ASSOCIATION OF SPINOPELVIC PARAMETERS WITH THE LOCATION OF LUMBAR DISC HERNIATION

Objective: To associate spinopelvic parameters, pelvic incidence, sacral slope, pelvic tilt and lumbar lordosis with the axial location of lumbar disc herniation. Methods: Retrospective study, which evaluated imaging and medical records of 61 patients with lumbar disc herniation, who underwent surgery with decompression and instrumented lumbar fusion in only one level. Pelvic incidence, sacral slope, pelvic tilt and lumbar lordosis with simple lumbopelvic lateral radiographs, which included the lumbar spine, the sacrum and the proximal femur. The affected segment was identified as the level and location of lumbar disc herniation in the axial plane with MRI scans. Results: Of 61 patients, 29 (47.5%) had low lumbar lordosis; in this group 24 (82.8%) had central disc herniation, 4 (13.8%) had lateral recess disc herniation and 1 (3.4%) had extraforaminal disc herniation (p<0.05). Of the 61 patients, 18 (29.5%) had low sacral slope; of this group 15 (83.3%) had central disc herniation and 3 (16.7%) had disc herniation in lateral recess (p<0.05). Conclusions: There is a trend towards greater load distribution in the anterior region of the spine when the spine has hypolordotic curve. This study found an association between low lordosis and central disc herniation, as well as low sacral slope and central disc herniation.


INTRODUCTION
Disc herniation is defined as a continuous process of disc degeneration that causes the nucleus pulposus to migrate beyond the physiological limits of the annulus fibrosus. 1 Lumbar disc herniations are more frequent in the fourth and fifth decades of life, with prevalence of 4.8% in men and of 2.5% in women over the age of 35. The disease is considered a world health problem due to its epidemiological importance, and the physical and functional incapacity that it can produce. [1][2][3][4][5] Its etiology is multifactorial, caused by the association of degenerative changes of the disc, which entail a reduction in the content of proteoglycans in the nucleus pulposus, with situations in which there is increased disc pressure, particularly in spinal flexion and rotation. 1,2 Spinal alignment was, for many years, assessed only in the coronal plane, but several publications have highlighted the importance of changes in the sagittal plane. The analysis of sagittal balance appeared recently, and has been being correlated with the presence of herniated disc, degenerative disc disease, and spondylolisthesis. [6][7][8][9][10][11][12][13] For the body balance in the sagittal axis, the pelvis and the spine are stabilizing factors between the head and the lower limbs. The alteration of these segments can change the behavior of adjacent segments in maintaining sagittal balance, leading to early degenerative changes, and greater energy expenditure. [6][7][8][9][10][11][12][13] The main radiological parameters used to evaluate spinopelvic balance are pelvic incidence (PI), sacral slope (SS), pelvic tilt (PT) and lumbar lordosis (LL). Pelvic incidence is an individual anatomical constant that does not vary according to the posture adopted. PI corresponds to the result of the sum of the sacral slope and pelvic tilt angles. [6][7][8][9][10][11][12][13] (Figure 1) Roussouly et al. 15 studied and classified lumbar lordosis in morphological patterns, and the corresponding degenerative progress of each curve, using pelvic incidence and sacral slope angle as reference. Patients with a low sacral slope (<35 o ) and slight pelvic tilt (<45 o ) have a tendency to present with early disc degeneration, central disc herniation or disc degeneration at multiple levels, due to the anterior displacement of forces. 7,8,11,15 This study aims to associate the spinopelvic parameters pelvic incidence, sacral slope, pelvic tilt and lumbar lordosis with the axial location of the lumbar disc herniation.

