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].