12.1 Why Is Sagittal Balance So Important in Spondylolisthesis?

Spondylolysis is a defect in the pars articularis of a vertebra. It can occur independently or in association with spondylolisthesis, most frequently at the level of L5-S1. Spondylolisthesis is the forward displacement of one vertebra with respect to the adjacent caudal vertebra. Spondyloptosis is a 100% translation of one vertebra on the next caudal vertebra.

This chapter focuses on developmental spondylolisthesis and on the less frequently encountered stress fracture, the two most frequent types seen in children, adolescents, and young adults. It is not meant to be a comprehensive review of spondylolisthesis but to focus on sagittal balance as it applies to this disorder and show how it can help to better understand physiopathology, classification, and treatment. It is assumed that the reader has the basic understanding of the disease.

In the past two decades, there has been much development in the understanding of the disorder, as significant new knowledge on sagittal spinopelvic balance has been acquired. In 2005, the Spine/Scoliosis Research Society summary statement in the Spine focus issue devoted to spondylolisthesis ¹ stressed the point that “…global sagittal plane alignment is important in both adult and pediatric patients with spondylolisthesis. In patients with high-grade developmental spondylolisthesis, this has provided a compelling rationale to reduce and realign the spondylolisthesis deformity, thus restoring global spinal balance and improving the biomechanical environment for fusion.” Improved understanding of the complex relationship between spondylolisthesis and human standing posture was gained by stopping to concentrate on the local L5-S1 junction and rather focus on the global sagittal picture using long-standing lateral sagittal X-rays of the spine and pelvis and, more recently, with full-body sagittal radiological images using EOS low-radiation technology ² (Fig. 12‑1).

From the evolutionary standpoint, two important observations indicate the importance of sagittal balance in the development of this disorder. First, although many authors have searched for a spondylolytic lesion at birth, a pars defect has never been reported in a newborn. The earliest cases of spondylolysis have been found in children between the ages of 6 weeks and 10 months.

In a prospective study of 500 first-grade children, Fredrickson et al ³ found a prevalence of spondylolysis of 4.4% at 6 years old, 5.2% at 12 years old, 5.6% at 14 years old, and 6% in adulthood. Thus, spondylolysis/listhesis is intimately linked to the standing posture. Tardieu et al ⁴ have studied how sagittal balance is acquired during bipedal gait acquisition by comparing neonatal and adult pelvises in 3D (Fig. 12‑2). During gait acquisition, the relationship between the sacrum and acetabula are modified through the mobility and malleability of the S-I joints, as indicated by very significant changes in pelvic incidence (PI) values. Before walking, hip flexion and anterior vertebral flexion induce an anterior location of the trunk center of gravity. After walking, femoral extension and lumbar lordosis induced by muscular actions increase sacral slope (SS) and PI, creating a backward displacement of the center of gravity behind the femoral heads, a basic characteristic of human standing posture, which will set and control all spinopelvic relationships from childhood to adulthood.

Second, spondylolysis has not been reported in quadrupeds, only in bipeds. Furthermore, there are no known cases reported in nonambulatory humans. Evolution from the quadrupedal to the bipedal posture in primates and humans has been allowed by progressive and very significant changes in the shape and position of the pelvis and spine and of their supporting ligaments and muscles (Fig. 12‑3). A quadruped has no lumbar lordosis and a more longitudinal and narrow-shaped pelvis. In sharp contrast, a human has a well-developed lumbar lordosis and a much “rounder” pelvic shape, a situation that has gradually evolved in primates along with the transition to the bipedal posture. These changes in shape and morphology of the pelvis are crucial to the understanding and management of spondylolisthesis, a disorder that is closely linked to the bipedal posture and associated with activities involving a lordotic effect on the lumbar spine, such as gymnastics.

12.2 Why Standard Spondylolisthesis Classification Systems, although Useful, Are Insufficient to Understanding Physiopathology and Help Guide Treatment

The most commonly used classification systems are the Meyerding, ⁵ the Wiltse, ⁶ and the Marchetti and Bartolozzi ⁷ classifications.

