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Technical Brief

Moment Measurements in Dynamic and Quasi-Static Spine Segment Testing Using Eccentric Compression are Susceptible to Artifacts Based on Loading Configuration

[+] Author and Article Information
Carolyn Van Toen

Orthopaedic and Injury Biomechanics Group,
Departments of Mechanical Engineering & Orthopaedics and
the International Collaboration on Repair
Discoveries (ICORD),
University of British Columbia,
818 West 10th Avenue,
Room 5000,
Vancouver V5Z 1M9, BC, Canada
e-mail: carolyn@mech.ubc.ca

Jarrod W. Carter

Origin Engineering LLC,
23403 E. Mission Avenue,
Suite #223,
Liberty Lake, WA 99019
e-mail: jc@oerecon.com

Thomas R. Oxland

Orthopaedic and Injury Biomechanics Group,
Departments of Mechanical Engineering & Orthopaedics and
the International Collaboration on Repair
Discoveries (ICORD),
University of British Columbia,
818 West 10th Avenue,
Room 5460,
Vancouver V5Z 1M9, BC, Canada
e-mail: toxland@interchange.ubc.ca

Peter A. Cripton

Orthopaedic and Injury Biomechanics Group,
Departments of Mechanical Engineering & Orthopaedics and
the International Collaboration on Repair
Discoveries (ICORD),
University of British Columbia,
6250 Applied Science Lane,
Vancouver V6T 1Z4, BC, Canada
e-mail: cripton@mech.ubc.ca

1Corresponding author.

Manuscript received November 18, 2013; final manuscript received October 10, 2014; accepted manuscript posted October 16, 2014; published online October 31, 2014. Assoc. Editor: Joel D. Stitzel.

J Biomech Eng 136(12), 124505 (Oct 31, 2014) (7 pages) Paper No: BIO-13-1544; doi: 10.1115/1.4028817 History: Received November 18, 2013; Revised October 10, 2014; Accepted October 16, 2014

The tolerance of the spine to bending moments, used for evaluation of injury prevention devices, is often determined through eccentric axial compression experiments using segments of the cadaver spine. Preliminary experiments in our laboratory demonstrated that eccentric axial compression resulted in “unexpected” (artifact) moments. The aim of this study was to evaluate the static and dynamic effects of test configuration on bending moments during eccentric axial compression typical in cadaver spine segment testing. Specific objectives were to create dynamic equilibrium equations for the loads measured inferior to the specimen, experimentally verify these equations, and compare moment responses from various test configurations using synthetic (rubber) and human cadaver specimens. The equilibrium equations were verified by performing quasi-static (5 mm/s) and dynamic experiments (0.4 m/s) on a rubber specimen and comparing calculated shear forces and bending moments to those measured using a six-axis load cell. Moment responses were compared for hinge joint, linear slider and hinge joint, and roller joint configurations tested at quasi-static and dynamic rates. Calculated shear force and bending moment curves had similar shapes to those measured. Calculated values in the first local minima differed from those measured by 3% and 15%, respectively, in the dynamic test, and these occurred within 1.5 ms of those measured. In the rubber specimen experiments, for the hinge joint (translation constrained), quasi-static and dynamic posterior eccentric compression resulted in flexion (unexpected) moments. For the slider and hinge joints and the roller joints (translation unconstrained), extension (“expected”) moments were measured quasi-statically and initial flexion (unexpected) moments were measured dynamically. In the cadaver experiments with roller joints, anterior and posterior eccentricities resulted in extension moments, which were unexpected and expected, for those configurations, respectively. The unexpected moments were due to the inertia of the superior mounting structures. This study has shown that eccentric axial compression produces unexpected moments due to translation constraints at all loading rates and due to the inertia of the superior mounting structures in dynamic experiments. It may be incorrect to assume that bending moments are equal to the product of compression force and eccentricity, particularly where the test configuration involves translational constraints and where the experiments are dynamic. In order to reduce inertial moment artifacts, the mass, and moment of inertia of any loading jig structures that rotate with the specimen should be minimized. Also, the distance between these structures and the load cell should be reduced.

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Figures

Grahic Jump Location
Fig. 1

(a) Overall free-body diagram. (b)–(d) Free body diagrams for m1, m2, and m3, respectively. (e) Orientation of the load cell outputs indicating the positive force and moment directions for those presented in Fig. 3 (F4 shear force, anterior positive; M4 bending moment, flexion positive). (f) Free-body diagram for calculating the loading environment at point C (synthetic specimens: where the specimen meets the top of the inferior potting material; cadaver specimens: the centroid of the inferior intervertebral disc). Positive force and moment directions for those presented in Figs. 4 and 5 are indicated by CC (axial force, compression positive), FC (shear force, anterior positive), and MC (bending moment, flexion positive).

Grahic Jump Location
Fig. 2

Photograph of the slider and hinge joint test configuration with the neoprene spring rubber specimen (durometer rating 75 A, diameter 4.45 cm, height 5.5 cm, McMaster Carr, Elmhurst, IL) potted in PMMA. Six-axis loads were recorded inferior to the specimen.

Grahic Jump Location
Fig. 3

Calculated and measured anteroposterior shear force at the load cell (F4: anterior positive) for the quasi-static (a) and dynamic (c) test with the roller configuration. Shear forces for the dynamic test with the roller configuration are the result of dynamic terms: mass and horizontal acceleration of the structures moving with the specimen (Eq. (7)). Calculated and measured sagittal moment at the load cell (M4: flexion positive) for the quasi-static (b) and dynamic (d) test with the roller configuration. The three terms of the calculated moment are also shown: compression term, linear acceleration term, and rotational acceleration term (Eq. (4)).

Grahic Jump Location
Fig. 4

Photographs of the hinge (part number 4388 80/20 Inc., Columbia City, IN) (a), linear slider (model HRW 35CA, THK, Schaumburg, IL) and hinge (b), and urethane roller (durometer rating 80 A, diameter 1.5 in., McMaster Carr, Elmhurst, IL) (c) joints. For posterior eccentric compression loading, extension moments are expected. Axial force (CC: compression positive) and sagittal bending moment (MC: flexion positive) at the inferior edge of the synthetic specimen for the hinge joint (d, g), hinge and slider joints (e, h), and roller joint (f, i). Plots for the quasi-static tests (5 mm/s: d–f) and dynamic tests (0.4 m/s: g–i) are shown.

Grahic Jump Location
Fig. 5

Axial force (CC: compression positive) and sagittal moment (MC: flexion positive) at the centroid of the inferior intervertebral disc of the human cadaver specimen for the dynamic compression tests with a roller configuration with posterior eccentricity (a—specimen 14, c—specimen 29, e—specimen 39) and with anterior eccentricity (b—specimen 3, d—specimen 5, f—specimen 7) [5]. The average donor age was 73 years (standard deviation 18 years), 11 specimens were from female donors and five were from male donors. For posterior eccentric loading, extension moments are expected and for anterior eccentric compression loading, flexion moments are expected. Note that a, c, e and b, d, f have different vertical scales.

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