Shown below, to the left, is an anatomic
drawing of the thoracic spine showing the relationships between
the paravertebral muscles on the left and the position of the
facet joints on the right (green). The red dots show where
the ribs (thoracic) or transverse processes (lumbar) are
located. The drawing on the right shows where pedicle
screws are placed. This occupies the area over the facet
joints and the only place for the bone placement becomes the red
dot areas.
In order to place fusion bone in the red dot
areas extensive dissection and retraction of the normal
paravertebral muscle mass is required. The devitalization
and scarring of these muscles has been so significant and so
common that the term "post-fusion disease"
has been coined to describe this phenomenon.
Studies have shown that in addition to muscle disruption from
dissection that muscle ischemia can occur from the effect of
retractor blade pressure during surgery and that the degree of
this injury can be directly determined by enzymatic monitoring.
It has also been noted that patients who had poor outcomes after
complex fusion surgeries were more likely to have persistent
pathologic changes in their paravertebral muscles as shown by
MRI scanning.
In the MRI scan to the left a multi-level
lumbar
complex fusion has been performed. The
paravertebral muscle mass has now been replaced by
scar tissue due to the disruptive injury produced by
dissection and retraction. This complication,
by itself, can be a continuing cause of
post-surgical back pain.
The illustrations to the left
demonstrate less invasive and lower risk means of
spine stabilization. By the use of a
self-tapping cage, rather than screws and rods, the
more medial area of the spine is made available for
bone placement.
Other surgical options attempting to
eliminate post-fusion disease have been the use of
"dynamic" stabilization devices such as that shown
here. These have a flexible component and do
not require associated bone deposition.
Other approaches have involved the more minimally invasive
placing of screws and rods percutanously or endoscopically.
The difficulty with these is less than optimal spine
stabilization and a inherent difficulty in achieving the
adequate decompression of impaired neurologic structures.
This is a quite important consideration because the primary, and
continuing, reason for
failure of spine surgery remains the
failure to adequately treat lateral
spinal stenosis.
