Introduction
The pelvic incidence (PI) angle was originally described by Beaupère et al. and it has been recently studied by many investigators
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3. The focus has primarily been on the sagittal alignment of the spine and pelvis
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6, Normal subjects, patients with scoliosis and those patients with isthmic spondylolisthesis were analyzed in their study
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9. The normal patients were divided into the pediatric, adolescent, adult and elderly groups
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6. Duval-Beaupere et al.
1 suggested that the PI has a positive correlation with lumbar lordosis and it is closely related with the sacral slope and pelvic tilt.
In the several studies that compared the PI between normal subjects and the patients with isthmic spondylolisthesis, it was reported that the PI was increasing as the grade of spondylolisthesis increased
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9. Since an increased PI could be one of the causative factors in the development and progression of spondylolisthesis, PI could be one of the parameters to check when evaluating and treating spondylolisthesis. However, the previous studies were limited to just L5 spondylolisthesis or there was no differentiation between L4 & L5 spondylolisthesis.
The purposes of this study is to assess the differences of the pelvic parameters between a normal control group and an isthmic spondylolisthesis group and between the groups of patients with L4 and L5 spondylolisthesis, as well as between groups of patients with single and bi-level spondylolisthesis. We then compared the groups to understand the correlation between the degree of slip of spondylolisthesis and the pelvic parameters.
Discussion
The causes of spondylolysis are considered to be multifactorial, and they include a hereditary predisposition and biomechanical factors. The precise mechanism of slippage after the development of an isthmic defect is unknown, yet there are many theoretical explanations for slip progression. The slip angle, slip grade and sacral inclination had been reported to be important parameters by which we can predict the slip progression. The sagittal pelvic tilt index was first suggested by Schwab et al.
10 to be an index that explains the positional relations between L5 and the hip axis, and it specifies the degree of the anteroposterior rotation of the pelvis. Schwab et al.
10 described rotation of the pelvis as developing with verticalization of the sacrum and anterior displacement of the hip joint, and this suggests a progression of spondylolisthesis by this index. A verticalization of the sacrum and anterior displacement of the hip joint is the same phenomenon as an increasing PT, which can be seen in this study. This develops as a compensatory mechanism for the anterior displacement of the vertebral body from the gravity line by the spondylolisthesis, but it is difficult to consider it as a parameter to predict the progression of spondylolisthesis.
The pelvic incidence was suggested by Duval-Beaupere et al.
1 to be an anatomical parameter that is correlated with such positional parameters as the sacral slope and pelvic tilt. Therefore, as the PI is increased, the PT and SS will be increased. Generally, the normal range of the PT is very narrow from 10 to 15 degrees, so if the PI is increased, the SS will be increased relatively much more than the PT and it will be the cause of increasing lordosis. If the lordosis is increased, there will be a large load to the posterior complex that will produce a spondylolysis and possible progression of spondylolisthesis. Progression of spondylolisthesis displaces the center of gravity of the pelvis anteriorly, which increases the PT and decreases the SS to compensate.
On comparison of the pelvic parameters between 48 subjects with isthmic spondylolisthesis and 30 normal subjects by Rajinics et al.
9, the SS, PT and PI in the spondylolisthesis group were significantly higher than those values in the normal group. In their study, the results for the PI were 66.5 degrees in the spondylolisthesis group and 54.0 degree in the normal group, and they described a correlation between the slip percentage and the PI (r=0.660). In their study, although 15 L4 spondylolisthesis subjects were included, the analysis was done without differentiating between L4 and L5 spondylolisthesis. However in our study, there was no statistical difference of the SS between the spondylolisthesis group & the normal control group, but a difference of the PI was present, which applied to the difference of the PT.
Hanson et al.
