Ridge of the pars interarticularis and the transverse process
Pedicle screw entry points and angulation in the thoracic spine can be divided into four groups based on the relation of the pedicle with the articular facets and the transverse process.
The entry points is in the intersection of the line drawn horizontally though the middle of the transverse process with a line drawn slightly lateral to the center of the articular facet.
The screw angulation is slightly medial and caudal.
The entry point is more cranial and medial compared to the T1-T3 entry points. The entry point is in the intersection of the line drawn horizontally through the upper 1/3 of the transverse process and the line drawn through the center of the articular facet.
The screw angulation is almost vertical.
The entry point is more cranial and medial compared to the T4-T6 entry points. The entry point is in the intersection of the line drawn horizontaly on the superior border of the transverse process and the line drawn slightly medial to the center line of the articular facet.
The screw angulation is vertical
These pedicles tend to be very narrow. The entry is in the intersection of a horizontal line drawn through the mammillary process or the upper 1/3 of the transverse process, and the vertical line drawn through the center of the articular facet.
The screw angulation is vertical or slightly lateral .
If one is not sure the entry point for the pedicle is correct, a small laminotomy with removal of the yellow ligament will allow palpation of the inner wall of the pedicle with either a pennfield or a ball-tip probe
2. Lumbar spine
The entry point of the pedicle screw is defined as the confluence of any of the four lines:
Lateral border of the superior articular facet
Mid transverse process
Opening of the cortex
Open the superficial cortex of the entry point with a burr or a rongeur.
A pedicle probe is used to navigate down the isthmus of the pedicle into the vertebral body.
The appropriate trajectory of the pedicle probe in the cranial caudal direction occurs by aiming for the contralateral transverse process.
Thereby paralleling the screws to the superior endplate.
The medio-lateral inclination will depend on the rotation of the vertebra. The main goal is to avoid medial penetration of the spinal canal superficially and lateral or anterior penetration of the vertebral body cortex at the depth of insertion. Ideally, the two screws should converge but stay entirely within the cortex of the pedicles and body.