Biochemical and biomechanical characterization of the cervical, thoracic, and lumbar facet joint cartilage in the Yucatan minipig

Abstract

Facet joint arthrosis causes pain in approximately 7 % of the U.S. population, but current treatments are palliative. The objective of this study was to elucidate structure-function relationships and aid in the development of future treatments for the facet joint. This study characterized the articular surfaces of cervical, thoracic, and lumbar facet cartilage from skeletally mature (18-24 mo) Yucatan minipigs. The minipig was selected as the animal model because it is recognized by the U.S. Food and Drug Administration (FDA) and the American Society for Testing and Materials (ASTM) as a translationally relevant model for spine-related indications. It was found that the thoracic facets had a ∼2 times higher aspect ratio than lumbar and cervical facets. Lumbar facets had 6.9-9.6 times higher % depth than the cervical and thoracic facets. Aggregate modulus values ranged from 135 to 262 kPa, much lower than reported aggregate modulus in the human knee (reported to be 530-701 kPa). The tensile Young's modulus values ranged from 6.7 to 20.3 MPa, with the lumbar superior facet being 304 % and 286 % higher than the cervical inferior and thoracic superior facets, respectively. Moreover, 3D reconstructions of entire vertebral segments were generated. The results of this study imply that structure-function relationships in the facet cartilage are different from other joint cartilages because biochemical properties are analogous to other articular cartilage sources whereas mechanical properties are not. By providing functional properties and a 3D database of minipig facet geometries, this work may supply design criteria for future facet tissue engineering efforts.

Keywords: Biomechanics; Cartilage; Characterization; Facet joint; Minipig; Structure-function relationships; Zygapophyseal joint.

Figure 1:

Entire minipig spine with the location of the C6-C7, T4-T5, and L5-L6 facet joints circled.

Figure 2:

3D reconstructions of total vertebrae. C6, T4, and L5 vertebrae are oriented so that the caudal side faces forward. C7, T5, and L6 vertebrae are oriented so that the sacral side faces forward. An image of each segment rotated 45° clockwise and a close of up the facet joint are also included. The dashed red line represents the outline of the cartilage surface. Corresponding .stl files are provided via Mendeley data as supplementary downloads.

Figure 3:

A. Orientation of morphometric measurements and B. morphometric characteristics of minipig facet joints. Values presented are mean+standard deviation for n=5–6. Joints not connected by the same letter are statistically different (p<0.05) from each other.

Figure 4:

Histology of minipig facet joints. H&E (left two columns), Safranin-O/fast green (center two columns), and picrosirius red (right two columns) were used to visualize structure, glycosaminoglycans, and collagen content, respectively. Scale bar=200μm.

Figure 5:

Biochemical properties of minipig facet joints. Values presented are mean+standard deviation for n=5–6. No statistical significance was observed among joints (p<0.05).

Figure 6:

Biomechanical characteristics of minipig facet joints. Tensile parameters examined were Young’s modulus, ultimate tensile stress (UTS), and strain at failure. Compressive properties examined were aggregate modulus, permeability, and shear modulus. Values presented are mean+standard deviation for n=5–6. Joints not connected by the same letter are statistically different from each other (p<0.05).