7 measured the pelvic parameters in 40 patients with isthmic spondylolisthesis and 40 normal subjects. There were 20 children and 20 adults in the group of normal subjects with an average age of 11.8 years and 60.0 years, respectively. They classified the Meyerding-Newman grades 1 and 2 into the mild spondylolisthesis group with grades 3 and 4 into the high grade spondylolisthesis group, and the average age of the groups was 26.6 years and 17.7 years, respectively. In their study, the average PI was 47.4° in the children's group, 57.0° in the adult's group, 68.5° in the mild spondylolisthesis group and 79.0° in the high grade spondylolisthesis group. The PI of the high grade spondylolisthesis group was greatest, and the PI of the mild spondylolisthesis group was second greatest. Additionally, there was a correlation between the grade of spondylolisthesis and the PI by the Meyerding-Newman score (p=0.03). However, in their study, there was no analysis of the SS and PT as a function of the PI, so there was no data on the differences of the SS and PT between the two groups.
Labelle et al.
8 measured the pelvic parameters in 214 patients with L5 isthmic spondylolisthesis, along with a control group of 160 normal subjests. These patients were classified into 5 groups by the Meyerding classification; on comparing each group, the PI, SS, PT and LL were found to be higher in the isthmic spondylolisthesis group. Additionally, as the degree of spondylolisthesis increased, the LL, PI and PT increased as well.
In our study, the PI in the spondylolisthesis group was higher than that in the normal group (61.8° and 49.1°, respectively, p<0.01), and the PT in the spondylolisthesis group was higher than in the normal group (21.4° and 11.0°, respectively, p<0.01). These results were in accord with the other reported results. Labell et al.'s conclusion that the SS of the spondylolisthesis group was higher than that of the normal group was not supported by our results. We estimated that the reason why the PT was increased, but the SS was not increased in our study was due to the compensatory increased pelvic tilt in the patients with spondylolisthesis. So it ultimately reduced the SS in the spondylolisthesis group and it made no difference between the study and the control group with the difference of the PI primarily depending on the PT. This can be more clearly seen in bi-level spondylolistheis, which has a greater PI and PT than one-level spondylolisthesis without any difference in the SS between the two groups. Our study had the age factor as a significant difference from Labelle's study, where the average age in his study was 16.8 years and the average age in our study was 56.2 years. It is generally assumed that the PT increases and the SS decrease with increasing age, which can be another reason for the lack of differences of the SS between the spondylolisheis group and the normal control group in our study.
The studies by Rajinics, Hanson, and Labelle have reported a correlation between the slip percentage and the PI according to the Meyerding grade
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9. In our study, we analyzed the correlations of the pelvic parameters with the slip percentage in 60 single level spondylolisthesis subjects by the Taillard method, with the result that there was a correlation between the slip percentage and the PI (r=0.293, p=0.023), and there was no correlation among the other parameters (p>0.05). We believe this fact can provide a theoretical basis for concluding that a high PI is correlated with slip progression. There was, however, only correlation between the pelvic incidence and the displacement in the L5 spondylolisthesis group (r=0.362, p=0.05), but not in the L4 spondylolisthesis group. In our studies, we could assume the reason we couldn't see correlation between the slip percentage and the pelvic parameters in L4 IS is that the anatomical configuration of the pelvis has an influence on the progression of slippage in L5, but it didn't influence on the slippage of L4. L5 is more stable than L4 due to the iliolumbar or iliotransverse ligament and the protected position of L5 below the iliac crest. In L4 spondylolisthesis, there might be other different independent factors such as segmental instability or concurrent disc degeneration at the level of the slip, which can have a greater influence on slip progression. Also, our results showed that there were no differences in the pelvic parameters between L4 and L5 spondylolisthesis.
However, the other result that the PI and PT were larger in two level spondylolisthesis than in one-level spondylolisthesis shows that pelvic parameters may have an influence on the slip progression of L4 IS, but we couldn't totally conclude that L4 IS is different from L5 IS. Therefore, although a high PI can be associated with the occurrence of spondylolysis at L4 and L5, L5 is regarded as more related to the progression of slip with L5's higher PI than that of L4.
Ultimately, even though there were limitations of our study, which include some young patients, we believe this subject needs further studies, including studies that will make comparisons not with a normal group, but with nonprogressed sponlylolysis patients.