25: Sacropelvic Fixation

Published on 23/04/2015 by admin

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Last modified 23/04/2015

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Procedure 25 Sacropelvic Fixation

Khaled Kebaish, Mostafa H. El Dafrawy

Indications

Long spinal fusion to the sacrum

• Rigid sacropelvic fixation serves as the foundation for thoracolumbar reconstructions that extend to the sacrum. It is often used in the treatment of complex lumbosacral spinal conditions.

Neuromuscular (NM) scoliosis

• The orthopedic spine literature supports pelvic fixation as the current standard of care of neuromuscular scoliosis correction. Conditions include Duchenne muscular dystrophy, spinal muscle atrophy, limb-girdle muscular dystrophy, myelomeningocele, and spinal cord injury. Indications for surgical treatment include unstable sitting posture, caused by coronal and/or sagittal imbalances and pelvic obliquity. Other indications for surgery include progressive kyphoscoliosis and compromising respiratory function.

Degenerative spinal deformities

• Spinal deformities involving the lumbosacral junction requiring corrective osteotomies are the most common indication for pelvic fixation. Oblique takeoff of L5, adult degenerative scoliosis, revision decompression surgery, and postlaminectomy flat back syndromes represent the majority of these cases. Along with advanced degeneration of the L5-S1 motion segment, these conditions cause lumbosacral instability, exerting huge biomechanical stresses on the construct; thus extending the fusion to the pelvis is a prerequisite to achieve and maintain the correction.

Spondylolisthesis (high grade)

• During reduction of grade 3 and 4 isthmic spondylolisthesis of the lumbosacral junction, strong consideration should be given to protect the sacral S1 screws both with interbody cages and pelvic fixation screws.

• Partial or complete sacrectomy in treatment of secondary and primary sacral tumors, including chordoma, chondrosarcoma, and some giant cell tumors, causes severe destabilization of the lumbopelvic region. Reconstruction requires restoring the continuity of the pelvic ring and spinal column integrity with a mechanical construct that can transfer loads from the spine to the pelvis.

• Fractures of the lower lumbar vertebra and sacral fractures with spinopelvic dissociation

Indications Pitfalls

• Variable anatomy of the sacrum and pelvis may add challenge to the procedure.

• The sacrum has poor bone quality, mainly cancellous and often osteoporotic.

• The sacropelvis functions as a unit; this segment must transmit the full weight of the body from the spine to one or both femoral heads. It must do this in a static environment, such as sitting or standing, and in a dynamic environment, such as walking and running.

• Large loads going through this segment, creating cantilever pullout forces, are the primary reasons for construct failure.

Indications Controversies

• Long fusion has been described by some authors as four levels extending to L2, while others define a long fusion as one that crosses the thoracolumbar junction.

• The long lever arm generated by incorporating more segments into the fusion requires additional fixation points to achieve a rigid construct at the lumbosacral junction.

• High risk of pseudarthrosis (9% to 41%), instrumentation failure (3% to 44%), and loss of lumbar lordosis (20% to 49%) are associated with fusion down to the sacrum.

• Terminating a long fusion at L5 might lead to subsequent degeneration of the L5-S1 disk and might impact sagittal balance.

• Degeneration of the sacroiliac (SI) joint, although so far not clinically significant, might be a concern in patients with implants violating the sacroiliac articulation (iliac and S2AI screws).

Biochemical Considerations

The lumbosacral junction is a transition from a highly mobile segment to a stiff segment, resulting in great stress concentrations when the biomechanics of this segment are altered by instrumentation and fusion. Forces include axial loading of up to 3 times body weight in activities of daily living; substantial shear, especially with a more vertical S1 end-plate alignment; flexion/extension moments; and torsion.

The following three concepts are crucial in understanding the biomechanics of lumbosacral fixation and its relevance to the stiffness of the cephalad construct.

Lumbosacral pivot point (McCord et al, 1992)

• Based on an ex vivo biomechanical model, McCord defined the pivot point for the flexural lever arm near the middle osseoligamentous column at the L5-S1 disk. Figure 25-1 shows the McCord pivot point in schematic sagittal and transverse sections in relation to S1, S2, and iliac screws.

• Stiffness increases as the implant extends anterior to this point.

Zones of sacropelvic fixation

• O’Brien and colleagues (2004) identified three distinct zones of the sacropelvic region. Fixation strength improves progressively from zone 1 to 3. Figure 25-2, A shows a schematic coronal section of the zones of sacropelvic fixation as defined by O’Brien. Figure 25-2, B shows a schematic sagittal section of different sacropelvic fixation techniques in relation to the three zones of O’Brien and the McCord pivot point).

• Zone 1: S1 vertebral body and the cephalad margins of the sacral alae

• Zone 2: the inferior margins of the sacral alae, S2, and the area extending to the tip of the coccyx

• Zone 3: both iliac bones

Triangulation of the screws

• Pedicular screws triangulation had been shown to significantly enhance load pullout. Directing the pedicle screws medially ensures maximum amount of triangulation, which increases stability at the bone–metal interface. With triangulation of screws, the strength of fixation is determined by the amount of bone within the area between the two screws (Figure 25-3).

FIGURE 25-1

FIGURE 25-2, A-B

FIGURE 25-3

Treatment Options

• S1 pedicular screws

• Sacral alar screws

• Iliosacral screws

• Galveston rods

• Iliac screws

• Transilial bar

• S2 alar iliac screws

Examination/Imaging

Pelvic obliquity and sacral inclination should be noted, especially in cerebral palsy patients.

Obtain spot lateral and true anteroposterior films of sacrum.

It is useful to visualize the sacral promontory, superior articular process (SAP), sacral foramina, and the first to third sacral segments.

For long spinal fusion to the pelvis, radiographic evaluation should include long-cassette, erect posteroanterior and lateral views to determine balance.

In patients with dural ectasia, variable anatomy or revision cases, attention should be paid to the proximity of the abnormality to the entry point of the screws to avoid injury of the neural elements.

Obtain a dual emission x-ray absorptiometry (DEXA) scan, especially in females greater than 50 years of age, because bone density correlates with screw pullout forces.

Intraoperative teardrop view of the pelvis allows safe placement of iliac and sacroiliac screws in the bony canal between the posterior superior iliac spine (PSIS) and anterior inferior iliac spine (AIIS). Figure 25-4 shows an intraoperative radiograph showing a teardrop view with the guidewire within an all-osseous channel between the PSIS and AIIS.

An intraoperative teardrop view is obtained by a combined obturator oblique–outlet view, with an approximately compound 45-degree anterior and 45-degree cephalad angulation of the beam. Figure 25-5 is a schematic representation of the position of the pelvis while obtaining a teardrop view with superimposed shadows of the PSIS and AIIS and iliac rim above the sciatic notch.

FIGURE 25-4

FIGURE 25-5

Portals/Exposures

The standard posterior midline approach is used as in fusion to the sacrum. Depending on the adopted technique of sacropelvic fixation, additional exposure might be necessary.

With S2AI screws, expose the S2 foramen.

With iliac screws, a separate fascial incision might be used over the PSIS.

Procedure A: S1 Pedicle Screws

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