Microscopic change in macroscopically normal equine cartilage from osteoarthritic joints

The objective of this study was to assess whether macroscopically normal articular cartilage taken from joints containing focal osteoarthritic lesions is histologically similar to articular cartilage taken from macroscopically normal joints. Metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints were obtained from 10 horses following euthanasia. Gross articular cartilage damage was scored and the cartilage assigned to one of two groups: (1) macroscopically normal cartilage from normal joints (control) and (2) macroscopically normal cartilage from diseased joints in which there were focal osteoarthritic lesions. Chondrocytes expressing specific cytokines and cytokine receptors were identified by immunohistochemistry. The total number of chondrocytes, and percentage of chondrocytes positive for these cytokines and receptors, was recorded in the superficial, middle, and deep cartilage zones. There was a significant increase in the expression of interleukin-1 beta in the superficial and middle zones and interleukin-18 receptor in the superficial zone in Group 2 compared with Group 1 control samples. A significant positive correlation also was found between the grade of osteoarthritis and the percentage of chondrocytes positive for interleukin-1 beta in the superficial and middle zones, and for interleukin-18 and interleukin-18R in the superficial zone. There was a significant increase in histology score for glycosaminoglycan loss in Group 2 compared with that in Group 1. In joints with focal osteoarthritis lesions, all the articular cartilage, even if macroscopically apparently normal, may have microscopic changes associated with osteoarthritis.