Meyerding described the simplest system of grading, and the least comprehensive classification, which is based only on the severity of the forward displacement of the cranial vertebra with respect to the caudal vertebra, with no consideration for the very important lordotic or kyphotic relation between the two. The caudal vertebra is divided into four parts. Grade I means a translation of the cranial vertebra of 0% to 25%, grade II of 25% to 50%, grade III of 50% to 75%, and grade IV of 75% to 100%. Grade V was added later, describing the ptosis of the cranial vertebra. Unfortunately, he did not specify the landmarks to use on the cranial vertebra. Bourassa-Moreau et al ⁸ have demonstrated that various landmarks have been used by many authors, creating significant variations on the interpretation of slip severity caused by the various lordotic and kyphotic relationship of L5-S1. Consequently, they proposed a standardized method, shown in Fig. 12‑4, which is recommended when using the Meyerding classification.

Wiltse divided spondylolisthesis into five types, based on radiological findings. It is useful to differentiate between the various etiologies of this pathology:

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  Type I (dysplastic) involves a congenital defect of the lumbosacral facet joints.

  
  
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  Type II (isthmic), where the lumbosacral facets are normal but the listhesis is caused by a defect in the pars. The subtypes are IIA with a stress fracture, IIB with a pars elongation, and IIC with an acute pars fracture.

  
  
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  Type III (degenerative) is secondary to degenerative osteoarthritis of the facet joints and intervertebral disk.

  
  
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  Type IV (traumatic) results from an acute fracture of posterior elements other than the pars interarticularis.

  
  
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  Type V (pathologic) is associated with destruction of posterior elements as result of a systemic or local bone disease.

Marchetti and Bartolozzi developed a classification system based on the developmental origin versus the acquired forms of spondylolisthesis, thereby providing some insight into the etiology and prognosis of spondylolisthesis:

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  Developmental spondylolisthesis is divided into two major types (high and low dysplastic), depending on the severity of bony dysplastic changes present in the L5 and S1 vertebrae and on the risk of further slippage. Dysplastic facet joints and spina bifida of L5 and/or S1 are frequent in both types, but, in addition, the high dysplastic type is associated with significant lumbosacral kyphosis (LSK), trapezoidal L5 vertebra, hypoplastic transverse processes, and sacral doming with verticalization of the sacrum, while the low dysplastic type is associated with a relatively normal lumbosacral profile, a rectangular L5 vertebra, preservation of a flat upper endplate of S1, and no significant verticalization of the sacrum.

  
  
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  Acquired spondylolisthesis is secondary to trauma, surgery, a pathologic disease, or a degenerative process: the traumatic form can be a result of either an acute or stress fracture. Typically, a stress fracture occurs in young athletes and is distinct from the isthmic dysplastic type of spondylolisthesis.

Thus, the Meyerding classification is useful, but insufficient, for grading severity of the displacement. The other two classifications are useful to identify the underlying pathology, but they are of little help in understanding physiopathology or guiding surgical treatment. As stated earlier, this chapter focuses on developmental spondylolisthesis (Marchetti and Bartolozzi) and on the less frequently encountered Wiltse type II stress fracture, the two most frequent types seen in children, adolescents, and young adults.

12.3 Etiology and Physiopathology Are Better Explained Using Sagittal Balance

The exact etiology of spondylolysis/listhesis remains unknown but it is most likely multifactorial, as various hereditary, traumatic, biomechanical, growth, and morphological factors have been reported to play a role. ⁹ They are shown in Fig. 12‑5. Among many of these factors, sagittal balance plays a crucial and central role, which will be discussed further.

12.3.3 Growth Plate Disturbance

Progression of spondylolisthesis mainly occurs during skeletal growth and is less likely after skeletal maturity. The risk of slip progression is generally low (4% to 5%). Factors associated with an increased risk of slip progression include female gender, presentation at a young age, severity of slip at presentation, developmental type, increased slip angle, and a high degree of bony dysplasia. If slip severity is less than 30% at presentation, then further slipping is unlikely.

