Alterations in Hoffa’s Fat Pad Induced by an Inflammatory Response Following Idealized Anterior Cruciate Ligament Surgery

Objective To determine whether inflammation following anterior cruciate ligament (ACL) reconstruction leads to long-term pathological changes in the infrapatellar fat pad (IPFP or Hoffa’s fat pad) which could compromise the integrity of the knee joint. Materials and methods Sixteen mature sheep underwent anatomic idealized ACL reconstruction surgery (ACL-R) and were sacrificed at 2 weeks ( n = 9) and 20 weeks ( n = 7) post-ACL-R. Five additional animals served as unoperated controls. A histological grading protocol was developed to quantify the changes in the IPFP post-injury. mRNA expression levels for key markers of inflammation, angiogenesis and tissue regeneration were assessed by qPCR. Results The IPFP exhibited altered cellularity and fibrosis at 2 and 20 weeks post-ACL-R. Immunohistochemistry detected macrophage-like cells in the IPFP which were increased at 20 weeks. Specific pro-inflammatory cytokines and IPFP specific adipokines exhibited changes indicating early inflammation mediated alterations. Elevations in CD105 mRNA levels at 2 weeks corroborated the increases in neovascularization observed in the IPFP following injury. Conclusions Sustained long-term pathological changes stemming from inflammation are present in IPFP tissue after ACL-R surgery and may compromise the long-term integrity of the knee joint.