Lumbopelvic Fixation Techniques

Iliac fixation adds biomechanical strength in situations where the lumbosacral junction is exposed to high shear forces. Lumbopelvic fixation should be considered to reduce strain on the S1 pedicle screws in high-grade lumbosacral spondylolisthesis (Meyerding grade III or higher) cases, L5/S1 pseudarthrosis cases ( ▶ Fig. 54.1), fusions extending from L2 (or cephalad) to S1, sacral fractures ( ▶ Fig. 54.2), and cases requiring sacrectomy owing to a fracture, tumor or infection. ¹^, ²^, ³ Fusion caudal to the sacrum may be useful to limit hardware failure, pseudarthrosis, and complications at the lumbosacral junction.

Fig. 54.1 Anteroposterior X-ray demonstrating lucency around the right S1 pedicle screw following L2 to S1 fusion not supplemented with iliac screw fixation or L5/S1 interbody fusion.

Fig. 54.2 Sacral insufficiency fracture after a L4 to S1 fusion in a patient with osteoporosis as demonstrated on follow-up computed tomography.

54.1.1 Sacral Anatomy and Biomechanics

Long fusions to the sacrum are challenging because it is difficult to achieve arthrodesis, maintenance of deformity correction, and provision of a stable foundation for realignment of the spine. Pseudarthrosis rates, particularly at L5/S1, and high perioperative and postoperative complications are well documented in the literature on reconstructive lumbosacral surgeries for complex deformity. ⁴^, ⁵^, ⁶^, ⁷^, ⁸^, ⁹^, ¹⁰ Additional perioperative risks include excessive blood loss, increased risk of durotomy during exposure for revision fusions, neurologic injury, and infection.

The difficulty in achieving successful fusion may be attributed to unique sacral anatomical characteristics, as well as unfavorable biomechanical forces of the lumbosacral junction. The sacrum consists of primarily cancellous bone with a thin cortical shell. The pedicles are large, and therefore, sacral pedicle screw instrumentation may fail to engage the cortical walls. Sacral pedicle screw fixation relies on capturing the anterior and superior margin of the S1 end plate to engage cortical bone. Furthermore, the dimensions of the sacrum limit the length of screws that can be used. In addition to anatomical characteristics that make sacral fixation difficult, the lumbosacral junction is a transitional zone from a highly mobile segment to a stiff segment, which results in high stress loads being placed across instrumentation in this region. To function successfully, this segment must transmit the weight of the upper body from the spine to the pelvis and then to the femoral heads. The forces applied across this segment include axial loading up to 3 times body weight in activities of daily living, substantial shear forces that increase with a more vertical S1 endplate alignment, flexion–extension moments and torsional rotation. ¹¹ Given these anatomical and biomechanical challenges, iliac screw fixation supplements sacral pedicle screw fixation in high biomechanical stress situations, likely improves the fusion rate across the lumbosacral junction, and reduces the chances for sacral insufficiency fractures.

54.2 Patient Selection

Clinical assessment is important in proper patient selection and decision-making regarding the use of pelvic fixation. The history-taking should focus on eliciting symptoms of lumbosacral pain or of L5/S1 radicular pain, as well as characterizing the progression and rate of deformity. Smoking history should be elicited as smoking is a risk factor for nonunion. Physical examination should focus on the deformity, inspecting the standing posture in the coronal and sagittal planes. Fixed sagittal plane imbalance can be elicited by asking the patient to straighten both the hips and knees, removing compensatory movements, thus revealing the true amount of positive sagittal balance. Lateral and forward bending X-rays may be performed to assess the flexibility of the curves. Often, local palpation over the L5/S1 facet joints posteriorly at the lumbosacral junction can elicit pain if the joints are arthritic. As patients with degenerative lumbar conditions often have coexisting arthrosis of the hip, the patient may also undergo a thorough hip evaluation to elicit range of motion and hip flexion contractures.

Long-cassette films are helpful for the evaluation of the spinal balance. Assessment of the sagittal vertical axis, pelvic incidence and lumbar lordosis for mismatch, along with retroversion of the pelvis with increased pelvic tilt may help in the operative planning. Instability can be evaluated through dynamic films. The fractional lumbar curve deserves attention in determining the lowest instrumented vertebra. To stop the fusion above the lumbosacral junction at L5, the surgeon should ensure that the distal end vertebra corrects to less than 10 degrees obliquity, be neutral in rotation, and lie within the stable zone. ¹² The L5/S1 disk should be free of any significant degenerative changes to stop the fusion short at L5 and avoid sacropelvic fixation. Magnetic resonance imaging and computed tomography (CT)-myelogram are useful in determining compression of neural elements. Dual-energy X-ray absorptiometry (DEXA) scans may be used to determine the quality of bone before surgery and may identify patients at risk of osteoporosis.

