Chuck,
I believe that it is a press fit for several reasons, however, I've not personally felt the resistance, so I don't know how tight it is. When they talk about fixation or integration of a prosthesis, they talk about primary and secondary fixation. Primary fixation is what you have on implantation. Secondary fixation is what you get after osteointegration of the device.
I believe that all of the coated metal devices will achieve excellent secondary fixation. I was in on Mr. Bee's revision surgery at Stenum to revise the undersized, poorly located and subsided Charite'. The revision took place at 10 weeks post-implantation. I got to see, first-hand, how tightly the plates had adhered to the vertebral bodies. You could see the bone that was grown into the coating at just 10 weeks. Pictures included below with Mr. Bee's permission.
The strongest primary fixation comes when you seriously attach the device to the bones. The old Bristol cervical disc with a flange and 2 screws at each level provides the most primary fixation I've seen in ADR. Now, Medtronic still opts for primary fixation, but has dropped one screw per level, so they don't interfere with potential multi-level procedures.
A keel provides more primary fixation because of all the surface area of the keel, adds to the total surface area that provides friction or surface tension that will resist migration. A loose fit would not provide as much primary fixation as a tight fit. Also, for it to be a loose fit, the keel cutter would have to chisel a slot that is wider than the keel. No motivation to do that. It would also increase the risk of damaging the body to use a very wide chisel. So opening the slot with a chisel, then opening just a little more with the keel.
I'm going to the NASS meeting Wendesday through Saturday. I'll look at the tools and talk it over with some of the surgeons. If I learn more, I'll post it here.
Mark
PS... sorry that this went from a question about the keel into fixation, but that is what the keel is about.
Charite' explanted at 10 weeks. See bony ingrowth. They were very strongly adhered to the vertebral bodies.
