Magnitude, response, and psychological determinants of placebo effects in chronic low-back pain: a randomised, double-blinded, controlled trial.

In this elegant study, the authors compare lidocaine injection of the zygapophyseal joint to saline injection (placebo). They then ask subjects whether they thought they got the block or the placebo.  Interestingly, the so called placebo injection was somewhat more effective than the lidocaine for pain reduction. In the group receiving the 'placebo,' those who thought they received the active block had significantly greater pain relief than those who (correctly) thought they got the 'placebo.'    

While this study clearly demonstrates the power of expectation, it's hard to say that the 'placebo' block was really just a placebo since all patients got injections of something and there was no 'no-treatment' control (not clear how you could even blind such a study).  However, regardless of semantics or its relevance to placebo studies, it seems as though this is very good evidence that this type of block is ineffective and should not be done.  Furthermore, it suggests that questioning individuals about their expectations for relief and whether they think they got the treatment would be a good way to evaluate interventions that seem to have a very large apparent placebo response.