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International Journal of Morphology

On-line version ISSN 0717-9502

Int. J. Morphol. vol.32 no.1 Temuco Mar. 2014

http://dx.doi.org/10.4067/S0717-95022014000100034 

Int. J. Morphol., 32(1):202-207, 2014.

 

Evaluation and Clinical Application of Sacral S1 Vestibule Measurements in Chinese Adults

 

Evaluación y Aplicación Clínica de las Mediciones del Vestíbulo Sacral S1 en Adultos Chinos

 

Qiang Dong*,**; Wei Tian*,** & Baotong Ma**

* Tianjin Medical University, Tianjin, China.

** Department of Orthopedics, Tianjin Hospital, Tianjin, China.

Correspondence to:


SUMMARY: This study aims to measure the anatomic parameters of the sacral 1 (S1) vestibule in Chinese adults and to discuss their clinical application during iliosacral screw fixation for pelvic posterior ring injury. Three-dimensional computed tomography (CT) reconstructions were performed on 36 individuals, and the parameters of their S1 vestibules were measured. Vestibular width (VW) was 25.15±2.91 mm, vestibular height (VH) was 20.94±3.03 mm, and mean vestibular size (VS) was 400.23±85.11 mm2. The mean angle of superior inclination was 30.85°±9.22°, and the mean anterior inclination (AI) was 13.91°±6.25°. VW and VS were significantly smaller in females than in males (p<0.05), but no statistical differences were found between the left and right sides. The S1 vestibules of Chinese patients are smaller than those reported for Caucasians. Therefore, the placement of iliosacral screws should be considered carefully based on the size, gender, and ethnicity of the patient. The anatomic parameters of females were much smaller than those of males and close to the minimum requirement for fracture fixation. Therefore, female Chinese patients who need iliosacral screws should undergo preoperative CT scans to measure S1 vestibule parameters to make individual operational plans.

KEY WORDS: Sacral vestibule anatomy; Sacroiliac joint; Bone screws.


RESUMEN: El estudio tuvo como objetivo medir los parámetros anatómicos del vestíbulo sacral 1 (S1) en individuos adultos chinos y discutir su aplicación clínica durante la fijación de tornillo iliosacral por lesiones del anillo pélvico posterior. Se realizaron reconstrucciones de tomografía computarizada tridimensional (TC) en 36 individuos y se midieron los parámetros de sus vestíbulos S1. El ancho vestibular (AV) fue 25,15±2,91 mm, la altura vestibular fue 20,94±3,03 mm y el tamaño medio vestibular (TV) fue 400,23±85,11 mm2. El ángulo medio de inclinación superior fue 30,85°±9,22° e inclinación anterior media fue 13,91°±6,25°. AV y TV fueron significativamente menores en las mujeres que en los hombres (p<0,05), sin embargo no se encontraron diferencias estadísticas entre los lados izquierdo y derecho. Los vestíbulos S1 de pacientes chinos son más pequeños que los reportados para los caucásicos. Por lo tanto, la colocación de tornillos iliosacros debe ser considerada cuidadosamente basada en el tamaño, el sexo y origen étnico del paciente. Los parámetros anatómicos de las mujeres eran significativamente más pequeños que los de los hombres y cercanos al requerimiento mínimo para la fijación de fracturas. Por lo tanto, pacientes de sexo femenino chino que requieren tornillos iliosacros deben ser sometidos a tomografías computarizadas preoperatorias para medir los parámetros del vestíbulo S1 a con el objetivo de programar un plan operativo individual.

PALABRAS CLAVE: Anatomía del vestíbulo del sacro; Articulación Sacro-iliaca; Tornillos para huesos.


 

Introduction

The sacral vestibule, first proposed by Carlson et al. (2000), refers to the three-dimensional (3D) screw space that is available in the narrowest part of the iliosacral screw channel. The sacral vestibule is located in the transition zone between the sacral wing and sacral body and serves as the entrance to the sacral vertebrae. The vestibule is divided into two components, the S1 and S2 vestibules. The smaller S2 vestibule is associated with limited tolerance for screw misdirection, which can increase the risk of nerve injury or spinal canal damage (Routt et al., 1997; Hinsche et al., 2002; van den Bosch et al., 2002). Therefore, S2 vestibules are rarely used for screw replacement. The S1 vestibule, located above the sacral foramina and between the sacral foramina and the slope of the sacral wing, is the isthmus of the transition zone between the sacral wing and the S1 vertebrae. The sacral wing forms the dome of the vestibule, and the upper margin of the sacral foramina is the bottom of the vestibule. On the front view, the vestibule is dumbbell-shaped, but it is an ellipse on the side view.

An in-depth understanding of the anatomy of the sacral vestibule is crucial to treating posterior pelvic ring injuries with iliosacral screws (Supplementary Fig. 1). The present research imaged the anatomy and measured 36 normal sacra in Chinese adults.

