Fig. 4

Correlation of observed improvement by infiltrations with qualitative facet joint degeneration and spinal stenosis classifications in the form of heat maps (left column) and boxplots (right column). a Facet joint degeneration (Weishaupt [20]), b neuroforaminal stenosis (Lee [25]) and c spinal stenosis (Schizas [30]). Pain relief obtained by the infiltrations is grouped into three categories: 1, no improvement (below 30%); 2, good (30–74%); and 3, excellent (75–100%) improvement. Colour intensity in the heat maps is shown according to absolute frequencies