Biomechanical characterization of the three-dimensional kinematic behaviour of the Dynesys dynamic stabilization system: an in vitro study

Abstract

The Dynesys, a flexible posterior stabilization system that provides an alternative to fusion, is designed to preserve intersegmental kinematics and alleviate loading at the facet joints. Recent biomechanical evidence suggests that the overall range of motion (ROM) with the Dynesys is less than the intact spine. The purpose of this investigation was to conduct a comprehensive characterization of the three-dimensional kinematic behaviour of the Dynesys and determine if the length of the Dynesys polymer spacer contributes to differences in the kinematic behaviour at the implanted level. Ten cadaveric lumbar spine segments (L2-L5) were tested by applying a pure moment of +/-7.5 Nm in flexion-extension, lateral bending, and axial rotation, with and without a follower preload of 600 N. Test conditions included: (a) intact; (b) injury; (c) injury stabilized with Dynesys at L3-L4 (standard spacer); (d) long spacer (+2 mm); and (e) short spacer (-2 mm). Intervertebral rotations were measured using an optoelectronic camera system. The intersegmental range of motion (ROM), neutral zone (NZ), and three-dimensional helical axis of motion (HAM) were calculated. Statistical significance of changes in ROM, NZ, and HAM was determined using repeated measures analysis of variance (ANOVA) and Student-Newman-Keuls post-hoc analysis with P<0.05. Implantation of the standard length Dynesys significantly reduced ROM compared to the intact and injured specimens, with the least significant changes seen in axial rotation. Injury typically increased the NZ, but implantation of the Dynesys restored the NZ to a magnitude less that that of the intact spine. The Dynesys produced a significant posterior shift in the HAM in flexion-extension and axial rotation. The spacer length had a significant effect on ROM with the long spacer resulting in the largest ROM in all loading directions without a follower preload. The largest differences were in axial rotation. A 4 mm increase in spacer length led to an average intersegmental motion increase of 30% in axial rotation, 23% in extension, 14% in flexion, and 11% in lateral bending. There were no significant changes in NZ with different spacer lengths. Typically, the short spacer caused a greater shift and a greater change in orientation of the HAM than the long spacer. The long spacer resulted in a ROM and a motion pattern, as represented by the HAM, that was closer to that seen in an intact specimen. The results of this study suggest that the length of the Dynesys spacer altered the segmental position and therefore affected kinematic behaviour.

Non-fusion instrumentation of the lumbar spine with a hinged pedicle screw rod system: an in vitro experiment

“…The intact ROM of the tested motion segments in the present study in all motion planes was comparable to the range of values reported in the literature for testing of specimens with pure bending moments [4,15,18,22,24,25].…”

“…Biomechanical studies have shown, that interspinous devices have a stabilising effect on decompressed segments in extension, but are hardly capable of stabilising a decompressed segment in axial rotation [7,13,23,26,29]. Laboratory in vitro studies of various posterior devices also mainly show a stabilising effect in flexion/extension and lateral bending and only a limited stabilising effect in axial rotation [3,18,20,[24][25][26]32].…”

Biomechanical evaluation of posterior lumbar dynamic stabilization: an in vitro comparison between Universal Clamp and Wallis systems

“…Although positive short-term results were obtained in carefully selected young patients, long-term follow-up results were not encouraging [20,21]. The Dynesys (Zimmer, Winterthur, Switzerland) is a popular device using titanium alloy pedicle screws and polycarbonate urethane spacers that surround tensioned polyethylene terephtalate (PET) cords on each side in order to reduce facet joint loading [22]. Recent studies have reported significant drawbacks and implant-related complications in pedicle screw-based instrumentation [23].…”

Effect of the cord pretension of the Dynesys dynamic stabilisation system on the biomechanics of the lumbar spine: a finite element analysis

“…However, the implant forces strongly depend on the stiffness of the Dynesys. Niosi et al [13,14] investigated the effects of the spacer length of the Dynesys. The results of this study indicated that a Dynesys with a long spacer typically caused an increase in range of motion (ROM) and a decrease in facet loads compared with those with a short spacer.…”