The Role of Pelvic Parameters on S2 Alar-Iliac (S2AI) Screw Trajectory

Presented at SMISS Annual Forum 2018
By Joseph Laratta MD
With Jamal Shillingford MD, James Lin MD, Comron Saifi MD, Steven Ludwig , Lawrence Lenke MD, Ronald Lehman MD,

Disclosures: Joseph Laratta MD A; Orthopaedic Science Research Foundation, Norton Healthcare. B; Evolution Spine Jamal Shillingford MD None, James Lin MD None, Comron Saifi MD None, Steven Ludwig A; AO Spine, K2M. B; DePuy Synthes, K2M. D; ASIP, ISD. F; DePuy Synthes, Lawrence Lenke MD A; Scoliosis Research Society, Evans Family, Fox Family Foundation, Setting Scoliosis Straight Foundation, EOS, AO Spine. B; DePuy Synthes, Medtronic, K2M. C; Fox Rothschild, LLP. F; Medtronic, Qualit, Ronald Lehman MD A; PRORP. B; Medtronic. C; Medtronic, Stryker, DePuy,


Spinopelvic fixation utilizing S2AI screws provides optimal fixation across the lumbosacral junction allowing for solid fusion, especially in long segment fusion constructs. Freehand placement of such screws relies heavily on the rich surrounding sacropelvic anatomy. To date, no study has examined the relationship between spinopelvic sagittal parameters and screw trajectory.


To determine the influence of pelvic parameters on S2AI screw trajectory.


The medical records and preoperative CT scans of 33 consecutive patients with degenerative lumbar pathology between 2015-2016 were reviewed. Preoperative standing X-rays were assessed to measure pelvic parameters including sacral slope, pelvic tilt, and pelvic incidence. Using 3-dimensional CT reconstructions, an ideal S2AI trajectory was defined as a start point between the S1 and S2 foramen with screw axis directed towards the anterior-inferior iliac spine on the sagittal plane. In the axial plane, the trajectory started at the lateral aspect of the S1/2 foramen and was directed through the narrowest portion of ilium. Sacral slope, horizontal angle, sagittal angle, intra-screw distance and estimated screw lengths were recorded.


The mean age at the time of surgery was 62.4+12.5 years and there were 14 (42.4%) female patients in the cohort. The average sagittal angle measured in the sagittal plane was 27.3+4.1°. The average horizontal angle measured in the axial plane using the posterior superior iliac spine as a reference was 35.9+3.9°. Maximum screw length and intra-sacral screw length were 109.7+16.4° and 33.6+6.4° respectively. Pelvic tilt was found to have a moderate inverse correlation with sagittal screw trajectory (r=-0.467, p-value=0.006). Pelvic incidence and sacral slope had weak correlations with sagittal screw angle. In the subgroup analysis, patients with high pelvic tilt >20° had a significantly lower sagittal screw trajectory compared to those with a normal pelvic tilt (24.9+3.7° versus 29.8+2.8°, p-value=<0.001).


Increases in pelvic tilt correlate to lower sagittal S2AI screw trajectories. This causal relationship is vital when planning for freehand S2AI screw placement.