Features on the dorsal surface of the sacrum

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1 Original Article Features on the dorsal surface of the sacrum Sayee Rajangam, Safeer Khan, Flossie Jayakaran Abstract Background and Aims: The present study was undertaken to report the observed variations in the features on the dorsal surface of the sacrum. Materials and Methods: Twelve female and 10 male sacra were used for the study. The studied features on the dorsum of the sacrum include sacral hiatus, spinous and articular tubercles (AT) of the median and intermediate sacral crests and sacral foramina. Results: The apex of the sacral hiatus was an inverted V in 10 and a U in 12 (54.5%) sacra. The apex was located between S3 and S4 in 12 (54.6%) and the base at S4 in 21 (95.5%) sacra. Spinal tubercles (ST) and AT were absent in two each of the sacra. Sacral foramina were seen five in one male sacrum along with ossified sacrococcygeal joint. In two female and two male sacra, sacralization of the 5 th lumbar vertebra were noted. Six features were a common occurrence in more than 50% of the female and male sacra: U-shaped apex; apex location between S3 and S4; base location below S4; medium length of hiatus with apex between S3 and S4; three numbers of ST; and four numbers of sacral foramina. Three features were commonly seen for the female sacra: U apex between S3 and S4; base below S4; and four numbers of AT. For the male sacra, the four common features were: V-shaped apex; base below S4; three AT; and circular sacral foramina. Conclusion: The implications from the findings of the present study are directed toward the importance of the variations in the features of the dorsal surface of the female and male sacra for any procedures involving caudal epidural anesthesia. Keywords: Articular/spinal/transverse tubercles, intermediate/lateral/median sacral crests, dorsal surface of sacrum, sacral hiatus Introduction The sacrum is a large triangular bone formed by the five fused sacral vertebrae (SV). Variations in sacrum and its components, especially the features on its dorsal surface, have been reported. [1,2] Clinical application depends on the knowledge available about the variations seen in the sacrum, which are important when interpreting the radiographs and when noting the precise sites of the pathological features of bone and soft tissue. Hence, Quick Response Code: Access this article online Website: DOI: ***** surface anatomy in the sacral region is considered to be an important aspect. [3] In 1942, it was reported that in obstetrics, the sacral hiatus is the region selected for the administration of continuous caudal epidural anesthesia (CEA). [4] During CEA, a frequent problem faced is placing the needle in the sacral canal through the sacral hiatus. The apex location is required for other procedures such as the caloscopy and trans-sacral endoscopy. But, locating its position is noted to be difficult because of the anatomical variations of the sacral hiatus, which may be due to ethnic and racial differences, genetic factors and also due to obesity. In view of its clinical importance, a number of studies have reported anatomical variations of the sacral hiatus. [5-9] In the present study, it was aimed to report the observed variations on the features in and around the sacral hiatus; any other features on the dorsal surface of the sacra and their association to each other as well as to the sex of the sacra. Department of Anatomy, International Medical School Bangalore, Karnataka, India Address for correspondence: Dr. Sayee Rajangam, Department of Anatomy, International Medical School, Bangalore , Karnataka, India. drsayee@gmail.com

2 Rajangam, et al.: Features on dorsal surface of sacrum 113 Materials and Methods In the Department of Anatomy, International Medical School, Bangalore, among the teaching material for Osteology, 34 sacra were available, of which 22 were selected and the rest were discarded because they were either incomplete or were damaged. Subjective sexing was done with the help of described features in the standard text books in anatomy. [2] The 22 sacra were identified as 12 female and 10 male sacra. The noted features on the dorsal surface of the dry sacra were: (1) the shape of the sacral hiatus based on the upturned end of its apex, (2 and 3) locations of the apex and the base of the sacral hiatus, (4 and 5) spinal tubercles (ST) and articular tubercles (AT) on the median and intermediate sacral crests, (6) number and shapes of sacral foramina and (7) any other features. For all features, the percentages were calculated. Results The observed features on the dorsal surface of the 22 sacra are listed in detail in Table 1 and Figures 1-3. = Sacral Hiatus: Apex: Shape: The shape of the apex of the sacral hiatus was grouped under two categories: inverted V or U. V shape was seen