Case StudyThe Association of Sacral Table Angle Measurements With Spondylolytic and Spondylolisthetic Defects at the Lumbosacral Articulation: A Radiographic Analysis
Introduction
The natural history of spondylolysis and spondylolisthesis has been extensively studied and described over the past several decades [1], [2], [3], [4], [5], [6]. However, the etiology of spondylolysis and subsequent slippage has been debated at great length and remains unclear [7], [8], [9], [10]. A multitude of radiographic parameters have been described to assess and/or prognosticate the sagittal deformity seen in association with spondylolisthesis [1], [11], [12], [13], [14], [15], [16], [17]. These measurements tend to be descriptive and none have proven an etiologic association with spondylolysis or listhesis through serial X-rays that show patients developing the deformity.
In addition to environmental factors, genetic predisposition may have an important role in the development of a pars defect [9], [18], [19], [20], [21]. The incidence of spondylolysis and spondylolisthesis in racially distinct groups has been reported to vary from as low as 3% in people of African descent to as high as 50% of the Inuits [18], [22], [23]. To date, there are no morphologic or radiographic parameters identified that predispose certain individuals to this disease process.
Whitesides, et al. [24] recently described a radiographic parameter, the sacral table angle (STA), proposed to be etiologically related to spondylolysis and subsequent listhesis. The STA is the angle created by a line drawn along the sacral end plate and a line drawn along the posterior aspect of the S1 vertebral body (Fig. 1). It has been postulated that an abnormally decreased, initial, and genetically based STA would place increased stress and shear forces across the more vertically oriented lumbosacral articulation, leading to a greater likelihood of fatigue failure at the pars interarticularis [24].
This measurement was originally described by Österman and Österman [25], in their study of rabbits, as the sacral end plate angle. Inoue et al. [13] later applied this measurement in a study of the Japanese population as the STA. Those authors were able to show a statistically significant decrease in STA in patients with slippage compared with those without a documented slip. In an archeological study, Whitesides et al. [24] found a lower STA in normal specimens from a genetically homogeneous group known to have increased occurrence rates of spondylolysis.
The purpose of this investigation was to assess the STA measurement as a predictor for the occurrence of L5 spondylolysis in Caucasian patients from the northeastern United States. The secondary objective was to determine whether the association between a decreased STA and isthmic spondylolisthesis is causative or an anatomic change seen as a result of secondary remodeling.
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Materials and Methods
Initiated in 1955, a prospective study to determine the incidence and natural history of spondylolisthesis was conducted on 500 first-grade children from Sayre, Pennsylvania [1]. Those subjects were followed longitudinally with serial radiographs through adulthood. The only criterion for inclusion in the study was that the child attend any one of several local public elementary schools in the years 1954 to 1957, when the study was conducted. Parental permission was obtained for each child in
Results
The mean STA for the index study group before the development of spondylolysis was 95° ± 5.5°. The mean STA for the control group was 97.5° ± 4.3°. No significant difference was found between groups (p > .05).
Four of the 6 index subjects developed spondylolisthesis subsequent to the occurrence of the pars defect. When analyzing the STA as a function of percent slippage with a regression model (Fig. 3, Table 1), a negative correlation was seen (r = 0.54). This supports a trend-wise decrease in
Discussion
The etiology of spondylolysis and spondylolisthesis has eluded researchers for decades. There have been numerous attempts to identify morphologic and radiographic parameters to predict the development of a pars defect and the potential for subsequent slippage [1], [2], [4], [12], [27], [28], [29]. Such prognostic factors would provide valuable information to patients with lytic pars defects, their parents, and the clinicians who treat them. This knowledge would be beneficial in caring for
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Author disclosures: RAT (none); BEF (none); TEW (none); WFL (grants from DePuy Spine, outside the submitted work).