Spinopelvic fixation

3 main concepts of spinopelvic fixation

Lumbosacral Pivot Point (McCord et al., 1992)

Definition:

The pivot point is at the center of the osteoligamentous column between the last lumbar vertebra (usually L5) and the sacrum.
It represents the "instantaneous axis of rotation" for lumbosacral biomechanics.

Construct stability increases as the fixation progresses more ventral to the pivot point

Zones of Sacropelvic Fixation (O’Brien et al., 2004)

Zone 1: S1 vertebral body and cephalad margins of sacral alae

Zone 2: Inferior margins of sacral alae, S2, and area extending to the tip of the coccyx

Zone 3: Both iliac bones

Implications: Extending fixation to more caudal and lateral zones (especially Zone 3, the ilium) provides greater construct stability

Triangulation of Screws

Directing pedicle screws medially for better pullout strength: triangulation increases the biomechanical strength of constructs by converging screws in a triangular orientation across multiple cortical layers.

Indications for Sacropelvic Fixation

Long fusions	Adult degenerative Scoliosis, Neuromuscular Scoliosis, Paralytic curves.
Spondylolisthesis	High grade slips
Deformity	Kyphotic deformity especially in cases of Ankylosed spine, Pelvic obliquity
Fractures	Sacral and sacropelvic fractures
Osteotomy	Osteotomies in lower lumbar spine
Others	Pseudarthrosis, Tumors, Lumbosacral dislocations, Osteomyelitis

Techniques and Construct Variations

Types of Sacropelvic Fixation

Zone	Technique/Implant
Zone 1	Sacral (S1) tricortical screws, S1 pedicle screws
Zone 2	Sacral alar screws, S2 pedicle screws, Sacral intrasacral rods
Zone 3	Iliac screws, S2-iliac screws, S2 alar-iliac (S2AI) screws

Key Points:

Multiple points of fixation using rods, cross-linking, and anterior column support improve stability.

Construct should aim for maximum purchase across these zones to resist physiological loading.

Positioning and Operative Reminders

Iliac crests must be symmetrical.

Adequate draping for lateral and distal access.

Ensure unobstructed imaging.

Mispositioning alters pelvic parameters.

Avoidance Zones:

Avoid placing screws near the (to prevent neurovascular injury)

Facies auricularis
Crista sacralis medialis
Anterior sacral foramen

Traditional sacropelvic fixation

Current Methods of Sacropelvic Fixation8 9 10

Feature	Traditional Iliac Screw	Anatomic Iliac Screw	Subcrestal Iliac Screw	S2-Alar-Iliac (S2AI) Screw	S1 Pedicle Screw	Sacral Alar Screw
Entry Point	At or near Posterior Superior Iliac Spine (PSIS)	Medial/caudal to PSIS, often in line with S1 pedicle screws	Medial aspect of iliac crest, inferior & medial to PSIS, beneath crest (subcrestal)	Between S1 and S2 dorsal sacral foramina, just lateral to the lateral sacral crest	Inferior border of superior facet of S1, base of the “V” sign	Lateral/inferior to S1 pedicle, into sacral ala
Trajectory	Lateral & slightly anterior toward Anterior Inferior Iliac Spine (AIIS)	Similar to traditional but in-line with S1 pedicle screws, toward AIIS	Lateromedial, deep under crest, parallel/sightly convergent with S1 screw, avoiding SI joint	Directed lateral and caudal—crossing sacral ala, SI joint, into ilium—aimed at AIIS	30° medial, toward the sacral promontory	Lateral/anterior, through the sacral ala
Advantages	- Strong biomechanical fixation- Long screw length- Familiar technique	- Low profile/less prominence- Easier rod connection (inline)- Less muscle dissection	- Nearly eliminates prominence- Minimal muscle disruption- Easier rod connection- Reduced wound complications	- Very low profile- Minimal soft tissue dissection- Rods align in-line with cephalad constructs- Lower implant prominence and wound issues- Avoids need for offset connectors- Better patient tolerance, especially in thin individuals	Strong fixation, large screw size, familiar, good biomechanics	Alternative when pedicles not usable; quick for fx
Disadvantages	- Prominent head can cause pain/skin breakdown- Offset connectors often needed- Extensive muscle dissection	- Shorter trajectory than traditional- Some risk of prominence	- Technically demanding- Must avoid SI joint violation- Less familiar technique	- Technically complex learning curve- Potential SI joint violation- May require fluoroscopy or navigation- Risk of neurological or vascular injury with incorrect placement	Risk of nerve or vessel injury; exposed hardware if prominent	Lower strength; shorter/smaller screw; precision needed

Sacral Fixation:

S1 Pedicle Screws

Entry Point:

Inferior border of the superior facet of S1 vertebra, typically close to the lateral aspect but can vary based on patient anatomy.
The entry is identified medially by the base of the "V" sign—formed by the lateral border of the superior articulating process and the sacral ala

Trajectory:

30° medial angulation, directed towards the sacral promontory. The screw path travels through the S1 pedicle into the vertebral body, generally parallel or slightly cranial to the S1 endplate

Pros:

Strong fixation due to dense trabecular bone
Large pedicle diameter accommodates larger screws
Familiar technique for most spine surgeons
Good biomechanical stability for most cases3

Cons:

Risk of misplacement leading to nerve or vascular injury2
Prominence or hardware-related pain if not well-seated
May not be sufficient in severe deformity or poor bone quality alone (may need augmentation)3

The left image is an anteroposterior radiograph of the spine highlighting the “V” sign on the right side (yellow curve) along with the entry point (yellow dot) for S1 pedicle screw insertion. The upper right image is a drawing (by C. Tannoury) of the posterior view of a male’s sacrum, including the “V” sign (red dotted lines), the inferior entry point (marked by an “X”), and the medial-lateral axis (green lines). The blue line represents the distance between the “V” sign and the entry point. The lower right image is a photograph of a cadaveric specimen. On the left side, the entry point is marked by a lead dot (yellow arrow) inferior to the “V” metallic wire; the L5 inferior facet was dissected and removed to highlight the S1 superior articular process. On the right side, the L5 inferior facet was preserved, the “V” metallic wire is highlighted by red dotted lines, and the entry point is marked by an “X” (medial-lateral axis; green lines).

