25: Sacropelvic Fixation

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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.

Sacrectomy

• 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.

Trauma

• 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

• 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

S1 pedicular screws can either be placed unicortical, bicortical, or tricortical. S1 screws should always be supplemented by distal fixation or an interbody fusion, because they are rarely used alone for fusion to the pelvis.

Unicortical fixation is not recommended because of the cancellous nature of the sacrum and a short capacious S1 pedicle. The screw can easily toggle, leading to loss of fixation and pullout.

Bicortical fixation has been the standard for many years. The classic trajectory has been parallel to the end plate of S1, with appropriate medial convergence to avoid the common iliac vessels.

Tricortical fixation by directing the screw toward the medial sacral promontory allows purchase of the dorsal cortex, the anterior cortex, and the superior end-plate cortex. This trajectory doubles the insertional torque of the bicortical screws inserted parallel to the S1 end plate.

Technique

• The starting point is just lateral to the base of the superior articular process. The starting point can be made with an awl or burr (Figure 25-8).

• To establish screw direction, a slightly curved large pedicle finder (gearshift–type) is used to sound the cancellous bone (Figure 25-9). The path of the screw should be directed anteromedially, approximately 30 to 40 degrees and superiorly toward the anterior tip of the sacral promontory. This is usually 15 degrees cephalad in the frontal plane or 5 to 10 degrees superior in the horizontal plane, depending on sacral inclination. The direction is toward the anterior tip of the promontory (Figures 25-10 and 25-11).

• The range of screw length is approximately 40 to 50 mm.

FIGURE 25-8

FIGURE 25-9

FIGURE 25-10

FIGURE 25-11

Procedure a Pearls

• The tricortical technique provides three potential points of fixation.

• The technique allows the surgeon to place longer screws into the sacrum than either the unicortical or bicortical technique.

• This technique permits the standard medial angulation, thereby allowing triangulation of the pedicle screws in the coronal and sagittal plane, increasing screw pullout strength.

• Using the large pedicle finder (Figure 25-9) is preferred over a small one or a drill because it allows more medialization of the S1 screw and, therefore, better triangulation.

Procedure a Pitfalls

• Forward trajectory will only allow a shorter screw and may place the iliac vessels and L5 nerve root at risk.

Procedure B: Sacral Alar Screws

This typically involves unicortical fixation into the cancellous bone of the ala. The screws are directed laterally in the ala. The internal iliac vessels, lumbosacral trunk, and sacroiliac joints can be injured if these screws penetrate the anterior cortex. Alar screws are currently used by some as supplemental fixation.

Technique

• The starting point is 5 mm cephalad to the S1 foramen and halfway between the distal aspect of the superior articular process and the S1 foramen (Figure 25-12, A).

• For the screw direction, the screw is angled laterally between 30 and 40 degrees and caudally 25 degrees (Figure 25-12, B). The more inferior the screw starting point, the less caudal the screw angulation.

• The screw length averages 35 mm.

FIGURE 25-12, A-C

Procedure B Pearls

• Biomechanical studies show improved fixation of sacral ala screws in conjunction with S1 pedicle screws as compared with S1 screws alone.

• The screws are connected to the same rod as the S1 screws.

Procedure B Pitfalls

• Sacral ala fixation is directed into low-density cancellous bone.

• Bicortical technique is not recommended, because the L5 nerve roots cross the sacral ala bilaterally, and the common iliac vessels are located directly on the anterior surface.

• Angulation of more than 20 degrees laterally puts the L5 root at risk for anterior cortex penetration.

• The L4 root is at risk as it crosses over the lateral one third of the anterior ala, with screw angulation more than 30 degrees laterally.

Procedure C: Iliosacral Screws

Iliosacral screws are placed through the outer table of the ilium, through the SI joint, and into the lateral sacrum. This bone channel has been well described and is regularly used in pelvic trauma. The cortical purchase at the inner and outer tables of the ilium and the entrance point to the sacrum provides greater stability to pullout. Iliosacral screws have biomechanical advantage over S1 fixation and alar screw fixation, but the development of easier and safer techniques has made their use less common.

Procedure D: Galveston Rods

In the 1980s, Allen and Ferguson (1982) introduced the Galveston technique to incorporate the ilium into the base of the fusion construct. Their instrumentation consisted of contoured smooth rods that were inserted from the posterior superior iliac spine, extending into each ilium between the inner and outer tables. The rods can be attached to the proximal levels by sublaminar wires and, more recently, by pedicle screws and hooks (Figure 25-13).

