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Pedicle Screw Misadventures
 
 

This illustration from the New York Times in December 2006 shows that spine implant screws and rods are expensive.  Screws are intended for placement within the vertebral pedicles.  When this, unfortunately, is not the case continuing patient problems as well as serious and permanent neurologic injury can be the result.
   
This patient had a five level front and back rigid instrumented fusion as a primary treatment for low back pain.  Screw placement has encroached on the vertebral endplate at two levels.  At the level above the fusion a classic "transitional syndrome" has been produced with associated collapse of the interspace and a beginning retrolisthesis.
In this case the superior pedicle screw of a one level front and back rigid fusion has penetrated the vertebral endplate and has begun to eroded part of the vertebrae above.  This is also a classic example of a  "transitional" syndrome.   Note that in addition to collapse of the interspace there is now a G1, almost G2, degenerative spondylolisthesis associated with nerve compression.
In the image to the left there is also an aberrant pedicle screw which, on the left side is completely outside (above) the pedicle and into the disc space.
In the case shown here the immediate post-operative CT scan shows a pedicle screw outside of the pedicle where it impaled a spinal nerve.  This patient continued to be disabled with a causalgic pain syndrome involving the right leg despite additional surgery.
In this example the pedicle screw on the left is outside of the pedicle and actually within the central spinal canal.   
A more extreme of screw misplacement is demonstrated here where the screw actually transverses the center of the spinal canal producing nerve injury.
It is generally accepted that 10-15% of patients having pedicle screw implantation have associated screw misplacement.  A recent study documented a misplacement rate of 29.2% using conventional imaging.  By the introduction of computer assistance the incidence of misplacement was decreased to 8.3% (Seller K, Urselmann L, Krauspe R: Prospective Screw Misplacement Analysis After Conventional and Navigated Pedicle Screw Placement, Biomed Tech (Berl), 50(9):287-92, Sept 2005)