S2 pedicle screw

S2 pedicle screw insertion raises a theoretical possibility of increasing the strength of fixation by supplementing S1 pedicle screws

Not recommended as it doesn’t offer any significant advantage.

S2 pedicles are dorsal to the sacral pivot (McCord point) and hence are unable to give any biomechanical advantage.8

Sacral Alar Screws

General

Sacral alar screws are usually unicortical screws taking the purchase laterally in the cancellous sacral ala.
Alar screws are used as supplemental fixations by many surgeons.

Entry Point:

between the S1 and 2 foramina.

Trajectory:

Directed laterally and anteriorly through the sacral ala.

35 - 40 degree laterally
Cephalad 15 - 20

End point

do not breech the anterior cortex as it can injury the L5 nerve

Pros:

Increases construct strength and stability when combined with S1 screws
Lower implant profile than iliac screws (less hardware prominence)
Avoids additional soft tissue dissection required by iliac screws
Does not interfere with iliac bone graft harvest
Lower risk of wound complications (compared to more prominent implants)
Placement can be done via a single midline incision
No need for offset connectors or separate fascial incisions
Avoids sacroiliac joint violation if placed correctly
Easier removal or salvage in cases of infection compared to iliac screws
High fusion rates observed in the study (solid fusion in 12 out of 13 cases)

Cons:

Provides less lateral/pelvic support than iliac screw fixation
Not as biomechanically robust for severe deformities or highly unstable constructs
Technically demanding placement with precise angulation needed (30–35° lateral, 15–20° cranial)
Risk of injury to L5 nerve root or internal iliac vessels if screw breaches anterior cortex or sacroiliac joint
Limited long-term and large-scale data on biomechanical performance compared to iliac screws

Lumbosacral Fixation Using the Diagonal S2 Screw for Long Fusion in Degenerative Lumbar Deformity: Technical Note Involving 13 Cases

Entry point of the S2 alar screw (arrow) and the midpoint of the line from the medial margin of the S1 dorsal foramen and the medial margin of the S2 dorsal foramen.

(A) The lateral trajectory of the S2 alar screw varied somewhat among patients but was typically between 30 and 35 degrees in the lateral planes. It did not penetrate the sacroiliac joint laterally or the S1 ventral foramen medially. (B) The superior trajectory of the S2 alar screw was a longer screw insertion and did vary somewhat among patients, but was typically between 15 and 20 degrees in the superior planes. It did not penetrate the anterior cortex as this could cause impingement of the L5 nerve root and injury to the internal iliac vessels.

Iliac Fixation:

General

Derivation of Galvenston technique

Entry point at PSIS

Directed towards ASIS

Lateral and caudal direction

Done with fluoroscopy guidance

Iliac screws different entry points:

Traditional

A free hand technique no need fluoroscopy until the end to check. When performing this screw can skip S1 pedicle screw

Incision made based on MIS technique for the L5 screw and extend incision so that screw is inserted in a mini open fashion.

Entry:

At PSIS.
The PSIS is osteotomized to prevent prominence of the screw head

Trajectory:

Directed toward the ASIS or Greater trochanter
Traversing thick bone above the sciatic notch.

Screw

Diameter is 7.0 or 8.0 mm.

Pros:

Maximizes screw length and purchase
Reliable biomechanical support.

Cons:

Often prominent →

increased soft tissue disruption,
risk of local pain and wound dehiscence,

Necessitating additional connectors
PSIS needs to be removed → more pain

Anatomic

Aks Subcrestal screw

Exposure:

Perform a subperiosteal dissection to expose the inner table of the iliac crest and identify the PSIS.
Less tissue dissection is required than in conventional methods since the entire iliac crest does not need to be fully exposed.

Entry:

Located 2–3cm caudal to the PSIS (posterior superior iliac spine), just below the S1 sacral foramen.

Caudal and medial to PSIS, flush with the S1 screw head.

This point sits 1.5–2cm beneath the iliac crest and above the SI joint, on the medial wall of the iliac crest.
It is more caudal and medial than the traditional entry point and aligns better with the lumbar pedicles.
The more caudal position places this screw in a wider cross-section of bone above the sciatic notch.

Trajectory:

The screw is aimed caudally (35–45°) and laterally toward the anterior inferior iliac spine (AIIS) or greater trochanter, following a trajectory similar to traditional iliac screws.
The trajectory is refined with tactile feedback: the tip should feel central in cancellous bone between the hard cortices of the crest.
The goal is to get a screw passage within 1.5 to 2 cm above the sciatic notch

Screw

Diameter is 7.0 or 8.0 mm.

Preparation:

Create a pilot hole with a ball-tip burr at the entry.
Use a straight long awl to deepen the pathway, following the planned angle.

Verification:

Confirm central placement with the "teardrop" radiographic view.
Check the screw track for breaches using a guidewire.

Insertion & Fixation:

The screw track is under-tapped by one size.
Use a fully threaded, polyaxial screw (≥7mm diameter, ≥75mm length preferred).
Plan S1 screws to be in-line with the subcrestal screw for easy, direct rod attachment without side-connectors.

Rod Engagement:

Optionally, use a MIS (minimally invasive) tubular sleeve as a reduction device to facilitate screw-rod engagement by cantilever reduction from caudal to cephalad.