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Accuracy and influencing factors of robot-assisted pelvic channel screw placement

This retrospective study of 283 patients demonstrates that robot-assisted pelvic channel screw placement achieves excellent overall accuracy (98.6%), with short screws showing significantly higher precision than trans-cortical screws and a BMI ≥30 kg/m² identified as a key risk factor for malposition.

Original authors: wenhui zhang, yuanyuan zhang, yuping yang, hui zhang, hongyu ma, huiping tai

Published 2026-07-25

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CC BY 4.0

Original authors: wenhui zhang, yuanyuan zhang, yuping yang, hui zhang, hongyu ma, huiping tai

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The High-Stakes Game of Pelvic Puzzle Pieces

Imagine the human body as a complex, high-performance vehicle. Sometimes, due to a bad crash or a nasty fall, the frame gets bent or broken. One of the most critical parts of this frame is the pelvis—the sturdy ring of bone at the base of your spine that holds everything together. When this ring cracks, it's a medical emergency. For decades, fixing these cracks has been like trying to thread a needle while wearing boxing gloves. Surgeons had to drill tiny screws through narrow, winding tunnels inside the bone to hold the pieces together. But these tunnels are right next to major nerves and blood vessels, and if a screw goes even a tiny bit off course, it can cause serious damage.

To solve this, doctors started using "robotic assistants." Think of these robots not as sci-fi Terminators, but as super-precise GPS-guided drill guides. Before the surgery, a computer takes a 3D picture of the patient's bones and maps out the perfect path for the screw. Then, a robotic arm holds the drill steady, following that map with sub-millimeter precision, far better than a human hand could do alone. The big question for the medical world was: Does this robot work for every type of screw path? And, is there a "safer" way to place these screws that doesn't require them to go all the way through the bone, which is like driving a car through a tunnel and out the other side, versus just parking it safely inside the tunnel? A team of researchers set out to find the answers by looking at hundreds of real-life cases.

The Robot's Perfect Scorecard

A team of surgeons at Gansu Provincial Hospital decided to put the robotic system to the test. They looked back at the records of 283 patients who had broken their pelvises and needed these high-tech screws between 2021 and 2025. In total, they placed 346 screws using the robot. The results were nothing short of impressive: the robot got it "perfect" 98.6% of the time. In the world of surgery, where "perfect" means the screw stayed entirely inside the safe bony corridor without poking out even a little bit, this is a huge win. Out of all those screws, only five went slightly off track, and even then, they only poked out a tiny bit (less than 2 millimeters)—about the width of a pencil eraser—and didn't hurt anyone.

Short vs. Long: The "Parking" Strategy

The most interesting discovery in this study was about the length of the screws. The surgeons compared two main strategies:

The "Through-and-Through" Screw: This is like driving a car all the way through a long tunnel until it pops out the other side. It requires extreme precision because if you aim slightly wrong at the start, you'll miss the exit or hit the tunnel wall at the end.
The "Short" Screw: This is like parking the car safely inside the tunnel. You don't need to exit the other side; you just need to stay well within the walls.

The study found that the "Short" screws were significantly more accurate. They achieved a perfect placement rate of 98.9%, while the "Through-and-Through" screws were slightly less accurate at 96.4%. While both are very good, the data suggests that if the bone is strong enough to hold the patient up, using the shorter screw is a safer bet. It's like choosing to park in a garage rather than trying to drive through a narrow alleyway; you have more room for error, and the risk of hitting a wall is lower.

The "Heavy Lifter" Factor: Why Weight Matters

The researchers also investigated why the few mistakes happened. They looked at factors like age, gender, and how the bone broke, but found those didn't really change the robot's accuracy. However, they did find a clear link to body weight. Patients with a Body Mass Index (BMI) of 30 kg/m² or higher were more likely to have a screw placement that wasn't perfect.

Why? Imagine trying to push a long, rigid stick through a thick, squishy layer of foam. If you are pushing through a thin layer, the stick goes straight. But if the layer is thick and squishy (like the extra soft tissue in patients with higher BMI), the stick can get pushed slightly off-course before it even hits the hard bone. The robot plans the path perfectly, but the "foam" of the body can nudge the drill bit just enough to cause a tiny deviation. This doesn't mean the surgery fails for heavier patients, but it does mean the team needs to be extra careful and perhaps use special techniques to keep the drill on track.

The Verdict: Safety First

The study concluded that robotic surgery is incredibly reliable for fixing broken pelvises, with an overall success rate of 98.6%. The patients who got the "short" screws did just as well, if not slightly better, in terms of healing and walking again six months later, compared to those with the long "through-and-through" screws.

The main takeaway for doctors is a simple rule of thumb: If the bone is strong enough to hold the patient, go with the shorter screw. It's like choosing the path with the wider guardrails. It reduces the risk of the screw poking out of the safe zone, especially in patients who carry extra weight. While the robot is a fantastic tool that makes these surgeries safer and more precise, the human team still needs to understand the body's unique shape and weight to get the best results. The study suggests that with the right strategy, these high-tech fixes are becoming the new gold standard for getting people back on their feet.

