I've had two disc herniations. The first one came on slowly over a couple years. The second one hit me suddenly. One day I was fine, and the next day I was off my feet in agony. Can you explain the difference?

We can only guess what might be happening at this point. Research into the cause of disc damage and stages of repair is ongoing. From what we do know, it appears that the mechanism for triggering disc degeneration differs depending on the injury.

In other words, does the damage comes from inside the disc or by forces and pressure on the outside? The effects of aging versus trauma may be different. Or perhaps repetitive mechanical trauma from doing the same activity or motion over and over is the major key to disc damage.

From a study in Sweden using pig models, we know what kind of damage a hole can do when poked in the disc. The researchers think a small hole results in minor leakage of fluid from the disc. Healing is faster, too. Maybe the healing in a small injury is more complete than a large injury.

With a larger hole poked in the disc, there's a faster loss of nucleus material right away. The site of the injury doesn't heal well, and the disc can't get enough nutrition. This may explain the differences in your two disc events, one may be a small injury with a slow leak, while the other was larger with slower healing.