The ultrastructure of the peri-articular osteophytes - an evaluation by scanning electron microscopy

Osteophytes are intra-articular osteochondral tissues, which are usually found at the margins of degenerating synovial joints. The aetiology or pathogenesis of this tissue has been a subject of protracted debate. The aim of this study was to offer a possible aetiology and or pathogenesis of this expendable yet important osteochondral tissue using the scanning electron microscopy to evaluate the structure of the cartilage mantle of osteophytes and the relationship of this mantle with that of the adjoining normal articular cartilage. Sections of periarticular osteophytes and osteophyte-normal articular cartilage composite tissues were obtained during total knee replacement for osteoarthritis (OA). These sections were routinely processed and examined using the scanning electron microscope with emphasis on the osteophytic cartilage mantle and the merger of the osteophyte and the adjoining normal articular cartilage. The cartilage mantle of osteophytes was found to be thinner but continuous with that of the adjoining normal articular cartilage. However, a longitudinal bar of acellular tissue was found to separate the subchondral bones of both tissues. The cellular (chondrocyte) arrangement in the osteophytic cartilage was similar to that of the adjoining normal articular cartilage. In addition, in the superficial layer, there was looping of collagen fibres between the normal articular cartilage mantle and that of the osteophytic tissues. The continuity between the cartilage mantle of osteophytes and the adjoining normal articular cartilage may suggest that in the formation of osteophytes, the chondrocytes from the adjoining normal articular cartilage greatly influence the mesenchymal tissue precursor of osteophyte to differentiate along a chondrocytic pathway initially. With subsequent vascular invasion probably from the periosteum, the neocartilage develops a bony core with a completely separate blood supply from that of the adjoining subchondral bone.