Table 1.

Common features and differences in bone loss between SpA and RA.

	SpA	RA
	(AS, PsA, reactive arthritis)
Erosions	•DIP, PIP joints •Evenly distributed •Small, Ω or tubule-shaped •Poorly demarcated •Periarticular site •Association with enthesitis and bone formation	•MTP, MCP, PIP, and wrist joints •Radial sites •U-shaped •Neatly demarcated •Joint margins •No association with enthesitis and bone formation
Periarticular osteopenia	•absent	•May precede bone erosion
Generalized bone loss	•Axial skeleton •Vertebral fractures •Association with ectopic new bone formation	•Axial and appendicular skeleton •Vertebral and non-vertebral fractures •No association with ectopic new bone formation
Bone remodeling	•↑ Bone resorption	•↑ Bone resorption •↓ bone formation

DIP, distal interphalangeal; MTP, metatarsophalangeal; MCP, metacarpophalangeal; PIP, proximal interphalangeal. RA erosions, Neatly demarcated and located at joint margins where the inflamed synovium is in direct contact with bone, erosions in RA are U-shaped and observed predominantly in metacarpophalangeal / metatarsophalangeal and proximal interphalangeal joints with a strong preponderance for radial sites; PsA erosion, Poorly demarcated, smaller in size and depth, Ω or tubule-shaped, and are more evenly distributed. They are located in the periarticular site in proximal and distal interphalangeal joints and are closely associated with bone formation.