Quadriceps Tendinopathy: This condition is characterized by pain and tenderness over the superior aspect of the knee, particularly over the quadriceps tendon. Difficulty with extension, especially from 90 degrees, is a common symptom due to the involvement of the quadriceps muscle and tendon in knee extension.

Prepatellar Bursitis: Inflammation of the prepatellar bursa can cause pain and swelling over the front of the knee, which might be tender to the touch. While it primarily affects the area in front of the patella, it could potentially cause discomfort that radiates to the superior aspect of the knee.
Patellofemoral Pain Syndrome (PFPS): This condition is characterized by pain around or behind the patella, often related to activities that put stress on the knee, like climbing stairs, squatting, or jumping. Although the primary complaint is usually around the patella, the pain can sometimes be perceived as more superior, especially if there's significant inflammation or if the patient has difficulty localizing the pain.
Iliotibial (IT) Band Syndrome: While IT band syndrome typically causes pain on the lateral aspect of the knee, in some cases, the pain can be more diffuse or even referred to the anterior or superior aspect of the knee, especially if the patient has a unique gait or running mechanics that put additional stress on the knee structures.

Septic Arthritis: Although less likely without a history of trauma or systemic symptoms, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It can present with pain, swelling, and decreased range of motion of the affected joint. The absence of systemic symptoms does not rule out septic arthritis, especially in immunocompromised patients.
Osteonecrosis of the Knee: This condition, also known as avascular necrosis, can cause sudden onset of knee pain without trauma. It's more common in older adults or those with risk factors such as long-term corticosteroid use. Early diagnosis is crucial for effective management.
Tumor: Bone or soft tissue tumors around the knee can cause atraumatic pain. While rare, missing a diagnosis of a tumor could have significant consequences, making it a "do not miss" diagnosis.

Plica Syndrome: This is a condition where the synovial folds within the knee joint become inflamed, causing pain and snapping sensations. It's less common and might be considered if other diagnoses are ruled out.
Hoffa's Syndrome: Inflammation of the infrapatellar fat pad can cause anterior knee pain that might be perceived as superior, especially if the patient has difficulty pinpointing the exact location of the pain. It's a less common cause of knee pain but should be considered in the differential diagnosis of anterior knee pain.