I hope someone on the forum might be able to offer some advice in saving a small indoor cactus.

About two months ago, the cactus fell over because it had become rotten at the base. After looking on the internet for advice on reviving it, I cut off the rotten part of the base and stabilised it in a mixture of sand and potting soil.

The cactus seemed to be doing quite well until two days ago when it fell over again. I noticed only limited evidence of roots--only 2mm long at best. The stem is now translucent and soft. The prickly top is still green and hard to the touch. Water has begun dripping from the bottom of the base of the stem.

Do you think the cactus can be saved, and if so what would be the best course of action to save/regrown it?

Here are some images of the cactus:

Showing the (still?) healthy top:
i46.servimg.com/u/f46/17/16/83/60/cactus10.jpg

As before, showing the top:
i46.servimg.com/u/f46/17/16/83/60/cactus11.jpg

Showing the rotten stem:
i46.servimg.com/u/f46/17/16/83/60/cactus12.jpg

Could the small round clusters at the top be removed and successfully regrown?

Definitely remove the offsets - the main stem is a goner and will just infect them if your leave it too long. When you remove them, sit them on some new DRY mix/sand. Don't keep them in too hot a positon as you won't want to water them until they've rooted down. You can even try a few without putting them on the mix, just sit them on a shelf somwhere and let them produce roots.

Good luck !

Good advice from Navajoa, I would do the same thing.

It looks to me like you have several Mamms grafted to a Hylocereus stock which has rotted. Just take all the pups off as suggested above and grow them out, they will grow no problem.

Thanks Navajoa and Hellonasty for the advice. I cut the Mamms off yesterday and there was no sign of rot in the upper part. I will leave them on a shelf until they show signs of rooting.

How long would you expect it to take before the roots start to grow?

There's no definite answer to that question. Temperature seems to have a lot to do with it; in warmer months my cuttings and offsets always seem to root a lot quicker than during the cooler part of the year. To assist with process you may wish to try using a rooting hormone powder.