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Experimental and clinical study of bone cement-reinforced pedicle screws for secondary screw placement in osteoporotic lumbar spine surgery

This study demonstrates that for secondary pedicle screw placement in osteoporotic lumbar spine surgery, a cement-augmented fenestrated screw technique with an optimal injection volume of 2.5 ml provides superior stability and clinical outcomes compared to conventional screw augmentation.

Original authors: Hao Li, Xiaofeng Chen, Weijun Guo, Zhuangxun Han, Xueyuan Chu, Qipeng Wei, Junxian Xie, Dongling Cai, Lixin Zhu

Published 2026-07-31

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CC BY 4.0

Original authors: Hao Li, Xiaofeng Chen, Weijun Guo, Zhuangxun Han, Xueyuan Chu, Qipeng Wei, Junxian Xie, Dongling Cai, Lixin Zhu

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

Imagine your spine is the main support beam of a skyscraper, holding up the heavy roof of your body. Sometimes, due to age or wear and tear, this beam gets a bit spongy and weak, a condition doctors call osteoporosis. To fix a broken or unstable section of this beam, surgeons often use a "scaffold" made of metal screws to hold everything together while the bone heals. But here's the tricky part: if the beam is too spongy, those metal screws can't get a good grip. They might wiggle loose, like a nail hammered into a piece of dry, crumbly bread. To solve this, surgeons inject a special liquid cement (a super-strong glue called PMMA) into the hole before putting the screw in. This turns the crumbly bread into a solid block of concrete, giving the screw something firm to hold onto.

But what happens if the surgeon tries to put a screw in, realizes the hole is too wobbly, pulls it out, and tries again? That's the "second try" scenario. Every time you pull a screw out of that spongy bone, you make the hole bigger and the surrounding area even weaker. It's like trying to fix a loose screw in a piece of wood that has already been stripped of its threads. The big question for doctors is: How much of that super-strong glue should they use the second time? Do they need more? How much is too much? And is there a special kind of screw that makes this whole process easier and safer? This is exactly the mystery a team of researchers set out to solve.

The Experiment: Testing the Glue in the Lab

To find the answers, the researchers started with a "dry run" using 30 real human spine bones from a laboratory. These bones were very weak, just like those of patients with severe osteoporosis. They divided the bones into two teams. The first team was the "First-Timers." The researchers drilled a hole, took the drill out, and injected different amounts of the glue—ranging from 1.0 ml to 3.0 ml—before screwing in a metal screw. The second team was the "Re-do Crew." For these bones, they drilled a hole, put a screw in, took it out, and then drilled a second time in the same spot (simulating a screw that failed and needed replacing). They also injected different amounts of glue for this group.

Then, they put the screws in a giant machine that pulled on them with increasing force until the screws ripped out of the bone. They measured exactly how much force it took to pull them out. The results were like a game of "Goldilocks." They found that using more glue generally made the screws hold tighter, but there was a point of diminishing returns. For the "First-Timers," injecting 2.0 ml of glue was just as strong as using 2.5 ml or 3.0 ml. However, for the "Re-do Crew," the story changed. Because the second hole was bigger and the bone more damaged, they needed a bit more glue to get the same strong hold. The sweet spot for the second attempt was 2.5 ml. Using less than that, and the screw was wobbly; using more didn't really help much more, but it increased the risk of the glue leaking out where it shouldn't.

The Real-World Test: Patients in the Operating Room

Next, the team moved from the lab to the hospital, looking at 78 real patients who had back surgery where one screw had to be re-inserted. They split these patients into two groups to see which tool worked better. The first group got "Conventional Pedicle Screws" (CPS). These are standard screws. If the glue needed to be added, the surgeon had to stop, remove the screw, and use a separate tube to inject the glue into the hole before putting the screw back in. It was a bit like trying to fill a cup with water while holding the cup upside down, then flipping it over.

