Introduction: Osteoarthritis (OA) of the apophyseal (facet) joints often appears to follow degenerative changes in the adjacent intervertebral discs. We test the hypothesis that facet joint OA is directly related to high compressive load-bearing resulting from disc degeneration.

Methods: Thirty six cadaveric thoraco-lumbar “motion segments” consisting of two vertebrae and the intervening disc and ligaments, were obtained from 22 human cadavers aged 64–92 yrs (mean 77 yrs). Each was subjected to a constant compressive load of 1.5 kN while the distribution of compressive stress was measured along the mid-sagittal diameter of the intervertebral disc, using a miniature pressure transducer, side-mounted in a 1.3 mm-diameter needle. Measurements of compressive “stress” were summed over area to give the compressive force resisted by the disc. This was subtracted from the applied 1.5 kN to indicate compressive load-bearing by the neural arch, including the apophyseal joints. After mechanical testing, the cartilage of each apophyseal joint surface was graded for degree of degeneration. Joints were then macerated, and each bony joint surface was scored for the following four degenerative changes, according to established criteria: marginal osteophytes, pitting, bony contour change, and eburnation. The four bone scores were summed and used to represent the severity of OA for that joint surface, and values were then averaged for the two facet joints (four surfaces) of each motion segment.

Results: Cartilage degeneration and summed bone scores both increased with age, and with each other (P< 0.01). Neural arch load-bearing ranged from 5%–96% (mean 45%) of the applied 1.5 kN compressive force, with values over 50% being found only in specimens with degenerated intervertebral discs. Facet joint summed bone score increased with neural arch load-bearing (P< 0.01), especially when the latter exceeded 50%.

Conclusion: High apophyseal joint load loading, equivalent to neural arch compressive load-bearing above 50%, is strongly associated with severe OA changes in the apophyseal joints. Associations were stronger for bone rather than cartilage changes, possibly because pathological load-bearing by the facet joints can occur between the tip of the inferior articular process and the adjacent lamina, substantially by-passing the articular (cartilage) surfaces.