This is a technique to prevent injury to the L4/5 facet when placing a L5 screw for L5/S1 fusion
Indications for Satellite Pedicle Screws
- Obese patients undergoing open Transforaminal Lumbar Interbody Fusion (TLIF).
- Specifically, patients with a Body Mass Index (BMI) greater than 30 and deep subcutaneous fat in the lumbar region thicker than 5 cm.
- The need for longer surgical incisions in obese patients to visualise landmarks and angulate screws medially, which carries higher risks of wound healing problems, increased blood loss, prolonged surgical time, and post-operative pain.
- Difficulty in achieving an optimal, converging pedicle screw trajectory due to the counterforces exerted by the erector spinae muscles and thick subcutaneous fat, especially at the L5-S1 level.
- Concerns with minimally invasive fusion techniques in obese patients, including lack of universal availability/expertise, high cost, high radiation exposure, and poor visibility of bony structures.
- For L5-S1 level
- Due to the thick fat and steep lordotic angulation that typically complicate conventional screw insertion in obese patients.
- For surgeons routinely performing open TLIF, it allows insertion of well-converging screws with limited dissection.
Technique
- Patient Positioning and Initial Exposure:
- The patient is positioned prone on bolsters on a radiolucent frame.
- A standard posterior midline exposure is performed to expose only the laminae and the two facet joints of the spinal segment to be fused (e.g., L5-S1).
- Pedicle Entry Point Marking (Main Wound):
- Within the main surgical wound, the pedicle entry point is marked using a monopolar cautery at the junction of the transverse process and the lateral facet joint.
- Pre-operative Planning for Satellite Incision:
- Axial MRI is used to determine the Medio-lateral (ML) point:
- A line is drawn along the pedicle trajectory from the skin, and the distance from this line to the midline (typically 6-7 cm) dictates the appropriate lateral position for the satellite incision.
- Mid-sagittal MRI is used to identify the Cephalo-caudal (CC) point:
- A line is drawn from the skin surface along the pedicle trajectory.
- A perpendicular line dropped from this point helps mark the CC point intra-operatively.
- Creating the Satellite Incision:
- Intra-operatively, a line is marked on the skin parallel to the main surgical incision, 6 to 7 cm away from it, based on the ML point planning
- A 1.5 cm incision is made along this line at the point corresponding to the CC point, confirmed with a C-arm image.
- This stab incision is deepened to the deep fascia, and the fascial incision is slightly lengthened to allow insertion of screws at two adjacent levels (e.g., L5 and S1) through the same skin incision.
- Freehand Screw Insertion:
- A pedicle entry awl is passed through the satellite incision towards the spine.
- In the main wound, a retractor (Hohmann's elevator or deep Langenbeck) gently retracts muscles to visualise the tip of the awl.
- The awl is guided towards the previously marked pedicle entry point.
- A pilot hole is created using a freehand technique, and its integrity is confirmed with a feeler passed through the satellite hole.
- A straight pedicle probe is then inserted in the same direction, guiding it through the pedicle into the vertebral body.
- The track is tapped and re-checked with a feeler.
- The appropriate pedicle screw is then inserted
- The same satellite incision can be used for subjacent screws (e.g., S1) by making minor adjustments in cephalo-caudal angulation.
- This method avoids the strong resistance from thick paraspinal structures and extensive dissection, enabling an optimal screw trajectory. The screws are inserted free-hand, avoiding radiation risks typically associated with image intensifiers.
- Rod Insertion and Wound Closure:
- A unilateral rod is persuaded through the main surgical wound.
- The poly-axial screw heads allow for convenient insertion of screw caps and rods through the main incision.
- Interbody fusion is performed through the main incision.
- The contralateral rod is then placed.
- The main wound is closed in layers, while the satellite incision is closed in a single layer.
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