General Session: Adult Spinal Deformity - Hall F

Presented by: A.S. Vosoughi

Author(s):

A. Seyed Vosoughi(1), A. Kiapour(1), A. Agarwal(1), V. Goel(1), J. Zavatsky(2)

(1) University of Toledo, Bioengineering, Toledo, OH, United States
(2) Spine & Scoliosis Specialists, Tampa, FL, United States

Abstract

Introduction: One major complication of pedicle subtraction osteotomy (PSO) is rod fracture, usually at the level of the PSO, with reported rates as high as 31.6%. The addition of satellite rods has been shown to decrease the rate of rod fracture. We evaluated different multi-rod instrumentation techniques, in a PSO model, using finite element modeling.

Methods: A validated T10-pelvis model was used to develop a 30° PSO at L3. In addition to standard rod instrumentation from T10 to pelvis, various satellite rod configurations including medially, laterally, and posteriorly affixed accessory rods, along with the short rod technique described by Gupta et al were simulated. In medially and laterally affixed multi-rod constructs, satellite rods were connected to the primary rods above the L2 and below the L4 pedicle screws. In the posteriorly affixed configuration, satellite rods were affixed to the L1 and L5 screws. In the short rod technique, two additional recessed short rods spanned the L3 PSO level from L2 to L4. Loads for the FE models were applied in all degrees of freedom.

Results: Adding 2 satellite rods medially, decreased flexion, extension, and axial rotation by 15%, 16%, and 8%, respectively. Lateral satellite rods decreased flexion, extension, and axial rotation by 11%, 12%, and 6%, respectively. Adding accessory rods posteriorly, reduced flexion and extension by 36%, lateral bending by 17%, and axial rotation by 10%. Utilizing the short-rod technique resulted in 11%, 4%, and 49% reduction in flexion, extension, and lateral bending motions, respectively, while the axial rotation motion increased by about 31%. Table 1 shows the magnitudes and locations of the maximum von Mises stress recorded on the rods. Adding medial, lateral, and posterior satellite rods reduced the force acting across the osteotomy site by 16%, 11%, and 37%, respectively, while the short-rod technique resulted in similar forces.

Discussion: Satellite rods increase the moment of inertia, which result in lower stress on the standard rods at the PSO region. The location of the maximum von Mises stress on the rods occurred adjacent to the domino connectors when used. Greater maximum von Mises stresses occurred on the medially affixed satellite rods at the PSO region, while in the laterally affixed multi-rod construct, greater stress was located on the primary rods. These data suggest a benefit in supplementing medial vs. lateral satellite rods at the PSO, so that if rod failure occurs, it should occur on the medially placed satellite rods, which are more easily replaced vs. primary rods. The short-rod technique, obviates the need for acute rod contouring at the PSO, leading to a reduction in the von Mises stresses. Additionally, except for the short-rod technique, all other multi-rod constructs decrease the magnitude of load acting on the osteotomy site, which may cause a delayed or non-union at the osteotomy site.

Location and magnitudes of the maximum stress