Differential Diagnosis for Knee Injury
- Single most likely diagnosis
- Patellofemoral Pain Syndrome (PFPS): This condition is characterized by pain around or behind the patella, especially with activities that load the patellofemoral joint like bending, squatting, or climbing stairs. The "pop" heard from the knee could be related to a sudden movement or misalignment of the patella, and the delayed swelling could be due to inflammation. The pain with bending and pushing outward on the knee (which could indicate a lateral tracking issue of the patella) supports this diagnosis.
- Other Likely diagnoses
- Meniscal Tear: Although the X-ray is negative, meniscal tears are not visible on X-rays and can cause pain with bending and twisting movements. The "pop" could be indicative of a meniscal tear, especially if it was accompanied by a sudden onset of pain.
- Ligament Sprain: A sprain of the medial or lateral collateral ligament could cause pain with pushing outward on the knee, depending on which ligament is involved. The delayed swelling could be consistent with a ligament sprain.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Septic Arthritis: Although less likely, septic arthritis is a medical emergency that requires prompt diagnosis and treatment. It can present with pain, swelling, and decreased mobility of the joint. The absence of systemic symptoms like fever does not rule out this diagnosis.
- Osteonecrosis: This condition, also known as avascular necrosis, can occur after a traumatic injury and may not be visible on initial X-rays. It's crucial to consider, especially if the patient has risk factors or the pain persists.
- Rare diagnoses
- Patellar Dislocation: Although the knee cap is described as mobile, a transient dislocation could have occurred, causing the "pop" and subsequent pain and swelling.
- Osteochondritis Dissecans: This condition involves a fragment of cartilage and bone becoming detached from the surrounding bone, which could cause locking, catching, or pain, especially with weight-bearing activities. It might not be immediately visible on X-ray, especially if the fragment is still in place.