My wife developed sciatica after her knee replacement.

As note, she is very heavy and does not exercise, so lack of fitness and weight are major contributors to both her knee and back issues.

As I understand it and having witnessed the progression, the recovery from the knee surgery was lengthy and more difficult then it might have been for a more fit and/or less heavy person and the result of the pain and recovery from the surgery caused an imbalance in walking as she favored the new knee. The gait issues can result in strains to the back and leg muscles with sciatica resulting. She probably had disc issues prior (as is common as we age), but it developed faster and more severely as a result.

The pain in the knee was certainly more severe then the sciatica, so the knee replacement was mandatory in her case, but with attending complications.