Technical Summary: Accuracy and Influencing Factors of Robot-Assisted Pelvic Channel Screw Placement

Problem Statement
Pelvic fractures, often resulting from high-energy trauma, require precise internal fixation to restore stability. While percutaneous channel screw fixation (sacroiliac, anterior column, and LC-II screws) is a mainstream minimally invasive technique, the complex pelvic anatomy and proximity to critical neurovascular structures result in a freehand malposition rate of 2%–15%. Although robotic-assisted systems have demonstrated high accuracy in spinal pedicle screw placement, their application in pelvic fractures faces specific challenges. Existing literature lacks large-sample clinical evidence comparing the accuracy of different screw subtypes, specifically "through-and-through" (trans-cortical) screws versus "short" screws, and fails to systematically identify factors contributing to malposition even within robotic-assisted workflows.

Methodology
This study conducted a retrospective analysis of 283 patients (346 screws) with acute unstable pelvic fractures (Tile type B/C) treated at Gansu Provincial Hospital between January 2021 and October 2025. All procedures utilized the "TianJi" orthopaedic robotic system combined with mobile C-arm fluoroscopy.

Grouping: Screws were categorized into eight groups based on type and trajectory: S1 and S2 trans-cortical, S1 and S2 short, antegrade/retrograde anterior column, and antegrade/retrograde LC-II.
Surgical Protocol: The workflow involved preoperative 3D CT planning, optical tracking fixation to the anterior superior iliac spine, robotic arm positioning, and percutaneous guidewire insertion followed by cannulated screw placement.
Outcome Measures:
- Accuracy: Assessed via postoperative CT within 24 hours using the Smith grading system (Grade 0 = accurate; Grade I–III = increasing degrees of cortical breach).
- Functional Outcomes: Evaluated at 6 months using the Majeed Pelvic Score.
- Statistical Analysis: Univariate analysis (Chi-squared or Fisher's exact test) was performed to identify associations between screw malposition and variables including screw type, age, sex, BMI, and fracture type. Multivariate analysis was not performed due to the low number of malposition events (n=5).

Key Results

Overall Accuracy: The system achieved an overall accuracy rate (Smith Grade 0) of 98.6% (341/346 screws). All five malpositioned screws were Smith Grade I (cortical breach ≤2 mm); no Grade II or III breaches occurred, and no neurovascular injuries were reported.
Short vs. Trans-Cortical Screws: A statistically significant difference was found between screw types. Short screws demonstrated an accuracy of 98.9% (92/93), significantly higher than trans-cortical screws at 96.4% (54/56) ( $P=0.035$ ).
Influencing Factors: Univariate analysis identified two significant risk factors for malposition:
1. Screw Type: Trans-cortical screws carried a higher risk than short screws.
2. BMI: Patients with a BMI $\ge$ 30 kg/m² had a significantly higher risk of malposition ( $P=0.031$ ).
- Age, sex, and fracture type showed no significant association with malposition.
Functional Outcomes: At 6 months, the excellent-good rate for the short-screw group was 91.4% compared to 85.7% for the trans-cortical group ( $P=0.097$ ). The authors note this difference was not statistically significant, potentially due to sample size limitations (power analysis suggested a minimum detectable difference of ~10.2 percentage points).
Causes of Malposition: Analysis of the five Grade I errors revealed four primary causes: oblique bone surfaces at the entry point (40%), minor tracker displacement (20%), patient positional changes (20%), and soft-tissue interference in obese patients (20%).

Key Contributions
The paper claims three primary novel contributions:

Systematic Comparison: It provides the first large-sample systematic comparison of accuracy between through-and-through and short screws in robotic-assisted pelvic surgery.
Risk Factor Identification: It identifies a correlation between high BMI ( $\ge$ 30 kg/m²) and increased screw malposition risk, a factor not previously highlighted in this specific context.
Strategic Recommendations: Based on the data, the authors propose actionable strategies, including a "three-step strategy" for screw selection and placement (preoperative measurement, intraoperative pre-drilling/pilot holes, and immediate postoperative verification) and specific countermeasures for obese patients (e.g., longer sleeves, high-rigidity guidewires).

Significance and Claims
The study concludes that robot-assisted pelvic channel screw placement offers excellent overall accuracy (98.6%). The authors assert that short screws achieve significantly higher precision than trans-cortical screws and should be preferentially selected when biomechanical fixation strength requirements are met, as this reduces the risk of malposition.

The paper emphasizes that while robotic systems effectively translate preoperative 3D planning into execution, technical nuances—such as entry point geometry and soft-tissue interference in obese patients—remain critical variables. The authors maintain a modest tone regarding the functional outcome differences, acknowledging that the lack of statistical significance in Majeed scores may be due to insufficient sample size rather than a lack of clinical relevance. They call for larger, prospective multicenter studies to confirm these findings and validate the proposed optimization strategies.