Repair of bone erosions in rheumatoid arthritis treated with tumour necrosis factor inhibitors is based on bone apposition at the base of the erosion.

Objectives: To investigate whether bone erosions in patients with rheumatoid arthritis (RA) show evidence of repair.

Methods: 127 erosions were identified in metacarpophalangeal joints 2-4 of the right hands of 30 RA patients treated with tumour necrosis factor inhibitors (TNFi) and 21 sex, age and disease activity-matched patients treated with methotrexate. All erosions were assessed for their exact maximal width and depth by high-resolution µCT imaging at baseline and after 1 year.

Results: All erosions detected at baseline could be visualised at follow-up after 1 year. At baseline, the mean width of bone erosions in the TNFi group was 2.0 mm; their mean depth was 2.3 mm, which was not significantly different from the methotrexate-treated group (width 2.4 mm; depth 2.4 mm). Mean depth of erosions significantly decreased after 1 year of treatment with TNFi (-0.1 mm; p=0.016), whereas their width remained unchanged. In contrast, mean depth and width of erosive lesions increased in the methotrexate-treated group. The reduction in the depth of lesions was confined to erosions showing evidence of sclerosis at the base of the lesion. Moreover, deeper lesions in the TNFi group were particularly prone to repair (-0.4 mm; p=0.02) compared with more shallow lesions.

Conclusions: Bone erosions in RA patients treated with TNFi show evidence of limited repair in contrast to bone erosions in patients treated with methotrexate. Repair is associated with a decrease in the depth of lesions and sclerosis at the bases of the lesions. Repair thus emerges from the endosteal rather than periosteal bone compartment and probably involves the bone marrow.