Histological Analysis of Bridge-Enhanced ACL Restoration (BEAR) Tissue in Ligament Retears

Background: Bridge-enhanced anterior cruciate ligament (ACL) restoration (BEAR) surgery has demonstrated, through clinical outcomes and magnetic resonance imaging evaluation, the ability to restore the injured native ACL. However, the integrity of the quality of the tissue restored has yet to be studied histologically in the setting of retear. Purpose/Hypothesis: The purpose of this study was to evaluate the histologic composition of BEAR procedure retear tissue in human patients. It was hypothesized that tissue biopsy specimens taken at the location of retear after BEAR procedures will resemble torn native ACL tissue. Study Design: Case Series; Level of evidence, 4. Methods: Patients undergoing revision ACL surgery after a failed BEAR procedure were consented for tissue biopsy at their treating institution. Tissue biopsy specimens were taken at several locations across the site of the retear, with cluster biopsy specimens taken at the retear site. Hematoxylin and eosin–stained slides were prepared and examined by a single pathologist. The intraoperative location of retear was recorded to be cross-referenced with the original site of tear based on the Sherman classification. Histologic parameters (0-4) included presence of acute inflammation, lymphocytes, eosinophils, macrophages, giant cells, organisms/evidence of infection, evidence of neovascularization, and fatty infiltrate. Foreign material, malignancy, ligamentous/fibrous tissue, and synovial tissue were assessed. The appearance of the tissue in relation to normal-appearing ACL was also evaluated. Results: Five patients were identified for inclusion. The retear of the BEAR and analysis occurred at a mean of 482.4 days (16 months) (range, 366-623 days [12-20.7 months]) after implantation. No cases demonstrated evidence of acute inflammation (neutrophilic infiltrate), while lymphocytic infiltrate was noted in all cases, ranging from rare (80%) to heavy infiltrate (20%). Foreign materials from previous procedures were present in 2 cases (40%) with associated histiocytic infiltrate and giant cells. Ligamentous tissue and synovial tissue were noted in all cases (100%) with evidence of neovascularization. Fatty tissue was identified in 3 cases (60%), while no cases showed evidence of organisms/infection or malignancy. Absence of grossly fibrotic tissue was noted at the biopsy sites. Conclusion: Histological evaluation of BEAR retears in human patients supports the hypothesis that the tissue restored by the implant is similar to native ACL collagen.