V sign reliable landmark for identifying S1 pedicle entry point under fluoroscopic guidance

Sacral (S1) pedicle screw misplacement in posterior percutaneous fixation (PPF) can be related to anatomical variability and a lack of reliable radiographic landmarks. It might lead to iatrogenic injuries to adjacent neurovascular structures and sometimes to the need for revision surgery.

Tony Tannoury et al conducted a study to highlight a reproducible anatomical landmark (the “V” sign) for the safe localization of the S1 pedicle entry point under fluoroscopy. Investigation performed at Boston Medical Center, Boston, Massachusetts. The study has been published in “JBJS Open Access.”

Human cadavers (n = 14) were dissected for the anatomical description of the “V” landmark and its relationship with the entry point of the S1 pedicle screw. The “V” landmark was defined medially by the lateral border of the superior articulating process of S1 and laterally by the posterior projection of the sacral ala. The mean distance was measured between the bottom point of the “V” landmark and the anatomical entry point to the S1 pedicle (V-S1 entry point distance). A similar measurement was conducted on computed tomography (CT) scans of 135 patients who underwent PPF using the “V” sign as a landmark for S1 pedicle screw placement (270 screws). These were retrospectively evaluated for appropriateness of S1 screw entry points and for proper S1 screw alignment and breaches.

Key findings of the study were:

• In the 14 cadavers, irrespective of the laterality and sex, the V-S1 entry point distance averaged 11.7 mm.

• On the medial-lateral axis, all entry points converged within 2 mm of a vertical line intersecting the base of the “V.”

• Additionally, the CT scan analysis (135 patients, 270 screws) revealed an optimal entry point for 100% of the screws and a 3.3% (n = 9 screws) breach rate.

• Six of the 9 identified breaches were minor, and only 1 (0.4% of the 270 screws) warranted revision.

The authors concluded that – “The “V” sign serves as a reliable anatomical and radiographic landmark for identifying the S1 pedicle entry point under fluoroscopic guidance. This landmark can help surgeons overcome the radiographic ambiguity of the sacral anatomy and ultimately reduces the rate of S1 pedicle screw misplacement.” 

Level of evidence: Diagnostic Level IV.

Further reading:

The “V” Sign A Reliable Anatomic and Radiographic Landmark for Posterior Percutaneous S1 Screw Placement Tony Tannoury, Aziz Saade et al JBJS Open Access 2023:e22.00079.http://dx.doi.org/10.2106/JBJS.OA.22.00079