Introduction. Pedicle subtraction osteotomies (PSO) are powerful techniques to correct sagittal and coronal malalignment. As PSOs are associated with high rates of nonunion and rod fracture, multi-rod constructs  (4-, 5-, 6-rods) are commonly used to stabilize PSOs. Multi-rod configurations can be created using “satellite” rods (not connected to primary rod) and/or "accessory" rods (connected to primary rod). This study aimed to assess biomechanics of laterally-based satellite rods, with and without additional accessory rods.

Methods. A previously validated 3D spinopelvic finite element model (T10-pelvis) with a 30° PSO at L3 was used (1). In the control model (2-Rods), two bilateral rods connected all levels (T10-pelvis) (Fig. 1). Three multi-rod techniques were modeled and analyzed: (1) laterally-based satellite rods with no accessory rods (4-Rods), (2) laterally-based satellite rods with one medial accessory rod (5-Rods), and (3) laterally-based satellite rods with two accessory rods (6-Rod) (Fig. 1). Global and PSO range of motion (ROM) were recorded. Rods’ von Mises stresses and PSO forces were recorded and the percent differences from Control were calculated.

Results. Laterally-based satellite rods decreased global ROM in flexion-extension in 4-Rod (8.6%-39.4%), 5-Rod (15.5%-43.7%), and 6-Rod constructs (22%-47.7%). Increasing rods from four to six led to reduction in global ROM in all motions. Compared to 2-Rods, laterally-based satellite rods increased PSO flexion and extension ROM by 18.8% and 211.6%, respectively.  Lower ROMs at the osteotomy site were observed for 5- and 6-Rods compared to 4-Rods. Laterally-based satellite rods (4-Rods) showed higher PSO force than 2-Rods (347.1N vs. 336N). However, additional accessory rods reduced the PSO force further to 327.8N (5-Rods) and 309.7N (6-Rods). All multi-rod models decreased von-Mises stresses on the primary rods at the PSO site. 5- and 6-Rods led to lower von Mises stresses in these areas. In 4-Rods, two critical stress locations were observed: adjacent to the PSO site and L5-S1. Adding the accessory rods (5- and 6-Rods) shifted the critical stress locations to connection points between primary rods and W-connectors in some motions.

Discussion. Laterally-based satellite rods across a lumbar PSO reduced stresses on primary rods. Adding accessory rods increased the construct’s rigidity and led to a lower global and PSO ROM. Moreover, 5- and 6-Rods resulted in forces on the posterior instrumentation being distributed across more components, which decreased von-Mises stresses at the PSO site compared to 4-Rod and 2-Rods. Although increased rigidity afforded by 5- and 6-Rods may be beneficial in reducing chances of rod breakage, it should be highlighted that due to higher stiffness, load may be transferred posteriorly and less load would be carried by the anterior vertebral column, which may not be most favorable for healing of the anterior column.

Fig. 1