Biomechanical comparison of multi-rod constructs by satellite rod configurations (in-line vs. lateral) and screw types (monoaxial vs. polyaxial) spanning a lumbar pedicle subtraction osteotomy (PSO): is there an optimal configuration?

Niloufar Shekouhi; Ardalan S Vosoughi; Joseph M Zavatsky; Vijay K Goel; Alekos A Theologis

doi:10.1007/s00586-022-07331-7

Biomechanical comparison of multi-rod constructs by satellite rod configurations (in-line vs. lateral) and screw types (monoaxial vs. polyaxial) spanning a lumbar pedicle subtraction osteotomy (PSO): is there an optimal configuration?

Eur Spine J. 2022 Nov;31(11):3050-3059. doi: 10.1007/s00586-022-07331-7. Epub 2022 Aug 6.

Authors

Niloufar Shekouhi¹, Ardalan S Vosoughi¹, Joseph M Zavatsky², Vijay K Goel¹, Alekos A Theologis³

Affiliations

¹ Engineering Center for Orthopedic Research Excellence (E-CORE), Departments of Bioengineering and Orthopaedic Surgery, University of Toledo, Toledo, OH, USA.
² Spine & Scoliosis Specialists, Tampa, FL, USA.
³ Department of Orthopedic Surgery, University of California - San Francisco (UCSF), 500 Parnassus Ave, MUW 3rd Floor, San Francisco, CA, 94143, USA. alekos.theologis@ucsf.edu.

PMID: 35932334
DOI: 10.1007/s00586-022-07331-7

Abstract

Purpose: Multi-rod constructs are used commonly to stabilize pedicle subtraction osteotomies (PSO). This study aimed to evaluate biomechanical properties of different satellite rod configurations and effects of screw-type spanning a PSO.

Methods: A validated 3D spinopelvic finite element model with a L3 PSO (30°) was used to evaluate 5 models: (1) Control (T10-pelvis + 2 rods); (2) lateral satellite rods connected via offsets to monoaxial screws (LatSat-Mono) or (3) polyaxial screws (LatSat-Poly); (4) in-line satellite rods connected to monoaxial screws (InSat-Mono) or (4) polyaxial screws (InSat-Poly). Global and PSO range of motions (ROM) were recorded. Rods' von Mises stresses and PSO forces were recorded and the percent differences from Control were calculated.

Results: All satellite rods (save InSat-Mono) increased PSO ROM and decreased primary rods' von Mises stresses at the PSO. Lateral rods increased PSO forces (LatSat-Mono:347.1 N; LatSat-Poly:348.6 N; Control:336 N) and had relatively lower stresses, while in-line rods decreased PSO forces (InSat-Mono:280.1 N; InSat-Poly:330.7 N) and had relatively higher stresses. Relative to polyaxial screws, monoaxial screws further decreased PSO ROM, increased satellite rods' stresses, and decreased PSO forces for in-line rods, but did not change PSO forces for lateral rods.

Conclusion: Multi-rod constructs using in-line and lateral satellite rods across a PSO reduced primary rods' stresses. Subtle differences in biomechanics suggest lateral satellite rods, irrespective of screw type, increase PSO forces and lower rod stresses compared to in-line satellite rods, which had a high degree of posterior instrumentation stress shielding and lower PSO forces. Clinical studies are warranted to determine if these findings influence clinical outcomes.

Keywords: Finite element analysis; Multi-rod construct; Pedicle subtraction osteotomy; Pseudarthrosis; Rod fracture.

Publication types

Research Support, U.S. Gov't, Non-P.H.S.
Research Support, Non-U.S. Gov't

MeSH terms

Biomechanical Phenomena
Bone Screws
Humans
Lumbar Vertebrae / surgery
Osteotomy
Pedicle Screws*
Range of Motion, Articular
Spinal Fusion*