Subchondral pre-solidified chitosan/blood implants elicit reproducible early osteochondral wound-repair responses including neutrophil and stromal cell chemotaxis, bone resorption and repair, enhanced repair tissue integration and delayed matrix deposition

Background: In this study we evaluated a novel approach to guide the bone marrow-driven articular cartilage
repair response in skeletally aged rabbits. We hypothesized that dispersed chitosan particles implanted close to the
bone marrow degrade in situ in a molecular mass-dependent manner, and attract more stromal cells to the site in
aged rabbits compared to the blood clot in untreated controls.
Methods: Three microdrill hole defects, 1.4 mm diameter and 2 mm deep, were created in both knee trochlea of
30 month-old New Zealand White rabbits. Each of 3 isotonic chitosan solutions (150, 40, 10 kDa, 80% degree of
deaceylation, with fluorescent chitosan tracer) was mixed with autologous rabbit whole blood, clotted with Tissue
Factor to form cylindrical implants, and press-fit in drill holes in the left knee while contralateral holes received
Tissue Factor or no treatment. At day 1 or day 21 post-operative, defects were analyzed by micro-computed
tomography, histomorphometry and stereology for bone and soft tissue repair.
Results: All 3 implants filled the top of defects at day 1 and were partly degraded in situ at 21 days post-operative.
All implants attracted neutrophils, osteoclasts and abundant bone marrow-derived stromal cells, stimulated bone
resorption followed by new woven bone repair (bone remodeling) and promoted repair tissue-bone integration.
150 kDa chitosan implant was less degraded, and elicited more apoptotic neutrophils and bone resorption than 10
kDa chitosan implant. Drilled controls elicited a poorly integrated fibrous or fibrocartilaginous tissue.
Conclusions: Pre-solidified implants elicit stromal cells and vigorous bone plate remodeling through a phase involving
neutrophil chemotaxis. Pre-solidified chitosan implants are tunable by molecular mass, and could be beneficial for
augmented marrow stimulation therapy if the recruited stromal cells can progress to bone and cartilage repair.

Uncontrolled Keywords

Animals; Biocompatible Materials; Blood Coagulation; Bone Resorption; Cartilage Diseases; Cartilage, Articular; Chemotaxis; Chitosan; Drug Implants; Extracellular Matrix; Female; Knee Joint; Male; Models, Animal; Molecular Weight; Neutrophils; Rabbits; Regeneration; Stromal Cells; Thromboplastin; Time Factors; Wound Healing; X-Ray Microtomography; Biocompatible Materials; Drug Implants; Chitosan; Thromboplastin; Cartilage repair; Bone marrow; Osteoclast; Collagen; Marrow simulation; Bone remodeling; Mesenchymal stromal cell; Micro-computed tomography