Residual or recurrent lumbar disc herniation: This condition is a common cause of postoperative weakness in the lower extremities, especially if the patient has a history of lumbar fusion. The herniated disc can compress the spinal nerves, leading to weakness, numbness, and falls.

Infection or abscess at the surgical site: Postoperative infections can cause weakness, pain, and instability, leading to falls. The proximity to the spinal cord and nerves makes this a plausible cause for lower extremity weakness.
Pseudarthrosis or hardware failure: If the lumbar fusion was not successful or the hardware used in the surgery fails, it can lead to instability and compression of the spinal nerves, resulting in weakness and falls.
Adhesive arachnoiditis: This condition involves inflammation and scarring of the protective membranes covering the spinal nerves, which can cause nerve compression and lead to weakness, pain, and falls.

Spinal epidural hematoma: Although rare, this condition is a medical emergency that requires immediate attention. A hematoma in the spinal epidural space can compress the spinal cord, leading to rapid onset of weakness, paralysis, and potentially permanent damage if not promptly treated.
Cauda equina syndrome: This is a serious condition that occurs when the nerves in the spinal canal are compressed, leading to loss of bladder and bowel control, as well as lower extremity weakness. It is a medical emergency that requires immediate surgical intervention.

Vitamin B12 deficiency: A deficiency in vitamin B12 can cause subacute combined degeneration of the spinal cord, leading to weakness, numbness, and gait disturbances. Although rare, it is an important consideration in patients with unexplained neurological symptoms.
Spinal cord infarction: This is a rare condition that occurs when the blood supply to the spinal cord is interrupted, leading to infarction of the spinal cord tissue. It can cause sudden onset of weakness, paralysis, and loss of sensation.