What is the differential diagnosis for distal anterior knee pain?

Differential Diagnosis for Distal Anterior Knee Pain

The differential diagnosis for distal anterior knee pain primarily includes patellar tendinopathy, patellofemoral pain syndrome, osteochondritis dissecans, Hoffa's disease (infrapatellar fat pad impingement), deep infrapatellar bursitis, and less commonly, tibial stress fractures or subchondral insufficiency fractures. 1, 2

Primary Diagnostic Considerations

Patellar Tendinopathy

• Pain localizes to the inferior pole of the patella and is exacerbated by activities involving repetitive loading of the patellar tendon, particularly jumping, navigating stairs, and prolonged sitting 1
• Physical examination reveals tenderness at the inferior patellar pole with the leg fully extended during resisted leg extension 1
• A decline squat test reproduces pain by placing increased load on the patellar tendon 1
• This condition occurs in both athletes and nonathletes, with repetitive loading placing jumping athletes at greatest risk 1

Patellofemoral Pain Syndrome

• This represents the most common cause of anterior knee pain, particularly in physically active individuals and adolescents 3, 4
• Patients typically present with insidious onset pain attributed to changes in activity and underlying neuromuscular impairments 3
• Weakness of the quadriceps muscle, especially during eccentric contractions, is usually present, with hypotrophy and reduced activity of the vastus medialis creating an imbalance with vastus lateralis 5
• Pain is often non-specific but may be associated with patellar maltracking or subluxation 5, 2

Osteochondritis Dissecans (OCD)

• Patients present with knee pain and swelling, often accompanied by mechanical symptoms including locking (motion halted), catching (motion partially inhibited), popping, or giving way 1
• Physical examination may reveal tenderness, effusion, loss of motion, or crepitus 1
• Standard radiographs (AP, lateral, sunrise/Merchant, and tunnel views) are the initial diagnostic option for patients with these symptoms 1

Hoffa's Disease and Fat Pad Pathology

• Infrapatellar fat pad impingement (Hoffa's disease) causes anterior knee pain and can be diagnosed on MRI, particularly with contrast enhancement 1
• Enhancing synovitis thicker than 2 mm in Hoffa's fat correlates with peripatellar pain 1
• Deep infrapatelular bursitis represents another fat pad-related cause of distal anterior knee pain 1

Additional Diagnostic Entities

Bone Marrow Lesions and Subchondral Pathology

• Subchondral insufficiency fractures most commonly involve the medial femoral condyle in middle-aged to elderly females and can present with anterior knee pain 1
• MRI identifies these fractures earlier than radiographs, which are often initially normal 1
• Bone marrow lesions (BMLs) are associated with increased knee pain, especially in males or patients with family history of osteoarthritis 1, 6

Other Structural Causes

• Medial plicae, adhesive capsulitis, ganglion cysts, osteophytes, and tumors can all present with chronic anterolateral or anterior knee pain 1
• Tibial stress fractures can be detected by MRI when clinical suspicion exists 1

Diagnostic Approach

Initial Evaluation

• Document focal tenderness location, presence of effusion, weight-bearing ability, gross deformity, palpable mass, and any mechanical symptoms 6
• Evaluate for quadriceps weakness and vastus medialis/lateralis imbalance through resisted extension testing 5
• Perform specific provocative tests: decline squat for patellar tendinopathy, resisted leg extension with palpation of inferior patellar pole 1

Imaging Strategy

• For acute presentations with trauma, obtain radiographs (minimum AP and lateral views) as the initial imaging modality 6
• For chronic anterior knee pain, radiographs should include frontal projection, tangential patellar view, and lateral view 6
• MRI should be considered when initial radiographs are normal but pain persists, when concomitant pathology is suspected (meniscal injury, ACL injury, articular cartilage injury), or when surgery is contemplated 1, 6
• MRI with contrast is more accurate for diagnosing Hoffa's disease, deep infrapatellar bursitis, and adhesive capsulitis 1

Critical Pitfalls to Avoid

• Do not order MRI without obtaining recent radiographs first, as this occurs in approximately 20% of patients with chronic knee pain and violates appropriate imaging algorithms 6
• Recognize that meniscal tears are often incidental findings in older patients and may not be the source of anterior knee pain 6
• Avoid attributing all anterior knee pain to patellofemoral syndrome without considering patellar tendinopathy, OCD, or fat pad pathology, which require different management approaches 1, 2
• Remember that the pathophysiology is multifactorial, combining biomechanical, neuromuscular, behavioral, and psychological factors, requiring comprehensive assessment beyond simple structural diagnosis 3, 4