The fusion of five vertebrae forms the sacrum and transfers the body weight from the trunk to the pelvis [1]. The sacral corridor (Sc) is the area on the sacral vertebra through which a screw is inserted to fix a sacral fracture. It (Sc) is a bony bridge that extends throughout the sacral segments. The bony corridors of the first, second, and third sacral segments are represented as first sacral corridor (S1c), second sacral corridor (S2c), and third sacral corridor (S3c), respectively (Fig. 1A).

The cephalocaudal (cc) dimension of the S1c is measured between the ala of the sacrum above and the first sacral foramina (F1) below (Fig. 1B). The S2c is bounded above and below by the F1 and second sacral foramina (F2), respectively. Similarly, the vertical limits of the S3c are the F2 and third sacral foramina (F3) (Fig. 1). All three Sc are bounded in front by the anterior sacral cortex (asc) and spinal canal (spc) behind (Fig. 2). The anteroposterior (ap) distance of each corridor was measured as the maximum distance from asc to spc. The neurovascular bundle lies in close vicinity of the sacral segments. So, to prevent nerves and vessels from being damaged, the screw needs to be placed uniformly within the bony sacral bridge. The anatomy of the upper sacral segment is highly variable [2, 3]. The total number of vertebrae in the lumbosacral region remains the same, but the fusion level of a transitional vertebra differs. Such variants are known as lumbosacral transitional vertebra (LSTV), which include sacralized fifth lumbar (L5) and lumbarized first sacral (S1) vertebra [4]. These variations may change the sacral inclination. Lack of awareness of adequacy of the intraosseous space (corridor) in these variable sacra leads to malpositioning of the screw. Routt et al. [2] described that screw malpositioning could be prevented with the help of a lateral sacral radiographic image. Due to limited bio-morphometric data, Gras et al. [5] detailed the precise sacral anatomy to look for anatomic variation in the European population. Some authors have given quantitative parameters to avoid the failure of the technique. Few morphometric studies on Sc have been investigated in different populations. The adequacy of the Sc varies in different ethnic groups and sex. Wagner et al. [3] studied Sc in European and Japanese populations, but their results cannot be generalized to the other ethnic groups [6]. Mendel et al. [7] reported a triangle ratio of 1.5 (anterior height and superior width ratio of S1 body), representing a reliable variable for screw placement. The sacral dysmorphic score was developed by some authors based on the angulation of the upper sacral segment [8]. Gardner et al. [9] quantified safe zones in normal and dysmorphic sacra using computed tomography (CT). Appropriate morphometric data of Sc are required by the surgeons to prevent screw malpositioning and neurovascular complications. The diameter of the implant required for sacral fracture fixation is 7.3 mm. The consideration of 2 mm of additional safe space on each side (7.3+2+2 mm) leads to a total anatomical corridor width of 11.3 mm (approx ≈12 mm). In the present study, the corridor ≥12 mm was assumed as adequate [6]. Limited studies exist regarding the Sc dimension in the Indian population [6]. In the present study, ten dimensions of the sacrum were measured, which included the cc and ap dimensions of the upper three Sc, length of the sacrum (L), the trans-sacral distance (TSD-S1, TSD-S2) at the level of the S1c and S2c, and iliosacral distance (ISD-S1) at the level of the first sacral segment (Table 1). The purpose of the study is to estimate the safe dimension of the corridor to avoid neurovascular damage during the fixation of sacral fractures in the Indian population. The study aimed to evaluate the cc and the ap dimension of the Sc in the upper three sacral segments (S1c, S2c, and S3c). The study further examines the effect of age and sex on the dimensions of the Sc at different sacral levels. In addition, the Sc dimensions of normal and LSTV sacra were explored.

This retrospective study was conducted in the Department of Anatomy to determine the dimension of the Sc of the upper three sacral segments. The research protocol was submitted for assessment and was approved by Institutional Ethics Committee of AIIMS Patna (IEC/2021/800 dated 16 September 2021).

The sample’s median age is 43 years (age range, 18–85 years). In the given sample, there are 91 males and 89 females. The median age of the male sample was 44 years, with an interquartile range of 31.0 years (range, 18–82 years), and the median age of the female sample was 41 years, with an interquartile range of 31.0 years (range, 18–85). A total of 154 sacra are normal (median age, 44 years [range, 18–82 years]), and the rest are LSTV (median age, 36.5 years [range 18–85 years]). The first corridor provided the maximum space (vertical and ap) compared to the others (Table 2). The details of descriptive statistics are given in (Table 2).