in 10 sacra (45.5%) and U shape in 12 sacra (54.5%). The 10 sacra with a V-shaped apex were seen in five male and female sacra. Among the 12 U-shaped sacral hiatuses, seven were female (58.3%) and five were male (41.7%) sacra. Apex: Location: The apex of the sacral hiatus was located between S3 and S4 in 54.6% (12) of the sacra (12) and, in the rest, it was between S2 and S3 (5,22.7%) or at S4 (5,22.7%). Among the 12 sacra with the location of the apex between S3 and S4, it was present in seven female (58.3%) and five male (41.7%) sacra. In the female, the location of the apex was not observed below S4. Base: Location: The base of the sacral hiatus was located at S4 in 21 sacra (95.5%) [12 female (57%) and nine male (43%)] and between S4- S5 in one male sacrum. Apex and Base of the Sacral Hiatus v/s Sex of the Sacra: The shape and the location of the apex of the sacral hiatus showed the association for six female sacra with U-shaped apex between S3 and S4. The shape of the apex and the location of the base of the sacral hiatus showed the association for Table 1: Features: Dorsal surface of sacra Shape Apex: Location Base: Location ST/AT Any other features V S2 and S3 Below S4 3/4; 4 Long hiatus V S2 and S3 Below S4 3/4; 4 Long hiatus. Cornua interrupted between S3 and S4 U S3 and S4 Below S4 3/4; 4 Sacralization: L5 U S4 Below S4 4/4; 4 Sacralization: L5 U S3 and S4 Below S4 3/5; 5 U S3 and S4 Below S4 3/4; 4 V S2 and S3 Below S4 2/4; 4 Long hiatus V S3 and S4 Below S4 -/- Fused STs of 1 st, 2 nd and 3 rd SV; Median sacral crest seen like a bony ridge; Ossified interspinous ligaments; S3 to S5: Posterior surface of their vertebral bodies visible through sacral hiatus V S2 and S3 Below S4 2/3; 3 Long hiatus U S3 and S4 Below S4 3/3; 3 U S3 and S4 Below S4 -/3; 3 Fused STs of 1 st, 2 nd and 3 rd SV; Median sacral crest seen like a bony ridge; Ossified interspinous ligaments; S3 to S5: Posterior surface of their vertebral bodies visible through sacral hiatus U S3 and S4 Below S4 4/3; 3 Male U S3 and S4 Below S4 3/4; 4 V S3 and S4 Below S4 3/3; 4 U S3 and S4 Below S4 3/3; 4 V S2 and S3 Below S4 -/- Smooth area on the region of the median and intermediate sacral crests Long hiatus Sacralization of L5 V At S4 Below S4 4/3; 3 Large S4 tubercle in the median crest Dorsally fused L5 above S1? sacralization V At S4 Below S4 3/3; 3 Large S2 tubercle in the intermediate crest U S3 and S4 Below S42 4/3; 3 Large S4 tubercle at apex of hiatus U At S4 Below S4 2/3; 3 Flat median crest below S2 tubercle V S3 and S4 Below S4 3/4; 4 Sacral vertebrae not fused Deep fossa lateral to auricular surface U At S4 S4 and S5 3/- Smooth area on the region of the intermediate sacral crest Long hiatus. 1 st SV: bifid spinous process 5 th sacral foramina present; because of the articulation between the transverse processes of the 1 st Co V and the inferolateral sacral angle and ossified bilateral sacrococcygeal ligament Note: (i) ST/AT: Spinal/transverse tubercles and (ii) For AT, the right and left side numbers are given September 2014 Vol 19 Issue 2 Journal of Mahatma Gandhi Institute of Medical Sciences

3 114 a Figure 1: Dorsal view: Sacral hiatus (a) V apex and long sacral hiatus (b) U apex and short sacral hiatus Figure 2: Sacrum: Dorsal view: Sacralization of 511, lumbar vertebra: five dorsal sacral foramina and incomplete spinal tubercles Figure 3: Sacrum: Dorsal view: 5 dorsal sacral foramen and sacralization of 1s CoV b seven female sacra with a U-shaped apex at S4. = Median Sacral Crest with ST: ST was absent in three sacra. The observed numbers of the ST ranged from two to four tubercles. ST v/s Sex of the Sacra: ST was absent in two female and one male sacra. The three numbers of ST were seen in six female and six male sacra. The female sacra with absent ST showed fused STs of 1 st, 2 nd, 3 rd SV; its median sacral crest was shaped like a bony ridge with ossified interspinous ligaments Rajangam, et al.: Features on dorsal surface of sacrum and the dorsal surface of the vertebral bodies of 3 rd to 5 th SV were visible in the sacral hiatus; one of them also had absent AT. In the two sacra with absent ST and AT, the apex was V shaped with a location between S3 and S4 and one had a U-shaped apex with a location between S3 and S4. The male sacrum with absent ST also had absent AT, with a smooth area in the region of the median and intermediate sacral crests; a long hiatus; sacralization of L5 and a V-shaped apex with a location between S2 and S3. ST v/s Apex of the Sacral Hiatus v/s Sex of the Sacra: ST with three numbers were associated in four female sacra to U-shaped apex and to both V- and U-shaped apex in three each of the male sacra, respectively. ST v/s Location of the Sacral Hiatus v/s Sex of the Sacra: ST with three numbers were associated to the location of the apex between S3 and S4 in four female sacra. In three each