An Anatomical Study of the Mid-Lateral Pars Relative to the Pedicle Footprint in the Lower Lumbar Spine

Abstract

An anatomic study that describes the relationship of the pedicle center to the mid-lateral pars (MLP) in the lower lumbar spine as a guide to pedicle screw placement. Describe morphometric data of the lower lumbar pedicles, the unique coronal pedicle footprints of L4 and L5, and their impact on the relationship of the pedicle center to the MLP. Traditional medial-lateral starting points for lumbar pedicle screws use the facet as an anatomic reference for all lumbar levels. The facet is often a difficult landmark to use secondary to degenerative changes and the desire to minimize damage to the facet capsule in the most cephalad level. These techniques can also result in pedicle violation particularly in the lower lumbar spine. Use of the nonarthritic MLP is proposed in this study as an alternative anatomic reference point for the pedicle center. Seventy-two pedicles (L3-S1) from embalmed cadaveric spines were used. Linear and angular dimensions of the pedicle were measured, including the degree of coronal pedicle tilt of L4 and L5. The center of the pedicle relative to the MLP and relative to the midline of the base of the transverse process was measured. The axial superior facet angle and angle of pedicle screw insertion were also measured. The minimum pedicle width was 10.9 and 12.4 mm and the coronal pedicle tilt was 36 degrees and 55 degrees for L4 and L5, respectively. A classification of 2 types of L5 pedicles relevant to pedicle center location was developed. In the medial-lateral direction, the pedicle center is 2.9 mm lateral to the MLP at L3 and L4. At L5, it is 1.5 and 4.5 mm lateral to the MLP for a type I and type II pedicle, respectively. In the superior-inferior direction, the pedicle center is 1 mm superior to the midline of the transverse process base for all lower lumbar levels. Significant differences between a type I and II L5 pedicle were a larger pedicle width and distance of the pedicle center to the MLP for a type II pedicle. The difference between the axial pedicle screw insertion angle and anatomic superior facet angles was 8 degrees from L4-S1. The MLP is a reliable anatomic reference point for the center of the pedicle in the lower lumbarspine. Consideration needs to be taken when inserting pedicle screws at L4 and L5 because of the degree of their coronal tilts and unique pedicle footprints. It is important to distinguish a type I from type II L5 pedicle as a type II pedicle is wider, has a more lateral pedicle center relative to the MLP, and has the potential for lateral screw placement while still remaining within the pedicle.