“Safe zone” is the area within the sacrum where the screw can be ideally placed to fix the fractured sacral segments without damaging the nerve roots and vessels lying in its vicinity. To pass a screw safely without a cortical breach, it must traverse the ipsilateral and contralateral osseous corridor, which must be of sufficient size [3, 10]. In the present study, S1c had the maximum corridor space, followed by S2c and S3c. The corridor space in normal sacra and LSTV showed no significant difference (P>0.05). The S3c is the least preferable corridor for the screw placement. The ap diameter of all three corridors showed sexual dimorphism (P=0.013, 0.0011, and <0.0001). The length of the sacrum also showed significant differences among males and females (P<0.00016). The ap dimension of the second sacral corridor (ap-S2c) has a moderate correlation with ap-S1c (R=0.519) and ap-S3c (R=0.677).

In time, authors have detailed the morphological divergences of the sacrum and its developmental malformations [1, 11]. These variations could change the symptoms and the surgical techniques during operations [8, 12]. The lumbosacral junction is one of the most variable regions of the spinal column. When the fifth lumbar vertebra (L5) fuses to the first sacral segment (S1), it is called sacralization of the lumbar. In contrast, the fusion of S1 to the L5 is known as the lumbarization of the sacrum. The LSTV includes sacralized L5 and lumbarized S1 [4]. The vertebral body develops by combining the caudal part of the sclerotome above and the cranial part of the sclerotome below. The intervertebral disc is formed by the mesenchymal cells. However, during the development, a variable fusion pattern between the caudal lumbar vertebra and the upper sacral segment has been observed [12]. Based on a study by Woods et al. [13], the corridor at the level of the first sacral body may be narrow or absent. Authors have reported a high rate of neurological complications during screw fixation. It is due to difficulty visualizing the complex anatomy of the upper sacral segments [9]. In the current study, the cc and ap diameter of the upper three sacral segments have been accurately determined in both normal and LSTV sacra. The surgeons should keep this data in mind to avoid errant screw placement and neurovascular injury. Some published data regarding the Sc’s critical dimensions are available in the literature with varying results. The reported dimension varies greatly among researchers and populations (Table 3). Wagner et al. [3] considered >12 mm an adequate corridor. They found 48% adequate (non-critical) corridor at the S1 level and 79% at the S2 level in the European and Japanese populations. In the present sample, 97.2% of the corridor at the S1 level and 29.4% at the S2 level were adequate. The results of Keereewan et al. [14] showed that all sacral dimensions of males were larger than those of females. Trikha et al. [6] reported that the cc and ap diameters were significantly higher in males at the S1c level (Table 3). The present study found no significant difference between males and females in the cc dimension (Table 2). Other authors also did not find a similar difference [3, 5, 15]. Trikha et al. [6] might have observed the difference due to a lower sample size. However, the ap diameter was significant at all three sacral levels (i.e., ap-S1c, ap-S2c, and ap-S3c) (Table 2). Age-related changes in vertebral dimensions have been mentioned by some authors [16]. The increase in vertebral body size among the elderly population due to periosteal apposition is a notable observation made by Junno et al. [16]. This (periosteal apposition) results in increased cross-sectional area (CSA) of the vertebral body. These changes are observed to be sex-specific, as a marked increase in CSA of the vertebral body is found only in elderly males. Our findings align with the results of Junno et al. [16] up to a certain extent. In the present study, the significant ap dimensions among the sexes may be due to the increased CSA in males. Understanding the age-related changes in vertebral dimensions becomes crucial for addressing implications for spinal health and function in elderly individuals. In the population aged 50 years or older, the prevalence of osteoporosis was reported to be 21% in females and 6% in males. Osteoporosis does reduce bone mineral density, leading to pelvic fractures in elderly females [17]. We agree with the authors, but age does not affect the Sc dimension according to our observation in the present study. In a study by Iga [15], 40.5% of the Japanese sample had adequate horizontal corridors (i.e., >10 mm) in S1 and 69% in S2. The rest of the sample had less than 10 mm (inadequate) of dimension in S1 (59.5%) and S2 level (31%). The present study shows that S1c is appropriate for screw placement in 97.8% of males and 96.6% of females. The S2c allows safe space only in 34% and 24% of Indian males and females, respectively.

The data belongs to the north Indian region, so it may not be generalized to other populations. This is the limitation of this study. The corridor dimensions were measured by one observer. Therefore, interobserver reliability is not provided. We have not included the fractured sacrum in this study, which could have altered the result. In the present study, morphometry was done in a three-dimensional reconstructed image. Whereas in hospitals, two-dimensional images are commonly used. A comparative study with other populations was not done due to a lack of data.

In conclusion, S1c has the largest cc and ap diameter in the Indian population. The ap diameter of the corridor at S1c, S2c, and S3c levels was significant among males and females (P<0.05). The length of the sacrum also showed sacral dimorphism (P<0.05). The screw can be placed in the first sacral segment taking the safe margin in both LSTV and normal sacra, avoiding cortical breach and neurovascular damage. Knowledge of the adequacy of the corridor is helpful to orthopedic surgeons for placing screws during the fixation of the sacral fracture.