Length of Construct

Although an appropriately sized construct for the pathology at hand is not exactly a complex technique, it is critical to optimizing patient outcome and avoiding revision surgery. Oversizing the construct means that the patient has undergone more surgery than is required and it has likely taken away normal motion and load sharing through the extrainstrumented segments. Undersizing the construct can place abnormal loads on the segments adjacent to the instrumentation and lead to adjacent segment failure. There are few rules to guide the surgeon in determining the length of the construct, but in general, the construct should include the Cobb angle of the curve and should not stop at the apex of a kyphosis. To put it a different way, instrumentation of spinal curves, whether in the coronal or sagittal plane, should include the entire curve so as not to place abnormal stress on a segment that is not normally acting in transition. Also, stopping an instrumented spine fusion at the junction between a mobile and a nonmobile segment should be done only with great thought and consideration of the stresses placed on the adjacent segment and the risk of a junctional kyphosis.

In adults, including the L5-S1 disc space in the fusion should be considered in any long construct that otherwise would have ended in the caudal lumbar spine. As outlined by Bridwell, indications for fusion to the sacrum in adults in a long construct include (1) L5-S1 spondylolisthesis, (2) previous L5-S1 laminectomy, (3) central or foraminal stenosis at L5-S1, (4) oblique takeoff of L5, and (5) “severe” degeneration of the disc.¹ Although fusing to the sacrum does decrease a significant amount of motion, there is a significant risk of adjacent segment degeneration when the construct is stopped at L5 in the adult degenerative deformity population. A retrospective study by Edwards et al. found that in a population of patients who had undergone a thoracolumbar construct that ended caudally at L5 who preoperatively had a “healthy” L5-S1 disc, 61% of patients had developed advanced degenerative disease at this level over a mean follow-up of 5.6 years.²

Because of the risk of pseudarthrosis at the L5-S1 space, many authorities have advocated for the use of interbody support at this level. Polly et al. found that L5-S1 interbody support increases biomechanical stability, restores junctional lordosis, improves the lumbosacral fusion rate, and increases disc and foraminal height, thus decreasing foraminal stenosis.³ In the adult deformity patients undergoing spine fusion, the restoration or maintenance of sagittal balance has been found to have a significant impact on outcome.⁴ Restoring junctional lordosis at L5-S1 through placement of an interbody graft is a powerful technique to correct sagittal imbalance. This is due to the long moment arm that occurs when the L5-S1 disc space angulation is altered relative to the C7 vertebral body on which sagittal balance is based. To maximize the footprint of the graft as well as the restoration of lordosis, an anterior lumbar interbody fusion (ALIF) is a better procedure than a transforaminal lumbar interbody fusion (TLIF) at L5-S1.⁵ One must weigh the benefit of a larger interbody device against the morbidity of an anterior approach when selecting an anterior approach versus a TLIF or posterior lumbar interbody fusion (PLIF).

Long constructs that extend to the sacrum can be problematic from a fixation standpoint. Although the S1 pedicles are large, the sacrum is composed of primarily cancellous bone, resulting in a decreased pull-out strength compared with other pedicle screws. This has prompted many to attempt to place bicortical pedicle screws to capture the anterior and posterior cortical bone to increase the strength of the screw. Lehman et al. showed that the highest bone mineral density, and therefore the greatest insertional torque for pedicle screws, was in the anterior sacral promontory, so that from a biomechanical standpoint, the strongest S1 screws are the so-called “tricortical” screws. When screws are placed in this fashion, there is almost a 99% increase in the insertional torque.⁶ In addition to an L5-S1 interbody and the placement of tricortical S1 pedicle screws, iliac screws have been advocated as another method to offload the S1 pedicles to allow for a solid arthrodesis at L5-S1. Indications for iliac screws include constructs greater than three levels that end in the sacrum, revision surgery for L5-S1 pseudarthorsis, high-grade spondylolisthesis, or trauma or pathology that does not allow for adequate sacral fixation.⁷ Kuklo et al. evaluated 81 patients who underwent fusion procedures that included the L5-S1 space using S1 and iliac screws. Approximately 50% (n = 42) were for isthmic spondylolisthesis, whereas the remainder (n = 39) were for long constructs to the sacrum. The researchers found that even in patients who had previous iliac crest grafts taken, 94% (34/36) had iliac screws placed without screw loosening or iliac crest fracture. Overall, the fusion rate, to include revision surgeries, was 95.1%.⁸

Junctional Kyphosis

One of the reasons to plan and execute an appropriate-sized construct is to avoid proximal junctional kyphosis (PJK). In 1999, Lee et al. looked at 69 patients with adolescent idiopathic scoliosis (AIS) who underwent fusion up to T3. The investigators defined PJK as greater than 5 degrees above the summed normal of the angular segments from the proximal instrumented segment to T2. They found a 46% incidence of PJK. As would be expected, a predictor of postoperative PJK was preoperative kyphosis of more than 5 degrees above the proposed proximal instrumented level, indicating that these levels should be included in the construct to avoid this complication.⁹

