Initial Imaging for Knee Locking and Giving-Out
Order a standard knee radiograph series that includes at least one frontal projection (anteroposterior, Rosenberg, or tunnel view), a tangential patellar view, and a lateral view of the affected knee 1.
Rationale for Plain Radiography First
Radiography is the appropriate initial imaging study for chronic knee pain with mechanical symptoms like locking and giving-out 1. The ACR Appropriateness Criteria (2018) explicitly establishes this as the standard first-line approach for evaluating chronic knee pain in adults. This 59-year-old woman's presentation with locking and instability ("giving out") falls squarely within guideline-recommended indications for knee radiographs 2, 3.
What the Initial X-rays Will Reveal
Plain radiographs can identify:
- Loose bodies that cause true mechanical locking
- Osteochondritis dissecans (OCD) lesions
- Degenerative changes and osteoarthritis (most common in this age group)
- Osteophytes that may cause locking (particularly superior patellar osteophytes that can lead to patellar dislocation) 4, 5
- Joint effusion (visible on lateral view)
- Patellofemoral malalignment or subluxation
- Fractures or prior injury signs
Critical Clinical Pitfall
"Locking" has two distinct meanings that determine your next steps:
- True mechanical locking = knee physically cannot extend or flex past a certain point (suggests loose body, meniscal tear, or osteophyte impingement)
- Pseudo-locking = sensation of catching or temporary hesitation that resolves (suggests patellofemoral pain syndrome or early degenerative changes)
The distinction matters because true locking with normal radiographs mandates MRI, while pseudo-locking may not 2.
Algorithm for Next Steps Based on X-ray Results
If radiographs show loose bodies, OCD, or significant abnormality:
- MRI without contrast is the next appropriate study 1
- MRI will evaluate menisci, articular cartilage, and ligamentous structures in detail
If radiographs are normal or show only joint effusion:
- MRI without IV contrast is usually the next indicated examination if pain and mechanical symptoms persist 1
- MRI is more sensitive than radiography for detecting meniscal tears, cartilage defects, subchondral insufficiency fractures, and bone marrow lesions
- Important caveat: In this age group (59 years), meniscal tears are often incidental findings—the majority of people over 70 have asymptomatic meniscal tears 1
If radiographs show osteoarthritis or chondrocalcinosis:
- Consider whether MRI will change management before ordering
- MRI may still be appropriate if there's concern for superimposed acute pathology (meniscal tear, subchondral insufficiency fracture)
Special Consideration for This Patient's Age and Sex
This 59-year-old woman is in the demographic at highest risk for subchondral insufficiency fractures (previously called spontaneous osteonecrosis of the knee), which most commonly involve the medial femoral condyle in middle-aged to elderly females 1. Initial radiographs are often normal, but MRI can identify these fractures early. If her pain is severe and localized medially, maintain high suspicion even with normal x-rays.
Why Not Start with MRI?
Approximately 20% of patients with chronic knee pain inappropriately receive MRI without recent radiographs 1. This is problematic because:
- Plain films are quick, cost-effective, and provide essential baseline information
- Many conditions (loose bodies, significant OCD, fractures, severe arthritis) are visible on x-ray and may not require MRI
- MRI findings must be interpreted in the context of radiographic findings
- Starting with x-rays follows evidence-based guidelines and is the standard of care