Lumbosacral Fixation

Indications of lumbosacral fixation

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 spinal fusion.

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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