Elsevier

Journal of Biomechanics

Volume 45, Issue 4, 23 February 2012, Pages 706-710
Journal of Biomechanics

Technical note
Pure moment testing for spinal biomechanics applications: Fixed versus 3D floating ring cable-driven test designs

https://doi.org/10.1016/j.jbiomech.2011.12.018Get rights and content

Abstract

Pure moment testing has become a standard protocol for in vitro assessment of the effect of surgical techniques or devices on the bending rigidity of the spine. Of the methods used for pure moment testing, cable-driven set-ups are popular due to their low requirements and simple design. Fixed loading rings are traditionally used in conjunction with these cable-driven systems. However, the accuracy and validity of the loading conditions applied with fixed ring designs have raised some concern, and discrepancies have been found between intended and prescribed loading conditions for flexion–extension. This study extends this prior work to include lateral bending and axial torsion, and compares this fixed ring design with a novel “3D floating ring” design. A complete battery of multi-axial bending tests was conducted with both rings in multiple different configurations using an artificial lumbar spine. Applied moments were monitored and recorded by a multi-axial load cell at the base of the specimen. Results indicate that the fixed ring design deviates as much as 77% from intended moments and induces non-trivial shear forces (up to 18 N) when loaded to a non-destructive maximum of 4.5 Nm. The novel 3D floating ring design largely corrects the inherent errors in the fixed ring design by allowing additional directions of unconstrained motion and producing uniform loading conditions along the length of the specimen. In light of the results, it is suggested that the 3D floating ring set-up be used for future pure moment spine biomechanics applications using a cable-driven apparatus.

Introduction

Pure moment testing has emerged as a preferred method to compare flexibility of cadaveric spine specimens in all anatomic directions when testing the efficacy of medical implants and innovative surgical techniques. The advantages to pure moment testing lie in its consistency as an accepted standard protocol across previous literature (Wilke et al., 1998, Wilke et al., 2001) and its ability to ensure uniform loading across all levels of the spinal column. Among the set-ups currently used to apply pure bending moments (Beaubien et al., 2005, Crawford et al., 1995, DiAngelo et al., 2004, Goel et al., 1988, Kotani et al., 2006, Melcher et al., 2002, Panjabi et al., 2007, Puttlitz et al., 2004, Schwab et al., 2006, Stanley et al., 2004, Wilke et al., 1994), the cable-driven (Acosta et al., 2008, Barnes et al., 2009, Panjabi, 2007) is relatively common due to its minimal requirements and ease of use. This system involves a loading ring attached to one end of the spinal segment and connected to the test frame's actuator head via a cable. In our group's experience using the cable-driven system with an initial fixed ring design, it became apparent that preserving cable co-linearity during testing (Eguizabal et al., 2010) demanded significant manual adjustment and posed a challenge for maintaining consistency in applied loading conditions. Prior work by our group indicated a discrepancy between applied and intended moment for this system in flexion-extension only (Crawford et al., 1995). We hypothesize that this discrepancy can be observed in other bending modes and minimized with a second-generation floating ring design to eliminate off-axis loads.

This study evaluates the accuracy with which this specific cable-driven system generates a pure moment and compares two different test set-ups—the historically used fixed ring design and a novel 3D floating ring. We hypothesize that the floating ring design will maintain a pure moment closer to the intended moment than the fixed ring set-up.

Section snippets

Biomechanical testing set-up

The applied loading conditions for the two different cable-driven pure moment set-ups were evaluated using an artificial lumbar spine model constructed from wooden cylinders simulating the lumbar vertebrae and neoprene rubber pads for the intervertebral disks.

The spine was tested quasi-statically in flexion–extension, axial rotation, and lateral bending using an established pure moment testing protocol (Crawford and Dickman, 1997). Moments were applied to a non-destructive maximum of 4.5 Nm in

Intended versus applied moments

There was an average percent difference from intended moments across all groups of 77.0%±17.9% in flexion–extension, 5.7%±3.2% in axial rotation, and 65.0%±15.6% in lateral bending for the fixed ring set-up, and 4.4%±1.7% in flexion–extension, 5.7%±3.5% in axial rotation, and 9.2%±2.1% in lateral bending for the 3D floating ring set-up.

Configurations that included posterior spinal fixation (PSF) deviated from intended moments an average of 9.2% more in flexion–extension, 0.3% less in axial

Discussion

This is a follow-up to a previous study (Eguizabal et al., 2010) that addressed the shortcomings of the fixed ring technique with a first-generation sliding ring design that allowed the ring to float in the sagittal plane necessary for flexion–extension. This second-generation 3D floating ring design allows an additional floating direction in the frontal plane for lateral bending.

Results show the potential of the fixed ring cable-driven system (Panjabi et al., 1994, Tzermiadianos et al., 2008)

Conflict of interest statement

All authors acknowledge that they have nothing to disclose in terms of personal relationships with other individuals or organizations that could inappropriately influence their work including those brought forth by employment, consultancies, stock ownership, honoraria, paid expert testimony, patent applications/registrations, and grants.

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