what does the following mean: T12-L1: Unremarkable. L1-2: There is mild facet arthropathy at this level. L2-3: There is mild facet arthropathy at this level. L3-4: There is facet arthropathy at this level with mild left neuroforaminal narrowing. L4-5: There is a broad-based disk bulge with facet arthropathy at this level. L5-S1: There is a broad-based disk bulge with left-sided protrusion at this level resulting in a mild bilateral neuroforaminal narrowing.

Summarized question: What causes mild facet arthropathy?