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Description
Introduction
Osteochondral defects present a significant challenge due to the non-regenerative nature of articular cartilage (AC). Current treatments are limited in size, availability and durability.[1,2] Regenerative medicine and bioprinting offer a promising solution to overcome these challenges.[3] Soft hydrogels can be mechanically reinforced with microfiber box structures created with melt electrowriting (MEW).[4] Biofabricated osteochondral implants have demonstrated stability in vivo after six months.[5] In view of the lack of the highly organized arcadic collagen type II structure in the repair tissue and the degradable nature of the polymer fibers, it is likely that the microfiber reinforcement will eventually be compromised. Therefore, this study investigates the application of non-degradable polypropylene (PP) microfiber reinforcement as an alternative to achieve long-term stability in an in vivo equine osteochondral defect model
Methods
Osteochondral implants were fabricated using a 1.2 mm high MEW PP microfiber (Æ 10mm) reinforcement of a cell-free gelMA as the cartilage phase, which was attached to a PCL bone anchor generated using extrusion-based bioprinting. (total dimensions: 6 mm diameter, 7.2 mm height). PP-reinforced constructs were compared to MEW PCL microfiber reinforcement constructs with the same dimensions. The implants were implanted in the trochlea of eight horses (Figure 1C). . The implants were evaluated in vivo in the equine osteochondral defect model. Seven horses received a PP-reinforced implant in one knee and a PCL control implant in the other. After six months, the constructs were explanted and implants, surrounding and opposing tissues were analyzed histologically.
Results
After six months, the fabricated implants demonstrated good integration with the surrounding cartilage tissue in the joint for both PCL and PP reinforced constructs, as was underscored by the abundant ingrowth of bone tissue in the pores of the implant. Histological analysis revealed tissue formation, predominantly fibrotic in nature in the cartilage compartment. Despite being cell-free at the time-point of implantation, cells had invaded the cartilage phase of the implants. Although, glycosaminoglycan (GAG) production was limited compared to the native tissue - with no significant difference between the PCL and PP groups - the PP group provided durable mechanical filling (equal to about 1/3 of the native tissue properties) of the defects to the same extend as the PCL group after 6 months of implantation.
Discussion
The reinforcement of the hydrogel-based cartilage phase with non-degradable PP microfiber structures provides equal mechanical support as PCL reinforced implants, These results highlight the potential of the use for non-degradable materials, such as PP, to provide long term mechanical support for osteochondral defects despite the absence of a highly organized collagen type II network.
References
[1] M. Howell et al., 2021
[2] J. Julin, et al., 2010
[3] R. Levato et al., 2020
[4] J. Visser et al., 2015
[5] M. de Ruijter et al., 2023
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