Cut the flexor tendon. cannot bend it, it sticks straight out. In fact it bends upwards quite a bit, as the extensor tendon is still pulling on it.
It was partially lacerated when a broken (thick) glass vase cut into the last joint of the left middle finger. I had stitches, and limited movement - but prognosis looked good. I was advised to use it and strengthen it. 2 weeks after the stitches were out, I was picking up my daughter's empty plastic blow-up kids pool (ie. fairly light) and it just ruptured and gave out.
At that point, I tried to move it, and moved it once (which I assume was the tendon pulling away from it's place and bending the finger in the process).
I'm not trying to blame my GP, I could have insisted on getting proper treatment - I didn't understand the risks fully, which is partly why I made this post, so if this happens to anyone else, they don't make the same mistake that I did.
Anyways, I'm positive, and am looking for the best path to take. As far as I can tell, here are my options:
1. Continue playing with a metal slide on that finger. slide on bass might be interesting... might get a unique style out of it - although there may be a reason I have never heard it done before

Guitar chords without a middle finger are tough, but can be faked out (only need to play a 1-3 strings of a chord at a time anyways, so no big deal)
2. As Music4Life209 said, just use the finger anyways. Problem is that my fingers bend back naturally (upwards, away from the palm) quite far. So I really have to crane my hand around to use the finger to press a note. It has been great for making mid-string barre chords on a guitar, but has a drawback right now... Might be a good idea to fuse the joint somehow - to a slightly downward angle... Seems extreme, but would also mean I can type and do other normal things with it.
2. Or play the bass on my lap so it doesn't matter about the finger curvature?
3. Go lefty. Flip the instrument upside down, and play like Hendrix used to. Or just switch to a lefty instrument (I'd have to re-teach myself in either case, so wouldn't matter which one - I wouldn't mind flipping back and forth depending on the song? so that is why considering the upside down approach). Problem is that finger-picking is lessened, but no big deal, could use 1,3,4 for picking.
Which option would you choose?