The Presacral Space and its Impact on Sacral Neuromodulator Implantation

Purpose: We describe the presacral space and its potential impact on sacral neuromodulator implantation and bowel injury. Materials and Methods: Parasagittal images containing bilateral sacral foramina (S2-S4) were examined on 45 pelvic magnetic resonance images. Images were excluded from analysis if they were poor quality or had any history causing distortion of normal anatomy. We measured the natural angle between the foramina and the dorsal skin to approximate the needle angulation during neuromodulator electrode placement. Using these angles we measured the distance from the skin to any bowel (D1), the skin to the dorsal sacrum (D2) and then calculated the distance from the dorsal sacrum to any bowel (D3). Results: Mean subject age was 45 years (range 19 to 78) and body mass index was 27.9 kg/m² (range 18.6 to 56.2). At S3 the mean foraminal angle and D3 were 46 ± 8.4 degrees and 27.4 ± 11.7 mm, respectively. Increasing age was moderately correlated to widening D3 at each foramina (r = 0.3, Pearson's p <0.05). Body mass index did not consistently vary with D3 at any foramina. Conclusions: Our measurements suggest that the presacral space can be expected to be approximately 27 mm at the level of S3 where the neuromodulator electrode is implanted. It is possible to encounter bowel while performing this implantation using standard techniques and equipment. We recommend the standard use of fluoroscopy during placement.