Technique

• The starting point is the posterior superior iliac spine.

• The rod orientation is approximately 30 to 35 degrees caudal and 20 to 25 degrees lateral, directed to the region above the sciatic notch.

FIGURE 25-13

Procedure D Pearls

• Most commonly used in NM scoliosis

• Implant is relatively inexpensive

Procedure D Pitfalls

• Difficult contouring

• High incidence of loosening and painful instrumentation requiring removal

Procedure E: Iliac Screws (Iliac Bolts)

Iliac screws were developed as an evolution of the Galveston technique. Iliac screws offer greater resistance to pullout secondary to the cancellous threads. Iliac screws may be partially or fully threaded and are connected to the longitudinal rods with special connectors. This is one of the most widely used techniques, when long fusions to the pelvis are performed. The modularity makes it possible to put more than one screw into the ilium, and screws can also be placed even in the face of a previous bone graft harvest. Figure 25-14 shows a schematic representation of lumbosacral fixation with S1 pedicular screws and two iliac screws.

Technique

• The starting point is slightly anterior to the PSIS. Figure 25-15 shows the intraoperative starting point of iliac screws over the posterior superior iliac spine.

• Two trajectories can then be taken for the screw direction. One trajectory is directed toward the superior portion of the acetabulum (Figure 25-16, Path A). Another path is directed toward the AIIS (Figure 25-16, Path B). The latter is preferred, because there is a lower risk for violation of the acetabulum and longer screws can be used. The trajectory is 25 degrees lateral and 30 degrees caudal. Fluoroscopy can be helpful in placement of iliac screws.

• The screw length is typically at least 80 mm, with a diameter of 7 to 8 mm.

FIGURE 25-14

FIGURE 25-15

FIGURE 25-16

Procedure E Pearls

• The iliac screws should be long enough to end anterior to the pivot point, thus allowing the force acting on them to change from inline pullout to cantilever bending.

• Partial excision of the PSIS allows the head of the screws to lie deeper within the ilium, preventing prominence.

• Iliac screws are directed toward the cortical bone of the sciatic notch, which is the strongest in the ilium.

Procedure E Pitfalls

• Iliac screws require an additional incision for placement.

• The procedure may decrease the volume of autogenous bone graft.

• The procedure requires cumbersome offset connectors (Figure 25-17).

• The procedure may cause pain because of prominence.

• The procedure requires removal in 22% of the patients (Tsuchiya et al, 2006).

FIGURE 25-17

Procedure F: Transilial Bar

In the 1960s, Harrington devised a sacral bar. At that time fixation to the bar was by hooks only, which did not control rotational stresses or flexion extension as well. Kostuik (1988) subsequently modified the Harrington sacral bar moments by using a transiliac bar fixed to S1 pedicle screws, to which proximal longitudinal members could be connected (Figure 25-18). Sacropelvic fixation using the transilial bar technique is an easy and effective method of achieving pelvic anchorage in long posterior spinal fusions but is not recommended without the use of anterior column support.

Technique

• The starting point is 1 to 2 cm anterior to both the posterior superior iliac spines.

• Regarding direction, the transilial bar is inserted in a retrograde fashion (inside–out method) to one iliac wing, then passed through both tables of the other iliac wing in a similar fashion. The bar is then attached to the S1 pedicle screws and subsequently attached to the rest of the construct.

FIGURE 25-18

Procedure F Pearls

• Partially burying the S1 screws allows ease of insertion of the bar into the iliac crest.

• Adding a gentle bend to the bar avoids impingement on the spinal canal.

Procedure F Pitfalls

• Sacral insufficiency fractures develop in osteoporotic patients. The mode of failure appears to be a shear/compression fracture at or below the S1 pedicle screws.

• Connectors can be cumbersome to place.

• There is a high rate of implant removal secondary to pain.

Procedure G: S2 Alar Iliac Screws (S2AI)

Pelvic fixation using the S2 alar iliac technique was described by Kebaish (2010) and by Sponseller and colleagues (2010) in the adult and pediatric populations, respectively. Use of the S2 alar iliac technique may address some of the issues with spinopelvic fixation. S2AI screws do not require a separate fascial or skin incision or the use of offset connectors. Placement of S2AI screws does not interfere with iliac crest harvest, while allowing the use of longer screws than the iliac bolts allow. The solid pelvic anchor provided by the S2AI technique allows performing corrective procedures at the lumbosacral junction, such as S1 and L5 osteotomies, thus achieving more linear correction of the sagittal vertical axis, and hence better sagittal restoration of sagittal balance.

