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