Histopathological study of the infrapatellar fat pad in the rat model of patellar tendinopathy: A basic study

Background Patellar tendinopathy is difficult to successfully treat. This study aimed to characterize the pathological changes of the infrapatellar fat pad (IPFP) in patellar tendinopathy (PT), and to investigate the influence of PT on the development of fibrotic changes in the IPFP. Methods Forty male Wistar rats were randomly divided into PT (n = 20) and control groups (n = 20). Bacterial collagenase I (patellar tendinopathy group) or saline (control) was injected, intratendinous, into the patellar tendon. Rats were sacrificed at week 12. The whole knee joint was sagittally sectioned and stained with hematoxylin–eosin and Masson's trichrome. The IPFP samples were graded according to cellularity, fibrosis, and vascularity. The whole IPFP and blue-stained area was measured. Mann–Whitney U tests were used to compare the between-group differences of each score and quantitative value. Results Scores for cellularity were three (2–3) and 0 (0–1) in the PT and control groups, respectively (P Conclusion The IPFP in the patellar tendinopathy group showed greater cellularity, fibrosis, and vascularity than the control group.