J Korean Neurosurg Soc.  2014 Jul;56(1):5-10. 10.3340/jkns.2014.56.1.5.

Feasibility of Bilateral Crossing C7 Intralaminar Screws: A Cadaveric Study

• Baek TH
• Kim I
• Hong JT
• Kim DH
• Shin D
• Lee SW

• KMID: 2067069
• DOI: http://doi.org/10.3340/jkns.2014.56.1.5

Abstract

OBJECTIVE
When the pedicle screw insertion technique is failed or not applicable, C7 intralaminar screw insertion method has been used as an alternative or salvage fixation method recently. However, profound understanding of anatomy is required for safe application of the bilaterally crossing laminar screw at C7 in clinic. In this cadaveric study, we evaluated the anatomic feasibility of the bilateral crossing intralaminar screw insertion and especially focused on determination of proper screw entry point.
METHODS
The C7 vertebrae from 18 adult specimens were studied. Morphometric measurements of the mid-laminar height, the minimum laminar thickness, the maximal screw length, and spino-laminar angle were performed and cross-sectioned vertically at the screw entry point (spino-laminar junction). The sectioned surface was equally divided into 3 parts and maximal thickness and surface area of the parts were measured. All measurements were obtained bilaterally.
RESULTS
The mean mid-laminar height was 13.7 mm, mean minimal laminar thickness was 6.6 mm, mean maximal screw length was 24.6 mm, and mean spinolaminar angle was 50.8+/-4.7degrees. Based on the measured laminar thickness, the feasibility of 3.5 mm diameter intralaminar screw application was 83.3% (30 sides laminae out of total 36) when assuming a tolerance of 1 mm on each side. Cross-sectional measurement results showed that the mean maximal thickness of upper, middle, and lower thirds was 5.0 mm, 7.5 mm, and 7.3 mm, respectively, and mean surface area for each part was 21.2 mm2, 46.8 mm2, and 34.7 mm2, respectively. Fourteen (38.9%) sides of laminae would be feasible for 3.5 mm intralaminar screw insertion when upper thirds of C7 spino-laminar junction is the screw entry point. In case of middle and lower thirds of C7 spino-laminar junction, 32 (88.9%) and 28 (77.8%) sides of laminae were feasible for 3.5 mm screw insertion, respectively.
CONCLUSION
The vertical cross-sectioned area of middle thirds at C7 spinolaminar junction was the largest area and 3.5 mm screw can be accommodated with 77.8% of feasibility when lower thirds were the screw entry point. Thus, selection of middle and lower thirds for each side of screw entry point in spino-laminar junction would be the safest way to place bilateral crossing laminar screw within the entire lamina. This anatomic study result will help surgeons to place the screw safely and accurately.

Keyword

C7; Cervical spine; Intralaminar screw

MeSH Terms

Adult
Cadaver*
Humans
Spine

Figure

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  Fig. 1 Measurement of maximal C7 intralaminar screw length (the length from the spino-laminar junction to the contralateral lamina-lateral mass junction).

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  Fig. 2 Measurement of C7 minimum laminar thickness.

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  Fig. 3 Measurement of C7 mid-lamina height.

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  Fig. 4 Spinolaminar angle of C7 specimens were measured by the digital photographs using Image J software.

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  Fig. 5 Cross sectional surface areas divided into 3 equal heights at screw entry point were measured by the digital photographs using Image J software.

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  Fig. 6 Measurements of C7 vertebra. 1. Mid-laminar height (mm), 2. Maximal screw length (mm), 3. Minimal laminar thickness (mm), 4. Spino-laminar angle (°), 5. Laminar height at spino-laminar junction.

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  Fig. 7 Measurement of maximal laminar thickness of upper 1/3, middle 1/3, and lower 1/3 at the spino-laminar junction.

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  Fig. 8 Drawing of ideal entry points of the bilateral crossing C7 intralaminar screws.

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