Baker's Cyst (Popliteal Cyst): This condition is characterized by a fluid-filled swelling in the posterior knee, which can cause pain and limit knee extension. The cyst can become large enough to cause mechanical obstruction, preventing full knee extension.

Meniscal Tear: Tears in the meniscus, especially the posterior horn, can cause posterior knee pain and mechanical symptoms such as locking or catching, which might prevent full knee extension.
Posterior Cruciate Ligament (PCL) Injury: Injury to the PCL can lead to posterior knee pain and instability, potentially causing difficulty in fully straightening the leg due to pain or mechanical instability.
Hamstring Strain or Tear: Significant injuries to the hamstring muscles can cause posterior knee pain and weakness, potentially limiting the ability to fully extend the knee.

Deep Vein Thrombosis (DVT): Although less common as a cause of posterior knee pain, DVT can present with pain and swelling in the leg and can be life-threatening if not diagnosed promptly. The inability to straighten the leg could be due to pain or swelling associated with the DVT.
Neoplastic Processes: Tumors, either benign or malignant, in the posterior knee can cause pain and mechanical symptoms, including limitation of movement.

Ganglion Cyst: A less common cause of posterior knee pain, ganglion cysts are benign lumps that can cause discomfort and limit movement if they are large enough or located in a critical area.
Osteochondroma: A type of benign bone tumor that can occur near the knee, potentially causing pain and mechanical symptoms if it interferes with the surrounding soft tissues or joint mechanics.
Infectious Processes (e.g., Abscess or Osteomyelitis): Infections in the posterior knee region can cause severe pain, swelling, and limitation of movement. These conditions are medical emergencies and require prompt diagnosis and treatment.