Causes of Infrapatellar Pain
Infrapatellar pain arises from multiple distinct pathologies affecting the infrapatellar fat pad (Hoffa's fat pad) and surrounding structures, with the most common causes being fat pad impingement syndromes, Hoffa's disease (inflammation and fibrosis), deep infrapatellar bursitis, patellar tendinopathy, and less commonly mass lesions such as ganglion cysts. 1
Primary Pathological Causes
Fat Pad-Related Pathology
The infrapatellar fat pad is an intracapsular, extrasynovial structure that is richly vascularized and innervated, making it a major pain generator when pathological 2. The key fat pad conditions include:
Hoffa's Disease: Characterized by inflammation, hypertrophy, and fibrosis of the fat pad in response to repetitive trauma 3, 4. This presents as chronic anterior knee pain with a burning or aching quality that worsens with activity 2. The fibrotic changes can be identified on MRI and confirmed arthroscopically 5.
Fat Pad Impingement Syndromes: Portions of the fat pad become trapped between the femoropatellar and femorotibial articular surfaces during knee movement 4. This mechanical impingement causes localized pain and restricted range of motion 6.
Deep Infrapatellar Bursitis: Inflammation of the bursa deep to the patellar tendon, often visualized better with contrast-enhanced MRI showing enhancing synovitis thicker than 2mm in Hoffa's fat 1.
Tendon Pathology
Patellar tendinopathy causes activity-related anterior knee pain, particularly at the inferior pole of the patella 7. Repetitive loading of the patellar tendon places jumping athletes at greatest risk, with pain exacerbated by stairs and prolonged sitting 7.
Other Structural Causes
According to ACR guidelines, additional etiologies diagnosed on MRI include 1:
- Adhesive capsulitis and anterior interval scarring
- Medial plicae
- Ganglion cysts (rare but documented in Hoffa's fat pad) 6
- Tumors and pigmented villonodular synovitis
- Osteophytes and osteonecrosis
- Discoid meniscus
Patellofemoral Joint Pathology
While technically peripatellar rather than strictly infrapatellar, patellofemoral conditions frequently present with overlapping anterior knee pain 1:
- Patellofemoral subluxation/dislocation with bone marrow edema
- Patellofemoral cartilage loss and osteoarthritis
- Lateral patellofemoral friction syndrome
Clinical Recognition
Look for burning or aching infrapatellar pain that reproduces with maneuvers causing fat pad impingement 2. Pain typically worsens with:
- Hyperextension of the knee
- Prolonged sitting
- Stair climbing
- Activities requiring repetitive knee flexion-extension
Physical examination should specifically test for fat pad tenderness and impingement signs, with pain localized to the infrapatellar region rather than diffusely around the patella 2, 4.
Diagnostic Approach
Start with plain radiographs to exclude bony pathology, followed by MRI without contrast as the gold standard for soft tissue evaluation 1, 8. Sagittal MRI sequences are most useful for assessing fat pad fibrosis, inflammation, edema, and mass lesions 2, 8.
Consider contrast-enhanced MRI when initial non-contrast imaging is equivocal, as it improves accuracy in diagnosing Hoffa's disease, deep infrapatellar bursitis, and adhesive capsulitis by demonstrating enhancing synovitis 1.
A diagnostic injection of local anesthetic into the infrapatellar fat pad followed by stress testing (treadmill running) can effectively confirm the fat pad as the pain source 5.
Important Clinical Pitfalls
Fat pad pathology is frequently missed because clinicians default to diagnosing patellofemoral pain syndrome without considering intra-articular structures 5, 9.
Increased fat pad stiffness occurs with knee flexion, particularly in osteoarthritis patients, and correlates with intermittent pain at mid-to-deep flexion angles 10. This biomechanical factor should be considered when evaluating pain patterns.
The fat pad can become a source of proinflammatory mediators (cytokines and adipokines) through immune cell infiltration, contributing to both local pain and potentially to knee osteoarthritis progression 3.
In chronic cases, metaplasia of the fat pad may result in osteochondroma formation 3, requiring surgical consideration.