A combination of back pain and numbness in the left arm can be associated with a herniated disk or injury to the neck and nerves associated with the neck, according to the New York Times. A herniated disk can also be referred to as a slipped disk.
When a disk is herniated, it is moved out of place or ruptured, and the spinal nerves are no longer protected. The lower back is the most common area for a disk injury, followed by the neck. Numbness along the arm can be associated with an injury to other tissues in the neck, not necessarily just an injury to a disk, as stated by the New York Times.