Bony landmarks in the sacral region: the posterior superior iliac spine and the second dorsal sacral foramina: a potential guide for sonography.

PURPOSE

Identification of the second dorsal sacral foramen (S2F) by sonographic imaging is a possible first step in localising the branches of the dorsal sacral rami. The aim of this investigation is to develop an imaging approach to assist the rapid identification of S2F using a well-known regional landmark, the posterior superior iliac spine (PSIS).

METHODS

Twenty-seven skeletal specimens were digitally imaged. Repeated measurements were undertaken of the angle and distance (D(1)) between PSIS and S2F, maximum width of S2F (D(2)), interforaminal distance between contralateral S2F (D(3)), distance between S1F and S2F (D(4)), and distance between S2F and S3F (D(5)). Sonographically guided needle placement was further undertaken to validate these osseous measurements in five intact cadavers.

RESULTS

In skeletal material, repeated measurements indicated high intraclass correlation coefficients. No statistically significant difference existed in any measure between sides. Combined measurements indicated that S2F was located 46.4 ± 14.9° from the PSIS in both sexes, with a statistical trend toward a greater angle in females. D(1) had a mean value 2.2 ± 0.62 cm but was significantly shorter in females. The mean values of D(2) and D(3) were 0.75 ± 0.18 cm and 2.98 ± 0.27 cm, respectively, with no significant difference between sexes. The mean value of D(4) was 1.42 ± 0.27 cm with a statistical trend toward a slightly smaller value in females. The mean value for D(5) was 1.28 ± 0.15 cm. Sonographically guided needle placement in cadavers tended to validate these osseous measurements.

CONCLUSIONS

S2F has a mean maximum width of 0.76 cm and lies approximately 2-3 cm from the PSIS, 45° inferior to the horizontal. The medial left and right borders of S2F are approximately 3 cm apart. The upper three ipsilateral dorsal sacral foramina are 1-1.5 cm apart. These measurements may be useful for sonographers imaging the dorsal sacral region and eventually, for the potential identification of neurovascular branches of the dorsal sacral rami.