The second group got "Fenestrated Pedicle Screws" (FPS). These are high-tech screws with tiny holes (fenestrations) built right into their sides. The surgeon could screw the whole thing in, and then inject the glue directly through the screw itself, letting it seep out the side holes to surround the screw like a protective shell. It was like having a sprinkler system built right into the screw.

The results showed that both groups got better over time, with less pain and better ability to move. However, the "Fenestrated" group had some clear advantages. Because the glue could flow out evenly through the side holes, it created a more uniform "concrete" shell around the screw. This meant the screws were less likely to loosen up later on. In fact, by the one-year mark, the group with the special hollow screws had a much lower rate of screws coming loose (about 12%) compared to the standard screw group (about 25.5%). While the difference wasn't huge enough to be a guaranteed rule for everyone, the trend was clear: the special screws held on tighter.

Also, the special screws made the surgery a bit smoother. The surgeons needed fewer X-rays to check their work because the process was simpler, and there was a slightly lower chance of the glue leaking into the wrong places (though this difference wasn't statistically huge, it was still a positive sign).

The Takeaway

So, what did this study teach us? First, if you have to put a screw in a weak bone for the second time, you need to be a bit more generous with the glue—about 2.5 ml instead of the 2.0 ml used for the first try. Second, if you are going to do a "re-do" screw placement, using those special hollow screws with side holes is a smarter choice. They act like a built-in delivery system, spreading the glue evenly to create a stronger grip and keeping the screws from wiggling loose as the patient heals. It's a small change in the tool kit that could make a big difference in keeping a patient's back stable and pain-free.

Technical Summary: Experimental and Clinical Study of Bone Cement-Reinforced Pedicle Screws for Secondary Screw Placement in Osteoporotic Lumbar Spine Surgery

Problem Statement

Lumbar degeneration requiring spinal fusion is frequently complicated by osteoporosis, which significantly increases the risk of pedicle screw loosening and fixation failure. While polymethylmethacrylate (PMMA) cement augmentation is a standard technique to enhance pull-out strength, clinical reality often necessitates secondary screw placement due to anatomical variations or initial placement failure. Repeated screw insertion causes additional cancellous bone loss and widens the screw tract, potentially altering the optimal cement dose and dispersion. Currently, there is a lack of data regarding the specific PMMA injection volume required for secondary screw placement compared to primary placement, and the comparative efficacy of conventional pedicle screws (CPS) versus fenestrated pedicle screws (FPS) in this specific context remains unclear.

Methodology

The study employed a two-part approach combining a basic science biomechanical experiment and a prospective clinical study.

1. Basic Research (Biomechanical Experiment)

Specimens: 30 fresh cadaveric lumbar vertebrae (L1-L5) with severe osteoporosis (BMD T-value < -2.5) were used, providing 60 pedicles.
Grouping: Pedicles were randomized into two main categories:
- Primary Implanting Group (Groups A-E): A single pilot hole was drilled. PMMA was injected in volumes of 1.0, 1.5, 2.0, 2.5, and 3.0 ml before screw insertion.
- Secondary Implanting Group (Groups F-J): A pilot hole was drilled, a screw was inserted and removed to simulate a failed placement, and a second screw was inserted along the same tract. PMMA was injected in the same five volume increments (1.0–3.0 ml).
Testing: After 24 hours of curing, specimens underwent axial pull-out testing using an MTS 858 mechanical test machine to determine the maximum axial pull-out strength ( $F_{max}$ ). Imaging (X-ray) assessed PMMA distribution.

2. Clinical Study

Population: 78 patients (aged >60) with severe osteoporosis undergoing single-segment L4-L5 posterior fusion who required intraoperative secondary screw placement for one pedicle.
Intervention: Patients were divided into two groups based on the screw type used for the reinforced fixation:
- CPS Group (n=43): Conventional Pedicle Screws. Cement was injected via a puncture tube into the tract after screw removal.
- FPS Group (n=35): Fenestrated Pedicle Screws. Cement was injected through the hollow channel and lateral foramina of the screw after insertion.
Protocol: Both groups received 2.5 ml of PMMA for the secondary screw (based on experimental findings). All patients received standardized anti-osteoporosis medication post-operatively.
Outcomes: Perioperative data (operative time, blood loss, fluoroscopy count, leakage), clinical scores (VAS for pain, ODI for disability), and radiological outcomes (screw loosening, intervertebral fusion) were recorded at 3 days, 3 months, 6 months, and 1 year.

