Tomographic sTudy of The s 2-alar-iliac screw Technique in Brazilian women

Objective: Lumbosacral fixation presents problems in its arthrodesis, mainly due to pseudoarthrosis. Iliac screws minimize this problem, however, they show problems in the operative wound. The S2-Alar-iliac (S2AI) screw presents a lower rate of these complications. The anatomical differences between the populations and the sexes analyzed in the literature justify the study of the S2AI screw technique in Brazilian women. Methods: A total of 14 adult female CT scans were analyzed by 4 evaluators. Results: The mean bone length was 131.8 mm, the largest bone diameter was 22.6 mm, and the smallest bone diameter was 22.6 mm. Conclusions: The data presented are compatible with the literature and may assist spine surgeons in choosing the best implant and surgical technique. Level of Evidence I; Diagnostic Studies — Investigating a Diagnostic Test (anatomical investigation).


InTRoduCTIon
The lumbosacral area is an anatomical region where problems occur in arthrodesis, often associated with nonunion and loss of fixation.Several historical alternatives have been described for the fixation of this region, such as Gavelston rods, 1 S1/S2 pedicle, S2-Alar, and iliac screws. 2 Although the latter method minimizes the fixation problem, it also gives rise to concerns regarding the surgical wound and the prominence of the screws. 3,4The S2-Alar-iliac (S2AI) screw was described with a view to reducing these problems and maintaining the quality of the fixation technique. 5,6e original description of the technique was made in the American pediatric and adult population. 5,6There is also a description in the Asian population, comprising mainly Chinese subjects, with 120 patients analyzed in the sum of two available studies. 7,8][12][13] Taking into account differences in the sacropelvic anatomy between sexes and ethnicities, further studies describing the passage of the S2AI screw in populations, other than those described above, are relevant.This study therefore aims to demonstrate the tomographic parameters of passage of the S2AI screw in a population of brazilian women.mETHodS Following approval by the Institutional Review Board, IRB -1,726,509, we retrospectively analyzed normal elective CT scans of the pelvis taken to investigate intra-abdominal pathologies.All the scans had to undergo reconstruction in the axial, sagittal and coronal sections with the possibility of three-dimensional reconstruction.The computed tomography equipment consisted of helical multichannel scanners.The scans were collected non-consecutively from 25 patients aged between 18 and 65 years, randomly selected from the year 2016.Of the total 25 patients, 11 were men and were excluded from the study.None of the patients selected had any bone deformity in the lumbosacral spine and pelvis.
The scans were analyzed by Isite (Phillips) ® , a company of Dutch origin, through three-dimensional reconstruction of the CT scans by two orthopedic and trauma residents, and two spinal surgery residents in their 5 th year.All the measurements were taken bilaterally.The mean of the measurements taken by the analyzers was calculated.
The data were standardized as follows: age in years; diameter, length and width measured in millimeters; angle measured in degrees.The S2AI screw parameters were measured as follows:

RESulTS
The study population consisted of 14 female patients aged 28-65 years (mean age 48.7 years).Table 1 shows the mean of the analyses performed by the four evaluators.The greatest bone length of the trajectory of the right and left screws was respectively 147.5 and 149.9 mm (final mean of 131.8 mm).The mean of the largest diameter of the bone through which the screw passes was 22.6 mm.The variation of the means was 24.4-19.2mm on the right side and 26.9-19.7 mm on the left.The evaluation of the smallest diameter of the bone, through which the screw passes in the iliac, showed that the final mean was 17.6 mm.

