Granulation Tissue Eroding the Subchondral Bone Also Promotes New Bone Formation in Ankylosing Spondylitis

Area of Science:

• Rheumatology
• Orthopedics
• Cell Biology

Background:

• Fibroblast-rich granulation tissue eroding subchondral bone is implicated in joint remodeling in ankylosing spondylitis (AS).
• The potential bone-forming capacity of this granulation tissue and its role in AS pathogenesis require further investigation.

Purpose of the Study:

• To determine if granulation tissue in AS possesses bone-forming capabilities.
• To identify bone-forming cells (osteoblasts) within this tissue.
• To assess adipogenic tissue transformation as a potential intermediate in AS bone formation.

Main Methods:

• Facet joints from AS patients, osteoarthritis (OA) patients, and controls were analyzed for subchondral granulation tissue.
• Expression of osteoblast markers (RUNX-2, CD56, type I collagen) and hypertrophic chondrocyte markers (RUNX-2, type X collagen, MMP-13) was assessed.
• Adipocytes and lipid accumulation were evaluated in stained sections.

Main Results:

• RUNX-2-positive cells expressing type I collagen, indicative of osteoblasts, were found in granulation tissue of AS and OA joints.
• New bone formation occurred in 91% of AS joints and 20% of OA joints.
• Granulation tissue in AS joints frequently contained adipocytes and lipid accumulations, with some bone marrow replacement by fat, but this was not linked to new bone formation.

Conclusions:

• Subchondral granulation tissue in AS contains osteoblasts.
• These osteoblasts contribute to new bone formation, leading to intraarticular ankylosis of facet joints in AS.