Short communicationPure moment testing for spinal biomechanics applications: Fixed versus sliding ring cable-driven test designs
Introduction
In vitro experimental assessment of spinal implant devices and surgical techniques is important to the development and design optimization of new clinical technologies. These studies typically compare spinal flexibility between intact and treated cadaveric specimens in multiple anatomic directions. Among the methods of acquiring such data, pure moment biomechanical conditions are preferred because they ensure uniform loading along the column of the spine easing the comparison of techniques and technologies with previous literature (Wilke et al., 2001; Wilke et al., 1998). A variety of systems are used to apply pure bending moments to the cadaveric spine, including 6-axis testing machines (Beaubien et al., 2005; Kotani et al., 2005; Wilke et al., 1994; DiAngelo et al., 2004; Schwab et al., 2006; Kotani et al., 2006; Panjabi et al., 2007a, Panjabi et al., 2007b; Panjabi, 2007; Crawford et al., 1995), suspended deadweights (Melcher et al., 2002; Puttlitz et al., 2004; Goel et al., 1988; Stanley et al., 2004), and cable-driven systems (Crawford et al., 1995; Acosta et al., 2008; Barnes et al., 2009). The cable-driven test set-up applies a couple via a continuous loop of cable that is wound around a “loading ring” attached to one end of the spinal segment. Since its development by Crawford in the mid-90s (Crawford et al., 1995), the cable-driven pure moment method has been widely adopted due to its simplicity and relatively low infrastructure requirements (it requires only a uni-axial testing frame). Recently, it has been called into question whether the cable driven pure moment test design is capable of generating a pure moment loading state (Panjabi, 2007).
The goal of this study was to determine whether the cable-driven pure moment apparatus generates accurate, consistent loading conditions for a range of possible multi-axial spinal testing scenarios. We considered differences in applied loads generated by the “fixed loading ring” design, which has been used historically for cable-driven pure moment testing(Crawford et al., 1995; Acosta et al., 2008; Barnes et al., 2009), and a modified “sliding loading ring” system that was developed by our group to address the limitations of the fixed ring design.
Section snippets
Biomechanical testing set-up
The applied loading conditions for two different cable-driven pure moment set-ups were evaluated using an artificial lumbar spine model. The 5-segment spine model was constructed from common laboratory materials. Briefly, this model consisted of wooden cylinders simulating the lumbar vertebrae (50 mm length×50 mm diameter) and neoprene rubber pads for the intervertebral discs (Shore durometer hardness 30, 15 mm height).
The artificial spine was tested quasi-statically in flexion and extension using
Results
For the fixed ring set-up, FE moments were 52–59% less than intended values (Table 1), and anterior–posterior shear forces reached 10.3 N. FE ROM decreased cranially-to-caudally for the 5 level specimen from 2.6 degrees at the L1/L2 to 0.4 degrees at the L4/L5 motion segment. Shorter spinal sections (i.e. 4, 3, and 2 levels) demonstrated resultant loads that were opposite in direction from the intended loading, i.e., extension moments while flexing forward, and had relatively higher
Discussion
The results of this study indicate that the standard fixed ring cable-driven pure moment system (Crawford et al., 1995) has the potential to deviate from a pure moment loading state and that our novel sliding ring modification corrects this error in the original test design. In this study, fixed ring system moments were 50–60% less than the intended values. This design also induced non-trivial shear forces, and non-uniform loading conditions were induced along the length of the specimen, as was
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.
Acknowledgements
Industrial Mechanical (Vacaville, CA) for manufacturing the sliding ring apparatus, and the UCSF/SFGH Biomechanical Testing Facility for funding support.
References (20)
- et al.
Moment-rotation relationships of the ligamentous occipito-atlanto-axial complex
Journal of Biomechanics
(1988) Hybrid multidirectional test method to evaluate spinal adjacent-level effects
Clinical Biomechanics (Bristol, Avon)
(2007)- et al.
Biomechanical comparison of three fixation techniques for unstable thoracolumbar burst fractures. Laboratory investigation
Journal of Neurosurgery: Spine
(2008) - et al.
Biomechanical pullout strength and stability of the cervical artificial pedicle screw
Spine (Phila Pa 1976)
(2009) - et al.
In vitro, biomechanical comparison of an anterior lumbar interbody fusion with an anteriorly placed, low-profile lumbar plate and posteriorly placed pedicle screws or translaminar screws
Spine (Phila Pa 1976)
(2005) - et al.
An apparatus for applying pure nonconstraining moments to spine segments in vitro
Spine (Phila Pa 1976)
(1995) - et al.
In vitro biomechanics of cervical disc arthroplasty with the ProDisc-C total disc implant
Neurosurgery Focus
(2004) - et al.
Multidirectional flexibility analysis of cervical artificial disc reconstruction: in vitro human cadaveric spine model
Journal of Neurosurgery Spine
(2005) - et al.
Multidirectional flexibility analysis of anterior and posterior lumbar artificial disc reconstruction: in vitro human cadaveric spine model
European Spine Journal
(2006) - et al.
Biomechanical testing of posterior atlantoaxial fixation techniques
Spine (Phila Pa 1976)
(2002)
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