Biomechanical study of rod stress after pedicle subtraction osteotomy versus anterior column reconstruction: A finite element study

Surg Neurol Int. 2017 Sep 6:8:207. doi: 10.4103/sni.sni_44_17. eCollection 2017.

Abstract

Background: In an effort to minimize rod fractures and nonunion in pedicle subtraction osteotomy (PSO) constructs, surgeons have adopted multirod constructs and interbody cages. Anterior column realignment (ACR) with posterior column osteotomies is a minimally invasive alternative to PSO in sagittal balance correction, however, there is a paucity of evidence with respect to rod survival.

Methods: Three-dimensional (3D) finite-element-model of a T12-sacrum spine segment was used to compare a 25° PSO at L3 and an ACR with a posterior column osteotomy and 30° hyperlordotic interbody cage at L3-4. The amount of overall T12-S1 lordosis correction was the same for each condition. Each simulation included cobalt chromium alloy primary rods with: (1) PSO; (2) PSO with an interbody cage (IB) at L2-3 (PSO+IB); (3) PSO with accessory (A) rods and IB at L2-3 (PSO+IB+A); (4) PSO with satellite (S) rods and IB at L2-3 (PSO+IB+2S); (5) ACR; 6) ACR with satellite rods (ACR + 2S). A 400 N follower preload was simulated for each condition.

Results: PSO condition had the largest rod stress of 286 MPa in flexion. Adding interbody support reduced the rod stress by 15%. The 4-rod constructs further reduced rod stress, with the satellite rod condition facilitating the largest reduction. The rod stress in the ACR+2S was equivalent to the PSO+2S, with 50% reduction in rod stress.

Conclusion: The rod stress with an ACR was comparable to a PSO coupled with interbody support. These results suggest an ACR is a viable MIS alternative to a PSO without the need for a large posterior osteotomy.

Keywords: Anterior column realignment; MIS; finite element analysis; pedicle subtraction osteotomy; rod fracture.