METHOD
A retrospective study, approved by the Institutional Review Board, in which imaging scans and medical records of 61 patients of the department were evaluated (Nº 35229314.4.0000.5065).
The inclusion criteria were patients with lumbar disc herniation who had undergone conservative treatment, with at least 20 physiotherapy sessions without improvement, patients who had undergone surgical treatment, or lumbar arthrodesis instrumented at only one level.
Patients without indication of surgical treatment, those who improved following physiotherapy, patients with previous lumbosacral surgery and those who had undergone surgical treatment at more than one level, as well as patients with other non-degenerative pathologies, were excluded from the study.
The spinopelvic balance was evaluated by measuring the angles of pelvic incidence, sacral slope, pelvic tilt and lumbar lordosis in the plain lumbopelvic radiographies with lateral views, which had to include the lumbar spine, the sacrum and the proximal third of the femur.
The angles in the radiographies were measured manually. Pelvic incidence (PI) was obtained by the angle formed, in the lateralview radiography, by the line perpendicular to the sacral plateau, which originates from its midpoint, and by the line that runs from the center of the sacral plateau to the midpoint of the segment that joins the two centers of the femur heads, known as the biocoxofemoral axis. Pelvic tilt (PT) is composed of the intersection of a vertical line, perpendicular to the ground, with the line that joins the bicoxofemoral axis to the center of the sacral plateau. Sacral slope (SS) consists of the intersection of a horizontal line that follows the ground, with the line that accompanies the sacral plateau. (Figure 1) The upper edge of L2 and the upper edge of S1 were used as a reference to measure lumbar lordosis. The sum of pelvic tilt and sacral slope angles is known to be equal to the value of pelvic incidence. [5][6][7][8][9][10][11][12][13][14][15][16] (Figure 2) The angle measurements were compared with the normal values for the adult population, which have already been determined by various authors. 5,10,[17][18][19] (Table 1

PI = PT + SS
The affected segment was identified with regards to the level and location of the herniated lumbar disc in the axial plane, according to the nomenclature of the Committee of the North American Spine Society (North American Spine Society, American Society of Spine Radiology and American Society of Neuroradiology) ( Figure 3) 20 with Magnetic Resonance Imaging (MRI) scans. The MRI scans of the patients' lumbosacral spines were assessed at T1 and T2 in the axial and sagittal sequences, by a single observer.
Frequencies were analyzed with a confidence interval of 95% (95% CI -Taylor Series). The results were represented numerically via absolute values, and some data were also expressed as percentages. The Kolmogorov-Smirnov test was applied to determine whether the distribution of probability of the data is normal, and this hypothesis was accepted. The Student's t-test was applied for comparisons between the means of the variables. The data analysis was performed using Microsoft Office/Excel 2007 software.
Among the 18 patients with low sacral slope, 15 (83.3%) presented with central herniation and three (16.7%) with recess herniation. The Binomial test proves that there is significant difference (p<0.05) ( Table 3 and Figure 6).  incidence. The association of pelvic incidence with the axial location of the herniated lumbar disc was statistically insignificant. The means of the sacral slope, pelvic tilt, pelvic incidence and lumbar lordosis parameters were compared for the central, recess, foraminal and extraforaminal disc herniations. There was statistical significance only for the comparison of lumbar lordosis (LL): central herniation X foraminal herniation. Patients with mean central herniation of 34.8 o of LL, foraminal herniation of 51.8 o . Student's t-test (p<0.05).

DISCUSSION
Roussouly and Pinheiro-Franco 7 carried out a literature review. When the lumbar spine is hypolordotic and flat, the contact force acts mainly on the anterior column (vertebral bodies and discs) and its distribution favors the resultant force perpendicular to the discs, increasing the pressure on the disc. On the other hand, if lumbar lordosis is hypercurved, the force acts mainly on the posterior elements (joints, spinous processes).
Several studies have assessed sagittal balance in patients with degenerative diseases -herniated lumbar disc, lumbar discopathy and spondylolisthesis, and Roussouly et al. 15 declared that patients with a low sacral slope (<35 o ) and slight pelvic tilt (<45 o ) have a tendency to forward displacement of forces and central herniated disc.
When the lumbar spine has increased lumbar lordosis, the Contact Force (CF) acts mainly on the posterior elements (joints, spinous processes). (Figure 8) On the other hand, if the lumbar spine is hypolordotic the CF acts mainly on the anterior column (vertebral bodies and discs), and its distribution favors the resulting force perpendicular to the discs, increasing the pressure on the disc. According to Roussouly, patients with structured hypolordosis (flatback syndrome) have a horizontalized disc and high disc pressure. These patients apparently have a high risk of developing central herniated disc. 7,8,11,13,15 A study by Rajnics et al. 13 affirmed that in patients with hypolordosis there is an increase in the compression force of gravity component. These increases in compression forces could speed up disc degeneration, corroborating our results.
In our study, of the 29 patients with low lumbar lordosis, 24 (82.8%) presented with central herniated disc, with a statistical difference (p=0.000). This association between low lordosis and central herniated disc is corroborated by the studies cited by Roussouly. 7,8,11,13,15 In our study, of the 18 patients with low sacral slope, 15 (83.3%) presented with central herniated disc, with statistical difference (p=0.008). This association is corroborated by the studies of Of the 61 patients, 23 (37.7%) presented with low pelvic tilt, 22 (36.1%) normal, and 16 (26.2%) high.