of the male sacra, the apex was located between S3 and S4 as well as at S4. ST v/s Apex of the Sacral Hiatus and its Location v/s Sex of the Sacra: ST with three numbers were associated with the U-shaped apex in four female sacra at the location between S3 and S4. = Intermediate Crest with AT: AT was absent in three sacra. The observed numbers of the AT when present ranged from 3 to 5. AT v/s Sex of the Sacra: AT was absent in one female and two male sacra. AT numbers were four in six female sacra and three in six male sacra. The features of the female and male sacra with absent AT and ST were already described. The 2 nd male sacrum with absent AT showed smooth area in the region of the intermediate sacral crest; long hiatus; 1 st SV with bifid spinous process; five sacral foramina; sacrococcygeal joint; ossified bilateral sacrococcygeal ligament and U apex at S4. AT v/s Apex of the Sacral Hiatus v/s Sex of the Sacra: AT with three numbers tubercles were associated with both U- and V-shaped apex in three each of the female sacra. AT with four numbers tubercles were associated to both U- and V-shaped apex in three each of the male sacra. AT v/s Location of the Sacral Hiatus v/s Sex of the Sacra: AT with three numbers were associated in four female sacra to the location of the apex between S3 and S4. In male sacra, the apex was located in three each either between S3 and S4 or at S4. ST v/s Apex of the Sacral Hiatus and its Location v/s Sex of the Sacra: AT with three numbers was associated with U-shaped apex in four female sacra at the location between S3 and S4. = Other features: - Long sacral hiatus was seen in six sacra, of which five had V-shaped apex with its location between S2 and S3 and the 6 th one with a U apex had its location at S4. Even though

4 Rajangam, et al.: Features on dorsal surface of sacrum 115 the apex was at S4, the sacral hiatus appeared long because it had ossified sacrococcygeal joint with five sacral foramina. Long sacral hiatus was present in four female and two male sacra, of which four female and one male sacra had V apex with location between S2 and S3 and the 2 nd male sacrum had U apex at S4. - The cornua of the sacral hiatus was interrupted in a female sacrum between S3 and S4 with V apex located between S2 and S3. - Sacralization of the 5th LV was present in four sacra. Among them, two were female sacra with U apex between S3 and S4 and at S4. The other two were in male sacra; one of them with V apex between S2 and S3 had absent ST and AT and smooth area in the regions of the median and intermediate sacral crests and the other male sacrum with V apex at S4 had a large 4 th ST. - Sacral foramina were four in number in 21 sacra, which included 12 female and nine male sacra. In one of the male sacrum, there were five foramina because it had sacrococcygeal joint and ossified bilateral sacrococcygeal ligament. Sacral foramina were deeper and circular in 10 male and five female sacra. Discussion The highlight of the sacral hiatus is its role in CEA as anesthetic solutions are injected into the sacral canal through the sacral hiatus. In obstetrics, CEA is in use for nerve block to relieve the pain during the 1 st and 2 nd stages of labor, and the advantage is that the anesthetic does not affect the infant. [1] In 1999, it was reported that caudal epidural block may have a 25% rate of failure. [10] The suggested common cause is the presence of anatomical variations not only at the level of the apex but also the deficiencies in the dorsal wall and the difficulty in palpating the apex in some patients. The anatomical landmarks found to be facilitatory in guiding the clinicians for successful caudal epidural block are the equilateral triangles between the two sides of the superolateral crest and the apex of the sacral hiatus (varying from 14.2% to 29%) [9,11] and the posterior superior iliac spines and sacral hiatus (51%). [12] The observations from the present study are discussed with the relevant studies from the literature. The shape of the sacral hiatus is observed to show variations from inverted V or U to irregular to dumbbell shapes, and the shape is found to be predominantly V or U. [9] The location of the apex of the sacral hiatus is also reported to show variations from S2 to S4 SV. The common finding is its position at S4 SV. [2,5,7] From Table 2, it is seen that the percentage occurrence of V-shaped apex is from 28.27% to 46.53% and that of the U-shaped apex is from 29.7% to 41.5%. In the present study, the shape of the apex of the sacral hiatus was either inverted V (45.5%) or U (54.5%). The percentage occurrence of V was within the reported range in the literatures. The higher occurrence of U apex in the present study is similar to the study by Nagar. [7] The occurrence of U apex is higher than the reports from the literature because, in the present study, the