Which imaging modalities are used postoperatively to assess hip/knee arthroplasty and which findings indicate complications?

• A. Ultrasound is the primary postoperative imaging modality to assess implant position.
• B. Plain radiographs to assess component position and loosening; CT/MRI for infection or soft tissue; look for lucency, subsidence, malalignment, loosening.
• C. MRI is used for all postoperative imaging to assess loosening.
• D. CT is never used postoperatively.

Answer: (B)

Postoperative imaging after hip or knee arthroplasty centers on confirming implant position and stability with plain radiographs. These are the first-line studies to detect signs of loosening or malposition. Look for periprosthetic lucency around the components, subsidence (the implant sinking or moving downward), malalignment, or progressive widening of lucent zones—each of these can indicate loosening or instability.

If infection or soft-tissue complications are a concern, additional imaging helps. CT provides detailed bone anatomy and is particularly good for assessing osteolysis, component positioning, and loosening around cemented or cementless implants when radiographs are inconclusive. MRI can evaluate soft tissues and potential infection or abscess around the prosthesis, though metal hardware creates artifacts; newer sequences mitigate this and improve soft-tissue assessment.

Ultrasound has limited use for evaluating implant positioning or loosening but is useful for assessing superficial soft tissue collections or guiding joint aspirates when infection is suspected.

Together, these modalities help distinguish mechanical complications from infectious or soft-tissue problems after arthroplasty.