Patellofemoral Pain Syndrome (PFPS): This condition is characterized by pain around or behind the patella, often associated with activities that put stress on the knee, such as climbing stairs, squatting, or jumping. The feeling of instability and buckling, along with mild arthritic changes on MRI but all structures intact, supports this diagnosis. PFPS is a common cause of knee pain and instability, especially in individuals with patellar malalignment or overuse.

Meniscal Tear or Strain: Although the MRI showed all structures intact, small meniscal tears or strains might not always be visible. Symptoms can include pain, locking, or a feeling of instability, which aligns with the patient's complaints.
Ligamentous Sprain: A sprain of the ligaments around the knee (e.g., medial or lateral collateral ligament, anterior or posterior cruciate ligament) could cause instability and pain. The fact that all structures appear intact on MRI does not entirely rule out a ligamentous sprain, as some sprains may not be visible, especially if the MRI was done after the acute phase.
Chondromalacia Patellae: This condition involves the softening of the cartilage on the underside of the kneecap, leading to knee pain and tenderness. It could contribute to feelings of instability, especially if the patella does not track properly.

Osteonecrosis of the Knee: Although less common, osteonecrosis (death of bone tissue due to lack of blood supply) can cause knee pain and instability. It's crucial to consider this diagnosis because it requires prompt treatment to prevent further bone collapse and potential need for surgical intervention.
Spontaneous Osteochondrosis of the Knee (SONK): This condition involves a fragment of bone and cartilage becoming detached, leading to pain and instability. It's essential to identify SONK, as it may require surgical intervention.
Infection or Septic Arthritis: Although the MRI does not suggest an infectious process, it's critical to consider septic arthritis, especially if there's any suspicion of infection (e.g., fever, recent trauma, or immune compromise). Septic arthritis is a medical emergency requiring immediate antibiotic treatment and possibly surgical drainage.

Neuropathic Pain: Rarely, neuropathic pain syndromes can cause knee pain and feelings of instability without clear structural abnormalities on imaging. Conditions like complex regional pain syndrome could be considered if other diagnoses are ruled out.
Tumor or Cyst: Benign or malignant tumors, or cysts around the knee, could potentially cause pain and instability. While rare, these conditions are important to consider, especially if the patient has a history of cancer or if symptoms persist without explanation.