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Technical Briefs

# Development of a Model Based Method for Investigating Facet Articulation

[+] Author and Article Information
Daniel J. Cook1

Department of Bioengineering, and Department of Neurological Surgery, University of Pittsburgh, Pittsburgh, PA 15212cook348@gmail.com

Boyle C. Cheng

Department of Bioengineering, and Department of Neurological Surgery, University of Pittsburgh, Pittsburgh, PA 15212

1

Corresponding author.

J Biomech Eng 132(6), 064504 (Apr 28, 2010) (6 pages) doi:10.1115/1.4001078 History: Received October 14, 2009; Revised January 08, 2010; Posted January 21, 2010; Published April 28, 2010; Online April 28, 2010

## Abstract

Reported investigations of facet articulation in the human spine have often been conducted through the insertion of pressure sensitive film into the joint space, which requires incision of the facet capsule and may alter the characteristics of interaction between the facet surfaces. Load transmission through the facet has also been measured using strain gauges bonded to the articular processes. While this method allows for preservation of the facet capsule, it requires extensive instrumentation of the spine, as well as strain-gauge calibration, and is highly sensitive to placement and location of the strain gauges. The inherently invasive nature of these techniques makes it difficult to translate them into medical practice. A method has been developed to investigate facet articulation through the application of test kinematics to a specimen-specific rigid-body model of each vertebra within a lumbar spine segment. Rigid-body models of each vertebral body were developed from CT scans of each specimen. The distances between nearest-neighboring points on each facet surface were calculated for specific time frames of each specimen’s flexion/extension test. A metric describing the proportion of each facet surface within a distance (2 mm) from the neighboring surface, the contact area ratio (CAR), was calculated at each of these time frames. A statistically significant difference $(p<0.037)$ was found in the CAR between the time frames corresponding to full flexion and full extension in every level of the lumbar spine (L1–L5) using the data obtained from the seven specimens evaluated in this study. The finding that the contact area of the facet is greater in extension than flexion corresponds to other findings in the literature, as well as the generally accepted role of the facets in extension. Thus, a biomechanical method with a sufficiently sensitive metric is presented as a means to evaluate differences in facet articulation between intact and treated or between healthy and pathologic spines.

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## Figures

Figure 1

Superior and inferior surfaces defined by the ROI for the left facet of an FSU (L1–L2)

Figure 2

VDM superimposed on single semitransparent FSU (L1–L2)

Figure 3

Digitization using four-marker probe

Figure 4

Distance mapping for full flexion and full extension sagittal view of a single specimen

Figure 5

Distance mapping for full flexion and full extension rear coronal view (same specimen as shown in Fig. 4)

Figure 6

Mean CAR during flexion/extension test (only the CAR for the superior surface at each intervertebral level is shown)

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