depends who's using the terms -
To a spine surgeon, instability means movemement that could cause serious harm and likely needs bracing and/or fusion. Instability could be from a fracture or dislocation, or disruption of the ALL and/or PLL causing spondylolisthesis - often seen with significant translocation on flexion and extension views.
To a chiro it means your spine moves, therefore it is unstable and needs manipulation. 😉
Spondylolisthesis simply means one vertebral body is malaligned anteriorly or posteriorly compared to the levels above or below it. The most common reason for this is probably facet arthropathy - in most cases it is benign. It could also come from pars defects and fractures, which when bilateral can create an unstable spine, needing bracing and possibly fusion. Xray often picks up pars defects, but you often want CT, SPECT or bone scan to confirm it.
Subluxation is most often a chiro term to say the spine is out of alignment and needs to be pushed back into shape. Since subluxation is a term that mainly refers to joints that are moving near dislocation, without completely dislocating, it should only be applied to the facet joints. I've never seen anyone truely demonstrate a facet joint subluxing. Dislocated after trauma - absolutely. Perhaps if one significantly ruptured a facet capsule it could sublux, but I've never seen it. I've even asked chiros to show me the subluxation they describe and I still don't see it. Maybe you have to believe it to see it?
