As such, the architecture of collagen scaffolds varies significantly among tissue-engineered cartilage products, but the effects of such architectures on construct mechanics and cell viability are not well understood. This study investigated the effects of local mechanical responses of collagen scaffolds on chondrocyte viability in tissue-engineered cartilage constructs. We utilized fast confocal microscopy combined with a strain mapping technique to analyze the architecture-dependent instabilities under quasi-static loading and subsequent chondrocyte death in honeycomb and sponge scaffolds. More specifically, we compared the isotropic and the orthotropic planes for each type of collagen scaffold. Under compression, both planes exhibited elastic, buckled, and densified deformation modes. In both loading directions, cell death was minimal in regions that experienced elastic deformation mode and a trend of increase in buckled mode. More interestingly, we saw a significant increase in cell death in densified mode. Overall, this study suggests that local instabilities are directly correlated to chondrocyte death in tissue-engineered cartilage constructs, highlighting the importance of understanding the architecture-dependent local mechanical responses under loading.

Understanding the Influence of Local Physical Stimuli on Chondrocyte Behavior.

Investigating the mechanobiology of chondrocytes is challenging due to the complex micromechanical environment of cartilage tissue. The innate zonal differences and poroelastic properties of the tissue combined with its heterogeneous composition create spatial- and temporal-dependent cell behavior, which further complicates the investigation. Despite the numerous challenges, understanding the mechanobiology of chondrocytes is crucial for developing strategies for treating cartilage related diseases as chondrocytes are the only cell type within the tissue. The effort to understand chondrocyte behavior under various mechanical stimuli has been ongoing over the last 50 years. Early studies examined global biosynthetic behavior under unidirectional mechanical stimulus. With the technological development in high-speed confocal imaging techniques, recent studies have focused on investigating real-time individual and collective cell responses to multiple / combined modes of mechanical stimuli. Such efforts have led to tremendous advances in understanding the influence of local physical stimuli on chondrocyte behavior. In addition, we highlight the wide variety of experimental techniques, spanning from static to impact loading, and analysis techniques, from biochemical assays to machine learning, that have been utilized to study chondrocyte behavior. Finally, we review the progression of hypotheses about chondrocyte mechanobiology and provide a perspective on the future outlook of chondrocyte mechanobiology.

Pathomechanism and Biomechanics of Degenerative Disc Disease: Features of Healthy and Degenerated Discs.

The human intervertebral disc (IVD) is a complex organ composed of fibrous and cartilaginous connective tissues, and it serves as a boundary between 2 adjacent vertebrae. It provides a limited range of motion in the torso as well as stability during axial compression, rotation, and bending. Adult IVDs have poor innate healing potential due to low vascularity and cellularity. Degenerative disc disease (DDD) generally arises from the disruption of the homeostasis maintained by the structures of the IVD, and genetic and environmental factors can accelerate the progression of the disease. Impaired cell metabolism due to pH alteration and poor nutrition may lead to autophagy and disruption of the homeostasis within the IVD and thus plays a key role in DDD etiology. To develop regenerative therapies for degenerated discs, future studies must aim to restore both anatomical and biomechanical properties of the IVDs. The objective of this review is to give a detailed overview about anatomical, radiological, and biomechanical features of the IVDs as well as discuss the structural and functional changes that occur during the degeneration process.

The influence of chondrocyte source on the manufacturing reproducibility of human tissue engineered cartilage.

Multiple human tissue engineered cartilage constructs are showing promise in advanced clinical trials but identifying important measures of manufacturing reproducibility remains a challenge. FDA guidance suggests measuring multiple mechanical properties prior to implantation, because these properties could affect the long term success of the implant. Additionally, these engineered cartilage mechanics could be sensitive to the autologous chondrocyte source, an inherently irregular manufacturing starting material. If any mechanical properties are sensitive to changes in the autologous chondrocyte source, these properties may need to be measured prior to implantation to ensure manufacturing reproducibility and quality. Therefore, this study identified variability in the compressive, friction, and shear properties of a human tissue engineered cartilage constructs due to the chondrocyte source. Over 200 constructs were created from 7 different chondrocyte sources and tested using 3 distinct mechanical experiments. Under confined compression, the compressive properties (aggregate modulus and hydraulic permeability) varied by orders of magnitude due to the chondrocyte source. The friction coefficient changed by a factor of 5 due to the chondrocyte source and high intrapatient variability was noted. In contrast, the shear modulus was not affected by changes in the chondrocyte source. Finally, measurements on the local compressive and shear mechanics revealed variability in the depth dependent strain fields based on chondrocyte source. Since the chondrocyte source causes large amounts of variability in the compression and local mechanical properties of engineered cartilage, these mechanical properties may be important measures of manufacturing reproducibility. STATEMENT OF SIGNIFICANCE: Although the FDA recommends measuring mechanical properties of human tissue engineered cartilage constructs during manufacturing, the effect of manufacturing variability on construct mechanics is unknown. As one of the first studies to measure multiple mechanical properties on hundreds of human tissue engineered cartilage constructs, we found the compressive properties are most sensitive to changes in the autologous chondrocyte source, an inherently irregular manufacturing variable. This sensitivity to the autologous chondrocyte source reveals the compressive properties should be measured prior to implantation to assess manufacturing reproducibility.