10 Spondylolisthesis: Classification and Natural History
L5-S1 spondylolisthesis is the forward displacement or slippage of the fifth lumbar vertebra on the sacral plateau. This anatomic condition was first described by Herbiniaux, a Belgian obstetrician, in 1782. L5-S1 spondylolisthesis may be acquired or developmental, and usually presents in early childhood. Spondylolisthesis may occur either associated with developmental defects of the anterior or posterior elements (developmental or dysplastic type) or after a spondylolysis, that is, a fracture of the pars interarticularis (acquired or isthmic type). The incidence of unilateral or bilateral pars interarticularis defects is reported to be ~ 4% in children at the age of 6 years, increasing to 6% in adults. 1 Spontaneous healing is almost only in a unilateral pars defect. Bilateral lysis may result in spondylolisthesis because of the mechanical failure of the posterior arch or because of a round sacral dome.
This chapter discusses our current knowledge of the natural history of spondylolisthesis, and reviews the most pertinent classification systems. The discussion of the natural history highlights the role of patient age at diagnosis and the potential worsening of the slippage and deformity throughout life. We also propose a new approach integrating patient age in the characterization and prognostication of spondylolisthesis.
The earliest spondylolisthesis classifications described the relationship between L5 vertebra and the sacrum, either the positioning aspects or anatomical aspects. These descriptive aspects were initially presented by the authors as etiological factors, by which they characterized two large families of spondylolisthesis, the isthmic and dysplastic types. More recently, the effect of the pathological relationship between L5 and S1 on the overall spinal alignment was analyzed and resulted in a classification that defines the therapeutic decision and refine the surgical strategy, emphasizing the need for lumbo sacral sagittal realignment. By differentiating low- and high-grade spondylolisthesis, Marchetti and Bartholozzi 2 categorized spondylolisthesis based on the severity of the condition. Later, Hresko and Labelle 3 integrated the recent findings on the spinopelvic balance, and thus were able to move beyond a purely descriptive classification to a more functional analysis. This new approach helped to guide surgical planning based on regional balance. The last version of the Spinal Deformity Study Group (SDSG) classification addresses treatment planning options, however it provides only a snapshot of the spondylolisthesis condition.
Indeed, the grades of spondylolisthesis sever ity and the occurrence of spinal imbalance describe the progression stages that may occur but may not serve as prognostic factors. Two criteria seem to be of prognostic value: patient age at onset and the presence of a rounded sacrum. We propose a new classification system that addresses both prognosis evaluation and treatment decision-making.
The earliest spondylolisthesis classifications described the relationship between L5 and the sacrum, either the positioning factors or the anatomic factors. These descriptive factors were also considered as etiological factors of the isthmic and dysplastic types of SPL. More recently, the effect of the pathological relationship between L5 and S1 on the overall spinal alignment was analyzed, which resulted in a classification system that addresses management decision making and surgical planning, with an emphasis on the need for lumbosacral sagittal realignment. 4
Meyerding Grading System
In 1932, Meyerding analyzed the demographic and clinical features of 207 patients presenting at the Mayo Clinic with spondylolisthesis. 5 He proposed a grading system that addresses vertebral slippage based on the degree of subluxation, from grade 1 (25%) to grade 4 (100%). Later, grade 5 was added, representing more than 100% of slippage or spondyloptosis. Meyerding strongly believed that trauma and mechanical overload strains such as obesity, pregnancy, and occupational demands were the main etiologic factors. He also cited the predisposing role of congenital posterior defects that are often associated with this condition. This classification was only descriptive, but it was the first attempt to address the severity of spondylolisthesis displacement.
This first attempt of classification can be criticized as it fails to assess accurately the degree of slippage when the sacrum is round. Indeed, the grading system proposed by Meyerding described a linear displacement of L5 over S1, and it cannot be applied in cases of a round or dome-shaped sacrum.
Segmental Anatomic Classifications: Dysplastic and Isthmic Spondylolisthesis
The classification system that is most often utilized is the one proposed by Wiltse 6 and later modified by Newman, based on the description of the mechanism that produced the spondylolisthesis. The Wiltse classification divided spondylolisthesis into five categories:
According to this classification, the most frequent types in children and adolescents are the dysplastic (type I) and the isthmic types (type II).
Type I is associated with congenital dysplasia of L5 or S1. Fredrickson et al 1 found a high frequency of spina occulta and S1 dysplastic changes associated with spondylolisthesis. The congenital anomalies of the posterior elements were recognized as key factors for the occurrence of progressive slippage. Fredrickson et al and later Beutler et al 7 described the frequency of progression based on the associated abnormalities. Lonstein 8 also compared these two types, and suggested that sacral doming was a congenital defect due to an anterior growth disturbance and was the main factor contributing to lumbosacral kyphosis.
Wiltse did not address the role of anterior dysplastic changes in the progression of spondylolisthesis. (We will discuss the importance of these changes later in the chapter.) Moreover, Wiltse did not differentiate pars elongation from true spondylolisthesis produced by the slippage of L5. This differentiation seems to us to be of clinical and therapeutical relevance, as closed spondylolisthesis with progressive sliding will progressively reduce the size of the canal and consequently induce spinal stenosis.
