13 Dysplastic High-Grade Spondylolisthesis
Spondylolysis and spondylolisthesis are a common cause of low back pain in the adolescent. Spondylolisthesis in the child presents with deformity of the lumbopelvic segment, mal-alignment of global sagittal balance, and clinical signs and symptoms in a range of severity. The term dysplastic high-grade spondylolisthesis describes both the severity and the etiology of the deformity. High-grade olisthesis encompasses a deformity of L5 that has slipped forward on S1 by more than 50%. Dysplasia describes an etiology that is related to malformation of the L5-S1 segment of the spine, which may include a high pelvic incidence (PI) with incomplete formation of the posterior elements of L5-S1 including the facet joints. Dysplastic high-grade spondylolisthesis at L5-S1 accounts for 15% of spondylolisthesis presenting before adulthood. 1 – 3 It affects children′s health-related quality of life (HRQOL), and presents the risk of deformity progression and development of neural symptoms and dysfunction.
The evaluation and management of high-grade dysplastic spondylolisthesis is controversial, especially regarding the goal of reducing the high-grade deformity and the role for interbody support. This chapter reviews the classification and etiology of spondylolisthesis in the child, and discusses the indications for, and details of, specific surgical techniques for the management of dysplastic high-grade spondylolisthesis in children.
Numerous classification systems have been proposed in the literature for the etiology and pathology of spondylolisthesis. More recent classifications helped us develop an understanding of the concepts of sacropelvic and spinopelvic balance and their role in guiding decision making in surgical planning.
Meyerding 4 Classification
Henry Meyerding of the Mayo Clinic devised in 1938 a simple classification system based on the percentage of slippage of the vertebra relative to each other. 5 Grade 1 entails a slippage of 25%, grade 2 a slippage of 50%, grade 3 a slippage of 75%, grade 4 a slippage up to 100%, and grade 5 a slippage of more than 100% or spondyloptosis. Low-grade olisthesis generally refers to slippage of grade 2 or less and high grade spondylolisthesis refers to slippage of grade 3 or more. The advantages of this classification system are that it is easy to remember and it is highly reproducible, with good inter- and intraobserver reproducibility. However, it does not include important descriptive variables of the slippage, such as the angular kyphotic deformation at the lumbosacral junction.
Wiltse and MacNab Classification
In 1976, Wiltse et al 2 described an etiology-based classification system for spondylolisthesis that includes five types of spondylolisthesis and three subtypes of the type II ( Table 13.1 ). The discussion in this chapter is mainly concerned with the type I, dysplastic spondylolisthesis.
Marchetti and Bartolozzi 7 Classification
This is an etiology-based system proposed in 1982 that classifies spondylolisthesis into two broad categories: acquired and developmental. The two categories are further subdivided ( Table 13.2 ). This chapter is mainly focused on the high dysplastic subtype in the developmental category.
Pathological (local or systemic)
Postsurgical (direct or indirect)
Mac-Thiong et al 8 and Labelle proposed a classification system that incorporated the concept of sagittal spinopelvic balance and sacropelvic balance to help guide surgical planning. This is a very important concept for our discussion of the treatment of dysplastic high-grade spon-dylolisthesis as it has therapeutic implications for patients.
The concept of sacropelvic balance culminated from the study of various patterns of high or low PI and high or low sacral slope (SS), which affect the biomechanics at the lum-bosacral junction in different ways. Hresko used cluster analysis to define two subsets of subjects with high-grade spondylolisthesis: patients with a balanced sacropelvic relationship had a high SS and a low pelvic tilt (PT), whereas patients with a low SS and a high PT had a retroverted pelvic and vertical sacrum along with lumbosacral kyphosis and sacropel-vic imbalance.
The mechanism of compensation for various degrees of lumbosacral imbalance depends on the severity of the imbalance. As imbalance develops, an increase in lumbar lordosis maintains the center of gravity over the hips. However, as the maximal lordosis is attained, the pelvis must be retroverted to keep the center of gravity over the hips. The relationship PI = SS + PT dictates that with the decrease in SS, the PT must increase to maintain balance. According to Mac-Thiong and Labelle, once the anatomic limit of these mechanisms is reached, sagittal spinal imbalance results, forcing the subject to lean forward.
The significance of the Mac-Thiong classification is that it suggests treatment options based on grade. This treatment algorithm, however, needs to be corroborated with more evidence to support it. The low-grade group is further divided into low PI/low SS (nutcracker subtype) or high PI/low SS (shear subtype). In general, those with low-grade spondylolisthe-sis according to the Mac-Thiong classification scheme are proposed to be best served by an in situ posterolateral fusion, which has a favorable fusion rate regardless of instrumentation.
The high-grade subtype is further divided into balanced pelvis or imbalanced pelvis, as described above. Patients with imbalanced pelvis are further subdivided into those with an imbalanced spine sagittally and those with a balanced spine sagittally. Although intraob-server reliability was high for this classification system, the interobserver reliability was only moderate because of the difficulty in classifying the degree of the dysplasia.
