Sacroiliac fixation in young children: practical considerations from a pediatric orthopedic perspective

Pediatric pelvic fractures are rare events in most trauma centers, yet when they present with multidirectional instability, they challenge even experienced orthopedic trauma surgeons. The immature pelvis behaves very differently from the adult pelvis, and our knowledge of fixation strategies in this population remains limited. For young children, especially those under six, the lack of evidence becomes even more apparent when we attempt to adapt sacroiliac fixation techniques commonly used in adults.

Pelvic injuries in the very young child

Most orthopedic surgeons rarely see unstable pelvic fractures in children. The elasticity of immature bone, thick periosteum, and the presence of cartilage contribute to a remarkable capacity for energy absorption. This protective anatomy is also the reason why, when a young child does sustain a pelvic ring injury with instability, the mechanism is usually significant and the injury pattern deserves great attention.

In practice, the Torode and Zieg Type IV fracture—complete disruption of the pelvic ring—is the pattern most likely to require surgical intervention. Closed treatment may be sufficient in many pediatric fractures, but when there is multidirectional instability, surgical stabilization becomes essential to restore alignment, protect vital structures, and allow safe mobilization.

While adult pelvic fixation principles often guide our decision‑making, it is immediately clear that adult implants and adult corridors cannot simply be transferred to small children. The sacrum is narrower, the safe corridors are different, and our options are dramatically more restricted.

The limits of applying adult SI fixation to small children

In adults, percutaneous sacroiliac fixation is well established. Surgeons can reliably use S1 and S2 corridors, place 7‑mm screws, and achieve robust fixation. Device companies design implants with this anatomy in mind, and the literature provides decades of biomechanical validation.

None of this applies to a three-, four-, or five‑year‑old child.

Even simple preoperative planning shows the challenge: the sacral corridors that easily accommodate a 7‑mm screw in adults may not exist in a toddler or preschooler. In fact, our previous CT-based study clearly showed that no standard corridor in children under six can safely accept a 7‑mm screw, even with a minimal safety margin.

This is not a small difference, and consequently it fundamentally changes the surgical strategy.

What we learned from our CT corridor study

In our prior research, in which we reviewed 170 pediatric CT scans, we measured the widths of the S1, oblique S1, and S2 corridors across three age groups. For children younger than six, two conclusions were clear:

1. A 7‑mm screw is unsafe in all corridors.
2. Even a 3.5‑mm screw is unsafe in the traditional S1 or S2 corridors.

The only corridor available for fixation in this age group is the oblique S1 corridor, and only when using a 3.5‑mm cannulated screw.

This finding provided helpful anatomic guidance, but it raised a critical clinical question: even if a 3.5‑mm screw is the only screw we can safely place, is it strong enough to do the job?

It was this question that led to the study discussed here.

Pelvic fixation: feasibility versus reliability

In children under six, pelvic fixation is meant to provide:

• temporary stability
• protection of the reduction
• safe conditions for healing
• prevention of long-term deformity

Surgeons are already familiar with using elastic nails, thin pins, and delicate implants in young patients. But sacroiliac fixation in a small pelvis is unique: the screw has to resist shear, rotation, and vertical instability forces across the pelvic ring. A screw that is anatomically possible may not be mechanically sufficient.

This is where clinical decision‑making becomes difficult. You can place a 3.5‑mm screw, but should you trust it?

What our biomechanical study tried to answer

To address this clinical uncertainty, our team created a pediatric pelvic model based on CT imaging of a five‑year‑old patient. Using this model, we simulated a Torode and Zieg Type IV fracture pattern and tested two different fixation strategies:

• a 3.5‑mm cannulated screw, placed in the oblique S1 corridor
• a 7‑mm cannulated screw used as a reference construct

By testing both under realistic load conditions, we could evaluate whether the smaller screw provided enough stability in the setting where it is anatomically the only option.

Stability, screw fatigue, and the importance of protection

Even though the engineering work was significant, the clinical implications are much more direct.

1. A 3.5‑mm screw can hold the reduction—initially

Both screw sizes kept the pelvic fracture aligned under the simulated bipedal load. There was no excessive fragment displacement, and overall pelvic stability was similar.

Surgeons can expect a reliable initial fixation with a 3.5‑mm screw when it is properly positioned in the oblique S1 corridor.

2. The limitation is not the reduction, it is the screw itself

The most important result from a practical standpoint was the stress profile. The 3.5‑mm screw showed a stress concentration exceeding the limits of the material, indicating a risk of deformation or breakage under repetitive loading.

The screw can hold the reduction, but it may not tolerate physiological repetitive load, especially if weight-bearing precautions are not strictly followed.

3. Postoperative management is critical

This is where the study becomes most valuable clinically. Although the 3.5‑mm screw can maintain the reduction, its susceptibility to fatigue means that strict postoperative weight‑bearing restriction is mandatory. Protecting the construct is just as critical as placing it correctly.

I make it a point to counsel families clearly from the beginning, coordinate closely with the physical therapy team, and use immobilization strategies that reduce load transmission across the sacrum whenever possible.

How this changes my own surgical practice

This research has changed the way I approach sacroiliac fixation in children under six. Some key points I now emphasize:

• I plan fixation assuming the screw is the weak link, not the bone.
• I give realistic expectations to families: stability is achieved, but the construct is fragile.
• I am meticulous about the trajectory of the oblique S1 screw. Tiny deviations matter in small anatomy.
• I implement a strict, clearly documented postoperative protocol for protected weight bearing.
• If instability is severe or additional injuries exist, I consider complementary stabilization strategies to reduce load on the screw.