Engineering complex orthopaedic tissues via strategic biomimicry

Abstract

The primary current challenge in regenerative engineering resides in the simultaneous formation of more than one type of tissue, as well as their functional assembly into complex tissues or organ systems. Tissue-tissue synchrony is especially important in the musculoskeletal system, wherein overall organ function is enabled by the seamless integration of bone with soft tissues such as ligament, tendon, or cartilage, as well as the integration of muscle with tendon. Therefore, in lieu of a traditional single-tissue system (e.g., bone, ligament), composite tissue scaffold designs for the regeneration of functional connective tissue units (e.g., bone-ligament-bone) are being actively investigated. Closely related is the effort to re-establish tissue-tissue interfaces, which is essential for joining these tissue building blocks and facilitating host integration. Much of the research at the forefront of the field has centered on bioinspired stratified or gradient scaffold designs which aim to recapitulate the structural and compositional inhomogeneity inherent across distinct tissue regions. As such, given the complexity of these musculoskeletal tissue units, the key question is how to identify the most relevant parameters for recapitulating the native structure-function relationships in the scaffold design. Therefore, the focus of this review, in addition to presenting the state-of-the-art in complex scaffold design, is to explore how strategic biomimicry can be applied in engineering tissue connectivity. The objective of strategic biomimicry is to avoid over-engineering by establishing what needs to be learned from nature and defining the essential matrix characteristics that must be reproduced in scaffold design. Application of this engineering strategy for the regeneration of the most common musculoskeletal tissue units (e.g., bone-ligament-bone, muscle-tendon-bone, cartilage-bone) will be discussed in this review. It is anticipated that these exciting efforts will enable integrative and functional repair of soft tissue injuries, and moreover, lay the foundation for the development of composite tissue systems and ultimately, total limb or joint regeneration.

Figure 1

Common orthopaedic tissue-tissue interfaces. Ligaments, such as the anterior cruciate ligament (ACL) in the knee (Modified Goldner’s Masson Trichrome), and tendons, such as the supraspinatus tendon in the shoulder (Toluidine blue), connect to bone via a fibrocartilaginous (FC) transition, which can be further subdivided into non-mineralized (NFC) and mineralized (MFC) regions (Von Kossa). The muscle-tendon junction (Modified Goldner’s Masson Trichrome) consists of an interdigitating band of connective tissue. Articular cartilage (AC), which can be subdivided into surface (SZC), middle (MZC), and deep (DZC) zones (Modified Goldner’s Masson Trichrome), connects to subchondral bone via a transitional calcified cartilage (CC) region (Von Kossa).

Figure 2

Scaffold design for ligament-interface-bone regeneration. Mimicking the stratified structure (Modified Goldner’s Masson Trichrome) and composition (FTIR-I: Fourier Transform infrared spectroscopy) of the native insertion, a tri-phasic scaffold (Phase A: PLGA mesh, Phase B: PLGA microspheres, Phase C: PLGA-BG microspheres) was designed for ACL-bone interface regeneration.^, This design allowed for spatial control over cell distribution (Fb: fibroblasts on Phase A, Ob: osteoblasts on Phase C, along with chondrocytes in a hydrogel in Phase B) enabled the formation of compositionally distinct yet structurally continuous tissue regions in vivo (Modified Goldner’s Masson Trichrome).

Figure 3

Scaffold design for tendon-interface-bone regeneration. A biphasic scaffold comprised of layered aligned PLGA and PLGA-HA nanofibers was fabricated by electrospinning, which led to phase-specific mineral deposition in vivo (Von Kossa, subcutaneous athymic rat model). The bilayer scaffold was subsequently tested in a rat rotator cuff repair model, disorganized scar tissue was observed in the single-phased controls (PLGA, PLGA-HA only). Interestingly, tendon-bone integration via an organized bilayer fibrocartilage interface was only observed with the biphasic design (Picrosirius red, Alcian blue).

Figure 4

Hydrogel-ceramic composite scaffold for osteochondral interface regeneration. Articular cartilage (AC) connects to subchondral bone via the osteochondral interface, which consists of calcified cartilage (CC, Modified Goldner’s Masson Trichrome). Analysis via Fourier Transform infrared spectroscopy (FTIR-I) reveals an exponential increase in mineral content between the articular cartilage and calcified cartilage regions. The hydrogel-HA composite design guided chondrocyte-mediated deposition of a mineralized matrix (Von Kossa) that is positive for collagen X (immunohistochemistry) and can be used in conjunction with cartilage grafts.