This 43-year-old construction worker presented with a painful, swollen, reddened foot and decreased ROM, a few days after stepping on a nail at a worksite. Here are a few points to remember about septic arthritis: • also known as pyogenic or infectious arthritis • most (90 per cent) are due to Staphylococcus aureus • in long bones, the organism lodges in the metaphysic, due to its high vascularity • organism spreads hematogenously to the vasculature of the synovial membrane • capsular distention ensues as a result of purulent exudate; the exudate interferes with cartilage nutrition, resulting in death of chondrocytes and causing a release of proteolytic enzymes; therefore, you get progressive destruction of cartilage and bone On X-ray: • early on – soft tissue swelling, joint capsule distention, obliteration of any juxta-articular fat planes • abscess formation, as a result of pus accumulation, produces increased juxta-articular soft tissue density • local periarticular osteoporosis (due to regional hyperemia, proteolytic enzyme release and disuse), often in a moth-eaten pattern • destruction of the articular cartilages invariably results in joint space narrowing, followed by a blurring of intra-articular cortical margins (Figure 1) • if the infection subsides early enough, articular surfaces may be remodelled, but there is usually some degree of residual articular deformity • if infection is not halted, articular cartilage is eventually completely destroyed, leading to bony ankylosis. •