Preparation and evaluation of an injectable HAp/Col I composite for promoting tendon-to-bone healing in ACL reconstruction

Tendon-to-bone healing is considered as a critical determinant influencing the quality of anterior cruciate ligament (ACL) reconstruction. The establishment of a microenvironment conducive to tendon-bone healing remains an urgent challenge for rapidly restoring the structural integrity and functional capacity of damaged tissue. In this study, an injectable hydroxyapatite/collagen type I (HAp/Col I) composite was developed and optimized. Its efficacy in ACL reconstruction was verified in a beagle dog model. During the early postoperative period, this composite successfully established an optimal regenerative microenvironment, thereby promoting cell proliferation, adhesion, and extracellular matrix secretion. Micro CT analysis revealed that the HAp/Col I composite significantly accelerated the mineralization process of local tissue and facilitated the contraction of the artificial bone tunnel. Tensile pull-out tests and nanoindentation experiments demonstrated that HAp/Col I composite enhanced both the macroscopic tensile pull-out strength of the ligament and the elastic modulus at the microscopic level. Furthermore, Raman spectroscopy and histological evaluations indicated that the tendon-to-bone interface exhibited a composition and structure closely resembling native tissue in terms of inorganic components and fiber alignment. Additionally, the injectable HAp/Col I composite consisted of safe, non-toxic materials, featured a convenient injection process, and was compatible with arthroscopic procedures, suggesting promising clinical application prospects.