Pedicle Screws Provide Superior Fatigue Resistance compared with lateral mass Screws in the subaxial cervical spine: A Paired Cadaveric Study
In a paired cadaveric study utilizing progressive cyclic cranio-caudal toggle fatigue testing, cervical pedicle screws demonstrated significantly superior fatigue resistance, maximum load capacity, and stiffness compared to lateral mass screws in the subaxial cervical spine.
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 human spine is a marvel of engineering, a flexible column that supports the weight of the head while allowing us to look around, nod, and tilt. When disease or injury damages the lower part of the neck, surgeons often need to stabilize the area using metal screws and rods. To hold these screws in place, they must grip the bone tightly enough to withstand the constant, subtle movements of daily life. For decades, the standard approach has been to anchor screws into the bony blocks on the side of the vertebrae, known as the lateral mass. However, these screws only engage the outer shell of the bone. An alternative method involves driving screws through the narrow, bony bridge of the pedicle and deep into the main body of the vertebra. This path offers a longer grip and contact with stronger inner bone, but it is technically more difficult to perform. The critical question for surgeons is not just whether the screw fits, but whether it will hold up over time as the patient moves, or if it will slowly loosen and fail under the repetitive stress of living.
A team of researchers at the University Medical Center in Göttingen set out to answer this question by testing the two methods side-by-side. They did not rely on computer models or simple pulling tests, which measure how hard it takes to yank a screw straight out. Instead, they wanted to see how the screws held up against the kind of repetitive, rocking motion that happens inside a living body. They took ten vertebrae from human donors, ranging from the third to the seventh level of the neck, and prepared them for testing. In a clever design, each vertebra served as its own control: on one side, they inserted a screw into the lateral mass, and on the opposite side, they placed a pedicle screw. This ensured that any differences in the results were due to the screw type and not variations in bone quality between different people.
The researchers then mounted these vertebrae in a machine that could apply a rhythmic, up-and-down rocking force to the head of each screw. They started with a very light load and increased the force slightly with every single cycle, mimicking the gradual accumulation of stress that leads to loosening. The test continued until the screw moved five millimeters from its original position, a point the researchers defined as failure. This method allowed them to measure exactly how much force the screw could withstand and how many cycles of movement it survived before giving way.
The results were clear and consistent. The pedicle screws proved to be significantly stronger than the lateral mass screws in every single case. On average, the pedicle screws withstood a maximum force of 252.4 newtons before failing, whereas the lateral mass screws gave out at 166.4 newtons. In terms of endurance, the pedicle screws survived an average of 2,573 cycles of rocking before reaching the failure point, while the lateral mass screws lasted only 1,636 cycles. This means the pedicle screws endured roughly 57 percent more movement before loosening. Furthermore, the pedicle screws remained stiffer throughout the test, resisting the wobble better than their counterparts. In every single pair of vertebrae tested, the pedicle screw outperformed the lateral mass screw.
The study also looked at how the density of the bone affected the results. While the researchers found that the advantage of the pedicle screw was most noticeable in bones with lower density, they cautioned that this observation was based on a small number of samples and requires further investigation. The primary takeaway remains the mechanical superiority of the pedicle screw under these specific testing conditions. The authors note that while this method offers a clear mechanical advantage, it is not without risks. The path through the pedicle is narrow and close to vital nerves and blood vessels, requiring precise surgical skill and careful planning. Therefore, the findings suggest that when a surgeon can safely place a screw through the pedicle, doing so may provide a more durable fixation for patients with unstable neck spines, offering a stronger anchor against the relentless motion of the body.
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