Fixation across the lumbosacral joint may be necessary for L5-S1
spondylolisthesis and numerous conditions requiring long segment
lumbosacral fixation including neuromuscular scoliosis with pelvic
obliquity, truncal paralysis as a sequelae of progressive muscular
weakness, scoliosis with associated degeneration at L4-L5 and L5-S1,
and painful spondylolisthesis or progressive deformity below a previous
Problems of lumbosacral fixation
Lumbosacral fixation is particularly susceptible to failure (as high as
30%) due to unfavorable biomechanical factors such as concentrated
stresses at the end of the instrumentation because of long moment arm
of large flexion moments, and weak holding power of the sacrum due to
its osteopenic nature. Also, inadequate patient factors such as
advanced age, poor general health, decreased muscle tone, and
neurologic or muscular impairment do exist.
Biomechanics of lumbosacral fixation
In situations of minimal instability wherein short segment of the
sacrum requires fixation, simple use of sacral pedicle screws may be
enough; however, more complex instrumentation is necessary for long
segment fixation. The paper by McCord et al has shown that the most
stable way of fixing a lumbosacral joint is to include fixation of the
ilium bilaterally that extends well anterior to the axis of the
rotation of the lumbosacral disc. Saer and Winter have shown reasonable
results with long fusions to the sacrum where the sacral pelvic
fixation was Galveston or Galveston-like fixation combined with an
anterior fusion of all lumbar segments.
The minimum requirements to achieve a long fusion to the sacrum include
the following: Ideally, one should have four-point fixation of the
sacrum and pelvis. This includes two bicortical S1 screws and two long
iliac screws. The iliac screws should measure at least 60 mm in length.
Also important is structural grafting at L4/L5 and L5/Sl to take stress
off the lumbosacral instrumentation. An anterior fusion is ideally
performed from T10 to the sacrum. Segmental fixation should include
each level without any gaps at L3, L4 and L5. As demonstrated by
Dekutoski and Transfeldt, ideally the sagittal alignment line should
fall through or behind the posterior aspect of the lumbosacral disc.
Failure to achieve all of these goals will likely result in either
pseudarthrosis or partial loss of fixation at the sacropelvic junction
which often results in a fixed sagittal imbalance/kyphosis problem.
Functional results of lumbosacral fixation
long fusion to the sacrum generally relieves back pain, if a solid
fusion is achieved with established coronal and sagittal balance.
Although mobility is lost, patients will report on increase in function
if they are converted from a positive sagittal balance to neutral or
negative sagittal balance.
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