Remaining growth is also an important predictor of progression. Previous studies support the role of a biomechanical weakness in the vertebral growth plate as an important mechanism in slip progression. In the growing child, increased stress in the L5-S1 disk can be associated with bony remodeling through the growth plates, particularly the upper and anterior endplate of S1. Involvement of the upper S1 endplate can further contribute to the progression of the spondylolisthesis in a process similar to the progression of Blount’s disease where progressive tibia vara develops as a result of asymmetric pressures on the growth plate in the upright position (Fig. 12‑6). This process explains the frequent development of sacral doming and the trapezoidal shape of L5 frequently encountered in high-grade spondylolisthesis (HGS). Once again, the standing posture is crucial to explain how these changes occur and how asymmetrical pressures on the anterior part of the growth plates create secondary doming of the sacrum in HGS. ¹⁰

12.4 Hip-Spinopelvic Balance and Morphology

Sagittal sacropelvic morphology and orientation modulates the geometry of the lumbar spine and, consequently, the mechanical stresses at the lumbosacral junction. In L5-S1 spondylolisthesis, it has been clearly demonstrated over the past decade that sacropelvic morphology is frequently abnormal and that, combined with the presence of a local lumbosacral deformity and dysplasia, can result in an abnormal sacropelvic orientation as well as in a disturbed global sagittal balance of the spine. ¹ These findings have important implications for the evaluation and treatment of patients with spondylolisthesis, especially for those with a high-grade slip.

When compared with normal populations, pelvic morphology is clearly abnormal, as indicated by PI, which is significantly higher ^{11 ,​ 12 ,​ 13} in spondylolisthesis, and the difference in PI tends to increase in a direct linear fashion as severity of the spondylolisthesis increases. ¹¹ The cause–effect relationship between pelvic morphology and spondylolisthesis remains to be clarified. By virtue of the relationship between morphology (PI) and spinopelvic balance (pelvic tilt [PT], SS, L5I, LSK, C7 plumb line, etc.) described in previous chapters, all other measures of spinopelvic balance are also significantly different in control populations compared to subjects with L5-S1 spondylolisthesis, ^{11 ,​ 13} especially in HGS where the relationship between the spine and the pelvis is distorted (Table 12‑1). Strong correlations are found between PI, L5I, PT, SS, and LSK and lumbar lordosis. ¹⁴ In high-grade slips with sacral doming, the measurement of PI, SS, and LSK is not as reliable, as they depend on the sacral plate, which is then irregular and domed. Contrarily, L5I (Fig. 12‑7) is not affected by sacral doming as it is measured with the superior L5 endplate, which offers a better inter- and intrarater reproducibility over the superior S1 endplate. ¹⁵ It is thus the preferred measurement to assess the disturbed relation between the spine and pelvis, particularly in HGS. Similarly, PT is a more reliable measure in HGS, as it does not depend on the sacral plate.

