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

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.

Novel posterior artificial atlanto-odontoid joint for atlantoaxial instability: a biomechanical study

“…The specimens were loaded using a testing system comprising cables and pulleys that applied pure moments and induced the flexion-extension, lateral bending, and axial rotation of C1-2. 25 Each moment was applied in 3 load-unload cycles to achieve a maximum torque of 1.5 Nm at a rate of 0.1 Nm/second. 14 The torque of 1.5 Nm was held constant for 10 seconds to stabilize the mechanical response.…”

Spine biomechanical testing methodologies: The controversy of consensus vs scientific evidence

“…The testing apparatus required for the application of pure moments is often considered to be more straightforward for developing in‐house when compared to the testing systems required to replicate complex 6DOF in vivo translations and rotations. There is substantial work based on both the application of pure moments, either with axial loads 128,136,137,142,150,155,160,161,163,164,166‐168,171,179,197,203‐206 or without axial loads, 7,39,63,134,137,162,174‐177,207‐214 and with using displacement control/stiffness test methods also being commonly used 23,32,43,125,126,148,165,215‐218 …”

“…in vivo translations and rotations. There is substantial work based on both the application of pure moments, either with axial loads 128,136,137,142,150,155,160,161,163,164,[166][167][168]171,179,197,[203][204][205][206] or without axial loads, 7,39,63,134,137,162,[174][175][176][177][207][208][209][210][211][212][213][214] and with using displacement control/stiffness test methods also being commonly used. 23,32,43,125,126,148,165,[215][216][217][218] Th...…”

Validation of a custom spine biomechanics simulator: A case for standardization

“…Spine biomechanics are often studied with human cadaveric specimens manipulated by mechanical test machines with optical motion capture systems to measure the vertebral rotations (Goel et al, 1995;Kelly and Bennett, 2013;Malcolmson et al, 2007;Mannen et al, 2015;Panjabi et al, 1985;Tang et al, 2012;Wilke et al, 2001Wilke et al, , 1998b. Some machines can apply a load in a single physiologic plane (pure moment) to better simulate in vivo biomechanics and facilitate comparison between laboratories (Wilke et al, 1994).…”

“…The University of Arkansas for Medical Sciences' Spine Biomechanics Simulator (SBS, Figure 1) can apply and measure loads in real-time. While vertebral motion is typically measured with an external motion capture system, the SBS can also measure the global motion of a specimen, or the displacement (both rotations and translations) between the two potted ends (Eguizabal et al, 2010;Galvis et al, 2017;Mannen et al, 2015;Tang et al, 2012). Each of the commercially available components of the system have quantified rotation errors, however the custom setup does not.…”