Chuck, I'll just address lumbar discs that I've seen implanted with a keel (ProDisc, MobiDisc, Activ-L, Kineflex). The procedures are strikingly similar, yet there are also substantial differences. With some of these devices I've only seen a few surgeries (Kineflex = 1), so I'll keep the discussion quite general. My understanding may not be fully baked. Also, here is the obligatory "I'm not a doctor and I may be completely wrong" statement.
After the level is exposed, a window is cut into the anterior ligament and annulus. The disc space mobilized and evacuated, then disctracted more, cleaned out more. Posterior decompression and cleanup is done as needed. (removal of osteophytes, remnants of old protrusions, etc...) Endplates are prepared - cartilagenous material moved. They expose bleeding bone to enhance integration.
Using flouroscopy, the midline is determined and marked. Trial implants (like blank prostheses) are inserted to verify proper sizing, angles, fit, etc... Once they have the proper fit, they are ready to make the cut. The keel cutter is a big chisel. It's very sharp and goes through the bone quite easily. The instrumentation varies greatly from one device to another. Remember that once the keel cut is done, it's done. There is no relocating it.
With the ProDisc, the trial implant... very snugly fitting (as the distraction is released)... location, size, angles all verified by flouro... the trial implant is the guide for the keel cutter. The chisel is advanced into the vertebral body. When they get near the back, they are watching it move very closely as they advance it. There is a stop to keep the keel cutter from going too far, but I've never seen that in play. The process is very easy to control. The surgeon is determining how far to go just by watching it. As he approaches the desired depth, he moves it with finer and finer control with lighter and lighter taps.
I suppose that is may be considered that the end of the keel cut is a stop, but I don't think of it that way. the surface tension on the keel and endplates is substantial and will keep the prosthesis from moving (when properly implanted).
Chuck, I hope this helps...
Mark
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