Sacrum

Normal Anatomy of Sacrum and Clinical correlation

1. Sacrum Dr. Mathew Joseph MBBS,MD(2 nd Year),BCCPM Junior Resident Department of Anatomy All India Institute of Medical Sciences - Rishikesh

2. Learning objectives Sacrum: Introduction Anatomical position Attachments Sexual dimorphism Ossification Clinical Correlation

3. Introduction Wedge shaped. Triangular fusion of five vertebrae. Posterosuperior wall of pelvic cavity. Wedged between two hip bones. Base : Superior(Articulates with L5 at lumbosacral angle) Apex: Inferior(Articulates with coccyx) Surfaces: Anterior/Pelvic Posterior/Dorsal 2 Lateral

4. Anatomical Position Sacrum is a midline bone plced between hip bones(on each side), 5 th Lumbar vertebra (Superiorly) and Coccyx (Inferiorly). Superior surface of the body of 1 st sacral vertebra slopes forward at an angle of 30 degree. Anterior surface of sacrum faces downwards and forwards. The upper end of sacral canal is directed upwards.

5. General Features Sacrum consists of: Base Apex Four Surfaces : a) Pelvic b) Dorsal c) Right Lateral d) Left Lateral 4. Sacral Canal

6. Base

7. Pelvic Surface Concave surface : Facing downwards and forwards. Features : Transverse ridges. Anterior sacral foramina: Ventral rami of upper 4 sacral nerves. Lateral mass: Lateral to foramina.

8. Pelvic Surface

9. Pelvic Surface Muscle Attachments: Piriformis Coccygeus Sacrospinal ligament.

10. Dorsal Surface Five vertical crests : a)Median sacral crest. b)2 intermediate sacral crests. c)2 lateral sacral crests. Sacral hiatus : U shaped gap at lower end.

11. Dorsal Surface

12. Dorsal Surface Muscle Attachments: Erector spinae . Multifidus . Gluteus maximus . Sacrotuberous ligament .

13. Lateral Surface

14. Sacral Canal Contents: Lower part of cauda equina . Filum terminale . Spinal meninges. Lateral sacral vessels. ** Dura and arachnoid extents up S2.

15. Sexual Dimorphism in Sacrum Features Male sacrum Female sacrum Length More Less 2. Ratio between the transverse width of body of 1 st sacral vertebra and the entire width of sacral base. More than 1/3 rd . Less thn 1/3 rd . 3. Auricular surface Relatively longer, upper three segments. Smaller, occupies only upper two segments of sacrum. 4. Anterior surface of sacrum Shallower Deeper 5. Sacral Index [ Breadth of the base X 100] Length Lesser Greater 6. Width Relatively narrower Wider 7. Curvature Uniformly curved Flattened in the upper part but sharply curved in the lower part.

16. Ossification Chondrification is initiated in the 5th gestational week and results in a cartilaginous vertebral column . Primary or enchondral ossification occurs in three primary ossification centers (central, neural, and costal) and forms the axial skeleton . In the sacrum, the costal ossification centers form a portion of the lateral mass. A total of six centers produce the sacral alae .

17. Bilateral neural ossification centers contribute to the neural arch and the posterolateral vertebral body. The central ossification center forms the midportion of the vertebral body. With secondary ossification, two epiphyseal plates provide accessory ossification to the superior and inferior portions of each sacral vertebral body. Disks separate the sacral vertebrae during childhood . The S3-4 and S4-5 disks fuse in late adolescence, and the remaining levels fuse during the 3rd decade of life.

18. Clinical Correlation Sacralisation : Congenital anomaly. Incorporation of the fifth lumbar (L5) or first coccygeal vertebra (C1) in the sacrum. Number of sacral foramina is increased unilaterally or bilaterally.

19. Lumbarisation : First sacral vertebra (S1) is separated from the sacrum and fused with the fifth lumbar vertebra (L5). Number of sacral foramina reduced to 3.