shape of the Table 2: Review: Sacral hiatus Shape Kumar et al., Nagar, 2004 [7] 1992 [5] (n = 270) Shape of sacral hiatus: Authors, year, n and/or % Aggarwal et al., Patel et al., Suma et al., Phalgunan and 2009 [13] 2011 [14] 2011 [8] (n = 150) Baskaran, 2013 [9] (n = 28) Present study, 2013 (n = 22) V 46.53% 73, 27% 31.57% 28.27% 10, 35% 10, 45.5% U 29.7% 112, 41.5% 40.35% 44% 10, 35% 12, 54.5% Irregular 38, 14.1% 15.78% 15.5% 10.1% 8, 28% Dumb-bell 36, 13.3% 7.01% 12.4% Bifid 4, 1.5% 4.38% 5.33% Apex location of sacral hiatus S2 9, 3.4% 4% 2.63% 0.93% 2, 7.1% S2 and S3 5, 22.7% S3 98, 37.3% 15% 14.03% 4.67% 13, 46% S3 and S4 12, 54.6% S % 147, 55.9% 64% 68.42% 68.42% 13, 46% 5, 22.7% S4 and S5 S5 9, 3.4% 15% 14.9% 14.9% Base location of sacral hiatus S4 29, 11.1% 10% 21, 95.5% S4 and S5 1, 4.5% S % 191, 72.6% 64.38% Coccyx 43, 16.3% 25.62% September 2014 Vol 19 Issue 2 Journal of Mahatma Gandhi Institute of Medical Sciences

5 116 apex of the sacral hiatus was divided only into two groups. In the present study, the location of the apex of the sacral hiatus was considered as between the SV because it was found to be difficult to pinpoint the exact position corresponding to or at the level of an SV, even with the help of a ruler; hence, it was considered as S2 and S3 or S3 and S4, except for the location at S4. The reported location of the apex in the literature extended from S2 to S5, and was of high percentage at S4, with a range from 46.4% to 59.3%. In the present study, in 54.6% of the sacra the apex was located between S3 and S4. The location of the base of the sacral hiatus was at S4 for 21 sacra (95.5%). In the literature, the location of the base extended from S4 to coccyx, and the common site was at S5, with a range from 64.38% to 83.17%. It is stated that the common arrangement for the formation of the sacral hiatus is by the laminae of the 5 th SV and, sometimes, the 4 th SV if it fails to meet in the midline; thereby, their vertebral bodies enter into the formation of the sacral canal. The hiatus is triangular or U shaped and is bounded on the sides by the sacral cornua. The size and shape of the hiatus depend on the number of the laminae that failed to fuse in the midline posteriorly. [1,2] From the findings, it is seen in the present study that the shape and length of the hiatus indeed depended on the number of the laminae that failed to fuse posteriorly in the midline. The apex location was noted between S2 and S3 in five, between S3 and S4 in 12 and at S4 in five. Then, accordingly, the laminae that failed to fuse are from S3 to S5. In the literature, the length of the sacral hiatus was described based on the actual measurements from the apex to the midpoint of the base. In a study, [7] the length was grouped into eight numbers starting from 0 to 10 mm to more than 51 mm, and the maximum number of 126 was noted for the middle groups with a length from 21 to 30 mm and 31 to 40 mm. In the present study, subjectively, based on the locations of the apex, the length of the hiatus was divided into three groups: Long, medium and short. The long hiatus was considered from S2 and S3, the medium hiatus from S3 and S4 and the short hiatus from S4. Long hiatus was present in five, medium hiatus in 12 and short hiatus in five. The sacra with the long or medium or short hiatuses may have had for their sacral canal as its anterior wall the posterior surfaces of the bodies of the 3 rd to 5 th SV. Rajangam, et al.: Features on dorsal surface of sacrum Additional observations from the present study In the present study, presence of ST and AT on the median and intermediate sacral crests were noted. Median sacral crest is an interrupted raised ridge with three or four ST, which represent the fused sacral spinous processes. The intermediate sacral crest situated medial to the dorsal sacral foramina and below the articular processes of the 1 st SV representing the fused articular processes of the 2 nd, 3 rd and 4 th SV is a row of four small AT. Lateral to the dorsal sacral foramina is the lateral sacral crest, a rough interrupted structure, which represents the fused transverse processes of the SV and the apex of the processes project as a row of TT. The inferior articular processes of the 5 th SV being a free structure project at the sides of the sacral hiatus as the sacral cornua and are connected to the coccygeal cornua by the intercornual ligaments (Standring 2008). [2] In the present study, as stated in the literature, the maximum numbers for the ST and AT were three and four, respectively. In one sacrum, the inferior articular processes of the 5 th SV were connected by the ossified intercornual ligaments with the 1 st Co V. The four pairs of the dorsal and ventral sacral foramina are situated lateral to the fused laminae of the SV; they communicate with the sacral canal through the intervertebral foramina