Authors of section

Authors

Ronald Lehman, Daniel Riew, Klaus Schnake

General Editor

Luiz Vialle

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Lateral mass screw insertion (Magerl technique)

1. Anatomical landmarks

The following anatomical landmarks are identified:

  • Lateral border of the lateral mass
  • Medial border of the lateral mass
  • Superior facet
  • Inferior facet

These landmarks form a square.

Anatomical landmarks identified during lateral mass screw insertion magerl technique

2. Screw entry point

The square outlined by the landmarks is divided by a horizontal and a vertical line.

Using anatomical landmarks to identify screw entry points in lateral mass screw insertion magerl technique

The screw entry point is found slightly inferomedial to the intersection of these lines.

Put entry point in medial-inferior quarter

3. Opening of the cortex

Penetrate the cortex with a thin burr or an awl.

Opening the cortex during lateral mass screw insertion magerl technique

4. Medio-lateral angulation

The drill trajectory should be aimed 25° laterally to avoid the vertebral artery, which is located directly anterior to the entry point.

Medio-lateral angulation during the lateral mass screw insertion magerl technique

5. Cranial-caudal angulation

To identify the craniocaudal angulation, a Penfield elevator is inserted in the facet joint which will be included in the fusion. The drill trajectory is then parallel to this elevator, avoiding compromise of the facet joints. Alternatively, the trajectory of the facet joints may be determined by lateral fluoroscopy.

Cranial-caudal angulation during lateral mass screw insertion magerl technique

6. Monocortical or bicortical screws?

Both mono- and bicortical screws may be used.

Utilizing 14 mm screws will be safer, but only monocortical purchase can be achieved.

Use of monocortical screws in lateral mass screw insertion magerl technique

A longer screw providing bicortical purchase will result in a more stable construct. However, the screw tip should not extend too far beyond the second cortex as it may compromise the nerve root.

Use of bicortical screws in lateral mass screw insertion magerl technique

7. Drilling

If a monocortical screw is planned, the drill is set for a 14mm screw hole.

When a bicortical screw is planned, the drill is used at high speed with meticulous handling. The drill bit is advanced only for a short distance, then pulled back before advancing again. This maneuver is repeated until the second cortex can be felt and crossed. Great care is taken to avoid over-penetration.

Alternatively, a drill stop may be used in 2mm increments. After each increase in effective drill length, the drill hole is probed to feel for penetration of the second cortex.

A screw length is selected, which corresponds to the final effective drill length required for bicortical penetration.

Drilling during lateral mass screw insertion magerl technique

8. Screw insertion

A screw of appropriate diameter (3.5 mm) and length is carefully inserted into the same created trajectory.

Note: C7 instrumentation can be either pedicle screw or lateral mass screw, depending on the patient's anatomy and the surgeon's preference.

C7 and T1 should have a pedicle screw (last inferior), not a lateral mass
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