In the isthmic type, the pars interarticularis is disrupted, first stage being spondylolysis (pars fracture without slippage). The most common mechanism is an acquired fatigue fracture. Wiltse et al 6 and later Fredrickson et al 1 demonstrated that spondylolysis was produced after the acquisition of bipedalism. The incidence of SPL increases during growth, from 5% at 7 years to 7% at skeletal maturity. 1 , 6 Wiltse suggested three subgroups based on the healing process: -spondylolytic stress fracture without signs of healing (fibrous tissue is seen surrounding the pars defect), -pars elongation (the healing process occurs on a dysplastic posterior arch), -acute traumatic fracture (which also could be included in type III). Pars elongation could also be included in the dysplastic type (type I), which is always associated with congenital dysplasia of the L5 or S1 posterior arches.
In 1997, Marchetti and Bartholozzi 2 proposed a new classification distinguishing developmental from acquired spondylolisthesis. In developmental spondylolisthesis, they defined two subgroups—low grade and high grade—based on the importance of the dysplastic changes. In low-grade dysplastic spondylolisthesis, the sacral plateau remains flat and L5 remains rectangular. High-grade dysplastic spondylolisthesis is characterized by a dome-shaped sacral end plate and a trapezoidal L5 vertebral body. The authors intended this classification to guide the management of spondylolisthesis patients, based on the risk of progression of the dysplastic changes.
Even though this classification may help in the management of high-grade dysplastic spondylolisthesis, it does not clarify either the spectrum of situations requiring surgical management or the decision of whether to fuse in situ or to perform surgical reduction.
The acquired spondylolisthesis group includes traumatic, iatrogenic, pathological, and degenerative spondylolisthesis related to an excessive loading stress applied to the pars interarticularis. Again, this classification does not help to determine the treatment strategy for stabilizing and normalizing spinal morphology and restoring spinal balance, even if it highlights some of the prognostic factors leading to spondylolisthesis aggravation process.
Neither the Wiltse nor the Marchetti-Bartholozzi classification was designed to guide decision making for spondylolisthesis treatment. It was not until the era of sagittal balance understanding, that the slippage phenomenon was integrated into the global spinal balance, helping to provide some critical therapeutic recommendations.
Global Anatomic Classification
New classifications integrating the pathological relationship between L5 and the sacrum into the overall context of spinal balance arose in the early 21st century, in order to differentiate balanced versus unbalanced situations. Once the spondylolisthesis has destabilized spinal balance, surgical management requires both a reduction strategy and stabilization.
During et al 9 were the first group to link the lumbosacral postural parameters. They described the pelvisacral angle, which is the complement angle to pelvic incidence (PI). In their original work, they were also the first to report a significant difference of the spino pelvic angles between spondylolisthesis patients and controls. 9
In 2004, Labelle and Roussouly′s group 10 found that specific pelvic shapes were associated with spondylolisthesis and that PI was correlated with the degree of slippage, suggesting that a high PI was a risk factor for spondylolisthesis. Later, some authors, such as Huang et al 11 and Whitesides et al, 12 criticized the prognostic value of PI. Roussouly et al, 13 therefore, differentiated two types of pelvic orientation that may produce spondylolisthesis by different mechanism: the shear type and the nutcracker type.
The shear type is characterized by a high PI, which increases the shear stress on the L5-S1 level due to a high sacral slope ( Fig. 10.1 ). More than 70% of lumbar lordosis is expressed in the lower arc of lordosis (L4–S1), which is generally equal to the sacral slope. Inoue et al 14 further confirmed the prognostic value of the morphology and orientation of the sacrum, emphasizing the importance of the shape of the sacral plateau. They also described the sacral table angle (STA), the angle between the sacral end plate and the posterior aspect of S1, and showed that in children and adolescents with L5-S1 spondylolisthesis the anatomy of the sacrum was different from that in the general population.
The nutcracker type is associated with a low PI. Spondylolysis is produced by the direct compression forces exerted by the L4 inferior facets on the L5 pars interarticularis. The association with a type 1 sagittal morphology (in the Roussouly classification 15 ) increases the constraints, as the lumbar lordosis is low and is mainly expressed at the L4, L5, and S1 levels ( Fig. 10.2 ). 16
In 2007, Hresko et al 3 proposed a new classification system integrating both the slippage grade and the sagittal spinopelvic balance. This new classification system evaluates progression, and addresses the treatment of decision making spondylolisthesis. The chief goal of this new classification was to integrate the contributions of previous classification systems, especially that of Bartholozzi and Marchetti, as well as the concept of spinopelvic balance, which is important in understanding human spinal pathology. With improved understanding of sagittal alignment and analysis of spinopelvic parameters, the factors of spinal balance and imbalance are now well characterized. Hresko et al identified two subgroups of patients with high-grade spondylolisthesis, based on whether the pelvis was balanced or unbalanced ( Fig. 10.3 )—low-grade spondylolisthesis being also balanced. The pelvic tilt and sacral slope values were similar between controls and the balanced pelvis patients. Unbalanced pelvis (retroverted) patients had a sagittal spinal alignment that differed from the balanced pelvis patients and the control group, suggesting that reduction techniques might be considered in patients with an unbalanced pelvis. In these patients, the importance of slippage was related to the following regional modifications: sacral doming, trapezoidal L5 vertebra, lumbosacral kyphosis (described by Dubousset 17 ) and increased L5 incidence. 18
However, this classification could only describe the consequences of high-grade spondylolisthesis based on the importance of sacral morphology modifications. But it is important also to address the sliding-mechanism concept that produces the L5-S1 spondylolisthesis. Indeed, the existence of a sacral dome, in association with dysplasia of the posterior elements, causes a rotational displacement of the L5 vertebral body on S1, from the moment when the mechanical failure occurs ( Fig. 10.4 ).