A surgical complication is a bad outcome from surgery, e.g., a back operation that results in increased pain.  A bad surgical outcome does not necessarily indicate that there was surgical negligence.  A bad outcome can be a rare, but known complication, of an operation.
The surgeon will most likely defend the claim by asserting that the bad surgical outcome was a known complication of surgery, and that the patient acknowledged and consented to the risk by signing a Consent to Surgery.
The Consent to Surgery will commonly contain a long list of bad outcomes of surgery, including paralysis, infection, nerve damage and even death. Frequently, the Consent to Surgery is so generic and broad that it provides no useful information for the patient.  In some rare situations, such as ophthalmological procedures (e.g., surgical removal of a cataract), the Consent to Surgery might explain the bad outcomes that can occur.