The first variety is a slow increase in curvature in a person's youth that progresses into adulthood and can be treated with braces during adolescence.
This can occur in both the upper and lower back and can be observed as a difference in shoulder height, ribs which might stick out, and/or a protrusion of the lower back.
As a person becomes older, this can worsen as the bones of the back degenerate and settle. This will most likely result in arthritis, which will lead to pain and stiffness with a shooting pain and numbness down the legs due to the nerves of the back being pinched.
The second variety occurs in adults due to the degeneration of the spinal discs. This usually occurs in the lower back and is often accompanied by a straightening of the natural curvature of the lower back. This is also accompanied by pain, stiffness, and numbness down the legs.
The third variety is often observed in patients who have undergone surgery, either for scoliosis or for degenerative low back conditions. This is usually seen as a flattening of the lower back as the curvature of the lower back goes away.
This condition makes it difficult to remain standing upright as the straightening of the lower back causes a person to lean forward.