In the adult deformity population, Kim et al. evaluated 161 patients with a minimum 5-year follow-up who had undergone long (more than five segments) dorsal constructs to determine the incidence and outcomes associated with PJK. The researchers found that at mean follow-up of 7.8 years, there was a 39% incidence of PJK defined either as a proximal junction sagittal Cobb angle of more than 10 degrees or as a proximal junction sagittal Cobb angle at least 10 degrees greater than the preoperative measurement. The time periods most notable for worsening of PJK were at 8 weeks postoperatively (59%) or after 2 years until final follow-up (35%). Scoliosis Research Society outcome measures did not show any significant difference in those with PJK as compared with those without except with self-image when PJK was more than 20 degrees. Age older than 55 years and combination anterior-posterior surgery were the only significant risk factors identified.¹⁰ The time course between the identification of PJK suggests two different populations, given that one group was relatively close to surgery, whereas the other was more remote. Further understanding of the similarities and differences of the early and late group may lead to changes in treatment strategies to better prevent PJK.

Several methods can be used to reduce the risk of PJK in addition to how to determine an appropriately sized construct. Some experts have advocated the use of percutaneous screws in the proximal construct to avoid iatrogenic trauma associated with exposure of the soft tissues, specifically the proximal facet joint. This technique involves placing percutaneous pedicle screws, typically through the fascia layer of the most proximal screws and subcutaneously passing the rod through these proximal screws. Although theoretically this helps protect the soft tissue envelope, there have been no studies to demonstrate its effectiveness on reducing PJK.

Another technique used to avoid PJK as well as strengthen the screw pull-out strength is vertebroplasty. Recently there has been some controversy over the effectiveness of this procedure in the setting of compression fractures, but the concept is applied differently in deformity surgery. At the proximal and sometimes distal screws, polymethylmethacrylate (PMMA) is injected under fluoroscopy into the screw tracks or through the screws themselves if the screw design allows it. Care is taken that there are no pedicle wall breaches allowing the egress of PMMA into the spinal canal or the production of embolic material. Once the PMMA is in place, typically 1 to 2 mL per side, then the pedicle screw is placed. Another scenario is to perform vertebroplasty at the level cephalad to the proximal instrumented vertebrae to prevent compression fractures leading to PJK (Fig. 151-1). Alternatively, this procedure can be performed in a postoperative setting. Preoperative PMMA augmentation of the planned proximal instrumented bodies is not recommended because it increases the difficulty of placement of the instrumentation.

FIGURE 151-1 This is an example of vertebroplasty in the supra-adjacent segments of a T10 to the pelvis construct in a patient with osteoporosis.

In a cadaveric study designed to look at effects of cement augmentation of pedicle screws compared with extension with a flexible rod, Tan et al. found that in a corpectomy model, cement augmentation significantly reduced the range of motion and resulted in a more stable construct.¹¹ In another cadaveric study, Becker et al. found that pedicle screws augmented with PMMA in an osteoporotic model had increased pull-out strength compared with non-PMMA augmented screws.¹² In a cost-effectiveness analysis published in 2008, Hart et al. evaluated 28 women older than 60 years of age who had undergone fusion to the thoracolumbar region. Fifteen of these patients had undergone vertebroplasty in the adjacent level cranial to the proximal instrumented vertebrae. Proximal collapse occurred in none of the patients who had PMMA augmentation and in two (15.3%) patients who did not. Assuming a 15% decrease in the incidence of this problem, the researchers determined that the cost to prevent a single proximal junctional collapse was $46,240 using vertebroplasty, whereas the cost of revising the instrumentation in a patient with proximal junctional collapse was $77,432.¹³

As with any spinal pathology, there is clinical and radiographic PJK. PJK seen on follow-up films that is not progressive and does not significantly alter the patient’s sagittal balance can be followed radiographically. However, progressive PJK that results in alterations in sagittal balance, instability, or becomes painful should be addressed. If the PJK occurs in the upper lumbar or lower thoracic spine, then the revision strategy focuses on extension of the fusion to either the lower thoracic spine (T10) or upper thoracic spine (T4), respectively. By crossing the thoracolumbar junction and not stopping at the apex of a curve, the surgeon can minimize an additional junctional failure. However, the repair strategy becomes more involved when the PKJ occurs in a construct that already ends in the proximal thoracic spine. Often, this requires extension into the cervical spine. At our institution, this is often done in a staged setting. In the first stage, dissection of the proximal instrumented level to the appropriate level needed in the cervical spine occurs with placement of lateral mass screws in the cervical spine and pedicle screws placed in the remaining uninstrumented thoracic levels. Appropriately sized osteotomies are performed across the kyphosis. In some cases, osteotomies of the 1st and 2nd ribs are also completed in the first stage. A temporary rod is then placed across the cervical junction so that distractive forces are not placed across the cervical spine but instead across the kyphotic proximal thoracic spine. The wound is closed, and the patient is placed in halo traction while being monitored in the ICU. Serial radiographs and examinations are performed while weight is added to the traction until either there is reduction of the kyphosis, a change in the examination, or unwanted proximal distraction. The patient is then locked into position with a halo vest. The patient returns to the operating room where the osteotomies are enlarged or additional osteotomies are placed if needed. Permanent rods are then placed and the construct is compressed across the osteotomies to achieve final correction.