In Vitro Biomechanical and Fluoroscopic Study of a Continuously Expandable Interbody Spacer Concerning Its Role in Insertion Force and Segmental Kinematics

Abstract

Study DesignIn vitro cadaveric study.PurposeTo compare biomechanical performance, trial and implant insertion, and disc distraction during implant placement, when two interbody devices, an in situ continuously expandable spacer (CES) and a traditional static spacer (SS), were used for transforaminal lumbar interbody fusion.Overview of LiteratureSevere degenerative disc diseases necessitate surgical management via large spacers to increase the disc space for implants. Next-generation interbody devices that expand in situ minimize insertion forces, optimize fit between vertebral endplates, and limit nerve root retraction. However, the literature lacks characterization of insertion forces as well as details on other parameters of expandable and static spacers.MethodsTen cadaveric segments (L5–S1) were divided into two groups (n=5) and implanted with either CES or SS. Each specimen experienced unconstrained pure moment of ±6 Nm in flexion–extension, lateral bending, and axial rotation to assess the contribution of CES and SS implants in biomechanical performance. Radiographic analysis was performed during trial and implant insertion to measure distraction during spacer insertion at the posterior, central, and anterior disc regions. Pressure sensors measured the force of trial and implant insertion.ResultsBiomechanical analysis showed no significant differences between CES and SS in all planes of motion. A total 2.6±0.9 strikes were required for expandable spacer trials insertion and 2.6±0.5 strikes for CES insertion. A total of 8.4±3.8 strikes were required to insert SS trials and 4.2±1.6 strikes for SS insertion. The total force per surgery was 330 N for CES and 635 N for SS. Fluoroscopic analysis revealed a significant reduction in distraction during implant insertion at the posterior and anterior disc regions (CES, 0.58 and 0.14 mm; SS, 1.04 and 0.78 mm, respectively).ConclusionsResults from the three study arms reveal the potential use of expandable spacers. During implant insertion, CESs provided similar stability, required less insertion force, and significantly reduced over-distraction of the annulus compared with SS.

Comparative Effectiveness of Expandable Versus Static Interbody Spacers via MIS LLIF: A 2-Year Radiographic and Clinical Outcomes Study

“…This results in indirect decompression of the neural elements. 10,11 Traditionally, static spacers have been utilized with this technique. However, static interbody spacers require excessive trialing, aggressive endplate preparation, and forceful impaction, which may lead to endplate damage and consequently, spacer subsidence.…”

“…However, static interbody spacers require excessive trialing, aggressive endplate preparation, and forceful impaction, which may lead to endplate damage and consequently, spacer subsidence. 11 Expandable technology was designed to expand vertically within the disc space, refuting the need for forceful impaction and minimizing iatrogenic overdistraction during insertion. 11 Additionally, there may be better disc height restoration and sagittal alignment correction compared with static spacers; however, studies comparing static spacers with expandable spacers are lacking.…”

“… 11 Expandable technology was designed to expand vertically within the disc space, refuting the need for forceful impaction and minimizing iatrogenic overdistraction during insertion. 11 Additionally, there may be better disc height restoration and sagittal alignment correction compared with static spacers; however, studies comparing static spacers with expandable spacers are lacking. 12 - 17 The importance of restoring radiographic parameters for improved functional outcomes has been well established.…”

Single-Level In Vitro Kinematic Comparison of Novel Inline Cervical Interbody Devices With Intervertebral Screw, Anchor, or Blade

“…Each specimen was thawed overnight and was affixed to a custom 6-degrees-of-freedom (6DOF) motion simulator for range of motion (ROM) testing, as described in the literature, 20 and modeled after the spinal loading simulator proposed by Wilke et al 21 The cranial (C2) and caudal (C7) portion of the specimen were affixed to a 6DOF motor gimbal assembly, which applied a pure, unconstrained rotational moment independently about the x -, y -, and z -axis corresponding to flexion-extension (FE), lateral bending (LB), and axial rotation (AR). The gimbal assembly is attached to the test platform, which includes linear air-bearing guide rails ( x - and z -axis) and pneumatic-controlled linear actuator ( y -axis) enabling pure, unconstrained translation.…”

Comparative Effectiveness of Laterally Placed Expandable versus Static Interbody Spacers: A 1-Year Follow-Up Radiographic and Clinical Outcomes Study

“…Kim et al [ 21 ] demonstrated in their retrospective study of 50 patients that the use of an uninterrupted, continuously expandable interbody device showed promising clinical outcomes and restoration of intervertebral DH. The expandable spacer has the advantage of being inserted in a collapsed state and continuously expanding the device in situ [ 22 - 24 ]. Advancements in expandable technology allow for the interbody cages to expand in height and lordosis in a controlled fashion.…”