sagittal angle s2ai
The mean angle on the axial plane for the right screw was 34.7 o , ranging from 38.1 to 29.8 degrees, while the mean angle on the left side was 32.3 degrees, ranging from 36.4 to 27.2 degrees.The final mean angle was 33.5 degrees.The final mean angle on the sagittal plane was 31.5 degrees, measuring 30.3 degrees on the right side and 32.7 degrees on the left.The maximum angle was 37.9 degrees for the right side and 41.8 degrees for the left, while the minimum angle was 25.2 degrees and 27.3 degrees, respectively.The distance from the skin had a final mean of 59.9 mm, with the mean ranging from 59.7 mm on the right to 60.1 mm on the left.Table 1 dISCuSSIon The pelvic fixation technique described by Kebaish 5 and Sponseller, 6 S2AI, appeared as an alternative in the attempt to resolve the problems of existing techniques.L5-S1 fixation has a high rate of pseudoarthrosis, and Kim et al. demonstrated a rate of 24% in the correction of adult scoliosis. 14A technique that minimized this problem was iliac fixation. 15However, this technique was not without problems.One of the complaints submitted is the prominence of the screw, with a withdrawal rate of 22% at one year 3 and a much higher rate after 5 years. 4Another drawback of the technique is the need for extensive soft tissue dissection, predisposing patients to a higher rate of infections and alignment with the rods.In the series of Kuklo et al, 15 4% of patients presented with surgical wound infection within 2 years.
In the pediatric population, the technique described by Sponseller 6 defines the entry point as being approximately 25 mm caudal to the upper endplate of S1 and 22 mm lateral to the midline, positioned at an angle of 40º laterally and caudally.His peer described the same technique 5 as being 2 to 4 mm lateral and 4 to 8 mm distal to the dorsal sacral foramen of S1.Despite the original descriptions, we noted minor variations in the medical literature showing the entry point, including studies in which the actual authors of the technique participated. 16,17ur entry point was between the midline between S1 and S2 and 2 mm lateral to the foramen, which closely resembles the technique described by Kebaish, going in the direction of the anterior inferior iliac spine. 8The purpose of this method of measurement was to facilitate the passage of the screw during the surgical procedure, with anatomical parameters that are easy to locate, thus avoiding the use of radioscopy.
The study by Kwan et al, 8 which analyzes the S2A1 screw trajectory in the Asian population (Chinese, Malaysians and Indians), showed a variation of 122.2 to 86.4 in the screw trajectory in women.Axial angles ranged from 39.5 to 50.2 degrees, depending on the longest or shortest screw trajectory.Another study that described 30 Chinese women obtained the following results: maximum diameter of 115.21 mm, mean sagittal angle of 35 degrees and mean axial angle of 36 degrees, and a mean diameter of 14.85 mm. 7Another study which shows the population of adolescents obtained results that are similar to the studies described, 9 such as a mean angle of 39 degrees on the axial plane and 38 degrees on the sagittal plane.In our study, the mean greatest screw length was 131.8 mm, the largest diameter 22.6 mm, and the mean axial and sagittal angles were, respectively, 33.5 and 31.5 degrees.Note that the difference shown is in millimeters, yet there is still a difference.This difference can be explained by the fact that the study subjects are of different races, presenting minor variations that may justify the result.Another possibility is the difference in size between the study participants; however, as not all studies present these data, it cannot be stated with more certainty whether this is indeed one of the justifications.
This study has limitations that must be taken into account for the interpretation of the data.The limited number of patients analyzed may not actually represent the Brazilian female population, leading to the difficulty of external validation.The data collection method, which is not uniform across all studies, also needs to be taken into account.Another point to be highlighted is that the programs used to analyze the CT scans are different, and may have differences between them, such as magnification.Non-inclusion of the weight and height of the patients undergoing the scan is a flaw in the data interpretation, as it makes it impossible to adequately match the size of the measurements with the size of the subject evaluated.
• Length: distance between the S2AI screw insertion point (between S1 and S2, 2 mm lateral to the paramedian sacral crest) and the anterior inferior iliac spine; (Figure 1) • Sagittal angle: angle of inclination of the S2AI screw in the sagittal section of the CT scan; (Figure 2) • Axial angle: angle of inclination of the S2AI screw in the axial section of the CT scan; (Figure 3) • Largest diameter: largest diameter between the external and internal iliac cortices; (Figures 4 and 5) • Smallest diameter: smallest diameter between the external and internal iliac cortices in its isthmic region; (Figures 4 and 5) • Distance from the skin: distance between the S2AI screw insertion point and the skin; (Figure 6)

Figure 4 .
Figure 4. Largest and smallest diameter of the iliac bone.

Figure 5 .
Figure 5. Largest and smallest diameter of the iliac bone from the S2AI screw.

Figure 6 .Tabela 1 .
Figure 6.Distance from the S2AI screw to the skin.