For the record, during the first operation last August, I tightened the two hip joints and two back joints, 1 and 2 in the following photo:

Joint 3 was not visible, as the incision was too high. This is a clear indication that the markings on the photo, on which I based the incision, are not sufficiently precise and should be disregarded. On the other hand, joints 1 and 2 (especially 2) had to be tightened, which led to a clear improvement. Static tests, repeated several times, with arms stretched forward were OK: the body no longer tended to bend spontaneously forward.
It was only after Geena had been standing up for several days without any problem, then sitting down, standing up again without any problem, and two sessions of changing clothes, again without any problem, that on the last occasion I felt that the back was supple again... So I thought that joint 2 must have loosened up again. But no, it's joint 3 that seems to be at fault.
I don't know for sure why the tests were OK at the 1st operation, but I think that tightening joint 2 tightened up the "spine", and that the extra effort over the two months following the operation loosened up joint 3.
The hips, on the other hand, are perfect after the August tightening.
The practical details of the back operation described in the first post are those adopted for this 2nd operation. There will therefore be no detailed description here.
The points to bear in mind are as follows:
- The incision is made over the old scar extending it downwards by 4cm, making a total length of 13cm :

- Joint control 2. Limited additional tightening (a quarter turn of the wrench).
A priori there was no further loosening of this joint, which was feared at the outset;
- Check joint 1, which had been checked during the 1st operation: tightening ok;
- By performing dorsal flexion movements while looking at and palpating the skeleton in the wound, we can clearly see that joint 3 is not tight enough;
- For joints 1 and 2, I can confirm that 13 mm wrenches should be used. You can only tighten on one side, that of the nut. On the other hand, joint 3 requires 16mm ring spanner, and you need two, as the opposite side turns when tightening one side;
- With the two wrenches in place (eye side) on joint 3 :

You need to apply a lot of pressure to the wrenches. This joint is really hard. I had to push as hard as I could and it wasn't enough. I had to finish by putting the doll forward on the bed, to get a better grip:

I had to finish clamping with a hammer! You have to clamp on both sides, alternately blocking one side and then the other. I did it first in this position, then after putting the doll back on the dollstand.
The tests carried out on and off the dollstand show that the tightening of the dorsal joints obtained is correct, i.e. firmer than that of the hips. I'm afraid, however, that this joint is unstable and tends to loosen up again over time.
This is why I would again recommend, for any body presenting the double problem of too-tight hips and too-soft back, to intervene also, and beforehand, on the hips to loosen them.
Gluing and wound closure were carried out in the same way as for the first operation.
The scar after bandage removal:

A small incision can be seen on the right, caused by the scissors used to cut the bandage. Mine are too sharp. It's better to use round-tipped scissors.
Today's tests show that Geena's back is quite stable. Convalescence should be short-lived!
Not having the means to modify my first post, I hope that the subject will be read up to this complementary post by those possibly interested, so that they can carry out all three operations (2 hips + 3 back joints) in the same operating time.
I will, of course, keep you informed of the post-operative progress should I find anything of interest.