Perspectives Commentary on: Adjustment of Suboptimally Placed Lumbar Pedicle Screws Decreases Pullout Strength and Alters Biomechanics of the Construct: A Pilot Cadaveric Study by Wadhwa et al. pp. 368-375.

The Enemy of Good Is Perfect Michael Y. Wang

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n this issue of WORLD NEUROSURGERY, Wadhwa et al. investigate the implications of repositioning lumbar pedicle screws on their pullout strength. Their working theory is that if an initial pedicle screw was placed poorly, creating a second trajectory would result in reduced purchase owing to the alterations in the bone during initial entry. This real-world problem likely occurs frequently, and the authors’ theory makes sense intuitively. They have now shown this biomechanically using a cadaveric model. In their study, using 6 cadavers, the authors tested the L4 and L5 vertebral segments with standard pedicle screw fixation applied using the Magerl technique. When testing the entire construct (L4-5 bilateral screws with connecting rods and no crosslink) for stiffness, there was little difference between optimally placed and revised constructs (with 1 “weakened” screw at L5). Only axial rotation was altered by 9.2%. The authors then tested the individual screws for pullout strength and found a 33% reduction from 900 N to 600 N in the L5 screw with a corrected trajectory. These findings are not surprising because there is only so much usable space in the vertebral pedicle. The authors have chosen L5 as their model for the defect, arguably one of the largest pedicles, and in real life no salvage option may be possible in a small pedicle, such as at the L1 level. Although every spinal neurosurgeon wants perfectly symmetric hardware on x-rays, there is a potential cost to this. For example, the University of Miami is situated in one of the most litigious pockets in the United States, and so my partners and I are likely to use more live fluoroscopic imaging to maximize the likelihood of good screw placement both initially and during the bone machining process than most other surgeons. However, the

Key words Biomechanics - Lumbar pedicle screw - Pullout strength -

effects of this additional radiation on both the surgeon and the patient must be considered. Another alternative is to leverage technology to maximize success on the initial bony cannulation. Use of technologies such as image guidance can assist greatly to these ends, but additional operative time and capital equipment costs have been barriers to widespread application of these technologies. For situations demanding maximal bony fixation (e.g., at the anchoring end of a long construct), reconstructing the initial bony defect is another possibility. Orthopedic surgeons have been using autologous “bone shims” for years. This method involves packing the bony defect with softer bone followed by screw insertion in an attempt to “back fill” any defects. Another option is to use cement augmentation in the vertebral body. In the United States, cement augmentation is limited to polymethyl methacrylate to fill the interstices of the bone and expand the surface area of contact between the implant and the patient. There are increasingly diverse methods for appropriate bony fixation. For example, cortical screws are growing in popularity as a primary method for spinal fixation. These appear quite different on x-ray than Magerl pedicle screws, and hybrids employing both methods in the same patient are now in use. One should be willing to accept radiographically nonideal screws as long as they are safely within the bone with good biomechanical purchase. The alternative screw paths such as the cortical trajectory could be useful as a salvage technique after a standard trajectory has been weakened. Ultimately, the key to success is a solid bony arthrodesis. Regardless of how strong the initial fixation of a pedicle screw is,

Departments of Neurological Surgery & Rehabilitation Medicine, University of Miami Miller School of Medicine, Miami, Florida, USA To whom correspondence should be addressed: Michael Y. Wang, M.D. [E-mail: [email protected]] Citation: World Neurosurg. (2015) 83, 3:299-300. http://dx.doi.org/10.1016/j.wneu.2014.10.015

WORLD NEUROSURGERY 83 [3]: 299-300, MARCH 2015

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the instrumentation will eventually loosen given effects of cyclic mechanical loading at the bone-implant interface. Surgeons should continue to pay close attention to proper bone grafting techniques including meticulous graft site preparation.

Citation: World Neurosurg. (2015) 83, 3:299-300. http://dx.doi.org/10.1016/j.wneu.2014.10.015 Journal homepage: www.WORLDNEUROSURGERY.org Available online: www.sciencedirect.com 1878-8750/$ - see front matter ª 2015 Elsevier Inc. All rights reserved.

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The enemy of good is perfect.

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