What are the differences between the lumbar and thoracic vertebrae?

There are several differences :

1. Size: Lumbar vertebrae are much larger than thoracic vertebra. All skeletal elements are larger in lumbar vertebra

2. Body: The vertebral body of lumbar vertebrae is larger and cylindrical / kidney shaped whereas that of a thoracic vertebra is smaller and heart shaped.

3. Verebral Foramen: Circular in thoracic Vertebrae and Triangular in Lumbar.

4. Thoracic Vertebra articulate with the ribs and via the ribs to the sternum. They are therefore much less mobile and less likely to develop degenerative osteoarthritis. They have 6 additional facets on them which the lumbar do not have. 2 hemi-facets on each side of the body (total 4) for articulation with its own rib and with the rib below and a facet on each transverse process (total 2) for articulation with the tubercle of its own rib.

5. The zygapohyseal joints (joints between articular facets) of the thoracic vertebra are directed vertically so they limit flexion and extension but allow rotation. The zygapophyseal joints of lumbar vertebra are circular and the articular processes interlock thereby limiting rotation and indeed flexion and extension. However a fair amount of flexion and extension does take place.

6. Combined together, the thoracic vertebra give the thoracic spine a concave curvature anteriorly whereas the lumbar vertebra produce a lordosis (convex anteriorly)

7. The L5 or lowermost lumbar vertebra has massive transverse processes for attachment of the iliolumbar ligaments which connect the lumbar spine to the pelvis.