Determination of annulus fibrosus cell response to tensile strain as a function of duration, magnitude, and frequency.

As clinical evidence suggests that mechanical forces can have both reparative and traumatic effects on the spine, we investigated responses to different magnitudes, frequencies, and durations of applied tensile strain in an in vitro system. We examined the interactions of inflammatory and mechanical stimuli on cells isolated from the annulus fibrosus. Rabbit annulus fibrosus fibrochondrocytes were cultured in the presence or absence of an inflammatory stimulus. Cells were exposed to various magnitudes and frequencies of tensile strain for 4 or 24 h, and mRNA expression of catabolic mediators of inflammation and matrix degradation was measured by quantitative real time PCR and compared to control cells. Conditioned media were analyzed for matrix metalloprotease activity and production of prostaglandin E(2) . Application of low magnitudes and frequencies of tensile strain resulted in down-regulation of catabolic mediators, particularly under inflammatory stress. However, loss of this protective effect was observed at higher frequency and magnitude, and after prolonged duration. These in vitro data confirm the existence of magnitude, frequency, and duration based effects, which determine biochemical response of disc tissue resulting in either anti- or pro-catabolic outcomes. This may help to explain the beneficial effects of motion-based therapies as well as the destructive effect of traumatic levels of applied strain.