First sacral nerve and anterior longitudinal ligament anatomy: clinical applications during sacrocolpopexy

Background The recommended location of graft attachment during sacrocolpopexy is at or below the sacral promontory on the anterior surface of the first sacral vertebra. Graft fixation below the sacral promontory may potentially involve the first sacral nerve. Objective The objectives of this study were to examine the anatomy of the right first sacral nerve relative to the midpoint of the sacral promontory and to evaluate the thickness and ultrastructural composition of the anterior longitudinal ligament at the sacral promontory level. Study Design Anatomic relationships were examined in 18 female cadavers (8 unembalmed and 10 embalmed). The midpoint of the sacral promontory was used as reference for all measurements. The most medial and superior point on the ventral surface of the first sacral foramen was used as a marker for the closest point at which the first sacral nerve could emerge. Distances from midpoint of sacral promontory and the midsacrum to the most medial and superior point of the first sacral foramen were recorded. The right first sacral nerve was dissected and its relationship to the presacral space was noted. The anterior longitudinal ligament thickness was examined at the sacral promontory level in the midsagittal plane. The ultrastructural composition of the ligament was evaluated using transmission electron microscopy. Height of fifth lumbar to first sacral disc was also recorded. Descriptive statistics were used for data analyses. Results Median age of specimens was 78 years and median body mass index was 20.1 kg/m². Median vertical distance from midpoint of sacral promontory to the level of the most medial and superior point of the first sacral foramen was 26 (range 22-37) mm. Median horizontal distance from the midsacrum to the first sacral foramen was 19 (range 13-23) mm. In all specimens, the first sacral nerve was located just behind the layer of parietal fascia covering the piriformis muscle, and thus, outside the presacral space. Median anterior longitudinal ligament thickness at the sacral promontory level was 1.9 (range 1.2-2.5) mm. Median fifth lumbar to first sacral disc height was 16 (8.3-17) mm. Conclusion Awareness of the first sacral nerve position, approximately 2.5 cm below the midpoint of the sacral promontory and 2 cm to the right of midline, should help anticipate and avoid somatic nerve injury during sacrocolpopexy. Knowledge of the approximate 2-mm thickness of the anterior longitudinal ligament should help reduce risk of discitis and osteomyelitis, especially when graft is affixed above the level of the sacral promontory.