How do doctors tell if a knee replacement is loose or infected?

Why infection is always considered first

An infected knee replacement needs a very different operation from a loose one. If infection is missed and the knee is revised as if it were simply loose, the infection usually returns. That is why almost every painful replacement is tested for infection before revision is planned, even without fever.

The usual tests

• X-rays: a series over months or years can show lines of loosening around the implant, wear or bone loss
• Blood tests: inflammation markers, which are often raised with infection but are not specific
• Joint aspiration: fluid drawn from the knee with a needle, tested for white cell count and grown in the laboratory for bacteria
• Further tests in selected cases: CT scans for implant position, bone scans and specialised fluid markers

Preparing for the tests

Antibiotics taken shortly before an aspiration can hide an infection, so tell your doctor about any recent courses. Bring previous X-rays, operation notes and implant stickers if you have them, especially if the original surgery was done outside Qatar. The aspiration is usually done under local anaesthetic and takes a few minutes.

Signs that need prompt attention

Seek urgent review for fever or shivering with a painful knee, wound leakage, rapidly increasing swelling, warmth or redness, or a sudden inability to bear weight. At Aman Hospital, Dr. Gelber arranges these tests and explains the results before discussing whether a one-stage or two-stage revision, or another approach, is appropriate.