Idiopathic Scoliosis: Surgical Screw Placement

Screw Placement

The C-Arm is placed into the operating field and positioned at the superior most vertebral body to be instrumented. It is imperative to have the C-Arm parallel to the spine to give an accurate image (Fig 1).

The vessels are located in the valley or middle of the vertebral body and serve as an anatomical guide for screw placement. The segmental vessels are grasped and ligated at the mid-vertebral body level with the electrocautery (Fig. 11).

Figure 11

Larger segmental vessels and the Azygous system may be hemoclipped and cut if necessary. The patient positioning is again checked to ensure they are still in the direct lateral decubitus position.

The “K” Wire Guide is placed onto the vertebral body just anterior to the rib head (Fig. 12).

Figure 12

The position is checked with the C-Arm to verify that the wire will be parallel to the endplates and in the center of the body. The inclination of the guide is checked in the lateral plane by examining the chest wall and the rotation. The guide should be in a slight posterior to anterior inclination. This will direct the wire away from the canal. If there is any doubt or concern about the anterior inclination, a lateral C-Arm image should be obtained to verify position. Once the correct alignment of the guide has been attained, the “K” wire is inserted into the cannula of the “K” Wire Guide that is positioned most centrally on the vertebral body. Drill the Guidewire to the opposite cortex, ensuring it is parallel to the vertebral body. The position is confirmed with the C-Arm as the wire is inserted. Care must be taken so the wire is not drilled through the opposite cortex. This can result in injury to the segmental vessels and the lung on the opposite side.

The most superior mark on the Guidewire presents a length of 50mm and the etched lines are at 5mm increments. The length of the “K” wire in the vertebral body can be determined by these marks. Start at the 50mm mark and subtract 5mm for each additional mark that is showing. For example, if there are 5 marks, in addition to the 50mm mark, the length of k-wire would be 25 mm. The calculation for this would be 50 - 5 -5-5-5-5. The 5mm subtractions are for the 5 marks showing. (Fig. 13)

Figure 13

The Wire Guide is removed and the Tap is now placed over the “K” wire onto the vertebral body. The largest diameter tap that will fit in the vertebral bodies, based on the pre-operative x-rays, should be used to maximize fixation strength. The distal end of the wire is grasped with a clamp (Fig. 14) and held as the tap is inserted so the wire will not advance. This is important in order to avoid a pneumothorax in the opposite chest cavity. Only the near cortex is tapped (Fig. 15). The C-Arm should be used to monitor tap depth and “K” wire position.

Figure 14	Figure 15

The appropriate sized screw, based on the “K” wire measurement and tap diameter, is placed over the wire with the Eclipse Screwdriver and advanced (Fig. 16). One should select a screw that is 5mm longer than the width of the vertebral body, as measured with the “K” wire, to ensure bicortical fixation. The wire is again grasped to avoid advancement while the screw is inserted (Fig. 14). The wire is removed when the screw is approximately ½ to ¾ across the vertebral body. The screw direction is checked with C-Arm as it is advanced and seated against the vertebral body (Photo 9). The screw should penetrate the opposite cortex for bicortical fixation (Fig.17).

Figure 16	Figure 17

Using each rib head as a reference for subsequent screw placements helps ensure the screws are in line and yield proper spinal rotation when the rod is inserted. Properly aligned screws will have the screw heads aligned in an arc. This alignment can be verified with a lateral image. (Photo 10)

Photo 9	Photo 10

All Cobb levels should be instrumented.

The side walls of the screws (saddles) are adjusted to be in line for receipt of the rod (Fig.18).

If a screw is sunk more than a few millimeters deeper than the rest of the screws, reduction of the rod into the screw head may be difficult. The C-Arm image can clearly show this as the screws are being inserted.

Once all the screws have been placed the surgicell is removed and the graft is inserted. The graft is delivered to the disc space using the Graft Funnel and Plunger (Fig. 19 and 20). The disc space should be filled all the way across to the opposite side.

Figure 18	Figure 19	Figure 20