I came across Comparison of the effectiveness of medicinal leech and TENS therapy in the treatment of primary osteoarthritis of the knee : A randomized controlled trial and did a double take.

I have seen leeches used in my hospital after finger reattachment to help decrease blood engorgement in the finger. But osteoarthritis? I would have thought it would be so Theodoric of York. But no pseudo-medicine ever dies.

TENS is Transcutaneous electrical nerve stimulation, a form of analgesia that is likely not all that effective. There are many forms of TENS, including high-frequency transcutaneous electrical nerve stimulation (h-TENS), low-frequency transcutaneous electrical nerve stimulation (l-TENS), neuromuscular electrical stimulation (NMES), interferential current (IFC), pulsed electrical stimulation (PES), and noninvasive interactive neurostimulation (NIN). A variety of styles that reminds me of acupuncture.

One meta-analysis suggests

Although the recommendation level of the other ES therapies is either uncertain (h-TENS) or not appropriate (l-TENS, NMES, PES and NIN) for pain relief…

and my go to for analysis of pain modalities, painscience.com, looked at all the meta-analyses and found that TENS

…doesn't look great: five disappointingly non-positive,16 two spankings for back pain and osteoarthritis, and just the one clearly positive review out of seven (which is also one of the oldest and perhaps the least picky).

and concludes

My official position for now is that it probably works well enough for some patients to be worth trying if you're desperate…but keep your expectations low. There are solid reasons to doubt that TENS can work well.

Not a ringing endorsement for TENS but a good comparator for another pseudo-medicine. I suspect that there is a general inaccurate belief in the medical world that TENS, like acupuncture, is effective. So if the pseudo-medicine is equal to TENS, which is science,  then the pseudo-medicine must be effective.  

The problem, of course, is comparing two interventions with no placebo will always show benefit, especially when comparing two placebos.

And they are equivalent: 

Leech therapy relieves symptoms in patients with osteoarthritis of the knee and is as effective as TENS therapy in the management of osteoarthritis of the knee.

But would you even expect leech therapy, aka hirudotherapy, to work? Leech saliva does have analgesic effects. After all, Charlie didn't know he had leeches until after he got out of the water. I would be skeptical that a topical application of leech spit would have efficacy in deep joint pain. As one study noted

There is still no definitive explanation for the pain-reducing effect of leech therapy

Suggesting there isn't one beyond placebo.  There is actually a randomized study that used an artificial leech (!). Patients had one leech treatment, two treatments separated by four weeks or an artificial leech. All three had improvement and not unexpectedly, those with two interventions had a better response. The more theatrical the placebo, the better the response.

As is always the case with pseudo-medical studies, they cannot wrap their heads around the idea that when an intervention is no better than placebo, it doesn't work,

Leech therapy can reduce symptoms caused by osteoarthritis. Repeated use of the leeches appears to improve the long-term results. We have not determined whether the positive outcome of the leech therapy is caused by active substances released during the leeching, the placebo effect, or the high expectations placed on this unusual treatment form.

The answer is staring them in the face but they can't see it.

TENS or leeches? Pick your placebo.