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

Su, Brian W. MD*; Kim, Paul D. MD*; Cha, Thomas D. MD*; Lee, Joseph MD*; April, Ernest W. PhD†; Weidenbaum, Mark MD*; Vaccaro, Alexander R. MD, PhD‡

Spine:
doi: 10.1097/BRS.0b013e3181a4f3a9
Anatomy
Abstract

Study Design. 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.

Objective. 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.

Summary of Background Data. 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.

Methods. 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.

Results. The minimum pedicle width was 10.9 and 12.4 mm and the coronal pedicle tilt was 36° and 55° 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° from L4–S1.

Conclusion. 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.

In Brief

This is an anatomic study that describes 2 different types of L5 pedicles and provides guidelines for using the mid-lateral pars (MLP) as a reference point to the pedicle center in the lower lumbar spine. The pedicle center is located 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. When placing pedicle screws, it is important to understand the unique elliptical pedicle footprint of L4 and L5 and to distinguish a type I from type II L5 pedicle.

Author Information

From the *Department of Orthopaedic Surgery, Orthopaedic Research Laboratory, NY Presbyterian Hospital, New York, NY; †Deparment of Anatomy, Columbia University, College of Physicians and Surgeons, New York, NY; and ‡Department of Orthopaedic Surgery, The Rothman Institute, Thomas Jefferson University, Philadelphia, PA.

Acknowledgment date: December 11, 2008. Acceptance date: January 8, 2008.

The manuscript submitted does not contain information about medical device(s)/drug(s).

No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

Address correspondence and reprint requests to Brian W. Su, MD, Department of Orthopaedic Surgery, NY Presbyterian Hospital, 622 West 168th Street, PH 11, New York, NY 10032; E-mail: Brianwsu@gmail.com

© 2009 Lippincott Williams & Wilkins, Inc.