My arthritic mare is wearing hers now. As far as I'm concerned they can't hurt. My vet agrees that they can work very well on various problems. Never used them on a spavin though
I've ued them on splints with fantastic results. A local vet from Rossdales recommends the N.E.W magnet boots and says that on his wife's horse they completely eliminated a splint. Not sure what effect they would have on a spavin, but worth a go.
mine has navicular in front and bone spavin behind - she has her bioflow boots on 247 - they go on front during night and hinds during day - she has the brushing boot type as lives out. She also has a magnetic rug on at night under her turnout - i find it has helped mine - it doesnt have to be right over the arthritic joint to help