Biomaterial Graft to Enhance Bone Ligament Enthesis Regeneration
Bone-ligament interface (enthesis) tissue engineering requires uniquely designed constructs in order to regenerate the multiple types of tissue within a miniscule region. These transitions include either bone, mineralized fibrocartilage, fibrocartilage, and bone (direct enthesis) or ligament and bone (indirect enthesis) connected by embedded tissue fibers. Tissue engineering approaches seek to mimic these physiological cues through biomaterials, growth factors, and cells. The objective of this study was to develop an enthesis scaffold and environment that could produce cell responses similar to the tissues found within the transition zones of the enthesis.
The first step in this process was determining the appropriate material. Silk was chosen as a base material for the entire scaffold as its structure allows for tunable mechanical properties. The silk bone portion of the scaffold was mineralized to mimic native bone. This mineralized scaffold combined with calcitriol produced enhanced osteogenic responses. These findings suggested that the bone scaffold and calcitriol could be effective for enthesis formation and that local high doses of calcitriol could be effective for tissue regeneration. The ligament portion of the scaffold was composed of silk combined with collagen in a unique aligned structure to mimic native ligament matrix. The silk/collagen scaffold produced an enhanced ligamentogenic response on stem cells when combined with ascorbic acid and tensile dynamic stimulation. This combination also significantly improved the mechanical strength of the scaffold.
The mineralized silk bone scaffold and silk/collagen ligament scaffolds were combined to form a stable enthesis graft. A uniquely designed dynamic stimulation system was created to stimulate the ligament portion in tension, but not the bone portion. This system also separated the culture media of the two portions. The static, control scaffold without stimulation or vitamins produced stem cell responses that are indicative of a direct enthesis with distinct zones that were spatially controlled. The enthesis scaffold with tensile stimulation without vitamins had a similar response, but it was not as controlled spatially. The enthesis scaffold with tensile stimulation and vitamins produced a response more indicative of an indirect enthesis with the addition of chondrogenesis. This research provides insights into how vitamins and mechanical stimulation can change the regenerative response of cells and can be tailored to which type of enthesis is required. This information can also aid in the development of preconditioning metrics and new tissue regenerative strategies.