Patellofemoral Pain Syndrome with Eccentric Loading
The most likely cause of your knee pain when walking downhill with swelling despite a normal MRI is patellofemoral pain syndrome (PFPS), specifically related to increased patellofemoral joint compression forces during eccentric loading of the quadriceps.
Why Downhill Walking Causes This Problem
Walking downhill creates 3-4 times greater patellofemoral joint compression forces compared to level walking 1. During downhill walking, your knee flexes to approximately 40 degrees while generating an extension moment of 2.6 Nm/kg body weight—more than double the forces seen in level walking 1. This biomechanical stress is comparable to running or squatting, making downhill walking particularly strenuous for the patellofemoral joint 1, 2.
Women experience even higher forces (70 N/kg body weight versus 50 N/kg in men) due to smaller moment arms, which may explain why this condition is more prevalent in females 1.
Why Your MRI Appears Normal
Standard MRI sequences may miss early patellofemoral pathology. The ACR guidelines note that patients with normal radiographs and anterior knee pain can have abnormal cartilage detectable only on advanced 3T MRI using quantitative T2 mapping 3. Your "normal" MRI likely means gross structural damage is absent, but early cartilage changes, bone marrow lesions (BMLs), or synovitis may be present but not reported 3.
Additionally, up to 97% of asymptomatic adults show some MRI abnormalities, with patellofemoral cartilage and bone marrow lesions present in 57% and 48% of knees respectively 4. This means MRI findings must be correlated with your specific symptom pattern—pain with downhill walking—rather than relying on imaging alone.
Specific Causes to Consider
Based on ACR guidelines, the differential diagnosis for anterior knee pain with normal initial imaging includes 3:
- Patellofemoral cartilage loss with associated BMLs (most likely given your symptoms)
- Hoffa's disease (fat pad inflammation)
- Deep infrapatellar bursitis
- Patellar tendinopathy
- Lateral patellofemoral friction syndrome
- Synovitis (may require contrast-enhanced MRI to detect)
Critical Next Steps
If Standard MRI Was Performed:
- Consider contrast-enhanced MRI if symptoms persist, as this is more accurate for diagnosing Hoffa's disease, bursitis, and synovitis—enhancing synovitis >2mm in Hoffa's fat correlates with peripatellar pain 3
- Request 3T MRI with T2 mapping if available, which can detect early cartilage degeneration not visible on standard sequences 3
Clinical Examination Focus:
- Palpate for focal tenderness at the inferior pole of the patella (patellar tendinopathy)
- Assess for effusion with ballottement or ultrasound confirmation 3
- Check for patellofemoral maltracking or subluxation patterns
- Evaluate vastus medialis oblique muscle strength asymmetry
Ultrasound May Be Appropriate:
US can confirm effusion, evaluate patellar and quadriceps tendons, and assess for synovitis—particularly useful if MRI access is limited 3.
Important Caveats
Downhill walking also impairs joint position sense (proprioception), which worsens after 30 minutes of descent 5. This proprioceptive deficit may contribute to ongoing symptoms and injury risk even after tissue healing begins.
Referred pain must be excluded: If knee examination and imaging remain unrevealing, obtain hip radiographs (especially if there's groin discomfort or limited hip rotation) and consider lumbar spine evaluation if there are radicular symptoms 3.
The swelling suggests either synovitis, effusion, or reactive bone marrow edema—all of which can occur with patellofemoral overload and may not be adequately characterized on standard MRI sequences 3.