What does signal changes in the posterior left medial menicus and posterior lateral meniscus mean?

Degeneration. Mri scans image tissue by measuring hydrogen atoms such as are found in water (h2o - 2 parts hydrogen, 1 part water) . As a meniscus ages the tissue degenerates and is replaced by fluid which is mostly water. Increased MRI signal ( increased fluid) usually means some degeneration of the meniscus which is a consequence of aging.
Not to worry . As we get older meniscus is more prone to degeneration after years of wear and tear. Signal change usually means this and doesn't mean tear.

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