Review Question - QID 213093

The most likely location for an errantly placed screw is anterior. This would lead to injury in the L5 nerve root, which would manifest with weakness in great toe extension.

Indications for iliosacral screw fixation include complete sacral fractures (i.e. fractures that extend through the sacrum in its entirety, anterior to posterior), sacroiliac joint disruptions, and combinations of these posterior pelvic injuries following reduction. The safe zone for screws is the area within the sacrum where the screws remain within the bone. The L5 and S1 nerve roots course from the spinal canal in an anterior-lateral-caudal and oblique direction. The L5 nerve root is located on the anterior-cranial sacral alar surface and may be injured by an errant screw that is placed too anteriorly. The S1 nerve root is located within the oblique neural tunnel, beginning at the spinal canal and exiting through the anterior sacral foramina between the upper and second sacral segments.

Kaiser et al. performed a study to evaluate anatomic determinants of sacral dysmorphism and implications for safe screw placement. They found the prevalence of upper sacral segment dysplasia at 41%. The major determinants of sacral dysmorphism are upper sacral segment coronal and axial angulations. They developed a sacral dysmorphism score, derived with the equation: (first sacral coronal angle) + 2(first sacral axial angle). They conclude that the sacral dysmorphism score quantifies dysmorphism and can be used in preoperative planning of IS screw placement.

Miller and Routt performed a review of variations in sacral morphology and implications for iliosacral screw fixation. They report that knowledge of the posterior pelvic anatomy, its variations, and related imaging is critical to performing reproducibly safe surgery. They report that the dysmorphic pelvis has several key characteristics. The upper portion is colinear with the iliac crests on the outlet view. Mamillary bodies (underdeveloped transverse processes) may be present as well. A residual upper sacral disk may be present along with an acute alar slope. They conclude that the surgeon must be knowledgeable about individual patient anatomy to ensure safe iliosacral screw placement.

Figure A is an illustration of a sacroiliac-style iliosacral screw. Illustration A is an inlet fluoroscopic image demonstrating the drill within the sacrum with an arrow indicating the location of the L5 nerve root. Illustration B is an axial CT of a patient with normal sacral anatomy through the first sacral and second sacral segments, respectively. Illustration C is an axial CT of a patient with dysmorphic sacral anatomy through the first sacral and second sacral segments, respectively. The L5 nerve roots are indicated by the white circles.

Incorrect Answers:
Answer 1: Weakness in knee extension would indicate injury to the L4 nerve root.
Answer 2: Weakness in ankle plantarflexion would indicate injury to the S1 nerve root.
Answer 4: Numbness to the medial leg would indicate injury to the L4 nerve root.
Answer 5: Numbness over the posterior leg would indicate injury to the S1 nerve root.