The knee joint cannot bend.

Under normal circumstances, the inability to bend the knee joint may be related to conditions such as knee arthritis, knee synovitis, meniscus injury, fractures around the knee joint, or adhesions around the patella after surgery, and requires symptomatic treatment. Specific details are as follows:

1. Knee Arthritis

Knee arthritis refers to inflammatory joint lesions caused by chronic injury, obesity, aging, metabolic abnormalities, or changes in nutrition. The main clinical features include redness, swelling, heat, pain, and functional impairment of the joint. It may lead to inflammation and pain around the knee joint, impairing the normal function of muscles and structures, thereby causing the aforementioned condition. Patients are advised to take medications such as ibuprofen sustained-release capsules or enteric-coated diclofenac sodium tablets as prescribed by a physician.

2. Knee Synovitis

Knee synovitis refers to degeneration of the synovial membrane due to mechanical or chemical irritation of the knee joint. Symptoms include joint pain, swelling, and functional impairment. It may cause joint effusion, restricting normal joint movement and resulting in the above-mentioned symptoms. Patients are advised to take medications such as celecoxib capsules or diclofenac sodium sustained-release capsules as directed by a doctor.

3. Meniscus Injury

Meniscus injury refers to rupture of the meniscus caused by rotational forces, compression, or underlying meniscal disease. It typically presents as severe knee pain, which interferes with normal knee joint movement, leading to the aforementioned condition. Patients are advised to rest adequately. In more severe cases, surgical interventions such as partial meniscectomy or meniscus repair may be required.

4. Fracture Around the Knee Joint

A fracture in the knee area may disrupt the structure and stability of the joint, impairing normal joint motion. It may also result in displaced bone fragments, bone shards penetrating the joint cavity, or damage to surrounding soft tissues, thus causing the aforementioned issues. Patients should seek timely medical attention and may require surgical open reduction and internal fixation for treatment.

5. Post-Surgical Adhesions Around the Patella

Post-surgical adhesions around the patella refer to scar tissue formed after surgery that adheres the patella to surrounding structures, restricting normal joint movement and causing the aforementioned symptoms. This condition may occur after knee surgeries such as arthroscopy or joint replacement. Timely medical evaluation is recommended, and surgical intervention may be necessary to release the adhesions.

Patients are advised to seek prompt medical attention when experiencing discomfort and to undergo standardized treatment under a physician's guidance, which helps promote recovery.