Anatomic and experimental basis for the insertion of a screw at the first sacral vertebra

Abstract

The authors present the anatomic and experimental basis of an original technique for screwing at the first sacral level employed in lumbosacral fusion. The anatomic studies were based on specimens from the anatomy museum, frozen sections of the sacrum and CT examinations with three-dimensional reconstruction and assessment of the density of the different structures of S1 in Hounsfield units (HU). The findings were that the ala and lateral portions of S1 contain yellow marrow forming what amounts to a fatty sphere bounded by the cortical bone of the sacroiliac joint, the linea terminalis and the spongy bone of the pedicles and of the body of S1. The experimental study was made by avulsion of sacral screws (system of Cotrel Dubousset), each of 7mm diameter. No screw perforated the sacral cortex. Three directions were tested. The insertion of a screw through the pedicle and body of S1 is advised, with the point of insertion below and lateral to the articular process of S1 and an oblique course forward and inward at an angle of 10 degrees to the sagittal plane. This internal obliquity is limited by the posterior prominence of the iliac ala.

L5 spinal nerve injury caused by misplacement of outwardly-inserted S1 pedicle screws

“…By contrast, several authors have recommended that placement of S1 pedicle screws in a central position should be avoided because of the risk this placement carries of damage to the iliac vessels, the sympathetic chain, and the lumbosacral trunk, which are all close to the sacrum [19][20][21][22][23][24]. Therefore, from both biomechanical and anatomical points of view, it is recommended that S1 pedicle screws should be inserted inwardly with an acceptable angle reported as about 30°to 40°, almost the same as that of the S1 facet angle [27][28][29][30][31]. However, in general, S1 pedicle screws are inserted from a medial entry point with an outward angle, because of the prominent dorsal overhang of the posterior iliac crest and paravertebral muscle mass [30,[32][33][34][35].…”

“…Therefore, from both biomechanical and anatomical points of view, it is recommended that S1 pedicle screws should be inserted inwardly with an acceptable angle reported as about 30°to 40°, almost the same as that of the S1 facet angle [27][28][29][30][31]. However, in general, S1 pedicle screws are inserted from a medial entry point with an outward angle, because of the prominent dorsal overhang of the posterior iliac crest and paravertebral muscle mass [30,[32][33][34][35]. If screws are inserted outwardly, they risk causing an L5 spinal nerve injury, and thus, great care should be taken.…”

Paraspinal Muscle Sparing versus Percutaneous Screw Fixation : A Prospective and Comparative Study for the Treatment of L5-S1 Spondylolisthesis

“…Moreover, the procedure can potentially injure the medial branches of the dorsal ramus at adjacent levels and at the level of fusion; this is because these branches are relatively fixed as they run beneath the fibro-osseous mamilloaccessory ligament 2) . In contrast, several authors have recently reported that the muscle paraspinal sparing approach causes less paraspinal muscle damage than the traditional midline approach, and has positive effects on postoperative trunk muscle performance 3,5) . The paraspinal approach can also result in a more medially-oriented S1 pedicle screw placement than traditional midline approach, which should lead to stronger fixation.…”

“…Furthermore, it is known that medially-oriented pedicle screw placement is necessary to obtain a secure anchor to the sacrum for screw fixation at the L5-S1 level 3) . However, for the conventional midline approach for screw fixation at the L5-S1 level, forceful retraction of the paraspinal muscles is required to achieve the proper lateral-to-medial screw trajectory due to coronal plane angle increase of pedicle.…”

Efficacy and Safety of Percutaneous Sacroplasty for Painful Osteoporotic Sacral Insufficiency Fractures

“…Sacral osseous characteristics may provide an additional explanation of the pathophysiology of sacral insufficiency fractures. In independent investigations, de Peretti et al using computed tomography (CT) 26 and Smith et al using quantitative CT, 27 have documented a higher trabecular density in the sacral body than the sacral ala. In a subsequent study combining cadaveric sections, faxitron imaging, and CT reconstructions, Peretz et al 28 demonstrated a cruciate trabecular pattern of highest density and reproducibility in the proximal sacral body.…”