Spinal Deformity Study Group Classification
The classification proposed by the Spinal Deformity Study Group (SDSG) is a modification of the Mac-Thiong Classification. 9 , 10 The current version of this system has been simplified when compared with the previous version. First, determine whether the slippage is low grade (Meyerding 1 or 2) or high grade (Mey-erding 3 or 4). Next, measure the sacropelvic parameters of SS and PT, and calculate PI. For low-grade spondylolisthesis, two subgroups are divided at a PI value of 60 degrees. For high-grade spondylolisthesis, the Hresko et al method is applied. Patients are assigned to these categories based on the threshold line defined in the cluster analysis. Above the line, the patient is classified as having a balanced pelvis (high SS/low PT); below the line, the patient is classified as having an un balanced pelvis (low SS/high PT). The global spinopelvic balance is also easily determined. If the C7 plumb line falls over or posterior to the femoral head, the spine is balanced. However, if it lies anterior to the femoral heads, the spine is unbalanced. Usually if the sacropelvis is balanced, the patient displays global spinal balance regardless of the grade. However, this may not be the case in patients with high-grade spondylolisthesis with an unbalanced pelvis. A reliability study was performed on this system as well. Using a computer-assisted method of identifying anatomic landmarks, the study was able to demonstrate high reliability in both inter- and intraobserver reliability ( Fig. 13.1 ).
The majority of patients with spondylolisthe-sis have lower grade degenerative or isthmic disease that can be treated conservatively. Surgery is appropriate for patients who have failed conservative measures and have a persistent neurologic deficit, intractable pain, instability, or positive sagittal balance with higher grade disease. The focus of this discussion in on the treatment of high-grade spondylolisthesis and on surgical techniques for managing dysplastic high-grade spondylolisthesis. The decision to treat high-grade spondylolisthesis with surgery is an area of controversy, and multiple techniques for reduction and fixation have been described.
Patients with high-grade slippage are usually symptomatic, with an imbalanced sacropelvic or spinopelvic relationship; surgical management is generally indicated. The primary goals of treatment in these patients are to relieve the patient′s pain and neurologic deficit and to restore the alignment when indicated. Maxi-mizing the fusion area by including anterior fusion or with transsacral fixation is typically necessary. Unbalanced deformities may necessitate reduction of the sagittal malalignment based on the proposed SDSG classification algorithm above, with the suggestion that reduction may be indicated for type 5 and type 6 deformities. In addition, inclusion of full-length lateral 36-inch scoliosis radiographs are imperative to further classify overall sagittal balance. 11 , 12
Significant controversy exists regarding the necessity of reducing the slippage, the timing of surgery, and the most effective means of achieving reduction. Although low-grade spon-dylolistheses treated with in situ fusion have shown relatively good outcomes, high-grade slippages with pelvic imbalance or spinal imbalance treated without reduction are thought to be prone to high rates of nonunion or slippage progression and patient dissatisfaction.
Advocates of in situ fusion report a high rate of back pain relief and a low risk of iatrogenic neurologic deficit. On the other hand, advocates of reduction report a higher fusion rate due to the increased surface area of bone, and improved sagittal balance. Poussa et al 13 found that the in situ fusion group had a 27% rate of pseudarthrosis. Prior studies have shown that patients who undergo reduction have satisfaction scores and outcomes similar to those of patients who undergo in situ fixation. Poussa et al also found that the in situ fusion patients did better in terms of their Oswestry Disability Index (ODI) score and had less adjacent disk degeneration and muscle atrophy as seen on magnetic resonance imaging (MRI); thus, Poussa et al concluded that in situ fusion is a safer procedure with good long-term outcomes.
Ruf et al 14 found improved radiographic parameters after reduction and fusion, and minimal risk of permanent nerve root damage. A recent systematic review of eight studies also found that reduction improved the overall spine biomechanics and was not associated with a greater risk of neurologic deficits when compared with in situ fixation.
Various methods of reduction and fusion of spondylolisthesis have been described in the literature. Some are of historical significance and are no longer practiced commonly. Some of the historical techniques included halo-femoral traction with pelvic suspension, anterior-posterior in situ fusion and placement of a pantaloons spica cast in hyperextension, use of halo skeletal traction in a staged manner, and use of Magerl′s external fixator for reduction. With advancement of surgical techniques and more powerful instrumentation, these historical techniques are no longer utilized.
Reduction of slippage includes reduction of the lumbosacral kyphosis at L5-S1 and reduction of the translation of the L5-S1 vertebrae. The highest yield reduction would be L5-S1 kyphosis in terms of restoring sagittal balance, whereas reduction of the translation helps to further improve the sagittal profile, but also increases the interbody contact area between L5 and S1, improving the fusion potential.
After reduction, fusion can be performed as interbody fusion or through a transsacral fibula graft spanning the L5 segment.
Additional points of fixation up to L4 and down to the pelvis may be needed.