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Lateral Mass Screws: Anatomy is the Key, Not Image Guidance!

Dickerman, Rob D. DO, PhD; Reynolds, Ashley S. RN; Bennett, Matthew T. MD; Morgan, Brent C. MD

Journal of Spinal Disorders & Techniques: February 2007 - Volume 20 - Issue 1 - p 109
doi: 10.1097/01.bsd.0000211289.44637.dd
Letter to the Editor

Neurosurgery Research Foundation of Texas, North Texas Neurosurgical Associates, Plano Presbyterian and Medical Center of Plano Hospitals Plano, TX

To the Editor:

We read with interest the article by Roche et al1 on Posterior Cervical Screw Placement without Image Guidance: A Safe and Reliable Practice. We find the title of the article somewhat misleading, in that, it is very common to be not able to visualize the lateral mass screws on patients even below C4. Thus, we only use fluoroscopy at the start of the surgery to insure proper alignment of the cervical spine and for localization of the levels for screw placement. We agree with Roche et al that the placement of lateral mass screws is safe and highly effective if, and only if, the surgeon is aware of neurovascular anatomy in the cervical spine and how to manage complications should they occur. We would like to add a few comments that support Roche et al and are the basics for our routine use of lateral mass screws, with virtually no serious complications to date. First, the most basic understanding of the lateral mass and its neurovascular content has been reviewed by several surgeons and there are several basic techniques in the trajectory for safe screw placement to avoid nerve root injury and vertebral artery injury.2–6 We typically use the Cherny et al6 technique with a screw trajectory of approximately 20 degrees laterally and 25 to 30 degrees rostrally. As with all surgeries, preoperative planning is of utmost importance and thus should the axial magnetic resonance imaging studies be insufficient for visualization of the vertebral artery, we then order axial computed tomography scans of the cervical spine to assess the foramen transversarium and rule out any possible anomalies.7 Second, a basic rule we follow, when placing lateral mass screws, is to start with right-sided screw placement; in case a vertebral artery injury does occur, it will typically not be the dominant vertebral artery injured. As previously mentioned, all surgeons should be able to appropriately manage such an injury.8 In terms of screw depth, a landmark study by Seybold et al,9 on the use of unicortical versus bicortical purchase for safety and stability, demonstrated that over 92% of unicortical screws were determined safe whereas only 68% of bicortical screws were safe.9 Surprisingly, when comparing pullout strength of the bicortical versus unicortical screws, there was no significant difference, leading support to the authors' conclusions that 11 mm of effective screw length is strong and safe.9 Lastly, a recent study by Katonis et al10 reported a risk of 0.01% for violating the foramen transversarium with a lateral mass screw after following 70 patients who underwent lateral mass screw fixation, with a total of 356 screws, of which only 5 screws minimally violated the foramen trasversarium without neurologic sequlae. In closing, we support Roche et al that fluoroscopy is not required for safe placement but clarify this support by urging surgeons to know the neurovascular anatomy and, most importantly, how to manage complications should they occur.

Rob D. Dickerman, DO, PhD

Ashley S. Reynolds, RN

Matthew T. Bennett, MD

Brent C. Morgan, MD

Neurosurgery Research Foundation of Texas, North Texas Neurosurgical Associates, Plano Presbyterian and Medical Center of Plano Hospitals Plano, TX

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REFERENCES

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© 2007 Lippincott Williams & Wilkins, Inc.