Injectable citrate-based polyurethane-urea as a tug-of-war-inspired bioactive self-expansive and planar-fixing screw augmented bone-tendon healing

Inspired by tug-of-war, a game-changing bone-tendon fixation paradigm was developed. Specifically, injectable citrate-based bioactive self-expansive and planar-fixing screw (iCSP-Scr) consisting of reactive isocyanate (NCO) terminalized citrate-based polyurethane, proanthocyanidin modified hydroxyapatite (HAp) and water (with/without porogen) was developed and administrated in the bone-tendon gap. Instead of the “point to point” tendon fixation by traditional interface screws, along with the moisture-induced crosslinking and expansion of iCSP-Scr within the confined space of the irregularly shaped bone-tendon gap, the tendon graft was evenly squeezed into the bone tunnel in a “surface to surface” manner to realize strong and stable bone-tendon fixation via physical expansion, mechanical interlocking and chemical bonding (between –NCO and the –NH2, –SH groups on bone matrix). The optimized iCSP-Scr exhibited rapid crosslinking, moderate expansion rate, high porosity after crosslinking, as well as tunable elasticity and toughness. The iCSP-Scr possessed favorable biodegradability, biocompatibility, and osteoinductivity derived from citrate, PC and HAp, it was able to promote osteogenesis and new bone growth inward of bone tunnel thus further enhanced the bone/iCSP-Scr mechanical interlock, ultimately leading to stronger tendon fixation (pull-out force 106.15 ± 23.15 N) comparing to titanium screws (93.76 ± 17.89 N) after 14 weeks’ ACL reconstruction in a rabbit model. The iCSP-Scr not only can be used as a self-expansive screw facilitating bone-tendon healing, but also can be expanded into other osteogenic application scenarios.