These are usually congenital. Meaning that you were born with it. Most of the time, partially fused vertebrae are the result of failure of segmentation while the person is in utero. The vertebrae do not completely separate.
There are quite a few congenital vertebral “deformities”. Some can cause significant problem, such a hemivertebrae, as they cause curvature in the spine. Unilateral bar formation can cause the spine to grow less on one side than the other, leading to a long spinal curve.
But, partial fusions usually do not cause a lot of problems.
However, there are other causes of partial fusion of the vertebrae. If there is significant disc degeneration, then the vertebral bodies can fuse together. Some inflammatory disorders, such as ankylosis spondylitis or psoriatic arthritis can cause fusion of the spine, but these have very specific appearances and usually involve the complete spine.
You should discuss the findings of your study with your physician. All findings have to be correlated with the patient’s history, symptoms, and physical exam.