I don't recall hearing of any problems that would lead me to believe that instability and ADR issues have to do with stresses on the prosthesis or on the bone/plate interface. The structures that keep a healthy spine from being unstable are also required to keep the ADR implanted spine stable. I believe that problems associated with spondy and ADR have to do with protection of the posterior elements... just as in a non-ADR implanted spine. Watching this evolve over the years, there has been much discussion about degenerative instability vs. isthmic instability. Degenerative instability comes from collapsing disc spaces... imagine the disc as a tire on it's side. As you let the air out, it becomes wobbly and unstable. ADR is tolerant of degenerative instability because as the disc space is restored, the system is 're-tensioned' and the instability is resolved. Isthmic instability is the result of a true structural issue (like a pars fracture). This will not be resolved by ADR and the abuse heaped on the posterior elements will not be halted.
Here is where a discussion of constrained vs. unconstrained prostheses is important. A more constrained prosthesis with a stationary joint like a ProDisc or Maverick may provide more centering force that will help to keep the joint stable even with source of instability unresolved. The patient that I'm asking to come and respond had a ProDisc implanted in spite of substantial spondy. It was a calculated risk and, I believe, one that paid off. Time will tell. I look forward to her update.
At this years SAS, Dr. Marnay presented positive results from a series of patients that had repairs of Pars defects in conjunction with ProDisc. I know a patient who had this done early in Marnay's learning curve... unsuccessfully. It will be interesting to see where this goes... how much instability is too much.... how will we know. As with all other things spine, the answer will not be clear. Some patients with gross instability will be successful... others with minor instability will not.
Here comes the "I'm not a doctor" disclaimer... take everything I write with a grain of salt...
Mark
|