Loose hinge.
I am 67 was dx last November But my curve in my spine was noticed in my 30`s when I had an xray for back problems which have bothered me ever since.
I was told then that I had been born with a curve.
My mum has this problem, when we 'land' her in a chair we make sure she's wedged in on the right hand side by the arm rest rather than sitting dead centre, it seems to prop her up. Can't be done on a dining chair with no arms mind.