FRI0193 Osteoblasts Promote New Bone Formation at Sites of Cartilage and Bone Erosions While Fat Marrow is Found Within the Subchondral Bone Marrow in Ankylosing Spondylitis
Authors: J. Bleil, R. Maier, A. Hempfing, J. Sieper, H. Appel, U. Syrbe
Journal: Annals of the Rheumatic Diseases (2015)

<h3>Background</h3> Ankylosing spondylitis (AS) is characterized by erosive bone damage followed by bone formation. A recent MRI study suggested that ankylosis of sacroiliac joints develops following repair of erosions with fat metaplasia and backfill as intermediary steps in this pathway [1]. <h3>Objectives</h3> In this study we used histological analysis of facet joints of AS patients to elucidate the mechanism of bone formation and the potential role of adipogenic tissue transformation. <h3>Methods</h3> Sections of facet joints which can also undergo ankylosis in AS were used from patients with AS, patients with osteoarthritis (OA) and from controls (CO). In safranin O stained tissue sections, the subchondral bone marrow area and sites of erosions were evaluated with regard to replacement by fibrous tissue, a pathological feature described before, and adipogenic tissue transformation. To identify putative bone forming cells we analyzed the expression of Runx2 a shared marker of chondrocytes and osteoblasts, and collagen type X and MMP13 as specific markers of hypertrophic chondrocytes as well as CD56 and collagen 1 as markers of osteoblasts within eroded areas of the subchondral endplate and the articular cartilage. <h3>Results</h3> Within the subchondral bone marrow adipogenic tissue transformation as well as fibrous transformation was found: 10/16 (62.5%) of CO joints, 18/29 (62%) of AS joints and 16/17 (94%) of OA joints showed replacement of bone marrow by fat marrow of various extend (up 100% of the bone marrow area/slide) while 0/16 (0%) of control joints, 12/29 (41.4%) of AS joints and 14/17 (82%) of OA joints showed transformation into fibrous tissue. Among AS joints, replacement of bone marrow by fibrous tissue was found more often in joints with cartilaginous fusion (5/6) compared to joints with bony fusion (4/15, p&lt;0.05) while there was no difference for the incidence of fat marrow (p&gt;0.05). At sites of subchondral bone and cartilage erosion we always found fibrous tissue. Runx2<sup>+</sup> cells were detectable at the borders of fibrous tissue in AS and OA joints. The lack of collagen type X and MMP13 expression but osteocalcin and collagen type I expression of these cells proved the osteoblastic nature of these cells. Formation of new bone was found at contact areas between fibrous tissue and eroded articular cartilage – a phenomenon occurring more often in AS joints than in OA joint, i.e. in 92% of AS joints and 27% of OA joints (p&lt;0.003) with fibrous tissue-cartilage contacts. <h3>Conclusions</h3> In AS, osteoblasts lining the fibrous tissue facilitate bone formation at sites of bone and cartilage erosions which promotes joint ankylosis. Fat marrow is rather found within the adjacent subchondral bone marrow. <h3>References</h3> Maksymowych WP, Wichuk S, Chiowchanwisawakit P, Lambert RG, Pedersen SJ: Fat metaplasia and backfill are key intermediaries in the development of sacroiliac joint ankylosis in patients with ankylosing spondylitis. Arthritis Rheumatol 2014, 66(11):2958-2967. <h3>Disclosure of Interest</h3> None declared