Removal of the infrapatellar fat pad and associated synovium benefits female guinea pigs in the Dunkin Hartley model of idiopathic osteoarthritis.

ABSTRACT

Background: Several tissues contribute to the onset and advancement of knee osteoarthritis (OA). One tissue type that is worthy of closer evaluation, particularly in the context of sex, is the infrapatellar fat pad (IFP). We previously demonstrated that removal of the IFP had short-term beneficial effects for a cohort of male Dunkin-Hartley guinea pigs. The present project was designed to elucidate the influence of IFP removal in females of this OA-prone strain. It was hypothesized that resection of the IFP would reduce the development of OA in knees of a rodent model predisposed to the disease. Methods: Female guinea pigs (n=16) were acquired at an age of 2.5 months. Surgical removal of the IFP and associated synovium complex (IFP/SC) was executed at 3 months of age. One knee had the IFP/SC resected; a comparable sham surgery was performed on the contralateral knee. All animals were subjected to voluntary enclosure monitoring and dynamic weight-bearing, as well as compulsory treadmill-based gait analysis monthly; baseline data was collected prior to surgery. Guinea pigs were euthanized at 7 months. Knees from eight animals were evaluated via histology, mRNA expression, and immunohistochemistry (IHC); knees from the remaining eight animals were allocated to microcomputed tomography (microCT), biomechanical analyses (whole joint testing and indentation relaxation testing), and atomic absorption spectroscopy (AAS). Results: Fibrous connective tissue (FCT) replaced the IFP/SC. Mobility/gait data indicated that unilateral IFP/SC removal did not affect bilateral hindlimb movement. MicroCT demonstrated that osteophytes were not a significant feature of OA in this sex; however, trabecular thickness (TbTh) in medial femorae decreased in knees containing the FCT. Histopathology scores were predominantly influenced by changes in the lateral tibia, which demonstrated that histologic signs of OA were increased in knees containing the native IFP/SC versus those with the FCT. Similarly, indentation testing demonstrated higher instantaneous and equilibrium moduli in the lateral tibial articular cartilage of control knees with native IFPs. AAS of multiple tissue types associated with the knee revealed that zinc was the major trace element influenced by removal of the IFP/SC. Conclusions: Our data suggest that the IFP/SC is a significant component driving knee OA in female guinea pigs and that resection of this tissue prior to disease has short-term benefits. Specifically, the formation of the FCT in place of the native tissue resulted in decreased cartilage-related OA changes, as demonstrated by reduced Osteoarthritis Research Society International (OARSI) histology scores, as well as changes in transcript, protein, and cartilage indentation analyses. Importantly, this model provides evidence that sex needs to be considered when investigating responses and associated mechanisms seen with this intervention.