Introduction: Lumbar pedicle screws are placed for internal fixation and help to enhance fusion. Optimal screws are medially directed, should be parallel or pointing to the superior endplate, and penetrate 50-80% of the vertebral body. "Suboptimal" pedicle screws can be inadvertently placed within the confines of the pedicle and vertebral body but are sometimes replaced to obtain a more acceptable post-operative image. We define a suboptimal screw to be in the pedicle and body and not violating bone, however not parallel to the superior endplate. We suspect these “cored-out” grooves left in the bone from the initial tap and screw placement compromise the integrity of the bone and thus the construct. We set up a cadaver study to test both screw pullout strength and biomechanics of the construct.
Methods: DEXA scans and L4-5 laminectomies were performed on 6 fresh-frozen cadaveric lumbar spines. We placed 2 optimal L4 pedicle screws, 1 optimal L5 screw and one suboptimal screw in L5. Axial rotation, flexion/extension and lateral bending were tested and the suboptimal screw was replaced in an optimal fashion and retested. Pullout strength was also performed on the revised screw and the contralateral screw.
Results: Axial rotation and flexion/extension were significantly different among the two groups. The mean difference in the pull out force was significantly different in the non-revised optimal screw group as compared to the revised optimal screw group (906.93 vs. 608.32N, p=0.031) ;in favor of the former. DEXA demonstrated 4 osteopenic and 2 osteoporotic specimens and difference in bone mineral density did not play a role in assessing either the biomechanical parameters or the pull out strength.
Conclusions: Great care is warranted to initially place the screw in an optimal position. Revising the suboptimal screw results in decreased pull out strength as well as altered biomechanical movements. Therefore, we propose a screw placed within the confines of the pedicle and vertebral body with adequate bone purchase but inadvertently placed non-parallel to the end plate should be left in its place.
Patient Care: It will hopefully stress the improtance of proper screw placement and what to do if the screws are not placed in a picture-perfect fashion.
Learning Objectives: By the conclusion of this session, participants should be able to describe the importance of optimally placed lumbar pedicle screws, discuss in small groups the methods of doing so, and identify and effective treatment of suboptimally placed screws.
References: 1. Skinner R, Maybee J, Transfeldt E, et al: Experimental pullout testing and comparison of variables in transpedicular screw fixation. A biomechanical study. Spine 15:195–201, 1990.
2. Krag MH. Biomechanics of transpedicle spinal fixation. In Weinstein JN, Wiesel SW: The Lumbar Spine. Philidelphia, WB Saunders, 1990. 916-940.
3. Zindrick MR, Wiltse LL, Widell EH, Thomas JC, Holland WR, Field BT, Spencer CW. A biomechanical study of intrapeduncular screw fixation in the lumbosacral spine. Clin Orthop 1986;203:99.
4. Vaccaro AR, Garfin SR. Pedicle-Screw Fixation in the Lumbar Spine. J Am Acad Orthop Surg 1995:3:263-274.
5. Ekström L, Hansson T, and Afonja A. The pull-out strength of intrapedicular screws in relation to the vertebral bone mineral content. Abstract, First World Conference on Biomechanics, La Jolla, California, August, 1990. (do not have)
6. Abshire BB, McLain RF, Valdevit A, Kambic HE. Characteristics of pullout failure in conical and cylindrical pedicle screws after full insertion and back-out. The Spine Journal 2001;1:408-414.
7. Rohmiller MT, Schwalm D, Glatter RC, Elalayli TG, Spengler DM. Evaluation of calcium sulfate paste for augmentation of lumbar pedicle screw pullout strength. The Spine Journal 2002;2:255-260.
8. Cook SD, Salkeld SL, Stanley T, Faciane A, Miller SD. Biomechanical study of pedicle screw fixation in severely osteoporotic bone. The Spine Journal 2004;4:402-408.
9. Paré PE, Chappuis JL, Rampersaud R, Agarwala AO, Perra JH, Erkan S, Wu C. Biomechanical Evaluation of a Novel Fenestrated Pedicle Screw Augmented With Bone Cement in Osteoporotic Spines. Spine 2011;36(18):E1210-E1214.
10. Kiner DW, Wybo CD, Sterba W, Yeni YN, Bartol SW, Vaidya R. Biomechanical Analysis of Different Techniques in Revision Spinal Instrumentation: larger diameter screws versus cement augmentation. Spine 2008;33:2618-2622.
11. Gelalis ID, Paschos NK, Pakos EE, Politis AN, Arnaoutoglou CM, Karageorgos AC, Ploumis A, Xenakis T. Accuracy of pedicle screw placement: a systematic review of prospective in vivo studies free hand, fluoroscopy guidance and navigation techniques. Eur Spine J 2011;August(07).
12. Slomczykowski M, Roberto M Schneeberger P. Radiation dose for pedicle screw insertion. Fluroscopic method versus computer-assisted surgery. Spine 1999;24:975-982.
13. Laine T, Schlenzka D, Mäkitalo K, Tallroth K, Nolte LP, Visarius H. Improved accuracy of pedicle screw insertion with computer-assisted surgery. A prospective clinical trial of 30 patients. Spine 1997;22:1254-1258.
14. Roy-Camille R, Saillant G, Mazel C: Plating of thoracic, thoracolumbar, and lumbar injuries with pedicle screw plates. Orthop Clin North Am 17:147–159, 1986