Zlotnick, H. M. | Search bioRxiv

bioRxiv2021

Fabrication and delivery of mechano-activedmicrocapsules containing osteogenic factors in alarge animal model of osteochondral injury

Chondral and osteochondral repair strategies are limited by adverse bony changes that occur after injury. Bone resorption can cause entire scaffolds, engineered tissues, or even endogenous repair tissues to subside below the cartilage surface. To address this translational issue, we fabricated poly(D,L-lactide-co-glycolide) (PLGA) microcapsules containing the pro-osteogenic agents triiodothyronine and {beta}-glycerophosphate, and delivered these microcapsules in a large animal model of osteochondral injury to preserve bone structure. We demonstrate that developed microcapsules ruptured in vitro under increasing mechanical loads, and readily sink within a liquid solution, allowing for gravity-based positioning onto the osteochondral surface. In a large animal, these mechano-active microcapsules (MAMCs) were assessed through two different delivery strategies. Intra-articular injection of control MAMCs enabled fluorescent quantification of MAMC rupture and cargo release in a synovial joint setting over time in vivo. This joint-wide injection also confirmed that the MAMCs do not elicit an inflammatory response. In the contralateral hindlimbs, chondral defects were created, MAMCs were locally administered, and nanofracture (Nfx), a clinically utilized method to promote cartilage repair, was performed. The NFx holes enabled marrow-derived stromal cells to enter the defect area and served as repeatable bone injury sites to monitor over time. Animals were evaluated 1 and 2 weeks after injection and surgery. Analysis of injected MAMCs showed that bioactive cargo was released in a controlled fashion over 2 weeks. A bone fluorochrome label injected at the time of surgery displayed maintenance of mineral labeling in the therapeutic group, but resorption in both control groups. Alkaline phosphatase (AP) staining at the osteochondral interface revealed higher AP activity in defects treated with therapeutic MAMCs. Overall, this study establishes a new micro-fluidically generated delivery platform that releases therapeutic factors in an articulating joint, and reduces this to practice in the delivery of therapeutics that preserve bone structure after osteochondral injury.

bioengineering↗

bioRxiv2021

Marked differences in local bone remodeling in response to different marrow stimulation techniques in a large animal

Marrow stimulation, including subchondral drilling and microfracture, is the most commonly performed cartilage repair strategy, whereby the subchondral bone plate is perforated to release marrow-derived cells into a cartilage defect to initiate repair. Novel scaffolds and therapeutics are being designed to enhance and extend the positive short-term outcomes of this marrow stimulation. However, the translation of these newer treatments is hindered by bony abnormalities, including bone resorption, intralesional osteophytes, and bone cysts, that can arise after marrow stimulation. In this study, three different marrow stimulation approaches -- microfracture, subchondral drilling, and needle-puncture - were evaluated in a translationally relevant large animal model, the Yucatan minipig. The objective of this study was to determine which method of marrow access (malleted awl, drilled Kirschner wire, or spring-loaded needle) best preserved the underlying subchondral bone. Fluorochrome labels were injected at the time of surgery and 2 weeks post-surgery to capture bone remodeling over the first 4 weeks. Comprehensive outcome measures included cartilage indentation testing, histological grading, microcomputed tomography, and fluorochrome imaging. Our findings indicated that needle-puncture devices best preserved the underlying subchondral bone relative to other marrow access approaches. This may relate to the degree of bony compaction occurring with marrow access, as the Kirschner wire approach, which consolidated bone most, induced the most significant bone damage with marrow stimulation. This study provides basic science evidence in support of updated marrow stimulation techniques for preclinical and clinical practice.

bioengineering↗