It all depends on your definition of "work".
Inversion therapy tables such as the Teeter EP550 inversion table affect the spine and, to a lesser extent, other joints by reversing the effect of gravity.
The idea is that the continuous effect of gravity compressing our spines and bodies day in day out has a negative effect on the health of our spine and joints and may even promote degenerative changes in the spine.
Gravity is of course needed to survive on earth and even has a beneficial effect on healthy beings.
The space program has reams of documentation on the negative effects of weightlessness (no gravity) on astronauts.
Without gravity, muscles become weak and bones begin to lose strength and calcification making them more prone to fracture.
So some exposure to gravity is indeed necessary.
The negative part of gravity is the constant compression from our heads pulling down toward the earth.
For the perfect specimen who is heavily muscled and has a perfectly health spine, gravity and the compression it causes will, most likely, not have any negative effect.
The problem is that most of us are not anywhere near the physical shape of the "perfect specimen" and we do feel the effects of gravity.
Inversion therapy has been around for a long time.
Remember the gravity boots from thirty or forty years ago?You had to be a gymnast just to get them hooked on the bar and you had better hope the bar was sturdy or you would end up on your head! The next version was easier to use but expensive and bulky making home use very impractical.
These units held the user by the thighs in what was known as a 90-90 position.
The user was bent forward at the hips or waist 90 degrees and secured by their knees being bent 90 degrees as well.
These machines worked pretty well except for the fact that considerable strength or agility was required to return to an upright position.
For someone suffering from a high level of pain, it could be very difficult to get out of the machine without further straining their back.