I'm going to explain what I think the theory is. I am not an engineer. I may be all wet. I can't engage in a discussion about this, I can just pass on what I think I understand.
ProDisc comes with lordotic angles, 6 and 11 degrees. The lower plate that the core snaps in to is flat. The upper plate has the angle. The purpose of the angle is to bring the plates of the prosthesis near parallel.
With the angled plate being the top one, that configuration effectively brings the surface of the upper plate closer to parallel to the lower plate. If the lower endplate is parallel to the floor and the the upper plate is parallel to the lower plate, the ball and socket will have zero sheer force acting upon it (from the pull of gravity... you can load in other ways.) If the upper endplate of the vertebral body is not parallel to the floor, then there will be sheer force acting upon the ball and socket. If you flip the disc, instead of the angled plate bringing the upper plate parallel to the lower plate; the angled plate below will bring the lower plate of the prosthesis closer to parallel to the floor. (Clear as mud?)
It makes sense, but there is a trade-off. I believe that the flipped configuration MAY increase the likelyhood of migration of the lower plate as the sheer force doesn't just disappear. It is reduced on the ball, but I believe it still exists or may even be increased on the lower plate. IMHO, migration used to be a more serious consideration before they got good at sizing, placement and before we had the coated plates. IMHO, while the trade-off needed to be resolved in terms of less risk of migration in the early days of ProDisc, improvements made it more reasonable to work the trade-off towards less sheer force on the actual joint instead of on the bone-metal interface. This should result in less significant wear.
Again... I may be all wet here and may have this completely wrong. Take this for what it's worth... just my layperson's opinion.
Mark
|