Key Results

Biomechanical Findings

Dose-Response: A strong positive correlation was observed between PMMA volume and $F_{max}$ in both primary ( $r=0.885$ ) and secondary ( $r=0.942$ ) placement groups.
Optimal Volumes:
- Primary Placement: No significant difference in $F_{max}$ was found between 2.0 ml, 2.5 ml, and 3.0 ml. The study identifies 2.0 ml as the preferred volume.
- Secondary Placement: No significant difference was found between 2.5 ml and 3.0 ml. The study identifies 2.5 ml as the preferred volume, noting that secondary placement requires 0.5 ml more cement than primary placement to compensate for bone loss.
Comparison: At identical volumes, secondary placement groups generally exhibited lower $F_{max}$ than primary groups, though the difference was not statistically significant.

Clinical Findings

Operative Metrics: The FPS group required significantly fewer intraoperative fluoroscopic images ( $p < 0.05$ ) and fewer cement injections compared to the CPS group. There were no significant differences in operative time, blood loss, or hospital stay.
Safety: Cement leakage occurred in 7/36 CPS cases and 2/33 FPS cases (not statistically significant). No pulmonary embolisms were reported.
Clinical Outcomes: Both groups showed significant improvement in VAS and ODI scores over time. However, at 3 and 6 months post-operatively, the FPS group demonstrated significantly lower VAS (back and leg pain) and ODI scores compared to the CPS group ( $p < 0.05$ ). By 12 months, differences between groups were no longer statistically significant.
Radiological Outcomes:
- Screw Loosening: The FPS group consistently showed lower loosening rates than the CPS group at all follow-up points (12-month rates: 12.00% for FPS vs. 25.54% for CPS), though differences were not statistically significant.
- Fusion: Intervertebral fusion rates were higher in the FPS group at all time points, but differences were not statistically significant.

Key Contributions

Quantification of Secondary Placement Needs: The study establishes that secondary screw placement in osteoporotic bone requires an additional 0.5 ml of PMMA (2.5 ml vs. 2.0 ml) compared to primary placement to achieve optimal stability, addressing a gap in current literature.
Biomechanical Validation: It confirms a positive correlation between cement volume and pull-out strength specifically in the context of re-drilled tracts.
Clinical Efficacy of FPS: The study demonstrates that using fenestrated screws for secondary placement offers superior short-term clinical outcomes (pain and disability scores at 3–6 months) and a trend toward lower loosening rates compared to conventional screws with external cement injection.
Operational Efficiency: The FPS technique was shown to reduce intraoperative radiation exposure (fluoroscopy) due to the ability to inject cement after screw insertion without repositioning instruments.

Significance and Claims

The authors claim that this study provides a clinically feasible and biologically significant protocol for managing secondary screw placement in osteoporotic spines. The primary significance lies in defining the optimal PMMA injection volume (2.0 ml for primary, 2.5 ml for secondary) to maximize fixation stability while minimizing leakage risks.

Furthermore, the paper posits that the FPS technique is superior to the CPS technique for secondary placement. The hollow channel and lateral foramina of FPS allow for more uniform cement distribution around the screw-bone interface, creating a larger contact surface and stronger fixation. This results in reduced screw loosening, higher fusion rates, and better short-term patient-reported outcomes. The study concludes that while both methods are viable, the FPS technique simplifies the procedure and offers a safety profile with lower leakage risks, making it a preferred option for secondary screw reinforcement in osteoporotic patients.

Limitations Acknowledged by Authors:
The study notes limitations including a 12-month follow-up period (lacking long-term 2–5 year data), a single-segment focus, lack of stratification by gender or vertebral size, and the inability to correlate adjustment frequency with loosening rates due to the study design.