Residual or recurrent lumbar disc herniation: This is a common cause of lower extremity weakness and falls after lumbar surgery, as the herniated disc can compress or irritate the surrounding nerves, leading to neurological symptoms.

Post-laminectomy syndrome: Also known as failed back surgery syndrome, this condition occurs when patients experience persistent or new symptoms after lumbar surgery, which can include lower extremity weakness and falls.
Epidural hematoma or abscess: These complications can occur after lumbar surgery and compress the spinal cord or nerves, leading to lower extremity weakness and falls.
Nerve root injury: Direct injury to the nerve roots during surgery can cause lower extremity weakness and falls.
Adhesions or scar tissue formation: The formation of adhesions or scar tissue after lumbar surgery can compress the nerves and cause lower extremity weakness and falls.

Spinal epidural abscess: A serious and potentially life-threatening condition that requires prompt diagnosis and treatment to prevent long-term neurological damage or death.
Cauda equina syndrome: A rare but serious condition that occurs when the nerves in the cauda equina are compressed, leading to severe neurological symptoms, including lower extremity weakness and falls.
Infection (e.g., osteomyelitis, discitis): Infections after lumbar surgery can cause lower extremity weakness and falls, and require prompt diagnosis and treatment to prevent serious complications.

Arachnoiditis: A rare condition characterized by inflammation of the arachnoid membrane, which can cause lower extremity weakness and falls after lumbar surgery.
Vascular complications (e.g., arteriovenous fistula): Rare vascular complications can occur after lumbar surgery and cause lower extremity weakness and falls.
Tumor recurrence or metastasis: Although rare, tumor recurrence or metastasis can cause lower extremity weakness and falls after lumbar surgery.