# A novel Z-shaped anti-rotation rod for atlantoaxial dislocation reduction: finite element analysis

## TL;DR

A new Z-shaped rod design improves atlantoaxial dislocation reduction compared to traditional methods, potentially reducing surgery risks.

## Contribution

A novel Z-shaped anti-rotation rod is proposed and evaluated for enhanced reduction force in atlantoaxial dislocation treatment.

## Key findings

- The PS-ZR system provides 1.67 to 3.68 times higher reduction force than the PS-CR system.
- Both configurations achieve over 98% reduction in C1-C2 segmental range of motion.
- The PS-ZR system experiences higher stress, particularly at the Z-shaped transition area.

## Abstract

C1-C2 pedicle screw-rod fixation (PSR) is widely used for atlantoaxial dislocations. However, its limited reduction capacity in refractory cases necessitates additional release surgeries, increasing operative risks including prolonged surgical time and expanded tissue damage. We developed a novel Z-shaped anti-rotation rod to improve reduction capability, but its biomechanical performance requires evaluation.

A nonlinear atlantoaxial instability three-dimensional (3D) C0-C3 finite element model was constructed using computed tomography images from a 25-year-old healthy male without a history of cervical spine diseases. Based on this model, two C1-C2 fixation configurations were simulated: conventional pedicle screw-rod (PS-CR) and pedicle screw-Z-shaped rod (PS-ZR). Reduction forces were measured and compared. Range of motion (ROM), stress distribution and peak stress values of the implants were recorded and compared under six loading conditions including flexion, extension, lateral bending, and axial rotation.

Both configurations achieved a greater than 98% reduction in the C1-C2 segmental ROM, with similar compensatory motions in adjacent segments. The reduction force of PS-ZR showed significant advantages (2–8 mm range), achieving a maximum reduction force of 88.544 N, which is 1.67 to 3.68 times that of PS-CR. The PS-ZR system experiences greater stress compared to the PS-CR system, escalating with Z-rod height. Regarding stress distribution and peak values of rods, the maximum stress on the PS-CR system was mainly concentrated at the connection between the rod and the screw nut while the maximum stress on the PS-ZR system was concentrated at the transition part of the “Z” shape.

Both PS-CR and PS-ZR configurations provide reliable and comparable stability. Compared to the PS-CR configuration, the PS-ZR configuration provides superior reduction force and stability, potentially reducing the need for additional release surgery and surgical time. This novel design has significant clinical implications for improving fixation techniques.