My Dendrobium amethystoglossum lost its roots, so I did this...

Please note that I made slits on the side of the plastic pot to increase
both drainage and the circulation of air around the media and roots.

This Dendrobium amethystoglossum grew relatively well for several years in my garden and bloomed a number of times in that time span.  Unfortunately, last year the media began retaining too much water, this caused the death of the root mass.  When I noticed something was wrong almost all the roots were gone.  So I took the plant from its pot, trimmed away the dead roots, removed the decayed media from the meagre remains of the root ball and repoted it.   I used as media the hardest part of the trunk of the tree fern.  I cut the pieces of tree fern by hand into medium sized flakes.  The flakes were put on the pot so that they would make a relatively loose open mass that could be easily penetrated by the roots.  This part of the trunk of the tree fern is particularly resistant to decay so I expect it will last a while.  The orchid started growing in April and now it is producing new roots.  I expect that in the next two years it will have a good root system and will start producing larger canes.