Ilio-sacral screw

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Status

Done

Parent item

TL spine procedures

Indications

• for vertical pelvic fracture

Principle

• A partially threaded lag screw is used for joint compression.
• A washer is necessary to maximize purchase on the ilium without burying the screw head in the bone.
• 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 plates, posteriorly or anteriorly
• Anterior arch fixation increases stability

Pre op planning

Patient's individual pelvic anatomy (normal vs. dysmorphic)

• Check CT for
• Site of injury, including its displacement and signs of instability
• Base-line anatomic features
Sacral anatomy variations
• Variable in utero segmentation of the lumbo-sacral somites produces abnormal (“dysmorphic”) upper sacral anatomy in approximately one-third of patients. These variations must be recognized because they compromise normally safe intraosseous pathways for ISS.
• Normal vs. dysmorphic sacral anatomy
• 3D reconstruction: The differences in sacral alar anatomy, with corresponding upper sacral segment deficiencies on the dysmorphic axial CT view.
• In such cases, the second sacral level may be better for an iliosacral screw
 

• Features of dysmorphic pelvis
• Sacral deformities may be unilateral or bilateral.
• Bilateral dysmorphism may be symmetrical, or not.
• This case show an individual with symmetrical upper sacral dysmorphism.
• The lumbosacral disc is near the level of the iliac crests, not below them as is normal.
• There is a residual disc between the upper and second sacral segments.
• The uppermost anterior sacral foramina are not round, as the lower ones are.
• The superior alar slope is steeper, from medial to lateral, and from posterior to anterior.
• The upper sacral alar anterior cortical limit appears as an indentation (white arrows) relative to the alar anterior cortical of the second sacral segment (yellow arrows).
• The upper and second sacral alar anterior cortical limits are different but easily seen on the inlet view.
• The surgeon must understand these differences and then visualize them under image intensifier in the operating room because iliosacral screws must remain posterior to these alar cortical limits.


 

Check inlet and outlet view angles on CT and then on C arm

• Individual variation of pelvic inclination is significant enough that customised inlet and outlet angles should be chosen for use during ISS.

• using Midline sagittal CT
• The inlet angle should be tangent to the anterior cortex of S1.
• The outlet angle is perpendicular to the “midline” of the trapezoidal S1 body.
• An optimal outlet view shows the upper border of the pubis overlying the second sacral vertebra.

• If the C arm inlet and outlet views structures (spinal canal (inlet view), S1 body (inlet view), sacral foramina in outlet view (outlet view)) cannot be seen, a safe trajectory for the iliosacral screw cannot be determined, cannot do this with a MIS approach
• can be difficult in
• Obese patients
• Extensive bowel gas
• Overlying radiographic contrast material

• Patient's specific SI joint injury configuration

• Optimal ISS type, location. and length

Selection of screw channels

• General
• Placement of iliosacral screws should be planned on pre-operative CTs, and confirmed with post-reduction fluoroscopic images obtained intra-operatively.
• ISS should be perpendicular to the plane of instability, entirely within the bone, and avoid the nerve root tunnels.
• For SI joint fixation, the ISS is inserted perpendicular to the SI joint, and extends beyond the midline of the sacral body.
• For sacral fractures, the ISS is horizontal, allowing it to be inserted to or through the contralateral SI joint, to optimize fixation on both sides of the sacrum.
 

• Patients with normal anatomy

• Patients with dysmorphic anatomy

• Need for enhanced fixation (see above)

Essential C-arm views

• AP
 

 

• Inlet views
• Following structures should be seen easily
• Spinal canal
• S1 body
• Location of S1 nerve root tunnels
• The root tunnels travel from central to anterolateral, exiting through anterior cortical lucencies.

• Outlet views
• Following structures should be seen easily
• Sacral foramina
• Location of S1 nerve root tunnels
• The appearance is similar to a hip spica cast, with its upper part formed by the midline sacral spinal canal, and the root tunnels being the legs of the spica.

• True lateral
• Centered on S1 is essential for IS screw insertion.
• The true lateral x-ray shows the midline profile of the sacral promontory, and the iliac cortical densities, which mark the anterosuperior surface of the ala.
• Confirm that the lateral view is indeed true, without rotation.
• The sciatic notch outlines should be superimposed.
• identify the S1 body and iliac cortical densities (ICDs), here overlapping correctly.
• C-arm set-up, and resulting images should finalized after the patient is anesthetized and positioned, before sterile drapes are applied.
• Location of S1 nerve root tunnels
• On the lateral view the S1 root tunnel is caudal/posterior to the iliac cortical density, beneath the superior cortex of the sacral ala.

Surgical technique

• 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.

• Incision
• 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.

• Guidewire insertion into ilium
• 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.
• RMK used a Jamshidi to get the entry and a hold on the Ilium and through the initial part of the sarcum, then only use guidewire rest into the sacrum
 

• Guidewire proceed into sacrum
• The guidewire is advanced 1 cm into the sacral ala.
• The position and trajectory of the guidewire is checked in inlet and outlet projection.
• 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.
 

• Screw insertion
• 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.
• 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.
• To help prevent such intrusion, use of a washer should be routine.
• In case of comminution, fully threaded screws may be preferred to avoid over-compression of a sacral fracture.
• They may also be used to supplement an initial lag screw.
• The guidewire is removed.

 

References

Iliosacral screw for SI joint