Bilateral Infrapatellar Knee Pain: Diagnosis
The most likely diagnosis is patellofemoral pain syndrome (PFPS), which is the most common cause of anterior knee pain in the outpatient setting and characteristically presents with retropatellar or peripatellar pain without mechanical symptoms like grinding, popping, or instability. 1, 2
Clinical Reasoning
The absence of grinding, popping, or instability effectively rules out meniscal tears, ligamentous injuries, and loose bodies, which typically present with mechanical symptoms 1. The bilateral infrapatellar location and lack of mechanical symptoms create a clinical picture highly consistent with PFPS 1, 2.
Key Diagnostic Features Supporting PFPS
- Pain location: Infrapatellar pain falls within the retropatellar/peripatellar distribution characteristic of PFPS 1, 3
- Bilateral presentation: PFPS commonly affects both knees due to systemic biomechanical factors like quadriceps weakness or muscle imbalances 4, 5
- Absence of mechanical symptoms: The lack of grinding, popping, or instability excludes structural pathology like meniscal tears, cartilage defects with loose bodies, or ligamentous instability 1
- Functional pattern: PFPS pain typically worsens with activities requiring knee flexion under load, such as stair climbing, squatting, or prolonged sitting 1, 3
Pathophysiology
PFPS results from imbalances in forces controlling patellar tracking during knee flexion and extension, leading to increased strain on peripatellar soft tissues and increased patellofemoral joint stress 2. The condition is multifactorial, with quadriceps weakness (especially vastus medialis), muscle imbalances between vastus medialis and vastus lateralis, and overuse being primary contributing factors 4, 5.
Differential Considerations
While PFPS is the primary diagnosis, several conditions warrant consideration:
- Patellar tendinopathy (jumper's knee): Presents with infrapatellar pain localized to the patellar tendon insertion, but typically unilateral and associated with jumping activities 1
- Hoffa's fat pad impingement: Causes infrapatellar pain but usually presents with localized tenderness and swelling of the fat pad 6
- Early patellofemoral osteoarthritis: Can present identically to PFPS, particularly in patients aged 45-55 years, even with normal radiographs 1
- Osgood-Schlatter disease: Causes infrapatellar pain at the tibial tuberosity in adolescents, not typical bilateral adult presentation 7
Diagnostic Approach
Obtain anteroposterior and lateral knee radiographs to exclude fractures, osteoarthritis, osteophytes, and loose bodies before confirming the PFPS diagnosis. 1, 3
Physical Examination Findings to Confirm
- Assess for knee effusion (typically absent in PFPS) 1
- Evaluate patellar mobility and tracking during knee flexion 2
- Test for peripatellar tenderness, particularly at the medial and lateral patellar facets 5
- Assess quadriceps strength, especially vastus medialis obliquus 4
- Perform single-leg squat to observe movement patterns and reproduce symptoms 8
When to Consider Imaging Beyond Radiographs
MRI is not initially indicated unless radiographs show abnormalities or symptoms fail to improve after 6-8 weeks of appropriate conservative treatment 1, 3. In patients aged 45-55 years with normal radiographs, MRI may reveal elevated T2 mapping values indicating early cartilage changes that still present clinically as PFPS 1.
Critical Pitfalls to Avoid
- Do not assume knee pathology without evaluating the hip and lumbar spine: Hip pathology commonly refers pain to the knee, and lumbar spine pathology must be considered when knee radiographs are unremarkable 1
- Do not order excessive imaging initially: The clinical presentation is sufficient for diagnosis when mechanical symptoms are absent and radiographs are normal 1, 3
- Do not overlook bilateral presentation significance: Bilateral symptoms suggest systemic biomechanical factors rather than isolated structural pathology 4