Internal anterior fixators for pelvic ring injuries: Do monaxial pedicle screws provide more stiffness than polyaxial pedicle screws?

Objectives: Little is known about the mechanical properties of internal anterior fixators (known as INFIX), which have been proposed as subcutaneous alternatives to traditional anterior external fixators for pelvic ring disruptions. We hypothesised that INFIX has superior biomechanical performance compared with traditional external fixators because the distance from the bar to the bone is reduced.

Methods: Using a commercially available synthetic bone model, 15 unstable pelvic ring injuries were simulated by excising the pubic bone through the bilateral superior and inferior rami anteriorly and the sacrum through the bilateral sacral foramen posteriorly. Three test groups were established: (1) traditional supra-acetabular external fixation, (2) INFIX with polyaxial screws, (3) INFIX with monaxial screws. Load was applied, simulating lateral compression force. Outcome measure was construct stiffness.

Results: The traditional external fixator constructs had an average stiffness of 6.21 N/mm ± 0.40 standard deviation (SD). INFIX with monaxial screws was 23% stiffer than the traditional external fixator (mean stiffness, 7.66 N/mm ± 0.86 SD; p = .01). INFIX with polyaxial screws was 26% less stiff than INFIX with monaxial screws (mean stiffness, 5.69 N/mm ± 1.24 SD; p = .05). No significant difference was noted between polyaxial INFIX and external fixators (mean stiffness, 6.21 N/mm ± 0.40 SD; p=.65).

Conclusions: The performance of INFIX depends on the type of screw used, with monaxial screws providing significantly more stiffness than polyaxial screws. Despite the mechanical advantage of being closer to the bone, polyaxial INFIX was not stiffer than traditional external fixation.