Third Sacral Foramina Morphometry for Sacral Neuromodulation

From the University of South Florida, Tampa, FL.
Journal of Pelvic Medicine and Surgery (Impact Factor: 1.09). 01/2013; 19(1):23-30. DOI: 10.1097/SPV.0b013e31827bfdb1
Source: PubMed


This study is aimed to define the geometry and location of the human S3 foramen, with respect to bony landmarks visible on ultrasound.
Computed tomographic (CT) image data from an institutional review board-approved database of de-identified pelvic CT images were analyzed. Points along the S3 foramina and bony sacrum were tagged, and their locations saved. The saved points were mathematically analyzed to determine the geometry and relative location of the S3 foramina with respect to other bony landmarks, specifically the sacral hiatus, and the sacral spinous processes, and the caudad aspect of the bilateral SI joints ("SI line"). Descriptive statistics were used to describe the geometry and aggregate location of the S3 foramina bilaterally. CT data sets were excluded if they had evidence of pelvic bone injury, prior bony fixation, severe osteoporosis, or other deformity.
One hundred thirty-three data sets met the inclusion criteria. The SI line was superior to the sacral hiatus for reliable S3 localization. The entire circumference of approximately 14% of the S3 foramina is located cephalad to the SI line. The sagittal angle of trajectory for S3 was approximately 70 degrees relative to the dorsal surface of the sacrum.
Clinical localization of the S3 foramen for sacral neuromodulator needle placement is best obtained when the needle tip is positioned 15 to 25 mm lateral to the sacral spinous processes and 0.0 cm to 25 mm caudad to the SI line, at the level of the dorsal sacrum surface. The findings presented in this study may be applied to improve the efficacy and accuracy of neuromodulator lead placement into the S3 foramen. This study provides rationale for the effectiveness of the crosshair placement technique and demonstrates the best location for needle repositioning when this technique is not initially successful.

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    ABSTRACT: Introduction and hypothesis: Standard external landmarks have been suggested as a guide for in-office percutaneous nerve evaluation (PNE), but validity of these landmarks has not been assessed. Our objective was to determine whether the standard 9 cm from the tip of the coccyx indicates the position of the S3 sacral foramen and whether other boney landmarks and measurements improved positioning. Methods: Measurements and distances between external boney landmarks were obtained in 22 embalmed cadavers. Spinal needles were placed 9 cm superior to the coccyx and 2 cm lateral to midline bilaterally. After dissection, internal measurements relating to sacral length, position of S3, and location of the needle in relation to S3 were recorded. Correlations among measured variables were assessed using descriptive statistics. Results: Mean distance from the tip of coccyx to S3 was 9.26 cm (±0.84), from S3 to midline 2.30 cm (±0.2); from needle to S3 1.25 cm, and needle placement was as likely to be placed above or below S3; and S2-S3 and S3-S4 interforamenal distance 1.48 cm (±0.30) and 1.48 cm (±0.24), respectively. Mean distance from S3 to sacroiliac joint (SIJ) was shorter than S2 to SIJ. All associations between external measurements and length from tip of coccyx to S3 were not significant. Conclusion: A distance 9 cm from the tip of the coccyx is a reasonable starting landmark for in-office blind PNE. However, given the variability in coccyx length, caution should be taken; also, sensory-motor response is necessary to confirm proper placement.
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