Technique (authors’ preference)

• The starting point is midway between the first and second sacral foramina (Figures 25-19 and 25-20).

• Screw direction

• The trajectory is aimed lateral, approximately 40 degrees to the horizontal plane and 20 to 30 degrees caudal, depending on the pelvic tilt. Figure 25-21 is a schematic representation of the S2AI screw direction in different planes—transverse (Figure 25-21, A), coronal (Figure 25-21, B), and sagittal plane (Figure 25-21, C).

• Fluoroscopy is helpful but not always necessary; an anteroposterior view showing the pelvis and the sciatic notch is most beneficial.

• The path of the drill should be within 20 mm proximal to the greater sciatic notch and aimed toward the AIIS.

• A 2.5-mm drill is used initially through the sacral ala, and once the SI joint is crossed, a 3.2-mm drill is used to guard against breakage.

• A teardrop C-arm view, at this stage, helps ensure the anteroposterior trajectory within the thickest part of the ilium, without cortical breach.

A polyaxial screw of an average 80 to 100 mm is used; the diameter is usually 8 to 10 mm, but never less than 8 mm to avoid screw breakage.

FIGURE 25-19

FIGURE 25-20

FIGURE 25-21, A-C

Procedure G Pearls

• Insertion of S1 screws first will ensure a starting point that allows in-line rod placement.

• Using the greater trochanter as a landmark allows safe placement without fluoroscopy in most patients. Figure 25-22 is an intraoperative view showing the direction of drilling of the S2AI screw toward the greater trochanter, which may be felt by the surgeon’s opposite hand.

• Tap drilling and ensuring there is a hard end point during advancement of the drill will prevent violation of the cortices of the ilium.

• Placing a guidewire after the initial drilling could minimize the use of a C-arm and guarantee a correct trajectory.

• A teardrop view will guarantee a correct trajectory in difficult pelvic anatomy (Figure 25-23, A).

• Keeping the screw pathway just above the greater sciatic notch allows the biggest screw diameter (Figure 25-23, B)

• Partially burying the screw head to avoid prominence allows, on average, 15-mm deeper implants than with iliac screws.

• S2AI screws can be placed percutaneously by a minimally invasive approach (Figure 25-24).

• Figure 25-25 shows postoperative radiography of double S2AI screws.

FIGURE 25-22

FIGURE 25-23, A-B

FIGURE 25-24

FIGURE 25-25

Procedure G Pitfalls

• Lateral cortical penetration is the most common misplacement.

• There appears to be no significant effect on the SI joint at 2 years; however, the long-term effect of violating the SI joint is unknown, and longer follow up may be needed.

Postoperative Care and Expected Outcomes

Complications of Pelvic Fixation

Misplacement and injuries to adjacent structures

• Injuries to structures in the sciatic notch, including the superior gluteal artery and the sciatic nerve, are rare.

• This complication can be avoided by paying close attention to the anatomy of the pelvis and using external bony landmarks. Some clinicians advocate placing a finger or a blunt instrument in the notch, although doing so would require additional dissection.

• Fluoroscopy can also be very helpful, especially in patients with unusual anatomy, and until the surgeon becomes more experienced with the procedure.

• Maintenance of a bony end point and the correct measurement, and choice of the screw size, will avoid medial or lateral cortical violation.

Implant prominence and loosening

• Implant prominence is a major challenge in small patients and in those with compromised soft tissues, as is the case with children with neuromuscular deformities.

• Loosening continues to be a problem with the Galveston technique but appears to be less common with S2AI and iliac screws. It may not cause symptoms, especially if the fusion is already complete.

Wound complications and infection

• Techniques requiring extensive surgical exposure are associated with a higher risk of wound complications, including infection.

• One study (Tsuchiya et al, 2006) of 81 patients treated with iliac screw fixation reported a 4% infection rate.

• A recent study (Sponseller et al, 2010) found no wound infection in 27 children who underwent pelvic fusion with the sacral alar iliac technique. This finding may have been related to the minimum dissection needed with this procedure, which preserved the soft tissue envelope.

Nonunion and implants failure