Which triad of radiological findings characterises tuberculous arthritis of a large joint such as the hip or knee?

• A Juxta-articular osteoporosis, rapid symmetric joint space narrowing, and ankylosis within months
• B Diffuse osteosclerosis, subchondral cysts, and early joint space loss
• C Periosteal reaction, central sequestrum, and involucrum formation
• D Periarticular osteoporosis, marginal erosion, and preservation of joint space until late disease ✓

Explanation

Phemister's triad describes tuberculous arthritis: juxta-articular osteoporosis, marginal erosions at the bare areas of the joint, and gradual narrowing of the joint space that is preserved until late because cartilage is destroyed slowly from the margins inward. This slow cartilage loss distinguishes tuberculosis from pyogenic septic arthritis, in which the joint space is lost rapidly within days to weeks.

Reference: Textbook of Orthopaedics by John Ebnezar, 6th ed.

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