A Computational Biomechanical Investigation of Posterior Dynamic Instrumentation: Combination of Dynamic Rod and Hinged (Dynamic) Screw

Abstract

Currently, rigid fixation systems are the gold standard for degenerative disk disease treatment. Dynamic fixation systems have been proposed as alternatives for the treatment of a variety of spinal disorders. These systems address the main drawbacks of traditional rigid fixation systems, such as adjacent segment degeneration and instrumentation failure. Pedicle-screw-based dynamic stabilization (PDS) is one type of these alternative systems. The aim of this study was to simulate the biomechanical effect of a novel posterior dynamic stabilization system, which is comprised of dynamic (hinged) screws interconnected with a coiled, spring-based dynamic rod (DSDR), and compare it to semirigid (DSRR and RSRR) and rigid stabilization (RSRR) systems. A validated finite element (FE) model of L1-S1 was used to quantify the biomechanical parameters of the spine, such as range of motion, intradiskal pressure, stresses and facet loads after single-level instrumentation with different posterior stabilization systems. The results obtained from in vitro experimental intact and instrumented spines were used to validate the FE model, and the validated model was then used to compare the biomechanical effects of different fixation and stabilization constructs with intact under a hybrid loading protocol. The segmental motion at L4-L5 increased by 9.5% and 16.3% in flexion and left rotation, respectively, in DSDR with respect to the intact spine, whereas it was reduced by 6.4% and 10.9% in extension and left-bending loads, respectively. After instrumentation-induced intradiskal pressure at adjacent segments, L3-L4 and L5-S1 became less than the intact in dynamic rod constructs (DSDR and RSDR) except in the RSDR model in extension where the motion was higher than intact by 9.7% at L3-L4 and 11.3% at L5-S1. The facet loads were insignificant, not exceeding 12N in any of the instrumented cases in flexion. In extension, the facet load in DSDR case was similar to that in intact spine. The dynamic rod constructions (DSDR and RSDR) led to a lesser peak stress at screws compared with rigid rod constructions (DSRR and RSRR) in all loading cases. A dynamic construct consisting of a dynamic rod and a dynamic screw did protect the adjacent level from excessive motion.

Lumbar Single-Level Dynamic Stabilization with Semi-Rigid and Full Dynamic Systems: A Retrospective Clinical and Radiological Analysis of 71 Patients

“…The most comparative results to a normal functional segment have been obtained by a full dynamic system in a cadaveric destabilized vertebra study wherein breaking and loosening of screws was shown to be less common because of reduced loading stress. 10 16 17) …”

A rehabilitation protocol for patients with lumbar degenerative disk disease

“…Biomechanical studies have shown that PTDS stabilizes the spine almost as strongly as rigid instrumentation systems (10,39). Oktenoglu et al reported a unique biomechanical study and found that a dynamic system (dynamic rod and dynamic screws) restores the instable spinal segments to almost normal ROM (15,29). There are recent studies on PTDS with encouraging clinical results showing that PTDS provides stabilization similar to posterior rigid stabilization with fusion surgery (10, 22,23,30,39).…”

Comparison of clinical and radiographic results between isobar posterior dynamic stabilization and posterior lumbar inter-body fusion for lumbar degenerative disease: A four-year retrospective study

“…Pedicle based dynamic rod devices consist of rigid/dynamic screws inter-connected with flexible or semiflexible rods and aim to preserve intact motion and balancing of load sharing between the anterior and posterior columns at the index level [14]. Several Pedicle based dynamic rod devices such as Graf ligament, Dynesys, AccuFlex rod and Medtronic PEEK rod have been widely studied since 1990s [15].…”