The biomechanical fundamentals of crosslink-augmentation in posterior spinal instrumentation

Abstract

Posterior screw-rod constructs can be used to stabilize spinal segments; however, the stiffness is not absolute, and some motion can persist. While the effect of crosslink-augmentation has been evaluated in multiple studies, the fundamental explanation of their effectiveness has not been investigated. The aim of this study was to quantify the parameters “screw rotation” and “parallelogram deformation” in posterior instrumentations with and without crosslinks to analyze and explain their fundamental effect. Biomechanical testing of 15 posteriorly instrumented human spinal segments (Th10/11—L4/L5) was conducted in axial rotation, lateral bending, and flexion–extension with ± 7.5 Nm. Screw rotation and parallelogram deformation were compared for both configurations. Parallelogram deformation occurred predominantly during axial rotation (2.6°) and was reduced by 60% (−1.45°, p = 0.02) by the addition of a crosslink. Simultaneously, screw rotation (0.56°) was reduced by 48% (−0.27°, p = 0.02) in this loading condition. During lateral bending, 0.38° of parallelogram deformation and 1.44° of screw rotation was measured and no significant reduction was achieved by crosslink-augmentation (8%, −0.03°, −p = 0.3 and −13%, −0.19°, p = 0.7 respectively). During flexion–extension, parallelogram deformation was 0.4° and screw rotation was 0.39° and crosslink-augmentation had no significant effect on these values (−0.12°, −30%, p = 0.5 and −0°, −0%, p = 0.8 respectively). In axial rotation, crosslink-augmentation can reduce parallelogram deformation and with that, screw rotation. In lateral bending and flexion–extension parallelogram deformation is minimal and crosslink-augmentation has no significant effect. Since the relatively large screw rotation in lateral bending is not caused by parallelogram deformation, crosslink-augmentation is no adequate countermeasure. The fundamental understanding of the biomechanical effect of crosslink-augmentation helps better understand its potential and limitations in increasing construct stiffness.

Cross-link augmentation enhances CFR-PEEK short fixation in lumbar metastasis stabilization

“…The radiolucency of CFR-PEEK permitted the positioning of the transverse rod just Frontiers in Bioengineering and Biotechnology frontiersin.org behind the lesioned vertebra. Previous characterizations of short fixations with metallic cross-links (Wahba et al, 2009;Cornaz et al, 2022) represented valid starting points, but could not be considered conclusive and rigorously transferred in spinal oncologic surgery due to the different materials used and the different conditions of the spine itself. Secondly, existing literature on CFR-PEEK instrumentation has focused so far on comparing their performance with metallic ones (i.e., titanium alloys) and agree on its promising responses; however, there is still a lack of systematic evaluation of different CFR-PEEK long and short stabilizations as has been largely done for metallic alloys in the ambit of thoracolumbar fractures (McLain, 2006;Aly, 2017;Li and Liu, 2017;de Andrada Pereira et al, 2021;Lai et al, 2022).…”

“…This new material could allow the use in oncologic surgery of transverse rods connecting the longitudinal rods, which is currently not recommended due to the radiopacity of metals. In spinal surgery, the transverse rod addition is commonly referred to as "cross-link augmentation" (Cornaz et al, 2022). In vitro studies have investigated the effects of metallic cross-link augmentation in long fixation for the treatments of fractures, by loading spinal segments along the three directions in space and by varying the number of crosslinks and their mutual position with respect to the rods (Lynn et al, 1997;Brodke et al, 2001;Cornaz et al, 2021).…”

Editorial: Innovations to improve screw fixation in traumatology and orthopedic surgery

“…The spine cycles several million times a year, and the loads applied to the instrumentation and its fixation are highly variable (Spirig et al, 2021). They also depend on the construct design (anchor density (Widmer et al, 2020), anchor rigidity (Cornaz et al, 2021;Cornaz et al, 2022) use or absence of anterior column support (Burkhard et al, 2021), which could influence the outcomes and affect the risks of mechanical complications such as fixation loosening, material breakage, and adding on problems such as proximal junctional kyphosis or proximal junctional failure. Optimal construct stiffness is unknown.…”