THE IPSILATERAL LAMINA AS A GUIDE FOR SCREW TRAJECTORY IN POSTERIOR INSTRUMENTATION OF THE SUB-AXIAL CERVICAL SPINE

Abstract

Aim: In sub-axial cervical vertebrae the lamina appears to project perpendicular to the ipsilateral pedicle axis, and forms a reliable trajectory for avoidance of vertebral artery injury in lateral mass (LM) screw placement: the aim is to confirm these observations.

Material and Methods: 51 digital cervical spine CTs (255 vertebrae; 25 female 26 male; mean age 37.4 range:18–80). Exclusions: Severe degeneration, malformation, tumour, trauma.

Measurement (axial view):

1. Angle of ipsilateral outer lamina cortex to pedicle axis

2. Virtual screw trajectory 2 mm from and parallel to the lamina was placed through the LM. Potential violation of the transverse foramen and LM width available for screw purchase was assessed

Results: Average lamina-pedicle angle (standard deviation):

• Females: Right: C3–84.8°(2.6), C4–85.2°(3.1), C5–86.7°(3.3), C6–89.2°(2.5), C7–92.3°(2.4);

• Left: C3–84.0°(3.1), C4–84.5°(3.9), C5–86.6°(3.7), C6–89.6°(2.6), C7–92.1°(2.3)

• No significant difference between males and females (P< 0.05)

• Violation of transverse foramen C3–C7: 0%

• LM width (trajectory parallel to LM) in millimetres (standard deviation):

• Males: Right: C3–5.5(0.7), C4–6.1(0.7), C5–6.8(0.8), C6–7.1(1.1), C7–6.1(1);

• Left: C3–5.2(0.8), C4–5.9(0.8), C5–7(1.2), C6–7.3(1.1), C7–6.3(1.4)

• Females: Right: C3–5.3(0.8), C4–5.5(0.9), C5–6.6(1.2), C6–6.3(1.3), C7–5.4(1.4);

• Left: C3–5.2(1), C4–5.7(1), C5–7.1(1.1), C6–6.5(1.3), C7–5.5(1.6)

Conclusion: The angle formed by the lamina and ipsilateral pedicle ranges from 84° at C3 to 92° at C7. Although the angle is not exactly perpendicular at all levels as hypothesised, the lamina forms a useful reference plane for pedicle screw insertion in the sub-axial cervical spine.

LM screws placed parallel to the lamina find sufficient LM width and are highly unlikely to injure the vertebral artery in bi-cortical placement. This technique appears favourable over conventional 30° LM placement.

Ethics approval: None needed

Interest Statement: None