Fig. 1. The anterior-posterior and lateral pictures of sacrum reconstruction.

Subject and method

Enrolled participants. A total of 36 Chinese adults (19 males and 17 females) had their normal sacrum subjected to a CT and their sacral vestibules were visualized and measured on both sides, for 72 samples. This study was conducted in accordance with the declaration of Helsinki. This study was conducted with approval from the Ethics Committee ofTianjin Hospital, Tianjin Medical University. Written informed consent was obtained from all participants.

CT scanner and post-processing software. All participants underwent a sacral scan on a spiral CT scanner (SOMATOM Sensation 10, SIEMENS, Germany) with 100 mA tube current and 120 kV tube voltage. All data were transferred into an image reconstruction system (Syngo CT 2006G, SIEMENS) to complete the post-processing procedure. The slice section was 1.0 mm and the interval was 1.0 mm.

CT measurements. The sacral CT data of all participants were reconstructed as 1.0 mm interval and 1.0 mm sections to build 3D sacral images (Fig. 1). The sections, which were vertical to the bone in the sagittal plane near the transition zone between the vertebrae of S1 and sacral wing, were saved to take a series of measurements as follows: The area of the sections was measured and the area of the smallest section was considered as the area of the vestibule (VS) (Fig. 2). The length of the long anteroposterior diameter, namely, the width of the vestibule (VW), was measured; the length of the short superior inferior diameter, namely, the height of the vestibule (VH) of the section, was also measured (Fig. 2). The angles of the anterior inclination (AI) and the superior inclination (SI) of the S1 vestibule were measured.

Statistical analysis. SPSS 17.0 was used to perform the statistical analysis. All data are presented as mean ± standard deviation. A Student's t-test was used to compare the results between different measurements. A p<0.05 was considered statistically significant for all statistical analyses.

Fig. 2. Measure the parameters of S1 vestibule.

Results

The average area (VS) of the S1 vestibules was 400.23±85.11 mm2 (196.96 mm2 to 607.94 mm2). The average area of the male S1 vestibules was 428.59±72.43 mm2 (277.65 mm2 to 607.94 mm2), and that of the female vestibule was 369.25±89.05 mm2 (196.96 mm2 to 598.97 mm2). The females had significantly smaller vestibule areas than males (p<0.05; Table I). The average width (VW) of the S1 vestibules was 25.15±2.91 mm (17.25 mm to 31.30 mm). The average VW of the male S1 vestibules was 26.82±2.49 mm (21.49 mm to 31.30 mm), and that of the female vestibules was 23.71 ± 2.93 mm (17.25 mm to 31.06 mm). The average height (VH) of the S1 vestibules was 20.94±3.03 mm (14.73 mm to 25.42 mm). The average VH of the male S1 vestibules was 21.08±2.66 mm (14.73 mm to 24.83 mm), and that of the female vestibules was 20.78±3.29 mm (15.56 mm to 25.42 mm). The average SI angle was 30.85°±9.22° (17° to 45°). The average SI angle in males was 30.72°±9.14° (17° to 45°), and the average SI angle in females was 31.17°±8.33° (17° to 43°). The average AI angle was 13.91°±6.25° (0° to 25°). The average AI angle in males was 14.35°±6.47° (0° to 25°), and the average AI angle in females was 13.57°±6.33° (2° to 24°) (Table II).

Table I Comparison of S1 vestibule anatomical parameters in male and female.

Table II. Comparison of anatomical parameters of left and right S1 vestibule.

Discussion

Posterior pelvic ring injuries include sacral fractures, sacroiliac complex factures or dislocations, iliac fractures, or a combination of these injuries (Reilly et al., 2003). Many fixation methods are currently used to treat posterior pelvic ring injuries clinically (Sagi et al., 2004; Schildhauer et al., 2002, 2003; Yinger et al., 2003; Ziran et al., 2003; Vanderschot et al., 2001; Kobbe et al., 2012), and the iliosacral screw (ISS) technique (Ponsen et al., 2006) has been widely used (Cole et al., 1996; Shuler et al., 1995; Sciulli et al., 2007; Gardner & Chip Routt, 2010; Gardner et al., 2009; Zwingmann et al., 2010; Kim et al., 2012; Oh et al., 2008) because of its reliable fixation strength, small wound, decreased bleeding, and low infection rate. Mears & Velyvis (2003) described the pelvis as a "three-column structure." ISS establish effective fixation between the middle and posterior columns that properly fits biomechanical characteristics (Figs. 3 and 4). However, the ISS technique requires high precision and has strict requirements for the direction and location of screws. In addition, the operation has to be adjusted according to the pelvic anatomy of each patient. Otherwise, improper positioning of screws might damage nerves and blood vessels. Such damage is caused by screws penetrating the screw channel, fixation failure, or complications from bone cuts, screw bending, or screw facture within the screw channel (Routt et al.; Altman et al., 1999; Ricci et al., 2003; Collinge et al., 2005). Therefore, surgeons who perform the operation must understand the anatomy and the spatial configuration of sacral screw channels (Gardner & Chip Routt; Zwingmann et al.; Mendel et al., 2012). The sacral vestibule, which is the narrowest part of the screw channel, is crucial for this operation.