and transmit the dorsal and ventral rami of the sacral spinal nerve. [1,2] In the present study, the four pairs of the dorsal and ventral sacral foramina were present in 21 sacra (95.45%); one sacrum (4.5%) had five foramina because of the presence of the sacrococcygeal joint. Coccygeal ankylosis was reported in 43 sacra (16.3%) in the study by Nagar. [7] Sacrum may have six vertebrae by the presence of an additional SV or by the incorporated 5 th LV or the 1 st Co V. Sacralization of the 5 th LV is observed to be incomplete and limited to one side. The transverse process of 5 th LV may be large and articulates with the posterolateral angle of the base of the sacrum. [1,2] In the present study, sacra with six vertebrae were present in five, of which four (two female and two male) (18.2%) had sacralization of the 5 th LV and the 5 th was articulated to the 1 st Co V in a male sacrum. In the literature, it is stated that the bodies of the 1 st and 2 nd SV may remain unfused, even though the lateral masses are fused, the laminae are completely or partially absent or there could be spina bifida

6 Rajangam, et al.: Features on dorsal surface of sacrum 117 occulta, i.e. the spinous process is affected with no abnormalities in the overlying soft tissues. [1] In the present study, spina bifida occulta was noticed in one male sacrum with ossified sacrococcygeal joint for the 1 st SV. It is known that the sacra may have variations in its features. Male sacra often have more than the five numbers of SV because of the addition of LV or Co V. [1,2] In the present study, two male sacra had addition of LV and one had Co V. In the present study, the observed features were listed as per the sex of the sacra. It could be stated that for the first time among the studies on sacra, such analysis has been attempted. The total number of the features were 107. Six features (5.6%) were common in their occurrence of more than 50% for both the female and the male sacra. Female sacra: U-shaped apex of the sacral hiatus in seven (58.3%); apex of the sacral hiatus located between S3 and S4 in seven (58.3%); base of the sacral hiatus located below S4 in 12 (100%); medium length of the sacral hiatus with apex between S3 and S4 in seven (58.3%); three numbers of ST in six (50%); and four numbers of sacral foramina in 12 (100%). Male sacra: U-shaped apex of the sacral hiatus in five (50%); apex of the sacral hiatus located between S3 and S4 in five (50%); base of the sacral hiatus located below S4 in nine (90%); medium length of the sacral hiatus with apex between S3 and S4 in five (50%); three numbers of ST in six (60%); and four numbers of sacral foramina in nine (90%). Three features (2.8%) only were seen with a more than 50% occurrence for the female sacra: U-shaped apex between S3 and S4 in six (50%); U-shaped apex with base location below S4 in seven (58.3%); and four numbers of AT in six (50%). Four features only were seen with more than 50% occurrence for the male sacra: V-shaped apex in five (50%); V-shaped apex with base location below S4 in five (50%); three numbers of AT in six (60%); and circular and deeper sacral foramina in 10 (100%). Features were present in the female sacra but absent in the male sacra and vice versa, and 31 (29%) features were absent both in the female and in the male sacra. The observed U apex and location between S3 and S4, and its association to the female sacra, is a valid point for CEA in female. The observed differences between the literature and the present study on the features of the sacral hiatus and the dorsal surface of the sacrum may be because of the sample size, the number of September 2014 Vol 19 Issue 2 features that were studied and sex determination of the sacra. Conclusion The present study has reported the observed features from 22 dry sacra. There were 12 female and 10 male sacra. The maximum numbers were seen for the U-shaped apex of the sacral hiatus and its location between S3 and S4 in 12 and the location of the base of the hiatus at S4 in 21 sacra. Both ST and AT were together absent in two and individually absent in two sacra. ST with three numbers were seen in 12: Six female and three male sacra. AT with four numbers were seen in six female and with three numbers in six male sacra. Sacralization of the 5 th LV was present in four sacra. Sacral foramina were four in 19 sacra and five in one sacrum because of the ossified sacrococcygeal joint. The features were associated with each other as well as with that of the sex of the sacra. The variations in the features reflected the structural and functional differences in the female and male sacra and the pelvis. The [resent study has added further data on the anatomical variations in the region of the sacral hiatus and on the dorsal surface of the female and the male sacra. 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