Postoperative pain management: why is it difficult to show that it improves outcome?

Intuitively it seems obvious that unrelieved, severe acute pain after surgery may have a number of undesirable effects 1. Many experienced clinicians maintain that the immediate postoperative course as well as long-term outcome may be influenced by the quality of pain relief after surgery or trauma 1, 2. However, there are conflicting data from published clinical studies concerning the beneficial effects of postoperative pain management on short-term and long-term outcome of surgery 3.