Fig. 3. Male, 24 years old, pelvic fracture because of traffic accident.

Fig. 4. The patient had the operation seven days after injury. The fracture of pelvic anterior
ring was fixed with reconstruction plate, while the posterior ring injury (iliosacal joint dislocation) was fixed
with iliosacal screws. X-ray shows the fractures had the anatomical reduction.

Carlson et al. measurements indicated that the average area of the vestibule is 330 mm2 to 630 mm2 (534 mm2) in males and 450 mm2 in females. The anteroposterior diameter of the outlet of the S1 vestibule is larger than its superoinferior diameter. The angle of the SI ranged from 19° to 45° and the angle of AI ranged from 0° to 25°. Noojin et al. (2000), measured the vestibules of 13 adults (6 men and 7 women) via CT and found that the average VW of all specimens was 28.05 mm (22.10 mm to 34.00 mm) and the VH was 27.76 mm (22.80 mm to 35.90 mm). Racial differences in sacral anatomy, especially among Chinese adults, could lead to unsatisfactory treatment results with a high risk of complications. Consequently, vestibule measurements from Chinese adults should be analyzed to determine whether different races have significant differences and to address the clinical significance of individual vestibule measurement.

The VW and VS of the S1 vestibule showed significant differences (p<0.05) between male and female subjects, but VH was not significantly different (p>0.05). The width and area of the S1 vestibule in males' were larger than those of in females. The proper location to insert iliosacral screws was parallel to the long diameter with the inclination angle of the vestibule; therefore, VW and VS are very important references for the operation. Given that both the VW and the VS of females were smaller than those of males, the insertion location, direction of the screw, and the position relationships between the screws are particularly limited for female patients. No significant difference in vestibule measurements was observed on the left and the right side of the body. The vestibule contralateral to the sacral fracture can be measured as a reference for the operation and can be used to determine the feasibility of the operation and the number of screws necessary for fixation.

Compared with previous reports, the anatomical parameters of the vestibules of Chinese adults were generally smaller than those of Caucasians. The VS of both Chinese males and females was about 20% smaller than that of Europeans and Americans. Thus, standards for operations need to be adjusted based on the individual characteristics of the patient, including ethnicity. Smaller screws are recommended for patients with smaller vestibules, as long as strength is sufficient. Ebraheim et al. (1997) suggested that the screw should be at least 5 mm away from the cortical edge to ensure patient safety. Based on this standard, the following conclusions were drawn: the smallest bone area for safely inserting two screws is 318.10 mm2 if a 6.5 mm hollow pulling screw is used, and the smallest transverse diameter of the section area is 23 mm with the smallest vertical diameter of 16.5 mm. The smallest bone area required for the safe insertion of one screw is 203.72 mm2, with both the smallest transverse diameter and the vertical diameter at 16.5 mm. Considering the sacral vestibule is the narrowest part of the 3D available screw space in the iliosacral screw channel, we can use the anatomical parameters of the vestibule as the minimum measurements for feasibly inserting different numbers of screws. The average vestibule width, height, and area in both sexes were all higher than the lowest requirements for inserting two 6.5 mm screws. Therefore, the sacral vestibule of Chinese adults generally allows the use of two iliosacral screws for fixation. Analyzing the data by gender, we found that VW, the anteroposterior diameter, was 23.74±1.59 mm in females, which is slightly larger than the lowest requirement for inserting two 6.5 mm screws. However, the VS of some women was smaller than the lowest requirement with the smallest vestibule area (196.96 mm2), even lower than the lowest required area for safely inserting a single screw. Therefore, special attention should be given to female patients who undergo iliosacral screw fixation. Routine CT scans or other imaging examinations should be conducted to obtain the specific anatomical parameters of the sacral vestibule of the patient and determine whether the vestibule area and long axis length allow safe insertion of one screw.

The anatomical parameters of the S1 vestibule of Chinese adults in the current study were generally smaller than those previously reported for other adults. The area of the vestibule was approximately 20% smaller than that found in previous studies, which indicates that more attention and accuracy are necessary for operations in this region. The anatomical parameters of the sacral S1 vestibule of females were significantly smaller than those of the males and close to the lowest requirement for fracture fixation (with two iliosacral screws). Therefore, female patients should undergo CT scans or other imaging examinations to measure the sacral anatomical parameters of the S1 vestibule for a more careful and precise operation.

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Correspondence to:

Wei Tian

Tianjin Medical University
Tianjin 300070
CHINA
Phone: +86-13820737331
Fax: +86-22-60362990

Email: weitiancn@163.com

Received: 19-10-2013
Accepted: 13-12-2013