Placebo effects are a fascinating subject. In so-called alternative medicine (SCAM), they are particularly important because much of SCAM seems to rely on little more than placebo effects. Therefore, I think this new paper is of some relevance to us.
The aim of this systematic review was to quantify the placebo effect of intraarticular injections for knee osteoarthritis in terms of pain, function, and objective outcomes. Factors influencing placebo effect were investigated.
Out of 2,363 records, 50 articles on 4,076 patients were included. The meta-analysis showed significant improvements up to the 6-month follow-up: Visual Analogue Scale (VAS)-pain −13.4 mean difference (MD) (95% confidence interval [CI]: −21.7/−5.1; P < 0.001), Western Ontario and McMaster Osteoarthritis Index (WOMAC)-pain −3.3 MD (95% CI: −3.9/−2.7; P < 0.001). Other significant improvements were WOMAC-stiffness −1.1 MD (95% CI: −1.6/−0.6; P < 0.001), WOMAC-function −10.1 MD (95% CI: −12.2/−8.0; P < 0.001), and Evaluator Global Assessment −21.4 MD (95% CI: −29.2/−13.6; P < 0.001). The responder rate was 52% (95% CI: 40% to 63%). Improvements were greater than the “minimal clinically important difference” for all outcomes (except 6-month VAS-pain). The level of evidence was moderate for almost all outcomes.
The authors concluded that the placebo effect of knee injections is significant, with functional improvements lasting even longer than those reported for pain perception. The high, long-lasting, and heterogeneous effects on the scales commonly used in clinical trials further highlight that the impact of placebo should not be overlooked in the research on and management of knee osteoarthritis.
The authors furthermore confirmed that “the main finding of this meta-analysis is that placebo is an important component of the effect of injective treatments for patients with KOA, with saline injections being able to provide relevant and long-lasting results not only in terms of pain relief but also with respect to stiffness resolution and function improvement. These results are both statistically and clinically significant and can be perceived by patients up to 6 months.”
I would dispute that!
To explain why it might help to read our 1995 BMJ paper on the subject:
We often and wrongly equate the response seen in the placebo arm of a clinical trial with the placebo effect. In order to obtain the true placebo effect, other non-specific effects can be identified by including an untreated control group in clinical trials. A review of the literature shows that most authors confuse the perceived placebo effect with the true placebo effect. The true placebo effect is highly variable, depending on several factors that are not fully understood. A distinction between the perceived and the true placebo effects would be helpful in understanding the complex phenomena involved in a placebo response.
In other words, what the authors picked up in their analysis (i.e. the changes that occurred in the placebo groups between the start of a trial and after placebo application) is not just the placebo response; it is, in fact, a combination of a placebo effect, concomitant interventions/care, regression towards the mean, natural history of the condition and possibly other factors.
Does it matter?
Yes, it does!
Placebo effects are not nearly as powerful and long-lasting as the authors conclude. And this means virtually all their implications for clinical practice are incorrect.