S3 Sacral-Alar Iliac Screw: A Salvage Technique for Pelvic Fixation in Complex Deformity Surgery

Sacropelvic fixation

The sacropelvis is not only an anatomically complex region but also a biomechanically unique zone transferring axial weights via the transitional lumbosacral junction and the pelvic girdle to the lower appendicular skeleton. When the sacral instrumentation alone is insufficient to achieve stability and solid arthrodesis across the lumbosacral junction, as in long-segment fusions, high-grade spondylolisthesis, deformity corrections, complex sacral/lumbosacral injuries, and neoplasms, sacropelvic fixation is indicated. Many modern sacropelvic fixation modalities outperform historical modalities, especially the conventionally open and percutaneous iliac and S2-alar-iliac screw (S2AI) fixation techniques. Novel screw insertion technologies such as navigation and robotics and modern screw designs aim to maximize the accuracy of screw placement and minimize complications. This review addresses the anatomy and biomechanics of the sacropelvic region as well as the indications, evolution, advantages, and disadvantages of various past and contemporary techniques of lumbosacral and sacropelvic fixation.

Three-dimensional digital anatomical measurement of modified sacroiliac screws

Purpose

To rebuild a model of the pelvis and effectively simulate the trajectory of modified sacroiliac screws, we measured the parameters of each screw and screw channel and assessed the safety and feasibility of the parameters in adults.

Method and materials

One hundred (50 males and 50 females) normal adult pelvic computed tomography (CT) images were randomly selected and imported into Mimics software to rebuild the three-dimensional pelvis model. In these models, each ideal channel of modified screws was simulated, and then we obtained the precise parameters of screws and channels using Mimics and Three-matic software.

Results

The results of the comparison (right vs. left) showed that there were no significant differences in any of the angles, radius or M1SI parameters (the first modified sacroiliac). However, one parameter (BS) of M2SI (the second modified sacroiliac), two parameters (AP and BS) of M3SI (the third modified sacroiliac), and three parameters (AP, BS, L) of M4SI (the fourth modified sacroiliac) were statistically significant (P < 0.05). The result of comparison (between genders) showed that there were no significant differences in M1SI and M2SI; in contrast, the radius, length and the α angle of M3SI and M4SI were significantly different between genders (P < 0.05), and the radius of M4SI required special attention. If the radius of the limiting screw channel was > 3.50 mm, 52 cases (52%, 24 males and 28 females) could not complete the M4SI screw placement among 100 samples. If the radius of the limiting screw channel was > 3.0 mm, a total of 10 cases (10%, 2 males and 8 females) could not complete the M4SI screw placement.

Conclusion

Through the measurement of 100 healthy adult real three-dimensional pelvic models, we obtained the parameters of each modified sacroiliac screw and measured the three angles of each screw based on international coordinates for the first time, which can instruct clinical application.

The posterior superior iliac rim screw as an adjunct to pelvic fixation in complex spinopelvic stabilization

BACKGROUND

Fixation to the ilium is a commonly used alternative or supplement to sacral fixation in complex spinopelvic reconstructions. This can be achieved with traditional posterior superior iliac spine or S2 alar-iliac screws. Posterior superior iliac rim screws may be considered to achieve or enhance pelvic fixation. The objective of this case series was to describe the use of and indications for posterior superior iliac rim screws.

METHODS

A retrospective review was performed of the medical records and radiographic data for three patients who underwent complex lumbosacral reconstructions involving the use of posterior superior iliac rim screws to enhance pelvic fixation. The cases included a 35-year-old with bilateral sacral fractures, a 43-year-old with extensive metastatic sarcoma to the sacrum, and a 48-year-old with multiple lumbar and pelvic fractures. An overview of the key surgical techniques is provided.

RESULTS

All three patients tolerated the procedure, without any unexpected post-operative complications or deficits. Radiographs at last follow-up showed stable fixation and no hardware issues.

CONCLUSIONS

The use of posterior superior iliac rim screws as an adjunct method of fixation in complex spinopelvic reconstructions is a feasible option that can be considered in the fixation armamentarium.

Sacropelvic fixation techniques - Current update

Sacropelvic is a complex junctional area owing to the complex regional anatomy and higher biomechanical stress. However extension of construct is indicated in cases with complex deformities, high grade spondylolisthesis, and complex fractures. The challenges remain which includes pseudoarthrosis and fixation failures. The fixation techniques have constantly evolved over time with better results with iliac screws and S2-alar-iliac screws. This article gives background on evolution, biomechanics, and recent update of use of robotics for sacropelvic fixation.