Numbness in the back of the left shoulder is commonly caused by cervical spondylosis and trauma, as follows: 1. Cervical spondylosis: mixed cervical spondylosis or neurogenic cervical spondylosis can lead to nerve conduction dysfunction, and if the nerve root on the left side is compressed, the sensation in the back of the left shoulder will be reduced, resulting in clinical symptoms of numbness. 2. Trauma: Trauma to the back of the left shoulder, such as a direct violent blow, can lead to severe local edema in the back of the left shoulder, resulting in nerve ischemia and numbness, accompanied by pain and local limitation of movement. An X-ray will reveal a comminuted fracture of the left scapula. The symptoms of numbness in the back of the left shoulder can be significantly relieved by timely consultation and treatment, and cervical spondylosis and trauma to the back of the left shoulder should be prevented.