In a static standing position, the way SS and PT balance refers to the concept of sacropelvic balance. Members of the Spinal Deformity Study Group (SDSG) have specifically investigated sacropelvic balance in low-grade spondylolisthesis (LGS) and HGS. Roussouly et al ¹⁶ proposed two different subgroups of sacropelvic balance observed in subjects with LGS, which could be related to the etiology. In their opinion, patients with high PI and SS have increased shear stresses at the lumbosacral junction, causing more tension on the pars interarticularis at L5, and ultimately a pars defect (Fig. 12‑8). Conversely, patients with a low PI and a smaller SS have impingement of the posterior elements of L5 between L4 and S1 during extension, thereby leading to repetitive impingement of the pars interarticularis of L5 by the posterior facets of L4 and S1 during extension movements, a “nutcracker” effect on the L5-S1 pars (Fig. 12‑8). The clinical relevance of these findings is that because PI is always much greater than normal in HGS, ¹² it is assumed that the risk of progression in the low-grade subgroup with a normal PI is much lower than in the subgroup with an abnormally high PI value. It is hypothesized that the subgroup with normal PI corresponds to acquired traumatic cases with an acute or stress fracture (Marchetti and Bartolozzi ⁷ classification) in subjects with a normal sacropelvic morphology, whereas the other subgroup with high PI is associated with more dysplastic developmental cases. As for HGS, Hresko et al ¹⁷ have identified two subgroups of patients: balanced versus unbalanced pelvis (Fig. 12‑9). The “balanced” group includes patients standing with a high SS and a low PT, a posture similar to the subgroup of normal individuals with high PI, whereas the “unbalanced” group includes patients standing with a retroverted pelvis and a vertical sacrum, corresponding to a low SS and a high PT. Each new subject with HGS can be classified by using the raw SS and PT values or, in borderline cases, by using the nomogram (Fig. 12‑10) provided by Hresko et al. ¹⁷ Recently, Sebaaly et al ¹⁵ have demonstrated that L5I and /or PT values can be used reliably and more easily to identify these two basic pelvic postures. Subjects with PT values ≤25° and/or L5I values ≤60° are in the balanced pelvis group. Subjects with PT values >25° and/or L5I values >60° belong to the unbalanced group.

In a static standing position, the way the spine and the pelvis balance themselves refers to the concept of spinopelvic balance. By using a postural model of spinopelvic balance showing the relationships between parameters of each successive anatomical segment from the thoracic spine to the sacropelvis, Mac-Thiong et al ¹⁸ have observed that a relatively normal posture is maintained in LGS, whereas posture is clearly abnormal in HGS. In HGS, the spinopelvic balance is particularly disturbed in the subgroup with an unbalanced sacropelvis, as described by Hresko et al. ¹⁷ They also reported that for most patients with spondylolisthesis (low grades and balanced high grades), the global spinopelvic balance (position of C7 vertebral body over the femoral heads) was relatively constant with the C7 plumb line projecting behind the femoral heads, regardless of the local lumbosacral deformity and particularly with the alignment of the C7 plumb line with respect to S1, indicating the predominant influence of the sacropelvis in the achievement of a normal global spinopelvic balance.

A few studies have correlated spinopelvic and sacropelvic balance with health-related quality of life (HRQoL) measures. Tanguay et al ¹⁹ have demonstrated that increased LSK has a significant association with a decrease in the physical aspect of the quality of life for patients with adolescent L5-S1 spondylolisthesis. The effect of LSK is particularly important for patients with HGS, independent of the slip percentage. Therefore, LSK values should be included in the routine evaluation of patients with spondylolisthesis, to fully appreciate the severity of the deformity and its clinical impact on the quality of life of patients. Glavas et al ²⁰ studied techniques of LSK measurement used in the literature and found that the technique described by Dubousset (DUB LSK; Fig. 12‑7) was the only measurement of LSK that showed a gradation along the spectrum from normal to HGS. They also found that DUB LSK was the most correlated to slip percentage. Moreover, they proved that it had the best inter- and intrarater reliability, and the upper sacral endplate doming does not interfere with its measurement. Correlation between DUB LSK and L5I is strong both in the normal population ¹⁵ and in the HGS population. Harroud et al, ²¹ in a cohort of subjects with HGS, have noted that an increasing positive sagittal alignment was related to a poorer SRS-22 total score, especially when the C7 plumb line (C7PL) is in front of the hip axis. Global sagittal alignment using C7PL should therefore always be assessed in patients with HGS.

Recently, Mac-Thiong et al ²² reported on the importance of evaluating the presence of hip flexum in the standing position using the proximal femoral flexion angle ([PFA]; Fig. 12‑11). PFA is significantly higher in HGS subjects compared to normals. A PFA ≥10° is proposed as a criterion to define abnormal PFA. PFA was increased in HGS and increased along with deteriorating sagittal balance and HRQoL.