Photo Credit: Alfred Senkumar
In posterior C1/C2 fusion using lateral mass screws (LMS) or pedicle screws (PS), screw stability is predominantly determined by cortical contact length and bone volume.
Atlantoaxial instability (AAI) is common in older patients, particularly after low-energy trauma like falls, according to a study published online in the Journal of Orthopedic Surgery and Research. Posterior C1/C2 fusion using lateral mass screws (LMS) or pedicle screws (PS) is a standard treatment, but the choice of screw type and placement remains debated, wrote Leon-Gordian Leonhardt, MD, and colleagues. Their study examined the biomechanical durability of LMS and PS fixation in mono- and bicortical positions in relation to bone microstructure.
The study authors analyzed 28 C1 vertebrae from older donors (50% female, mean age 80.8 years) using high-resolution computed tomography to assess bone microstructure. They then tested mechanical stability by cyclic loading in a cranio-caudal direction. The study compared the impact of bone volume and cortical contact length on screw fixation.
Results showed that PS trajectories had greater bone volume and cortical contact length than LMS, contributing to higher stability. PS required more force and a greater number of cycles before loosening, making them mechanically superior to LMS. Additionally, Dr. Leonhardt and colleagues found that differences between mono- and bicortical placement were minimal except in initial stiffness. Cortical contact length and bone volume were the most critical factors influencing screw stability.
The authors note that these findings align with previous research showing that PS provides stiffer fixation than LMS, particularly under cranio-caudal loading. While some literature questions the necessity of bicortical positioning due to neurovascular risks, this study suggests that bicortical positioning may offer advantages for older patients with compromised bone quality.
“Whether to use PS or LMS in mono- or bicortical position needs however to be allocated on an individual basis as anatomical conditions or high injury risks for surrounding soft tissue might rule out any other treatment,” they noted.
The study provides valuable clinical insights. Since PS engages more trabecular and cortical bone, they are preferred for achieving stable fixation. Bicortical positioning can enhance initial stiffness but does not significantly alter long-term mechanical properties. Ultimately, maximizing bone volume engagement and cortical contact should be prioritized to prevent screw loosening.
“Overall, fixation with PS appears to be biomechanically superior to fixation with LMS,” Dr. Leonhardt and colleagues concluded. “Even though an advantage of bicortical positioning over monocortical positioning could only be shown regarding the initial stiffness of the PS, our data suggest that bicortical anchorage could be recommended in particular in older patients.”
“Traversed bone volume and the cortical contact length of screws should be maximized in clinical use to increase screw stability, as they correlate strongly with their biomechanical load-bearing capacity.”
