1 Introduction topenlarge
Iliosacral screw (ISS) fixation is a fluoroscopically guided, percutaneous procedure. Its primary use is for fixation of satisfactorily reduced sacral fractures or sacro-iliac joint disruptions. Anatomic reduction must be obtained before ISS insertion.
Screw placement and angulation will depend on the injury and the patients specific anatomy (variations are common).
Principles of ISS fixation for sacral fracture
ISSs for sacral alar fractures are usually oriented transversely because this is the path of a transsacral channel. Screw channels for sacral fractures are narrow and precise screw placement is therefore essential.
ISS for sacral fractures can be unilateral, extending far into the contralateral sacral ala, transsacral, or transiliac extending beyond the contralateral ilium where a suitable nut on the screw threads can enhance fixation.
Compression of a comminuted alar fracture may injure a nerve root within the fracture zone. Thus, a fully threaded (position) screw may be preferable to a lag screw for ISS fixation. However, a screw that maintains excessive distraction increases the risk of nonunion. A washer is advisable for secure fixation on the ipsilateral side.
Fixation failure may occur, especially with grossly unstable injuries. Fixation can be enhanced by one or more of the following:
- 1 or 2 additional screws (S1 or S2 level)
- Supplementary posterior plates
- Spinopelvic fixation (especially for comminuted transforaminal vertical sacral fractures and bilateral injuries).
- Anterior pelvic ring fixation increases stability
2 Preoperative planning top
An explicit, written pre-operative plan is strongly encouraged. In addition to overall assessment and preoperative planning, specific considerations are required for ISS.
- Patient's individual pelvic anatomy (normal vs. dysmorphic)
- Patient's specific sacral fracture configuration
- Optimal ISS type, location, trajectory, and length
- Selection of screw channels
- Need for additional fixation (see above)
More details on preoperative planning is found here.
3 Iliosacral screw insertion topenlarge
Landmarks for stab incision
On the true lateral projection, identify the S1 body and iliac cortical densities (arrow) here overlapping correctly.
If double contours of the iliac cortical densities (see arrows) are visible as shown here, it is likely that the radiographic central ray is striking the pelvis obliquely. The C-arm should be adjusted to obtain a true lateral view, with superimposed ICDs and S1 cortices.
(This assumes that inlet and outlet views show that the pelvic reduction is anatomic.)
Location of screw entry point
The entry point should be anterior in S1 and inferior to the iliac cortical density (ICD), which parallels the sacral alar slope, usually slightly caudal and posterior. The ICD thus marks the anterosuperior boundary of the safe zone for an iliosacral screw which may injure the L5 nerve root if it penetrates this cortex.
An instrument handle can help to target the desired entry site for the guidewire.
A stab incision is made at the identified site. The underlying tissues are dissected down to bone, by spreading with an appropriate blunt clamp, or with scissors if necessary.
There should be sufficient room for a protective drill sleeve, and for the planned screw and washer.
A guidewire is tapped 2-3 mm, or drilled (oscillating mode preferred) into the planned screw entry point. This is monitored by X-ray on the true lateral projection.
Note that with a true lateral view, the power source and chuck must be removed from guidewire or drill to assess their position.
The guidewire is advanced 1 cm into the sacral ala according to chosen screw channel.
The position and trajectory of the guidewire is checked in inlet and…
When the guidewire tip is just lateral to the neural foramen on the outlet view, pause to confirm that its position is satisfactory.
The desired trajectory is within but close to the anterior alar cortex on the inlet view, and cranial to the ventral foramen of the 1st sacral nerve root.
If the trajectory of the guidewire would compromise either the sacral foramen or the spinal canal, the guidewire is removed and then reinserted from a similar entry point but in the corrected trajectory.
Alternatively, the guidewire is left in place as a reference and a new one is inserted along the correct trajectory.
When a safe trajectory for the guidewire is confirmed, it is further advanced to the contralateral lateral border of the first sacral body.
When the guide wire reaches the center of the sacral body, the position is again verified in true lateral, inlet, and outlet view.
The wire must be far enough from cortices and neural foramina to accommodate the thread diameter of the planned screw.
The guide wire can then be advanced across the midline.
The screw length is measured with a ruler or gauge suitable for the guidewire, and compared with the pre-operatively estimated length.
An appropriate screw hole is drilled over the guidewire, which should remain anchored in the bone, if it has been advanced far enough beyond the intended screw tip site.
The chosen cannulated lag screw is inserted with a washer.
In case of comminution, fully threaded screws may be preferred to avoid over-compression of the sacral fracture and the neuro foramina. They may also be used to supplement an initial lag screw.
The guidewire is removed.
Pifall: Intrusion of screw head into illium
Excessive tightening of the screw will cause intrusion of the screw head into the ilium.
Such errors, which compromise fixation, are more likely with osteoporotic bone.
The use of a washer may help prevent this error.
With the C-arm rotated so its central ray is tangential to the ilium at the ISS insertion site, an excellent view of bone surface, washer, and screw head is available. This permits precise tightening of the screw without intrusion of washer and screw head through the iliac cortex.
4 Check of osteosynthesis topenlarge
Upon completion of the ISS procedure, use multiple C-arm images to assess
- Reduction of sacral fracture
- Overall pelvic ring alignment & symmetry
- Position of each screw, within cortical boundaries, and avoiding intra-osseous neural pathways
- Confirm that all guidewires and reduction aids have been removed
- Alignment and fixation of any associated ring injuries
Assessment of ISS requires AP, Inlet, outlet, and true lateral images of the posterior pelvis. Especially for bilateral injuries, radiographs that show the entire pelvic ring are necessary to confirm correction of complex deformities.
Make sure that any additional injury sites have been treated as planned.