A 38-year-old woman with a 6-month history of progressive left knee pain and swelling presents to the orthopedic clinic. Physical examination reveals joint effusion and restricted range of motion. Which is the most common imaging modality used as the first-line investigation to evaluate internal derangement and soft tissue pathology of the knee joint?

## First-Line Imaging for Knee Joint Pathology ### Why MRI is the Gold Standard for Soft Tissue Assessment **Key Point:** Magnetic resonance imaging (MRI) is the most common and preferred first-line imaging modality for evaluation of internal derangement, ligamentous injuries, meniscal tears, and cartilage pathology of the knee. **High-Yield:** MRI of the knee provides: - Superior soft tissue contrast resolution - Multiplanar imaging (axial, sagittal, coronal) - Excellent visualization of menisci, cruciate ligaments, and collateral ligaments - Assessment of articular cartilage and bone marrow - Detection of joint effusion and synovitis - No radiation exposure - Non-invasive - High sensitivity and specificity for meniscal and ligamentous tears ### Clinical Pearl In a patient with chronic knee pain and effusion, MRI can identify the specific pathology (meniscal tear, ACL/PCL injury, cartilage damage, or synovitis) without the need for invasive arthroscopy, allowing for targeted conservative or surgical management. ### Imaging Modality Comparison for Knee Pathology | Modality | First-Line? | Soft Tissue Detail | Bone Assessment | Radiation | Cost | |----------|-------------|-------------------|-----------------|-----------|------| | **MRI** | **Yes** | **Excellent** | Good | **No** | High | | CT | No | Fair | Excellent | Yes | Moderate | | Plain X-ray | No | Poor | Good | Yes | Low | | Arthroscopy | No | Excellent (direct) | Poor | No | High | **Mnemonic: MRI SOFT** — **M**ultiplanar, **R**adiation-free, **I**nternal structures; **S**oft tissue detail, **O**ptimal contrast, **F**irst-line, **T**issue characterization ### When to Use Alternative Modalities - **Plain X-ray:** Initial screening to exclude fractures, assess joint space narrowing, or evaluate for osteoarthritis - **CT:** Evaluation of complex fractures, bone anatomy, or when MRI is contraindicated - **Arthroscopy:** Therapeutic intervention (meniscectomy, ACL repair) or when MRI findings are inconclusive [cite:Park 26e Ch 12]