Cervical Spondylosis: This condition is a common cause of bilateral upper arm numbness due to degenerative changes in the cervical spine, which can lead to nerve root compression. The presence of mild osteopenia and the absence of significant bony lesions or facet joint abnormalities support this diagnosis, as cervical spondylosis often occurs in the context of degenerative changes rather than overt bony pathology.

Thoracic Outlet Syndrome: This condition involves compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib), which can cause numbness in the arms. The symptoms can be bilateral and might not necessarily correlate with significant radiographic abnormalities.
Radiculopathy: This refers to damage or irritation of the nerve roots, which can cause numbness, weakness, or pain in the arms. Given the patient's symptoms of bilateral upper arm numbness, radiculopathy affecting multiple nerve roots could be a consideration, especially if there are underlying conditions affecting the spinal nerves.

Multiple Myeloma: Although the imaging does not show discrete bony lesions, multiple myeloma can sometimes present with diffuse osteopenia and minimal or no focal lesions on initial imaging. It's crucial to consider this diagnosis due to its potential severity and the need for early intervention.
Metastatic Disease: Similar to multiple myeloma, metastatic disease to the bone can sometimes present subtly, with osteopenia being an early sign. The absence of discrete bony lesions does not rule out metastatic disease, especially in patients with a known history of cancer.
Spinal Cord Compression: This is a medical emergency that requires immediate attention. Although the vertebral body heights and alignment are preserved, and there are no discrete bony lesions, spinal cord compression can occur due to soft tissue abnormalities (e.g., epidural abscess, hematoma) not visible on the provided imaging or due to ligamentum flavum hypertrophy in the context of cervical spondylosis.

Vitamin Deficiency (e.g., Vitamin B12 Deficiency): Certain vitamin deficiencies can cause neurological symptoms, including numbness in the extremities. While less directly related to the radiographic findings, vitamin deficiencies are important to consider in patients with unexplained neurological symptoms.
Amyotrophic Lateral Sclerosis (ALS): A progressive neurodegenerative disease that affects nerve cells in the brain and the spinal cord, causing loss of muscle control. ALS can present with bilateral symptoms, although it typically progresses in an asymmetric manner. The diagnosis would not be suggested by the radiographic findings but could be considered in the differential for bilateral upper limb symptoms.