54.3 Preoperative Preparation

Fusion across the lumbosacral junction demands meticulous attention to preoperative imaging findings. The width of the pedicles at L5 and S1 can be measured for appropriate screw selection from preoperative CT scans. Osteoporosis can also be anticipated based on averaging the Hounsfield units of CT cross-sections through the operated levels. ¹³ The surgeon may use this information as an aid to determine whether iliac screw placement is required to protect the sacral screws. CT-myelogram imaging may be helpful in revision instrumented cases by detailing the location and adequacy of previous decompressions. It is important to identify any prior instrumentation utilized during a previous fusion to accommodate incorporation or removal as needed.

Fusions across the lumbosacral junction are known to be associated with increased blood loss, operative time, and complication rate. ¹⁴^, ¹⁵ The surgeon should carefully evaluate the ability of the patient to tolerate the operation. If excessive blood loss is expected or the planned surgical time is longer than 10 hours, then breaking the procedure into stages may be considered. In staged procedures, we perform interbody work through anterior or lateral approaches followed by pedicle screw fixation, decompression, and osteotomy as needed on a later date. Antifibrinolytic agents may be considered to help minimize blood loss. ¹⁶ Communication with the anesthesiology team about operative duration, anesthetic needs with neuromonitoring, expected blood loss and plans for intraoperative replacement, and extubation plans at the end of the procedure are part of a successful operative plan.

A key to successful fusion across the lumbosacral junction is achieving circumferential fusion at this mobile segment. Previous studies have demonstrated that successful posterior lateral fusion across this segment does not prevent the cessation of movement across the joint. ¹⁷ Glazer et al demonstrated that adding anterior femoral ring allografts or threaded interbody cages significantly improved the intervertebral stiffness in a posterior instrumented spine. ¹⁸ This result correlates with a significantly higher fusion rate compared with posterior only fixation at the lumbosacral junction. ¹⁹

54.4 Operative Procedure

Complex adult deformity surgery forces the surgeon to carefully consider the sequence of the steps given the lengthy nature of surgery. The operating room must be set up to maximize efficiency during the procedure. We prefer the use of a Jackson table, as it best frees the abdomen of compression and establishes lordosis. The fluoroscopy unit is made available for optimal placement of screws. Screw placement is checked with intraoperative CT when available.

The posterolateral approach consists of a complete exposure of the L5 transverse process and the superior and posterior aspects of the sacral ala. Care must be taken to avoid the first dorsal sacral foramina and injury to the sacral nerve roots. We often use Cobb periosteal elevators to bluntly dissect soft tissue off the dorsal surface of the sacrum rather than Bovie electrocautery that may cause an inadvertent durotomy. If iliac screws are to be placed, we prefer a muscle-splitting approach between members of the erector spinae group to avoid extensive stripping of muscle from the ala of the sacrum. The interval between sacrospinalis and longissimus/iliocostalis is reached from the midline through the delamination of the various layers of the erector spinae from the midline lumbodorsal fascia. This interval leads to the inner surface of the posterior iliac crest and identification of the posterior superior iliac spine (PSIS).

54.4.1 Iliac Screw Fixation

The iliac screw entry point we use is 1 cm cephalad and medial to the PSIS ( ▶ Fig. 54.3). The starting point is recessed by 2 to 3 cm to avoid prominence of the screw head. We place iliac fixation using a freehand technique, using the blunt gearshift pedicle probe to pass between tables of the iliac crest. The morphology and trajectory of the iliac screw path have been well characterized with documented safety of screws 80 mm long. ²⁰^, ²¹ The screws are directed along a path from the PSIS to the anterior inferior iliac spine (AIIS) just cephalad to the sciatic notch. We have found that violation of the sciatic notch is unlikely with a freehand technique, as a dense cortical ridge of bone overlies the notch. The obturator outlet view (teardrop view) may be helpful in determining correct trajectory ( ▶ Fig. 54.4). We typically avoid placing the probe beyond 90 mm to prevent violation of the superior acetabulum. Screw diameter ranges from 7 to 10 mm based on patient anatomy. The ability to place iliac fixation ipsilateral to iliac crest harvest has been well documented. ²² The screw is usually placed before harvesting the autograft. At least 1 to 2 cm of bridging bone is typically left between the path of the iliac screw and the harvest site.

Fig. 54.3 Intraoperative view demonstrates starting points for iliac and S2 alar iliac screw fixation. The iliac starting point is 1 cm cephalad and medial of the posterior superior iliac spine. The iliac screw is already placed on this patient above the probe. The probe points the S2AI starting point , which is inferolateral to the S1 dorsal foramen, just medial the sacral iliac joint.

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